Cacat pansat

Two-Stroke Marine Diesel Engine Variable Injection Timing System Performance Evaluation And Optimum Setting For Minimum Fuel Consumption At Acceptable

NOx Levels

Dimitrios T. Hountalas*1

E-mail: [email protected]

Spiridon Raptotasios1


Antonis Antonopoulos1


Stavros Daniolos2

E-mail:

[email protected]

Iosif Dolaptzis2

E-mail:

[email protected]

Maria Tsobanoglou2

E-mail:

m.tsobanoglou@ minervamarine.com

1 National Technical University of Athens, School of Mechanical Engineering, Internal Combustion Engines Laboratory, Heroon Polytechniou 9, Zografou Campus 15780 Athens, Greece

2 Minerva Marine Inc, 141-143 Vouliagmenis Avenue Voula, 16673 Athens, Greece

ABSTRACT Currently the most promising solution for marine propulsion is

the two-stroke low-speed diesel engine. Start of Injection (SOI) is of

significant importance for these engines due to its effect on firing

pressure and specific fuel consumption. Therefore these engines are

usually equipped with Variable Injection Timing (VIT) systems for

variation of SOI with load. Proper operation of these systems is

essential for both safe engine operation and performance since they

are also used to control peak firing pressure.

However, it is rather difficult to evaluate the operation of VIT

system and determine the required rack settings for a specific SOI

angle without using experimental techniques, which are extremely

expensive and time consuming. For this reason in the present work it

is examined the use of on-board monitoring and diagnosis techniques

to overcome this difficulty. The application is conducted on a

commercial vessel equipped with a two-stroke engine from which

cylinder pressure measurements were acquired. From the processing

of measurements acquired at various operating conditions it is

determined the relation between VIT rack position and start of

injection angle. This is used to evaluate the VIT system condition and

determine the required settings to achieve the desired SOI angle.

After VIT system tuning, new measurements were acquired from the

processing of which results were derived for various operating

parameters, i.e. brake power, specific fuel consumption, heat release

rate, start of combustion etc. From the comparative evaluation of

results before and after VIT adjustment it is revealed an improvement

of specific fuel consumption while firing pressure remains within

limits. It is thus revealed that the proposed method has the potential

to overcome the disadvantages of purely experimental trial and error

methods and that its use can result to fuel saving with minimum effort

and time.

To evaluate the corresponding effect on NOx emissions, as

required by Marpol Annex-VI regulation a theoretical investigation is

conducted using a multi-zone combustion model. Shop-test and NOx-

file data are used to evaluate its ability to predict engine performance

and NOx emissions before conducting the investigation. Moreover,

the results derived from the on-board cylinder pressure

measurements, after VIT system tuning, are used to evaluate the

model’s ability to predict the effect of SOI variation on engine

performance. Then the simulation model is applied to estimate the

impact of SOI advance on NOx emissions. As revealed NOx

emissions remain within limits despite the SOI variation (increase).

Keywords: Two-stroke marine diesel engine, Diagnostic technique,

Variable Injection Timing (VIT), Multi-zone combustion model, NOx

emissions, engine performance, brake specific fuel consumption.

NOMENCLATURE A Area (m2)

adel Ignition delay constant

cr Radiation constant (W/m2K4)

cv Specific heat capacity under constant volume (J/kg K)

D Cylinder bore (m)

f Number of cycles per second

hc Heat transfer coefficient (W/m2K)

k Turbulent kinetic energy (J)

kith Forward reaction rate constant for the “ith” reaction

ṁ Mass flow rate (kg/s)

m Mass (kg)

p Pressure (N/m2)

Q Heat (J)

Ri One way reaction rate for the “ith” reaction

Spr Integral value in ignition delay correlation

t Time (s)

T Temperature (K)

V Volume (m3)

Wi Indicated power (W)

Proceedings of the ASME 2014 12th Biennial Conference on Engineering Systems Design and Analysis ESDA2014

June 25-27, 2014, Copenhagen, Denmark

ESDA2014-20528

1 Copyright © 2014 by ASME

mailto:[email protected]





mailto:m.tsobanoglou@%20minervamarine.com

z Number of cylinders (-)

Abbreviations

CA Crank angle

IMEP Indicated Mean Effective Pressure

IMO International Maritime Organization

LHV Lower Heating Value

MCR Maximum Continuous Rating

rpm revolutions per minute

SMD Sauter Mean Diameter

SOI Start of Injection

TDC Top dead center

VIT Variable Injection Timing

Greek Letters

δrc Equivalent cylinder ring clearance (m)

εt Viscous dissipation rate per unit mass (W/kg)

θ Crank angle (deg)

φeq Equivalence ratio

Subscripts

cumul Cumulative

f Fuel

g Gas

gros Gross

hl Heat loss

net Net

w Wall

INTRODUCTION The major issues that have to be addressed for the large-scale

two-stroke diesel engines, which are the primary solutions for marine

vessel propulsion and for power generation (in specific applications),

are reduction of fuel consumption and NOx emissions. A common

technique, which is extensively used in these engines, to achieve

greater fuel economy is Variable Injection Timing (VIT). The VIT

system is used to modify Start of Injection (SOI) in order to control

the maximum combustion pressure and achieve optimum

performance. Thus, proper operation of VIT system, is of significant

importance for two-stroke diesel engines, since it can result to

reduced bsfc and optimized engine operation at a wide operating

range, while maintaining at the same time NOx emissions at

acceptable levels [1].Variable Injection timing is also used for small

adjustments to consider for the effect of fuel oil quality [2].

