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Commercial Vehicle Air Consumption:
Simulation, Validation and
Recommendation
PARSA BROUKHIYAN
BETHUEL KARANJA
Master of Science Thesis
Stockholm, Sweden 2017
Commercial Vehicle Air Consumption: Simulation, Validation and
Recommendation
Parsa Broukhiyan Bethuel Karanja
Master of Science Thesis MMK 2017:99 MKN 194
KTH Industrial Engineering and Management
Machine Design
SE-100 44 STOCKHOLM
I
Examensarbete MMK 2017:99 MKN 194
Luftförbrukning i kommersiella fordon:
Simulering, validering och rekommendationer
Parsa Broukhiyan
Bethuel Karanja
Godkänt
2017-06-13
Examinator
Ulf Sellgren
Handledare
Ulf Sellgren
Uppdragsgivare
Scania CV AB
Kontaktperson
Robert Skaba
Sammanfattning
I denna rapport beskrivs ett examensarbete som genomfördes på bromsavdelningen på Scania
CV AB. Projektet innefattar utveckling av en numerisk modell (i Matlab) som beräknar och
förutspår luftförbrukningen i en lastbil under olika körcykler. I rapporten beskrivs det tester
och experiment som gjordes för att ta fram nödvändiga uppgifter för utvecklingen av
modellen. Sedan presenteras modellen som skapades och alla valideringstester som
genomfördes. Modellen är gjord så att användaren kan kombinera olika
komponentkombinationer för lastbilar med olika lastningskonfigurationer och körcykler.
Slutligen används modellen för att utvärdera luftförbrukningen i lastbilar under särskilt
ansträngande körcykler.
Den utvecklade modellen visade sig vara pålitlig och korrekt med en felmarginal på 7% med
avseende på mängden luft som konsumeras. Med dess hjälp kunde flera rekommendationer
ges om hur luftförbrukningen i kommersiella fordon kan förbättras. De bästa
komponentkombinationerna hittades också och presenteras i denna rapport
Nyckelord: Numerisk modell, Luftförbrukning, Pneumatiskt System, Simulering, Broms,
Luftfjädring
II
III
Master of Science Thesis MMK 2017:99 MKN 194
Commercial vehicle air consumption: Simulation, validation and recommendations
Parsa Broukhiyan
Bethuel Karanja
Approved
2017-06-13
Examiner
Ulf Sellgren
Supervisor
Ulf Sellgren
Commissioner
Scania CV AB
Contact person
Robert Skaba
Abstract
This report details the work done in a master thesis project. The project was conducted at the
Brake Performance Department at Scania CV AB. The project involves the development of a
numerical model (in Matlab) that calculates and predicts air consumption in a truck under
different drive cycles. The report first details tests and experiments done so as to acquire the
necessary information for the development of the model. The report then presents the model
that was created and delves into tests that were conducted for its validation. A model is
created that allows the user to select different component combinations on the trucks along
with different loading scenarios and drive cycles. Finally the model is used to evaluate air
consumption in trucks during particularly strenuous cycles.
The model developed is found to be reliable and accurate to with 7% with regard to amount of
air consumed. With its help, several recommendations on how air consumption in commercial
vehicles can be improved are made. The best components’ combination is also found and
presented.
Keywords: Numerical model, air consumption, pneumatic system, simulation, brake, air
suspension
IV
V
FOREWORD
This thesis is written as completion to the master program Engineering Design: Machine
Design track at KTH Royal Institute of Technology in Stockholm. In this chapter the authors
would like to acknowledge help, assistance, cooperation and inspiration, important for the
presented master thesis project provided by others.
We would like to thank Scania for giving us this opportunity to work on such an exciting
project. We are extremely thankful to our supervisor at Scania, Robert Skaba, and our
colleagues, Richard Vadasz Martin Sundberg, Tomas Nordin, Arne Lindqvist, Erik Stugholm,
David Johansson, Stefan Karlberg, Johanna Tikka Turunen and Henrik Svensson for their
valuable comments and technical assistance. We would also like to acknowledge Youssef
Saliba, Oskar Wernerson and Fredrik Larsson for their valuable assistance with setting up so
many of the experiments we conducted. Our thanks are also extended to IT support of Scania
specially Stefan Despotovic, our supervisor at KTH, Ulf Sellgren, for giving us the
opportunity to make this work happen and Fredrik Karlsson, our department manager, for all
his support.
Finally we thank all of our family and friends for their support and encouragement during the
entire time we were working on this thesis report. We will be always indebted to them.
Parsa Broukhiyan and Bethuel Karanja
Stockholm, June 2017
VI
VII
NOMENCLATURE
Here are the notations and abbreviations that are used in this Master thesis.
Notations
Symbol Description
A Acceleration (m⋅s-2)
Fbrake Braking force (N)
Fl Flow (NLPM)
m Mass (kg)
M Molar mass (kg⋅mol-1
)
P Pressure (Pa)
Q Flow (l⋅s-1)
R Gas constant (J⋅mol-1⋅K-1
)
𝑅𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 Specific gas constant (J⋅kg-1⋅K-1
)
t Time (s)
T Temperature (K)
υ Specific volume (m3/kg)
V Volume (m3)
Abbreviations
APS Air Processing System
AST Air Suspension Tanks
NLPM Normal litres per minute
OEM Original Equipment Manufacturer
VIII
IX
TABLE OF CONTENTS
Sammanfattning ...................................................................................................................... I
Abstract .................................................................................................................................. III
FOREWORD ........................................................................................................................... V
NOMENCLATURE ............................................................................................................... VII
TABLE OF CONTENTS ....................................................................................................... IX
1 INTRODUCTION .............................................................................................................. 1
1.1 Background .............................................................................................................. 1
1.2 Purpose ..................................................................................................................... 1
1.3 Planned deliverables ............................................................................................. 2
1.4 Delimitations ............................................................................................................ 2
1.5 Chosen methodology ............................................................................................ 2
2 FRAME OF REFERENCE .............................................................................................. 5
2.1 Pneumatics ............................................................................................................... 5
2.1.1 Air characteristics ........................................................................................... 6
2.2 Airflow Units ............................................................................................................. 8
2.3 Vehicle Pneumatic System ................................................................................... 9
2.3.1 Air Compressor ................................................................................................ 9
2.3.2 Air Processing System .................................................................................. 9
2.3.3 Service Brake Circuits (1 & 2) .................................................................... 12
2.3.4 Parking Brake Circuit (3) ............................................................................. 13
2.3.5 Auxiliary Circuit (4) ....................................................................................... 13
2.3.6 Air Suspension Circuit (5) ........................................................................... 14
2.3.7 Brake chambers ............................................................................................. 15
2.3.8 Air Bellows ...................................................................................................... 16
2.4 Previous Work ....................................................................................................... 17
2.4.1 Mapping of air consumption for heavy vehicles ................................... 17
2.4.2 Investigation of Air Volumes and Pressure Levels in Air Brake
Systems .......................................................................................................................... 17
2.4.3 Survey of Air Consumption on B6x2B ..................................................... 18
3 REQUIREMENT SPECIFICATION ............................................................................. 19
4 THE PROCESS .............................................................................................................. 21
X
4.1 Model ........................................................................................................................ 21
4.2 Available data ........................................................................................................ 23
4.2.1 Compressors .................................................................................................. 23
4.2.2 Air consumption by air suspension ......................................................... 23
4.3 Experiments ........................................................................................................... 25
4.3.1 Gas Law Experiment ..................................................................................... 26
4.3.2 Flow metre analysis ...................................................................................... 27
4.3.3 Parking brake chamber ................................................................................ 30
4.3.4 Overflow valve: opening pressure ............................................................ 33
4.3.5 Overflow valve: flow ..................................................................................... 34
4.3.6 IDU: Compression and regeneration ....................................................... 35
4.3.7 APS behaviour ............................................................................................... 35
4.3.8 Service brake .................................................................................................. 39
5 RESULTS ........................................................................................................................ 41
5.1 User interface ......................................................................................................... 41
5.1.1 Main figure ....................................................................................................... 41
5.1.2 Input figures .................................................................................................... 42
5.1.3 Output figures ................................................................................................ 46
5.2 How the model works .......................................................................................... 50
5.2.1 The model (pBk’s) code ............................................................................... 50
5.2.2 User data .......................................................................................................... 52
5.2.3 How to feed the data into pBk .................................................................... 54
5.2.4 The model (pBk) file ...................................................................................... 54
5.2.5 Simulation ....................................................................................................... 55
5.2.6 Failures ............................................................................................................. 57
6 DISCUSSION .................................................................................................................. 59
6.1 Impact of assuming that p-brake release is isothermal ............................. 59
6.2 Evaluation of the model ...................................................................................... 61
6.2.1 The harsh cycles, high load ....................................................................... 61
6.2.2 The very harsh cycles, high load .............................................................. 64
6.2.3 The whole drive .............................................................................................. 67
6.2.4 The harsh cycle, medium load ................................................................... 69
6.2.5 The very harsh cycle, medium load ......................................................... 70
XI
6.2.6 Causes of error .............................................................................................. 71
6.3 Evaluation of the tests ......................................................................................... 72
6.4 Further discussion ............................................................................................... 73
6.4.1 Results of further investigation ................................................................. 75
6.5 A discussion about APS2 ................................................................................... 77
7 RECOMMENDATIONS ................................................................................................. 79
7.1 Compressor ............................................................................................................ 79
7.2 Service brake activation ..................................................................................... 80
7.3 Using the best combination for refuse trucks .............................................. 80
8 CONCLUSIONS ............................................................................................................. 81
9 FUTURE WORKS .......................................................................................................... 83
10. REFERENCES ............................................................................................................ 85
XII
1
1 INTRODUCTION
This report presents a thesis project carried out at Scania CV AB. The project is carried
out by two students from the Machine Design department at KTH. It is carried out at the
Brake Performance department (RTCS) at Scania under supervision of one of the
development engineers there. This chapter describes the background of the problem tackled in
the project, the purpose of the project, delimitations and chosen methodology.
1.1 Background
Scania, founded in 1891, is a leading manufacturer of heavy trucks and buses. [1]. Given
its market, it is under constant pressure to design and produce commercial vehicles that offer
high efficiency and fuel savings, while simultaneously providing great performance. Several
functions on the vehicles they produce are driven by compressed air. Examples of these
include air suspension, brakes, seat adjustment etc. It must be ensured that adequate supply of
compressed air is available and that it is used in an efficient way.
In the systems mentioned above, compressed air is used as the energy transferring medium.
Compressed air is usually provided by a compressor that is coupled to the engine. The air is
then transferred to, and stored, in reservoirs under high pressure from where it can be supplied
to the different systems when needed.
Air circuits are one of the most important parts of the trucks. Using compressed air, Scania
provides comfort (air suspension) and safety (air brake system) for their customers. Hence, air
consumption is one of the important things that heavy truck manufacturers are dealing with.
Lower air consumption will mean a lighter and cheaper truck in the first place. Additionally, it
will provide more space on the truck for other components if needed. Mapping total air
consumption of a truck under different drive cycles would help Scania to better understand
the demands of their products and thus aid in their improvement.
1.2 Purpose
The purpose of the thesis project is to give an overview of how compressed air is
consumed in a truck by different functions. Once this is done, the aim will then be to
determine whether the intended drive cycle profile for a specific vehicle type is feasible per
customer wishes, for a given type of air compressor. If the cycle is not feasible, it should be
determined why and after how long it will fail. What pneumatic function will fail? How can
the air usage be altered or rearranged to help the customer fulfil their demand? The accuracy
of the simulation is to be evaluated using an actual truck.
The investigation will include a simulation of the whole vehicle’s pneumatic system’s air
consumption, taking into account the individual air consuming constituents, air compressor
2
duty cycle control strategy, air compressor type and vehicle drive cycles. The independent
variables that define the air consumption tendencies of the highest consuming devices will be
studied and confirmed through performance testing. The results will finally be compared and
verified with actual vehicle level testing.
1.3 Planned deliverables
At the end of the project, a numerical model for the entire vehicle’s pneumatic system
done in MATLAB will be delivered. The model will be used to calculate the air consumption
for given vehicles and drive cycles. The model should allow the user to choose between
several component (e.g. brake chambers and air suspension bellows) and factor (e.g. axle load
) combinations that will result in unique systems as the user desires. The model should also
allow the user to choose, or possibly create, different driving cycles.
As the project also delves into the efficiency of the current systems, suggestions on
possible improvements will also be made.
1.4 Delimitations
The project focuses on air consumption by trucks and tractors and as such will not delve
into air consumption by trailers that could be connected to said trucks/tractors. The project
will prioritize the larger air consumers which are the brakes, air suspension and desiccant
regeneration. After these have been simulated the accuracy of the model will be evaluated and
if necessary more systems will be added. If this will not be necessary then more effort will be
spent on fine-tuning the model.
The reason for these delimitations is so as to be able to complete the project within the given
time frame.
1.5 Chosen methodology
The first part of the project involves learning about the different systems that are driven by
compressed air. This will be done through reading reports and other documentations written
about them and also by talking to the persons who work on the systems. Once a thorough
understanding has been obtained, the following phase will involve creating numerical models
that calculate the air consumed by the different systems. These models will be based in part
on pneumatic laws and principles and in part on results from experiments that have been
previously conducted by the RTCS department. Where needed, and possible, measurements
will be done on actual trucks or bench test to either gather more information for model
creation or to verify the models created.
3
Throughout this process, the respective systems will be analysed for potential improvements.
To evaluate the validity of any potential improvements arrived at, they will be tested either in
the numeric model created or on actual trucks/bench tests.
4
5
2 FRAME OF REFERENCE
The reference frame is a summary of the existing knowledge and former performed
research on the subject. This chapter presents the theoretical reference frame that is
necessary for the performed research.
2.1 Pneumatics
Pneumatics deals with the study of the behaviour and application of compressed air [2]. A
pneumatic system transmits and controls energy through the use of a pressurized gas. Air is
commonly used by drawing it from the atmosphere and reducing it in volume by compression,
thus increasing its pressure. Compressed air is usually used by acting on a piston to provide
useful mechanical energy [3].
Some advantages and reasons for the wide use of compressed air in industry (including
trucks) are [3]:
Availability
Most factories and industrial plants have a compressed air supply in working areas and
portable compressors can serve more remote situations.
Storage
Since air is compressible it can be stored in tanks to be used as both the energy storage
medium and the actuating fluid
Simplicity of Design and Control
Pneumatic components have a simple design and are easily fitted to provide extensive
automated systems with comparatively simple control.
Economy
Installation is of relatively low cost due to modest component cost. There is also a low
maintenance cost due to long life without service.
Reliability
Pneumatic systems are a long established technology that allow for use of off the shelf
components. They can function even with leakages where as in hydraulics even small
leakages may lead to system failure.
Environmentally Clean
It is clean and with proper exhaust air treatment can be installed to clean room standards.
Safety
It is not a fire hazard in high risk areas, and the system is unaffected by overload as
actuators simply stall or slip. Pneumatic actuators do not produce heat, other than friction.
6
2.1.1 Air characteristics
Air is a mixture of several gases and its behaviour can be approximated with the help of
the ideal gas law.
