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This paper develops an improved empirical autoignition model that includes the cool-flame phenomenon. It is calibrated for primary reference fuel (PRF) blends from 0 to 100 octane and also blends of methanol with 80 PRF. Methanol does not exhibit a cool-flame and hence the blends provide insight regarding the probable blending profile of multi-component gasolines and future synthetic fuels. The model was calibrated using ∼1500 detailed chemical kinetic simulations mapping a wide pressure, temperature and air-fuel ratio domain. Besides being a computationally elegant autoignition predictor for engine simulations, the model also provides a technically defensible structure for encapsulating experimental data from autoignition research devices such as rapid-compression machines.

An adaptation of the procedure originally developed by Twu and Coon for blend octane prediction is described. The technique is based on a graded index describing an aspect of the negative temperature coefficient (NTC) autoignition behaviour of a fuel. It is further postulated that the fuel's NTC behaviour can be linked to the transition state activation energy barriers involved in the first internal hydrogen abstraction by the alkylperoxy free radical. Density-functional theory (DFT) calculations were employed to assess this hypothesis and the results were able to explain the difference between the ignition behaviour of a number of selected fuel components. The calculated NTC assignments, which were directionally consistent with the DFT results, were used successfully to determine the blend octane rating.

The knock-limited spark advance (KLSA) data for various engines and fuels were analysed using a comprehensive engine model to simulate the pressure-temperature history of the end-gas. Regression techniques were used to match the engine data with a three-stage Arrhenius model of the fuel ignition delay and to deduce parametric information regarding the behavioural characteristics of the system. The validity of the analysis results was cross-checked by classifying the fuels in terms of linear paraffins, iso-paraffins, olefins, aromatics or alcohols and subjecting specific examples of these classes of fuels to a detailed chemical-kinetic analysis to determine the essential characteristics of their associated auto-ignition delays. A further boundary condition for the analysis was provided by the octane numbers (RON and MON) of the fuel.

An understanding of the ignition delay behaviour of spark ignition fuels, over a wide range of temperatures and pressures, was an essential prerequisite for an ongoing pursuit to develop a fundamentally-based predictive octane model for gasoline blends. The ignition delay characteristics of certain model fuel compounds such as linear and iso-paraffins, olefins, aromatics and alcohols were investigated by means of chemical kinetic modelling, employing CHEMKIN 3.7 using detailed molecular oxidation mechanisms obtained from the literature. The complexity of these mechanisms necessitated the parallel investigation of reduced kinetic models in some of the applications. Reduced kinetic models were also used to describe the blending behaviour of selected binary combinations of the model fuels. The complex ignition delay response in the temperature/pressure domain that was predicted by the detailed kinetic analyses was reduced to a simple system of three, coupled Arrhenius equations.

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.

This paper describes the preliminary findings arising from a project to develop a blend-property model for gasoline. An engine model was used in conjunction with various auto-ignition models, including the Shell model, to analyze engine test results and the ASTM guide curves that link octane number to compression ratio. Good correlation for both RON and MON was achieved for 40

The Sasol Slurry Phase Distillate (SPD) process provides an opportunity to convert the world's abundant natural gas reserves into a conventional liquid fuel that is easily transportable and marketable. The high quality diesel produced by the Sasol SPD process could either be used on its own or as a blending component. Blending Sasol SPD diesel with crude oil derived diesel will improve the properties of the crude oil derived diesel so that it can meet the more stringent, environment and engine technology driven, diesel quality and emissions specifications. The properties of the Sasol SPD diesel and blends of Sasol SPD diesel and an on-highway, 2D-grade diesel fuel from the USA were compared to current and proposed specifications for high quality diesel which included CARB and Premium Diesel specifications from the USA. The compatibility of the Sasol SPD diesel with various elastomers was found to be similar to other low aromatic diesel fuels.

Transient emission tests were performed to compare emissions using fuels produced by the Sasol Slurry Phase Distillate Process, to those with US diesel fuels. A heavy-duty, four stroke, 1991 emission level diesel engine was used. Two variations of the Sasol Slurry Phase Distillate (SSPD) fuels were tested, along with fuels meeting the US 2-D and CARB specifications, as well as three blends comprising various concentrations of SSPD fuel in the 2-D fuel. It was found that the SSPD fuels produced significantly lower emissions than the 2-D and CARB fuels in all four regulated emission categories. The blended fuels generally reduced emissions in proportion to the amount of SSPD fuel in the blend. Tests were also performed at retarded injection timing settings with the SSPD fuel, which has a cetane number in excess of 70. It was found that a further reduction in NOx emissions could be obtained, without significantly compromising particulate emissions or specific fuel consumption.