A typical two-stroke low-speed marine diesel engine is

optimised for operation in the region of 80% to 85% of full load [2-

4]. Currently, due to the global financial slowdown, the marine

industry is looking into methods for operating cost and mostly fuel

consumption reduction without negative environmental impact.

Towards this direction it has been introduced the slow steaming

concept [5]. However, marine diesel engines are not designed to

operate in this region and VIT can become a useful tool to improve

performance.

For most applications the VIT system is adjusted so that start of

injection is kept constant at low load (usually up to 40% of full load)

and then advances up to ~ 85% load from where it starts to reduce to

avoid excessive peak firing pressures [6]. As already mentioned the

load where the engine is designed to have its minimum bsfc and its

peak firing pressure (~85% of full load) is the “breakpoint” i.e. the

load where the VIT system is set up to start retarding the start of

injection. For this reason there is no significant change in the

maximum combustion pressure in the region of 85% to 100% of full

load. In Figure 1, it is depicted the typical adjustment of the VIT

system and the corresponding variation of peak firing pressure [6].

Figure 1 Variation of VIT index and maximum cylinder pressure

for the entire load operating range [6].

Variable Injection Timing can be achieved either by a

Mechanical-Pneumatic (older) system, or by an Electro Pneumatic

system which is implemented in newer engine designs. The essential

difference between the two systems is the use of the “breakpoint” for

the pressure rise control. For the mechanical system the “breakpoint”

is fixed, while for the electrical VIT system it is variable and is

controlled from the scavenge pressure. For high scavenge pressure

and as a result high compression pressure, the maximum combustion

pressure can rise above the design point. Therefore the “breakpoint”

must be shifted to a lower load point. On the contrary, for low

scavenge pressure the breakpoint can be shifted to higher load, for the

optimum peak firing pressure to be achieved [6].

The optimal tuning of the VIT system is essential to obtain the

required SOI setting for optimum performance. The use of common

experimental methods (trial and error, coupled with continuous

measurements) on large-scale diesel engines to achieve this requires

significant effort and time, which results as well to significant cost.

Towards this the combination of experimental and computational

techniques can become a useful tool that can be applied efficiently

with minimum effort.

Therefore, the results from a technique which has been

developed and applied on a two-stroke diesel engine are presented in

the present study. The SOI was adjusted through the VIT system

using cylinder pressure measurements acquired at sea, under actual

conditions. The cylinder pressure measurements were processed

using an existing, well validated, diagnostic technique [7,8], from

which -beyond diagnosis and tuning- results are generated for various

operating parameters such as brake power, specific fuel consumption,

heat release rate, start of combustion etc.

Primarily, the measurements acquired at various loads with the

existing VIT settings, were processed, to determine the condition of

the engine and the setting of the VIT system. Using this methodology

the relation between VIT rack position and start of injection angle

(referred to as VIT scale) was determined and used to make proposals

for optimum tuning and/or adjustment of individual cylinders. This is

extremely important since it is made easy to define the required VIT

rack adjustment to achieve a requested SOI angle without use of trial

and error techniques. After this the VIT setting was modified and

measurements were repeated at nearly the same operating conditions.

The results before and after the VIT system modification were

evaluated and revealed a potential fuel saving without peak firing

pressure exceeding the design limits. The investigation also revealed

improper operation of the VIT system on one engine cylinder.

However, for two-stroke marine diesel engines the reduction of

bsfc is affected by the well-established tradeoff between bsfc and


NOx [3,4]. Injection advance decreases bsfc but at the expense of

NOx increase at the engine exhaust. Considering the NOx emission

regulations which are set by Marpol Annex VI [9,10], an additional

investigation is necessary to ensure that NOx emissions after VIT

system tuning will remain within limits.

In the present work this investigation is conducted using an

existing well validated multi-zone combustion model [11-16] initially

developed for high-speed DI diesel engines and modified to properly

describe the processes of the entire two-stroke operating cycle [17-

19]. For field applications it will be obviously required to finally

conduct emission measurements onboard vessel. Towards this

direction it is first demonstrated the model’s ability to predict the

engine performance and NOx emissions at various loads, using data

acquired from the engine shop-tests and NOx file. From the analysis

of results it is demonstrated that the simulation model manages to

predict NOx emissions adequately and for this reason it is used to

estimate the corresponding effect of SOI at the test conditions. The

target is to determine if the NOx values, after SOI advance via the

VIT system, remain within the IMO Marpol Annex-VI limits foreseen

for the specific engine design.

The results derived from the investigation indicate a clear fuel

saving potential, while at the same time both engine performance

(peak firing pressure) and NOx emissions are maintained within

limits. Moreover, it is revealed that the diagnostic technique can be

successfully used for evaluation of VIT system operation and

especially for optimum tuning to achieve minimum fuel consumption

at acceptable NO values. Furthermore, having defined the relation

between VIT index and SOI it is possible to conduct investigations

for optimum SOI setting if the engine will operate in the low load

region according to the slow steaming concept. As clearly shown this

is achieved with minimum effort and low cost which is promising for

field applications. Last but not least it is demonstrated the combustion

model’s ability to predict both engine performance and NOx

emissions of a two-stoke marine diesel engine at various operating

conditions and SOI settings which is promising for its use as a tool

for engine development.

