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A method for determining the exceptionally high cetane number value of SasolChevron GTL diesel is described. The conventional ASTM D613 method is inadequate at such high cetane number ratings where the reproducibility exceeds ± 8 numbers. The ignition delay of a selection of primary and secondary reference fuels were modeled and characterized using a combustion bomb apparatus and this information was used to calibrate a virtual cetane engine model. CFD simulations of the combustion bomb apparatus was used to validate the calculation process using n-heptane as the reference fuel. The analytical treatment was applied to Sasol GTL diesel and the cetane number was deduced as 86.9 with a 95% confidence interval of ±1.3.

The paper presents a statistical analysis of burn rates observed in a single cylinder spark-ignition engine. Parameters in a functional description of the burn rate were correlated to fuel blend composition and engine parameters. The analysis indicated that burn rate parameters were inter-linked and did not change in isolation. The use of sound statistical experimental design and analysis proved worthwhile in revealing tendencies that may otherwise have been misinterpreted. The features of the burn rate were discussed in lieu of the engine design, blend properties and test conditions.

A recently approved method for cetane determination using the Ignition -Quality Tester (IQT™) is based on an ignition delay measurement in a combustion bomb apparatus, which is empirically correlated to cetane number. The correlation assumes that all fuels will respond to the different pressure and temperature domains of the IQT™ and the cetane test engine in the same way. This assumption was investigated at a more fundamental level by conducting IQT™ measurements at different pressure and temperature points and characterising the ignition delay of the fuel in terms of an Arrhenius autoignition model. The fuel model was combined with a mathematical model of the cetane engine and the concept was evaluated using a variety of test fuels, including the diesel cetane rating reference fuels. The analysis technique was able to accurately predict the cetane number in all cases.