A Hydrocarbon Autoignition Model for Knocking Combustion in SI Engines 971672
The comprehensive engine simulation code, WAVE, is extended to include a knock sub-model. A hydrocarbon autoignition model based on a degenerate chain-branching mechanism that constitutes the basic kinetic framework was modified and coupled with WAVE's engine thermodynamic environment for this purpose. Making use of this modified hydrocarbon autoignition model and the flow based in-cylinder heat transfer model in WAVE, the original rapid compression machine (RCM) experiments of Shell can be reproduced reasonably well. In addition, a spatially and temporally resolved end-gas thermodynamic model was developed to allow a more accurate calculation of the end-gas temperature over the combustion chamber wall. The developed end-gas thermodynamic-driven knock model further assumes the existence of a pseudo-boundary-layer temperature profile which is linearly distributed between the unburned end-gas and the wall. A properly chosen characteristic temperature inside the thermal boundary layer was used to drive the chemical kinetics of the end-gas autoignition sub-model. The developed knock model is able to give a quantitative prediction of the spontaneous oxidation of the unburned mixture ahead of the flame front during the normal flame-propagating combustion. For example, excellent agreement can be obtained between the model predictions and the knock results obtained from a single-cylinder research engine with a central-ignition chamber running on premixed primary reference fuels and air. These predictions include the spark advance for onset of knock, knock phasing and knock intensity when different primary reference fuels were used.