BRIEF DESCRIPTION OF THE DIAGNOSTIC TECHNIQUE

The proposed diagnostic technique is based on the processing of

measured cylinder pressure data using a two-zone modeling approach

for the corresponding thermodynamic calculations. The two-zone

modeling, which is based purely on thermodynamics, has been

extensively validated in the past. As mentioned, in the present study

the diagnostic technique is used as a tool to determine engine tuning

and performance at the present conditions and then investigate the

possibility for fuel saving via SOI optimum adjustment. Furthermore,

using the diagnostic technique, it is determined the relation between

VIT index and SOI angle which is important for VIT performance

evaluation and optimum SOI setting. For this reason it is required to

determine the following parameters:

To evaluate the results it is necessary to estimate and compare

the fuel consumption of the engine before and after VIT adjustment.

However, it is difficult to acquire measurements on field with the

required accuracy especially when fuel consumption differences in

the region of 1-2% are involved. Another difficulty is that it is also

impossible to have exactly the same operation between the two test

conditions. For this reason it was decided to estimate and compare

the bsfc values which are derived from the diagnostic methodology.

For this reason it is given herein, only a brief description of the

diagnosis methodology and special focus to the methodologies for

TDC position determination, cylinder power estimation and

determination of fuel oil consumption form the heat release rate.

Having determined the TDC it is also possible to estimate the ignition

angle and from this to derive an estimate for the SOI angle which is

used to determine the VIT scale and the SOI setting of engine

cylinders enabling proper VIT adjustment.

Determination of TDC Position A significant advantage of the present technique which makes it

suitable for field applications is that it does not require measurement

of TDC position, which is a time consuming procedure with specific

difficulties. The precise determination of the TDC is a crucial

parameter, since an error results to an incorrect pressure diagram and

to significant errors on several thermodynamic calculation results

such as IMEP (Indicated Mean Effective Pressure), indicated power

etc. [20 - 23]. For example an error of 0.5 CA deg on the estimated

timing of TDC, results to a fault in the calculated indicated engine

power of up to ~4%. For this reason it is employed a thermodynamic

methodology for TDC estimation developed by the authors in the past

and extensively validated by both lab and field measurements. TDC

estimation is based on the processing of the measured compression

part of the pressure diagram which is then compared to the calculated

one using the embedded simulation model mentioned above. TDC

position is estimated when the difference between the two curves is

minimized using a special constants determination methodology

(multi-parameter determination) where TDC is considered to be an

additional unknown constant together with the main parameters that

affect the compression stroke i.e. effective CR, initial pressure at

exhaust valve closure, blow-by and heat transfer. The error of the

proposed method is in the range of ±0.2 degrees of crank angle which

is adequate for accurate estimation of cylinder power and heat release

rate.

Estimation of Cylinder Brake Power The measured mean cylinder pressure trace is used to calculate

the indicated power output for each cylinder. The TDC position

determined from the aforementioned methodology is used to convert

the measured “p-t” signal into a “p-V” one from, the integration of

which it is derived the indicated power as follows [3]:

�̇�𝑖 = 𝑧 ∙ (∮𝑝𝑑𝑉) ∙ 𝑓 (1)

Then, using the mechanical efficiency, which is available from the

engine shop tests, the corresponding break power is determined.

Estimation of Cylinder Fuel Flow Rate For a multi-cylinder engine operating on the field, it is extremely

difficult to determine the fuel flow rate with the required accuracy. To

overcome this difficulty a method has been developed in the past to

estimate the fuel mass flow rate of each cylinder. The method is

based on the processing of the measured cylinder pressure diagram.

An estimate for the actual amount of fuel mass burned inside the

combustion chamber is obtained from the heat release rate analysis

procedure from the following [3,24]:

�̇�𝑓 =𝑄𝑔,𝑐𝑢𝑚𝑢𝑙

𝐿𝐻𝑉𝑓𝑢𝑒𝑙 (2)

where (LHVfuel) is the lower heating value of the fuel used and

(Qg,cumul) represents the cumulative gross heat release obtained from


the integration of the instantaneous gross heat release given from the

following relation: 𝑑𝑄𝑔𝑟𝑜𝑠

𝑑𝜃=𝑑𝑄𝑛𝑒𝑡𝑑𝜃

+𝑑𝑄ℎ𝑙𝑑𝜃

(3)

while the instantaneous heat losses to the cylinder walls are estimated

from: 𝑑𝑄ℎ𝑙𝑑𝜃

= 𝐴 ∙ [ℎ𝑐 ∙ (𝑇𝑔 − 𝑇𝑤) + 𝑐𝑟 ∙ (𝑇𝑔4 − 𝑇𝑤

4)] (4)

and the instantaneous mean gas temperature (Tg) is obtained from the

perfect gas state equation and the measured cylinder pressure value.

The mass of the cylinder charge is estimated from the simulation

model (open cycle simulation including gas exchange) using the

measured values of inlet pressure, temperature etc., while constant hc

and the corresponding cylinder wall temperature are obtained from

the model constant estimation procedure. The specific methodology

has been validated, by laboratory experiments and a great number of

field tests on both marine and stationary engines, and its accuracy is

in the region of ±1.5%. However when used on a comparative basis,

such as in the present investigation, where bsfc before and after VIT

system tuning is compared, the relative variation of values between

the two different test cases of the same engine is in the range of

±0.5%.

BRIEF DESCRIPTION OF THE COMBUSTION MODEL FOR NOx EMISSIONS ESTIMATION

As mentioned the model embedded in the diagnostic technique

is a two-zone which is adequate for performance studies. For NOx

emission studies and especially for the investigation of the SOI effect,

it is made use of a well validated multi-zone combustion model. The

specific model has been extensively applied and validated in the past

in a number of light and heavy duty DI diesel engine configurations

[11-16]. For the specific application modifications were necessary to

make its use possible on slow speed 2-stroke diesel engines [17-19].

