Autoignition of isomers of pentane, hexane, and primary reference fuel mixture of n-heptane and iso-octane has been studied experimentally under motored engine conditions and computationally using a detailed chemical kinetic reaction mechanism. Computed and experimental results are compared and used to help understand the chemical factors leading to engine knock in spark-ignited engines. The kinetic model reproduces observed variations in critical compression ratio with fuel molecular size and structure, provides intermediate product species concentrations in good agreement with observations, and gives insights into the kinetic origins of fuel octane sensitivity. Sequential computed engine cycles were found to lead to stable, non-igniting behavior for conditions below a critical compression ratio; to unstable, oscillating but nonigniting behavior in a transition region; and eventually to ignition as the compression ratio is steadily increased. This transition is related to conditions where a negative temperature coefficient of reaction exists, which has a significant influence on octane number and fuel octane sensitivity.

JP-8 is a jet fuel widely used by the U.S. military. The military has called for JP-8 to be used in all internal combustion applications, including compression ignition engines. To understand the combustion of JP-8 in these engines, an accepted procedure is to develop combustion models of simple mixtures of hydrocarbons, called surrogates, and their components. As part of a program to develop such models, the auto-ignition behavior of n-decane in a motored engine has been investigated. In-cylinder pressure was measured to indicate the overall reactivity behavior and quantify the effects of engine operational parameters on the auto-ignition of n-decane. Additionally, exhaust gas composition was analyzed by GC/MS to identify and measure stable intermediate species to deduce the key reaction pathways leading to auto-ignition. Furthermore, a new method that uses only pressure data was proposed and developed to identify the start of combustion. By applying this method, the in-cylinder conditions for pre-ignition point were used to predict the ignition of n-decane. Based on the in-cylinder conditions of pre-ignition point, a general pre-ignition limit line for n-decane was generated, taking dilution of residual gas, equivalence ratio, and compression ratio into account. This pre-ignition limit line will be useful for predicting the pre-ignition initiation during the oxidation of n-decane. Furthermore, it is a proof of the concept "pre-ignition limit line", which may be useful when generalized for all hydrocarbons. The measured species profiles from this study may be used in future work to develop detailed and reduced kinetic models for the auto-ignition and oxidation of JP-8 surrogate fuels.

Autoignition of the five distinct isomers of hexane is studied experimentally under motored engine conditions and computationally using a detailed chemical kinetic reaction mechanism. Computed and experimental results are compared and used to help understand the chemical factors leading to engine knock in spark-ignited engines and the molecular structure factors contributing to octane rating for hydrocarbon fuels. The kinetic model reproduces observed variations in critical compression ratio with fuel structure, and it also provides intermediate and final product species concentrations in very dose agreement with observed results. In addition, the computed results provide insights into the kinetic origins of fuel octane sensitive.