2.1.1.1 Ideal Gas Model
The ideal gas model has many applications in engineering [4]. An ideal gas is defined as
any gas whose P-υ-T relationship is of the form:
𝑃υ = 𝑅𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐T (1)
in which P is the pressure (in Pascal), υ is specific volume (in m3/kg), Rspecific is specific gas
constant (in J kg-1
K-1
) and T is the temperature (in K) of the gas.
Just what pressure and temperature give ideal-gas behaviour depends on the gas and the
amount of deviation from Eq. (20) that one will accept. Figure 1 shows the deviation from
ideal-gas behaviour for several gases. In general one can say that, if the temperature is well
above the critical temperature (132 K) and the pressure is well below the critical pressure
(37.7 bar), the ideal-gas model is accurate in defining the behaviour of air [5].
The defining equation Pυ = RspecificT can be put into other useful forms. If m is the mass of a
sample of gas (in kg) occupying volume V (in cubic meter), multiplication by the mass yields:
𝑃𝑉 = 𝑚𝑅𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐𝑇 (2)
Denoting the number of moles of the gas by n and observing that the mass m is related to the
number of moles and the molar mass M by:
𝑚 = 𝑛𝑀 (3)
Knowing that:
𝑅 = 𝑀𝑅𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 (4)
Eq. (2) can be expressed as:
𝑃𝑉 = 𝑛𝑅𝑇 (5)
7
Figure 1 Test of the ideal-gas approximation for several gases [5]
It can be seen from Figure 1 that air can be considered as an ideal gas within 2.5 % error
while having a temperature of 300 K and a pressure between 0-100 atmospheres.
2.1.1.1.1 Special cases
There are some special cases of the ideal gas law. Boyle’s law, Charles’s law and
Avogadro’s law represent these cases [6].
Boyle’s law
If the quantity of gas and the temperature are held constant then:
𝑛𝑅𝑇 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (6)
From Eq. (5&6):
𝑃𝑉 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (7)
A very common situation is that P, V and T are changing for a fixed quantity of gas, in which:
8
𝑛𝑅 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (8)
From Eq. (5&8):
𝑃𝑉
𝑇= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
(9)
Thus, the system before and after the changes in P, V and/or T can be compared:
𝑃1𝑉1
𝑇1=
𝑃2𝑉2
𝑇2
(10)
2.2 Airflow Units
Volumetric flow is defined as the volume of fluid flowing per unit of time. It can be
calculated as the product of the cross-sectional area and average velocity of the flow passing
through an area. But it is common in the industry to indicate the capacity of an air compressor
in normal litres per minute or NLPM [7]. NLPM is basically a universal unit for flow. It
represents the volume of air [normal litres] being transferred in one minute. Normal litre
means the amount of air that would occupy a one litre capacity at atmospheric pressure and
temperature of 0 °C. It is possible to convert the volumetric flow rate from this normal state to
other states (different pressure and temperatures) and vice versa using Eq.(11) below.
𝑞𝑁 = 𝑞𝑥 ∙
𝑃𝑥
𝑃𝑁∙
𝑇𝑁
𝑇𝑥
(11)
qx = Volumetric flow rate at X conditions of temperature and pressure. (l/s)
qN = Volumetric flow rate at 0 °C and 1 atm. (Nl/s)
Px = Pressure at X conditions of temperature and pressure. (bar)
PN = Pressure at 0 °C and 1atm. (1.01325 bar)
Tx = Temperature at X conditions of temperature and pressure. (K)
TN = Temperature at 0 °C and 1 atm. (273 K)
Eq. (11) does not take humidity into account [8].
It is common that manufacturers specify the flow rate of an air compressor in Free Air
Delivery (FAD). The only way this differs from the described condition (normal state) is that
the temperature used is 20 °C instead of 0 °C [7].
9
2.3 Vehicle Pneumatic System
The pneumatic system of the vehicle consists of a compressor coupled to the engine, the
pneumatic circuits that it feeds with compressed air and the Air Processing System that
controls this feeding. These different parts of the pneumatic system are presented in this
section.
2.3.1 Air Compressor
The compressor is coupled to the engine through a gear system and when it is running it
pumps compressed air into the pneumatic system. The compressors that are used in the trucks
and trailers are of the reciprocating variety.
The principal parts of a reciprocating air compressor are the same as that for an engine. The
reciprocating air compressor may be single-acting (air is admitted to one side of the piston
only), or double-acting (air is admitted to each side of the piston alternatively), and may be
single-stage or multi-stage. In a multi-stage compressor, the air is compressed in several
stages instead of compressing the air fully in a single cylinder. This is equivalent to a number
of compressors arranged in series. The pressure of air is increased in each stage. Multi-stage
compressors will provide higher pressure [9].
2.3.2 Air Processing System
The APS cleans and dries the air that comes from the compressor and also controls how air
is allocated to the different pneumatic circuits in the vehicle [10]. The APS can be divided
into three modules as shown in the figure below.
Figure 2. The three modules of the APS [7]
A description of the functions of each module is provided in the following subsections.
10
2.3.2.1 Air Dryer
The air dryer module contains the following components:
Figure 3. Drawing and schematic of the air dryer module
1. Desiccant container
The desiccant container, contains the desiccant which captures the moisture in the air
coming from the compressor thereby drying it. The desiccant itself is dried in the regeneration
phase where dry air is blown back from the compressed system, through the desiccant and out
through a drain valve.
2. Check valve
This valve prevents the air from flowing back upstream.
3. Pressure limiting valve
This valve limits the pressure in the parking brake and trailer brake circuits to protect
pressure-sensitive components located therein.
4. Safety valve
This valve opens at high pressures and is meant to keep the pressure in the pneumatic
system from getting too high.
5. Drain valve
This valve opens during the regeneration phase to dry the desiccant [10].
11
2.3.2.2 Circuit Protection Valve
The circuit protection valve module contains several protection valves that determine at
what pressures, and therefore what sequence if starting with 0 bar in the system, the respective
circuits will be fed. These pressures are shown in the table below.
Table 1. Opening and closing pressures of the circuit protection valves
Circuit Opening pressure (bar) Closing pressure (bar)
Rear service brake 7.5 ≤4.5
Front service brake 7.5 ≤4.5
Trailer brake and parking brake 6 – 7.5
Given that front and rear
service are ≥ 7.2
≤4.0
Auxiliary circuit 7.5 ≤4.5
Air suspension 8.5 ≤4.5
Note that the parking brake circuit only opens after the service brakes circuits have reached
7.2 bar. This is to prevent the driver from being able to release the parking brake before there
is enough pressure to safely operate the service brakes.
This module also contains the regeneration solenoid valve and a solenoid valve for
compressor control. The former, when activated, allows air to flow from the compressed air
system, through the desiccant and out thorough the drain valve. The latter is activated when
there is zero pressure in the signal line and it in turn activates the compressor.
2.3.2.3 Control Unit
This module contains temperature and pressure sensors, a circuit board and a connection to
the vehicle’s CAN.
2.3.2.4 Pressure levels in the APS
The APS decides when to activate the compressor and when to run the regeneration phase
depending on system pressure and IDU level. The IDU is discussed in the next section.
Ordinarily, if the pressure is below the cut-in pressure, the compressor will turn on and it
turns off once the pressure has climbed to the cut-out pressure. During engine braking, if the
pressure is below the overrun cut-in pressure, the APS takes advantage of the ‘free’ kinetic
energy and turns on the compressor. The compressor turns off at the overrun cut-out pressure
in this case. From cut-in pressure and above, the regeneration can be turned on if the
compressor is not running and the IDU has reached a particular level. Slightly below the cut-
in pressure, regeneration will only be turned on if the compressor has been on for too long and
the IDU has reached a very high level. If the pressure is very much below the cut-in pressure
12
the compressor will be turned on regardless of whether or not the desiccant needs to
regenerate [10].
2.3.2.5 The IDU
IDU stands for Integrated Desiccant Use. It is an indication of how much moisture the
desiccant has collected. It is given in normal litres (NL). When it reaches a particular level the
desiccant has to be dried by blowing dry air back through the desiccant.
2.3.2.6 APS VARIANTS
There are currently two models of APS, the APS1 and APS2. The difference between the
two is that APS2 is a newer and improved model that has a slightly different software. Each
of these models has several variants. The two most common are the advanced variant and the
high capacity variant. The main difference between these two for the APS1 is that the latter
has a dedicated regeneration tank. This means that during regeneration a higher air flow can
be driven through the desiccant resulting in a faster, but less efficient, regeneration phase [10].
When it comes to the APS2 the high capacity variant has two desiccant containers [11]. This
means that the high capacity variant is able to regenerate one desiccant while simultaneously
feeding air from the compressor to the system through the other.
2.3.3 Service Brake Circuits (1 & 2)
These circuits operate the front and rear service brakes. A simplified schematic of an
example is shown in Figure 4 below. The lines coloured green feed the service brakes on the
front axle (circuit 2) while the ones coloured red feed those on the rear axles (circuit 1).
Figure 4 Simplified schematic of the front and rear service brake circuits [7]
13
The circuits each have their own pressure tanks and are connected to the service brake
module. The service brake module monitors the position of the brake pedal and sends a signal
to the Electronic Braking System (EBS) of when and how much to brake. This signal is
electrical but there is also a pneumatic connections that offers a redundancy in case of failure
of the electrical system. A description of how the brake chambers work is provided in
subsection 2.3.7.
2.3.4 Parking Brake Circuit (3)
Unlike the service brakes, a single circuit feeds the parking brakes on both the front and
rear axles. The parking brake circuit may or may not have its own reservoirs. When it does
not, it is fed from the APS and the service brake reservoirs. This circuit is also used for
feeding air to the pneumatic system of the trailer. Even though trailers are not within the
scope of this project, the feeding lines to the trailer must be considered as they are pressurized
together with the parking brake lines.
Parking brakes use a single acting spring loaded cylinder. Unlike the service brakes,
compressed air is used to release the parking brake instead of actuating it.
2.3.5 Auxiliary Circuit (4)
The auxiliary circuit on the vehicle has three main branches. The first branch feeds
systems in and around the cab such as steering wheel adjustment, cab suspension and seat
suspension. The second branch feeds systems connected to the engine. These include the
pneumatic circuits that control the Engine Gas Recirculation (EGR) valve that lets exhaust gas
back into the engine so as to lower the combustion temperature and reduce NOx emission, the
waste gate valve that regulates the speed of the turbocharger and the exhaust brake that
provides supplementary braking by causing pressure build-up in the exhaust line. The final
branch feeds systems connected to the transmission. These include the pneumatic circuits that
control the retarder, clutch and differential lock [7]. This final branch is built as a separate
circuit (6) on APS2.
An example of an auxiliary circuit is shown in below.
14
Figure 5. Example of an auxiliary circuit
2.3.6 Air Suspension Circuit (5)
This circuit feeds the air suspension in the vehicle. Air suspension is used to allow for
features such as chassis height adjustment or retraction of the tag axle so as to increase the
load on the drive axle and thereby improve traction [7]. Air springs also offer spring travel
that is independent of the loading condition of the vehicle and improve vibration damping in
loaded and unloaded conditions [12]. An axle fitted with air suspension might have two or
four bellows. Not all the axles in a vehicle need to be fitted with air suspension. The
schematic shown in Figure 6 represents a 6x2 truck where only the rear axles have air
suspension.
15
Figure 6 Example of an air suspension schematic
In the new generation of trucks and tractors (NCG) the front axle is fitted with hybrid
suspension meaning that it has both leaf springs and air bellows. This means that through part
of the range of travel of the suspension, the load is split between the air bellows and the leaf
springs. Only after the suspension has extended past a predetermined height do the air bellows
carry all the load. This height corresponds to the camber height of a free leaf spring.
2.3.7 Brake chambers
On axles that contain both a parking and a service brake, the same cylinder is used to
activate both brakes. The cylinder contains two chambers as shown in the figure below:
Figure 7 Brake cylinder with both parking brake and service brake chambers [13]
16
On the left side of the cylinder, there is the parking brake chamber that is spring loaded. When
there is no pneumatic pressure in this chamber, the spring is at full extension and the brake is
activated (top right). When pressure is introduced the spring is compressed and the piston is
retracted (top left) deactivating the parking brake. On the right part of the cylinder there is the
service brake chamber. When pressure is introduced in this chamber it expands pushing out
the piston and activating the service brake (bottom left). The brake cylinder shown in the
above figure also has a mechanical release feature. This allows the parking brake to be
released by winding a screw that is attached to the piston (shown in red, bottom right).
2.3.8 Air Bellows
Air bellows or air springs are used to both provide suspension and allow for chassis height
adjustment. The front axles of the trucks can be fitted with one of three different types of air
suspension bellows.
These are:
1. Normal bellow
2. Low bellow
3. Extra low bellow
These bellows offer different height adjustment limits and have different sizes. As such, air
consumed by the air suspension will not only depend on the load and the height adjustment
but also on the type of bellow installed. Previous tests have been conducted at Scania that
investigate and map air consumed as a function of height and load.
The rear axle uses either a two bellow configuration or a four bellow configuration. The
bellows in these two configurations differ. Both configurations are shown below:
Figure 8 Two bellow configuration (only one side of axle shown) [14]
17
Figure 9. Four bellow configuration (only one side of the axle is shown) [15]
There is also a light tag axle suspension, that is lighter and has completely different bellows.
2.4 Previous Work
This section provides brief descriptions of other works done at Scania that are pertinent to
this project.
2.4.1 Mapping of air consumption for heavy vehicles
This is a master thesis project that aims to identify which systems contribute most to
average air consumption and which contribute most to peak air consumption. To do this the
author installs flow metres on the different pneumatic circuits and measures the air they
consume under different driving conditions. His finding is that which systems consume the
most air depends on the drive cycle of the vehicle.
The project also delves into whether it is possible to estimate air consumption based on
pressure drop in the air reservoirs. This is done by measuring the pressure in the reservoirs
and then using the Ideal gas law to estimate how much air is being consumed. These values
are then compared with the actual values obtained through the flow metres. The finding is that
it is in fact possible to estimate air consumption from pressure drops in the pneumatic circuits.
2.4.2 Investigation of Air Volumes and Pressure Levels in Air
Brake Systems
This is also a master thesis project. The project involves the investigation of the effect of
the subdividing the air storage into several reservoirs and their strategic placement around a
truck. The project also investigates the possibility of reducing the storage volume by
18
increasing the storage pressure. In the report a numeric model for the brake system is
developed and evaluated.
2.4.3 Survey of Air Consumption on B6x2B
This is a report of field tests conducted on a 6x2 truck. The tests involve the measurement
of air consumption under different conditions. Dynamic tests are conducted when driving on a
highway, a country road, up and down a hill and finally on a hilly landscape. When driving in
these different environments, the air consumed by the different pneumatic circuits is measured
using flow metres. Here as well it is found that both the amount and proportion of air
consumed by the different systems depends on the driving conditions.
Static tests are also conducted when the vehicle is at a standstill. These tests measure how
much air is used to: release the parking brake, apply the service brakes, adjust the chassis
level, transfer load to traction axle and lift the tag axle.
19
3 REQUIREMENT SPECIFICATION
As mentioned in section 1.3, the plan is to deliver a numerical model of a truck’s
pneumatic system. There are some requirement specifications for the planned deliverable
which will be discussed in this chapter.