The model is a three-dimensional multi-zone one where the fuel

jet is divided into zones using a concentric consideration as shown in

Figure 2a and Figure 2b. The number of axial direction depends on

the injection duration and the calculation time step used. In the

present work, five zones are used in the radial direction and eight in

the circumferential direction. Each zone has its own history of

temperature, composition etc., while the pressure inside the engine

cylinder is considered to be uniform. The condition inside each zone

is calculated from the first law of thermodynamics and the

conservation equations for mass and momentum. In the following

sections, it is provided a brief description of the sub-models used.

Figure 2.a Zone formation on the “r-z” plane normal to injection

direction

Figure 2.b: Zone formation on the “x-r” plane

Heat Transfer For the estimation of the characteristic velocity, necessary for

the heat transfer calculations, a turbulent kinetic energy viscous

dissipation rate k∼εt model is used [3,25,26]. Having determined the

characteristic velocity and the heat transfer coefficient the

instantaneous heat rate is obtained from eq. (4), where Tg is the bulk

temperature of the fuel jet defined by (where index k denotes the ‘kth’

zone of a total number n):

𝑇𝑔 =∑ 𝑚𝑘 ∙ 𝑐𝑣𝑘 ∙ 𝑇𝑘𝑛𝑘=1

∑ 𝑚𝑘 ∙ 𝑐𝑣𝑘𝑛𝑘=1

(5)

so that the heat exchange rate obtained from eq. (4) is then distributed

to the zones according to their mass, temperature and specific heat

capacity as follows,

𝛥�̇�𝑘 =�̇� ∙ (𝑚𝑘 ∙ 𝑐𝑣𝑘 ∙ 𝑇𝑘)

∑ 𝑚𝑘 ∙ 𝑐𝑣𝑘 ∙ 𝑇𝑘𝑛𝑘=1

(6)

The Jet Model After initiation of fuel injection, zones start to form and

penetrate inside the combustion chamber. The zone velocity along the

jet axis is obtained from correlations providing the penetration length

of the fuel jet inside the cylinder [3,27]. The zone velocity at the jet

periphery is estimated using the radial distance of the zones from the

jet axis. The effect of air swirl on zone velocity is also considered,

using the local components of air velocity in the radial and axial

directions and the momentum conservation equation on both axes.

From the previous considerations using momentum and mass

conservation it is estimated the position of each zone inside the

combustion chamber. After wall impingement the wall jet theory of

Glauert is used to determine the jet history on the cylinder walls [28].

Air Entrainment into the Zones

Momentum conservation is adopted to estimate the amount of air

entrained into the zones [11,12]. The effect of injection pressure

variation on the jet formation mechanism is taken into account by

considering the independent initial velocity of each zone, based on

the instantaneous injection and cylinder gas pressures.

Droplet Evaporation and Breakup The injected fuel is distributed to the zones according to the

instantaneous injection rate and inside each zone it is divided into

packages (groups) where the droplets have the same Sauter Mean

Diameter (SMD).Inside each zone a chi squared distribution is used

to describe the distribution of the fuel droplet diameter. For the

evaporation process the model of Borman and Johnson is followed.

z

r

(1,1)

(2,1)

(3,1)

(4,1)

(1,2)

(1,3)

(1,4)

(1,5)

(1,6)

(1,7)

(1,8)

OX

r

(1,2,3)

(1,1,3)

(1,3,3)


Ignition-Combustion Model Ignition commences after an ignition delay period which is

given by the following relation [3,29]:

𝑆𝑝𝑟 = ∫1

𝑎𝑑𝑒𝑙 ∙ 𝑃𝑔−2.5 ∙ 𝜑𝑒𝑞

−1.04 ∙ 𝑒𝑥𝑝 (5000 𝑇𝑔⁄ )

∙ 𝑑𝑡 = 1

1

0

(7)

where (φeq) is the local equivalence ratio of the fuel air mixture inside

the burnt zone, (Tg) is the local temperature in K, and (Pg) the

cylinder pressure expressed in bar. Constant (adel) depends on the

ignition quality of the fuel and mainly on its cetane number.

Having evaporated and mixed with the entrained air and the

existing combustion products the evaporated fuel is ready for

combustion after the ignition delay period. The amount of air

entering a zone mixes with the evaporated fuel and the

combustion rate, which is derived using an Arrhenius type

expression, depends strongly on local temperature and on the

concentration of O2 and evaporated fuel.

Cylinder Blow-by Blow-by has a significant effect on the compression and

combustion-expansion part of the pressure diagram [3,24]. According

to the simplified model approach, the blow-by rate is modelled

assuming an equivalent blow-by area between the cylinder rings and

the cylinder wall. The mass flow is then calculated using isentropic

compressible flow relations. The equivalent blow-by area A is equal

to:

𝐴 = 𝜋 ∙ 𝑑 ∙ 𝛿𝑟𝑐 (8)

where δrc is the equivalent cylinder-ring clearance that defines the

level of cylinder liner–ring wear.

Gas Exchange For the simulation of the inlet and exhaust manifolds and the

calculation of the mass exchange rate between them and the engine

cylinder, the method of filling and emptying is used, which provides

good results [3,27], for constant pressure turbocharging systems that

are used for large scale marine two stroke diesel engines. Provision is

taken in the present model to simulate also the operation of the

turbocharger and the air-cooler.

