Analysis of auto-ignition characteristics of low-alcohol/iso-octane blends using combined chemical kinetics mechanisms

Knocking combustion is mainly caused by auto-ignition of the end gas in spark-ignition (SI) engines. It not only leads to severe engine damage, but also can reduce thermal efficiency and constrain engine performance. As high-octane gasoline additives, the auto-ignition characteristics of low-alcohols, such as methanol, ethanol and n-butanol, and their blends with iso-octane fuels are analyzed under the conditions of the end gas. Moreover, the effects of what mechanism that promots and inhibits low-alcohols effects on auto-ignition characteristics of iso-octane is studied based on detailed chemical kinetics mechanisms. First, the low-alcohols mechanisms are validated and selected by comparing the predicted ignition delay times of the end gas with the corresponding experimental data under different target conditions of lower temperature and high pressure. From there, the auto-ignition characteristics of low-alcohol are analyzed to predict the knock tendency of fuels. In addition, the reactivity of low-alcohol/iso-octane blends is analyzed by comparing the CA50 contours of different fuels under various intake temperature and pressure conditions. It is found that the n-butanol and iso-octane have stronger auto-ignition characteristics than methanol and ethanol, revealing that methanol and ethanol have higher anti-knock properties. For further explaining the effect of low-alcohols on the anti-knock performance of low-alcohol/iso-octane fuels, the key reaction pathways for the low-temperature reaction of low-alcohol/iso-octane fuels is studied. Studies show that metastable products, such as aldehydes and ketones, produced by the chain propagation can delay the auto-ignition process and reduce the low temperature reactivity of blended fuels, when methanol and ethanol fuels are added to iso-octane. But the reaction rate of the system is accelerated, and the ignition time of the blended fuel is significantly earlier than that of iso-octane after the addition of n-butanol in iso-octane.