Efficiency Impact on Operating Costs of Mobile Machines

38.2

Efficiency Impact on Operating Costs of Mobile Machines

Robert Rahmfeld Sauer-Danfoss GmbH & Co. OHG, Neumuenster, Germany

Craig Klocke Sauer-Danfoss (US) Company, Ames, IA

ABSTRACT

Incessantly rising fuel costs and planned pollution legislations like Tier 4/Eu IIIB are increasing the focus on efficiency in the mobile machinery market strongly, especially for hydrostatic pumps and motors. This paper discusses the impact of hydrostatic unit design on the driveline efficiency and operating costs of mobile machines. Reliable and precise loss models of hydrostatic units are essential for this investigation, driving requirements for special attention to efficiency measurement and modeling. The energy saving from reducing the needed cooling power combined with additional losses, results in lowering direct operating costs in the range of thousands of $ per year.

INTRODUCTION

Continuously rising fuel costs and the future perspective are making the efficiency of hydrostatic units a major sales argument today in a majority of the mobile machine markets. The planned pollution legislations for 2011, in particular the Tier 4/Eu IIIB, put an additional focus on reduced power losses. Therefore, the selection of hydrostatic pumps and motors for mobile machine drivelines is also becoming a key focus, because the power losses are mainly caused by the design principle of the rotating group (kit). The direct impact of hydrostatic unit efficiencies on the machine operating costs depends on the main working conditions for the hydrostatic units in the corresponding drivelines, combined with the amount of work per year. Reduced power losses of an optimized hydrostatic drive line lead consequently to:

reduced cooling power needs, that is in addition combined with reduced losses for the cooling system (e.g. fan),

more available power, that will increase productivity of the machine, or a noticeable reduction in direct fuel costs for the end customer.

Based on precise and robust loss models derived from suitable efficiency measurements, the loss power reduction potential can be analyzed for typical operating points or working cycles (e.g. in simulation). Therefore, this paper also describes the scientific background of the

derived and used measurement as well as modeling approach for hydrostatic units. In fact, the current ISO4409 and ANSI/NFPA standards for efficiency measurement of hydrostatic pumps and motors provide many deficiencies and uncertainties, especially when closed circuit units are evaluated – but this is where efficiency really matters according to the resulting power losses. Based on these, simulation and measurement results for typical operating conditions of mobile machines will be presented and compared according to state-of-the-art hydrostatic drivelines (pump and motor). The results will prove the relevance of component selection for efficiency.

EFFICIENCY MEASUREMENT AND MODELING OF HYDROSTATIC UNITS

The basis for a suitable loss modeling and analysis is a reliable efficiency measurement of hydrostatic units. The measurement accuracy of all relevant sensors deserves special attention to ensure a small confidence interval for the resulting efficiency and power losses. The applied measurement concept is based on ISO4409, [1], and contains also the strategy that comparison measurements (also to competition) are always done on the same test stand with the same measurement equipment and at the same time. However, ISO4409 shows some essential weaknesses and uncertainties, especially when closed circuit units are to be measured. Some of these points are listed below.

The location of the main flow meter is normally recommended to be in high pressure. But almost no flow meter manufacturer world-wide is capable of calibrating a flow meter under high pressure, and it can be concluded that for pressures > 200 bar, the accuracy is generally unacceptable.

The recommended hydraulic setups lead to a high primary power need for the electric motor, and in addition, a high amount of cooling power. This high energy flow makes the measurement and temperature control very difficult.

The standards do not mention how the hydrostatic units are to be adjusted according to the displacement volume.

The standards do not include information or recommendations about flushing of the unit.

These standards do not contain definitions of the partial efficiencies (volumetric and torque).

A suitable rule for the realization of steady-state conditions and sufficient sample points as well as time is missing.

Another aspect is the run-in prior to the measurements, which is not specified.

It can be concluded that the available standards are from a time period where the attention on efficiency and power losses was not too high, because all the points above are relevant sources for misleading comparisons.

In [1] appropriate methods are presented how these issues can be resolved; leading to a robust and reliable efficiency measurement result, with efficiency confidence intervals lower 0.5%, see Fig. 1. One key element is an energy-efficient test setup which simplifies the temperature control in the circuit significantly. The temperature or viscosity in the lubricating gaps as well as in the housing is often a major error source, especially when comparing hydrostatic units. Another point is the flow measurement in low pressure and in case drain, which leads to a much better flow measurement accuracy and robustness than in high pressure.