Scavenging Model Practically all slow-speed, marine diesel engines are two-stroke

turbocharged, thus the scavenging process is of great importance for their operation [3]. For this reason a two-zone scavenging model has been developed which during gas exchange divides the cylinder contents into two parts, one consisting of fresh entrained air and a second consisting of combustion products from the previous cycle and fresh entrained air.

One part of the amount of air entering the cylinder, at a certain time instant during intake, escapes to the exhaust manifold directly, while the remaining one enters the fresh air and combustion products zone. Likewise, during scavenging, the total amount of exhausted cylinder mass to the exhaust manifold is taken partially from the fresh air zone and the combustion products one [27]. At the end of the scavenging process perfect mixing between the two zones is assumed resulting to only one zone which is a mixture of fresh entrained air and combustion products from the previous cycle.

Nitric Oxide Formation Model In order to calculate the formation of nitric oxide inside each

zone, a chemical equilibrium scheme is used to calculate the concentration of various components under equilibrium conditions. Due to the very high temperatures existing inside the zones chemical dissociation takes place. Inside each zone the following eleven species are assumed to exist: O2, N2, CO2, H2O, H, H2, N, NO, O, OH, CO. NO formation is widely assumed to be a non-equilibrium process controlled by chemical kinetics. The most commonly used scheme for NO formation is the extended Zeldovich mechanism [1,3,24,27]. This mechanism is comprised of the reactions, along with their related forward and reverse reaction rate constants that govern NO formation. Finally the NO formation rate in each zone is expressed by the following differential equation:

1

𝑉∙𝑑([𝑁𝑂]𝑉)

𝑑𝑡=2 ∙ (1 − 𝛽2) ∙ 𝑅1

(1 + 𝛽 ∙𝑅1

𝑅2+𝑅3)

(9)

β = [NO] / [NO]e, where [NO] is the actual concentration and [NO]e

the corresponding equilibrium one inside each zone.

DESCRIPTION OF THE ENGINE AND THE OPERATING POINTS CONSIDERED

The experimental investigation was conducted on a commercial

vessel powered by a large-scale, two-stroke, six-cylinder, marine

diesel engine. The characteristic technical data of the engine are

summarized inTable 1.

Table 1 Engine specifications

Cylinder Bore 700 mm

Piston Stroke 2674 mm

MCR speed 91 rpm

MCR power 16860 kW

In Table 2 are shown the operating points considered. The first

three measurements were acquired, using the reference VIT setting.

Then after the analysis of derived results the VIT rack was adjusted to

a higher value compared to normal i.e. advanced SOI (measurement

No. 14 ~ 85% load)and then to a lower value compared to normal

(measurement No. 16 ~ 60% load).

Table 2 Test cases examined

Test

Case

Engine

Speed (rpm)

Load

(%)

Engine

Power (kW)

VIT

setting

1 85.6 86.0 14494 Normal

2 84.2 80.3 13547 Normal

3 76.2 59.6 10042 Normal

14 85.5 85.2 14363 Increased

16 75.4 58.1 9795 Decreased

Cylinder pressure measurements were acquired using a highly

accurate air-cooled transducer (Kistler 6613CP). For each

measurement 50 continuous operating cycles were recorded using a

sampling rate of 0.5 deg CA. Measurements were acquired and stored

using a high speed sampling system (i.e. USB/AD Card) and a

portable PC with the installed diagnostic software.

As far as the investigation for the effect of SOI on NOx

emissions is concerned, the data provided in the official NOx

Technical file were used corresponding to 25%, 50%, 75%, 85% and


100% of full engine load. The official NOx Technical file

corresponds to the NOx emission data of the parent engine.

VIT INDEX-SOI ANGLE SCALE DETERMINATION The first set of measurements acquired for the reference engine

VIT setting was used to evaluate the engine condition and to

determine the relation between VIT index and SOI angle. The

measured cylinder pressure data are processed to estimate the TDC

and to convert the measured “p-t” signal into a crank angle based “p-

φ”. The resulting mean cylinder pressure traces for each cylinder for

the two operating test cases of ~60% load and ~85% load are

depicted in Figure 3 and Figure 4 respectively.

Figure 3 Cylinder pressure traces for ~60% load before VIT

modification

Figure 4 Cylinder pressure traces for ~85% load before VIT

modification

In Figure 3 and Figure 4, it is shown, that with the exception of

cylinder No.3, there are no significant differences between the

cylinder pressure traces revealing fairly uniform cylinder operation.

The small differences observed are mainly due to differences in

fuelling rate and SOI. However, the significant difference for cylinder

No.3 is attributed to the fact that VIT system of this cylinder does not

function properly as noted below. The processing of measured

cylinder pressure data provided information for the following

parameters: brake power, specific fuel consumption, heat release rate

and cylinder ignition and injection angle. To validate the findings fuel

consumption was monitored using the installed flow meters revealing

a good coincidence between measured and estimated data.

The estimated injection angle and the recorded VIT rack setting

were used to determine VIT scale i.e. the correlation between VIT

rack position and start of injection angle. In Figure 5, are given the

corresponding values of injection angle and VIT index (mm) for the

first three measurements using the normal VIT setting for all

cylinders. As revealed there exists a clear linear correlation with a

slope of ~ 0.4 CA deg/10mm. Using this value it is now possible to

define the required VIT setting to achieve a specific SOI angle

avoiding thus the use of trial and error methods which is extremely

difficult for such applications due to the size of the engines.

