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A literature review was conducted to survey recent research on the effects of fuel properties on exhaust emissions from gasoline and diesel vehicles, on-road and off-road. Most of the literature has been published in SAE papers, although data have also been reported in other journals and government reports. A full report and database are available from the Coordinating Research Council (www.crcao.org). The review identified areas of agreement and disagreement in the literature and evaluated the adequacy of experimental design and analysis of results. Areas where additional research would be helpful in defining fuel effects are also identified. In many of the research programs carried out to evaluate the effect of new blendstocks, the fuel components were splash blended in fully formulated fuels. This approach makes it extremely difficult to determine the exact cause of the emissions benefit or debit.

This paper reports the test results from the DPF (diesel particulate filter) portion of the DECSE (Diesel Emission Control - Sulfur Effects) Phase 1 test program. The DECSE program is a joint government and industry program to study the impact of diesel fuel sulfur level on aftertreatment devices. A systematic investigation was conducted to study the effects of diesel fuel sulfur level on (1) the emissions performance and (2) the regeneration behavior of a continuously regenerating diesel particulate filter and a catalyzed diesel particulate filter. The tests were conducted on a Caterpillar 3126 engine with nominal fuel sulfur levels of 3 parts per million (ppm), 30 ppm, 150 ppm and 350 ppm.

The Coordinating Research Council conducted a program to measure the reversibility of fuel sulfur effects on emissions from California Low Emission Vehicles (LEVs). Six LEV models were tested using two non-oxygenated conventional Federal fuels with 30 and 630 ppm sulfur. The following emission test sequence was used: 30 ppm fuel to establish a baseline, 630 ppm fuel, and return to 30 ppm fuel. A series of emission tests were run after return to 30 ppm to ensure that emissions had stabilized. The effect of the driving cycle on reversibility was evaluated by using both the LA4 and US06 driving cycles for mileage accumulation between emission tests after return to 30-ppm sulfur fuel. The reversibility of sulfur effects was dependent on the vehicle, driving cycle, and the pollutant. For the test fleet as a whole most but not all of the sulfur effects were reversible.

The Coordinating Research Council conducted a program to measure the effect of fuel sulfur on emissions from California Low Emission Vehicles (LEVs). Twelve vehicles, two each from six production LEV models, were tested using low mileage as-received catalysts and catalysts aged to 100k by each vehicle manufacturer using “rapid-aging” procedures. There were seven test fuels: five conventional fuels with sulfur ranging from 30 to 630 ppm, and two California reformulated gasoline (RFG) with sulfur of 30 and 150 ppm. Reducing fuel sulfur produced statistically significant reductions in LEV fleet emissions of NMHC, NOx and CO. Comparing conventional fuel and California RFG at the same sulfur level: California RFG had lower NMHC and NOx emissions and higher CO emissions, but only some NMHC and NOx differences and none of the CO differences between conventional and California RFG were statistically significant.

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.

Exhaust emissions of three vehicles fueled with compressed natural gas (CNG) were compared with emissions of three counterpart gasoline vehicles. The natural gas vehicles were tested on four CNG fuels covering a wide range of pipeline natural gas compositions. The gasoline vehicles were tested on AQIRP Industry Average gasoline and a reformulated gasoline meeting California 1996 regulatory requirements. Nonmethane hydrocarbon (NMHC) and toxic air pollutant emissions of the CNG vehicles were about one-tenth those of their counterpart gasoline vehicles, while methane emissions were about ten times those of the gasoline vehicles. Carbon monoxide (CO) and nitrogen oxides (NOx) emissions were more variable among the three vehicle pairs. CO emissions ranged from 20 to 80% lower with CNG than with gasoline, and NOx ranged from 80% lower with CNG to equivalent to gasoline.

Exhaust and evaporative emissions from three flexible/variable fuel vehicles (FFV/VFV) were measured as the vehicles operated on E85 fuel (a mixture of 85% ethanol and 15% gasoline) or on gasoline. One vehicle was a production vehicle designed for ethanol fuels and sold in 1992-93 and the other two vehicles were prototypes which were recalibrated 1992 model year methanol FFV's. The gasolines tested were Industry Average Fuel A and a reformulated gasoline Fuel C2 that met California 1996 regulatory requirements. The gasoline component of Fuel E85 was based on the reformulated gasoline. The major findings from this three-vehicle program were that E85 reduced NOx 49% compared to Fuel A and 37% compared to Fuel C2, but increased total toxics 108% (5 mg/mi) and 255% (20 mg/mi), respectively, primarily by increasing acetaldehyde. The NOx effect was significant for both engine-out and tailpipe emissions.

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.

