Pressure-Based Knock Measurement Issues

Paper #:
  • 2017-01-0668

Published:
  • 2017-03-28
Abstract:
The pressure oscillations that occur immediately after knock onset (the first ~200-500 µs) are shown to be primarily dominated by content at frequencies much higher than typical knock frequencies (8-20 kHz for the engine in this study). These high frequency oscillations are generated by two distinct, but inseparable sources: 1) The autoignition wave front hitting the transducer is an impulse, which, by definition, includes content at all frequencies, and 2) The impulsive impact of the autoignition front excites strong resonance oscillations at the natural frequency of the piezoelectric pressure transducer. Almost all of the typically used knock intensity metrics are focused, partially or entirely, on the time window immediately after knock onset. The high frequency noise in the first few hundred microseconds after knock onset can significantly impact single-value metrics like the maximum amplitude of pressure oscillation (MAPO), and moderately affect the integral-value methods based on average energy in either the time (AEPO) or frequency domains (AEFD). A new knock intensity metric is introduced based on the sustained oscillations, i.e. oscillations after the initial period that is adversely affected by the impulse (332 µs was used for this study). The agreement between knock intensity determined from three separate pressure transducers installed in the chamber were examined with MAPO, AEFD, and the new knock intensity metric; it was shown that the new knock intensity metric exhibits the strongest relationship between the three transducers.
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