This study examines intermediate temperature heat release (ITHR) in homogeneous charge compression ignition (HCCI) engines using blends of ethanol and n-heptane. Experiments were performed over the range of 0–50% n-heptane liquid volume fractions, at equivalence ratios 0.4 and 0.5, and intake pressures from 1.4 bar to 2.2 bar. ITHR was induced in the mixtures containing predominantly ethanol through the addition of small amounts of n-heptane. After a critical threshold, additional n-heptane content yielded low temperature heat release (LTHR). A method for quantifying the amount of heat released during ITHR was developed by examining the second derivative of heat release, and this method was then used to identify trends in the engine data. The combustion process inside the engine was modeled using a single-zone HCCI model, and good qualitative agreement of pre-ignition pressure rise and heat release rate was found between experimental and modeling results using a detailed n-heptane/ethanol chemical kinetic model. The simulation results were used to identify the dominant reaction pathways contributing to ITHR, as well as to verify the chemical basis behind the quantification of the amount of ITHR in the experimental analysis. The dominant reaction pathways contributing to ITHR were found to be H-atom abstraction from n-heptane by OH and the addition of fuel radicals to O2.
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