To program the automatic performance control loops of the modern cars at the design stage, numerical result of engine variables effecting knock intensity is the most important tool. Although general aspects of the effect of engine operating conditions on knock have been studied in previous investigations, precise numerical results of that, are not available. The procedures proposed in the past to provide the requirements of the models either predicting or measuring knock are complicated and the details are not presented.
In the current work, on the basis of the ‘sampling frequency theorem’ and in regard to the measured frequency of the knocking-pressure fluctuations, a fast on-line data acquisition system was designed and developed. The system was used on a variable compression ratio research engine using two different fuels. Large number of the engine pressure crank angle data were recorded at different operating conditions. Knock, intensity measurement was carried out using a developed criteria obtained by a careful analysis of pressure oscillations at 0.25 degree crank angle intervals. This lead to identification of cycles having specified knock intensity over a wide range of operating conditions. Effect of engine variables on knock intensity was therefore measured numerically for both fuels. Using the simple criteria to study the effect of engine operating conditions on the intensity of knock produced the results which agreed with the earlier experimental findings.