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3-D CFD Analysis of the Mixture Formation Process

in an LPG DI SI Engine for Heavy Duty Vehicles

Gisoo Hyun and Mitsuharu OgumaNEDO (New Energy and Industrial Technology Development Organization

Shinichi GotoNational Institute of Advanced Industrial Science and Technology

ABSTRACT

This work aimed to develop an LPG fueled direct injection

SI engine, especially in order to improve the exhaust

emission quality while maintaining high thermal efficiency

comparable to a conventional engine. In-cylinder directinjection engines developed recently worldwide utilizes the

stratified charge formation technique at low load, whereas

at high load, a close-to-homogeneous charge is formed.

Thus, compared to a conventional port injection engine, a

significant improvement of fuel consumption and power

can be achieved. To implement such a combustion

strategy, the stratification of mixture charge is very

important, and an understanding of its combustion

process is also inevitably necessary.

In this work, a numerical simulation was performed using

a CFD code(KIVA-3), where the shape of a combustion

chamber, swirl intensity, injection timing and duration, and

so on were varied and their effects on the mixture

formation were investigated. The conclusions include the

fuel injection conditions such as injection timing and

duration showed relevant influences on the stratification of

mixture charge. And, it was also clarified that the in-

cylinder flows such as swirl and tumble significantly

enhance the mixture formation process, forming a rich

charge around the spark plug and a lean one near the

cylinder wall. This eventually leads to the improved

emission characteristics in an LPG direct injection SI

engine.

INTRODUCTION

Air pollution with the exhaust emission is still a serious

problem, and an international concern has been risen for

its control and restriction. Therefore, energy conservation

with high efficiency and low emission are important

research topics for development of engine system.

Recently, the engine which uses alternative fuels such as

natural gas (CNG, LNG), LPG(Liquefied Petroleum Gas),

DME(Dimethyl Ether), GTL(Gas to Liquids), and hydrogen

is actively developed to solve these problems[1][2][3].

Especially, LPG is paid to attention as a useful alternative

fuel which can be substituted from production from not

only the oil refinement but also the gas refinement to oil

In addition, as LPG is excellent in the exhaust emission

performance, LPG vehicles are being rapidly developed

as an economical and low pollution car.

Recently, several works have been carried out for injecting

the fuel directly into the combustion chamber to meet the

low emission standard and high efficiency[4][5][6]. And

those methods have been used in practical gasoline

engines. However, a large amount of unburned

hydrocarbon emission is a new problem with GDI engines

when they operate near the stoichiometric mixing

condition.

On the other hand, for LGP engine, though lean and

stratification combustion are effective means to high

efficiency of LPG engine by the In-cylinder direct injection

method, it must be clarified the behavior of LPG spray

mixture formation process and combustion process fo

development of high efficiency LPG engine system. This

work aimed to develop an LPG fueled direct injection S

engine. The present work used computational fluid

dynamics (CFD) is to examine the changes that occur in

the in-cylinder flow field, mixture preparation and

combustion due to injection conditions, swirl intensity and

geometry of combustion chamber. A numerical simulation

was performed using a CFD code(KIVA-3) , where

combustion chamber shape of bathtub and dogdish type

is used.

COMPUTATIONAL METHOD ANDCONDITIONS

PROBLEM DEFINITION The investigation is carried ou

on development of two-valves spark ignition engine with


Surface definition of the cylinder and ports assembly is

shown in Figure 1. Here, exhaust valve opening and

closing time are -236.0 BTDC and 14.0 ATDC, and intake

valve are -21.0 BTDC and 231.0 ATDC, respectively.


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Computational results are presented and discussed for

the intake, compression and combustion periods,

respectively. These are included the distribution of the

air/fuel equivalence ratio in the vicinity of fuel injector due

to the change of injection conditions. And flow fields is

investigated during the intake and compression periods.

Each result are analyzed in three cutting planes as

defined in Figure 1. Plane A is the symmetry plane of the

cylinder and plane B is normal to plane A at the cylinder

axis. Plane C is a horizontal plane just at the vicinity of

the fuel injector.

COMPUTATIONAL APPROACH - The KIVA- software

package [7][8] was used here, which is a numerical

analysis code for transient, reactive, multiphase, three-

dimensional flows. The KIVA- is able to solve not only the

behavior of fuel spray and the evaporation process but

also the combustion process of the internal combustion

engine. And, a consideration was undertaken for the

convenience of the calculation and the display by adding

an improved analytical function for unstructured grid and

the interface function between the pre- and post-processorin this work.

