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A study has been made of an automotive direct injection diesel engine designed to reduce exhaust emissions, particularly NOx and particulates, without performance deterioration. Special emphasis has been placed on air-fuel mixing conditions controlled by the fuel injection rate, the intake swirl ratio, and the intake boost pressure. By means of increasing the injection rate, ignition delay can be shortened enough to improve particulate emissions at retarded injection timings. Enhancing the intake swirl velocity contributes to the reduction of soot emission in spite of the deterioration of NOx emission. Supercharging can favorably enhance diffusion combustion resulting in improved fuel economy for retarded injection timings and reduced emissions. As a result, a good compromise can be achieved between fuel economy and exhaust emissions by increasing the injection rate along with retarding the injection timing. Supercharging was found to be more favorable than swirl enhancement.

An experimental study has been made of a single cylinder, direct-injection diesel engine having a re-entrant combustion chamber designed to enhance combustion so as to reduce exhaust emissions. Special emphasis has been placed on controlling the inert gas concentration in the localized fuel-air mixture to lower combustion gas temperatures, thereby reduce exhaust NOx emission. For this specific purpose, an exhaust gas recirculation (EGR) system, which has been widely used in gasoline engines, was applied to the DI diesel engine to control the intake inert gas concentration. In addition, supercharging and increasing fuel injection pressure prevent the deterioration of smoke and unburned hydrocarbons and improve fuel economy, as well.