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.

The effects of residual gas trapping on part load performance and emissions of a spark ignition direct injection engine fuelled with wet ethanol

“…In these cases, the recompression temperatures probably enabled some level of fuel reforming, while the mean in-cylinder gas temperature during combustion was high enough to complete the THC oxidation process. As shown in a previous study [27], the reforming process transforms the fuel molecules into smaller and highly active radicals which can facilitate the oxidation reactions during combustion. Similar THC emission behaviour occurred for E80W20, where the increased water content may had contributed to the ethanol reforming process.…”

“…Alternatively, the use of negative valve overlap (NVO) through the early closure of the exhaust valves has been proposed as an effective means of trapping hot residuals and promoting HCCI and/or controlled auto-ignition (CAI) combustion. In particular, experiments have reported the formation of highly active radicals that enhance the auto-ignition process when direct injecting fuel during the NVO recompression phase [25]- [27]. This fuel reforming promoted by the hot residuals has showed no benefit in increasing the HCCI/CAI operation range with wet ethanol [16], [17].…”

Estratégias Aprisionamento De Gases Residuais Para Motores De Ignição Por Centelha Com Etanol De Alta Eficiência

“…Com o propósito de alcançar uma boa relação entre eficiência indicada e eficiência de combustão, um estudo de otimização sobre o período de NVO foi previamente conduzido para todas as cargas ensaiadas. A injeção direta foi avaliada nos períodos optimizados de NVO[17]-[19].Para os ensaios ER[20], apenas uma das válvulas de exaustão foi utilizada durante o processo de exaustão. A segunda válvula foi utilizada apenas durante o período da admissão para efetuar a re-indução de gases de escape no cilindro.…”