Autoignition of n-pentane and 1-pentene: Experimental data and kinetic modeling

Abstract

Autoignitions of n-pentane and 1-pentene are studied by rapid compression between 600 and 900 K at high pressure. Both hydrocarbons show a two-stage ignition and a negative temperature coefficient region (NTC). However, 1-pentene is less reactive. Ignition temperature limit is 50 K higher; cool flames and NTC are weaker and confined to a narrower temperature range. Chemical analyses are performed on the reacting mixture for fuel consumption and cyclic ethers. n-Pentane and 1-pentene give very different distribution patterns for cyclic ethers. 2-Methyltetrahydrofuran dominates the n-pentane pattern, whereas propyloxirane is by far the major cyclic ether formed by 1-pentene. Detailed mechanisms based on a common skeleton scheme are developed and used to simulate the experiments. They are validated for ignition delay times, cool flame intensities, and cyclic ether distributions. Good results are obtained for 1-pentene only if (1) direct addition channels of OH and HO₂ to the double bond are included and (2) if a higher rate constant for the decomposition of the hydroperoxyalkyl radicals into cyclic ethers is used when this radical is formed by direct HO₂ addition instead of isomerization of alkylperoxy radicals. The sensitivity analysis of the low-temperature scheme for 1-pentene points out that the total ignition delay time is dependent upon the competition between the decomposition channels of hydroperoxyalkyl radical into the branching sequence and into alkenes. The cool flame delay time is less sensitive but depends mainly upon the decomposition rate of unsaturated ketohydroperoxides.