1

Evaluation of Pilot Injections in a Large Two-Stroke Marine

Diesel Engine, Using CFD and T-φ Mapping

Panagiotis Kontoulis, *Christos Chryssakis and Lambros Kaiktsis

Dept. of Naval Architecture and Marine Engineering

National Technical University of Athens (NTUA)

Heroon Polytechniou 9, GR-15773 Zografou, Greece

Key Words: Marine Engines, CFD, Combustion, Emissions, Pilot Injection

ABSTRACT

In 2002 the European Commission adopted a European Union strategy to reduce atmospheric emissions from

seagoing ships. The strategy reports on the magnitude and impact of ship emissions in the EU, and sets out a number of

actions to reduce the contribution of shipping to health and climate change. One possible approach for the reduction of

NOX and soot emissions of marine diesel engines is the use of multiple injection strategies, similar to the ones used in

automotive diesel engines. In this way, diesel combustion could be optimized with respect to pollutant emissions,

without compromising fuel efficiency.

Our interest is in investigating the potential for emissions reduction and overall optimization of combustion in large

two-stroke marine diesel engines, using numerical simulation. In this context, we study the effects of advanced injection

strategies by utilizing Computational Fluid Dynamics (CFD) tools. We use the KIVA-3 code as the modeling platform,

with improved models for spray breakup, autoignition and combustion. Here, we report first results, corresponding to

pilot injections, which are visualized for the fuel injection and combustion processes, and are also mapped on

temperature - equivalence ratio charts (T-φ maps). This analysis reveals important information on pollutant formation

mechanisms in large marine diesel engines, and suggests that fuel savings with simultaneous reduction of soot

emissions may be feasible.

INTRODUCTION Substantial efforts have been made recently in order

to develop new strategies for the in-cylinder reduction of

soot and nitrogen oxides (NOX) emissions from diesel

engines. In small engines for automotive applications,

these strategies include multiple fuel injections to

improve fuel-air mixing and achieve combustion under

conditions that do not favor emissions formation [1-5].

The main goal of this approach is the enhancement of

fuel atomization and evaporation in order to create a

more homogeneous fuel-air mixture in the cylinder (with

reduced fuel-rich areas, where soot is produced). In

addition, maintaining combustion at temperatures lower

than approximately 2300 K is important for reducing the

nitrogen oxides formation.

Further development of two-stroke marine diesel

engines can benefit from the knowledge acquired from

the study of small engines; thus, similar strategies can

possibly be employed to reduce pollutant emissions

without sacrificing fuel economy. This is increasingly

important, after the European Union’s efforts to reduce

the environmental footprint of seagoing ships, both in

terms of emissions and fuel consumption, which directly

affects carbon dioxide (CO2) emissions [6]. Moreover,

reduction of the vast fuel consumption of marine diesel

engines is of utmost importance due to the increasing

fuel prices.

CFD analysis can be a valuable tool in gaining

insight into the engine aerothermochemistry, as it offers

the possibility to visualize the fuel injection, spray

atomization and evaporation, fuel-air mixing, as well as

the development of the combustion process. In this

context, the parameter combinations that favor emissions

formation can be identified by using T-φ analysis of the

fuel-air mixture, first introduced by Kamimoto et al. [7],

and subsequently also used for Low Temperature

Combustion (LTC) studies [8]. T-φ maps are isocontours

2

of produced soot and NOX concentrations in a

temperature – equivalence ratio plane, obtained from 0-D

chemical kinetics simulations. The methodology was

extended by Bergman and Golovitchev [9], who created

transient emission maps, taking into account both soot

precursors and the varying pressure during the engine

cycle.

The objective of the present work is to investigate

the effect of injection characteristics on the combustion

process in large two-stroke marine diesel engines, with

respect to thermal efficiency and emissions formation. In

particular, the effect of pilot injections on engine

performance and emissions is evaluated. The analysis is

based on 3-D CFD simulations, used for the construction

of temperature - equivalence ratio (T-φ) maps, in order to

characterize emissions formation.

COMPUTATIONAL APPROACH The geometry corresponds to the RTA58T Sulzer

engine. Each cylinder has three injectors, located

symmetrically on the periphery of the cylinder head;

each injector has five orifices, to enhance dispersion of

the liquid fuel into the cylinder. In general, the injection

is in a co-swirl direction. The main engine characteristics

are given in Table 1.

