Pressure Trace Knock Measurement in a Current S.I. Production Engine

The use of cylinder pressure transducers in engine control systems will permit optimum performance under all operating conditions. Previous research has shown that it is possible to automatically detect and evaluate knocking combustion based on low frequency (1 point per crank angle degree) pressure data from research and production engines. However, the previous work was done in a single cylinder research engine and a production engine with relatively slow combustion and large knock pressure peaks.

In this study, a spark-plug-mounted pressure transducer and an in-cylinder flush mounted pressure transducer were used to monitor the combustion pressure in a modern four cylinder engine during knocking and normal full load operation over a speed range of 1800 RPM to 4000 RPM. This engine features much more rapid combustion and much smaller knock pressure peaks. Pressure trace knock detection algorithms are compared with the output of the standard knock sensor (accelerometer) mounted on the intake manifold of the engine. It was found that the third derivative knock detection algorithm is able to adequately resolve knocking cycles and to provide a measure of knock severity directly from analysis of the pressure traces without being affected by stray vibrations from other sources. Due to the narrowness of the pressure peak caused by rapid combustion, the third derivative knock indicator is calculated from higher frequency (3.3 points per crank angle degree) pressure data rather than 1 point per degree.