The role of additives as sensitizers for the spontaneous ignition of hydrocarbons

Abstract

The response, following rapid compression, of the spontaneous ignition of normal butane to the effect of small additions of isopropyl nitrate and di-t-butyl peroxide was studied. Comparisons were also made with the effects of ethanal and of diethyl ether, which are amongst the most readily oxidisable organic compounds. Compositions of up to 5% of the additive in the fuel were studied, which was itself in stoichiometric proportions in “air”. Supplementary studies were made of the combustion of each of these compounds in the absence of butane, and of other selected compositions in which the “additive” components were more prominent. The results relate to the improvement of cetane numbers of Diesel fuels in response to the inclusion of organic peroxides or nitrates as additives. Ignition delays in gases compressed beyond 750 K became extremely short when the peroxide or nitrate were present at less than 5% of the butane, but the oxygenated organic compounds had hardly any effect on the ignition delay when present in such small amounts, despite their far greater reactivity than normal butane. The distinction in behaviour arose because the thermally unstable compounds were able to undergo oxidative decomposition readily and release heat during the final stages of compression. This led to thermal criticality. By contrast, ethanal and diethyl ether were able only to oxidise via a degenerate chain branching reaction, which did not develop sufficiently rapidly for ignition to take place on a sub-millisecond timescale, and was not enhanced very merkedly by the interaction with normal butane. It appears that the choice of a Diesel fuel additive should be guided by the ability of the additive to maximise the initial reaction rate and overall exothermicity through a kinetic interaction between it and the primary fuel. The type of interation that takes place may be controlled by the numbers of free radicals and the nature of primary products from the additive decomposition or oxidation.