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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.

One major noise source of diesel engines are timing gear impacts. The structure vibration excitation mechanisms of the timing gears of different 3-cylinder diesel engines are investigated. To this end, the engine timing gears are equipped with acceleration sensors for simultaneous measurements of the rotary gear motion. The peripheral acceleration component shows impacts between the different gear pairs which are due to the alternating camshaft torque and alternating fuel injection pump torque. The dependence of these excitation mechanisms on e.g. engine speed, load, start of pump delivery and valve clearence will be described. Additional findings on the impact generation are obtained by analysing many consecutive cycles of the peripheral gear acceleration. By comparing the peripheral acceleration measurements taken on the rotating gears with the structure vibration measurements on the gear bearings, the impact transfer into the engine block structure is roughly quantified.

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.

A combustion engine of a given displacement will develop less power when running on hydrogen compared to gasoline or natural gas operation due to less heating value of the fuel air mixture in the combustion chamber. This drop in output exists for external mixing of hydrogen and air prior to intake valve closure. However, external mixing does not require intricate engine modification. Without substantial investment, supercharging is an effective method to increase the output of a hydrogen engine which uses the simple technique of external mixing of hydrogen and air. AVL stationary type research engine was used to investigate the percentage gain in output and thermal efficiency, knock limited combustion air ratios, NO emission and combustion characteristics at different supercharging pressures. The performance of the supercharged hydrogen engine is also compared with that of naturally aspirated hydrogen engine.

The pressure time history in the combustion chamber of an IC-engine is seen as a sound phenomenon which - attenuated by the engine structure - is radiated as combustion noise. A distinction has been made between the direct and indirect combustion noise. The subject “transmission rate/direct combustion noise” has been investigated by the example of an air-cooled single cylinder diesel engine. To this end short-range noise intensity measurements as well as cylinder pressure measurements at several points in the combustion chamber have been carried out simultaneously. The evaluation method chosen considers the considerable cyclic fluctuations from working cycle to working cycle of the cylinder pressure level to check the applicability of a transmission rate. Within a frequency range of 0.5 to 9.5 kHz the correlation coefficients of sound intensities and cylinder pressure excitation have been determined in 500 Hz band-width for 50 successive working cycles.