1. INTRODUCTION Knock is an abnormal combustion phenomenon in SI engines that imposes a fundamental limit on the eficiency of SI engines [1, 2, 3]. Knock is caused by the autoignition of the end-gas ahead of the lame front and depends on the pressure and temperature history of the end-gas as well as on the anti-knock quality of the fuel. Fuel anti-knock quality has to be described by empirical measures because the fundamental autoignition chemistry cannot be adequately quantiied for practical fuels because of their complexity. It is traditionally measured by Research and Motor Octane Numbers, RON and MON, of the fuel used. The RON test is run in a single-cylinder CFR (Cooperative Fuels Research) engine at an engine speed of 600 rpm and an intake temperature of 52° C while the MON test is run at 900 rpm and with a higher intake temperature of 149°C. These tests are run in accordance with the procedures set in [4] - ASTM D2699 for RON and ASTM D2770 for MON. The octane scale is based on two parafins, n-heptane and iso-octane. Blends of these two primary components are referred to as primary reference fuels (PRF) and deine the intermediate points in the RON or MON scale. The RON or MON is the volume percent of iso-octane in the PRF. Thus a blend of 95% of iso-octane and 5% of n-heptane by volume is assigned the octane number of 95 in both RON and MON scales. A gasoline is assigned the RON (or MON) value of the PRF that matches its knock behavior in the RON (or MON) test. In fact the reining industry has developed over the past eighty years or so on the assumption that PRFs are suitable knock surrogates for practical gasolines. However, the autoignition chemistry of non-parafinic components in gasoline is different from that of PRF and RON or MON describes the anti-knock behavior of the gasoline only at the RON or MON test condition. The true anti-knock quality of a gasoline is best described by an Octane Index, OI [3, 5, 6, 7] which is deined as (1) An Alternative Method Based on Toluene/n-Heptane Surrogate Fuels for Rating the Anti-Knock Quality of Practical Gasolines Gautam Kalghatgi, Robert Head, Junseok Chang, Yoann Viollet, Hassan Babiker, and Amer Amer Saudi Aramco ABSTRACT As SI engines strive for higher eficiency they are more likely to encounter knock and fuel anti-knock quality, which is currently measured by RON and MON, becomes more important. However, the RON and MON scales are based on primary reference fuels (PRF) - mixtures of iso-octane and n-heptane - whose autoignition chemistry is signiicantly different from that of practical fuels. Hence RON or MON alone can truly characterize a gasoline for its knock behavior only at their respective test conditions. The same gasoline will match different PRF fuels at different operating conditions. The true anti-knock quality of a fuel is given by the octane index, OI = RON −KS where S = RON − MON, is the sensitivity. K depends on the pressure and temperature evolution in the unburned gas during the engine cycle and hence is different at different operating conditions and is negative in modern engines. In this paper we propose that the gasolines are ranked against toluene /n-heptane mixtures (toluene reference fuel, TRF). The gasoline is assigned a Toluene Number (TN), which is the volume percent of toluene in the TRF which matches the gasoline for knock in the CFR RON test. Since TRFs will have comparable sensitivity to gasolines for the same RON, they will have comparable OI at different engine conditions and the TN will describe the knock behavior of the gasoline reasonably well at all different conditions. The paper describes the method and shows supporting experimental data. The other advantage of such a method is that it will enable a better quantitative description of fuels, such as ethanol mixtures, with RON > 100 and hence beyond the maximum of the current octane scale. CITATION: Kalghatgi, G., Head, R., Chang, J., Viollet, Y. et al., "An Alternative Method Based on Toluene/n-Heptane Surrogate Fuels for Rating the Anti-Knock Quality of Practical Gasolines," SAE Int. J. Fuels Lubr. 7(3):2014, doi:10.4271/2014-01-2609. 2014-01-2609 Published 10/13/2014 Copyright © 2014 SAE International doi:10.4271/2014-01-2609 saefuel.saejournals.org Downloaded from SAE International by Gautam Kalghatgi, Tuesday, October 28, 2014