Knock characteristics of commercial LPG, with 83…87% propane and 9…15% alkenes have been investigated in a spark ignition engine for heavy duty vehicles. While knock characteristics of propane have been already studied, it was considered that there is at present a lack of information concerning a possible synergistic interaction in the end gas of propane with the alkenes, the second important component of LPG. Different conclusions existing on some issues, like the correlation between knock intensity and knock onset time on an individual cycle basis, emphasizing our insufficiencies in information, were also suggestive for our study. At last, special problems resulting from the need to make best use of the recorded pressure data, like the best way to isolating the high frequency component, were also addressed. The engine was operated over a range of conditions and cylinder pressure data from several hundreds successive cycles were stored for a statistical study of correlations between knock involved parameters.
Engine knock was characterized in terms of intensity and knock occurrence crank angle (time), based on the filtered cylinder pressure data. It was found that knock characteristics of commercial LPG are very much similar to those previously found for propane. The end gas has to be compressed to a high pressure for knocking to occur, and the knock threshold pressures for LPG and propane are close. With the same knock onset crank angle and pressure, knock intensity in individual cycles can be totally different, in relation with variable knock events. Knock occurrence crank angle correlates well with the normal combustion history (50% burn crank angle, 50% burn pressure) on a mean base, and also on an individual cycle basis. For a given engine operating condition, the knock variability increases with the knock intensity. Knock intensity correlates satisfactorily with the normal combustion history (50% burn time) on a mean basis and cycles with faster heat release are likely to result in higher knock intensity. Correlation of knock intensity with the occurrence time, on an individual cycle basis, is poor, very much like in the case of pure propane; however the general trend of knock intensity increasing for earlier knock occurrence is apparent.