This paper deals with the effect of combustion chamber shape and the role of pressurized injection in high-speed direct-injection diesel engines. First, the previously reported good performance and emission characteristics of the reentrant chamber were confirmed in a single-cylinder engine test. To obtain a better insight into this excellence, a high-speed gas-sampling method was applied to determine the local fuel-air equivalence ratios and mass fractions of substances having higher boiling points during combustion. The results showed that even at a retarded injection the reentrant chamber suppressed the outflow of gas into the clearance space from containing a lot of higher-boiling-point substances, like raw fuel and carbonaceous matter, thereby assuring a less heterogeneous state than the ordinary deep-bowl chamber. This is attributed partly to the suppressed outflow of unburnt gas from the cavity and partly to the enhanced mixing near the entrance. In addition, the effect of the pressurized injection was also studied on the same test engine using a specially designed injection system consisting of a pressure booster piston. The test results show that at a high injection pressure the particulate emission can be reduced without increasing nitricoxide emission if an adequate nozzle orifice diameter is utilized.