7. EFFECT OF INJECTION TIMING AND FUEL ALLOCATION RATIO ON ENGINE PERFORMANCES
Figure 11 shows the effect of injection timing and fuel allocation ratio on combustion characteristics and engine performances on IDI diesel engine. In this case, NO emission increased with the maximum value of ROHR when the injection timing was advanced up to 40｡?TDC and it was reduced when the injection timing was advanced over 40｡?TDC. Smoke emission tendency was opposite to NO emission tendency. By the two-stage injection, NO emission was greatly reduced. Smoke emission with two-stage injection increased when most of the fuel was injected by the first injection. However, smoke emission was reduced when the fuel allocation ratio was 0.25 and the first injection timing was 80｡?TDC.
Figure 12 shows the effect of injection timing and fuel allocation ratio on combustion characteristics and engine performances on DI diesel engine. The swirl ratio of DI diesel engine used in this study was low and matching of injection nozzle and piston cavity was less suitable for combustion. Therefore, smoke level and fuel consumption was very high and NO emission was very low as compared with conventional engines. With two-stage injection, smoke emission and fuel consumption increased and NO emission was reduced compared with single-stage injection. Tendencies of NO emission and the maximum value of ROHR were similar as IDI diesel engine. Ignition delay became long, when the first injection timing was advanced. There was no great difference on combustion period of DI diesel engine with two-stage injection as compared with single-stage injection. Moreover, combustion period was unrelated with injection timing and fuel allocation ratio.
8. CONCLUSIONS
The combustion characteristics of IDI and DI diesel engines with early stage injection and two-stage injection were investigated. The result are summarized as follows.
(1) When the injection timing was early, ignition in IDI diesel engine with two-stage injection was mainly controlled by the cylinder temperature even if the amount of the first injection was small.
(2) Ignition in DI diesel engine with αF = 1.00 or 0.75 occurred at before the second injection. However, with αF = 0.50 or 0.25, ignition occurred after the second injection.
Fig.10 Effect of injection timing and over all excess air ratio on ignition timing (Two-stage injection)
