The diagnostic model proposition of the engine vibration signal

Vibroacoustic signal of reciprocating engine carries diagnostic information that is not efficiently used because of complicated signal processing methods for separation of useful components and noise elimination. The measured vibrations are a mixture of periodic waves due to the rotating components and transient waves due to the reciprocating components of the engine and pressure forces. Engine vibrations may be described in several ways depend on the diagnostic purpose. For detecting non-uniform cylinder operation, for instance, the methods for rotating machines can be considered. Many transients occur in vibration signal of the combustion engine like exhaust and inlet valve operations, fuel injection, combustion, piston slap. To analyze them the time-frequency methods can be used. Diagnosing of some damages, like valve clearance changing or valve burning, can be done using the diagnostic model. This model should be rather simple and sensitive to mentioned damages. In modelling two approaches can be considered. The first one is based on the real physical parameters of the system. The second one is making the abstractive statistical model of signal with abstractive parameters. In the paper the autoregressive model is proposed. Identifying model is finding periodically time-varying coefficients of the equivalent filter. All the damages in the system make changes of the coefficients. The model presented in the paper is illustrated with the signal of spark ignition combustion engine.