It is very important for the elucidation of the combustion

processes to understand the state of the fuel-air mixture

and the combustion in the combustion chamber. In this

research, numerical simulation was conducted to examine

the influence of combustion chamber shape in the mixture

formation and the combustion process. Combustion

chamber shape of bathtub and dogdish type was

calculated in this simulation. n-Butane was used as a

main fuel, and engine speed was set at 1500 and 2800

rpm. The mixture formation process and the combustion

process in cylinder were observed by fuel injection at the

comparatively early stage of injection timing. The injection

pressure is 10.0MPa. The number of calculation mesh in

BDC is about 30,500 for bathtub type, and about 30,400

for dogdish type. And the detailed fuel-air mixture

formation and combustion processes in each cavity was

observed with respect to mainly the flow. The fuel injection

conditions of the engine and calculation parameters are

shown in Table1.

Table 1. Engine specifications

and calculation conditions

Fuel Butane

Bore stroke 108mm x 115 mm

BathtubPiston cavity

Dogdish

Compression ratio 10.0

Pressure 10.0MPa

Timing 120, 90, 60BTDCInjection

Duration 30, 40, 50CA

Connecting rod length 185.0mm

Maximum intake valve lift 11.83mm

Exhaust valve opening -236.0BTDC

Exhaust valve closure 14.0ATDC

Intake valve opening -21.0BTDC

Intake valve closure 231.0ATDC

Engine speed 1500, 2800 rpm

Swirl ratio (S/R) 1.97, 3.73

Fig.1 Computational domain, geometry of combustion chamber

and definition of cutting planes A, B and C

Fuel injector

Dogdish type

Plane C

Plane B

Plane A

Spark plugBathtub type

Intake Port

Exhaust Port


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RESULTS AND DISCUSSION

Simulation results are presented and considered under

separating the intake, mixture preparation and combustion

processes. Especially, injection condition was changed to

achieve mixture stratification in compression process, and

their influence was also considered.

INTAKE PROCESS Figure 2 presents the results of

calculating flow pattern in intake port and cylinder duringthe overlap period(-21.0BTDC~14.0ATDC) and intake

process. Here, the plane A, B and C is defined in Fig.1. A

is the symmetry plane of the cylinder and plane B is

normal to plane A at the cylinder axis. C is a horizontal

plane just at the vicinity of the fuel injector. Moreover, the

intake port is designed by helical shape, in order to

generate the swirl flow easily. In addition, the combustion

chamber is set up at the position where offset was done

for the cylinder center, and squish is generated easily fo

the bathtub type combustion chamber.

In overlap period, though the flow in combustion chamberhas not changed greatly because valve lift of intake and

exhaust valve is still small, the gas exchange situation is

Combustion chamber

of bathtub type

Combustion chamber

of dogdish type

Crank angle

TDC

70ATDC

120ATDC

(a)

(b)

(c)

Low High

(a)

(b) (c)

(a)

(b)

(c)

Low High

(a)

(b) (c)

(a)

(b)

(c)

(a)

(b)

(c)

Fig.2 Flow fields in overlap period and intake process


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observed through both ports. Moreover, when crank angle

advances and intake valve opens greatly, air for

combustion is entered into the cylinder through intake port

while forming the large scale tumble flow. The shape of

intake port is designed by helical shape as mentioned

above, swirl flow is accompanied, too.

During the intake process, though a great difference is not

seen about flow pattern between bathtub and dogdish typecombustion chamber, the swirl flow becomes strong a

little for bathtub type. Therefore, it is thought that such

flow influences on the following mixture formation and

combustion process.

MIXTURE PREPARATION - The mixture stratification is

the most important factor in In-cylinder injection engine.

Therefore, it is necessary to examine the mixture

formation process by changing the combustion chamber

shape. Thus, the geometry of the piston cavity was

changed as shown in Fig.1. And the fuel-air mixture

formation process in each cavity was observed with

respect to the flow as shown in Fig.3. And fuel injectionconditions have a great influence on mixture formation and

stratified combustion in the cylinder. Therefore, it is

important to control adequately the fuel fluid in association

with the flow in the combustion chamber. Then, the shape

of a combustion chamber, swirl intensity, injection timing

and duration were varied and their effects on the mixture

formation and combustion process were investigated as

shown in Fig.4, 5 and 6.

Mixture formation processes in each combustion chamber

shape - Figure 3 presents the distribution of velocity vector

Fig.3 Distribution of velocity vector and fuel concentration for different combustion chamber in plane A

Crank angle

300CA

Injection Finish

310CA

330CA

340CA

Ignition Start

HighLowCombustion chamber

of bathtub type

Velocity vector Fuel concentration

HighLowCombustion chamber

of dogdish type

Velocity vector Fuel concentration

Fig.4 Equivalence ratio distribution near

the spark plug for the swirl intensity

240.0 270.0 300.0 330.0 360.0

Crank angle [Deg.]

