Block Vibration as Indicator of Combustion Behaviour in a Direct Injection Diesel Engine

In-cylinder pressure fluctuations predominantly characterize the combustion development of the premixed phase in diesel engine combustion; on the other hand, it is well known that fuel injection pressure, number of injections and related timings and quantities, strictly affect combustion phenomenology. On the basis of the relation between combustion and in-cylinder pressure fluctuations, the present work aims at understanding the possibility to recognize, based on the engine block vibration, the harmonic components due to the combustion development, in order to diagnose a possible malfunction of the injection system. Therefore, a set of experimental tests using a common rail 1929 cm3 diesel engine has been performed. In-cylinder pressure and block vibration signals were measured using a piezoelectric transducer and two accelerometers mounted on the engine block. Time-frequency analysis has been used in addition to the classical Fourier analysis to define the degree of correlation between the in-cylinder pressure, the rate of heat release and the vibration signals. The effects of the injection pressure and timing variation have been recognized on the frequency content of the vibration signal. The effectiveness of this approach has been proved simulating a real malfunctioning of the injection system.