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Study of the stratified charge and stable combustion in DI gasoline engines
ELSEVIER
JSAE Review 16 (1995) 363-368
Study of the stratified charge and stable combustion in DI gasoline engines Seiko Kono Technical Research Center (Yokohama), Mazda Motor Corp., Moriya-cho 2-5, Kanagawa-ku, Yokohama, 221 Japan Received 9 January 1995
Abstract
A new combustion method for DISC engines was developed. It has a double structure combustion chamber known as "Caldera". This method makes possible a perfect un-throttling operation, and a fuel consumption equal to a diesel engine is achieved. In the output of the DISC engine, a stoichmetric combustion and a high torque are achieved by controlling the fuel injection timing with an electro-magnetic injection device. With regard to emission regulations, a heavy EGR including residual gas greatly decreases NOx and HC emissions simultaneously.
L Introduction
In the case of a direct injection stratified charge (DISC) engine, the load is controlled by the amount of injection fuel instead of by induction throttling. The DISC engine is able to improve the thermal efficiency significantly, especially at light loads, for the following four reasons: (1) decrease of pumping loss, (2) improvement of cyclic efficiency by being close to the air cycle, (3) decrease of heat loss because of low mean temperature in the cylinder, (4) high compression ratio standing a preventing knocking. Many types of DISC engine have been developed so far. The MAN-FM type developed in Germany and the TCCS type developed by Texaco are well known as high completion DISC engines, which are shown in Fig. 1 [1-31. Kono et al. have paid attention to realizing certain ignition and stable combustion under wide operating conditions, and proposed a DI stratified charge and stable combustion method [4,5]. The spark plug is located at the center of the cylinder in order to increase the air consumption rate in the cylinder, and the piston has a cavity just below the spark plug in order to form a stable mixture around the electrode. The piston has a cavity, whose shape
is known as "Caldera," which has a double structure. The center cavity has the role of obtaining certain ignition and the outer cavity has the role of a main chamber. The "Caldera" combustion method achieves stable combustion over a wide range of operating conditions for both ignition timing and injection timing, which makes it possible to select the best ignition and fuel injection timing for thermal efficiency. Fuel consumption equal to a diesel engine was achieved [5]. The operating area demanded by automotive engines is quite broad. A high torque and high horse-power at a high
Inj. N o z z l e ~
wjrl
s,,,k Plu, ~-N~k
]
l.Sprslr "~:~ll~t / 2.11ixture 3.Fleme 4.Burned ....
TCCS
Spark
Plug
MAN-FM
Fig. 1. Typical DISC engines (MAN-FM and TCCS).
0389-4304/95/$09.50 © 1995 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved
SSDI 0 3 8 9 - 4 3 0 4 ( 9 5 ) 0 0 0 3 2 - 1
JSAE9537934
364
S. Kono /JSAE Review 16 (1995) 363-368
1~
ct°r
Pulse delay generator
Oscilloscope
~
I I unsen,
~
[ I
~-'J Pipe
Motor
Engine Vessel
Fig. 2. Fuel injection system. Flash
revolution speed are demanded especially in a gasoline engine. The new combustion method satisfies emission regulations which are becoming more severe. In this study, the possibility of the "Caldera" combustion method meeting the demands of automotive-engines is investigated, setting the target that a fuel consumption equal to diesel engines and an out-put equal to gasoline engines are compatible.
2. Measurements and calculation method
2.1. Test engine A fuel injection system has to satisfy injection characteristics required for a wide range of operating conditions. The test engine has a fuel injection system as shown in Fig. 2 and a mechanical super charger. The characteristics of these devices are decided according to engine demands which are investigated by means of spray formation analyses. Table 1 shows the main engine specifications. With a fuel injection system, the mass of injected fuel, injection timing and injection pressure are controlled manually, corresponding to revolution speed and load. The super charger is operated only at high load. A piezo type pressure transducer is set at one cylinder, and pressure measurements are taken. 2.2. Analyses of mixture formation There are many factors and parameters which control D.I. stratified charge combustion, but the main consideration is how to form a mixture in the combustion chamber, especially around the spark plug electrode, and how to control the mixture formation according to engine operating conditions. Because fuel is injected into the cylinder during the compression stroke in the DISC engine, analysis of spray
Table 1 Main Engine Specifications Bore Stroke Compression ratio Injection pressure
84.5 mm 74.2 mm 11.5 10-25 MPa
light
if<
[--1
N:Bomb
Fig. 3. Spray visualizing equipment.
