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Development status of small direct Injection diesel engine at Isuzu Motors Ltd. is reviewed. There is much difficulty in combustion optimization of small DI engines, due to small combustion chamber volume, large surface to volume ratio, wide engine speed range and so on, Novel ideas in the area of injection system, combustion chamber and induction swirl were tried to solve these problems and their effects are presented here. Our prototype DI engines which adopted some of these ideas has turned out to have better fuel economy by about 15 percent, 2-3 dB(A) higher noise level than IDI and almost the same power output performance as IDI. As to exhaust emissions, they have a possibility to conform to ′86 California emission standards, in inertia weight classes up to 2625 LBS. The remaining problem areas are noise emission, durability of injection pump and cabin heater performance.

During our investigations using an ion-gap, we observed a rise of the ion-current before the injection event. We then tried to trap some portion of the fuel spray in the swirl-chamber to cause early ignition in the next cycle in order to get the effect of pilot injection. As a result, a very smooth pressure rise was attained and the objectionable knocking noise at idling was eliminated. However, we had to make compromises when taking advantage of this combustion mechanism, as there was a trade-off between noise level and HC emission.