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.