Requirement specifications were written after a discussion between the project authors and
the stakeholders. Stakeholders are people/person/firm who/which are/is [16]:
affected by the activities or results of the project
influencing, supporting or resisting the outcome
with a personal, financial or professional interest in the outcome
In this case the stakeholder is the RTCS (brake performance) department at Scania CV AB.
The requirement specifications of this project, along with their importance and difficulty, can
be found in the table below:
Table 2. Requirement Specifications
Requirement Specifications Importance Difficulty
Working model 9 3
Have an explainable error 9 9
Relatively quick simulations 9 9
Simulation of Service brake front circuit 9 3
Simulation of Service brake rear circuit 9 3
Simulation of Parking brake circuit 9 1
Simulation of Air Suspension circuit 9 9
Simulation of Auxiliary circuit 3 3
Simulation of transmission circuit 1 3
Having a user friendly product 3 9
Have different changeable variables 9 1
Allow user to enter choose different drive cycles 9 3
Display the pressure in the various circuits 9 3
Display which cycle and which function fails 9 3
Display air in the system and air consumed 9 3
20
21
4 THE PROCESS
Upon completion of the literature review, development of the model was started. Input for
the model is obtained both from data previously collected at the RTCS department and on
data obtained from tests and experiments conducted by the authors of this report. This
chapter goes through the gathering of this data and the creation of the model.
4.1 Model
The main deliverable of this thesis project is a model of the whole pneumatic system of
Scania trucks, being able to change different variables, which will be talked about. The
purpose of this model is to observe the behaviour of the pneumatic system of the truck (i.e.
changes in pressure of the system and the amount of air that it has and it is consuming),
without the need of doing time consuming experiments which can be sometimes costly as
well. As mentioned in section 1.3 the method which was used to simulate the pneumatic
system of the trucks is MATLAB.
Three different concepts (approaches) were generated in the beginning of the thesis:
MATLAB Simulink approach
In this approach, the model would have been simulated in Simulink framework. The
advantage of this approach was that Simulink has the power to calculate the air consumption,
air flow, pressures and every other variable, if the proper inputs are given to it. The problems
that would have been there by choosing this approach were threefold:
1. Flexibility
Compared to the other, approach this approach has less flexibility (see below). The model
that is expected needs to allow the user as much flexibility as possible. The user should be
able to change the variables as much as they wants, as fast as possible, and in the easiest way
possible.
2. Accuracy
Simulink uses its own pre-programmed functions to get solutions and the accuracy of these
are not know to the users.
3. Verification
In the end of the modelling, a verification of what the model is showing had to be done.
Not many changes could have been done if the verification was off by an unacceptable
amount of error, since Simulink was meant to do most of the simulation which the
programmers wouldn’t have had direct access to it. In this case the refining of the model
might have been extremely challenging or impossible.
22
Because of the discussed problems, this approach was rejected after concept evaluation.
Real-time model
Real-time model is a model that can execute at the same rate as a real time clock (e.g. a
clock on the wall). The approach of having a real-time model was decided to be the main
concept of the project’s delivery after a concept evaluation process. However a third concept
came out after refining the requirement specifications of the model.
The idea of having a real-time simulation came up to give the users the chance of experience
driving the truck in the way the want in a real time pace while all the different pressures on
different circuits, IDU values and also the amount of air that system had and the amount of air
being used at each moment could be observed. This concept was developed for about a month
after which, it was decided to be substituted with the last concept (see below). The reason of
such decision was the realization, that the model needs to be as fast as possible. The
requirement specification was to get all the information at once in a quick time and observe
and save them if needed, then change the variables to see how they affect the air consumption
in the truck’s pneumatic system. In contrast to what was required this concept was too slow
and it took the same amount of time as driving a truck and recording all the data using a CAN
drive. Hence, a new (final) concept came up:
MATLAB fast simulator (pBk air consumption application)
MATLAB fast simulator of the pneumatic system of the truck was the first name that came
up for this concept. Later on it was named the “pBk air consumption application” (from now
on this concept will be referred as pBk for simplicity). Being similar to the second concept
(real-time model), pBk had the advantage of fulfilling the need of simulating as fast as
possible. Instead of having a real-time model which users have to command every single
moment, pBk provides a one-time result by getting a recorded cycle (see 5.2.2) from the user
as an input. This way the users only need to drive a truck for a short amount of time
(depending on their purpose), recording the needed values, feeding it to pBk, defining all the
variables the way they want which includes the number of times they want their cycle to be
repeated (see 5.1.2) and get the results in a reasonable amount of time. pBk will then tell the
user at which cycle the truck started to have insufficient amount of air and also which
pneumatic function has failed to function due to that insufficiency. The user can then easily
change the variables to see if a way of fixing the problem is possible. Experimenting this in
real life will take about a week for each different combination of variables.
The speed of the application (pBk) is known to be 360 times faster than real life time, if the
truck has enough air for doing the wanted drive cycle as many times as defined by the user
(see 5.2.6). This means that given a data of one hour (i.e. 3600 s) as an input, pBk will give
the simulation results in about ten seconds. Users can evaluate the application from time to
time to make sure it works by just driving for a long period of time (e.g. one, two hours or
even more) once and record the required values (both inputs and outputs) for pBk (see 5.2.2).
pBk can then run it 360 times faster than the driving time and give the results. The user can
make sure that the application works properly if the results gotten by pBk and the recorded
23
values are having more or less the same values (i.e. in a range of an acceptable error for the
user).
4.2 Available data
Plenty of the information that was used in the development of the model is obtained from
data available at RTCS from experiments that had already been conducted by other engineers
before the beginning of this project. This data is discussed below.
4.2.1 Compressors
Several different types of compressors are used in the Scania trucks. The compressor is
attached to the engine through a gear system. The gear ratio depends on the engine used.
Datasheets for the different compressors are available that give values for free air delivery at
given engine speeds and back pressures. Since only a limited number of these values are
given, interpolation is used to estimate the air delivered at any rpm and pressure within the
possible range. The percentage difference between the values obtained after interpolation and
those provided by the manufacturers is about 2% for all compressors. This difference is
caused by the fact that the function generated through interpolation does not go through all the
experimental values.
4.2.2 Air consumption by air suspension
For each of the types of bellows named in section 2.3.8, there exists data from tests where
the pressure drop in the air suspension tanks is measured as the height is adjusted from 0 mm
(with the chassis resting on the bump stops and the bellow pressure at 0 bar) to the maximum
possible height. For each bellow type these tests are done with three different loads. An
example is shown in Figure 10 below. It is the data recorded for normal bellows.
24
Figure 10. Air consumption for a normal bellow
So as to be able to use this data in the model, allowing the user to input whatever load and
height they desire, high order polynomials are used to interpolate the data. The interpolation is
done in two steps. First for each load, air consumed by the bellows as a function of chassis
height is interpolated using an 8th
order polynomial. This polynomial is then evaluated over
regular intervals. As an example, for the normal bellows at a load of 5.22 tonnes, readings of
pressure drop were taken at 38 irregularly spaced intervals from the minimum height to the
maximum height. After interpolation and evaluation, a value for air consumed is made
available for every millimetre.
After interpolation over height adjustment, the obtained values are then used to obtain a linear
interpolation of the data over the available loads for each height. Since the loads used in the
test are all larger than the minimum and less than the maximum that the user would want to
choose, the data is also extrapolated to include these minimum and maximum load values. As
an example the normal bellow tests were performed for the loads 5.2, 7.0 and 8.5 tonnes but
the model should allow the user a load range of 4 to 10 tonnes.
Only data from NGS bellows is used in the model. This is because it was the only complete
set available. The little data available for NCG bellows would not have been enough to
complete the interpolations mentioned above. This means that if this model is used for an
NCG truck the air consumption by air suspension might not strictly correlate. It is however
considered to be a good approximation.
25
4.2.2.1 Error evaluation
After the two rounds of interpolation and extrapolation, the percentage difference between
the statistically obtained values and the experimental values is calculated. This difference is
caused by the fact that the function generated through interpolation does not go through all the
experimental values. The average of this difference is found to be 1.4, 2.1 and 6.2 for normal,
low and extra low bellows respectively. The differences on the rear axles are 0.6 and 1.5 for
the two-bellow and four-bellow configuration respectively. These differences are considered
small enough to be negligible.
4.2.2.2 Calculating the pressure in the air bellows
In the model, it is necessary to know the pressure in the bellows as this determines
whether or not the pressure in the system and/or extra air tanks is enough to operate the air
suspension. The pressure in the air bellows depends primarily on the load acting on the
bellows but also on the height extension of the bellows. Several tests have been conducted at
the RTCS department that measure this relationship. From these tests, linear equations that
relate pressure to load at a given height are derived for all types of bellows. As an example,
for the low bellows on the front axle, a linear equation based on three loads (unloaded, half
loaded and fully loaded) has been derived for each height extension between 10 and 120 mm
at 10 mm intervals.
It would be rather difficult and complicated to use all these equations in the model. In an act
of simplification, the average of these equations is calculated to give a single equation that
averages the effect of height and takes load as a singular input to give bellow pressure as an
output. The difference between the individual equations and this calculated average is
calculated and is found to be less than 10% in almost all instances which is deemed to be
acceptable. The only time when this difference exceeds 10% is in the case of the extra low
bellows for a height extension of 130 mm where the difference is 18%. For this reason two
equations are used for the relationship between load and bellow pressure for extra low
bellows so as to decrease this difference. One equation is valid for height extensions 0 to 115
mm and the other from 115 mm to 130 mm. All other bellows use one equations.
4.3 Experiments
Several tests and experiments were conducted both on actual trucks and in the lab so as to
provide information to be used either in the development of the model or in its evaluation.
These tests and experiments are presented below. Please note that from now on, unless
otherwise stated, all pressure values given represent relative and not absolute pressure.
26
4.3.1 Gas Law Experiment
This experiment is to evaluate the accuracy of gas law equations. This needs to be done
since the gas law is a tool that will be heavily used in the simulation of air consumption in the
truck.
4.3.1.1 Method
The experiment setup is shown in the figure below.
Figure 11 Setup for the gas law experiment
At the beginning of the experiment the second valve is closed and the pressure in Volume 1 is
increased to the desired value. After this the first valve is closed and the second valve is
opened so that the pressure in both tanks equalizes. The equalisation pressure is then
recorded. The experiment follows a half factorial design where the sizes of and pressures in
Volume 1 and Volume 2 are varied as shown in the following table.
Table 3. The test set to be followed during the experiment
Test No. 1 2 3 4 5 6 7 8
P1 9 9 9 9 6 6 6 6
P2 3 0 3 0 3 0 3 0
V1 60 60 40 40 60 60 40 40
V2 20 4 4 20 4 20 20 4
4.3.1.2 Results and analysis
If it is assumed that air is behaves like an ideal gas and that the process of equalization is
isothermal, the following equation can be set up.
𝑃1 ∙ 𝑉1 + 𝑃2 ∙ 𝑉2 = 𝑃𝑒𝑞 ∙ (𝑉1 + 𝑉2) (12)
The table below presents the theoretical equalization pressure as calculated for each test, the
actual equalization pressure as measured by the pressure sensor and percentage difference
between the two.
27
Table 4 Comparison between experimental and theoretical equalization pressures
Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8
P_eq
experimental
7.44 8.39 8.30 5.88 5.78 4.44 4.94 5.49
Peq
theoretical
7.51 8.48 8.41 5.97 5.82 4.46 4.49 5.57
Percentage
difference
0.88 1.00 1.13 1.26 0.52 0.28 0.84 1.27
As can be seen the largest percentage difference is only 1.27%. This is deemed an acceptable
deviation and as such the ideal gas law will be used in the rest of the project.
4.3.2 Flow metre analysis
The purpose of this experiment is to test the accuracy and precision of flow metres. To do
this, the test is similar to that presented in the previous section but flow metres are used as the
instruments of measurements. The setup is as shown in the following figure.
Figure 12 Setup for flow metre tests
The flow metres require developed flow so as to give accurate measurements and as such long
pipes are installed at their entrance so that the flow has time to acquire a fully developed
velocity profile. The setup is as shown in Figure 13.
28
Figure 13. Flow metres with long pipes attached to the entry and shorter ones to the exit
A pressure sensor is used so as to know when to shut off the flow. For tests where the starting
pressure in volume 2 is zero, valve 2 is shut at the beginning and air is let into volume 1 until
the desired pressure is obtained. During this time flow1 (flow through flow metre 1) is
recorded. After this, valve 2 is opened and the flow 2 is recorded. For tests where the starting
pressure in volume 2 should be about 3 bar, valve 2 is initially open and air let into the entire
system until pressure builds up to around 3 bar. Both flow1 and flow2 are recorded. After this,
valve 2 is shut and more air is let into volume 1 to increase the pressure to the desired value.
Flow1 (now denoted as flow1_2) is recorded.. In the third step, valve 2 is opened and flow2
(now flow2_2) is recorded. The flow metres give readings in NLPM and these are integrated
over time to give normal litres. The tests follow the same factorial design presented in Table
3 but each test is done twice.
4.3.2.1 Results and Analysis
The tables below show the results obtained from the tests. The flow through each flow
metre is integrated over time to give the amount of air that flows through the flow metre. The
integral of flow1 is denoted as Air1, that of flow2 as Air2 and so on and so forth.
29
Table 5. Results from the flow metre experiment
Test 1 Test 2 Test 3 Test 4 Test 5 Test 6 Test 7 Test 8
P1 6 9 6 6
P2 0 0 3 0
V1 40 40 40 60
V2 4 20 20 20
Air1 171 177 267 262 138 139 277 274
Air2 10.2 10.8 82 77 46.5 45.7 57.2 55. 3
Air1_2 83.2 65.4
Air2_2 17.8 15.4
Table 6. Continuation of results from the flow metre experiment
Test 9 Test 10 Test 11 Test 12 Test 13 Test 14 Test 15 Test 16
P1 9 6 9 9
P2 0 3 3 3
V1 60 60 60 40
V2 4 4 20 4
Air1 399 388 140 152 188 180 96.6 94.0
Air2 17.5 17.1 6.27 7.68 47.1 47.0 7.53 7.87
Air1_2 129 116 258 267 175 152
Air2_2 5.68 4.49 59.5 58.7 11.2 9.25
It can be assumed that the temperature of the air in the tanks at the start of each test is room
temperature which during the tests was around 22 ºC. The pressure is equivalent to the
atmospheric pressure. Given this, the amount of air (Vn), in normal litres, at the start of each
test can be calculated according to:
𝑉𝑛 = 𝑉𝑥 ∙
273
273 + 22
(13)
where Vx is the capacity of each volume.
For tests where the starting pressure in volume2 is 0, the theoretical value of flow2 (Fl2) can
be calculated using the following formula;
∫ 𝐹𝑙2 𝑑𝑡 =
𝑉𝑥,2
𝑉𝑥,1 + 𝑉𝑥,2∙ (∫ 𝐹𝑙1 𝑑𝑡 + 𝑉𝑛,1 + 𝑉𝑛,2) − 𝑉𝑛,2
(14)
It is this value that is compared to the integral of flow2 value obtained from the tests so as to
gauge how accurately the flow metres measure flow.