Table 1: Main engine characteristics

Bore Diameter [m] 0.58

Stroke [m] 2.416

Injection System Common Rail

IPC [Deg. CA aTDC] -96

EVO [Deg. CA aTDC] 120

Engine Speed [RPM] 105

A modified version of the KIVA-3 code [10-11] has

been used. The modifications include the fuel

atomization, ignition, combustion, and NOX formation

models. In particular, spray modeling involves a cascade

drop breakup model [12-13], while ignition modeling

utilizes an ignition progress variable, obtained from the

solution of a corresponding transport equation [14]. NOX

modeling is based on a detailed chemical scheme,

implemented on top of the extended Zeldovich

mechanism [15]. Tetradecane (C14H30) has been used as

fuel. The present code has been used extensively by the

NTUA group in the simulation of flow and combustion

in two-stroke marine diesel engines [16-17].

In the simulations, a full 360o geometry has been

selected, in lieu of a 120o sector, to allow for variation of

the injection profile between the injectors, a possibility

provided by the common rail injection systems. The

computational mesh, validated in previous studies [16],

consists of approximately 85,000 cells at Intake Port

Closing (IPC). Only the closed part of the engine cycle is

computed, i.e. from -96 deg. CA aTDC (after Top Dead

Center) at IPC to 120 deg. CA aTDC at Exhaust Valve

Opening (EVO).

The initial swirl velocities are prescribed in terms of

a Bessel function, and correspond to a proper value of

swirl number, defined as the ratio of the angular velocity

of the swirl to the angular velocity of the engine

crankshaft:

engine

swirlSN

ωω

= , with fengine πω 2=

where f is the engine frequency. Note that ωswirl

corresponds to a wheel-flow (linear) profile of exactly

the same angular momentum as the Bessel function

profile.

The initial conditions for pressure and temperature

have been determined from a try-error procedure, by

comparing the computed cold-flow pressure against an

experimental pressure trace. The reference conditions,

corresponding to a continuous injection profile, are

summarized in Table 2. Here, the Start Of Injection (SOI)

and injection duration are those of an actual Sulzer

engine.

Table 2: Reference engine operating conditions

Intake Pressure [bar] 3.52

Intake Temperature [K] 330

SOI [Deg. CA aTDC] 2

Injection Duration [deg. CA] 15.2

Engine Load 100%

The computed pressure and rate of heat release traces

of the reference case are presented in Figures 1 and 2,

and compared to experimental data. The experimental

rate of heat release curve is based on the measured

pressure, and is obtained from a thermodynamic code (G.

Weisser, private communication). Note that here

normalized values of the rate of heat release are

presented:

( )( ) totchem

chem

QCAd

QdROHR

,

100⋅=

While the agreement between experiment and

simulation is very good regarding the pressure, the rate

of heat release curves indicate that the computation

predicts a slower burning in the late stages of combustion,

resulting in an overall longer combustion duration. This

suggests the requirement for further physical model

development.

Regarding the use of T-φ maps, we clarify that in the

present work transient maps have not been used for

combustion analysis; instead, for simplicity, one static

map is utilized, corresponding to the data reported in

Kitamura et al. [18], obtained for a pressure of 60 bar

and a residence time of 2 ms, which corresponds to 1.3

deg. CA for the current engine speed.

The T-φ maps indicate the concentrations of

produced NOX and soot, as functions of local

stoichiometry and temperature, thus identifying the

conditions that promote low-emissions engine operation.

It should be noted that for soot the interpretation is less

straightforward than for NOX, since the maps do not

provide information on the soot oxidation process, as

discussed subsequently.

3

-40 -20 0 20 40 600

20

40

60

80

100

120

140

160

Crank Angle [deg.]

Cylin

der P

ressure

[bar]

Experiment

Computation

Figure 1: Reference case pressure histories for

experiment and computation.

-20 -10 0 10 20 30 40 50 60-1

0

1

2

3

4

5

6

7

8

Crank Angle [deg.]

Ra

te o

f H

ea

t R

ele

ase

Experiment

Computation

Figure 2: Reference case normalized rate of heat release

histories for experiment and computation.

