A comparative reactivity study of 1-alkene fuels from ethylene to 1-heptene

A comparative reactivity study of 1-alkene fuels from ethylene to 1-heptene was performed using ignition delay time (IDT) measurements from both a high-pressure shock tube and a rapid compression machine, at an equivalence ratio of 1.0 in ‘air’, at a pressure of 30 atm in the temperature range of 600 K–1300 K. At low temperatures (< 950 K), the results showed that 1-alkenes with longer carbon chains showed higher fuel reactivity, with 1-pentene being the first fuel to show negative temperature coefficient (NTC) behavior followed by 1-hexene and 1-heptene. At high temperatures (> 950 K), the experimental results showed that all of the fuels except propene showed very similar fuel reactivity, with the IDT of propene being approximately four times longer than for all of the other 1-alkenes. To analyze the experimental results, a chemistry mechanism was developed using consistent rate constants for these alkenes. At 650 K, flux analyses showed that hydroxyl radicals add to the double bond, followed by addition to molecular oxygen producing hydroxy–alkylperoxy radicals, which proceeded via the Waddington mechanism or alternate internal H-atom isomerizations in chain branching similar to those for alkanes. The major chain propagation reaction pathways that compete with chain branching pathyways mainly dproduce hydroxyl rather than hydroperoxyl radicals, which explains the less pronounced NTC behavior for larger 1-alkenes compared to their corresponding alkanes. At 1200 K, flux analyses showed that the accumulation of hydroperoxyl radicals is important for the auto-ignition of 1-alkenes from propene to 1-heptene. The rate of production of hydroperoxyl radicals for 1-alkenes from 1-butene to 1-heptene was higher than that for propene, which is due to the longer carbon chain facilitating hydroperoxyl radical formation via more efficient reaction pathways.