Figure 5 VIT Index - Injection Angle Interrelation as determined

from the methodology

Based on the findings it was then decided to modify the advance

of SOI by ~1deg CA and investigate the corresponding effect on peak

firing pressure and bsfc at 85% load. Following this SOI was retarded

by ~1deg CA compared to the reference value and measurements

were repeated for ~60% load. In Figure 6 and Figure 7 are given the

corresponding cylinder pressure traces for all cylinders where it is

noticeable the different behavior of No.3 cylinder. In the next section

it is described by detail the effect of SOI on the cylinder pressure

trace for the two test cases examined (i.e. advance and retard by ~1

deg CA).

Figure 6 Cylinder pressure traces for ~60% load after VIT

Decrease Compared to Reference

60 90 120 150 180 210 240 270 300

Crank Angle (deg)

0

20

40

60

80

100

120

140

Pre

ssu

re (

ba

r)

Cyl No 1

Cyl No 2

Cyl No 3

Cyl No 4

Cyl No 5

Cyl No 6

60 90 120 150 180 210 240 270 300

Crank Angle (deg)

0

20

40

60

80

100

120

140

Pre

ssu

re (

ba

r)

Cyl No 1

Cyl No 2

Cyl No 3

Cyl No 4

Cyl No 5

Cyl No 6

1 2 3 4 5 6 7

VIT Index (-)

-1

0

1

2

3

Inje

ctio

n A

ng

le (

de

g)

AT

DC

Y = -0.4 * X + 2.4

60 90 120 150 180 210 240 270 300

Crank Angle (deg)

0

20

40

60

80

100

120

140

Pre

ssu

re (

ba

r)

Cyl No 1

Cyl No 2

Cyl No 3

Cyl No 4

Cyl No 5

Cyl No 6


Figure 7 Cylinder pressure traces for ~85% load after VIT

Increase Compared to Reference

EFFECT OF VIT TUNING ON ENGINE PERFORMANCE

In the present section it is investigated the effect of SOI

variation on engine performance. For this reason a comparative

evaluation is conducted for test cases 1-14 (load ~ 85%) and 3-16

(load ~ 60%). In Figure 8 and Figure 9 it is shown the effect of VIT

variation on the ignition angle of each cylinder for the two loads

examined i.e. ~60% load and ~85% load respectively. In both cases

the variation of ignition angle is as initially estimated i.e. ~1 deg CA

which reveals the validity of the applied methodology. On the other

hand the ignition angle of cylinder No.3 remains constant, which

verifies the previous statement i.e. that the VIT system of specific

cylinder does not function properly.

Figure 8 Cylinder ignition angle before and after VIT

modification at ~60% load

Figure 9 Cylinder ignition angle before and after VIT

modification at ~85% load

To better understand the effect of VIT variation on cylinder

pressure it is compared the cylinder pressure trace of one cylinder

before and after adjustment. For this comparative evaluation, cylinder

No.4 was chosen as the representative cylinder since its power and

fuel consumption presented the lowest deviation from the mean value

of the six cylinders. In Figure 10 and Figure 11 it is given the

comparison of cylinder pressure traces before and after VIT

adjustment. As observed SOI advance (for ~85% load test case)

results to significant increase of peak firing pressure, while the

injection retard (for ~60% load test case) has the exact opposite

effect. The peak firing pressure value is acceptable since it does not

exceed the 145 bar limit.

Figure 10 Cylinder No.4 pressure trace before and after VIT

modification for ~60% load

60 90 120 150 180 210 240 270 300

Crank Angle (deg)

0

20

40

60

80

100

120

140

Pre

ssu

re (

ba

r)

Cyl No 1

Cyl No 2

Cyl No 3

Cyl No 4

Cyl No 5

Cyl No 6

1 2 3 4 5 6Cylinder Number

0

1

2

3

4

5

6

Ignitio

n A

ng

le (

de

g)

AT

DC 60% Load(normal VIT)

60% Load (decreased VIT)

VIT does not function


0

1

2

3

4

5

6

Ignitio

n A

ng

le (

de

g)

AT

DC 85% Load(normal VIT)

85% Load (increased VIT)


60 90 120 150 180 210 240 270 300

Crank Angle (deg)

0

20

40

60

80

100

120

140

Pre

ssu

re (

ba

r)

60% Load (normal VIT)



Figure 11 Cylinder No.4 pressure trace before and after VIT

modification for ~85% load

In Figure 12 and Figure 13, it is summarized the effect of VIT

variation on the peak firing pressure of all cylinders. It is observed

once again that the cylinder No.3 firing pressure remains unaffected.

Figure 12 Effect of VIT modification on peak firing pressure at

~60% load

Figure 13 Effect of VIT modification on peak firing pressure at

~85% load

In Figure 14 and Figure 15 it is given the corresponding effect of

SOI variation on the net heat release for cylinder No. 4 for ~60% load

and ~85% load respectively. Obviously SOI variation affects the start

of combustion as also observed in Figure 8 and Figure 9, while the

net HRR is practically shifted towards expansion when SOI is

retarded and to the reverse direction when SOI is advanced. On the

other hand there is no noticeable effect on combustion rate and

duration for the specific SOI variation level which is most possibly

attributed to the slow speed of the engine and the relatively low

ignitions delay when expressed in deg CA. From the processing of

the heat release rate histories and the knowledge of the heat loss rate,

it is estimated the fuel consumption of each cylinder using the heating

value of the fuel used.