When it comes to tests where the starting pressure in volume 2 was around 3 bar, the
following equation is used to calculate the theoretical value of flow2_2
30
∫ 𝐹𝑙2_2 𝑑𝑡 =
𝑉𝑥,2
𝑉𝑥,1 + 𝑉𝑥,2∙ (𝐹𝑙1 + 𝐹𝑙1,2 + 𝑉𝑛,1 + 𝑉𝑛,2) − (𝑉𝑛,2 + 𝐹𝐿2)
(15)
The table below shows the theoretical and experimental values alongside the percentage
difference.
Table 7. Comparison of the experimental and theoretical flow results
Test Number Experimental air amount [NL]
Theoretical air amount [NL]
Percentage difference
Flow
1 10.2 15.6 41 100
2 10.8 16.1 40 100
3 82.0 88.9 7.1 500
4 77.0 87.2 13 450
5 17.8 27.4 43 250
6 15.4 22.4 37 200
7 57.2 69.2 19 600
8 55.3 68.6 22 550
9 17.5 25.0 35 200
10 17.1 24.2 35 225
11 5.68 10.5 60 80.0
12 4.49 9.13 68 60.0
13 59.5 64.3 7.8 325
14 58.7 64.8 9.7 375
15 11.2 17.1 42 140
16 9.25 14.5 44 200
As can be seen in Table 7 above, for tests with the same volumes and almost same pressures,
the percentage difference between the expected values and the measured values is almost the
same. However, between tests with different volumes and/or pressures, the percentage
difference varies considerably. From these results it can be inferred that readings collected
with flow metres depend on pressure and the amount of air that flows through them. . It
appears that the larger the expected flow, the smaller the percentage difference becomes. A
possible explanation for this is that with smaller amounts of air flow, the flow does not have
enough time to build into developed flow which is a prerequisite if the flow metre is to give
accurate readings.
Since the pressure sensors give reasonably accurate measurements regardless of amount of air
flow, they are the instruments of measurement that will be relied upon during the course of
this project.
4.3.3 Parking brake chamber
The purpose of this experiment is to determine the air consumed during pressurization of
the spring chamber so as to fully release the parking brake using both a 24”/30” and a 24”/24”
31
brake chamber. The experiments aims to both measure the capacity at full actuation and the
air consumed at different pressures.
4.3.3.1 Method
The brake chamber is connected to a tank through a tap that serves the purpose of the relay
valve in the truck as it is opened when the brake chamber is to be pressurized. A pressure
limiting valve that keeps the pressure in the brake chamber below 8.0 bars will also be used.
This is to imitate the pressure limiting valve in the APS that serves the same purpose. The
setup will be as shown in the figure below.
Figure 14 Schematic of the experiment assembly
A second normally closed (NC) valve will be used to depressurize the brake chamber without
having to depressurize the entire system. In this way it has a function similar to a quick
release valve.
4.3.3.2 Results and Analysis
The following tables show the results from the tests. P1 is the pressure in the tank before
the brake chamber has been pressurized and P1_a is the pressure after pressurization. P2 is the
pressure in the brake chamber as measured by the second pressure sensor. Tanks with
different capacities were for the two different brake chamber sizes.
32
Table 8. Results from pressurization of 30" parking brake chamber
Test
number
P1 Cylinder 1 Cylinder 2
P1_a P2 P1_a P2
1 12 10.6 8.0 10.8 8.0
2 11 9.6 8.0 9.7 8.0
3 10 8.7 8.0 8.8 8.0
4 9 7.7 7.6 7.8 7.8
5 8 6.9 6.9 6.9 6.9
6 7 6.0 6.0 6.0 6.0
7 6 5.3 5.3 5.3 5.2
Table 9. Results from pressurization of 24" parking brake chamber
Test number P1 P1_a P2
1 12 10.5 8.0
2 11 9.6 7.9
3 10 8.5 7.9
4 9 7.6 7.6
5 8 6.8 6.8
6 7 5.9 5.9
7 6 5.1 5.1
Once P2 has been measured, the capacity of the brake (Vbrake) chamber can be calculated
according to the following formula:
𝑉𝑏𝑟𝑎𝑘𝑒 =𝑃1 − 𝑃1𝑎
𝑃2𝑉𝑡𝑎𝑛𝑘 (16)
From the above results the capacity of the 30” parking brake chamber was found. This value
deviates from that provided by the chamber manufacturer (Wabco) by 12%. This difference is
possibly caused by equipment error as the two pressure sensors did not always show the same
value even when they were connected to the same pressure; a difference of about 0.5 bar was
observed. Another possible error is in the size of the tanks.
As for the 24 type brake, the percentage difference between the measure value and that
provided by the manufacturer was 6%. This difference is rather negligible and is probably
caused by the aforementioned reasons.
In the Matlab model created, the air consumed is calculated according to the case it belongs to
as described in section 6.1. Using the starting pressure in the tanks and the volumes of both
the tanks and the brake chamber, the theoretical pressure drop can be calculated and compared
to the one obtained experimentally. The table below shows this comparison.
33
Table 10. Comparison between theoretical and experimental results
Test no. Theoretical P1_a Experimental P1_a Experimental P1_a
1 10.9 10.6 10.8
2 9.9 9.6 9.7
3 8.9 8.7 8.8
4 7.9 7.7 7.8
5 7.0 6.9 6.9
6 6.1 6.0 6.0
7 5.2 5.3 5.3
Table 11. Continuation of the comparison between theoretical and experimental results
Test no. Theoretical P1_a Experimental P1_a
1 10.6 10.5
2 9.6 9.6
3 8.6 8.5
4 7.7 7.6
5 6.8 6.8
6 6.0 5.9
7 5.1 5.1
As it can be seen from these tables, the theoretical values are very close to the experimental
ones if not similar. This is a validation of the model that is used to calculate the pressure drop
in the system when the parking brake is released.
4.3.4 Overflow valve: opening pressure
Some trucks and trailers have extra air tanks that exclusively feed the air suspension.
Between these tanks and the APS, an overflow valve is installed. The purpose of this
experiment is to determine at what pressure the overflow valve opens given different
downstream pressures.
4.3.4.1 Method
The overflow valve is connected between two tanks as shown in the figure below.
Figure 15. Setup for experiment testing the opening pressures of the overflow valve
34
Volume 2 is pressurized up to the desired pressure and then valve B is closed. Since the
overflow valve has an integrated check valve, air cannot flow from Volume 2 into Volume 1.
After this, Volume 1 is pressurized. The pressure will increase steadily until the overflow
valve opens at which point the pressure levels off.
The closing pressure is determined by filling both volumes to a pressure that ensures that the
overflow valve is open and then opening valve C to let air out of the secondary side. The
closing pressure is observed when the pressure on the primary side stops decreasing.
4.3.4.2 Results
It was learned that less up-stream pressure was needed to open the valve if the down-stream
pressure was higher. The exact values were recorded and used in the model. The value for the
closing pressure was also obtained and used in the model.
4.3.5 Overflow valve: flow
It is necessary to know at what rate the extra air tanks are filled when the overflow valve is
open. For this purpose tests are done to measure the flow at different upstream and
downstream pressures.
4.3.5.1 Method
The setup is as shown in the following figure:
Figure 16. Setup for tests to measure flow through the overflow valve
With Valve B closed Volumes 1 and 2 are pressurized to their respective desired pressures
then valves A and C are closed. Valve B is then opened and air flows through the overflow
valve until either the pressure equalizes or the valve closes. The pressure drop (ΔP) in Volume
1 (V1) is recorded and with this the flow (q) through the overflow valve can be calculated
according to the following formula.
𝑞 =
∆𝑃
∆𝑡∙ 𝑉1 ∙
273
𝑇𝑎𝑡𝑚
(17)
35
4.3.5.2 Results
It was seen that for the same downstream pressure, the higher the upstream pressure the
higher the flow. This is to be expected since a higher upstream pressure means that the
overflow valve opens more. However, for higher upstream pressures, the flow falls with an
increase in downstream pressures. This is because flow is also dependent on pressure
difference. The results of this experiment were interpolated to give a function that calculates
flow as a function of upstream and downstream pressures.
4.3.6 IDU: Compression and regeneration
It is necessary to model the IDU. Since the IDU is part of the software of the APS which is
made by an OEM outside Scania, it is not known how the counter works. So as to better
understand how it works, some tests were conducted. In a stationary truck (called Apple), the
compressor was allowed to pump air into the pneumatic system while the engine RPM and
system pressure were recorded. Using the function described in section 4.2.1 this information
was then used to deduce how much air the compressor must pump into the system for the IDU
value to go up by one unit. To collaborate this finding, data recorded from earlier test
conducted at RTCS was analysed in a similar fashion and the ratio of compressed air into
system to IDU value increase was found. In the model this ratio can be adjusted to whatever
the user desires.
So as to calculate how much air was used during regeneration, the system pressure and purge
valve state were checked. The purge valve is open only when the desiccant is regenerating.
Using a flow rate [l/min] vs pressure [bar] graph provided, it was possible to tell how much
air is being used for regeneration and this can be compared to the drop in IDU.
The ratio between air used for regeneration and drop in IDU was found for both the data
collected from Apple and that available from earlier tests.
4.3.7 APS behaviour
The APS is made by a company called Wabco. Wabco makes the software in the APS’s
ECU and does not share it with Scania. As such it is rather challenging to simulate the
behaviour of the APS in the model. In an effort to gain a better understanding of the APS,
data previously recorded at RTCS was analysed and some tests were run.
Plenty of data with recordings of CAN signals was available for analysis. One of these data
was recorded by Tomas Björnelund [17]. The data was analysed and compared to the
behaviour described in the available APS file [10] and presented in section 2.3.2.4. It was
discovered that the two did not match and that in the truck the APS behaviour was more
complicated that stated in the APS file. The following conclusions were drawn:
36
The behaviour of APS is directly dependent on the pressure of the system and the IDU value.
This relationship however appears to be rather complicated and is at times seemingly random.
During the analysis, six different system pressures seemed to always recur with three different
IDU values. To best describe different situations let it be assumed that the pressure of the
system is below nine bar and try to fill the system with more air so that the pressure rises to its
highest possible value. Note that this behaviour was found analysing this specific data [17],
meaning that this might not be valid when analysing other data to figure out APS’s behaviour.
Hence, the six different pressures are changeable variables in pBk, so that they can be
controlled by the user accordingly.
The compressor always compresses if the pressure is below IntReg no matter what IDU value
the desiccant has. IntReg stands for Initial Regeneration. Coming up from IntReg, the first
point that the APS checks the IDU value is when the system pressure reaches FReg. FReg
stands for First Regeneration, which is the first pressure at which regeneration can occur.
What determines if regeneration should happen or not at this point is the IDU value. If the
IDU value is equal to or more than the IDU value in which the APS is saturated, the APS will
start regenerating otherwise it will just compress until the system pressure gets to CutOut. At
CutOut, the APS will again check the IDU value. If it is equal to or higher than half of
saturation value, the APS will regenerate until the system pressure drops to SReg. At SReg,
the APS will again check the IDU value. If it is equal to or more than half of the IDU
saturation value; it will regenerate to CutIn. Otherwise, it will compress until the system
pressure gets to CutOut again and act as mentioned before. At CutIn pressure, if the IDU
value is again equal to or more than half of the IDU saturation value, the APS will regenerate
to FReg and behave as mentioned before. Otherwise, it will compress until the system
pressure gets to CutOut again and act as mentioned before. If at CutOut pressure the IDU
value is lower than half of the IDU saturation value, the APS will turn the whole system off
(neither regeneration nor compression), until the system pressure drops to SReg. At that point
the same scenarios as mentioned before will happen.
There is a mode in which the system pressure can go higher than the CutOut value and that is
if the truck is in overrun. In that case, it is known that the APS tries to compress air as much
as it can and tries to get the system pressure to the overrun cut-out, if its pressure is lower than
the overrun cut-in pressure. While the pressure of the system is between the overrun cut-in
and cut-out, the APS will not compress even in overrun.
This APS behaviour is summarized in the following flowchart.
37
Figure 17. Flowchart describing APS behaviour
The flowchart does not describe fully and accurately the behaviour observed. There were
some times when changes from compression to regeneration or to idling and vice versa
occurred at pressure and IDU levels not covered by this chart. These occurrences seemed to
be random as no specific pattern could be discerned. The same analysis was repeated on a
different set of available data (recorded by Tomas Björnelund [17]). The results were similar,
with some seemingly random occurrences being observed here as well. Further investigation
was not done due to the time limits and priorities. All in all, the chart shown in Figure 17 was
the best behaviour model that could be derived given the available data and time. Further
investigation could be done, to create a more complete model that gives a more accurate
simulation of APS behaviour.
Simulating APS2-HighCapacity, which works quite different than APS2-Advanced could be
done in quite similar ways (testing on a truck). This was not an option since a truck having an
APS2-HighCapacity was not available for testing. Hence, a bench test was done. The results
of the said bench test was not enough for being able to coming up with a flowchart or getting
to know the complete complex behaviour of APS2-HighCapacity. In fact, the test was only
enough to get to know how APS2-HighCapacity behaves more or less. The following
behaviour was recognised after observing an APS2-HighCapacity’s behaviour which were
being fed by a pump for half an hour (it is noteworthy to mention that one of the problems of
the test was that the capacity of the pump was not known).
APS2-HighCapacity has two desiccants as said before (see section 2.3.2.6). Since each
desiccant has an IDU value, APS2-HighCapacity has two different IDU values known as
IDU1 and IDU2. There is a cartridge valve which switches between these two desiccants this
cartridge valve can make the following configurations possible:
Regeneration in desiccant1 and compression in desiccant2
Compression in desiccant1 and regeneration in desiccant1
There are three different situations that might happen during the running time of the vehicle
which comes from the same configurations:
Regeneration in desiccant1 and no compression nor regeneration in desiccant2
Compression in desiccant1 and no compression nor regeneration in desiccant2
No compression nor regeneration in desiccant1 and regeneration in desiccant2
No compression nor regeneration in desiccant1 and compression in desiccant2
38
No compression nor regeneration in both desiccants
So as an example when regeneration is happening in desiccant1 and no compression nor
regeneration is happening in desiccant2 the situation of the cartridge valve is the same as
when regeneration is happening in desiccant1 and compression is happening in desiccant2. So
basically cartridge valve can only have the two configurations which said which makes all the
five situations to happen. One important thing to note from the said configurations is that
there are two configurations that can never happen:
Regeneration in both desiccants
Compression in both desiccants
The behaviour of APS2-HighCapacity was observed having IDU value of zero for both
desiccants and cartridge valve configuration of compressing (or no regeneration nor
compressing) into the first desiccant and regenerating (or no regeneration nor compression) in
the second one. From now on for simplicity this configuration will be referred to as cartridge
config.1 and the other configuration will be referred to as cartridge config.2. The behaviour
which was observed is briefly explained in the next paragraph.
