Autoignition of binary fuel blends of n-heptane and C7 esters in a motored engine

Autoignition of binary fuel blends of n-heptane and C7 esters, i.e., n-heptane/methyl hexanoate blend and n-heptane/methyl 3-hexenoate blend, was studied in a modified CFR engine at an equivalence ratio of 0.25 and an intake temperature of 155 °C. Heat release analyses showed that the n-heptane/methyl hexanoate blend exhibits stronger cool flame response than the n-heptane/methyl 3-hexenoate blend within the test range of this study, suggesting that methyl 3-hexenoate is less reactive than methyl hexanoate at low temperatures due to the presence of a double bond in the aliphatic chain. Based on quantitative product analyses, major oxidation pathways of methyl 3-hexenoate and of methyl hexanoate were identified. Consistent with the existing literature, at low to intermediate temperatures, methyl hexanoate was observed to undergo paraffinic low temperature oxidation pathways with the ester functional group remaining largely intact, while methyl 3-hexenoate exhibits olefinic oxidation characteristics. Moreover, it was observed that a key feature in the oxidation of methyl 3-hexenoate is the attack of radical species to the double bond in its aliphatic chain, resulting in the formation of unsaturated esters, an epoxy ester, and an aldehyde.