Testing a Third Derivative Knock Indicator on a Production Engine

In a previous paper, a knock indicator based on the third derivative of the cylinder pressure trace has been developed. This knock indicator measures the rate at which pressure trace curvature changes from positive to negative during the knock peak. Since it is dealing only with the general shape of the pressure trace, it gives a measurement of combustion severity based on low frequency data such as is commonly used for engine cycle analysis. This low frequency sampling makes it useful for adding knock analysis to existing engine analysis programs without changes of equipment or significant increases in computational effort. It may also make it useful for future on-board engine controls using limited frequency response or heavily filtered pressure transducers.

Results of using this knock indicator as a diagnostic tool on a multi-cylinder engine with a spark plug-mounted transducer are presented. It is shown that the third derivative knock indicator is a consistent pressure record knock diagnostic comparable with other suggested knock measurement methods, particularly considering the problems encountered with pressure trace distortion and resonance caused by a spark plug-mounted transducer.