Shahlari, A. and Ghandhi, J., "Pressure-Based Knock Measurement Issues," SAE Technical Paper 2017-01-0668, 2017, doi:10.4271/2017-01-0668.
Highly time resolved measurements of cylinder pressure acquired simultaneously from three transducers were used to investigate the nature of knocking combustion and to identify biases that the pressure measurements induce. It was shown by investigating the magnitude squared coherence (MSC) between the transducer signals that frequency content above approximately 40 kHz does not originate from a common source, i.e., it originates from noise sources. The major source of noise at higher frequency is the natural frequency of the transducer that is excited by the impulsive knock event; even if the natural frequency is above the sampling frequency it can affect the measurements by aliasing. The MSC analysis suggests that 40 kHz is the appropriate cutoff frequency for low-pass filtering the pressure signal. Knowing this, one can isolate the knock event from noise more accurately. Four time windows are identified for a knock event: (1) pressure rise due to flame propagation; (2) a rapid but resolved pressure rise that is ~50 µs in duration; (3) a transducer shock period that is highly contaminated by noise that is estimated to last ~300 µs; and (4) a sustained ringing period that is well resolved. Data during the transducer shock period need to be eliminated from all measures of knock because they do not represent the cylinder pressure - even when filtered. The commonly used maximum amplitude of pressure oscillation suffers from this problem. New procedures for knock onset and knock intensity characterization are proposed. The knock intensity metric uses the exponential decay envelope of the sustained oscillations to estimate the magnitude of the initial knock event. This metric was shown to correlate well between the different transducers.