The combustion noise generated by DI diesel engines can be clearly reduced during both steady-state and transient operation in applying a pilot injection. After optimization, a slight increase in fuel consumption is found in the upper load range. The pilot injection also tends to reduce the NOx emissions. An increase in black smoke emissions is considered to be the main drawback with pilot injection. High-speed Schlieren photographs of injection and combustion phenomena within a pressurized chamber show that the higher black smoke emissions may be due to the combustion of the main injection quantity that occurs in a mixture that is insufficiently prepared and with nearly no delay due to the pilot injection. On the basis of these findings, it is concluded that a high degree of atomization and rapid vaporization of the main injection quantity must be accomplished. To achieve these goals better, a separate injection nozzle for the pilot quantity is preferred to an injection system with a single injection nozzle, providing both the pilot and the main quantity. Therefore, rather simple injection systems with a separate pilot injector can be developed that provide a constant pilot quantity and controlled pilot injection time over the entire engine map.