FUEL INJECTION PARAMETERS The effect of pilot injections on engine performance

and emissions is investigated by comparing against the

reference case of continuous injection. In the present

study, the injection timing and duration of the pilot

injection are varied, and the effects on engine efficiency

and emissions are investigated. In a first series of

computations, the total fuel mass of the reference case is

maintained. Following the experience of corresponding

studies on automotive diesel engines [1-5], an 8% of the

total mass is injected before TDC, as a pilot injection,

while the beginning of main injection is held constant at

2 deg. CA aTDC. Due to its short duration, the pilot

injection is modeled as a triangle, in accordance with

previous work on characterization of pilot injection

profiles [19].

The injection velocity of the reference case is

maintained in the main injection. As a result of fuel mass

conservation, the main injection duration is reduced to

14.2 deg. CA. The start of injection (SOI) has been

varied from -20 deg. CA aTDC to -5 deg. CA aTDC, in

increments of 5 deg. CA. The pilot injection durations

(PID) tested for each case are 2 deg. CA, 3 deg. CA, and

4 deg. CA. The injection profiles and velocities are

illustrated in Figures 3 and 4, respectively.

-25 -20 -15 -10 -5 0 5 10 15 200

1

2

3

4

5

6

7

8

9

Crank Angle [deg.]

Mass Inje

ction R

ate

Reference

SOI = -20 deg. CA aTDC

SOI = -15 deg. CA aTDC

SOI = -10 deg. CA aTDC

SOI = -5 deg. CA aTDC

PID = 3 deg. CA

Figure 3: Normalized injection profiles (% per deg. CA).

-15 -10 -5 0 5 10 15 200

100

200

300

400

500

600

Crank Angle [deg.]

Spra

y V

elo

city [m

/s]

SOI = -10 deg. CA aTDC

PID = 3 deg. CA

PID = 4 deg. CA

PID = 2 deg. CA Pilot Mass 8%

Figure 4: Spray injection velocity histories.

COMBUSTION AND EMISSIONS

FORMATION ANALYSIS The heat release rates for computed cases with pilot

injection are shown in Figure 5. The shape of all curves

in the main combustion phase, characterized by two

maxima, is due to the interaction of spray with the

swirling flow of combustion products, and is

representative of co-swirl injection [16]. In Figure 5a, the

heat release rates for pilot injections with duration of 3

deg. CA are presented. For early injection cases (-20 deg.

CA aTDC, -15 deg. CA aTDC), there is a considerable

time delay between the SOI and the start of ignition,

which allows for better fuel-air mixing. It is also

interesting to note that the profile of the heat release

curve for SOI equal to -20 deg. CA aTDC is substantially

different than the profiles for later injections. This is

attributed to the longer mixing time that creates a more

homogeneous and leaner mixture, as well as to the lower

temperatures in the cylinder during combustion.

In Figure 5b, the heat release rates for SOI equal to

-15 deg. CA aTDC and three different injection durations

(namely 2 deg. CA, 3 deg. CA, and 4 deg. CA.) are

presented. In the case of shorter duration, i.e. 2 deg. CA,

an earlier ignition occurs, with a higher peak of heat

release for the pilot phase. This suggests that the fuel

atomization is much stronger, leading to faster

evaporation rates and higher local φ values.

Computational results are presented in Figures 6 and

7, showing relative increase in useful work, and NOX

4

emissions levels, respectively, as functions of the

injection parameters (SOI, PID). The reference case

(Case A) is also illustrated, for comparison. We note that,

for the reference case, our results for NOX concentration

are in good agreement with the experimental value of

1400 ppm, provided by Wärtsilä Switzerland.

Inspection of Figures 6 and 7 reveals that, among the

computed cases, engine operation is mostly improved for

a pilot injection at -15 deg. CA aTDC, with duration of 3

deg. CA (Case B). Under these conditions, the work

output is increased by 1.5%, with a NOX emissions

concentration raise of only 2.2%, compared to the

reference case. The cylinder pressure for Case A and

Case B is presented in Figure 8. We note that, under real

operating conditions, a limit of 150 bar is imposed for

the maximum cylinder pressure, to avoid excessive

mechanical strain on the engine components. This

constraint has not yet been considered, and will be

imposed in the immediate next steps of the present study.

-20 -10 0 10 20 30 400

1

2

3

4

5

6

7

8

Crank Angle [deg.]