formation under high ambient pressure is important. The spray is observed with a constant volume vessel in which the ambient pressure is changed from atomospheric pressure to 2.0 MPa, according to the compression ratio. A direct photograph is taken at 10000 frames per second under an ambient pressure corresponding to the in-cylinder pressure at injection timing. Figure 3 shows the observing apparatus. A flash-light, which flashes at the same time as the injection start, is scattered by the spray. Observation of the spray lacks information of the vaporized mixture and fuel wetting the cavity wall. In order to supply this information and to analyze spray and mixture formation in the combustion chamber under engine operating conditions, the CFD code " K I V A " , which has been developed by Los Alamos Laboratory is applied [6]. The " K I V A " has a discrete particle model in which the spray is modelled as a set of fuel liquid particles. The movement of each particle is solved by a 3-D fluid field, taking the vaporization and combination of particles into consideration.
3. " C a l d e r a " combustion method
3.1. " Caldera" combustion chamber Keeping the mixture around the spark plug electrode for enough time for the flame kernel to grow is the key to realizing stable combustion over a wide operating range. The "Caldera" combustion chamber, as shown in Fig. 4, is proposed. This chamber is constructed with a center cavity, corresponding to a pre-combustion chamber, and an outer cavity corresponding to a main chamber. Fuel is injected from the circumference of the chamber to the center cavity, and the spark plug is located above the cavity, in order to achieve certain ignition. Mixture overflows from the center cavity and spreads over the outer cavity riding on a swirl flow. 3.2. Mixture formation analysis in "Caldera" Spray and mixture formation in the "Caldera" chamber are calculated on the assumption of actual engine operating
365
S. Kono/JSAE Reuiew 16 (1995) 363-368
ion
21°
BTDC
10° BTDC
Fig. 6. Mixture formation in conventional DISC chamber. conditions. Table 1 shows the engine specifications and operating conditions. In-cylinder flow is calculated on the assumption of a solid swirl whose ratio is 2.5 at BDC. The swirl ratio is measured by means of the impulse swirl meter method. The top figures of Fig. 5 show distribution of liquid spray at 30 ° and 25 ° BTDC. The middle figures show two cross sections of flow vector graph at 16 ° BTDC. The bottom figures show distributions of mixture concentration
30 °
BTDC
25 °
BTDC
16° BTDC
at 20 ° and 10 ° BTDC. The shaded area indicates a combustible mixture. W e found that a stable mixture stays at the electrode location during 10 ° C.A., and certain ignition and stable combustion can be fully expected. 3.3. Mixture formation in conuentional DISC chamber
Spray and mixture formation in the conventional DISC chamber, as shown in Fig. 1, are analyzed by calculations on the assumption of actual engine operating conditions. In the case of a conventional DISC engine, fuel is injected along the chamber wall in the swirl flow direction. In the case of fuel injected in the reverse direction, mixture formation is also analyzed. Table 1 shows the engine specifications and operating conditions. Figures 6 and 7 show distribution of mixture concentration in a chamber at 20 ° and 10 ° BTDC. Each cross section shows a vertical and horizontal section at the spark plug electrode position. The shaded area indicates a combustible mixture. The interval of 10 ° C.A. corresponds to the duration between ignition and a stable flame kernel forming.
~ 21 ° BTDC
10° BTDC
Fig. 5. Mixtureformation at basical operated condition.
21° BTDC
tion
lO ° BTDC
Fig. 7. Mixture formation in case of reverse injection direction.
