Numerical and experimental characterization of knock occurrence in a turbo-charged spark-ignition engine

In this paper, knock intensity is deeply studied through experimental tests carried out on a turbo-charged spark-ignition engine. The experimental methodology is based on the analysis of the pressure signals detected within the engine combustion chamber. In order to evaluate knock intensity, fast Fourier transform (FFT) and bandpass filtering techniques have been used to process the cylinder pressure values acquired in five hundred consecutive cycles. Resonance frequencies have been found at about 8.0 kHz, 13.5 kHz and 18.5 kHz. The maximum amplitude of pressure oscillations (MAPO) has been calculated for every engine cycle. In order to discriminate between knocking cycles and free knock cycles, MAPO values are compared to threshold values. These values have been determined following a statistical approach described in the paper. An index of knock intensity, that takes into account both the extent of knocking events and the cycleto- cycle variation has been introduced. Thus, at different engine operating points, the knock limited spark advance can be found. At the end, a numerical analysis of the combustion process has been carried out in order to find a relationship between the knock occurrence and the combustion chamber geometry. A 3-D computational model, based on AVL FIRE v2011 code, has been utilized. The 3-D model is able to predict the auto-ignition zones. By matching these zones and the map of mixture distribution, it is possible to predict the location of the most dangerous areas within the combustion chamber. Furthermore, comparisons of calculated and measured data provide sound information about the importance of pressure transducer position in terms of knock intensity quantifying.