Fuel Economy of Gasoline Vehicles

Vehicle Fuel Economy

Variation by Class

Everyone recognizes that some vehicles give better fuel economy than others. Figure 1 compares the city fuel economy ranges for 12 classes of 1999 vehicles. The vehicles were tested in a laboratory (see box at left) using the FTP 75 driving cycle.* The highest average city fuel economy – 23 mpg for a subcompact car – is 50 percent better than the lowest average city fuel economy – 15 mpg for a 4-wheel-drive pickup truck.

Figure 1
Range and Average of City Fuel Economy by Vehicle Class
U.S. Department of Energy (DOE) Estimates for 1999 Model-year Vehicles ¹

There also are big differences among vehicles within a given class. Subcompact and compact cars with the best fuel economy are almost 300 percent better than those with the worst. And for nine other classes, the best performance is at least 100 percent better than the worst.

These differences are larger than any other fuel economy differences discussed in this bulletin. Selecting an efficient vehicle is the most important decision drivers concerned about fuel economy can make. No operating choices will have as big an impact. Of course, there are trade-offs. Vehicles with better fuel economy tend to be smaller, lighter, and have less power.

* To arrive at their estimates, DOE lowers the city result by 10 percent and the highway result by 22 percent to account for the differences between the laboratory driving cycles and actual driving on the road.

Design and Lubrication

Fuel economy focuses on the energy in gasoline that's converted to work at the wheels. But it's also informative to look at the energy that's lost. This alternative view provides insight into how fuel economy might be improved by design changes. The gasoline internal combustion engine is relatively inefficient. In an actual driving cycle, it converts 62 percent of the chemical energy in gasoline to heat and only 38 percent to mechanical energy. And only about one-third of the mechanical energy reaches the wheels to do useful work (Figure 2).

The energy losses show that the potential routes to better fuel economy involve:

• Increasing the thermal efficiency of the engine
• Decreasing energy losses within the vehicle
• Decreasing the work needed to move the vehicle

Figure 2
Simplified Vehicle Energy Balance ²
Passenger Car in EPA Urban Cycle (FTP 75)

An engine's thermal efficiency can be increased by raising the compression ratio. However, increases much beyond today's ratios of 9:1 or 10:1 have diminishing returns because of emissions, friction, and gasoline octane considerations. Thus, most of the design efforts focus on decreasing energy losses within the vehicle and the work needed to move the vehicle. For example, decreasing vehicle weight reduces inertia, and making the vehicle's profile more aerodynamic reduces air resistance. Some of the benefits obtained by changes in the engine and drive train are complex. ³ The recent trend to four-valve-per-cylinder engines results in better fuel economy if automobile designers take advantage of the horsepower increase to use smaller-displacement engines, an option that decreases vehicle weight without sacrificing performance. Increasing the number of valves provides other fuel economy benefits: "...the greater valve area...reduces pumping losses, and the more compact combustion chamber geometry and central spark plug location allow an increase in compression ratio." ⁴

Friction is an important cause of energy loss within the vehicle. Mechanical engineers are reducing friction by redesigning various engine components, including pistons, piston rings and rods, and the valve train. Lubrication engineers are developing energy-conserving motor oils.

Motor oils reduce friction in two ways:

• The bulk oil separates opposing metal surfaces to prevent contact (hydrodynamic lubrication).
• Friction-modifying additives alter metal surfaces so friction forces aren't as great when there is contact (boundary lubrication).

Two-thirds of the friction losses in an engine are estimated to occur during hydrodynamic lubrication and one-third during boundary lubrication or mixed hydrodynamic/boundary lubrication. The new energy-conserving motor oils are designed to reduce friction losses from both types of lubrication by tailoring the viscosity characteristics of the base oil and the chemistries of the friction-modifying additives.

Here is one author's estimates of the impact of engine friction on fuel economy. ⁵

• Up to 28 percent increase in fuel economy, if engine friction were completely eliminated (not possible).
• Up to 5.8 percent increase in fuel economy for real driving cycles, if an improved motor oil reduced engine friction 50 percent.

Keep in mind that the results depend on the specific engine being considered and the speed and load conditions imposed by the driving cycle.

Operation and Maintenance

While the fuel economy sticker on a new vehicle attempts to predict the city and highway results that will be achieved by a typical driver, the actual fuel economy will depend on the driving conditions and on how the vehicle is operated and maintained. Figure 3 estimates the average and maximum effects of some of the common factors that influence fuel economy.

Climate

Winter weather conditions can combine to lower fuel economy 20 percent, compared with the summer. In cold weather, a richer fuel-air mixture is required to start and warm up the engine. Also, much of the warmup may be done at idle (zero fuel economy) because of the need to defog or defrost windows. (During defogging/defrosting, many vehicles not only heat the air but use the air conditioner to dehumidify it.) More energy is required to overcome the resistance created by the higher viscosities of cold lubricants – engine oil, transmission fluid, and differential lubricant. Head winds increase air resistance; rain necessitates windshield wiper use; water or snow on the road increase tire-rolling resistance; and bad weather in general requires slowing to less fuel-efficient speeds.

In summer, head winds increase air resistance, and higher temperatures increase air conditioner use.

Terrain and traffic

Extra energy is needed to climb a grade, and not all the fuel economy decrease is made up on the downgrade because the engine, at minimum, is fast idling. Bumper-to-bumper driving in traffic jams means more idling and more driving at less fuel-efficient speeds.

Vehicle maintenance

Crankcase oil viscosity higher than the recommended grade increases engine friction. A clogged air cleaner increases pumping work. Wheels out of alignment, non-radial tires, and underinflated tires increase tire rolling resistance.

Driver behavior

Fuel economy is decreased by unnecessary idling, jackrabbit starts and other instances of hard acceleration, excessive highway speeds, carrying unnecessary cargo, and mounting carriers and other attachments on the outside of the vehicle.

Fuel

Gasoline's effect on fuel economy is discussed in the following sections.

Figure 3
Estimates of Fuel Economy Reductions Caused by Various Operating Conditions ⁶

Condition	Comparison	Percent Reduction in Fuel Economy
		Average	Maximum
Temperature	20°F vs. 77°F	5	13
Idling/warmup	Winter vs. summer	Variable with driver	20
Defroster	Extreme use	Same as air conditioning for some vehicles
Head wind	20 mph	2	6
Uphill driving	7% grade	19	25
Poor road conditions	Gravel, curves, slush, snow, etc.	4	50
Congested traffic	20 mph vs. 27 mph average speed	11	15
Highway speed	70 mph vs. 55 mph	N/A	25
Acceleration rate	"Hard" vs. "easy"	12	20
Wheel alignment	0.5 inch	<1	10
Tire type	Non-radial	<1	4
Tire pressure	15 psi vs. 26 psi	3	6
Air conditioning	Extreme heat	21	N/A
Windows	Open vs. closed	Unknown but likely small

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