S. Kono /JSAE Review 16 (1995) 363-368
366
E ~ 20 I 8~ i-
_m
~
'
15
60 "[
so 8
40 ._~
g~
~
30
m
20
8O 60
'
350
"~
~
40 20
300
250
t6" BTDC
10" BTDC
0LL (O
Fig. 8. Mixture formation at idling condition.
DISC 200
150 2
f
I
3
4 5 IMEP (MPa)
I 6X10 -1
Fig. 10. Characteristics of fuel consumption.
1 30 ° BTDU
10" BTDC
Fig. 9. Mixture formation at high-load condition (5000 rpm).
Comparing these two types, the reverse type has a better mixture formation at 20 ° BTDC than the conventional type. In both types, though, a good mixture cannot be achieved at 10° BTDC. It is difficult for the mixture to remain during 10° C.A. at the electrode location. These types of engine have to control ignition and fuel injection timing exactly or ignite with a strong spark device.
effect is calculated, over a wide range of operating conditions, which are summarized in Table 2. Figure 8 shows the distribution of mixture concentration under idling conditions. Between 16° and 10° BTDC, a good mixture remains in the center cavity. Stable ignition and combustion can therefore be expected.
3.5. Mixture formation at high revolution speed Figure 9 shows the distribution of mixture concentration at 5000 rpm and high load. After ignition timing, sufficient mixture forms at the electrode location. Mixture is badly distributed at the cylinder head, because of ascending flow caused by the piston movement. Mixture is quenched in the piston cavity by squish flow which is favorable to knocking, but is unfavorable to achieving high torque equal to a homogeneous charge. The fuel vaporizing ratio is quite low (from 50 to 70%) in the calculations. In the actual engine, fuel vaporizes much more and the distribution of mixture is more close to a homogeneous charge.
4. "Caldera" DISC engine's performance
3.4. Mixture formation at idling
4.1. Fuel consumption characteristics
The "Caldera" chamber has a significant effect on stable and stratified charging at the electrode location. The
Figure 10 compares the indicated specific fuel consumption (ISFC) at 1500 rpm, between a "Caldera" DISC
Table 2 Calculating (operating) conditions Figs. 5 - 7 Fig. 8 Fig. 9
Reverse speed
Mass of fuel
Injection time
Injection speed
~/c
Swirl ratio
1500 rpm 700 5000
15 mm3/str 5 50
40 ° BTDC 30 o 170 °
105 m / s 90 120
85% 80 140
2.5 2.5 2.5
S. Kono /JSAE Review 16 (1995) 363-368
engine, whose specifications are shown in Table 1, and a port injection engine, operating under stoichiometric conditions, which has the same displacement as the DISC engine. Fuel injection timings, ignition timings, the 10-90 %mass burning duration, and the air/fuel ratio are also shown in Fig. 10. Stable combustion is obtained even at low load under un-throttling conditions. The ISFC does not increase at low load. On the contrary, the ISFC slightly increases at middle load, because combustion deteriorates which is caused by the combustion chamber geometry which causes a two step combustion, and by poor spray mixing. However, for NO x emission reduction, slightly slower combustion is desirable. The A / F ratio varies between 30 and 80 at this operating area. At low load, more than 25% reduction of ISFC is obtained, so that the initial target that fuel consumption is equal to a diesel engine is achieved. 4.2. Max torque characteristics
Figure 11 shows the maximum indicated mean effective pressure of the DISC engine at 1000-5000 rpm, compared with a port injection engine which has the same displacement and no super charger. The A / F ratio is set at stoichometry in the case of the DISC engine. In the case of a port injection engine, the A / F ratio is about 13. IMEP of A = 1 of the port injection engine is also compared. The volumetric efficiency in both engine and injection timing of the DISC engine are plotted in Fig. 11. The DISC engine has a volumetric efficiency 1.5 times the port injection and achieves a torque which meets volumetric efficiency. The in-cylinder mixture formation is made close to stoichmetric by early fuel injection. At low revolution speed, the A / F ratio is set about 18 to prevent soot generating, and sufficient torque is not
367
~
80
o t~ rr rr (.9 tu
40
60
20 40 f
~
W/0-EGR W/-EGR
0
OL
I 1
I 2
I L 3 4 IMEP (MPa)
I
5X10 -I
Fig. 12. Characteristics of emissions.