Rate

of H

eat R

ele

ase

SOI = -5 deg. CA aTDC

SOI = -10 deg. CA aTDC

SOI = -15 deg. CA aTDC

SOI = -20 deg. CA aTDC

Reference

PID = 3 deg. CA

(a)

-20 -10 0 10 20 30 400

1

2

3

4

5

6

Crank Angle [deg.]

Rate

of H

eat R

ele

ase

PID = 2 deg. CA

PID = 3 deg. CA

PID = 4 deg. CASOI = -15 deg. CA aTDC

(b)

Figure 5: Computed histories of normalized rate of heat

release for: (a) constant pilot injection duration (3 deg.

CA) and variable start of injection, (b) constant start of

injection (-15 deg. CA aTDC) and variable pilot injection

duration.

-25 -20 -15 -10 -5 0 50

2

4

0

0.5

1

1.5

2

PID [deg. CA]

SOI [deg. CA aTDC]

(W/W

ref-

1)*

10

0

Figure 6: Relative, with respect to reference case,

increase in computed useful work, versus start and

duration of pilot injection.

-25 -20 -15 -10 -5 0 50

1

2

3

4

5

1600

1700

1800

1900

PID [deg. CA]

SOI [deg. CA aTDC]

NO

x c

oncentra

tion [ppm

]

Figure 7: Computed final NOX concentrations versus

start and duration of pilot injection.

-100 -50 0 50 1000

20

40

60

80

100

120

140

160

180

Crank Angle [deg.]

Cylin

de

r P

ressu

re [

ba

r]

Case B

Reference

Figure 8: Computed pressure histories for reference case

and one case with pilot injection.

The above results have motivated a second series of

computations, with the aim of reducing fuel consumption.

Here, the injection characteristics of Case B are

maintained (SOI=-15 deg. CA aTDC, PID=3 deg. CA),

but the total injected fuel mass has been reduced so that

the total work output is the same as in Case A. The

simulations predict that the fuel consumption can be

reduced by 1.7%, in which case, NOX concentration is

also reduced by 1.1%. Thus, the above case, called

A

B

B

A

5

hereafter Case C, allows achieving the desired result in

terms of engine output, while reducing the fuel

consumption and maintaining the same NOX emission

levels. The results for Cases A, B, and C are summarized

in Table 3.

Table 3: Characteristics of cases A, B, and C

Case A Case B Case C

SOI [deg. CA aTDC] 2 -15 -15

Pilot Inj. Duration

[deg. CA] 0 3 3

Pilot Mass 0 % 8.0 % 7.86%

Main Mass 100% 92.0 % 90.44%

Work Output 100% 101.5% 100%

NOX [ppm] 1630 1665 1611

Max Pressure [bar] 149.08 158.97 157.73

Detailed information on the fuel-air mixing and the

combustion process can be revealed by post-processing

the simulation results and visualizing the flow. The fuel

injection process, along with local temperatures and

equivalence ratio values, in a horizontal plane at the

location of injectors, are visualized in Figure 9 for Case

C. In addition, analysis of combustion in T-φ maps for

Cases A and C is presented for the selected points in

time: here, the local values of temperature and

equivalence ratio of all computational cells are placed in

the maps.

A close observation of the maps indicates that the

fuel injected during the pilot injection is mixed with air,

and the resulting mixture does not penetrate map areas

that favor either NOX or soot formation. The fuel injected

during the pilot injection is entirely burnt before the start

of the main injection. This is shown in the T-φ map for 2

deg. CA aTDC, at which time the cylinder charge is

characterized by equivalence ratio values close to zero

(practically no fuel present), while the temperature

ranges from 700 to 1500 K. The part of main combustion

is similar for both cases, but in Case C the main injection

event lasts only 14.2 deg. CA, as opposed to 15.2 deg.

CA for Case A. One may thus infer that less soot is

formed in Case C. This effect is shown in the last two

rows of Figure 9 (at 17 deg. CA aTDC and 19 deg. CA

aTDC), where soot formation appears to drop

substantially for Case C. Furthermore, it can be deduced

that in Case C, due to more favorable conditions (higher

temperature and pressure), soot oxidation starts earlier.

Thus, we expect that in Case C the combined effect of

lower soot production and earlier start of oxidation leads

to reduced soot concentration at the exhaust.

