Autoignition chemistry in a motored engine: An experimental and kinetic modeling study

Autoignition of isomers of pentane, hexane, and primary reference fuel mixtures 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, nonigniting 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. Improvements in the detailed kinetic reaction mechanism include better treatments of dihydroperoxide radical species and more accurate thermochemical quantities, which lead to better reverse reaction rate expressions.