obtained. The torque is estimated at 1.0 [B.u.] soot level. Because soot is not detected over 2000 rpm, soot generates under poor spray mixing and rich mixture combustion at a low revolution speed. 4.3. Emission characteristics
In this study, the effects of a heavy EGR which is composed of residual gas and cold EGR are investigated in order to decrease both NO x and HC. The effects at low load especially are studied, because CO: concentration is very low and and the effect of EGR is small. A heavy EGR decreases the total mass of emission and HC emission, because of increasing in-cylinder temperature, and NO x emission because of EGR effects which are well known. Figure 12 shows the test results. When the EGR ratio goes above a certain value, combustion deteriorates and HC emission increases. At a demand value greater than the EGR ratio, NO x emission decreases to below 1 g / K w h , reducing the HC emission.
200 [
150I-j
L~ 1°°t .~
5. Conclusions
5010L
1 ~o DISC 100
XlO -1
Port_..=~._.~lnjection
~ 12 ~ to ~
~50
DISC (2,=1)
16
8 6 I
1
I
I
I
r
I
2 3 4 5 6X103 Engine Revolution Speed (rpm)
Fig. 11. Characteristics of max. torque.
"~
>
The spray and mixture formation in the combustion chamber of DISC engines, which are most important in realizing a DISC engine, were studied by means of observation and computing analyses. The result shows that the new combustion method known as "Caldera" realizes certain ignition and stable combustion. The possibility of the "Caldera" combustion method meeting the demands of automotive engines was investigated, and the following conclusions were obtained: (1) The "Caldera" chamber keeps the mixture in the center cavity around the spark plug electrode during the time (10 ° CA), when the flame kernel grows, and achieves stable ignition and combustion with conventional spark plugs. Computing analyses suggest that it is difficult for
368
S. Kono /JSAE Review 16 (1995) 363-368
conventional DISC engines to achieve stable ignition and combustion. (2) The "Caldera" combustion chamber realizes unthrottling operation perfectly over a wide range of operating conditions. (3) The ISFC is improved by more than 25% at low load, compared with a port injection engine which has the same displacement and is operated at stoichmetric conditions. (4) Early fuel injection at high load and high revolution speed makes it possible to operate at stoichmetric condition, and to get sufficient torque corresponding to volumetric efficiency. (5) Heavy EGR decreases emissions, especially NOx emissions significantly at low load. The NO x emission decreases to below 1 g/Kwh, reducing HC emissions. (6) Sufficient torque cannot be achieved because of soot generation at low revolution speed. Improvements of spray mixing remain a problem. References [1] Dictionary of Automotive Engineering (in Japanese), JSAE, p. 61. [2] Lewis, J.M., UPS Multifuel Stratified Charge Engine Development Program-Field Test, SAE Paper No. 860067, 1986. [3] Schapertons, H. et al., VW's Gasoline Direct Injection (GDI) Research Engine, SAE Paper No. 910054, 1991. [4] Kono, S., Action between Spray and In-Cylinder Flow in D.I. Engines (in Japanese), JSAE, Vol. 22, No. 2, April 1991. [5] Kono, S. et al., Development of Stratified Charge and Stable Combustion in D.I. Engines (in Japanese), JSAE, Vol. 24, No. 3, July 1993. [6] Amsden, A.A. et al., KIVA: A Computer Program for Two- and Three-Dimensional Fluid Flows with Chemical Reactions and Fuel Sprays, Los Alamos National Laboratory Report LA-10245-MS, 1985.
| ' ~ r
r
~ 1 1
I
l
l
I I I
better Rolls Royce, Maserati, Saab, A s t o n Martin, lnnocenti, J a g u a r , a n d Lotus h a v e o n e t h i n g in c o m m o n . NGK s p a r k plugs. All J a p a n e s e car m a k e r s also s p e c i f y NGK as original e q u i p m e n t a n d f a c t o r y - a u t h o r i z e d r e p l a c e m e n t parts. T h e u n i q u e c o p p e r c o r e in e v e r y NGK plug e l i m i n a t e s o v e r h e a t i n g a n d r e d u c e s fouling. T h a t m e a n s i m p r o v e d h e a t dissipation for p e a k p e r f o r m a n c e under any operating conditions. No w o n d e r NGK is r e c o g n i z e d as t h e p e r f e c t plug for a n y e n g i n e .