Effects of gasoline sulfur content on emissions were measured in a fleet of ten 1989 model year vehicles. Two ranges of sulfur content were examined. In a set of five fuels, reducing sulfur from 450 to 50 ppm, reduced fleet average tailpipe emissions of HC, NMHC and CO each by about 18%, and reduced NOx 8%. The largest effect on HC and CO emissions was observed in FTP Bag 2. This and the absence of any significant effect on engine emissions indicate that sulfur affected the performance of the catalytic converters. The response of HC and NMHC to fuel sulfur content was non-linear and increased as sulfur level was reduced. In the second set of three fuels, reducing sulfur from 50 to 10 ppm reduced HC and NMHC by 6% and CO by 10%, but had no significant effect on NOx. The effects on HC, NMHC and NOx were not significantly different from predictions based on the prior fuel set. The reduction in CO was larger than predicted.

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.

Effects of methyl tertiary-butyl ether (MTBE) and tertiary-amyl methyl ether (TAME) on emissions were compared in a fleet of ten 1989 model year vehicles. Test fuels containing 11.5 vol.% MTBE or 12.7 vol.% TAME were blended in a base fuel representing federal emission certification fuel. The oxygen content of both fuels was about 2.0 wt.%. No significant differences were found between the two fuels in exhaust mass HC, NMHC, CO, or NOx; in exhaust or evaporative toxic air pollutants, benzene, 1,3-butadiene, acetaldehyde, or total toxic emissions; or in evaporative hot soak emissions. The only differences found to be significant at the 95% level were in mass and estimated reactivity-weighted diurnal evaporative emissions, for both of which the TAME fuel was about 24% lower than the MTBE fuel; and in formaldehyde emissions, which were 28% higher with the TAME fuel.

Emission effects of gasoline hydrocarbon components distilling above 300°F were investigated to determine whether the effect of 90% distillation temperature (T90) found in an earlier Auto/Oil Program study is due to fuel distillation properties or to hydrocarbon composition, and also to determine whether the T90 effect is linear. Twenty-six fuels were tested in two sets. In Matrix A, the independent variables were catalytically cracked (FCC) and reformate stocks with nominal distillation ranges of 300 to 350, 350 to 400 and 400+°F. In Matrix B, the independent variables were a reformate stock (320 to 370°F), a heavy alkylate (330 to 475°F), and a light alkylate distilling below 300°F, which was used to vary fuel T50 at fixed levels of T90. Exhaust mass and speciation were measured using ten 1989 vehicles of the Auto/Oil Current Fleet. Tailpipe hydrocarbon emissions were found to increase nonlinearly with progressive addition of the heavier components.

Fuel effects on exhaust emissions of a sample of seven high emitting vehicles were studied. The vehicles had various mechanical problems and all ran fuel rich. The degree of enrichment varied between tests, and strongly affected mass emissions. Variable enrichment can cause incorrect apparent fuel effects to be calculated if not accounted for in data analysis. After variable enrichment was compensated for, the percentage effects of fuel oxygen, RVP, and olefins were largely in agreement with prior findings for normally emitting vehicles. Reducing fuel sulfur and T90 may have less benefit on hydrocarbon emissions in these high emitters than in normal emitters, and reducing sulfur may have less benefit on CO emissions. Reducing aromatics may be somewhat more helpful in reducing hydrocarbon and CO emissions in the high emitters.

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.

Prior studies of the effect of gasoline composition and physical properties on automotive exhaust and evaporative emissions have been reviewed. The prior work shows that the parameters selected for investigation in the Auto/Oil Air Quality Improvement Research Program (AQIRP) - gasoline aromatics content, addition of oxygenated compounds, olefins content, 90% distillation temperature, Reid vapor pressure, and sulfur content - can affect emissions. Effects have been observed on the mass of hydrocarbon, CO, and NOx emissions; on the reactivity of emissions toward ozone formation; and on the emissions of designated toxic air pollutants. The individual effects of some of the AQIRP parameters have been studied extensively in modern vehicles, but the most comprehensive studies of gasoline composition were conducted in early 1970 vehicles, and comparing the various studies shows that fuel effects can vary among vehicles with different control technology.

In this portion of the Auto/Oil Air Quality Improvement Research Program, ten 1989 model vehicles were tested using two fuels with different sulfur levels. These tests were run to determine instantaneous effects on exhaust emissions, not long-term durability effects. The high- and low-sulfur fuels contained 466 ppm and 49 ppm sulfur, respectively. Mass exhaust emissions of the fleet decreased as fuel sulfur level was reduced. Overall, HC, CO, and NOx were reduced by 16, 13, and 9 percent, respectively, when fuel sulfur level decreased. This effect appeared to be immediately reversible. Engine-out mass emissions were unaffected by changes in the fuel sulfur content, therefore, tailpipe emissions reductions were attributed to increased catalyst activity as the sulfur level was reduced.

An overview of Phase 1 of the Auto/Oil Air Quality Improvement Research Program is presented. Specific information is provided on each of the individual test fuel matrices that were conducted to investigate vehiclelfuel “system” effects on emissions. Procedures for sampling exhaust, evaporative, and running loss vehicle emissions are described, as well as techniques developed for speciation of individual hydrocarbons. Air quality models to project ozone reduction potential of reformulated gasolines and methanol, and economic studies to estimate the relative cost-effectiveness of the vehiclelfuel alternatives are also briefly explained.

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.