Distillation Adjustment: An Innovative Step to Gasoline Reformulation 910382
Four vehicle emissions test programs demonstrated the beneficial effects of modifying gasoline distillation characteristics on vehicle emissions. The initial program tested two modern vehicles with a wide variety of fuels, including the testing of individual fuel blending components as if they were gasolines. A second program tested five modern vehicles with a matrix of fuels which varied aromatics and distillation characteristics independently. The third program tested two premium unleaded gasolines in 20 vehicles with a variety of emissions control system. One of the fuels represented an average production gasoline, and the other was a reformulated gasoline designed to produce lower emissions. The fourth program tested some of the same fuels and an eight-vehicle subset from the third program at an independent laboratory. Besides exhaust emissions testing, the independent laboratory tested these vehicles to evaluate the effect of reformulated gasoline on evaporative emissions.
All of the results indicate that a gasoline's distillation characteristics have a large impact on exhaust hydrocarbon (HC) emissions. Decreasing the overall distillation characteristics of a gasoline (that is lowering the temperatures at which all portions of the gasoline vaporize) decreases HC emissions. The Driveability Index (1)* sufficiently describes the overall distillation characteristics of a gasoline and correlates well with the exhaust hydrocarbon changes indicated in these programs. The largest impact of modifying a gasoline's distillation characteristics is apparent in the hydrocarbon emissions in the first phase (cold transient) of EPA's Federal Test Procedure.
THE QUEST FOR LOWER VEHICLE EMISSIONS has necessitated the redesign of vehicles and gasolines for more than 20 years. This quest will certainly continue because tighter fuel and vehicle emissions specifications have been promulgated in the new Clean Air Act and in new California regulations. Achieving lower emissions will require an ongoing joint effort between the auto and oil industries to redesign their products to form an optimum fuel-vehicle system.
In the short term, however, a number of steps have been made to modify some gasolines to produce lower emissions in present day vehicles. These gasoline modifications have come to be known as gasoline reformulations. These early reformulations usually have involved lower Reid Vapor Pressure (RVP) and the addition of oxygenates to the fuels. The major impact of these modifications is to reduce evaporative hydrocarbon (HC) emissions and to reduce carbon monoxide (CO) emissions, respectively. Other reformulations have also reduced benzene emissions by reducing fuel aromatics and benzene concentrations.
Thus far, these reformulations have been generally made in a single unleaded grade of gasoline. There are a number of factors which have limited the oil industry's ability to make these modifications to all of its gasoline grades. Some of these factors include the supply of oxygenates, the necessary processing equipment to make particular components, and the economics of displacing butane from the gasoline pool.
Besides reducing aromatics and benzene, adding oxygenates, and decreasing the RVP, one additional step in gasoline reformulation has recently been introduced. This step involves reducing the overall distillation characteristics of a fuel, which reduces a vehicle's exhaust HC emissions.
Decreasing a gasoline's overall distillation characteristics (making gasoline more volatile throughout its entire distillation range) improves the ability of an engine to vaporize the fuel, especially during a cold start and warm-up operation.
The more readily the fuel vaporizes, the more the air-fuel mixture can become homogeneous and the more likely the complete mixture will be in its flammability region. The more homogeneous and flammable the mixture is, the better the chances for complete and efficient combustion. If the combustion is not complete, the level of unburned exhaust HC is greatly increased. Therefore, improving the chances of gasoline vaporization by decreasing the overall distillation characteristics of the gasoline should decrease exhaust hydrocarbons. This effect should be most noticeable during a cold start and during warm-up operation, when the vaporization of the fuel is the most difficult.
There are many ways to describe the overall distillation characteristics of a gasoline, and a number of different equations have been developed. Most of these equations have been created to relate a gasoline's distillation characteristics to how smoothly a vehicle responds to throttle changes during a vehicle's warm-up period. The most recent equation defines a Driveability Index (DI) as 1.5 × T10 + 3.0 × T50 + T90, where T10, T50, and T90 are the temperatures at which 10%, 50%, and 90% of a gasoline evaporates in the ASTM D 86 procedure (1). This parameter was used in this analysis to correlate exhaust HC reductions to the overall distillation characteristics of a gasoline.