Autoignition studies of C5 isomers in a motored engine

This study explores the autoignition characteristics of three C5 isomers, namely n-pentane, 2-methylbutane (iso-pentane) and 2,2-dimethylpropane (neo-pentane). These measurements are intended to enhance understanding of C5 autoignition chemistry, and provide experimental data to guide improvements to a general hydrocarbon oxidation mechanism. To that end, the autoignition behavior of these three C5 isomers was investigated in a modified CFR engine at an intake temperature of 120 °C and a fixed engine speed of 600 rpm to determine the critical compression ratio (CCR) at which hot ignition occurs. To find the critical compression ratio, the engine compression ratio (CR) was gradually increased to the point where CO in the engine exhaust rapidly decreased and significant high temperature heat release was observed, while holding equivalence ratio constant. Fundamental ignition behaviors such as the CCR and the calculated percentage of low temperature heat release (%LTHR) demonstrate the impact of chain length and methyl substitutions on ignition reactivity. The %LTHR shows a stronger two stage heat release for n-pentane than for neo-pentane observed at critical ignition conditions. In contrast, single stage heat release is observed for iso-pentane, leading to the weakest overall oxidation reactivity of the three isomers. Key reaction paths forming conjugate alkenes and C5 oxygenated species control the autoignition reactivity of n-pentane and iso-pentane within the low temperature and NTC regimes. However, neo-pentane forms no conjugate alkene due to its unique molecular structure, and instead produces iso-butene to retard its oxidation.