Comparison of oxidation and autoignition of the two primary reference fuels by rapid compression

Abstract

New experimental data on autoignition delays and product distributions during two-stage autoignitions for the two primary reference fuels n -heptane and iso -octane (2,2,4-trimethylpentane) have been obtained by rapid compression in the low and intermediate range of temperature for enginelike conditions of stoichiometry and dilution. The lower reactivity of iso -octane has been compensated by a four times increase in pressure. A good correlation between our data and that published is obtained when the compressed charge density of the core gas is considered. Both fuels show many common features in this temperature range: a marked negative temperature coefficient region that shifts to higher temperatures as the pressure is increased and a similarity in the nature of the intermediate species. However, the importance of the cool flame zone is greater for n -heptane, and the negative temperature coefficient region extends toward higher temperatures. The evolution of the main intermediate products formed during the two-stage autoignition is presented and discussed according to a common generic mechanism that takes into account the various isomerizations of alkylperoxy radicals and scissions of the hydroperoxyalkyl radicals. Cyclic ethers are important intermediates. For both hydrocarbons, tetrahydrofurans are the major O heterocycles formed in cool flames, especially in the case of iso -octane. The observed high selectivity in the lower alkenes demonstrates the importance of β carbon-carbon scission of the hydroperoxyalkyl radicals that leads to terminal alkenes in the case of n -heptane and to methylpropene and substituted pentenes in the case of iso -octane. These channels have to be taken into account in the improvement of detailed mechanisms for good predictions of pollutants.