Heat release rates due to autoignition, and their relationship to knock intensity in spark ignition engines

Abstract

Net chemical heat-release rates have been estimated experimentally, throughout the combustion, from a single-cylinder gasoline engine, running on paraffinic and aromatic fuels. These rates are compared for autoigniting and nonautoigniting cycles and, by means of a differencing procedure, the heat release rate due to autoignition found. Comparison of heat release rates in the propagating flame and in autoignition show that in knocking combustion, almost half the total energy release can occur in autoignition. Pressures were measured with transducers and gas temperatures prior to autoignition by the CARS technique. The measurements enabled volumetric autoignition heat release rates to be obtained. When plotted against the reciprocal temperature of the unburned gas just prior to autoignition, an activation temperature and Arrhenius constant were obtained for each fuel in a single global expression for the autoignition heat release rate. These constants are reexpressed in terms of the actual temperature and fuel concentration to give a more accurate kinetic representation. This is used in an analysis of the conditions necessary for the pressure wave generated at autoignition to couple with the chemical kinetics sufficiently to lead to a developing detonation. At the inception of knock, for the same temperatures and pressures, the maximum autoignition heat release rates for the paraffinic fuel are two to three times those for the aromatic fuel. Because of this, the paraffinic fuel is more prone to developing detonation than is the aromatic fuel.