Experiments were carried out to collect in-cylinder pressure data and microphone signals from a single-cylinder test engine using spark timingsbefore, at, and after knock onset for toluene reference fuels. The objective was to gain insight into the phenomena that determine knock onset, detected by an external microphone. In particular, the study examines how the end-gas autoignition process changes as the engine's spark timing is advanced through the borderline knock limit into the engine's knocking regime.
Fast Fourier transforms (FFT) and bandpass filtering techniques were used to process the recorded cylinder pressure data to determine knock intensities for each cycle. Two characteristic pressure oscillation frequencies were detected: a peak just above 6 kHz and a range of peaks in the 15-22 kHz range. The microphone data shows that the audible knock signal has the same 6 kHz peak. At audible knock onset, cycles begin to have knock intensities (based on the maximum peak to peak amplitude of the bandpass filtered pressure signal) greater than about 2 bar at this 6 kHz frequency band. This behavior occurs in a larger fraction of cycles as the spark timing is advanced through the borderline knock limit. The peak to peak amplitudes of the pressure oscillations in the 15-22 kHz frequency band are significantly larger than those in the 6 kHz frequency band. Thus a larger fraction of cycles (the majority) show knock intensities above the 2 bar level in the higher frequency band at the audible knock detection point. At knock onset and with more intense knock, the cylinder pressure and microphone knock intensities correlate well.
Our results show that prior to knock onset, autoignition occurs in a substantial fraction of the engine's cycles, producing higher amplitude pressure oscillations at frequencies above the audible threshold, but much smaller amplitude pressure oscillations in only a small percentage of cycles at the audible 6 kHz frequency. At knock onset conditions, cycles with significantly higher knock intensities (above about 2 bar, peak to peak) at the 6 kHz frequency start to occur. These cycles directly excite engine block vibrations and produce the audible knock signal. At more intense knocking conditions, as the spark is advanced, the fraction of cycles rapidly autoigniting, their knock intensity, and the audible knock signal amplitude, all increase.