André Swarts

Correlating Auto-Ignition Delays And Knock-Limited Spark-Advance Data For Different Types Of Fuel

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. These data required cautious treatment because the knocking criterion which is specified for the ASTM octane test method differs fundamentally from that used in a typical KLSA test. Issues relating to high-speed knock and fuel composition are addressed in this paper and some very relevant anomalies regarding the octane rating of aromatic and olefinic fuels are explained.

Understanding the Relation Between Cetane Number and Combustion Bomb Ignition Delay Measurements

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.

An Investigation of the Ignition Delay Character of Different Fuel Components and an Assessment of Various Autoignition Modelling Approaches

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. This simplified expression was used to emulate experimental data that were obtained for the model fuels in a combustion bomb apparatus, the IQT™, as well as data from a single cylinder CFR engine under knocking conditions. A combination of the various approaches has led to new insights regarding the blending behaviour of various classes of fuel molecules in regard to their collective resistance towards autoignition. This is a critical requirement for understanding and modelling the chemical ignition delay as reflected by octane numbers.

A Further Study of Inconsistencies between Autoignition and Knock Intensity in the CFR Octane Rating Engine

Careful consideration of the development and operation of the ASTM knock detection system on the Cooperative Fuels Research (CFR) octane rating engine has shown that the pressure fluctuations, brought about by autoignition of the end-gas, do not contribute to measurement of knock intensity. The analyses of a variety of fuels at standard knock intensity revealed that knock intensity measured on the CFR engine is related to the rate of change of pressure prior to knocking and is consistent with the description and operation of, not only the original bouncing pin, but also the modern day electronic CFR knock measurement system. It was concluded that the use of octane number data to directly infer information about the autoignition behaviour of fuels should be done with caution.