Understanding the Blending Octane Behaviour of Unsaturated Hydrocarbons: A Case Study of C4 Molecules and Comparison with Toluene

Abstract

Octane number (ON) is an important empirical parameter for developing and optimizing internal combustion engine (ICE) for knock resistance. Primary reference fuels (PRF) comprising iso-octane and n-heptane are the simplest gasoline surrogates. C4 hydrocarbons: butane isomers (n-butane and isobutane), butene isomers (1-butene, 2-butene and isobutene) and 1,3-butadiene are the smallest hydrocarbons with isomeric, saturated, unsaturated and conjugated bond structures, which makes them good candidates for understanding the blending octane behavior of saturated and unsaturated hydrocarbons. In this study, the blending octane behaviors of six PRF60 & C4 hydrocarbon mixtures were systematically investigated. Two state-of-the-art kinetic models were used by merging the latest LLNL and KAUST gasoline surrogate model with the AramcoMech 3.0 model, which contain the detailed and well-validated oxidation kinetics for PRF and all C4 hydrocarbons. Homogeneous gas-phase ignition delay times (IDTs) of stoichiometric fuel/air mixtures were simulated at wide range of pressure (20 ? 50 atm) and temperature (600 ? 1400 K) conditions. Three correlation equations are employed from the latest literature studies to predict the research octane number (RON) and motor octane number (MON) of all blends based on these calculated IDTs. Compared with the experimentally measured ON of all blends obtained from the American Society for Testing Materials (ASTM), the best correlation conditions and corresponding errors were identified for (a) each blend; (b) each kinetic model; (c) both RON and MON. With the highest degree of unsaturation in its structure, 1,3-butadiene was found to be the strongest octane number enhancer. Moreover, based on a polynomial correlation, a TPRF (PRF + toluene) blend was formulated by matching the RON and MON of PRF plus 1,3-butadiene blend, for a comparative analysis. Finally, the reactants? consumption profile, flux and sensitivity analysis were simultaneously performed for explaining the chemistry behind the blending octane behavior of the PRF blends with 1,3-butadiene and toluene.