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Several methods to reduce ignition delay period were tested in combination with a high pressure injection and effects on combustion improvement were examined. It was found that the reduction of ignition delay does not give so much improvement at the usual injection timing before TDC, but when the injection timing is considerably retarded or when the original ignition delay is relatively long, shortening of the ignition delay is effective to reduce pre-mixed combustion and NOx emission. Further, assuming the combustion system which conforms to the 1983 Japanese regulation as the reference system, it was found that the combination of pilot injection and high injection pressure, simultaneously reduces NOx by approximately 35% and smoke by 60-80% without worsening the fuel economy.

This paper reports on research works of ACE towards the most appropriate injection and combustion system for heavy-duty direct injection diesel engines. Selected items for the study are the effect of nozzle hole diameter, injection rate pattern, swirl ratio, and supercharging under high pressure fuel injection. According to those experimental results, the combination of over 150MPa injection pressure with controlled injection rate, smaller nozzle hole diameter, and quiescent combustion systems shows the best performance and emission. The mechanisms of the combustion improvement are discussed from the turbulent mixing viewpoint, including the results of combustion observation.