Effect of recess shape on combustion performance and knocking characteristics for a downsized gasoline rotary engine

The combustion of a downsized boosted spark ignition rotary engine (RE) is significantly affected by the rotor recess shape owing to the influence this structure has on the flame propagation and the turbulence flow. However, comprehensive assessments on both knocking and combustion performance of an RE for different recess shapes are generally lacking. Therefore, a three-dimensional dynamic simulating model coupling with the suitable turbulent model and the reduced chemical kinetic mechanism was established and validated. The numerical model was implemented for evaluating the influences of varying recess shapes on combustion performance and knocking characteristics of a downsized RE. Results showed that the end-gas auto-ignition of the RE mainly occurred in the trailing side of the elongated chamber due to the unidirectional flow field and a long distance away from spark plugs. It should be noted that the present results indicated that the knocking intensity was directly proportional to heat release rate. Moreover, the size and quantity of the vortex caused by the recess shape during the combustion process were the fundamental reason for determining the knocking and performance of the RE. Importantly, suppressing the formation of the vortexes in the elongated chamber during the combustion process could significantly reduce the knocking intensity of the RE, and then improved engine performance. In addition, under the present operating conditions, the elongated chamber structure with the leading deep recess was found to reduce the knocking intensity by 75% compared to that with the maximum knocking intensity, which increased indicated mean effective pressure and indicated thermal efficiency of the downsized RE. Therefore, considering knocking characteristics and combustion performance, the elongated chamber structure with the leading deep recess was the best choice for the RE used in boosted conditions.