Ignition of alkyl nitrate/oxygen/argon mixtures in shock waves and comparisons with alkanes and amines

Abstract Nitrates are well-known to promote the ignition of diesel fuel, achieved by the formation of chain-initiating radicals. The exothermic nature of the decomposition is also known to enhance ignition. The medium- to high-temperature combustion of n -propyl, isopropyl, n -butyl, isobutyl (2,2-dimethylethyl), and isoamoxyl (2,2-dimethylpropyl) nitrate has been studied using a shock-tube. Ignition-delay times were measured at 710–1660 K behind reflected shock waves with final pressures of ∼350 kPa. Thermodynamic data ( C p , S , and H ) for all species studied, in the form of NASA polynomials, were calculated or taken from the literature. Fuel concentration was maintained at 1 vol. % with the oxygen concentrations ranging from a minimum of 2 vol. % up to a maximum of 8 vol. %, the diluent being argon. The experimental conditions were as identical as possible for all the nitrates studied to allow comparisons of the effect on ignition of variations in the molecular structure. Under these fixed conditions, a division was noted in the measured ignition-delay times between molecules differing only in the structure of the hydrocarbon “backbone,” either branched or straight-chain. In addition, ignition-delay times were affected when the functional group of the molecule was changed. Hydrocarbons and amines were examined as specific structural analogues of the nitrates, with amines having ignition-delay times intermediate between the nitrates and hydrocarbons. Branched isomers exhibited relatively longer ignition delays, attributed to the production of chain-inhibiting methyl-radicals. Under fuel-rich conditions, the ignition delay increased with molecular length. Nitrates decompose via the formation of an alkoxyl-radical intermediate. The alternative routes to decomposition for these intermediates will determine ignition delays through either chain-inhibition or propagation, under fixed experimental conditions. A combination of thermo-chemical and chemical-kinetic arguments is presented to outline the most probable decomposition pathways for the alkoxyl-radicals produced from the five nitrates examined. A scheme is presented for the decomposition of the largest, 2,2-dimethylpropoxyl radical.