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.By increasing the EGR ratios, beat release rates during premixed combustion, which is characterized by rapid burning due to ignition delay and which significantly governs NOx formation, can be suppressed more efficiently than by retarding fuel injection timings. Furthermore, combined effects of EGR and supercharging achieved a considerable improvement in combustion along with a reduction in the NOx.The results show that NOx can be reduced almost in proportion to the EGR ratio increase, and that an approximately 50% NOx at a 20% EGR ratio can be achieved without deteriorating smoke and unburned hydrocarbon emissions with appropriate intake boost pressure and fuel injection parameters.