Research on combustion cycle-by-cycle variations under high load in SACI mode and application potential of i-EGR coupling air dilution when fueled with methanol and gasoline

Spark assist compression ignition (SACI) is a efficient combustion mode but roughness combustion under high load limits its application. This test analyzed the combustion parameters variation coefficient and quantified the correlation between combustion parameters with spark ignition ratio under high load SACI mode for engine fueled with methanol and gasoline, further studied the potential of i-EGR combining air dilution on improving combustion performance. Results showed that fueled with methanol can reduce in-cylinder pressure and pressure rise rate, and relieve the combustion roughness. However, compared with gasoline engine, the combustion parameters was more sensitive to ignition timing and i-EGR rate, especially for combustion phase parameters like ignition delay, combustion center and so on. Introducing small amount of high-temperature exhaust gas into cylinder will induce rougher combustion, but cylinder pressure significantly reduced and combustion phase parameters fluctuated with i-EGR rate increasing. Under high i-EGR rate, methanol as an oxygenated fuel was more beneficial to maintain stable combustion. I-EGR coupling air dilution can significantly improve the economic performance under high load SACI mode, fuel economy improvements of up to about 17.4% and 19% compared to the origin point when fueled with gasoline and methanol. EGR combining air dilution can simultaneously improve BSNO X , BSTHC, and BSCO emissions.