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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 paper studied the burn rate of fuels in the CFR engine at standard knock intensity. Burn duration was found to increase with compression ratio, and knocking pressure traces exhibited a distinct change in slope, thought to be the onset of knock. A criterion was developed to identify this knock-point. The knock-point was related to the mass fraction bunt and it was found that the mass fraction burnt at the knock-point decreases as the compression ratio decreases, to as little as 30%. It is proposed that the nature of knock in the CFR engine is unique in that a large fraction of the trapped mass participates in the autoignition. The paper also presented a functional descriptor for the mass fraction burnt and illustrated the suitability thereof through the application in an engine model.