Fig. 1: Combined mode setup for efficiency measurements [1]

Based on the combination of a variety of complex losses in hydrostatic pumps and motors, a physical loss model is not effective and consequently gives inaccurate results, see [2] and [3]. Mathematical interpolation models (of the measurement points) were found to be most appropriate, because all loss sources are regarded next to sufficient measurement points as input. In the VDMA project “Driveline”, the prediction possibilities were checked for the powersplit drive Fendt Vario ML160 [6], which was measured on the driveline test stand at the IFL institute of the Technical University of Braunschweig, see Fig 2. Here, the measurements for different load powers are compared with simulations done at the IFAS institute at the RTH Aachen, by using the developed loss models of the highly efficient used wide angle (45°) hydrostatic bent axis units. Note that

these simulation results are the initial results at that time without fine tuning. I.e. it is possible to predict fuel consumption of hydrostatic drivelines with reliable loss models. And hydrostatic loss models need special attention because these units are normally the main loss sources in hydrostatic drive lines.

Fig. 2: Comparison of simulation and driveline measurements of the Fendt Vario ML160 transmission

for different loads; lines: simulation, points: measurements [5]

ENERGETIC COMPARISON OF HYDROSTATIC DRIVELINES

With these models, or with direct measurement results from the efficiency tests in specific operating conditions, an energetic analysis and optimization of drivelines is possible, especially to compare different component designs. This will be exemplarily done for a typical harvester (Table 1a) and crawler (Table 1b).

Table 1a: Basic data of harvester

Basic Data

Empty weight 20.000 kg

Load weight 7.000 kg

Speed max. 30 km/h

Diesel engine max. 270 kW

Transmission Front axle

Speed Demands

1st Gear 10 km/h

2nd

Gear 30 km/h

Tractive force at low speeds

1st Gear 120 kN

2nd

Gear 35 kN

Motor Pump

EM

aux.

Pump

High Pressure

Low Pressure

Betriebspunkte

Die

se

lmo

torm

om

en

t [N

m]

Mess 0kW Sim 0kW Mess 20kW Sim 20kW Mess 40kW Sim 40kW

Mess 60kW Sim 60kW Mess 80kW Sim 80kW Mess 100kW Sim 100kW

Diesel engine input torque [Nm]

Loading Cycle Points

0 kW

20 kW

40 kW

60 kW

80 kW

100 kW

Measurements

Simulation

Table 1b: Basic data of crawler

Basic Data

Weight 15.000 kg

Sprocket diameter 378 mm

Speed max. 10 km/h

Diesel engine 150 kW, 2250 rpm

Transmission Dual path

Tractive force

Earthmoving 170 kN

Transport 25 kN

For the H1 basis hydrostatic system of the harvester following data was taken into account:

H1 System – 18° pump, 32° motor

Swashplate pump H1P 115 ccm, 18° maximum angle, 9 pistons

Bent axis motor H1B 160 ccm, 0-32° angle, 9 pistons

2-step gear, gears (including axis): i1 = 105,3; i2 = 31,8

Pump distributing gear: i = 1,55

The mechanical part was modeled with the VDMA „Driveline“ simulation library [5]. For the hydrostatic part the loss models were developed based accordingly under the following considerations:

Circuit temperature 80°C, equals to 11 cSt for a 46 standard mineral oil

Focus on hydrostatic part (pump and motor) and therefore neglecting:

- hose losses

- motor control system

Regarding of pump control and motor flushing system by taking into account the charge pump (reduction of pump input power)

In case of a full hydrostatic transmission (driveline) as in Fig. 3, the power flow is in sequence through pump and motor. The total efficiency of this sub-system is consequently the multiplication of the pump and motor efficiencies. Therefore, the energetic balance was done between the pump input and the motor output. By this,

the energetic impact of the used hydrostatic components can be analyzed in detail.

Fig. 3: Energetic balance of the hydrostatic driveline part

The driveline efficiency for the hydrostatic part tot as well as the hydrostatic power losses can be formulated to:

inPumpe

outMotor

totP

P

,

, (1)

outMotorinPumpS PPP ,, (2)

The result of a typical driveline analysis in form of the drive curve efficiency for the harvester (based on a total driveline calculation) can be found in Fig. 4. In this example, the important working areas like middle tractive force at high speeds in 1st gear (harvesting) as well as medium pressure at maximum speed in 2

nd gear

(transport) are reaching a hydrostatic driveline efficiency of 75-84%.

