One of the main problems experimenters are faced with in studying the diesel engine noise is the separation between the contributions of direct exciting forces, i.e. the combustion noise, and of indirect exciting forces, i.e. the so-called mechanical noise.
In this paper an approach based on complementary techniques is presented, by which the combustion noise is investigated and separated from other vibrational phenomena in a running engine, analyzing the evolution of the time history as a function of engine speed and load, as well as the correlation between pressure cycles and noise.
In fact a combination of techniques, such as near field intensity measurements, multiple and partial coherence methods, time-domain signal processing, can be successfully applied in order to improve the required discrimination.