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The IDI diesel engine still offers a substantial development potential. One major advantage is its low fuel consumption and, hence, its low CO2 emission compared to gasoline engines. The disadvantage of its higher noise emission, however, requires particular attention in the development stage. By means of modern signal analysing and signal processing methods in combination with computer simulation methods new tools for the development of low noise Diesel engines are available. The noise emission of IDI diesel engines has on average been reduced by about 5 to 8 dBA within the last 15 years. This trend will continue further despite the introduction of more and more light weight design components. Today's IDI diesel engine is mainly dominated by high noise levels in the frequency range about 1600 to 2000 Hz. In-depth measurements show that this is generally caused by a high combustion excitation (Helmholtz-resonance) and, in addition, structure weaknesses of the crankcase.

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.

Combustion noise emitted from diesel engines is characterized by short, loud and therefore annoying peak values of the sound pressure which vary from cycle to cycle. These cyclic variations have been observed and investigated for several diesel engines; depending on the combustion system and the operating conditions, the bandwidth of these variations can reach 10 dBA at maximum. Variations in the sound pressure can generally be related to variations in the combustion excitation. Simultaneous acoustical and multi-optical fiber measurements in the combustion chamber of a DI diesel engine showed correlations between combustion noise variations and flame development. Variations within the injection system do not explain the combustion noise variations.