The present work aims at developing and setting up a methodology in which non-intrusive measurements (engine block vibration) are used for monitoring combustion characteristics (combustion diagnosis, combustion development).
The engine block vibration appears as a very complex signal in which different sources can be identified, since every moving component or physical process involved in the operation of the engine produces a vibration signal (exhaust valve open/close, inlet valve open/close, fuel injection, combustion, piston slap). Aimed at monitoring the engine running condition, the information carried by the vibration signal has to be broken down into its various contributions and then they have to be related to their respective excitation sources.
Concerning combustion-induced vibration, experimental measures has been at first devoted to the selection of the best location where to place the piezoelectric accelerometer. Such a location has to guarantee the highest sensitivity as regards the combustion structural contribution (due to suddenly applied pressure forces inside the cylinder when combustion process takes place) in the overall engine block vibration signal. A bounded frequency range has been determined, in which high correlation values characterize the relation between the vibration signal and the in-cylinder pressure.
A second series of experimental tests has been carried out with the aim of extending the obtained results to the engine complete operative field.
In the paper, the experimental apparatus is at first described; then, the time and frequency domain processing technique is presented and representative results of the investigation are reported and discussed (different values of engine speed and load conditions have been imposed).