Investigations into the influence of internal and external exhaust gas recirculation on the combustion stability in an optical gasoline spark ignition engine

Combustion stability is the major concern for engines operated in highly diluted conditions, particularly during the mode transition between controlled autoignition and spark ignition. In this research, studies were performed to investigate the influence of the dilution of internal exhaust gas recirculation and the dilution of external exhaust gas recirculation on early flame development and combustion stability in a single-cylinder optical engine. It is found that a higher external exhaust gas recirculation rate slows down the early flame development, which is responsible for the higher cyclic fluctuation of combustion. The cyclic variation in the normalized flame area matches well the coefficient of variation of the early flame development period, which decreases with development of the flame. The dilution of the internal exhaust gas recirculation shows a more complicated influence on the combustion than the dilution of the external exhaust gas recirculation does. Although more hot residual gas is trapped in the cylinder, the increase in the internal exhaust gas recirculation rate does not contribute to the promotion of the in-cylinder thermal state in the studied cases. However, with a moderate increase in the internal exhaust gas recirculation rate from 9.8% to 13%, faster initial flame kernel formation is observed, which benefits the combustion by accelerating the early flame development, advancing the combustion timing and stabilizing the combustion. With the internal exhaust gas recirculation rate further increased to 19.6%, a sharp decrease in the mean expansion speed of the flame front results in a retarded early flame development and a slower heat release, which leads to severe deterioration in the combustion stability. In addition, by substituting part of the external exhaust gas recirculation with internal exhaust gas recirculation, an advanced combustion timing, a shorter combustion duration and an improved combustion stability can be achieved; this implies that a higher total exhaust gas recirculation tolerance can be achieved with the same limitation of the coefficient of variation in the indicated mean effective pressure. This result can be instructive in optimizing the control strategy of the valve timing and the external exhaust gas recirculation rate during the mode transition between controlled autoignition and spark ignition.