Influence of the spray pattern on combustion characteristics in a direct injection engine

ELSEVIER

JSAE Review 18 (1997) 401 421

O

Technical Notes

Influence of the spray pattern on combustion characteristics in a direct injection engine Takuya Shiraishi a, Mamoru Fujieda b, Minoru Oosuga a ~'Automotive Systems Group, 3rd Department of Systems Research, Hitachi Research Laboratoo', 2520 Takaba, Hitachinaka-shi, lbaraki, 312 ,Japan bAutomotive New Technology Developing Center, Automotive Products" Division Hitachi, Ltd., 2520 Takaba, Hitachinaka-shi, lbaraki, 312 Japan Received 6 January 1997

I. Introduction Requirements of engine power train control systems of automobiles are shifting to lower fuel consumption and lower exhaust emission from the previously demanded higher output, because of concern for the environment. It is necessary to improve thermal efficiency by using a learn-burn and higher compression ratio for lower fuel consumption and to improve response of fuel transport for lower exhaust emission, especially at cold starting. Direct injection for a spark ignition engine is seen as a promising technique to meet these new requirements [1]. In this paper, the effects of spray pattern and injection timing on combustion characteristics are investigated experimentally, making a direct injection system for a spark ignition engine for trial.

2. Experiment Figure l shows the engine testing apparatus. The engine is a modified diesel engine which has one cylinder and a displacement volume of 0.487 (1). Combustion takes place in the piston cavity which is in the top of the piston. The compression ratio is changed from 22 to 14 by enlarging the cavity volume. The engine has one intake valve and one exhaust valve per cylinder. The intake port is a helical type which generates intense swirl in the cylinder. The fuel injector is set at the cylinder center and a spark plug is set near the fuel injector. The fuel injector is a unit-type, remade for use with gasoline. In order to change the mixture prepared in the cylinder, injector nozzles which vary in number (n), diameter (4)d) and spray angle (0) are tested (Table 1). In this paper, fuel injectors are indicated by d-n-O. The fuel

supply system is also modified for use with gasoline. Fuel is pressurised by a piston-type pump which is driven by a motor moving at a constant number of revolutions per minute. Pressure pulsation in the high pressure pipe is controlled by an accumulator and the fuel supply pressure is 10 MPa. The engine revolves at 1600 r/min and intake manifold pressure is 101.3 kPa. Fuel injection timing is varied from the intake stroke to the compression stroke, output power is measured by an electric dynamo- meter and emissions are measured by an exhaust gas analyzer.

3. Results and discussion

3.1. Influence of nozzle spray angle Figure 2 compares the characteristics of emissions when the injection timing is 0(ATDC) and 0 is varied. When n is 1, the spread angle of the spray is about 5'~. For the fuel injector 0.22 1-0, HC and CO emissions are large at the same A/F (air fuel ratio). Besides, NO emission is the least. These observations mean incomplete combustion. For the fuel injector 0.15 4 30, HC emission is the least. For the fuel injector 0.15-4-60, CO emission is the least. This is because when 0 is bigger, HC emission tends to increase because fuel spray does not enter the piston cavity. At the same time CO emission tends to decrease because dispersion of fuel spray becomes better. When 0 is smaller, HC emission tends to decrease because dispersion of fuel spray becomes smaller and fuel spray enters the piston cavity. At the same time, CO emission tends to increase because the fuel spray becomes very concentrated. Figure 3 compares the A/Fo (lean-limit A/F) when 0 is varied. A/Fo is the air fuel ratio for a sudden increase of

0389-4304/97:$17.00 1997 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved PII S 0 3 8 9 - 4 3 0 4 ( 9 7 ) 0 0 0 3 0 - 1

JSAE9735835

402

Technical Notes / JSAE Review 18 (1997) 401 421

Fuel supply system '

Table 1 Specifications of injection nozzle

.~

~

n

~Inje~i~k plug

"

~

Piston

I I'

n

d (min)

1

0.22

0 (deg)

0.10 0.15 0.15 0.20

Schematic

60 60 30 30

.-

Fig. 1. Experimental apparatus. Xl03 01 = 30

0.15

02 = 60

3x 10 A [ ~ /f

~2

1600rpm,WOT ~\

it = 0 ~ATDC)

~,-,r~ ~ k

D:0.22-1-0

~-',~

~: 0.15-4-30

"~

28

1600rpm,WOT

26 24 22 2O i

O ~)1 o

d-n

14

16

18

20A/F 22

-~-"-"P

24

26

0.15-4

0.15-4

30

60

0.15 -6 0 1=30 0 2=60

0.15-8 0 1=30 0 2=60

I

28

Fig. 3. Comparison of lean limit air fuel ratio A/F o.