0

0.3

0.6

0.9

1.2

1.5

Equivalence

ratio[-

]

Ba thtub- swirl1. 97 15 00 rpmBathtub-swirl3.73Dogdish-swirl1.97

Dogdish-swirl3.73

1500rpm

240.0 270.0 300.0 330.0 360.0

Crank angle [Deg.]

0

0.3

0.6

0.9

1.2

1.5

Equivalence

ratio[-

]

Ba thtub- swirl1. 97 28 00 rpmBathtub-swirl3.73Dogdish-swirl1.97

Dogdish-swirl3.73

2800rpm


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and fuel concentration for each combustion chamber in

plane A. A large vortex is formed at the periphery of the

injected fuel fluid by shear with surrounding air; the vortex

grows while entraining surrounding air. It is considered

that surrounding air and small droplets, as well as the fuel

vapor which has already evaporated, are entrained into the

fuel fluid by these vortices, and move with the fuel fluid

Fuel fluid of this kind impinges on the bottom of the piston

cavity, and progresses along the wall. This flow

characteristic can be observed for all combustion chambe

The piston cavity does not show any effect on fuel fluid

before impingement. However, different behaviors can be

observed after impingement with different combustion

chamber shape. In the case of bathtub type, the vortex

flow after impingement is comparatively weak, because

the vortex flow is attenuated on the other side wall of thenozzle. However, in this case, large scale tumble flow is

observed to be formed until near the ignition timing

(340CA) with the influence of squish flow. Rich mixture is

formed with such flow in the vicinity of the spark plug. In

the case of dogdish type, though growth of a large scale

vortex continues without collapsing after impingement on

the piston cavity wall, the scale is small. Moreover, in this

type, the tumble flow is also formed though the scale is

smaller than bathtub type.

It is understood from the figure of the fuel concentration

distribution that fuel fluid progresses getting on the above

mentioned flow. For bathtub type, after forming a richmixture in vicinity of the wall on the opposing side of the

nozzle, the fuel fluid progresses along the cavity wall, and

rich mixture is formed at the vicinity of the spark plug in

ignition timing. In the case of dogdish type, though the

fuel concentration distribution keeps growing up along the

piston cavity wall, fuel fluid does not move well to the

vicinity of the spark plug in ignition timing as compared

with the bathtub type.

Influence of swirl intensity Fig.4, 5 and 6 present the

distribution of local equivalence ratio within 10.0mm of the

spark plug. The mixture distribution at the vicinity of the

spark plug may be considered in more detail. Figure 4 is

the results of observing the influence that swirl intensity

gives to the mixture formation process, when swir

intensities are 1.97 and 3.73 and engine speeds are

1500rpm and 2800rpm, respectively.


the spark plug for the injection duration

240.0 270.0 300.0 330.0 360.0

Crank angle [Deg.]

0

0.3

0.6

0.9

1.2

1.5

Equiv

alence

ratio[-

]

Bathtub-Dinj30 1500rpmBathtub-Dinj40Bathtub-Dinj50Dogdish-Dinj30

Dogdish-Dinj40Dogdish-Dinj50

1500rpm

240.0 270.0 300.0 330.0 360.0

Crank angle [Deg.]

0

0.3

0.6

0.9

1.2

1.5

Equivalencer

atio[-

]

Bathtub-Dinj30 2800rpmBathtub-Dinj40Bathtub-Dinj50Dogdish-Dinj30

Dogdish-Dinj40Dogdish-Dinj50

2800rpm


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In the case of engine speed 1500rpm and swirl intensity

1.97, rich mixture in the vicinity of the spark plug is

formed at the range of 330CA to 340CA. The rich mixture

reaches earlier at the vicinity of the spark plug as an

increase of the swirl intensity. But this tendency with the

stronger swirl intensity (3.73) becomes smaller for both

types of combustion chamber.

In the case of engine speed 2800rpm, equivalence ratio in

the vicinity of the spark plug decreases overall. Moreover,there is no great influence by the difference of swirl

intensity, as a little difference is appeared.

Influence of injection duration Figure 5 shows the

influence of injection duration on the mixture formation

process. The simulation conditions are the same as Fig.4,

so that, swirl intensities are 1.97 and 3.73, and engine

speeds are 1500rpm and 2800rpm, respectively. Injection

duration is changed to 30CA, 40CA and 50CA. Local

equivalence ratio at the vicinity of spark plug decreases as

injection duration becomes long for both types of

combustion chamber. Moreover, the highest value of

equivalence ratio appears early as injection durationshortens. And the local equivalence ratio decreases with

increasing engine speed as shown in Fig.4.