Fig. 4: Efficiency simulation results of harvester drive curve, including iso lines for constant efficiency values –

hydrostatic part (pump, motor)

Diesel Engine

Wheel

PPump,in PMotor,out

Hydrostatic Driveline

0 5 10 15 20 25 300

20

40

60

80

100

120

140

0.550.60.70.750.650.45

0.8

0.50.550.60.70.75

0.650.45

Systemwirkungsgrad des hydrostatischen Teils - Mähdrescher 270 kW, H1-System

Fahrgeschwindigkeit [km/h]

0.8

0.7

50.5

0.7

0.550.6

0.65

0.45

0.8

0.8

0.7

5

0.7

5

0.7

0.7

0.6

50.60.5

0.5

5

0.6

50.6

Zugkra

ft [

kN

]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Gang 2Gang 1

Hydrostatic system efficiency [%] – H1 System

vehcile speed [km/h]

trac

tive

forc

e [k

N]

1st gear 2nd gear

0 5 10 15 20 25 300

20

40

60

80

100

120

140

0.550.60.70.750.650.45

0.8

0.50.550.60.70.75

0.650.45

Systemwirkungsgrad des hydrostatischen Teils - Mähdrescher 270 kW, H1-System

Fahrgeschwindigkeit [km/h]

0.8

0.7

50.5

0.7

0.550.6

0.65

0.45

0.8

0.8

0.7

5

0.7

5

0.7

0.7

0.6

50.60.5

0.5

5

0.6

50.6

Zugkra

ft [

kN

]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

For analyzing the impact of the hydrostatic units design on driveline efficiency and power losses in typical applications, a common and new state-of-the-art comparison system was selected with the following parameters. Hereby, it was made sure that the output performance of the machines is the same with both systems (H1 and Comparison system below), what leads to almost the same operating parameters for the hydrostatic units, because of the similar size or displacement.

New Comparison System – 20° pump, 25° motor

Swashplate pump 110 ccm, 20° maximum angle, 9 pistons

Bent axis motor 150 ccm, 0-25° angle, 9 pistons

The results for both hydrostatic systems (H1 vs comparison system) in the harvester (Table 2a) are shown in Fig. 5a and 5b. In Fig. 5a the efficiency advantage and in Fig. 5b the power loss advantage of the H1 system is shown over vehicle speed, respectively. It is obvious that the power loss savings are quite high numbers, depending on the machine power. The H1 system advantages (pump, motor) can be explained through the design principle, for the bent axis unit by:

higher angle ratio of 32° compared to 25°, and therefore a more compact rotating kit with smaller lubricating gaps, when comparing similar displacements,

lighter ring sealed pistons caused by the point above and the synch joint principle, leading to lower centrifugal and friction forces,

flow optimized valve segment.

On the pump side, the H1 power loss savings can be explained by (see Fig. 6a and 6b):

stepped slipper design, with an additional small sealing room, leading to lower friction and leakage in a wide operating range,

welded closed cavity pistons (closed hollow piston), which lead to lower dead volume (causing continuously compression losses), and lower piston mass, what is especially essential in swash plate pumps due to continuous axial acceleration of the pistons during stroking.

The results prove and are somehow a logical consequence of the direct unit comparisons from the efficiency tests on one test stand, see [1] and [8]. For the machine application and end customer the main question is how often and long the operation is done in specific conditions, meaning inside the efficiency plots. Therefore, a consideration of the main machine

operating points is reasonable for a simple and effective comparison result. In Table 2a, the results for the harvester are shown for the main typical operating points harvesting and transport. These conditions were also measured in the efficiency test on the same test stand, and therefore Table 2a contains also a comparison between simulation loss model and direct measurement, in order to judge the model prediction quality. The results show that the H1 system power loss advantage is about 10 kW for harvesting and 3.5 kW for transport (both from measurement values). In case of typically 400 hours/year harvesting and 200 hours/year transporting, this leads to a saving of round about 4700 kWh.

Fig. 5a: Comparison of H1 system and Comparison system for harvester (Table 1a) according to efficiency

Fig. 5b: Comparison of H1 system and Comparison system for harvester (Table 1a) according to power

losses

[%] [kW]

0 5 10 15 20 25 300

2

4

6

8

10

Efficiency Advantage of H1-System to Comparison System

Driving velocity [km/h]

250 kW

200 kW

150 kW

100 kW

75 kW

0 5 10 15 20 25 300

5

10

15

20

Power Loss Advantage of H1-System to Comparison System


250 kW

200 kW

150 kW

100 kW

75 kW

[kW][%]

[%] [kW]

0 5 10 15 20 25 300

2

4

6

8

10

Efficiency Advantage of H1-System to Comparison System


250 kW

200 kW

150 kW

100 kW

75 kW

0 5 10 15 20 25 300

5

10

15

20

Power Loss Advantage of H1-System to Comparison System


250 kW

200 kW

150 kW

100 kW

75 kW

[kW][%]

When now taking into account a typical Diesel engine fuel consumption of 215 g/kWh at a Diesel price of 1,25 €/liter (Europe), then this saved energy equals to round about 1500 €/year fuel cost savings as well as ca. 3 tons less CO2.

This significant saving increases in a noticeable way with more working hours per year, of course, as well as in cases of drivelines with multiple pumps and/or motors.