The APS starts compressing air (via pump) into the first desiccant until IDU1 gets to 40, at
that point APS checks the system pressure. If it was more than IntReg it will change the
cartridge config. to config.2, otherwise, it will continue pumping into desiccant1 until
desiccant1’s IDU value gets to 70 (known as saturation value for desiccants). After getting to
70 it will definitely change the cartridge config. to config.2. The same thing will happen to
desiccant2 with this difference that when its IDU value gets to 40, other than the pressure,
APS also checks whether or not desiccant1’s IDU value is above 40. If it was above 40, it will
continue pumping into desiccant2 and regenerating desiccant1. If not APS changes the
cartridge config. to config.1. This will happen all the time. Now the problem with this, is that,
this way none of the IDU values will go above 70, but in reality (the bench test), IDU values
go higher than 70. The reason of this happening was not further investigated, but one thing
that may cause this, is having a high capacity for the pump. If the said behaviour of APS2-
HighCapacity is correct, the only way (as far as came to authors minds) for IDU values to get
higher than 70, is that the compression flow is higher than the regeneration flow so that when
one of the IDU values, say IDU1 reaches 70 and the cartridge config. changes, desiccant1
doesn’t have enough time to regenerate, since the compression flow is so high so when
desiccant2’s IDU value reaches 70, desiccant1’s IDU value is 45 or 50, after a while of
having this situation (due to having high air consumption), a time will come that desiccant1
reaches IDU value of 70 while desiccant2’s IDU value is 68. Continuing this for some more
time first one of the desiccant then the other one’s IDU value (having no choice), goes above
70. At this point which was observed at the bench test (when both desiccant IDU values are
above 70, which the authors call it high air consumption for APS2-HighCapacity), it was seen
that cartridge config. changes each time, when the desiccant which was being regenerated, has
completed regenerating for 40 IDU values (this value changed during the test, but was 40
most of the times).
39
As mentioned before this test was not enough to know the exact behaviour of APS2-
HighCapacity and was just a way of testing and getting to know how it works and what
different modes (situation) it has, so that it could be somehow simulated. Due to time factor,
also not having a truck which uses APS2-HighCapacity and also having a bit of hard time
doing more bench tests (the bench test was in a special room that a specialist was doing his
tests there, hence, every time that a bench test wanted to be done, it should have been fixed
with him (if he has time or not), since he should have been present, because of some
configurations and also responsibility reasons), further tests (investigations) couldn’t be done
on APS2-HighCapacity’s behaviour. Further investigations are highly recommended (see
chapter 9).
4.3.8 Service brake
Unlike the parking brake that is either fully disengaged or fully engaged, the service can be
applied to varying levels depending on how much the driver depresses the brake pedal. This
means that different amounts of air are consumed depending on how fast the driver wants to
decelerate. For the purpose of the model it is assumed that the amount of air fed into the
service brakes is proportional to the pressure in the brakes which is in turn proportional to the
braking force provided. It is further assumed that the braking force provided by the brakes is
proportional to the mass of the truck multiplied by the deceleration it will experience i.e.
𝐹𝑏𝑟𝑎𝑘𝑒 = 𝑘 ∙ 𝑚 ∙ 𝑎 where k is the proportionality constant and ‘a’ is the deceleration. What all
these assumptions mean is that, for example, the same amount of air is consumed when
slowing a truck down from 30 km/h to 25km/h in 1 second as when slowing down a truck that
weighs twice as much from 20 km/h to a complete stop in 8 seconds assuming that both truck
are on a flat surface and only the surface brake is used.
The tests were conducted on the truck called Apple. Before the test, the service brake circuit
was reconfigured so that the tanks that fed the service brakes were disconnected from
everything else. This means that the pressure drop in these tanks was caused only by air going
into the service brakes. Pressure sensors were installed to measure this pressure drop. Pressure
sensors were also installed on the communication lines between the foot brake module and the
EBS computer since the air consumed in these lines comes from the APS and as such is not
included in the pressure drop in the tanks. Since the volume of the lines is known, the pressure
increase in them can be used to calculate how much air is consumed to send the braking signal
to the EBS computer.
4.3.8.1 Results and analysis
The air that left the tanks was added to the air used to send the braking signal to the EBS
computer and this sum was plotted against the product of the truck’s mass and its
acceleration. This plot is shown in Figure 18 below.
40
Figure 18. Service brake air consumption
It appears that the relationship between air consumed and m*a is linear. The red line is a line
of best fit that will be used in the model to predict the air consumed by the service brakes for
a given m*a value. From the data obtained the average percentage difference between any
point and the line is about 8%. This value is deemed to be acceptable especially since the
service brakes consume a rather small amount of air compared to parking brakes and air
suspension meaning that an 8% discrepancy in the service brake air consumption will have a
negligible effect on overall air consumption.
Possible causes for these variations include heating of the brake pads. At higher temperatures,
the coefficient of friction between the brake pads and callipers falls, meaning that more
pressure, and therefore more air, should be applied to give the same deceleration. Other
possible causes include small variations in slope and changes in air resistance due to wind.
Upon talking to several drivers it was learned that it is common for drivers to step on the
service brake pedal before releasing the parking when the truck is stationary. This means that
there are times when the service brake is used without causing a deceleration. To estimate
how much air is used when the driver does this, a test was conducted where the driver
imitated this behaviour and the pressure drop in the service brake tanks was measured.
41
5 RESULTS
The main result of this project is the model itself. Different parts of the model and the
whole general view of how it works will be discussed in this chapter.
5.1 User interface
The model (pBk) has been coded using uicontrols (user interface controls) in MATLAB.
Using uicontrols, it is possible to build a user friendly model that has a combination of
buttons, popup menus, texts, edit boxes and figures.
The model consists of one main figure (pBk’s main figure, shown in Figure 19), four input
figures (variable definers) and three output figures (results).
5.1.1 Main figure
The main figure of pBk the air consumption app. is a window consisting of several text,
edit and pushbutton uicontrols. In this figure the user can define some of the variables and run
the app. Variables like the initial system pressure, the type of APS being used, the type of
compressor being used, the volume of the service brake circuit tanks etc. (see Figure 19); can
be defined directly in this window. This window also gives the user access to all other figures.
There are several different variables that can be defined by accessing those figures.
42
Figure 19. The main figure of pBk app.
5.1.2 Input figures
The input figures are also known as variable definers. As the name implies, these figures
give the user the possibility to change the variables they want. As stated before, four different
input figures are available for pBk. The user can open each one of them by clicking on the
relevant pushbutton on the main figure which is specifically assigned to open the wanted
figure.
43
5.1.2.1 Truck info. figure
On this figure the user can define the truck’s model (e.g. 6x2 EBS Drum), whether or not
the truck has parking brakes in front, load on front and rear axles separately and finally the
load distribution ratio between the traction and tag axles. The app. will then calculate how
much load there is on each axle. The model (pBk) also calculates the sprung weight for each
axle as the total load on each axle minus the mass of the axle. In other words, sprung load is
the total load which will lie on the suspension of the truck. The sprung weight is later used to
calculate the pressures in each bellow during the simulation (see section 4.2.2.2).
The model (pBk) also gets the number of front, traction and tag axles from the truck model
that the user chose. These will be later used for air consumption calculation since it will
directly affect the air consumption for the air suspension and parking brake circuits in the
model. Some trucks have a ‘disengaging parking brake’ feature for the front which can be set
to on or off by setting ‘Front PBrake automatic release feature’ drop down menu to yes or no
respectively. The trucks that have this feature use air to release the front parking brakes while
kneeling right after the driver engages the parking brake. The truck starts to vent air so as to
engage the front parking brake, but this feature cause air to start being pumped back in, before
the brake fully engages, so as to once again disengage it. Less air is used in this case than if
the parking brake had fully engaged.
Truck info. figure is shown in Figure 20 below:
Figure 20. Truck info. figure.
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5.1.2.2 Air suspension heights figure
On this figure, the user can define different heights and kneeling speeds of the truck with
which the cycles will happen. These heights consist of driving height, kneeling distance and
kneeling speed both for the front and the rear. On top of that, the user can also feed in his/her
cycle file (i.e. a pre-defined excel file (see 5.2.2)). This can be done using the Browse
pushbutton and choosing the desired file. After that, the user has to ask pBk to generate their
selected file. This is done by clicking on “Generate the specified cycle” pushbutton.
Note that the driving height is defined as the distance between the top of the bellows and the
truck’s bump stop for front axles in pBk. Also, the more rows the user’s file has, the longer it
takes pBk to generate a file suitable for the app. (referred to as a pBk file). It is approximated
that every 13200 rows on the data file will take pBk about one minute to convert to a pBk file.
The model (pBk) shows air consumption, pressure etc. at each 0.1 second interval. This 0.1
second is known as ‘Moment’ in pBk. This moment is set to 0.1 as the default of the program,
however, this can be changed by the user if need be, according to the frequency of his/her
recorded data. Having a moment of 0.1 seconds, 13200 rows translate to 1320 seconds, which
is about 22 minutes. So, with a moment of 0.1 seconds, the program can generate a pBk file
from the user’s file with a speed 22 times faster than the real-time. Obviously this will take
much more time if the frequency of the recorded data is higher thus making the moment
smaller. Hence, it is recommended for the users to have a frequency of ten Hertz while
recording their data.
pBk gives the user the option to save the pBk file which they just generated so that the next
time they can use the same recorded data without the need of one again generating and
converting it to a pBk file which usually takes a long time if the data file is large. Having the
pBk file saved somewhere in their computer folders, the users can easily feed in those files
instead of their recorded data to save a lot of time. This way is almost 50 times faster than
generating a pBk file from raw recorded data. Feeding in the pBk file can be easily done by
clicking on “Open pBk file” pushbutton on Air Suspension Heights figure and choosing the
pBk file which was saved by pBk using the “Save pBk file” pushbutton earlier on. The ‘Air
suspension heights figure’ is shown below:
45
Figure 21. Air Suspension Heights figure.
5.1.2.3 APS Config. figure
APS config. figure is a figure in which the user can change the five variables described in
section 4.3.7 (IntReg, FReg, CutIn, CutOut and SReg) to change the model’s APS behaviour.
This figure is shown below:
46
Figure 22. APS Config. figure
5.1.3 Output figures
The whole purpose of the pBk app. is to have a model which can provide the user with
some outputs (e.g. systems pressure, parking brake circuit pressure, system air, air
consumption and etc.), without the need of time consuming experiments on a truck. Other
than that, the user wants to be able to change the variables that were described in section 5.1.2
to try different situations and truck properties with the same or different cycles to see how air
consumption changes due to these changes in variables. The latter can be done in input figures
(see section 5.1.2). Each of these changes could take several days to set up if the user wants to
do them in field tests with trucks. To satisfy the former, pBk uses output figures. Output
figures are used to show all the output data that was mandated in the requirement specification
(see section 5.2.2 and chapter 3). The three different output figures in pBk are explained
below.
5.1.3.1 Pressure graph
‘Pressure graph’ shows the system pressure, parking brake pressure (circuit 3), pressure in
the air suspension tanks (if any) and maximum bellow pressure (between front and rear axles).
It also shows marks for not having enough pressure for parking brake or air suspension in
case of failure during the current cycle. Note that all circuits combined except parking brake
circuit form the system (pressure in all these circuits are assumed to be always the same). An
example of this graph is shown below:
47
Figure 23. An example of a pressure graph
5.1.3.2 Air Consumption graph
‘Air consumption graph’ contains system air (all circuits combined, air suspension tanks
excluded), air being consumed at each moment and indicators that show that there is not
enough air for parking brake or air suspension in case of failure. An example of this graph is
shown below:
Figure 24. An example of air consumption graph
48
5.1.3.3 IDU graph
‘IDU graph’ shows the IDU value simulated by pBk. If the user has selected an APS with
just one desiccant the value for IDU2 remains firmly at zero but if a two desiccant APS was
chosen then the value changes accordingly. An example of this graph is shown below:
Figure 25. Example of an IDU graph
5.1.3.4 Control figure
This figure gives the user the possibility to hide or show different curves on different
output figures. The user can do this by checking or unchecking the checkbox next to the name
of each curve. Figure 26 is the same graph as Figure 23 with air suspension tank pressure vs
time being hidden. Note that in ‘Control figure’, Pt is the curve for system pressure vs time,
PPBEt is for parking brake circuit pressure error (i.e. not having enough air for parking brake)
vs time, PASEt is for air suspension tanks pressure error vs time (i.e. not having enough air
for air suspension), ASTPt is for air suspension tanks pressure vs time. Other than that there is
a button for generating a report on this figure. Report is described in section 5.1.3.5.
49
Figure 26. An example of ‘Control figure’
5.1.3.5 Report figure
This figure will popup upon clicking on ‘’Generate a report’’ on Control figure (see Figure
26). This figure gives the user a chance to have a full report of what happened during the
simulation. The report will tell the user whether or not the pneumatic system failed during the
simulation and, in case of failure, in which cycle it happened and which function failed. On
top of that the report shows total air consumption and also the distribution of it between the
circuits, regeneration and leakage in NL and also percentage. The report also lets the user
know how much air was delivered into the system by the compressor during the simulation
and also the time length of the simulated cycle. An example of this figure is shown below:
50
Figure 27. An example of the ‘Report’ figure by pBk
Note that all the output figures except the control figure have the ability to be saved as a
figure (for later use in MATLAB) or as JPEG or even PDF if needed.
5.2 How the model works
As described earlier in 5.1, the model consists of different parts and has different
changeable variables. In this part a brief and general description of what inputs should be
provided by the user and how the code simulates the cycles is made.
5.2.1 The model (pBk’s) code
As mentioned before pBk is a model coded in MATLAB. The model (pBk) consists of
several scripts including one main script and several supplementary scripts (functions).
5.2.1.1 The model (pBk’s) main script
The main script which is about 2700 lines is the heart of the program. The program will
start by running this script. This script consists of input and output figures and the whole user
interface (see 5.1). The whole simulation is done by running this script (having a while loop
which runs until simulating the number of cycles that the user wants is completed). The script
simulates the cycles by going back and forth between the supplementary scripts as needed.
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5.2.1.2 Supplementary scripts of pBk
There are seven supplementary scripts in pBk that parallel to the main script:
1. Scripts aircon and aircon_interpolate
The scripts aircon and aircon_interpolate are contain two different functions that together.
The function aircon_interpolate runs whenever the user changes the air suspension types (e.g.
two or four bellows for rear). The program will choose the correct excel sheet for the
specified air suspension type and run aircon_interpolate to interpolate for loads, heights and
air consumption as explained in section 4.2.2. After this whenever a simulation for air
suspension is needed the code will run the aircon function. This function will determine how
much air should be consumed going from height a to height b given a particular load and air
suspension type.
2. Scripts airdel and airdel_interpolate
The scripts airdel and airdel_interpolate also contain two different functions that work
together. The function airdel_interpolate will run whenever the user changes the compressor
type to interpolate the air delivered for different back pressures and engine speeds as
described in chapter 4.2.1. After that, at each moment, airdel will run to determine how much
air should be delivered to the system at that moment if the compressor is on.
3. Scripts overflow and flow_interpolate
The scripts overflow and flow_interpolate as well contain two different functions which
work together. The flow_interpolate function interpolates the flow through the overflow valve
for different upstream and downstream pressures as described in sections 4.3.4 and 4.3.5 After
that at each moment overflow function will determine whether or not the valve is open and
also, if open, how much the flow [NLPM] is, given the previous state of the valve
(open/close) and upstream and downstream pressures (system and air suspension tanks
pressure respectively).
4. Script cycle_definer
The cycle_definer script generates pBk files (see section 5.2.4). The script gets its needed
data from the user (see 5.2.2) and converts it to a unique pBk file. From the speed and service
brake status provided by the user, pBk will calculate the deceleration, a, for each service
brake that happens via cycle_definer, using Eq. (18). These decelerations will then determine
how much air should be consumed in that specific moment, for applying service brake (see
section 2.3.3). Note that there are some times that service brake is activated without having
any deceleration (like when the driver wants to release the parking brake and start driving
he/she usually uses the service brake). At these points the air being consumed was found,
through experimentation.