Influence of injection timing Figure 6 shows the

distribution of local equivalence ratio near the spark plug

for injection timing. Injection timing is changed to

Tinj=60BTDC(300CA), 90BTDC ( 270CA) and 120BTDC

(240CA), respectively. At 1500rpm, equivalence ratio in

the vicinity of spark plug becomes higher as injection

timing retard. However, in the case of 2800rpm, this

tendency is little difference, while it has the highest value

at Tinj=60BTDC. Moreover, the influence of engine speed

shows the same tendency as mentioned above.

CONCLUSIONS

This work aimed to develop an LPG fueled direct injection

SI engine. The present work used computational fluid

dynamics (CFD) is to examine the changes that occur in

the in-cylinder flow field, mixture preparation and

combustion due to injection conditions, swirl intensity and

geometry of combustion chamber. A numerical simulation

was performed using a CFD code(KIVA-3) , where


is used. The results of this work can be summarized asfollows :

1. During the intake process, though a great difference is

not seen about flow pattern between bathtub type and

dogdish type combustion chamber, the swirl flow

becomes strong a little for the bathtub type.

2. The rich mixture reaches earlier at the vicinity of the

spark plug as an increase of the swirl intensity. But

this tendency with the stronger swirl intensity

becomes smaller for both types of combustion

chamber.

3. Local equivalence ratio at the vicinity of spark plug

decreases as injection duration becomes long for both

types of combustion chamber. Moreover, the highes

value of equivalence ratio appears early with

shortening the injection duration. And the loca

equivalence ratio decreases with increasing engine

speed.

4. Equivalence ratio in the vicinity of spark plug becomes

higher as injection timing retard.

REFERENCES

1. Goto, s., Lee, D., Shakal, J., Harayama, N., Honjyo

F. and Ueno, H., Performance and Emissions of an

LPG Lean-Burn Engine for Heavy Duty Vehicles

SAE Paper No.1999-01-1513, 1999.

2. Stavinoha, L. L., Alfaro, E. S., Dobbs, H. H.

Villahermosa, L. A. and Heywood, J. B., Alternative

Fuels: Gas to Liquids as Potential 21st Century TruckFuels, SAE Paper No.2000-01-3422, 2000.

240.0 270.0 300.0 330.0 360.0

Crank angle [Deg.]

0.0

0.3

0.6

0.9

1.2

1.5

Equivalenceratio

[-

]

1500rpmSolid : BathtubPlain : DogdishInjection timing

120BTDC

Injection timing

90BTDC

Injection timing

60BTDC

240.0 270.0 300.0 330.0 360.0

Crank angle [Deg.]

0

0.3

0.6

0.9

1.2

1.5

Equivalence

ratio

[-

]

2800rpmSolid : BathtubPlain : Dogdish

Injection timing

90BTDC

Injection timing60BTDC

Injection timing

120BTDC


the spark plug for the injection timing


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3. Hyun, G., Oguma, M., Alam, M., Ehara, R. and Goto,

S., Spray Characteristics and Exhaust Emissions of

a Diesel Engine Operating with the Blend of Plant Oil

and DME, Proc. 6th Annual Conference on Liquid

Atomization and Spray Systems-Asia (ILASS-Asia

'99), pp. 253-258, 2001.

4. Kuwahara, K., Ueda, K. and Ando, H., Mixing Control

Strategy for Engine Performance Improvement in a

Gasoline Direct Injection Engine, SAE Paper

No.980158, 1998.5. Preussner, C., Doring, C., Fehler, S. and Kampmann,

S., GDI: Interaction Between Mixture Preparation,

Combustion System and Injector Performance, SAE

Paper No.980498, 1998.

6. Harada, J., Tomita, T., Mizuno, H., Mashiki, Z. and Ito,

Y., Development of Direct Injection Gasoline Engine,

SAE Paper No.970540, 1997.

7. Amsden, A. A., ORourke, P. J. and Butler, T. D.,

KIVA- : A Computer Program for Chemically Reactive

Flows with Sprays, Los Alamos National Laboratory

Report LA-11560-MS, 1989.

8. Amsden, A. A., KIVA- : A KIVA Program with Block

Structured Mesh for Complex Geometries, Los

Alamos National Laboratory Report LA-12503-MS

1993.

9. Soltani, S. and Veshagh, A., CFD Analysis of Effec

of Staggered Intake Valve Timing on Mixture

Preparation and Combustion in a Four-valve S

Engine, 1999 Spring Technical Conference ASME,

Paper No.99-ICE-169, 1999.

CONTACT

Gisoo Hyun, Dr.Engrg.

Clean Power System Research Group

Energy Utilization Research Department,

National Institute of Advanced Industrial Science &

Technology

1-2 Namiki, Tsukuba, Ibaraki 305-8564, Japan

Tel:+81-298-61-7863, Fax:+81-298-61-7275

E-mail: [email protected]

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Written 8 Hyun