The Great Race

The Technological Challenges of More Fuel-Efficient Vehicles

With the price of oil hovering around $70 a barrel and gasoline around $3 a gallon, the automotive industry is facing political pressure to increase fuel economy. Short of eliminating the internal combustion engine, there’s no quick technological fix that would dramatically reduce petroleum consumption. Stricter standards could only be met by re-engineering a variety of vehicle components.

Automakers are currently required to meet a fleet-wide average fuel economy[1] of 27.5 miles per gallon for passenger cars and 22.2 mpg for light trucks. The U.S. Senate energy bill passed in July 2007 calls for raising CAFE standards to 35 mpg by 2020 for cars and SUVs.

Corporate average fuel economy standards — dubbed CAFE — were first enacted by Congress in 1975, in the aftermath of the 1973 Arab oil embargo. At the behest of Congress, the National Highway Traffic and Safety Administration sets the standards; the U.S. Environmental Protection Agency calculates the average fuel economy for each manufacturer.[2]

Chart 1 - click to enlarge
Source: National Highway Traffic and Safety Administration,
http://www.nhtsa.dot.gov/cars/rules/cafe/overview.htm

CAFE was intended originally to reduce U.S. demand for foreign oil. The goal has since expanded to include environmental protection, e.g., reductions in emissions of carbon dioxide.

The U.S. auto industry spends an estimated $21 billion each year on fuel efficiency and alternative fuel technologies, as well as vehicle design and safety.[3] In a letter to employees of Daimler Chrysler AG, executive Tom LaSorda estimated that it would cost $11.2 billion over five years to raise the company’s fleet average to 36 mpg by 2022 and to 30 mpg for light trucks by 2025, as proposed by Michigan Sen. Carl Levin.[4] President George W. Bush has called for a 40 percent increase in CAFE, to 35 mpg within 10 years, while other legislation pending in Congress would set the standard at 52 mpg by 2030.

According to the National Research Council, the average fuel economy of new passenger cars nearly doubled between 1970 and 1982, and increased by 50 percent in new light trucks during the same period.[5] Additionally, the NRC reported that the entire U.S. light-vehicle fleet increased 66 percent by 1992.[6]

These improvements in fuel efficiency have been achieved, in part, by downsizing vehicles, the use of lighter auto body materials such as plastics, aluminum and fiberglass, and body construction, i.e. casting the vehicle body and underlying frame as a single component.

Simple physics dictates that it takes less fuel to operate a lighter vehicle. In 1975, the average U.S. passenger car weighed 4,380 pounds compared to 1,676 pounds for European cars and 1,805 pounds for Asian cars.[7] The "weight gap" had narrowed dramatically by 2000, when the average American vehicle was 75 pounds lighter than the average European car and just 245 pounds heavier than the average Asian car.[8]

Vehicle aerodynamics have also improved. The lower the wind resistance, the less fuel required to propel a vehicle. The majority of newer models are rounded rather than boxy, prompting more than one reviewer to liken them to jellybeans.

Reducing engine friction also improves fuel efficiency. Engineers thus have designed fewer moving parts and developed better lubricants. Additional fuel savings have come from a transition to front-wheel-drive power trains. According to the National Resource Council, only 1.3 percent of U.S. passenger cars were front-wheel drive in 1975, compared to 17 percent of Asian models and 46 percent of European models.[9] The majority of U.S. vehicles built today are front-wheel drive.

Advancements have also been made in fuel delivery systems. Direct fuel injection has improved upon the relative inefficiency of carburetors by delivering a more precise measure of gasoline to each cylinder. Less than 1 percent of U.S. passenger cars were fuel-injected in 1975, compared to 14 percent of Asian vehicles and 39 percent of European vehicles.[10] Today, fuel injection has replaced the carburetor in nearly all vehicles rolling off U.S. assembly lines.

Fuel efficiency still can be increased by further improvements in fuel-injection technology. Although most conventional American vehicles employ electronic fuel injection or multi-port injection systems, a direct injection system would burn less fuel and generate less heat by delivering the fuel directly to the cylinder rather than routing it through an intake valve. Direct injection technology is widely available in Europe, in both gasoline and diesel engines. But U.S. emissions standards have limited the introduction of direct-injection diesels here.[11]

Currently, only 12 percent to 20 percent of the gasoline burned in an internal combustion engine is used to propel the vehicle. Two-thirds of the energy created by combustion is lost as heat, which is why engine coolants are necessary. The remainder of the energy is lost to engine friction and the powering of air conditioning and a host of other features. Losses also occur during idling and deceleration because fuel continues to combust even as the vehicle coasts or is no longer in motion.

Chart 2 - click to enlarge

The full benefits of recent technological advances have yet to be realized. For example, fuel savings of 3 percent to 6 percent are expected from cylinder deactivation, in which some cylinder valves in V-8 and V-12 engines are closed to prevent fuel injection when the car no longer requires acceleration. Even greater efficiency gains — from 5 percent to 10 percent — are expected from variable valve lift and timing, which more precisely regulate the oxygen/fuel mixtures injected into an engine’s cylinders.

