Future developments of IDI diesel engines aim at a further reduction of the engine exhaust and noise emissions with no penalty on the fuel consumption. In this paper it is demonstrated that the combustion noise of IDI diesel engines is of great importance for the emitted total engine noise. Any reduction in combustion noise leads to an improvement in the noise behaviour of diesel engines. There is a large potential for the improvement of noise emission if the significant cycle-to-cycle variations in combustion noise can be reduced. These cyclic variations can be reduced e.g. by an optimized injection system which is part of future low emission, high efficiency combustion sytems.