Figure 14 Effect of VIT modification on the net heat release rate

of cylinder No.4 at ~60% load

Figure 15 Effect of VIT modification on the net heat release rate

of cylinder No.4 at ~85% load

To derive an estimate for the effect of SOI and thus VIT on fuel

consumption, it is used its effect on the bsfc as already mentioned,

which allows direct comparison. In Figure 16 and Figure 17, is

depicted the comparison of cylinder bsfc before and after VIT

adjustment for the two load cases examined. As expected, the ~1 CA

deg SOI retard results to an increase of bsfc by ~1g/kWh (mid-load

test case ~60%), while ~1 CA deg SOI advance (at ~ 85% load)

results to decrease by ~ 1.7g/kWh bsfc, which for the current power

setting (i.e. ~85% load) is reflected to a daily fuel saving potential of

~0.6 tn.

60 90 120 150 180 210 240 270 300

Crank Angle (deg)

0

20

40

60

80

100

120

140

Pre

ssu

re (

ba

r)

85% Load (normal VIT)



90

95

100

105

110

115

120

125

130

Fir

ing p

ressu

re (

bar)

60% Load(normal VIT)




110

115

120

125

130

135

140

145

150

Fir

ing p

ressu

re (

bar)




140 160 180 200 220 240

Crank Angle (deg)

-50000

0

50000

100000

150000

200000

250000

He

at R

ele

ase

Ra

te (

J/d

eg

) 60% Load (normal VIT)


140 160 180 200 220 240

Crank Angle (deg)

-50000

0

50000

100000

150000

200000

250000

He

at R

ele

ase

Ra

te (

J/d

eg

) 85% Load (normal VIT)



Figure 16 Cylinder bsfc before and after VIT modification at

~60% load

Figure 17: Cylinder bsfc before and after VIT modification at

~85% load

COMBUSTION MODEL CALIBRATION –VALIDATION As mentioned an important issue for Marine Diesel Engines are

NOx emissions which have to be maintained inside specific limits as

implied by Marpol Annex-VI. SOI obviously affects strongly NOx

emissions and for this reason it is necessary to investigate the

expected impact of SOI variation in the magnitude considered in the

present work. This is conducted using a simulation tool capable for

predicting performance and emissions of DI diesel engines. But

before this it is necessary to investigate and demonstrate model’s

ability to predict adequately NOx tailpipe emission values for two-

stroke marine diesel engines.

For this reason it is examined herein, the multi-zone combustion

model’s ability to predict performance and NOx emissions at various

load points. For this reason as already mentioned use is made of the

data provided in the official NOx file of the engine at 25% (57.3

rpm), 50% (72.2 rpm), 75% (82.7 rpm), 85% (86.2 rpm) and 100%

(91 rpm) of the maximum continuous rating (MCR). It is to be noted

that for 85% load NOx emissions were not available. To evaluate

models predictive ability it is given the comparison between the

calculated and measured values of: brake power output in Figure 18,

peak firing pressure and compression pressure in Figure 19, brake

specific fuel consumption in Figure 20, and NOx emissions in Figure

21.

Figure 18 Comparison between calculated and measured brake

power output vs. engine speed

Figure 19: Comparison between calculated and measured peak

firing pressure and compression pressure vs. engine load

Figure 20: Comparison between calculated and measured brake

specific fuel consumption vs. engine load


195

197

199

201

203

205

bsfc

(g

/kW

h)




190

192

194

196

198

200

bsfc

(g

/kW

h)



50 60 70 80 90 100

Engine Speed (rpm)

0

3000

6000

9000

12000

15000

18000

Bra

ke

Po

we

r (k

W)

Experimental

Calculated

20 30 40 50 60 70 80 90 100 110

Load (%)

40

50

60

70

80

90

100

110

120

130

140

150

Pe

ak F

irin

g P

ressu

re (

ba

r)

Experimental

Calculated

40

50

60

70

80

90

100

110

120

130

140

150

Co

mp

ressio

n p

ressu

re (

ba

r)

20 30 40 50 60 70 80 90 100

Load (%)

160

170

180

190

200

210

220

BS

FC

(g

/kW

h)

Experimental

Calculated


Figure 21 Comparison between the calculated and measured NOx

emissions vs. engine load.

As observed from Figure 18 Figure 19 and Figure 20 the model

manages to predict with reasonable accuracy the main engine

operating parameters without constant tuning since they are

calibrated at 75% load and are then maintained constant. As far as

NOx emissions are concerned it is obvious from Figure 21 that NOx

are predicted quite well even though the relative error is higher, as

expected, when compared to the corresponding ones for engine

performance parameters. This is encouraging since the main target

when investigating emissions is basically to capture trends.

MODEL APPLICATION FOR SOI VARIATION EFFECT ON ENGINE PERFORMANCE AND NOx EMISSIONS

Having evaluated model’s ability to predict both engine

performance and NOx emissions it is then applied to estimate the

corresponding impact of SOI variation. The theoretical investigation

is conducted for the 85% load point i.e. measurements 1 and 14

respectively. In Figure 22, Figure 23 and Figure 24 it is given the

comparison between the measured values of each cylinder and

calculated values of break power, peak firing pressure and bsfc vs.

ignition angle. Use of ignition angle was preferred instead of SOI

since it is directly provided form the diagnosis methodology. On the

other hand the SOI angle is estimated form the ignition angle and an

estimate of the ignition delay using equation (7). As observed the

model predicts the effect of SOI variation on the engine performance

and most important the trend. This is an indication that the simulation

model can be used as a tool to examine the effect of SOI variation on

engine performance and NOx emissions.