Advancements in "supercharging" also could improve fuel consumption by 5 percent to 7 percent. Increasing accelerating power (torque) can increase fuel efficiency because it takes more energy and, therefore, more fuel for a vehicle to accelerate than it does to cruise.

New transmission technologies are also emerging, including the five-speed automatic and the continuously variable transmission. A five-speed automatic allows an engine to work more efficiently because the additional gear requires less energy from the engine, thereby improving fuel consumption by 2 percent to 3 percent. The CVT can reduce fuel use up to 8 percent by varying the energy supplied to the drive train depending on driving conditions.

Advances in tire and wheel manufacturing are expected to reduce the rolling friction that occurs between the vehicle and the road. Reducing rolling friction is projected to increase fuel efficiency by up to 1.5 percent.

The gains in fuel efficiency from re-engineering will take time to achieve because of the development cycle of new models. According to the National Research Council:

The widespread penetration of even existing technologies will probably require four to eight years. For emerging technologies that require additional research and development, this time lag can be considerably longer. In addition, considerably more time is required to replace the existing vehicle fleet (on the order of 200 million vehicles) with new, more efficient vehicles. Thus, while there would be incremental gains each year as improved vehicles enter the fleet, major changes in the transportation sector’s fuel consumption will require decades.[12]

Automakers also are engineering alternative-fuel vehicles. Hybrid vehicles, for example, combine a gasoline-powered engine and an electric motor. The gasoline engine is used for acceleration, after which the electric motor kicks in. The electric motor is powered by batteries that are recharged, in part, through the process of "regenerative braking." Regenerative braking involves capturing the friction between the wheels and the brake pads of a decelerating vehicle, and converting that energy into electricity for battery storage.

The Chevy Volt, a General Motors Corp. concept vehicle, uses an electric motor to power the wheels and a small gas engine to charge the generator. The Volt is said to travel 40 miles per hour on electricity alone, while conventional hybrids switch to the gasoline engine when the vehicle speed exceeds 25 mph.

GM announced in May that it would begin production of the Volt by 2010, although the vehicle will require development of new lithium ion battery technology.

Ultimately, GM is engineering the Volt platform to take a hydrogen fuel cell. Hydrogen promises three times the energy content of gasoline per gallon, and the only tailpipe emission is water. Current hydrogen concept vehicles include Honda’s FCX and the BMW 7 series. But there remain complex challenges to overcome before mass production of such vehicles can occur. Extracting hydrogen from water by electrolysis is currently a hugely energy-intensive process. Nonetheless, Larry Burns, GM’s head of research and development, recently expressed confidence that the company will mass-produce hydrogen-cell vehicles by 2020.

"Clean diesel" technology is another alternative for improving fuel economy. The soot-belching diesels of years past have been replaced by turbocharged direct-injection technology that makes new diesels significantly cleaner and more powerful. In general, they improve fuel efficiency by 30 percent. They also cost less than hybrids. In Europe — where gasoline prices hover above $7 a gallon — diesel-powered vehicles comprise more than 50 percent of the new car market.

"Flex-fuel" vehicles also are coming to market. They can run on conventional gasoline or E85, a mixture of 85 percent ethanol and 15 percent gasoline. U.S. cars made after 1990 can use E10 ethanol without any modifications. But because E85 is more corrosive than gasoline, it requires a specially designed fuel system and additional fuel sensors to ensure a clean burn.

Price shocks at the gas pump, environmental awareness and concerns about U.S. reliance on foreign oil are driving demand for fuel efficiency improvements. But even absent stricter federal mandates, significant improvements in fuel economy already are in the pipeline, with more efficient vehicles comprising a growing share of the nation’s fleet.


[1] Corporate Average Fuel Economy (CAFE).

[2] National Highway Traffic and Safety Administration, http://www.nhtsa.dot.gov/cars/rules/cafe/overview.htm.

[3] Wards Automotive Facts and Figures 2005, National Science Foundation. Information is accessible to nonsubscribers at http://www.weberchevrolet.com/sale/american.pdf.

[4] Wall Street Journal, “Michigan’s Levin Prepares Fuel-Economy Measure,” by Mike Spector, June 14, 2007.

[5] National Research Council, "Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards, page 14. Available online at http://www.nap.edu/books/0309076013/html/.

[6] National Research Council, page 15.

[7] National Research Council, page 15.

[8] National Research Council, page 15.

[9] National Research Council, page 15.

[10] National Research Council, page 15.

[11] At 5 parts per million, the U.S. diesel emissions standard adopted in 2006 is the strictest in the world. By contrast, the European standard is 50 ppm. Automakers were hesitant to introduce new diesel engines into the U.S. market until a standard was adopted. Volkswagen, for example, introduced a direct injection diesel to the U.S. market, but was forced to withdraw it because it did not meet the new emissions standard. The 5 ppm standard gave automotive engineers a target to shoot for, however, and Volkswagen and other manufacturers plan to bring direct-injection diesels to the U.S. market as early as next year.

[12] National Research Council, p. 5.