SPARK PI.UG ¢O., LTD.
JSAE Review 16 (1995) 363-368
Study of the stratified charge and stable combustion in DI gasoline engines Seiko Kono Technical Research Center (Yokohama), Mazda Motor Corp., Moriya-cho 2-5, Kanagawa-ku, Yokohama, 221 Japan Received 9 January 1995
Abstract
A new combustion method for DISC engines was developed. It has a double structure combustion chamber known as "Caldera". This method makes possible a perfect un-throttling operation, and a fuel consumption equal to a diesel engine is achieved. In the output of the DISC engine, a stoichmetric combustion and a high torque are achieved by controlling the fuel injection timing with an electro-magnetic injection device. With regard to emission regulations, a heavy EGR including residual gas greatly decreases NOx and HC emissions simultaneously.
L Introduction
In the case of a direct injection stratified charge (DISC) engine, the load is controlled by the amount of injection fuel instead of by induction throttling. The DISC engine is able to improve the thermal efficiency significantly, especially at light loads, for the following four reasons: (1) decrease of pumping loss, (2) improvement of cyclic efficiency by being close to the air cycle, (3) decrease of heat loss because of low mean temperature in the cylinder, (4) high compression ratio standing a preventing knocking. Many types of DISC engine have been developed so far. The MAN-FM type developed in Germany and the TCCS type developed by Texaco are well known as high completion DISC engines, which are shown in Fig. 1 [1-31. Kono et al. have paid attention to realizing certain ignition and stable combustion under wide operating conditions, and proposed a DI stratified charge and stable combustion method [4,5]. The spark plug is located at the center of the cylinder in order to increase the air consumption rate in the cylinder, and the piston has a cavity just below the spark plug in order to form a stable mixture around the electrode. The piston has a cavity, whose shape
is known as "Caldera," which has a double structure. The center cavity has the role of obtaining certain ignition and the outer cavity has the role of a main chamber. The "Caldera" combustion method achieves stable combustion over a wide range of operating conditions for both ignition timing and injection timing, which makes it possible to select the best ignition and fuel injection timing for thermal efficiency. Fuel consumption equal to a diesel engine was achieved [5]. The operating area demanded by automotive engines is quite broad. A high torque and high horse-power at a high
Inj. N o z z l e ~
wjrl
s,,,k Plu, ~-N~k
]
l.Sprslr "~:~ll~t / 2.11ixture 3.Fleme 4.Burned ....
TCCS
Spark
Plug
MAN-FM
Fig. 1. Typical DISC engines (MAN-FM and TCCS).
0389-4304/95/$09.50 © 1995 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved
SSDI 0 3 8 9 - 4 3 0 4 ( 9 5 ) 0 0 0 3 2 - 1
JSAE9537934
364
S. Kono /JSAE Review 16 (1995) 363-368
1~
ct°r
Pulse delay generator
Oscilloscope
~
I I unsen,
~
[ I
~-'J Pipe
Motor
Engine Vessel
Fig. 2. Fuel injection system. Flash
revolution speed are demanded especially in a gasoline engine. The new combustion method satisfies emission regulations which are becoming more severe. In this study, the possibility of the "Caldera" combustion method meeting the demands of automotive-engines is investigated, setting the target that a fuel consumption equal to diesel engines and an out-put equal to gasoline engines are compatible.