From Table 2a it becomes also clear that the model accuracy is quite good, comparing power losses by model and measured power losses of the hydrostatic units, especially when considering that 2 hydrostatic models (pump and motor) are behind the calculations.

Fig. 6a: H1P stepped slipper

Fig. 6b: H1P welded closed cavity piston

Table 2a: Evaluation of typical operating conditions for harvester in Table 1a, comparison of pump/motor loss

model and measured data

In addition, Table 2b shows the main results for the crawler from Table 1b, whereby here only the direct results from the unit efficiency measurements are shown. Note that the crawler is a dual path machine, meaning that each sprocket is driven by a separate hydrostatic transmission (with the same unit sizes of the H1 and Comparison system), so 2 pumps and 2 motors are to be regarded. The resulting power loss saving of round about 4 kW for earthmoving can be transferred into a saved energy of 6000 kWh with 1500 h/year working performance in this condition. I.e. the fuel and operating costs savings are similar here compared to the harvester, approximately 2000 €/year with the assumptions above.

It has to be noted that the loss power differences based on the design principle of hydrostatic units is of course

not constant over all operating parameters. Generally, the power loss savings are higher when the units are transferring more power. This comes from the relation between efficiency and power losses, because the power loss difference can be much different although the efficiency difference is almost the same – this depends on the operating condition and shows that efficiency is a often a misleading number. The consideration of power losses is therefore generally recommended for this type of analysis.

Harvesting: v = 6 km/h

p = 300 bar

Power Losses by Model [kW]

H1 System Pump: 21,8 Motor: 12,5

= 34,3

Comparison system Pump: 25,0 Motor: 18,3

= 43,3

Transport: v = 30 km/h

p = 200 bar


= 27,8


= 30,6

Harvesting: v = 6 km/h

p = 300 bar

Measured Power Losses [kW] (Efficiency Measurement)


= 33,6


= 43,4

Transport: v = 30 km/h

p = 200 bar


= 27,4


= 30,9

Table 2b: Evaluation of typical operating conditions for crawler in Table 1b, comparison of pump/motor loss

measured data

Earthmoving: v = 2.3 km/h

p = 300 bar

Measured Power Losses [kW] (Efficiency Measurement)

H1 System Pumps: 24,9 Motors: 6,0

= 30,9

Comparison system Pumps: 25,3 Motors: 9,5

= 34,8

Continuously and unavoidably rising fuel costs will certainly increase the money value of this cost saving effect in the future, what will increase the benefit for the end customer once more. Note that the resulting reduction of cooling power, also by reduced combustion

additional lubricating step

engine losses, will in fact lead to more power loss savings, because the cooling system (e.g. fan) power consumption can be also reduced.

In other words: the selection of hydrostatic components (pumps and motors) and the corresponding design for hydrostatic drivelines has a significant impact on the system power losses. In some cases, the Diesel engine size may be down-sized without reducing machine performance, this is especially interesting for machines in the 56 kW range, because below this value the new legislations apply in a different way. But once again it has to be noted that the resulting power loss savings depend on the unit size and the performed power in the relevant operating conditions.

SUMMARY AND CONCLUSIONS

In this paper, the impact of hydrostatic unit design on the driveline efficiency and machine operating costs has been analyzed by simulation and measurements. Reliable efficiency measurement and modeling of hydrostatic pumps is the essential basis for this analysis. In fact, today more attention than in the available international standards has to be paid to reliable efficiency measurement and modeling. For two example machines, typical harvester and crawler, it was shown that the H1 system has relevant power loss savings compared to a new state-of-the-art comparison system. These differences can be explained by the main design differences of the rotating group. This power loss saving is however the logical consequence of the efficiency measurements and direct comparisons on one test stand at the same time, see [8] – for the machine impact there is only the question how often and how long the hydrostatic units operate in these plots during the main operating points or working cycles. Next to the direct fuel operating costs, the found power loss saving affects the cooler size as well as the power needed for this, and in addition, the Diesel engine loss according to this power amount. In fact, the fuel costs saving could be calculated to thousands of € or $. This potential proves that efficiency of hydrostatic components has really become a major sales argument in the mobile machinery market.

REFERENCES

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Measurement and Modeling - Essential for

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(Modeling, Analysis and Design of Hydrostatic

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CONTACT

Dr. Robert Rahmfeld

E-Mail: [email protected]

Craig Klocke

E-Mail: [email protected]

DEFINITIONS, ACRONYMS, ABBREVIATIONS

P [kW] power

PS [kW] power losses

v [km/h] driving velocity

tot [-] total efficiency

p [bar] delta pressure

Documents

Efficiency Impact on Operating Costs of Mobile Machines