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| Table 7.1 |
| Comparison of DISC engine combustion process parameters with Spray, Wall and Flow controlled approaches, 2000 rpm, imep =2.8 bar |
| |
Combustion parameter |
Spray controlled |
Wall controlled |
Flow controlled |
| 1 |
Injection timing, ºbtdc (approx.) |
60 |
50 -55 |
40 |
| 2 |
Ignition timing after injection ends, º CA |
3 to 5º |
35 -40 |
10 -15 |
| 3 |
Combustion duration, º CA
0 -5% heat release
0- 50%
0- 85%
85-90 % |
20 – 25
45
75-80
10
|
10
30
50-55
30
|
10
25
50-55
5
|
In the spray controlled method formation of mixture takes longer hence initial heat release rates are small and the combustion duration corresponding to 0 -5% heat release is much longer compared to the other two methods. . On the other hand in the wall controlled approach, although the fuel initially evaporates quite rapidly due to hot walls, but it takes longer to evaporate all the injected fuel Thus, the end of combustion is significantly delayed with the wall controlled method compared to other methods. The flow controlled method has the shortest combustion duration due to better mixture preparation and high intensity of fluid motion and turbulence. In practice, the production DISC engines use a combination of both the wall and air flow controlled strategies to obtain good ignition and combustion characteristics. Mitsubishi engine uses a specially designed piston cavity and tumble air motion while Toyota engine employs a specially designed piston cavity and air swirl.
Modes of DISC Engine Operation
The early DISC engines such as Ford PROCO and TCCS attempted to operate in stratified charge mode throughout the engine operation range but it proved to be unsuccessful and acceptable engine performance and emissions over the entire speed-load range could not be obtained. With mechanical injection system precise control of injection timing also could not be maintained. The current direct injection gasoline engines operate in the stratified charge mode at part loads and up to medium speeds while at high engine loads the engine are made to operate as stoichiometric homogeneous charge engines. This is controlled by fuel injection timing.
Stratified Charge Operation: Fuel is injected late in the compression stroke There is not enough time for fuel to fully mix with air and a stable charge stratification at part loads at the time of ignition is obtained. Engine operates unthrottled and very lean overall air-fuel ratios reaching up to 50:1 are used.
Homogeneous Stoichiometric Operation: Fuel is injected early in the intake stroke to allow adequate time before ignition for fuel evaporation, mixing and formation of homogeneous mixture At high loads, early fuel injection timing is used to operate the engine in stoichiometric homogeneous mode.
Homogeneous Lean Operation: Engine operates homogeneous lean in the mid-load range. This is the transition zone from stratified mode at low loads to stoichiometric operation at full engine load.
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