2. Measurements and calculation method
2.1. Test engine A fuel injection system has to satisfy injection characteristics required for a wide range of operating conditions. The test engine has a fuel injection system as shown in Fig. 2 and a mechanical super charger. The characteristics of these devices are decided according to engine demands which are investigated by means of spray formation analyses. Table 1 shows the main engine specifications. With a fuel injection system, the mass of injected fuel, injection timing and injection pressure are controlled manually, corresponding to revolution speed and load. The super charger is operated only at high load. A piezo type pressure transducer is set at one cylinder, and pressure measurements are taken. 2.2. Analyses of mixture formation There are many factors and parameters which control D.I. stratified charge combustion, but the main consideration is how to form a mixture in the combustion chamber, especially around the spark plug electrode, and how to control the mixture formation according to engine operating conditions. Because fuel is injected into the cylinder during the compression stroke in the DISC engine, analysis of spray
Table 1 Main Engine Specifications Bore Stroke Compression ratio Injection pressure
84.5 mm 74.2 mm 11.5 10-25 MPa
light
if<
[--1
N:Bomb
Fig. 3. Spray visualizing equipment.
formation under high ambient pressure is important. The spray is observed with a constant volume vessel in which the ambient pressure is changed from atomospheric pressure to 2.0 MPa, according to the compression ratio. A direct photograph is taken at 10000 frames per second under an ambient pressure corresponding to the in-cylinder pressure at injection timing. Figure 3 shows the observing apparatus. A flash-light, which flashes at the same time as the injection start, is scattered by the spray. Observation of the spray lacks information of the vaporized mixture and fuel wetting the cavity wall. In order to supply this information and to analyze spray and mixture formation in the combustion chamber under engine operating conditions, the CFD code " K I V A " , which has been developed by Los Alamos Laboratory is applied [6]. The " K I V A " has a discrete particle model in which the spray is modelled as a set of fuel liquid particles. The movement of each particle is solved by a 3-D fluid field, taking the vaporization and combination of particles into consideration.
3. " C a l d e r a " combustion method
3.1. " Caldera" combustion chamber Keeping the mixture around the spark plug electrode for enough time for the flame kernel to grow is the key to realizing stable combustion over a wide operating range. The "Caldera" combustion chamber, as shown in Fig. 4, is proposed. This chamber is constructed with a center cavity, corresponding to a pre-combustion chamber, and an outer cavity corresponding to a main chamber. Fuel is injected from the circumference of the chamber to the center cavity, and the spark plug is located above the cavity, in order to achieve certain ignition. Mixture overflows from the center cavity and spreads over the outer cavity riding on a swirl flow. 3.2. Mixture formation analysis in "Caldera" Spray and mixture formation in the "Caldera" chamber are calculated on the assumption of actual engine operating
365
S. Kono/JSAE Reuiew 16 (1995) 363-368
ion
21°
BTDC
10° BTDC
Fig. 6. Mixture formation in conventional DISC chamber. conditions. Table 1 shows the engine specifications and operating conditions. In-cylinder flow is calculated on the assumption of a solid swirl whose ratio is 2.5 at BDC. The swirl ratio is measured by means of the impulse swirl meter method. The top figures of Fig. 5 show distribution of liquid spray at 30 ° and 25 ° BTDC. The middle figures show two cross sections of flow vector graph at 16 ° BTDC. The bottom figures show distributions of mixture concentration
30 °
BTDC
25 °
BTDC
16° BTDC
at 20 ° and 10 ° BTDC. The shaded area indicates a combustible mixture. W e found that a stable mixture stays at the electrode location during 10 ° C.A., and certain ignition and stable combustion can be fully expected. 3.3. Mixture formation in conuentional DISC chamber
Spray and mixture formation in the conventional DISC chamber, as shown in Fig. 1, are analyzed by calculations on the assumption of actual engine operating conditions. In the case of a conventional DISC engine, fuel is injected along the chamber wall in the swirl flow direction. In the case of fuel injected in the reverse direction, mixture formation is also analyzed. Table 1 shows the engine specifications and operating conditions. Figures 6 and 7 show distribution of mixture concentration in a chamber at 20 ° and 10 ° BTDC. Each cross section shows a vertical and horizontal section at the spark plug electrode position. The shaded area indicates a combustible mixture. The interval of 10 ° C.A. corresponds to the duration between ignition and a stable flame kernel forming.