Figure 22 Comparison between the calculated and measured

brake power of 85% load vs. ignition angle

Figure 23 Comparison between the calculated and measured

peak firing pressure of 85% load vs. ignition angle

Figure 24 Comparison between the calculated and measured bsfc

of 85% load vs. ignition angle

The corresponding estimated effect of SOI variation on NOx

emissions is depicted in Figure 25. As expected, SOI advance results

to NOx emissions increase which is approximately 4-5%/1 CA deg of

SOI variation. This is encouraging, since this percentage corresponds

20 30 40 50 60 70 80 90 100

Load (%)

10

15

20

25

Exh

au

st

NO

x (

g/k

Wh

)

Experimental

Calculated

0 1 2 3 4

Ignition Angle (deg) ATDC

10000

12000

14000

16000

18000

Bra

ke

Po

we

r (k

W)

Experimental

Calculated

0 1 2 3 4


100

110

120

130

140

150

160

Pe

ak F

irin

g P

ressu

re (

ba

r)

Exprimental

Calculated

0 1 2 3 4


150

170

190

210

230

250

BS

FC

(g

/kW

h)

Exprimental

Calculated


to a ~0.7 g/kWh increase on NOx emissions which for the specific

engine is expected to create no problem for engine NOx file. This

conclusion is valid for similar engine designs.

Thus, as revealed, there is a clear fuel saving potential using SOI

advance, without the risk of exceeding the peak firing pressure and

NOx emission limits. It is also concluded from the results that there is

a potential for application of an even higher SOI advance (more than

one degree of crank angle) for a greater reduction of bsfc, since the

forthcoming effect on NOx is not significant.

Figure 25 Estimated effect of ignition angle variation on NOx

emissions.

CONCLUSIONS In the present study, it is investigated the potential for reducing

bsfc of a two-stroke marine diesel engine using SOI advance, without

exceeding peak firing pressure and NOx emission limits. Towards

this direction, a diagnostic technique has been applied on a two-

stroke marine diesel engine on-board a commercial vessel, where SOI

was adjusted using the VIT system. At the beginning cylinder

pressure measurements were acquired at three different operating

load points and processed to derive various engine operating

parameters. The derived values for injection angle along with the

measured VIT index values were used to evaluate VIT system

behaviour and to determine the engine VIT scale (i.e. the correlation

between VIT index and start of injection angle which is: SOI = -0.4 · VIT + 2.4). For the present case the VIT scale was estimated to be

~0.4 CA deg/10 mm.

Estimation of the VIT scale enabled direct determination of the

required VIT adjustment to achieve a specific SOI value without use

of trial and error techniques. The evaluation of SOI values after VIT

system adjustment verified this. In the present application SOI has

been modified by ~1 deg CA around its nominal value. From the

analysis of performance data, the effect of injection timing variation

on bsfc was determined. As revealed the fuel saving is ~1.7 g/kWh

which corresponds to a fuel saving of ~0.6 tn/day the value of which

can be increased to ~1 tn/day if SOI is further advanced by an

additional crank angle degree. Most important it has been observed

that this is achieved without significant increase of peak firing

pressure. From the comparison of the heat release rate before and

after VIT adjustment there is no noticeable effect on combustion rate

and duration since HRR is practically shifted to the left or the right

compared to TDC. Furthermore from the analysis it was also

identified that the VIT system of Cylinder No3 does not function

properly.

For NOx emissions an existing well-validated multi-zone

combustion model (since the one embedded in the diagnostic

technique is a two-zone one) was used to evaluate the effect of SOI

variation. Towards this direction, the model was initially validated for

its ability to predict overall engine performance and NOx emissions

for various load points using the normal SOI. The data for the

validation were derived from the official NOx file of the engine. The

validation revealed model’s ability to predict accurately engine

performance and NOx emissions for various engine loads. Following

this the model was applied to predict the corresponding effect of SOI

on performance and emissions at 85% load where NOx values are

higher. As revealed the simulation predicts adequately the effect of

SOI on engine performance enhancing the reliability for NOx

predictions. As observed SOI variation in the range of 1 deg CA

results to an increase of NOx emission by ~4-5 % which corresponds

to NOx variation of ~0.7 g/kWh. This variation creates no danger for

the NOx file of the engine while it permits even higher SOI variation

increasing the fuel saving potential.

Thus the investigation conducted reveals the existence of a fuel

saving potential at acceptable peak firing pressure and NOx values.

Moreover, it is revealed that the proposed methodology can be

successfully applied as a tuning (optimum VIT setting) and diagnosis

(detection of VIT system performance and scaling) tool, with

minimum instrumentation and effort, without the disadvantages of

purely experimental trial and error methods. Last but not least it is

demonstrated the multi-zone combustion model’s ability to predict

engine performance and NOx emissions of a two-stoke marine diesel

engine at various operating conditions and the effect of SOI. This

reveals that a potential exists for its use as a tool to assist engine

development studies and for optimum tuning of two-stroke marine

diesel engines.

ACKNOWLEDGMENTS Special thanks are attributed to Minerva Marine Inc. for

supporting the on-board vessel measurement campaign and for

providing valuable data and technical comments.

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-2 -1 0 1 2 3 4


12

13

14

15

16

17

18

NO

x (

g/k

Wh

)

0

5

10

15

20

25

dN

Ox (

% c

ha

ng

e)

dNOx (%)

NOx (g/kWh)


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