Preignition chemical reactions leading to autoignition and knock in spark-ignition engines

Studies investigating the knock phenomena for different hydrocarbon fuels have been conducted. The experiments were carried out in a single-cylinder research engine using a skip-fire strategy. The engine manifold conditions were used to vary end gas conditions to map critical conditions for autoignition. The results from these studies were used to develop models for the fuel breakdown and oxidation pathways which cause autoignition. Initial experiments investigated the effect of fuel structure on autoignition. n-Butane, isobutane and mixtures of the two hydrocarbons were used as fuels in a fired engine. When using n-butane and isobutane, heat release was observed during a non-fired cycle. The heat release was attributed to the occurrence of chemical reactions during the compression process. Upon analysis of the gas samples, it was observed that during heat release, decomposition of the C4 oxygenated species predominated. For isobutane, the pathway leading to the formation of methanol was speculated to be associated with the heat release. A computational code, utilizing a detailed chemical kinetic model, postulated the chemical mechanisms responsible for the autoignition of n-butane and isobutane. Experiments were conducted to investigate the autoignition process for higher molecular weight fuels-pentane, hexane, heptane, isooctane and mixtures of n-heptane and isooctane. A motored engine approach was utilized in these studies. Using carbon monoxide formation as an indicator of the extent of chemical reactivity, results show that n-heptane and n-hexane tend to autoignite without much chemical reactivity prior to autoignition. There is a noticeable difference between straight chain fuels and their equivalent primary reference fuel mixtures with respect to (i) the inlet autoignition temperatures and (ii) the amount of chemical reactivity prior to autoignition. In-cylinder gas samples were analyzed for stable intermediates formed during the n-pentane oxidation using the motored engine technique. Results show that for n-pentane the pathways leading to the formation of dihydroperoxides is favored for product formation. There is a predominance of low temperature reactions leading to autoignition, but the presence of alkenes, at conditions just prior to autoignition, suggests that Negative Temperature Coefficient chemistry is also important.