21 Nov 1995

The influence of gasoline mid-range to back-end volatility on exhaust emissions

Report no. 95/61: This report is a literature review of recent published studies evaluating the effects of mid-range to back-end volatility on regulated emissions from gasoline powered vehicles. It is concluded that there are no wholly definitive data defining exactly which distillation parameters are the true causative factors in influencing tail-pipe emissions, partly due to the necessary physical constraint of a certain degree of intercorrelation between adjacent distillation parameters, e.g. T50 and T60 or E100 and E110.

There is, however, a balance of evidence suggesting that the effect is best described by parameters in the mid-range region, typically between T50 and T70, or E100 and E120. Back-end chemical compositional effects on emissions have been studied by CONCAWE and AQIRP.

CONCAWE found that changing back-end composition from aromatics to paraffins and then to olefins gave relatively small but progressive reductions in HC and CO and increases in NOx emissions from catalyst cars. Fuel Volumetric Air Demand (FVAD) i.e. kg of air per litre of fuel, was thought to describe the magnitude of transient air fuel ratio excursions and hence emission effects observed, although intercorrelation between FVAD and other fuel parameters prevented firm conclusions. Distillation effects were found to be somewhat more important than back-end compositional effects for HC and CO emissions, but for NOx the opposite was true.

The AQIRP Phase 2 Heavy Hydrocarbon study reported no strong effects of back-end chemical composition on emissions. The reason for the rather different findings of the two programmes is not clear.

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