𝑎 =
𝑉2 − 𝑉1
∆𝑡
(18)
52
in which a is the acceleration, V1 and V2 are the speed when ‘’Service brake active’’ got
activated and the speed right after it got deactivated respectively and Δt is the time difference
between these two moments.
In addition, cycle_definer, defines the heights at each moment depending on the users input
into the ‘Air Suspension Heights’ figure and the parking brake status signal (called ‘Parking
brake active’ on the CAN). The script works on the assumption that that whenever the parking
brake gets activated, the truck has to kneel and that whenever it gets released, it should rise.
Lastly, the cycle_definer function converts the ‘’Parking brake active’’ and ‘’Service brake
active’’ values given by the user to the values that can be used by the program. pBk changes
‘Service brake active’’ column by putting a ‘1’ whenever service brake changes from
deactivated to activated. The same thing will happen to ‘’Parking brake active’’ with the
addition that it puts a 2 whenever it gets released as well.
5.2.2 User data
The first thing that the code needs after being run is the user data. The user should have
recorded data for a given amount of time to feed into the model. The data should be fed into
pBk as an excel sheet with some desired variables. These variables are listed below:
Vehicle’s speed
Engine speed (Rpm)
Parking brake active
Service brake active
All the variables listed above can be found on the truck’s CAN signals. ‘Parking brake active’
and ‘Service brake active’ are values from the truck’s CAN signals showing whether or not
the corresponding brake is activated. A value of ‘1’ means the brake is activated and ‘0’
means that it is not. After getting these variables from CAN signals, Moment values should be
fed into the excel sheet manually. An example of this excel sheet is shown below:
53
Figure 28. An example of the data which can be fed into pBk
There are several ways of getting this data to feed into the model:
1. Recording new data
The first, and the best, way is for the user to drive for the amount of time they want, in the
way they want their cycles to be, and record the required variables.
Note that while recording the frequency used for recording (known as sample rate), should be
fed into the text label named ‘’Sample Rate [Hz, 1/s]’’ in pBk’s main figure (see 5.1.1). The
user can then feed the recorded data into the model and repeat it as many times as they want.
On top of this, they can change the different variables in the model to see if they can improve
the air consumption of the truck and also see the effect that each variable has on the outcome.
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2. Using previous data
The user can just use data recorded by any other person which has the required variables,
put them in an excel sheet like Figure 28 and feed it into pBk.
3. Making your own data
The last way to have a valid data for pBk is by making your own data. If wanted, the user
can make their very own data. The user can make their own excel sheet with the required
variables. In this way, the user can have a chance to create their data however they desire.
5.2.3 How to feed the data into pBk
After creating the data sheet, the user can easily feed it into the model using the ‘Air
Suspension Heights’ figure (see 5.1.2.2). The user can click on Browse and browse to the
made data, wherever it has been saved. After selecting the file the user should then click on
‘’Generate the specified cycle’’. A message saying ‘’pBk data has been generated.’’ will show
up when the model has generated the file. This generated data can be saved as a pBk file that
could be used without the need of generating again for future works (see 5.2.4). Now that the
simulation file is ready, the simulation can be started by clicking on the ‘Run’ button on the
bottom right corner of pBk’s main figure (see 5.1.1).
Note that all other variables can be changed separately however the user wants via different
input figures and the main figure (see 5.1.2 & 5.1.1). The user has to make sure the sample
rate is correct and that it matches the sample rate of the recorded data before generating pBk
data.
5.2.4 The model (pBk) file
A pBk file is a simple excel sheet file which can be read directly by pBk. This means,
having this type of file, the user doesn’t even need to generate any data, since a pBk file is
data that has already been generated. The user can simply open this type of file by clicking on
‘Open pBk file’ and choosing the correct file from any directory on the computer. A pBk file
is a file which was once generated by the user and been saved for future works. This feature is
helpful when the user has a big data recorded which takes a lot of time to be generated (for
information about the speed of pBk please refer to ‘MATLAB fast simulator (pBk air
consumption application)’in section 4.1). Instead of generating the pBk file each time the user
wants to run simulations with the same file after closing pBk, the user can save it and open it
for simplicity in his/her future simulations. How a pBk file is made from user input data was
described in section 5.2.1.2. An example of a pBk file is shown below (this is the pBk file
generated from user data shown in Figure 28):
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Figure 29. pBk file generated from user data shown in Figure 28
5.2.5 Simulation
After generating the wanted pBk file and making sure all other variables are as wanted, the
user can run the model (i.e. start the simulation) by clicking on the ‘’Run’’ button on the main
figure. Having the correct pBk file, pBk will go through every single row of the file and run
the simulation accordingly. The model (pBk) will run a while-loop until it reaches the wanted
time and then finishes the simulation and plots the outputs (see sections 5.2.1.1 & 5.1.3).
Whenever needed, pBk refers to one of the supplementary codes (see section 5.2.1.2), goes
into them and calculate the wanted variable. Each time the while-loop runs, pBk will go into
the ‘airdel’ function to calculate how much air will be delivered by the compressor in case it
is activated.
5.2.5.1 Service brake air consumption
For service brake air consumption calculation, pBk has a function in the main script.
Having the coefficients from interpolating the service brake experiment (see section 4.3.8.1),
pBk will then calculate how much air should be used for that specific service brake and
acceleration. This will happen by checking the pBk file. Whenever service brake column
shows 1, it means that service brake is being used. After that pBk will check if there is any
acceleration for the service brake or not. If not a predetermined amount of air will be
consumed according to the service brake experiment. If an acceleration value is available pBk
will calculate the corresponding air consumption.
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5.2.5.2 Air Suspension air consumption
The pBk files have four columns named Height_f1, Height_f2, Height_b1 and Height_b2
which show the knelt height in the beginning of the data. These heights will rise up to driving
heights according to the kneeling speed (which applies to both kneeling and rising) given by
the user. Height_f1 and Height_f2 are heights of the front axle before the moment starts and
after the moment finishes respectively at each row of the pBk file. Height_b1 and Height_b2
are for the rear (back) axles. Note that pBk assumes that while kneeling, all the axles go down
together and while rising all the axles come up together which is not the case in real world. In
reality while kneeling, front and the tag axle kneel and the traction axle rises and while rising,
front and the tag axle rise and the traction axle kneels (at least this is what was observed on
the truck, Pierre, which was used for the evaluation experiments. It might be different in
different trucks). If in a row Height_f1 is less than Height_f2 or Height_b1 is less than
Height_b2, pBk will calculate the air needed to go from Height_f1 to Height_f2 and/or from
Height_b1 to Height_b2 using the ‘aircon’ function and according to what configuration the
user has chosen for the simulation (i.e. type of front and rear air suspension if any).
5.2.5.3 Parking brake air consumption
Parking brake air consumption is less complicated. PBk goes to another function inside its
main script whenever the pBk file shows a 1 on the parking brake column (i.e. the parking
brake should be released). After that it will calculate the air consumed using the equations
explained in section 6.1. Note that according to the configuration that the user has chosen, the
air consumption will differ. Also if the user has chosen the parking brake disengaging feature
for the front parking brakes (see 5.1.2.1), whenever the pBk file’s parking brake column
shows 2 (i.e. the parking brake should be engaged), the parking brake air consumption
function will run, this time only to release the brakes in front to do the height adjustment.
For each repetitions of the while loop, after calculating the air consumed by service brake,
parking brake via the functions explained in sections 5.2.5.1, 5.2.5.2 and 5.2.5.3, if needed,
pBk will decide if in that specific moment the air is being pumped in by the compressor or not
and also if regeneration is happening or not, according to APS simulation (see section 4.3.7).
Then according to what pBk has decided (i.e. compressor active or regeneration or neither), a
new IDU value will be calculated. After that new pressure will be calculated according to the
IDU ratio chosen by the user and IDU values before and after starting the moment. After that
the air consumed by the auxiliary circuit and leakage (again chosen by the user) will be
converted to pressure using Eq. (11). This pressure will then be subtracted from the pressure
calculated from IDU changes. To calculate total air consumed in that moment, the difference
of the pressures before starting the moment and after it, combined with Eq. (11) will be used.
In the end of each while-loop for each moment, the function overflow (see section 5.2.1.2)
will run to decide whether or not the air suspension circuit’s overflow is open. If the code
returns 1 (i.e. overflow is open), from the flow given by the same function, the new pressure
in the air suspension tanks will be defined.
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5.2.6 Failures
In the simulation, circuits 3 and 5 can fail if they do not have enough pressure to function.
If circuit 5 (i.e. air suspension circuit) fails, the code will continue to run through the cycle but
without truck being able to rise (same as reality). The code will then show this failure by
putting black stars on the places that failure has happened in output figures: ‘Pressure graph’
and ‘Air Consumption graph’. If the failure is in circuit 3 (i.e. parking brake circuit), the code
will just come out of the main loop, pump air into the system until the system has enough air
for releasing the parking brake and then go back and continue the pBk file. The same thing
will happen in reality; if the pressure in the system is low enough, the driver is forced to wait
for the pressure to rise. The driver cannot start moving the truck since the parking brake can’t
be released at all. The time that was spent pumping air instead of running through the pBk
file, in case of parking brake circuit’s failure, and the percentage of it compared to the total
time of the simulation will then be shown in the output figure: ‘Report figure’ (see 5.1.3.5).
58
59
6 DISCUSSION
6.1 Impact of assuming that p-brake release is
isothermal
In the model developed, it will be assumed that the process of releasing the parking brake
is isothermal as this allows for easier calculations since temperatures will not need to be
included. To quantify the potential effect of this, a Matlab script that compares the air
consumed assuming an isothermal process and air consumed with temperature changes was
created. The script considers three cases.
Case 1: Pressure in the system before brake release is below the cut-off of the
pressure limiting valve.
Figure 30. Simplified schematic representation of the parking circuit
Consider the system above, the subscript ‘sys’ represents the air tanks and service brake
circuit. ‘PL’ is the pressure limiting valve that controls the pressure in the parking brake
circuit. Subscript ‘P’ represents the part of parking brake circuit that is always pressurized and
subscript ‘b’ represents the part of the circuit that has to be pressurized in order to release the
parking brake.
Since in this case the pressure in the tanks is below the cut-off of the pressure limiting valve,
after the parking brake is released, all parts of the system will have the same pressure. If an
isothermal reaction is assumed, the following equation holds true,
𝑃𝑠𝑦𝑠 ∙ (𝑉𝑠𝑦𝑠 + 𝑉𝑝) + 𝑃𝑎𝑡𝑚 ∙ 𝑉𝑏,𝑏 = 𝑃𝑠𝑦𝑠,𝑎 ∙ (𝑉𝑠𝑦𝑠 + 𝑉𝑝 + 𝑉𝑏,𝑎)
The additional subscript ‘b’ here represents ‘before parking brake release’ and ‘a’ represents
‘after parking release’.
However, if an isothermal reaction cannot be assumed, the ideal gas law can be used as
follows:
𝑃𝑠𝑦𝑠,𝑏 ∙ (𝑉𝑠𝑦𝑠 + 𝑉𝑝)
𝑇𝑠𝑦𝑠,𝑏+
𝑃𝑎𝑡𝑚 ∙ 𝑉𝑏
𝑇𝑎𝑡𝑚=
𝑃𝑠𝑦𝑠,𝑎 ∙ (𝑉𝑠𝑦𝑠 + 𝑉𝑝 + 𝑉𝑏)
𝑇𝑠𝑦𝑠,𝑎
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To check for the maximum possible variation, Tatm is varied from 243K to 323 K.
Accordingly, Tsys,b is varied between 0-80⁰C. The equalization pressure, Tsys,a is assumed to
range between Tsys,b-10 K to Tsys,b. The beginning pressure of the system is also varied from 6
bar to 8.5 bar. The percentage between the isothermal case and the most extreme of the cases
with the varying temperature is found to be 4.3%. This is a negligible difference, especially
considering that these extreme temperature variations are unlikely of occur. As such, a model
that assumes an isothermal process is deemed acceptable.
Case 2: Pressure of the system after brake release is above the cut-off of the
pressure limiting valve
For this case the pressure in the brakes will be equal to the cut-off pressure of the pressure
limiting valve (PPL) when the brakes are released. The pressure upstream of the pressure
limiting valve will remain above PPL. The equation for an isothermal process is as follows:
𝑃𝑠𝑦𝑠,𝑏 ∙ 𝑉𝑠𝑦𝑠 + 𝑃𝑃𝐿 ∙ 𝑉𝑝 + 𝑃𝑎𝑡𝑚 ∙ 𝑉𝑏 = 𝑃𝑠𝑦𝑠,𝑎 ∙ 𝑉𝑠𝑦𝑠,𝑎 + 𝑃𝑃𝐿(𝑉𝑏 + 𝑉𝑝)
If the process is not isothermal then the following equation holds:
𝑃𝑠𝑦𝑠,𝑏 ∙ 𝑉𝑠𝑦𝑠
𝑇𝑠𝑦𝑠,𝑏+
𝑃𝑃𝐿 ∙ 𝑉𝑝
𝑇𝑎𝑡𝑚+
𝑃𝑎𝑡𝑚 ∙ 𝑉𝑏
𝑇𝑎𝑡𝑚=
𝑃𝑠𝑦𝑠,𝑎 ∙ 𝑉𝑠𝑦𝑠,𝑎
𝑇𝑠𝑦𝑠,𝑎+
𝑃𝑃𝐿(𝑉𝑝 + 𝑉𝑏)
𝑇𝑎𝑡𝑚
In this case, the percentage difference between the two is about 3.1% which can also be
considered negligible.
Case 3: System pressure starts above cut-off of pressure limiting valve and
ends up below
For this case the isothermal equation is as follows:
𝑃𝑠𝑦𝑠,𝑏 ∙ 𝑉𝑠𝑦𝑠 + 𝑃𝑃𝐿 ∙ 𝑉𝑝 + 𝑃𝑎𝑡𝑚 ∙ 𝑉𝑏 = 𝑃𝑠𝑦𝑠,𝑎(𝑉𝑠𝑦𝑠 + 𝑉𝑝 + 𝑉𝑏)
If the process is not isothermal then it can be represented with the following equation:
𝑃𝑠𝑦𝑠,𝑏 ∙ 𝑉𝑠𝑦𝑠
𝑇𝑠𝑦𝑠,𝑏+
𝑃𝑃𝐿 ∙ 𝑉𝑝
𝑇𝑎𝑡𝑚+
𝑃𝑎𝑡𝑚 ∙ 𝑉𝑏
𝑇𝑎𝑡𝑚=
𝑃𝑠𝑦𝑠,𝑎 ∙ (𝑉𝑠𝑦𝑠 + 𝑉𝑝 + 𝑉𝑏)
𝑇𝑠𝑦𝑠,𝑎
In this case the percentage difference is found to be 4.6% #negligible.
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6.2 Evaluation of the model
To evaluate the model, tests were done on a refuse truck (called ‘Pierre’) and the data
obtained was compared to what the model outputs. During the tests the driver aimed to
simulate what happens when during garbage collection.
