Numerical modeling of the propagating flame and knock occurrence in spark-ignition engines

The present paper reports multidimensional numerical simulations of knock occurrence in internal combustion engines. Knock occurrence in spark-ignition engines was examined within the context of a model of autoignition of hydrocarbon/air mixture. which has been extended by including chemical reactions for the propagating flames and an extended chemical model for the cool flames. Special attention was given to the influence of the propagating flame on the autoignition onset. Knocking occurrence is the self-ignition of the end gas as a result of combined effects of the end-gas compression by the moving piston in the compression stroke and by the accelerating propagating flame and expanding combustion products. It is recognized that the autoignition onset is accompanied by an acceleration of the propagating flame that acts as an accelerating piston emanating pressure waves in the end gas. Given the initial fuel/air mixture concentration, temperature, and pressure, the developed model was used to calculate temperature, pressure, species concentration as a function of crank angle, combustion mixture, exhaust gas recycled and engine speed, and the time of the autoignition onset. The model was validated using the experimental data obtained with a Ricardo test engine and excellent agreement was achieved between the modeling predictions and the observed experimental data. In particular it was shown that the increase of the engine speed results in the decrease of the knock onset tendency, allowing the engine to operate with higher compression ratio without knocking.