~ 21 ° BTDC
10° BTDC
Fig. 5. Mixtureformation at basical operated condition.
21° BTDC
tion
lO ° BTDC
Fig. 7. Mixture formation in case of reverse injection direction.
S. Kono /JSAE Review 16 (1995) 363-368
366
E ~ 20 I 8~ i-
_m
~
'
15
60 "[
so 8
40 ._~
g~
~
30
m
20
8O 60
'
350
"~
~
40 20
300
250
t6" BTDC
10" BTDC
0LL (O
Fig. 8. Mixture formation at idling condition.
DISC 200
150 2
f
I
3
4 5 IMEP (MPa)
I 6X10 -1
Fig. 10. Characteristics of fuel consumption.
1 30 ° BTDU
10" BTDC
Fig. 9. Mixture formation at high-load condition (5000 rpm).
Comparing these two types, the reverse type has a better mixture formation at 20 ° BTDC than the conventional type. In both types, though, a good mixture cannot be achieved at 10° BTDC. It is difficult for the mixture to remain during 10° C.A. at the electrode location. These types of engine have to control ignition and fuel injection timing exactly or ignite with a strong spark device.
effect is calculated, over a wide range of operating conditions, which are summarized in Table 2. Figure 8 shows the distribution of mixture concentration under idling conditions. Between 16° and 10° BTDC, a good mixture remains in the center cavity. Stable ignition and combustion can therefore be expected.
3.5. Mixture formation at high revolution speed Figure 9 shows the distribution of mixture concentration at 5000 rpm and high load. After ignition timing, sufficient mixture forms at the electrode location. Mixture is badly distributed at the cylinder head, because of ascending flow caused by the piston movement. Mixture is quenched in the piston cavity by squish flow which is favorable to knocking, but is unfavorable to achieving high torque equal to a homogeneous charge. The fuel vaporizing ratio is quite low (from 50 to 70%) in the calculations. In the actual engine, fuel vaporizes much more and the distribution of mixture is more close to a homogeneous charge.
4. "Caldera" DISC engine's performance
3.4. Mixture formation at idling
4.1. Fuel consumption characteristics
The "Caldera" chamber has a significant effect on stable and stratified charging at the electrode location. The
Figure 10 compares the indicated specific fuel consumption (ISFC) at 1500 rpm, between a "Caldera" DISC
Table 2 Calculating (operating) conditions Figs. 5 - 7 Fig. 8 Fig. 9
Reverse speed
Mass of fuel
Injection time
Injection speed
~/c
Swirl ratio
1500 rpm 700 5000
15 mm3/str 5 50
40 ° BTDC 30 o 170 °
105 m / s 90 120
85% 80 140
2.5 2.5 2.5
S. Kono /JSAE Review 16 (1995) 363-368
engine, whose specifications are shown in Table 1, and a port injection engine, operating under stoichiometric conditions, which has the same displacement as the DISC engine. Fuel injection timings, ignition timings, the 10-90 %mass burning duration, and the air/fuel ratio are also shown in Fig. 10. Stable combustion is obtained even at low load under un-throttling conditions. The ISFC does not increase at low load. On the contrary, the ISFC slightly increases at middle load, because combustion deteriorates which is caused by the combustion chamber geometry which causes a two step combustion, and by poor spray mixing. However, for NO x emission reduction, slightly slower combustion is desirable. The A / F ratio varies between 30 and 80 at this operating area. At low load, more than 25% reduction of ISFC is obtained, so that the initial target that fuel consumption is equal to a diesel engine is achieved. 4.2. Max torque characteristics
Figure 11 shows the maximum indicated mean effective pressure of the DISC engine at 1000-5000 rpm, compared with a port injection engine which has the same displacement and no super charger. The A / F ratio is set at stoichometry in the case of the DISC engine. In the case of a port injection engine, the A / F ratio is about 13. IMEP of A = 1 of the port injection engine is also compared. The volumetric efficiency in both engine and injection timing of the DISC engine are plotted in Fig. 11. The DISC engine has a volumetric efficiency 1.5 times the port injection and achieves a torque which meets volumetric efficiency. The in-cylinder mixture formation is made close to stoichmetric by early fuel injection. At low revolution speed, the A / F ratio is set about 18 to prevent soot generating, and sufficient torque is not
367
~
80
o t~ rr rr (.9 tu
40
60
20 40 f
~
W/0-EGR W/-EGR
0
OL
I 1
I 2
I L 3 4 IMEP (MPa)
I
5X10 -I
Fig. 12. Characteristics of emissions.
