Utilising the resonance frequency of the engine vibration sensor in diagnostics of the exhaust valve leakage

A vibroacoustic signal of an internal combustion engine contains several diagnostic information, which are -for the most part - not utilised since they require complex processing methods in order to separate useful data and to eliminate disturbances. The way of analysis ofsuch complex signal depends on the diagnostic aim. It is a rich source of information not only concerning the combustion process, but also various mechanical defects occurring in the driving system (valves, clutch, gearbox). Wear of elements as well as small mechanical defects cause the adaptation of the control system to new parameters. Thus, in many cases these adaptive systems can be masking mechanical defects. Burning out of exhaust valves or an improper valve clearance are examples of such defects. The method of diagnostics of leakages in a system: exhaust valve - cylinder, on the basis of measuring the vibration signal of the engine head is proposed in the paper. The results of the analysis of the engine vibration signal under conditions of a defected exhaust valve and an increased valve clearance are presented. The active diagnostic experiment was performed on the engine of Fiat Punto 1.2. During road tests the vibrations acceleration signal was recorded as well as certain auxiliary signals used for a synchronisation and identification of engine timing. The natural acceleration of vibrations in the resonance frequency range of the piezoelectric sensor was utilized. The vibration signal was pre-processed i.e. filtered, synchronously averaged and windowed. Satisfying diagnostic results of the valve defect or the improper valve clearance were obtained on the basis of the analysis of the engine vibration acceleration signal envelope. The signal measures explicitly indicating the valve condition were determined. The method, after certain modifications, can be applied for the on-line valve diagnostics.