6.2.1 The harsh cycles, high load
In the first part of the test, the driver would drive harsh. Every time he stopped he would
apply the parking brake and open the driver’s side door. This would cause the truck to kneel.
After a brief amount of time he would then close the door and release the parking brake. This
would cause the truck to rise back up to driving height. For this test the truck was heavily
loaded.
The data recorded during these tests can be seen in the figure below.
Figure 31. Data obtained from evaluation test 1
In this graph, and in the ones that follow, AST stands for air suspension tanks. As expected it
can be seen that the pressure deviations occur in sync with the speed cycles. To give a clearer
view of the pressure deviation, the same graph is shown below with the speed curve excluded.
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Figure 32. Same figure as above but without speed
The engine rpm, vehicle speed and other relevant data (see section 5.2.2), were entered into
the model and the result is shown in the figure below.
Figure 33. Results from the model with conditions of evaluation test 1
It can be seen from the graphs that the overall trend is the same; in the beginning the air
suspension tank pressure increases and decreases along with the service brake tanks’ pressure
but later in the test it stops increasing and only decreases. This is because in the beginning the
overflow valve between the APS and the AST is open but later when the pressure drops it
closes.
The air consumed in the actual test is calculated according to the following formula:
𝐴𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 = 𝐴𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 + (𝑉𝑠 ∙ (𝑃𝑠,𝑎 − 𝑃𝑠,𝑏) + 𝑉𝑎𝑠 ∙ (𝑃𝑎𝑠,𝑎 − 𝑃𝑎𝑠,𝑏))
∙ (273/𝑇𝑎𝑡𝑚)
(19)
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Where 𝑃𝑠,𝑏 and 𝑃𝑠,𝑎 are the pressures of the service brake circuit before and after running the
cycles respectively. 𝑃𝑎𝑠,𝑏 and 𝑃𝑎𝑠,𝑎 are the pressures of the extra air suspension tanks and 𝑇𝑎𝑡𝑚
is the atmospheric temperature. The model is off by 6.4% with regard to air consumed.
This is a very good result. However it can be noticed that curves in the two graphs are not the
same. One particular difference is that in the actual data the parking brake pressure falls at the
beginning of each cycle. In the second graph, from the model, this is not the case in the
beginning. This is because the model assumes instantaneous pressure equalisation between
the service brake and parking brake circuits. Even though this is actually not what happens, it
has no effect on overall consumption since equalisation will anyway happen before the
following cycle starts. In NCG trucks since there is no parking brake tank, this behaviour will
not occur at all anyway.
Another noteworthy difference is that the pressure in the air suspension tanks drops at the
beginning of each cycle in the first graph but in the second it only drops if it is greater than or
equal to the pressure in the service brake tanks. A possible reason for this is an unexpected
behaviour of the double check valve in the air suspension tank that selects between the extra
air tanks and the APS (air from the service brake tanks). In the model it is assumed that this
valve always opens up the tanks with the higher pressure. In reality this does not appear to be
the case. It appears that when the air suspension tanks are at a lower pressure, about 10% of
the air that feeds the air suspension is drawn from them. The exact cause of this was not
further investigated. A hypothesis for what could be causing it was however put forth. It could
be that the air flow through the double check valve from the APS causes a dynamic pressure
that is less than that of the AST causing the ball in the double check to move and open up the
AST. The model was adjusted so that it would replicate this behaviour during simulations.
The pressure curves after this change was implemented in the model are shown in the
following figure.
Figure 34. Pressure drop with edited model
In this case since none of the pneumatic systems fail, the amount of air consumed remains the
same. What do change however, are the final pressures in the respective circuits. The AST
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pressure will now be lower whereas the service brake tanks pressure will be higher. This can
be observed in Figure 34 above.
6.2.2 The very harsh cycles, high load
The above analysis was repeated on a set of more aggressive cycles and the respective
graphs are shown below. Note that these cycles will not happen in real-time driving (garbage
collecting) since they are really harsh and fast (impossible to be done). These cycles were just
done to make at least one of the functions (circuits) to fail. Failing means, not having enough
air to function properly.
Figure 35. Pressure drop for a more aggressive cycle
From this data, among others, it was discovered that the truck first fails to rise up to drive
height before the system pressure has fallen below the front bellows’ pressure. This was rather
surprising as such a failure would have been expected only after the system pressure has
equalized with that in the front. Upon further investigation it was discovered that this is
caused by the overflow valve in the circuit protection valve just before the air suspension
circuits is almost closed. What this means is that the air can barely flow from the APS into the
air suspension circuit and then into the bellows. After this was learned, the model was
changed so that failure in the air suspension would occur if either one of two conditions was
met: either the system pressure dropped below the found pressure (the pressure in which the
overflow in circuit protection valve for air suspension circuit is barely open) or the system
pressure dropped below the bellow pressure, whichever happened first. The result from the
model is shown below in Figure 36 below.
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Figure 36. Pressure drop according to the model for the more aggressive cycle
In both Figure 35 and Figure 36, there is failure of the air suspension. In the results from the
model, Figure 36, the time when it fails is clearly outlined, this happens in the fourth cycle. It
happens since the system pressure drops below the circuit protection, overflow valve’s is
barely open. In the actual data, this is harder to observe. It is characterised by the flat troughs
in the system pressure curve which are a result of the fact that as soon as air is compressed
into the system it goes directly to the air bellows which is why the system pressure never
increases. This failure becomes clearer when one looks at the chassis height signal.
Figure 37. Bellow extension during the very harsh cycles
As can be seen from this figure the truck is unable to get back to the drive height during the
third cycle and those thereafter. The percentage difference with regard to air consumed by
these cycles in reality and the model is 0% which is extremely good.
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To make the comparison between the model and the actual data clearer, the service brake and
air suspension pressures of both sets of data are plotted together in the following figure:
Figure 38. The pressures from the model and real data plotted together
From this figure it can be seen that the system pressure from the model follows the one from
the actual data very closely. On average, the pressure from the model deviates from the actual
one by 2.7%. This is a very good correlation. One of the reasons why the curves do not
perfectly match is because the failure in the air suspension tank does not always occur at the
same pressure. This is because this failure is due to the near closing of the overflow valve in
the circuit protection valve in the APS that feeds the air suspension circuit. The open and
closing of this valve is a complex dynamic behaviour that is not modelled in the Matlab code.
Instead a specific pressure for when the failure occurs is chosen and implemented.
When it comes to the AST pressures, the difference between them is rather large. By the end
of the cycles the pressure from the model deviates from the real one by 13%. This is because
in this case it appears that more air than just 10% is drawn from the AST when they are at
lower pressure than the service brake tanks during height adjustment. Due to the limited time
available, no efforts were made to remedy this discrepancy.
So as to test the feasibility of repeating the same cycle several times to represent longer
driving with similar recurring cycles, data from just the first cycle shown in Figure 35 is fed
into the model and the cycle is repeated 10 times. The result of this simulation is shown in the
following figure.
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Figure 39.Result of running the same cycle ten times
As can be seen in this figure the air suspension fails in the third cycle just as in Figure 36.
The air consumption in this case differs 5% from what was previously calculated by the
model when data from the ten cycles was fed in. What this shows is that asking the model to
repeat the same cycle several times to simulate a long drive time with recurring cycles is
reasonable and it gives a good approximation. Of course how good an approximation it is
depends on how the cycles in the real world are; the more similar they are, the better the
approximation.
6.2.3 The whole drive
The two different sets of cycles presented in sections 6.2.1 and 6.2.2 above were recorded
on the same day; they were part of one large drive. The speed profile for this drive is shown in
Figure 40 below.
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Figure 40. Speed during the whole drive
The pressure deviations during this drive are shown in Figure 41 below.
Figure 41. Pressure changes over the whole cycle
The result of running the whole cycle through the model is shown in Figure 42. Result
from simulation of the entire evaluation cycle below.
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Figure 42. Result from simulation of the entire evaluation cycle
The differences between Figure 42 and Figure 41 are the same as those discussed in
section 6.2.1. The model has an error of 1% when it comes to air consumed. To allow for
easier comparison, the pressures from both the model and real data are plotted in the same
graph in Figure 43 below.
Figure 43. Real and simulated pressures for the whole drive cycle
6.2.4 The harsh cycle, medium load
The harsh cycles were repeated once again but this time with medium load on the truck.
The graph below shows the pressures from both the model and the real data.
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Figure 44. Pressures for harsh cycles, medium load
Just as before the system pressure of the model is very similar to the real one but there is a
rather large difference between the AST pressures. The average deviation of the system
pressure is 3.3% while for the AST pressure it is 8.5%. The truck is off by 2% with regard to
the air being consumed.
6.2.5 The very harsh cycle, medium load
The very harsh cycle was also repeated with medium load on the truck and the results were
as shown in below:
Figure 45. Pressures for harsh cycles, medium load
The average error of the system pressure is 2.7% while that of the AST pressure is 15%.
The air consumed has a percentage difference of -1%.
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6.2.6 Causes of error
From the section above it can be seen that the model gives acceptable simulations of the air
consumption in the truck. However, when it was discovered that the graphs of the recorded
data did not fully correlate with those produced by the model, several investigations were
launched so as to find the cause of the error. The following were considered.
6.2.6.1 Compressor
It was considered possible that the datasheets from whose data the model calculates the air
delivered might be incorrect. To check if this was the case a test was done where in stationary
truck, the pressure in all the circuits was lowered to about 1 bar and then the compressor was
allowed to fill them up with a controlled engine speed. Using the following equation the air
delivery rate of the compressor was calculated and compared to the numbers provided in the
datasheet, and used in the model.
𝑎𝑖𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 = (∆𝑃𝑓 ∙ 𝑉𝑓 + ∆𝑃𝑏 ∙ 𝑉𝑏 + ∆𝑃𝑝𝑏 ∙ 𝑉𝑝𝑏) ∙ 273/𝑇𝑎𝑡𝑚 (20)
where ∆𝑃 is the increase in pressure, V is the volume and the subscripts f, b and pb
represent the front service brake, rear service brake and parking brake circuits respectively.
The following table shows the percentage error between the data sheet figures used in the
model and the calculated values for the four engine speeds that were tried.
Table 12. Results from compressor test
Engine Speed (rpm) Percentage error
600 2
1000 6
1500 0
2000 0
From this table it can be seen that the difference between the values that the model uses
and what the compressor actually delivers is rather low. Considering the fact that the truck
spends most of the time stationary (for refuse trucks, while collecting garbage), with the
engine running at 600 rpm, the error in air delivered in the model can be assumed to be about
2%.
6.2.6.2 Simplifications
Many simplifications have been that lead to the model not giving accurate results. As an
example, the model does not take into account the change in bellow extension and pressure
caused by the braking. When the driver brakes, because of inertia, more force is transferred to
the front bellows causing their pressure to increase and their extension to decrease. How
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exactly this affects air consumption has not been investigated and the effect is altogether
ignored in pBk.
6.2.6.3 Combination of all errors
As mentioned in their respective sections, the different systems have errors and though
these errors lie within the accepted margins, they build up to contribute the inaccuracy of the
model.
6.3 Evaluation of the tests
The tests that were conducted were very helpful in the evaluation of the model; a lot could
be learned from them. However, some shortcomings were also discovered. For one, since the
cycles are so aggressive, the compressor is almost always running. This means that it was
rather hard to judge how well the model simulates the APS in other regimes where it would
have frequently switched between compression, regeneration and idling.
It would have also been good to conduct some tests with the truck unloaded as this might
have given a better understanding of the failure in air suspension at lower bellow pressures.
Both the medium and high loads result in rather high pressures in the front bellows.
During the test it was noticed that when the truck was kneeling the pressure in the front
bellows fell considerably. This is because the chassis came to rest on the bump stop which
takes some of the load off the bellows causing their pressure to decrease. When the parking
brake is released air must first come into the bellows and build up the pressure before the
truck can rise back to the driving height. This means that the air suspension will consume
more air than if it hadn’t rested on the bump stops. The data available for air consumption by
air suspension start with the bellows having no pressure at all (see section 4.2.2). So as to
allow for a more accurate calculation of air consumption, the available data is shifted along
the x-axis to give the graph shown Figure 46 below.
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Figure 46. Normal bellow air consumption with shifted data
Compared to Figure 10, air consumption in this graph does not start at 0 NL for 0 mm. The
sharp rise in air consumption at the beginning has been removed. This is done so that in the
model when the chassis height is 0 mm, there is already some pressure in the bellow (note that
in the model air consumed during height adjustment is calculated as the difference in air
consumption at both heights according to the graph in the figure above). Even though, this
gives a more accurate model, the starting bellow pressure does not match that of the real truck
and as such air consumption by the air suspension is a source of error in the model.
6.4 Further discussion
After the evaluation phase, the idea of changing the variables of the model to see the
effects of each came into mind. An engineering design way of doing so, is to make a factorial
experiment. Due to the time factor no experiment was designed. Instead, changing one
variable at a time relative to a base-line simulation and observing the effects was done.
The idea was to compare everything to a base-line simulation. After some deliberation, the
base-line chosen was a simulation which fails somewhere in its mid-point (halfway through
the simulation). Also, since some evaluation and tests were done, it was preferred that the
base-line be one of the tested cycles. Hence, the evaluated cycles from section 6.2.2 was
chosen with some alterations. Most of the trucks specifications, such as number of parking
brakes and type of air bellows, used in the model were the same as those on the truck used
(Pierre). The starting pressures were set higher than in the actual test and the IDU value was
set to zero. The ambient temperature was set to 10°C (temperature of a normal day in
Stockholm). On top of these differences, the sum of the parking brake and service brake
circuits’ volume on Pierre was put solely on the service brake circuit and the parking brake
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circuit volume set to zero in the model. This was decided since all the new trucks will only
have tanks on their service brake circuit. The driving heights and kneeling heights were
changed as well to the ones that will be used in the NCG trucks; from now on the trucks that
will be produced do not kneel all the way to the bump stop. The model gives better results
when simulating a truck that does not kneel lower than 15 mm (enough to touch the bump
stop).
The simulation takes place in 20 cycles and the base line model fails on 12th
cycle. After
changing the variables one at a time, the results which are shown below, were observed and
recorded:
Table 13. Results of changing pBk variables one at a time
Factor Change
Failed
cycle Base line simulation - 12
APS APS2-Advanced APS2-HighCapacity 8
Compressor High capacity Normal capacity 8
Compressor High capacity Low capacity 6
Service brake circuit tanks
volume Normal High 15
Air bellow front Low –> Low Hybrid 15
Air bellow front Low –> Extra low 9
Air bellow front Low –> Extra low Hybrid 16
Air bellow front Low –> Normal 16
Air bellow front Low –> Normal Hybrid
DNF (Did
Not Fail)
Air bellow front Low –> None DNF
Air bellow rear 2 bellow –> None DNF
Air bellow rear 2 bellow –> 4 bellow 14
Environment temperature °C 10 –> -5 12
Environment temperature °C 10 –> -20 12
Environment temperature °C 10 --> 25 12
Environment temperature °C 10 --> 40 12
Air suspension circuit tanks
volume
Normal to low (service brake tanks normal
to high) 11
Air suspension circuit tanks
volume Normal to high 17
Engine type Changed to type 1 (gear ratio increased) 19
Engine type Changed to type 2 (gear ratio decreased) 9
Parking brake tanks Low to high 11
Parking brake in front Without –> With (Disc) 10
Parking brake in front Without –> With (Drum) 9
Disengaging front parking brake
feature Without –> With (Disc) 8
Disengaging front parking brake
feature Without –> With (Drum) 7
No. of rear axles with parking 2 –> 1 18
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brake
No. of rear axles with parking
brake 2 --> 1 add disc parking brake to front 15
No. of rear axles with parking
brake
2 --> 1 add disc parking brake and release
feature to front 11
Load on front axles and rear
axles High Normal 17
Load on front axles and rear
axles Normal Low 20
Load distribution on the back 60/40 –> 65/35 12
Load distribution on the back 60/40 –> 70/30 12
Kneeling and driving heights (f-
d, f-k, r-d, r-k) New trucks config to old trucks config DNF
Kneeling and driving heights (f-
d, f-k, r-d, r-k)
New trucks config to old trucks config
(bump stop touch) 8
Note that in Table 13 whenever a failure happened it was the air suspension function which
failed (meaning that it didn’t have enough air to rise up again).
