KNOCK IN DUAL FUEL ENGINES: A COMPARISON BETWEEN DIFFERENT TECHNIQUES FOR DETECTION AND CONTROL

In dual fuel engines operating on gas mode knock represents one of the major constraints on performance and efficiency, because it limits the maximum value of the engine compression ratio and of the boost pressure. The detection of abnormal combustion onset and the evaluation of knock intensity is therefore a crucial issue in engine development. In this work two different categories of knock detection methods, based both on frequency domain manipulations of the cylinder pressure signal and on cylinder head vibration analysis, were extensively compared through an experimental investigation carried out on a Wartsila W50DF engine. After a detailed literary review, the following three knock indicators were chosen to be examined through the experimental analysis: • maximum peak to peak value of the band-pass filtered pressure or vibration signal; • mean square value of the band-pass filtered pressure or vibration signal; • integral of the absolute value of the first derivative of band-pass filtered pressure or vibration signal. Different criteria for the identification of knocking cycles were evaluated, based on the comparison of the individual cycle knock indicator level with a constant threshold or on a statistical approach. While constant threshold approach was shown to be suitable for in cylinder pressure methods at constant engine load and speed (as for genset applications), the use of a statistical approach appeared to be mandatory for a fixed propeller pitch engine applications. Moreover the statistical approach turned out to be more reliable and robust in case of use of vibration based methods and therefore more suitable for the implementation on mass-produced engines. Finally, by means of a proper choice of filtering frequencies and of the accelerometer position, the influence of the engine transfer function on the vibration signal was remarkably reduced, thus allowing an easier and more reliable detection of knocking cycles, as well as a ranking of knocking cycles on the base of their intensity, thus paving the way to future finer engine control strategies development