Therefore, the ignition timing of IDI diesel engine in this condition (αF = 0.5) was similar with the ignition timing of the single-stage injection. As a result, combustion period of IDI diesel engine became long as compared with DI diesel engine.
6. EFFECT OF INJECTION TIMING AND EXCESS AIR RATIO ON IGNITION DELAY
Figure 9 shows injection/ignition timings diagram with the over all excess air ratio. Fuel spray for IDI diesel engine was supplied in the swirl chamber, and most of fuel spray was stayed in the chamber. The over all excess air ratio of IDI diesel engine at ignition timing was calculated from supplied fuel in the swirl chamber and the air inside the chamber at the ignition timing. Then in a case of early stage injection, the over all excess air ratio in the swirl chamber increased during a compression process because of an increase of the air inside the chamber. Fuel for DI diesel engine was supplied in the cylinder. The over all excess air ratio of DI diesel engine was calculated from fuel and all of air in the cylinder. Then the over all excess air ratio was constant after the injection. In this figure, symbol of solid circle ● shows ignition timing of IDI diesel engine and Gothic number is the over all excess air ratio at the ignition timing. Open circle ○ shows DI diesel engine's ignition timing and Italic number is the over all excess air ratio at this timing. In the figure, periods from injection timing line to ignition shows ignition delays.
As for the DI diesel engine, ignition timing was advanced when the injection timing was advanced from 10｡?TDC to 40｡?TDC. However, it was almost constant at 24｡?TDC when the injection timing was advanced over 40｡?TDC. Ignition timing of IDI diesel engine had similar tendency with that of DI diesel engine. However, the over all excess air ratio of IDI diesel engine was different from DI diesel engine. Therefore when injection timing was advanced from 10｡?TDC to 40｡?TDC, ignition timing was mainly controlled by fuel-air mixing process. It was controlled by the cylinder temperature when the injection timing was advanced over 40｡?TDC.
As for the cases of two-stage injection, injection timing and the over all excess air ratio at ignition are shown in Fig.10. Diagonal dotted line is the injection timing of Pump 1 and horizontal dotted line is the injection timing of Pump 2. Fig.10-(a) shows the results in which the 75% of fuel was injected in the first injection timing (αF = 0.75). In this figure, the over all excess air ratios were higher than those in Fig.9 because of the first injection being reduced. However, the ignition timings of IDI and DI diesel engines had similar tendencies with the cases of single-stage injections. Therefore in these cases, ignition timings were controlled by similar factors with the cases of single-stage injections.
Figure 10-(b) shows the ignition timing with half of fuel (αF = 0.50) being injected with the first injection and the over all excess air ratio at ignition. When the injection timing was changed from 10°ATDC to 30°BTDC, ignition timings of IDI and DI diesel engines were advanced. However, when the injection timing was advanced over 30°BTDC, ignition timing of DI diesel engine was late and it occurred after the second injection. This phenomenon was mainly controlled by the spray of the second injection. Ignition timing of IDI diesel engine had similar tendency with the case of single-stage injection. Therefore, when the injection timing was advanced over 40°BTDC, ignition timing of IDI diesel engine was mainly controlled by the cylinder temperature, even if the fuel allocation ratio was 0.5.
Figure 10-(c) was the results, where the quarter part of fuel (αF = 0.25) was injected with the first injection. Ignition in DI diesel engine occurred at after the second fuel injection. Therefore in this condition, ignition timing was mainly controlled by the fuel of the second injection. However, ignition timing of IDI diesel engine with αF = 0.25 was similar with the single-stage injection.
From the reason mentioned above, ignition delay of IDI diesel engine was mainly controlled by fuel-air mixing process when the injection timing was advanced from 10｡?TDC to 40｡?TDC, and mainly controlled by the cylinder temperature when the injection timing was advanced over 40｡?TDC. However, ignition timing of DI diesel engine, was mainly controlled by the over all excess air ratio when the fuel allocation ratio was low.
Fig.7 Effect of injection timing on combustion characteristics (Single injection)
Fig.8 Effect of injection timing on combustion characteristics (Two-stage injection)
