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The paper deals with the implementation of a diagnosis procedure able to reveal if anomalies in the combustion process of a spark ignition engine appear and then to identify the responsible cylinder. The procedure, based on the processing of the exhaust pressure signal in both time and frequency domains, has been developed aimed at overcoming the lacks others diagnostic methodologies have exhibited when the malfunction appears during some particular engine operative conditions. At first, the effectiveness of the method has been shown by considering steady-state conditions: experimental measurements have been carried out on a spark ignition engine running in a test cell by fixing different conditions of engine speed and load. In this paper the attention has been devoted to steady conditions in which the engine operating is characterized by a large amount of irregularity; engine transient conditions have been analysed, too. The results of both steady and transient tests are presented.

In automotive engines, combustion process substantially contributes to noise emission. The complex interaction between the excitation force (i.e. the cylinder pressure profile) and the characteristic response of the engine structure is responsible for engine block vibration and then for noise radiation. Aimed at obtaining a better understanding of the causes which mainly contribute to the combustion noise generation, a processing technique has been developed and set up, in which the trend of the mean frequency of the pressure trace in the cylinder is computed by using a sliding window of the signal. The analysis of such a trace retains great importance in the strategies devoted to control the combustion noise quantity and quality, since it allows to extract useful information to assess the contribution of the different phenomena which characterize the combustion process, in terms of their amplitude, frequency and time distribution.