In order to predict knock onsets of multi-cylinder engines, the reduced chemical kinetic model developed by Cowart et al. was modified using this modified model, the effects of various factors of engines on knock occurrence were discussed. The modified model, of which reaction rate constant was optimized using sensitivity analysis, represents not only the knock onsets and appearance of two-stage ignition but also a steep temperature rise at auto-ignition. Using this model, the knock onsets of a 4-cylinder engine were predicted under various operating conditions of residual gas ratio, revolution speed, intake port design and fuel composition. The predictions were in good agreement with the measurements for all operating conditions. The prediction results suggest that the temperature and pressure histories of unburned gas have a great effect on the knock onset, whereas the chemical role of residual gas and the intake flow have little effect. Furthermore, the effects of the combustion period and the unburned gas temperature before spark ignition on the torque at the trace knock limit were predicted. The results show that the torque at the trace knock limit is increased by rapid combustion and decreased by the temperature rise before spark ignition. Under certain conditions, it was shown that the increase in torque for the combustion duration shortened by about 25% was negated by the temperature rise of about 15K before spark ignition.
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