Further analysis of the soot formation process

appears in Figure 10, where three-dimensional soot

formation areas inside the combustion chamber are

shown. These plots have been constructed by defining a

function that describes the likelihood of soot formation,

based on the T-φ maps. The form of the above function

accounts for the observed linear dependence of soot

formation on φ, as well as for the corresponding

parabolic dependence on mixture temperature. In

particular, in a first step, the following parabolic function

of temperature has been defined, to account for the effect

of temperature on soot formation:

cTbTaTy +⋅+⋅= 2)(

Here, the coefficients a, b, and c, were calculated from

the solution of a 3×3 system of linear equations,

corresponding to the data pairs (T, y)=(1500, 0), (2000,

1), (2500, 0). Thus, for temperatures lower than 1500 K

and higher than 2500 K there is no soot formation, while

the temperature effect is maximum for 2000 K. By

multiplying y(T) with the equivalence ratio, φ (to account

for the linear dependence of soot formation on φ), the

following function, named hereafter Soot Formation

function, SF, is defined:

−+−⋅= 15

125

22

000,250

1TTSF φ

For values SF>3, the above function approximates

the soot formation area of the T-φ map with

concentration higher than 5%. An isosurface with SF=3

has been constructed that includes all areas with SF>3

(which are more likely to contribute to soot production),

and is presented in Figure 10.

Figure 10 reveals that soot formation begins at

approximately 4 deg. CA aTDC, i.e. 2 deg. CA after the

start of main injection. One can observe that at this point

soot formation areas are slightly larger in Case C, due to

the higher temperatures inside the cylinder. However, the

soot formation areas rapidly become of comparable size

and shape, an observation which is in accordance with

the T-φ maps in Figure 9, regarding the main combustion

phase. A difference becomes obvious at 17 deg. CA

aTDC and later on, when, for case C, fuel injection

terminates. As indicated previously, even though soot

oxidation rates are not taken into account in the present

work, it can be concluded that reduced soot emissions

are achieved in Case C, in comparison with the reference

Case A.

CONCLUSIONS The effects of pilot injections in large two-stroke

marine diesel engines have been studied in this work,

using CFD and analysis of the combustion and emissions

formation process with T-φ mapping. It has been

demonstrated that, by adding a pilot injection,

appropriately timed, fuel savings of the order of 1.5%

can be achieved, without increasing NOX emissions.

Furthermore, analysis of the fuel-air mixture indicates

that soot formation can possibly also be reduced. To

illustrate the significance of the fuel economy, it is noted

that the present estimates correspond to approximately

one tone of fuel/day of engine operation, for an

8-cylinder engine of this type. Such an engine can propel

a tanker ship of 130,000 tones deadweight with 14

Knots.

6

T-φ – Case A T-φ – Case C Temperature – Case C Equiv. Ratio – Case C

Figure 9: T-φ maps for cases A and C, for different values of crank angle, and color-coded contours of temperature and

equivalence ratio in a horizontal plane at the location of injectors.

7

Figure 10: Soot formation areas (corresponding to values

of the soot formation function SF>3) for Case A and

Case C, for different values of crank angle.

Acknowledgements

The second author would like to acknowledge the

financial support by a Marie-Curie International

Reintegration Grant, agreement nr. 207232. We would

like to thank Dr. G. Weisser of Wärtsilä Switzerland for

helpful discussions, and for providing us with the engine

data.

Contact Information:

Mr. Panagiotis Kontoulis: nm03622@central.ntua.gr

Dr. Christos Chryssakis: cchryssa@naval.ntua.gr

Prof. Lambros Kaiktsis: kaiktsis@naval.ntua.gr

NOMENCLATURE aTDC after Top Dead Centre [deg. CA]

CA Crank Angle [deg.]

EVO Exhaust Valve Opening [deg. CA]

f Engine Frequency [s-1

]

IPC Intake Port Closing [deg. CA]

p Pressure [bar]

Qchem Combustion Heat Release [J]

Qchem,tot Total Combustion Heat Release [J]

PID Pilot Injection Duration [deg. CA]

ROHR Rate Of Heat Release [% per deg. CA]

SF Soot Formation function [ - ]

SN Swirl Number [ - ]

SOI Start Of Injection [deg. CA]

T Temperature [K]

φ Equivalence ratio [ - ]

ωengine Angular velocity of engine crankshaft [s-1

]

ωswirl Angular velocity of swirl [s-1

]

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