Noise Optimization of Diesel Engines with New Combustion Systems 2009-01-2081

The Euro 6 emission standard requires a strong reduction of NOx and soot emissions for future Diesel engines. One of the ways to reach the Euro 6 standard for new Diesel engines is to adopt the low NOx combustion concept with new injection strategy, but this kind of combustion can give higher combustion noise and worse stability in transient conditions. This paper describes some of the new methodologies developed by Renault for controlling and optimizing Diesel combustion noise, particularly for engines with low NOx combustion modes.

In steady working conditions, it was found that a homogeneous combustion mode gave high level of combustion excitation particularly in the 1000 Hz octave band. Thus improvement should be made in the engine structural attenuation (SA) in this frequency range in order to limit engine noise deterioration. This requires not only the technical solutions for improving the structural attenuation, but also reliable methods for measuring engine SA. By using an innovative method based on synchronous Wiener filtering, engine SA can be measured in a fast and accurate way for frequency ranges above 800 Hz, but in the low frequency range, the accuracy is not always satisfactory. For this reason, a new technique has been implemented in the measurement process which enables good results to be obtained even in the low frequency range. Technical solutions for improving engine SA can then be evaluated accurately.

For Diesel combustion noise in transient driving conditions, a new measurement method and a new indicator have been developed which enable engine transient combustion noise to be quantified in repeatable conditions. Firstly, several representative driving conditions have been standardized, and secondly a new criterion has been defined which takes into account the effect of mechanical noise on the perception of combustion noise. In this way, transient combustion noise of any new engine can be controlled and compared to existing engines, and therefore improvement can be achieved during the early stages of the engine development process.