4. EFFECT OF TWO-STAGE INJECTION ON ROHR
Figure 5 shows the effect of two-stage fuel injection on ROHR. In this figure, fuel allocation ratio was αF = 0.5 (half of fuel was supplied by the first injection). The maximum value of ROHR of IDI diesel engine increased when the injection timing was advanced up to 40°BTDC. However, it decreased when the injection timing was advanced over 40°BTDC. The maximum value of ROHR of DI diesel engine decreased when the injection timing was advanced over 20°BTDC.
Ignition timing of IDI diesel engine was advanced by an advance of the injection timing. However in DI diesel engine, ignition timing was late when the injection timing was advanced over 40｡?TDC. Diffusion of fuel in DI diesel engine was promoted because of an extension of ignition delay period. It resulted an over lean mixture at the condition of extremely long ignition delay. Therefore, when the first injection timing was very early, ignition in DI diesel engine did only occur at after the second injection.
5. EFFECT OF INJECTION PATTERN ON COMBUSTION CHARACTERISTICS
Figure 6 shows definitions of parameters for combustion characteristics. Upper part of the figure is ROHR line and lower part is injection pressure line. Ignition delay period was defined from the first injection timing to the timing that ROHR became positive. Combustion period was defined between the timings of ROHR changing to positive, and of ROHR being zero.
Figure 7 shows effect of injection timing on ignition delay, combustion period and ROHR with single-stage injection. There was no difference between IDI diesel engine and DI diesel engine on the ignition delay. The maximum value of ROHR of each engine increased when the injection timing was advanced up to 40｡?TDC. However, the maximum value of ROHR decreased when the injection timing was advanced more than 40｡?TDC.
Figure 8 shows effect of injection timing on ignition delay, combustion period and ROHR with two-stage injection. In this condition, fuel allocation ratio of the two-stage injection was 0.5. The maximum value of ROHR in IDI diesel engine increased when the injection timing was advanced up to 40｡?TDC. It decreased when the injection timing was advanced over 40｡?TDC. However, the maximum value of ROHR in DI diesel engine was slightly changed even if the injection timing was advanced. Compared with IDI diesel engine, the maximum value of ROHR on DI diesel engine was low level.
Ignition delay on IDI diesel engine with two-stage injection had similar tendency with the case of single-stage injection, but ignition delay period of DI diesel engine became longer than that of IDI diesel engine when the injection timing was advanced over 40｡?TDC.
As mentioned before, fuel concentration in the swirl chamber was high as compared with the concentration in DI diesel engine. Therefore, ignition timing in IDI diesel engine was earlier than DI diesel engine when the injection timing was advanced over 40｡?TDC. In this early stage injection, ignition in DI diesel engine occurred after the second injection and combustion period became short. Fuel concentration of IDI diesel engine was high enough to be ignited.