obtained. The torque is estimated at 1.0 [B.u.] soot level. Because soot is not detected over 2000 rpm, soot generates under poor spray mixing and rich mixture combustion at a low revolution speed. 4.3. Emission characteristics
In this study, the effects of a heavy EGR which is composed of residual gas and cold EGR are investigated in order to decrease both NO x and HC. The effects at low load especially are studied, because CO: concentration is very low and and the effect of EGR is small. A heavy EGR decreases the total mass of emission and HC emission, because of increasing in-cylinder temperature, and NO x emission because of EGR effects which are well known. Figure 12 shows the test results. When the EGR ratio goes above a certain value, combustion deteriorates and HC emission increases. At a demand value greater than the EGR ratio, NO x emission decreases to below 1 g / K w h , reducing the HC emission.
200 [
150I-j
L~ 1°°t .~
5. Conclusions
5010L
1 ~o DISC 100
XlO -1
Port_..=~._.~lnjection
~ 12 ~ to ~
~50
DISC (2,=1)
16
8 6 I
1
I
I
I
r
I
2 3 4 5 6X103 Engine Revolution Speed (rpm)
Fig. 11. Characteristics of max. torque.
"~
>
The spray and mixture formation in the combustion chamber of DISC engines, which are most important in realizing a DISC engine, were studied by means of observation and computing analyses. The result shows that the new combustion method known as "Caldera" realizes certain ignition and stable combustion. The possibility of the "Caldera" combustion method meeting the demands of automotive engines was investigated, and the following conclusions were obtained: (1) The "Caldera" chamber keeps the mixture in the center cavity around the spark plug electrode during the time (10 ° CA), when the flame kernel grows, and achieves stable ignition and combustion with conventional spark plugs. Computing analyses suggest that it is difficult for
368
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conventional DISC engines to achieve stable ignition and combustion. (2) The "Caldera" combustion chamber realizes unthrottling operation perfectly over a wide range of operating conditions. (3) The ISFC is improved by more than 25% at low load, compared with a port injection engine which has the same displacement and is operated at stoichmetric conditions. (4) Early fuel injection at high load and high revolution speed makes it possible to operate at stoichmetric condition, and to get sufficient torque corresponding to volumetric efficiency. (5) Heavy EGR decreases emissions, especially NOx emissions significantly at low load. The NO x emission decreases to below 1 g/Kwh, reducing HC emissions. (6) Sufficient torque cannot be achieved because of soot generation at low revolution speed. Improvements of spray mixing remain a problem. References [1] Dictionary of Automotive Engineering (in Japanese), JSAE, p. 61. [2] Lewis, J.M., UPS Multifuel Stratified Charge Engine Development Program-Field Test, SAE Paper No. 860067, 1986. [3] Schapertons, H. et al., VW's Gasoline Direct Injection (GDI) Research Engine, SAE Paper No. 910054, 1991. [4] Kono, S., Action between Spray and In-Cylinder Flow in D.I. Engines (in Japanese), JSAE, Vol. 22, No. 2, April 1991. [5] Kono, S. et al., Development of Stratified Charge and Stable Combustion in D.I. Engines (in Japanese), JSAE, Vol. 24, No. 3, July 1993. [6] Amsden, A.A. et al., KIVA: A Computer Program for Two- and Three-Dimensional Fluid Flows with Chemical Reactions and Fuel Sprays, Los Alamos National Laboratory Report LA-10245-MS, 1985.
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