Analysis of the effect of direct injection of alcohol fuel on minor heat release reactions and controlled autoignition combustion

Abstract

In this study, systematic investigation was carried out on the effect of alcohol on heat release reactions during the negative-valve-overlap period in a single-cylinder direct-injection gasoline engine equipped with an air-assisted injector. The fuels used included gasoline, gasoline–ethanol blends, gasoline–methanol blends, and pure ethanol and methanol. To achieve controlled autoignition combustion, the negative-valve-overlap method was employed to trap hot exhaust gases into which fuel was injected directly. An air-assisted injector injected fuel together with a small amount of air immediately after the exhaust valves were closed. The in-cylinder pressure was recorded and then the heat release was analysed. The occurrence of the heat release reactions were identified through the in-cylinder pressure and heat release analysis. In particular, the effect of the alcohol fuels on the degree of heat release reactions during the negative-valve-overlap period was investigated. In addition, its influence on the initiation and heat release of the main combustion process was analysed. Alcohol fuels were found to produce more heat release reactions during the negative-valve-overlap period. Methanol exhibited the most pronounced heat release. This resulted in advanced autoignition and faster combustion of alcohol–gasoline blended fuels, except for pure ethanol and methanol. Gasoline and low-alcohol fuel blends were characterised by hybrid combustion of initial slow flame propagation and faster autoignition combustion later on. The autoignition combustion became dominant as the percentage of alcohol increased.