An Investigation of Knock in a Spark Ignition Engine Using LPG

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. Previous study has emphasized that the knock characteristics of propane, currently known, seem to be significantly influenced by a probable synergistic interaction in the end-gas between propane and the alkenes, the second important component of commercial LPG. This study was continued by considering the influence of the criteria used for determination of the knock occurrence time. The rate of heat release and the temperatures of the unburned fuel-air mixture have been estimated by a two-zone analysis of the cylinder pressure trace. Based on these estimates we have investigated the variability of knock intensity and knock onset time, as well as the correlation with pressure and end-gas temperature at the knock onset, on individual cycles basis. Thus the influence of the condition imposed for knock onset upon this correlation was emphasized. A simple model for knock prediction was also developed.