The instantaneous power carried out by the frequency components of the in-cylinder pressure results from the rate of energy released during the whole development, and in particular the first stage, of the Diesel combustion. On the other hand, the non-linear pressure fluctuations, caused by the steeped fronted waves that develop after the mixture ignition, characterize predominantly the first stage of the combustion development in Diesel engines. These pressure waves are transmitted to the engine block and then to its surface according to the block damping characteristics. The aim of the present work is then to analyze in detail the relation existing between the block vibration signal and the in-cylinder pressure, and, in this way, to monitor continously the behaviour and the quality of the combustion. Therefore, a series of experiments has been performed on a direct injection common rail Diesel engine, varying the combustion characteristics. In particular, these variations were obtained changing the main injection timing and pressure. During experiments, the in-cylinder pressure and block vibration signals were acquired, while the heat release rate was derived. The instantaneous power of the high-pass filtered portion of the acquired signals has been estimated through the S-method, and the effect of the injection parameters in both the in-cylinder pressure and the vibration signals has been recognized and related with the combustion development. A real malfunctioning of the injection system was then simulated, so that the effectiveness of the method was proved.
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