Detailed kinetic models for the low-temperature auto-ignition of gasoline surrogates

In the context of the search for gasoline surrogates for kinetic modeling purpose, this paper describes a new model for the low-temperature auto-ignition of n-heptane/iso-octane/hexene/toluene blends for the different linear isomers of hexene. The model simulates satisfactory experimental results obtained in a rapid compression machine for temperatures ranging from 650 to 850 K in the case of binary and ternary mixtures including iso-octane, 1-hexene and toluene. Predictive simulations have also been performed for the autoignition of n-heptane/iso-octane/hexene/toluene quaternary mixtures: the predicted reactivity is close to that of pure iso-octane with a retarding effect when going from 1to 3-alkene. Introduction While gasoline and diesel fuel have a "near-continuous spectrum" of hydrocarbon constituents, surrogates composed of a limited number of components have to be defined in order to develop detailed kinetic models. The following strategy to develop them has been proposed [1]: “1. Feasibility. Candidates in the formula must have known detailed kinetic mechanisms. 2. Simplicity. Mainly limited for computational capabilities to normal paraffins with less than 12 carbon atoms, monocyclic paraffins with less than 8 carbon atoms, and simple aromatics such as benzene, alkyl-benzenes and naphthalene. 3. Similarity. The surrogate is required to match practical fuels on both physical and chemical properties: (i) volatility (boiling range and flash point), (ii) sooting tendency (smoking point and luminous number), (iii) combustion property (heat of combustion, flammability limits and laminar premixed mass burning rate). 4. Cost and availability.” The hydrocarbon constituents of gasoline contain from 4 to 10 atoms of carbon and can be divided in five main families, namely linear alkanes (n-paraffins, for about 10% mass in an European gasoline), branched alkanes (iso-paraffins, for about 30%), cyclic alkanes (naphtenes, for about 3%), alkenes (olefins, for about 20 %) and aromatic compounds (for about 35%) [2]. Since normal-heptane and iso-octane are primary reference fuels (PRF) for octane rating in spark-ignited internal combustion engines, the n-heptane/iso-octane blend (PRF mixture) has long been the most commonly proposed surrogate to reproduce low-temperature oxidation of gasoline (e.g. [3]). More recently, models have been proposed for a PRF/toluene blend [4,5], as well as for a n-heptane/iso-octane/ 1-pentene/toluene/methylcyclohexane mixture [6]. The low-temperature auto-ignition of a ternary iso-octane/1-hexene/toluene blend and of the related binary mixtures has also been recently investigated: while satisfactory modeling was obtained for the iso-octane/1-hexene mixture, the agreement deteriorated for the other blends [7-8]. No experimental data are available for hydrocarbons mixtures including an alkene in which the double bond is not the first one. The purpose of the present paper is to describe new models for the low-temperature auto-ignition characteristics of n-heptane/iso-octane/hexene/ toluene blends for the different linear isomers of hexene, to validate that related to 1-hexene using the existing experimental data and finally to use them to study the influence of the position of the double bond on the reactivity of n-heptane/iso-octane/hexene/ toluene mixtures representative of gasoline. Description of the model A recent mechanism for toluene from our team [9] has been added to models generated using the EXGAS system for n-heptane/iso-octane/hexene mixtures. The mechanisms generated by EXGAS for the oxidation of n-heptane and iso-octane taken individually or in mixtures have been previously tested [10]. Models for the oxidation of the different linear isomers of hexene have been recently developed by Bounaceur et al. [11]. All these models have been validated in the same temperature range as the present study. In the case of 1-hexene, the full mechanism involves 1257 species and includes 5803 reactions, while a mechanism for the oxidation of n-heptane/iso-octane/toluene would only includes 2575 reactions. It is worth noting that the presence of a double bond in alkene molecules involves an important complexity of the chemistry of low temperature oxidation. The radicals directly deriving from the ha l-0 03 76 72 4, v er si on 1 20 A pr 2 00 9 Author manuscript, published in "European Combustion Meeting 2009 (ECM 2009), Vienne : Autriche (2009)"