Gasoline Fuel Economy
Energy in Gasoline
A gasoline engine is powered by the energy released when gasoline burns. Thus, it is not a surprise that gasoline containing more energy gives better fuel economy. The energy content of gasoline can be expressed in a variety of units; one of the most common is British thermal units per gallon (Btu/gal).
While anomalous results have been documented for some vehicles (see below), there is compelling evidence that the fuel economy of most vehicles tracks the energy content of the gasoline. Figure 4 shows the average laboratory fuel economy for two fleets of vehicles fueled with two different groups (matrices) of gasolines. The energy contents of the gasolines in each group were changed by varying hydrocarbon composition and oxygenate content.
That the results fall in two straight lines demonstrates that average fuel economy is proportional to energy content, and that the impact of gasoline composition on fuel economy is limited to its affect on energy content. The separation between the two lines – slightly less than 2 mpg – represents the improvement that the automobile manufacturers made in fuel economy from the 1985 model year to the 1989 model year.
Figure 4
Relationship Between Fleet Average Fuel Economy and Energy Content 7
Composition Changes
Gasoline is composed of hundreds of hydrocarbons, each with its own energy content. The energy content of the gasoline as a whole is the weighted sum of the energy contents of the component hydrocarbons. The relationship between gasoline fuel economy and energy content is advantageous because it can be used to predict how composition changes will affect fuel economy.
Replacing some hydrocarbons with others may change the energy content of a gasoline. A more significant change in energy content occurs when a gasoline is oxygenated – when some hydrocarbons are replaced by oxygenate(s). Oxygenates are combustible liquids that contain oxygen. At present, the most common oxygenates are ethanol and methyl tertiary-butyl ether (MTBE). Ethyl tertiary-butyl ether (ETBE) and tertiary-amyl methyl ether (TAME) are used to lesser extents. Because oxygenates can be viewed as being "partially burned" hydrocarbons, they have lower energy contents than hydrocarbons, and, consequently, oxygenated gasolines have lower energy contents and lower fuel economies than conventional gasolines.
An ethanol concentration of 6.0 percent by volume is needed to achieve the 2.1 percent by mass average oxygen content required by federal RFG.* Adding this amount of ethanol to conventional gasoline lowers the volumetric energy content by 2.0 percent.** In general, the percent decrease in volumetric energy content of an oxygenated gasoline is about equal to its oxygen content in units of percent by mass.
The previous section mentioned that some of the gasolines in Figure 4 contained oxygenate. Figure 5, which recasts Figure 4 to identify those gasolines, shows that the presence of MTBE (14-15 percent by volume) or ethanol (9.6 percent by volume) did not alter the relationship between fuel economy and energy content for either fleet.
* More ethanol – 10 percent by volume (3.5 percent by mass oxygen) – is often used because tax laws encourage it and an Environmental Protection Agency (EPA) waiver allows it.
** Calculated assuming energy contents of 76,000 Btu/gal. for ethanol and 114,000 Btu/gal. for conventional gasoline.
Figure 5
Oxygenate Status of Gasolines in Fuel Economy Test
Normal Variation
A modern refinery is a collection of individual plants, each designed to perform a different manufacturing process. Several of these plants produce material (a stream) suitable for use in gasoline. Because no single stream has all the required properties, finished gasoline is a blend of up to eight different streams. While all gasolines sold in a given geographical region meet the same requirements, a different combination of streams, i.e., a different recipe, may be used for different batches. There are several reasons:
- Within a refinery, the properties and availability of the blending streams change with changes in the crude oil feed and in the operating status of the plants that produce them.
- Within a refinery, different recipes are used for the different octane grades.
- Different refineries use different crude oil feeds and have different combinations of plants and blending streams.
- Summer and winter gasolines have different specifications for emissions and performance reasons.
As a result, the fuel economy of gasoline in a given market may vary from day to day and service station to service station.
The EPA collected information on the energy contents of conventional gasolines. 8 Compared to the average, the range of energy contents was -4.0 percent to +2.2 percent.* The EPA also looked at seasonal differences. The average energy content of summer gasoline was 1.8 percent higher than the average for winter gasoline.
A driver has no way of knowing whether the fuel economy of the gasoline at a specific station on a given day is average, or slightly above or below average. And because these variations are small, fuel economy measurements cannot separate them from the operating changes that also affect fuel economy.
* The source does not indicate whether the results are for one octane grade or a mix of grades.
Don't Buy a Higher-octane Gasoline to Improve Fuel Economy
Octane and energy content are not related. Premium-grade gasoline doesn't necessarily have a higher energy content, especially if it is oxygenated.
The exception to the above advice is when a lower-octane gasoline doesn't satisfy the octane requirement of the vehicle's engine. The abnormal combustion that announces itself as knocking reduces engine efficiency. Using a higher-octane gasoline that eliminates knocking will improve both engine performance and fuel economy.
Many newer vehicles with an electronic control module (ECM) also have a knock-sensor device. When the sensor detects knocking, the ECM retards the engine's ignition timing to eliminate the knocking. This happens so quickly that the driver never hears the knocking. But retarding timing decreases power and fuel economy. A higher-octane gasoline may improve the performance of knock sensor-equipped vehicles that have less power than when new.
Reformulated Gasoline
The EPA requires reformulated gasoline (RFG) in a number of ozone-nonattainment areas. Federal RFG must be oxygenated year-round to 2.1 percent by mass oxygen, average. In addition, the EPA requires gasoline in some carbon monoxide-non-attainment areas to be oxygenated during the winter months to a minimum of 2.7 percent by mass oxygen. Finally, California's Air Resources Board (CARB) requires a different RFG throughout California. California RFG must be oxygenated year-round in those areas where federal RFG also is required. A recent regulatory change eliminated the requirement for oxygenated RFG in most other areas of California.
From their effect on energy content, the oxygenates in RFG would be expected to lower fuel economy a few percent. However, RFG specifications require other compositional changes as well. The combined changes are so complex that it isn't feasible to estimate their effect on energy content. Fortunately, there are several direct comparisons of the fuel economies of RFG and conventional gasoline. They show that the average fuel economy of RFG is 2 percent to 3 percent lower.
| Road Test |
Change in Fleet Average Fuel Economy for RFG a, % |
Footnote Reference |
| California ARB |
-2.4 |
9 |
| Chevron |
None b |
10 |
| CSAA c |
-2 |
11 |
| Wisconsin DNR d |
-2.8 |
12 |
- a Compared to conventional gasoline
- b Comparison gasoline also was oxygenated
- c California State Automobile Association
- d Wisconsin Department of Natural Resources
This decrease is superimposed on the normal day-to-day and season-to-season variations in fuel economy previously discussed. However, these variations may be smaller for RFG than for conventional gasoline because there are more restrictions on the composition of RFG.
Individual Vehicle Exceptions
When RFG gasolines were introduced, some drivers complained of fuel economy decreases of 10 percent, 15 percent, and even 20 percent. In fact, these complaints were the impetus for the Wisconsin road test. As reported above, the Wisconsin fleet average fuel economy decreased 2.8 percent, compared to conventional gasoline, for three versions of RFG. However, the RFG fuel economy of some of the individual vehicles in the eight-vehicle fleet changed more. The changes ranged from -14 percent to +6 percent. The University of Wisconsin observed an even greater range of changes when they tested the same vehicles in a laboratory using six versions of RFG and two different driving cycles. 13
The failure of the individual vehicles to respond consistently to fuel and driving variations may be further evidence of how difficult fuel economy is to measure. But the results raise the possibility that there are vehicles on the road with RFG fuel economies both significantly better and worse than those predicted by gasoline energy content.
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