Autoignition of pentane isomers in a spark-ignition engine

Abstract

This paper describes a study on the autoignition of three pentane isomers (n-, neo- and iso-pentane) in a Cooperative Fuel Research (CFR) engine operating at standard, ASTM knocking conditions. The Research Octane Numbers (RONs) of these three fuels are first measured and compared to historical data. Autoignition of pentane/air mixtures in the CFR engine are then simulated using a two-zone model with detailed chemical kinetics. Initial and boundary conditions for these kinetic simulations are systematically calibrated using engine simulation software. Two published, detailed kinetic mechanisms for these fuels are tested with a published NO sub-mechanism incorporated into them. Simulations using both of these mechanisms demonstrate autoignition in the engine for all three pentanes, and that residual NO promotes autoignition, as found in previous studies. Differences between these two mechanisms and the engine experiments are nonetheless observed, and these differences are consistent with those observed in simulations of published rapid compression machine (RCM) data. Comparison of the RCM and the CFR engine modelling also suggests the need for high accuracy experiments and high-fidelity models due to the significant impact that small differences in autoignition timing can potentially produce in real engines.

Introduction

There are three isomers of pentane: normal-pentane, iso-pentane (2-methyl-butane), and neo-pentane (2,2-dimethyl-propane). Pentanes are significant constituents of gasoline and particularly contribute to gasoline's required vapour pressure. Their content in gasoline typically varies from 10 to 40% by volume worldwide [1], [2]. Despite this, the combustion chemistry of pentanes has not been studied as much as that of C1–C4 alkanes or C7–C8 reference fuels. Their chemistry is often embedded in the mechanisms of longer chain alkanes without being properly validated, even though experimental data for their oxidation are available from a number of studies [3], [4], [5], [6], [7].

This paper therefore examines the autoignition of these three pentanes during a standard, octane rating test in a Cooperative Fuel Research (CFR) engine. Research Octane Numbers (RON) are first measured for the three isomers as it appears that they have only been reported previously by the American Petroleum Institute (API) more than 50 years ago [8], with more recent engine experiments being restricted to motoring studies of iso-pentane [9], [10]. The autoignition kinetics are then studied at the standard RON test conditions using low order engine modelling, and these results are also compared to modelling of published rapid compression machine (RCM) data.