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Engine-out and tailpipe exhaust, and hot soak evaporative emissions of two reformulated test gasolines and an Industry Average reference gasoline were compared in four vehicle fleets designed for progressively lower emission standards. The two reformulated gasolines included: 1) a gasoline meeting 1996 California Phase 2 regulatory requirements, and 2) a gasoline blended to the same specifications but without an oxygenated component. These two gasolines were compared with the Auto-Oil Air Quality Improvement Research Program's (AQIRP) Industry Average gasoline representing 1988 national average composition. The vehicle fleets were the AQIRP Older (1983 to 85MY) and Current (1989MY) vehicle fleets used in prior studies, and two new AQIRP test fleets, one designed to 1994 Federal Tier 1 standards and a prototype Advanced Technology fleet designed for lower emission levels of 1995 and later.

The effect of gasoline olefin composition and content on urban ozone was estimated using the Urban Airshed Model (UAM), emission measurements for a base fuel, and projected emissions for two hypothetical fuels with reduced olefin content. The projected emissions for the hypothetical fuels were developed using regressions developed from Auto/Oil Air Quality Improvement Research Program (AQIRP) Phase I testing, a vapor headspace model and other information. Ozone modeling was conducted for Los Angeles in year 2010 and Dallas-Fort Worth and New York in year 2005. When all olefins were removed from the base fuel, the light-duty vehicle contribution to peak hourly ozone was reduced by 8 to 12%. This corresponds to a projected reduction of 0.6 to 0.8% in total peak ozone from all sources. Removing only light (C5) olefins provided 67 to 78% of the peak ozone benefit from removal of all olefins.

The analysis of data from the Auto/Oil Air Quality Improvement Research Program (AQIRP) study of the effect of aromatics, MTBE, olefins, and T90 on mass exhaust emissions from current (1989) vehicles was extended to include individual vehicles during individual operating modes. The results of the modal data analysis agree with and complement results which have been reported previously by AQIRP. Beyond this, attention is focused on three fuel compositional changes where the effect on emissions shows a reversal in sign depending on the vehicle operating mode chosen.

The chemistries controlling autoignition of primary reference fuels (n-heptane/isooctane binary mixtures) and binary olefin/paraffin mixtures have been inferred from experimental motored-engine measurements. For all n-heptane/isooctane and olefin/paraffin mixtures, each component of the mixture reacted via parallel intramolecular mechanisms with the only interactions being via small labile radicals. The octane qualities of the neat components appears to be dictated not by the initial reaction rate of the fuel, but by the reaction rate of the subsequent fuel-product reactions. In contrast, the blending octane quality of a component appears to be dictated more by the rate of the initial fuel reactions. The abnormally high blending octane qualities of olefins result from them having high rates of initial fuel reaction combined with slow rates of subsequent fuel-product reactions.

In this pilot study conducted by the Auto/Oil Air Quality Improvement Research Program, engine-out and tailpipe speciated hydrocarbon emissions were obtained for three vehicles operated over the Federal Test Procedure on two different fuels, both of which were speciated. The fates of the fuel species were traced across the engine and across the catalyst, and relationships were developed between engine-out and tailpipe hydrocarbon emissions and fuel composition. These relationships allowed separating the fuel's contribution to engine-out and tailpipe hydrocarbon emissions into two parts, unreacted fuel and partial oxidation products. Specific ozone reactivities and toxic air pollutants were analyzed for both engine-out and tailpipe emissions. Vehicle-to-vehicle, fuel-to-fuel, and bag-to-bag differences have been highlighted.

Autoignition chemistries of several paraffins, olefins, and aromatics were examined in a motored engine at different engine conditions. Paraffin chemistry was dominated by “negative-temperature coefficient” (NTC) behavior which became more pronounced at lower pressures, higher temperatures, and shorter reaction times. In contrast, olefin and aromatic chemistries did not exhibit NTC behavior. Measured pressures and calculated temperatures at fired octane rating conditions showed slightly lower pressures, higher temperatures, and lower reaction times at Motor octane rating conditions when compared to Research conditions. Therefore, paraffins would have a more pronounced NTC behavior under Motor rating conditions than under Research conditions.