6.4.1 Results of further investigation
The results of the above experiment will be briefly discussed in this section. There were
three main purposes for this experiment:
1. Finding the variable with the largest effect
2. Finding the variable with the least effect
3. Finding the best combination
Note that as said before, this experiment would have gotten better (more accurate and
trustworthy) results if it followed a factorial design. The results of this experiment are just
observations of the effects that changing each variable (one at a time) has on the air
consumption (mostly the failing cycle).
6.4.1.1 Variables with the largest effect
The variables that had a large effect are listed below. The list is in descending order with
regard to the amount of effect each variable had.
Taking out air suspension (different bellows)
Changing the bellow type
Changing parking brake configs. (e.g. adding or removing them)
Changing the APS type
Changing kneeling and driving heights
Changing the loads on the axles
Changing the compressor
Changing the volume of the tanks
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It should be once again mentioned that these are just observations from the results of a test
which could have been done in a more scientific way (factorial design), which was not done
due to the limited time available.
6.4.1.2 Variables with the least effect
There were two variables which seem to have rather low effect on air consumption and
consequently which cycle fails. These two variables were the ambient temperature and the
load distribution ratio between the tag and the traction axles.
6.4.1.3 The best combination
It was observed that having normal hybrid bellows on the front axle gives the least air
consumption of all amongst all other bellows (except no bellows, obviously). On top of that,
obviously having no bellows (or having them without kneeling) on the rear axles will reduce
the air consumption considerably. Keeping in mind that the IDU value in most of the
experiments (all of them beside the ones which used APS2-HighCapacity) goes too high
(resulting in high humidity air), the best combination seems to be having normal hybrid
bellows for front air suspension, no bellows for the back (using instead spring suspension) and
APS2-HighCapacity to have dry air during the cycles. However it still seems to not complete
20 very harsh cycles for Pierre which rests on the bump stop when kneeling, but this will not
be happening in the trucks produced from now on). Figure 47 shows the pressure graph for
the best combination.
Note that this was chosen due to the fact that changing compressors capacity is currently not
possible. It is obvious that raising the compressor’s capacity will give better results (see
Figure 48).
Figure 47. Very harsh cycles , High loaded, repeated two times with the best combination config. (i.e. Normal
Hybrid air suspension for front, no kneeling on rear with APS2-HighCapacity)
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Figure 48. Very harsh cycles , high loaded, repeated two times with the best combination config. (i.e. Normal
Hybrid air suspension for front, no bellows on rear with APS2-HighCapacity and having a capacity of 1.5 times
more than the capacity of the normal compressor used for Pierre)
6.5 A discussion about APS2
After the experiments, it was seen that even though APS2-HighCapacity will reduce the air
consumption at high system pressures, it will increase it significantly when it comes to low
pressures (below IntReg) observed on harsh, high air consuming, cycles. This will happen
since, in high air consuming cycles the compressor duty cycle will become 100% even in
APS2-Advance, because of not having enough pressure in the system (APS decides to pump
air in all the time, without regeneration happening). Duty cycle is the percentage of time in a
full cycle during which an air compressor is running. In this case, APS2-HighCapacity is
disadvantageous when it comes to air consumption. This is because APS2-HighCapacity,
despite having a compressor duty cycle of 100% in this case, regenerates as well. This will
consume more air because no regeneration happens in these cases while using APS2-
Advanced. This will eventually result in a sooner failure in the system. On the other hand, in
these situations APS2-Advanced will most probably feed humid air into the system which is a
disadvantage compared to APS2-HighCapacity which will give dry air. With these advantages
and disadvantages, it is a tradeoff between the two APS types. A more thorough research on
the advantages and disadvantages between these two types can be done, to see which one
would be better to use on Scania trucks. This research is out of the scope of this project.
Succinctly put, for high air consuming cycles, there will be a tradeoff between having dry air
and having more air when it comes to choosing APS2 type. Trucks with APS2-HighCapacity
will have less but dry air while trucks with APS2-Advanced will have more air but with high
humidity.
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7 RECOMMENDATIONS
Before starting the project, a goal of making some recommendations for the high air
consumption in the refuse trucks was set. During the process of the thesis, many observations
were made and some recommendations were written down to be used for this section. These
recommendations are briefly explained below:
7.1 Compressor
Through the process of this project, the idea of having a second compressor on the truck or
finding a better compressor for the trucks was considered and researched. On top of this,
research on whether or not two-stage compressors can improve the pneumatic system was
done. The conclusions of these investigations and thoughts will be discussed in the following
sections.
Second compressor
As mentioned in section 6.4, using a second compressor can even make the truck go
through 20 very harsh cycles. Another thing that was found is that if a second compressor is
added and combined with APS2-HighCapacity, not only will there be dry air for all the cycles
but also the system pressure will be always above CutIn. Even when using a capacity of 1.5
times that of one compressor, the truck will survive 20 very harsh cycles (this time going
below IntReg from time to time). This means that adding a second compressor and combining
it with APS2-HighCapacity will ensure improvements on the system and might also make the
compressors’ duty cycles a bit lower so that they can last longer. The influence that having a
capacity of 1.5 times the capacity of a normal compressor will have on the pressure behaviour
of the truck can be seen when comparing Figure 48 to Figure 47.
Two-stage compressor
At the moment all Scania trucks are using single-stage compressors. In this project, the
possibility of using two-stage compressors for the trucks was briefly investigated. The
advantages [9] & [18] are as follows:
1. Reduction in power required to drive the compressor
2. Limits the gas discharge temperature
3. Limits pressure differential
4. Increased volumetric efficiency. Volumetric efficiency of an air compressor is the
ratio of the actual volume of the free air at standard atmospheric conditions discharged
in one delivery stroke, to the volume swept by the piston during the stroke. The
standard atmospheric conditions (S.T.P.) is actually taken as atmospheric pressure and
temperature of 15°C.
5. Reduced leakage loss because of reduced pressure difference on either sides of the
piston and valves
6. Provides effective lubrication because of lower temperature range
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7. It gives more uniform torque, hence, a smaller size flywheel is required
All in all, the most important finding of this investigation was that the two-stage
compressor will last longer than a single-stage one. If the duty-cycle of a compressor goes
above 50% (which is the case when there is high air consumption), the temperature increases
greatly which can cause lubrication oil close to the piston rings to evaporate and form an
aerosol that flows past the piston rings. This causes increased wear in the compressor [19]. As
such, a two-stage compressor will work best in less harsh conditions (i.e. lower temperature
air etc.) with lower duty cycle. Unfortunately, no information regarding whether or not two-
stage compressors have a higher air delivery was found. So, two-stage compressors will
extend the compressors’ life but will presumably not solve the high air consumption problem.
Other than this, two-stage compressors will have the following disadvantages:
1. Increase in the complexity of the compressor body
2. Increase in piping, equipment, valves and space needed
3. Significant increase in manufacturing and operating costs.
7.2 Service brake activation
It was observed that the parking brake circuit is the second highest air consumer in‘Pierre’.
On top of that, refuse trucks are high air consuming trucks. An idea of using service brakes
instead of parking brake while collecting garbage was generated under the course of this
project.
In this concept, a lever will be added to the truck which can activate the service brake
without pushing the pedal. This lever should only be used when the driver has completely
stopped the vehicle but wants to activate the service brake instead of the parking brake
because of the high air consumption that the parking brake will have. The driver should be
aware that they should never leave the truck without engaging the parking brake even though
this lever is on. On top of that, the truck will still beep even though this lever is on whenever
the driver open his door if the parking brake is not activated. This concept has its own
advantages, like being simple and cheaper to build. On the other hand, it might be less safe
and some countries might have legislations that will prohibit its use.
7.3 Using the best combination for refuse trucks
The last recommendation put forth, comes from the results of the experiment which was
explained in chapter 6.4. According to the results obtained from changing the variables in
pBk, the best combination seems to be having type 5 bellows for front axles, no air bellows on
rear axles and APS2-HighCapacity. Also, type 1 air suspension for the rear axles was found to
be better than type 2 air suspension with regard to air consumption. However, type 1 air
suspension might have disadvantages such as increase in cost. The disadvantages of type 1 air
suspension were not researched; this was deemed to be out of scope.
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8 CONCLUSIONS
The conclusions are listed as bullet points below:
The air delivered by the air compressors on the truck Pierre is less than the air being
consumed by the truck in very harsh garbage collecting cycles having high load on the
truck which will result to having low air in the system. This can be solved by using the
best combination found in this project for refuse trucks.
Having two compressors or a compressor having twice as much capacity as the one
which is being used now can greatly improve the pneumatic system.
Highest air consumers for garbage collecting trucks are air suspension and parking
brake. After these two, service brake and auxiliary or sometimes regeneration have the
highest air consumption, but significantly lower than the first two highest air
consumers.
When it comes to harsh garbage collecting cycles, APS2-HighCapacity has the
advantage of delivering dry air and the disadvantage of having lower air due to
regenerating compared to APS2-Advanced which will have more air but with high
humidity.
The delivered model (pBk) can be trusted within ±7% for air consumed and +2 cycles
when it comes to determining the failed cycle according to the evaluations which were
done.
The model can be further developed and can be a helpful tool for engineers working
with air consumption in commercial vehicle. They can use it to do simulations instead
of performing time consuming and presumably costly experiments.
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83
9 FUTURE WORKS
The model developed in this project can definitely be improved in many ways and some of
these are presented in this chapter.
One thing that can be said about the model developed in this project is that it can be
improved to make it more accurate, precise, up to date and more extensive (i.e. more
variables, more truck varieties, etc.). In other words, this program is far from perfect so plenty
of future work can be done on it. The most crucial future works of all will be mentioned
below as bullet points:
Simulating the air suspension circuit’s overflow valve which is inside APS, by doing
experiments (like the one which was explained in sections 4.3.4 and 4.3.5) and then
transforming the results into a code and putting them into the script.
Doing more field tests for further evaluation of the model. More evaluations seem to
be needed to make sure everything works properly and the results of the model are
trustworthy enough. The cycles which were done for the evaluation of the model are
harsh cycles with high load. Evaluating the model with milder cycles and lower loads
would be a good idea.
Doing more experiments on APS2-HighCapacity. These tests can be done as both
bench tests and field tests with trucks. Testing on a truck will results more reflective
of reality. After this, an APS2-HighCapacity simulation can be done again in a better
way (if necessary).
Evaluating the model with a truck which uses APS2-HighCapacity. It is crucial to
make sure that the model represents APS2-HighCapacity as well as APS2-Advanced,
since APS2-HighCapacity might be used on so many Scania trucks from now on.
Simulating the auxiliary and power transmission circuits by contacting the
responsible department and getting help from them to make the model more
complete and more precise.
Doing a factorial design on the changeable variables and variants to see which one is
the variable with the largest effect on the air consumption and deriving more
suggestions and recommendations from the result.
Adding more variables and truck models (e.g. 8x2, 4x2 etc.) into the model.
Have a meeting with the colleagues that will use this model in future, explain to them
how to use the model if needed and ask them, what they think can be added to the
model. As mentioned before there are so many things that can be added to the model
and the improvement process of the model can carry on indefinitely. Since Scania
has specialists that would want to use this model in the future, knowing what they
want and providing it to them by adding them to the model can be the best approach
for further developing the model at some point.
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85
10. REFERENCES
[1] Scania AB, “Heritage,” 2017. [Online]. [Accessed 02 February 2017].
[2] S. R. Majumdar, Pneumatic Systems: Principles and Maintenance, New Delhi: Tata
McGraw-Hill Publishing Company Limited, 1995.
[3] N. F. Rd., “BASIC PNEUMATICS,” in a manual for fluid power components and
practical applications, Indianapolis, SMC Pneumatics Inc., 1997, pp. 4-5.
[4] H. N. S. D. D. B. M. B. B. MICHAEL J. MORAN, “FUNDAMENTALS OF
ENGINEERING THERMODYNAMICS,” Danvers, John Wiley & Sons, Inc., 2011.
[5] H. C. P. WILLIAM C. REYNOLDS, “ENGINEERING THERMODYNAMICS,”
McGraw-Hill, Inc., 1977, pp. 240-242.
[6] M. Blaber, “General Chemistry I,” in a virtual textbook, Florida, Florida State
University, 1996.
[7] R. Vadasz, “Mapping air consumption for heavy vehicles,” Stockholm, 2015.
[8] Atlas Copco, “Sustainable Productivity,” in Compressed Air Manual 7th edition, Wilrijk,
Atlas Copco Airpower NV, 2010.
[9] L. P. R. C. P. a. L. C. J. Karamchandani, “Elements of Heat Engines,” BARODA,
ACHARYA PUBLICATIONS, 1997, pp. 342-354.
[10] Scania CV, “10-25 APS, Air processing system,” 2015.
[11] WABCO, “Scania APS 2 RFI -Technical Description,” Stockholm, 2011.
[12] Wabco, Air Springs for Commercial Vehicles, 2010.
[13] E. Bergenlid and E. Stugholm, “Investigation of Air Volumes and Pressure Levels in Air
Brake Systems,” Linköping University Electronic Press, Linköping, 2016.
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[14] Scania, Assembly drawing rear susp, 2007.
[15] Scania, Air susp Rear N/L Assy Dwg, 2003.
[16] C. Petersen, “The Practical Guide to Project Management,” bookboon, 2013, pp. 20-23.
[17] T. Björnelund, “Investigation of high air consumption on tipper truck,” Scania CV AB,
Haugesund, 2008.
[18] H. P. B. a. J. J. Hoefner, “Reciprocating Compressors,” in Operation & Maintenance,
Houston, Gulf Publishing Company, 1996, pp. 20-36.
[19] A. Kits, “Master thesis,” in Test method for oil- and particles carry over from a
compressor to a pneumatic system, LINKÖPING, LINKÖPINGS UNIVERSITET, 2011,
p. 11.
[20] T. Björnelund, “Investigation on high air consumption on refuse collection truck,” Skien,
Scania CV AB, 2008.
![Fuel consumption improvement of vehicle with cvt …performance curves of vehicle engine. Fuel consumption is calculated using specific fuel consumption (SFC [g/(PS*h)]) which is expressed](https://img.dokumen.tips/doc/110x75/5e7f06b7ce1b45666d12bbe3/fuel-consumption-improvement-of-vehicle-with-cvt-performance-curves-of-vehicle-engine.jpg)
