Study on the correlation characteristics of auto-ignition and knocking in high compression ratio spark ignition engine for isopropanol-butanol-ethanol (IBE) blends by dilution strategies

It is crucial to improve the thermal efficiency of engines and the application of alcohols to achieve carbon neutrality goals. However, challenges such as knock limit the use of high compression ratio engines. The dilution strategy has been proven to be an effective means of suppressing knock. In addition, to further overcome the limitation of knocking on the thermal efficiency of high compression ratio alcohol engines, the core relationship between auto-ignition and knock should be clarified. This article explores the effects of different dilution strategies and ratios (Rd) on the auto-ignition process of IBE. Meanwhile, the relationship between key parameters of auto-ignition and knock, as well as their correlation with combustion, performance, and emissions was innovatively explored. It was found that cycles with earlier auto-ignition timing occur before knock cycles, indicating that auto-ignition is a necessary condition for knock. MAPO had a negative correlation with auto-ignition timing and positive correlation with CI ratio and CI stage. Among them, the CI stage had a greater impact on MAPO than others. Regardless of Rd, CA50 and CA10-90 were highly correlated with MAPO and CI stage but cannot directly determine the auto-ignition timing and CI ratio. The MAPO and CI stages were the main causes of ITE transformation. There was no significant correlation between BSCO and BSHC emissions and MAPO, auto-ignition timing, CI ratio, and CI stage. In addition, reducing MAPO and prolonging the CI stage are effective measures to maintain low BSNOx.