Fig. 2. Comparison of exhaust emissions as a function of A/F.

3.2. Influence offuel injection timing HC emission due to a misfire. For the fuel injector 0.15-4 30, A/Fo is 22 and for fuel injector 0.15-4-60, it is 26. The reasons are probably as follows. When 0 is 30 °, a thick fuel liquid film is formed because the area on which fuel spray impinges on the piston cavity is small. When 0 is 60 °, a thin fuel liquid film is formed, vaporization is hastened and vaporized fuel is dispersed in the piston cavity because the fuel spray impinges on a large area on the piston cavity. But when n is, namely 0 is 30 and 60 °, A/Fo becomes smaller than when 0 is only 60 ° as there is narrow spray such that 0 is 30 (deg). It is seen that vaporized fuel must be dispersed widely in the piston cavity in order to form a homogeneous mixture.

When injection is in the first half of the intake stroke, fuel spray is injected into the piston cavity efficiently, but the fuel liquid film tends to be formed because of a shorter distance from the nozzle to the piston. When injection is in the second half of the intake stroke, fuel spray is injected into the cylinder widely. When injection is during the compression stroke, fuel spray tends to be prevented from dispersing and it concentrates in the piston cavity because air density in the cylinder becomes bigger. Figure 4 shows the relationship of injection timing, A/F and exhaust gas emissions when the amount of injected fuel is constant. The fuel injector is the

Technical Notes / JSAE Review 18 (1997) 401 421

403

Xl03

×103

1600rpm,WOT ×103

0.15-4-30

o : It=0(ATDC)

\

o

v ; It=236

:01 3L

r.) Ok

--'~

..

×103 3[ O~ 21i~

0 t-

1600rpm,WOT 0.15-4-30

20

19

~ 0

I 50

i

100 It (ATDC)

~

1

150

200 0 t

Fig. 4. A/F and exhaust emissions as a function of injection timing.

I

I

I

I

I

I

I

I

15

20

25

30 A/F

35

40

45

50

Fig. 5. Exhaust emissions as a function of A/F,

0.15 4-30. As the injection timing becomes bigger, the HC emission tends to decrease. This is because the piston is moving down and fuel spray is injected into the cylinder widely. When injection timing is at about 150(ATDC), the NO emission increases. This is probably because a rich mixture is partially formed and the A/F distribution is not homogeneous, so NO emission increases in the combustion region. When injection timing is after 150(ATDC), the NO emission tends to decrease and CO emission tends to increase. When the time interval between injection timing and spark timing becomes smaller, the time needed to vaporize fuel becomes shorter, too. This results in unstable combustion. Figure 5 shows the relationship of A/F and exhaust gas emissions when injection timing is varied. When injection timing is at 0 and 120(ATDC), both CO and NO emissions are similar. Judging from the characteristics of NO emission, the mixture in the cylinder is homogeneous. When the injection timing is 236 and 270(ATDC) in the compression stroke, A/F at the maximum NO emission is shifted to a bigger A/F. When injection timing is 236(ATDC), the A/F is 22, and at 270(ATDC) the A/F is

46. It is reasonable to assume that vaporized fuel is partly concentrated near the spark plug and the mixture is stratified.

4. Conclusions

The following points were seen in this study on mixture preparation in a direct injection engine. (1) The lean-limit air fuel ratio was 26 when injection timing was 0(ATDC) and spray angle was 60(deg). (2) The direct fuel injection engine could be changed from homogeneous to stratified charge by changing the fuel injection timing.

Reference [1] Iiyama, A. and Muranaka, S., Current status and future perspective of DISC engine (in Japanese), Proceedings of New Generation Gasoline Engine Symposium (No. 9402), No. 9431030, pp. 23 29 (1994).