ACOUSTIC CHARACTERISTICS OF THE ABNORMAL COMBUSTION IN A HIGH-COMPRESSION RATIO, SPARK-IGNITION ENGINE

Abstract

Engine knocking is often identified by an operator based on the combustion sound emanating from the cylinder block during engine calibration in a test cell. Such human-hearing-based acoustic knock determination is considered the most reliable real-time knock-monitoring method, and it is necessary for selecting an accurate spark timing. In this study, the characteristics of this combustion sound were investigated. To this end, a normal engine calibration experiment was conducted. The engine sound was recorded under various operating conditions by using a copper tube attached to the cylinder block and the microphone of a smartphone. The measured signals were subjected to acoustic analyses, including Fast Fourier transform, smoothing, spectral analysis, and autocorrelation, to compare the sounds recorded at the knock borderline with those recorded in the cases with significantly advanced spark timing. The results of these analyses revealed several features that distinguished the knock sound from the normal combustion sound. The knock sound exhibited distinctions in a high-frequency band between 6,000 and 8,000 Hz. These features can be used to develop effective knock classification models.