More efficient vehicles that run on lower carbon-emitting fuels are critical to meeting energy security and climate protection goals. EESI promotes improvements in vehicle fuel economy, while working to accelerate a transition from petroleum-based fuels to other liquid and non-liquid “fuels” derived from renewable sources.
Improvements in materials, aerodynamic design, and drive train (engine/transmission) efficiency have the potential to substantially increase the average fuel economy of the U.S. vehicle fleet. Full deployment of currently available technology could achieve gains of 40 percent, according to several studies. Other anticipated improvements now under development are estimated to bring total efficiency up to 80 percent. Greater vehicle efficiency will be important even as new fuels and energy sources are developed.
Auto fuel economy is significantly enhanced with a variety of technologies to increase drive train (engine/transmission) efficiency, improve aerodynamics and reduce vehicle weight. For more detail, see below. Properly maintaining the vehicle also helps. Keeping a car's oil filled at the proper level, doing on time oil changes and engine tuning, and ensuring properly inflated tires, all significantly improve fuel economy.
Driving behaviors also impact fuel economy. Removing unneeded accessories (i.e. roof racks) and cargo, smooth acceleration and deceleration, and use of high gears when at a steady speed, are some examples which reduce fuel consumption.
The driving behavior with perhaps the largest potential contribution to fuel efficiency, across nearly all vehicle types, is idle reduction. Idling for long periods of time wastes fuel and poses a health risk to people in and around the vehicle. For example, a typical idling school bus diesel engine burns about half a gallon of fuel per hour. Vehicle diesel engines are generally tuned for optimum operation at traveling speeds, so they combust fuel less efficiently when idling. Idling results in more pollutants per gallon, and these pollutants are concentrated in one place because the vehicle is stationary. These are typically places where people, such as vulnerable school children, are gathered. Diesel exhaust contains particulate matter which lodges in lung tissue when inhaled and is believed to cause or exacerbate numerous health problems, including cancer, asthma, reduced lung function, and premature death.
Drive Train (Engine/Transmission) Efficiency
Continuously Variable Transmission
A pair of variable diameter pulleys, linked by chain or belt, create an infinite amount of gear ratios. This limits revolutions per minute for any given speed, reducing fuel consumption.
A subset of the engine’s cylinders deactivate while cruising and re-activate under heavy acceleration. Engines with six or more cylinders can use this technology without a noticeable drop in performance.
Direct Fuel Injection
Fuel is injected directly into the cylinder barrel, providing more efficient combustion than when fuel and air are mixed outside the cylinder.
Engines shut down and instantaneously restart for short stops (red lights) rather than idle.
Turbochargers and Superchargers
Fans – powered by engine exhaust and the engine – force compressed air into the cylinders, allowing more fuel to be burned per piston stroke, producing more power and using less total fuel.
Variable Valve Timing and Lift
Valves control the flow of fuel and air into, and the flow of exhaust out of the cylinders in the engine. This technique allows the distance the valves travel to change in accordance with the speed of the engine.
Aerodynamics and Weight Reduction
Using carbon fiber and lighter metals enables manufacturers to reduce vehicle weight and increase engine efficiency, while maintaining durability and strength.
Lower vehicle frames are exposed to less drag since less air is exposed to the rough underside of the vehicle and the bottom of the tire tread. An air dam fitted below the front bumper also reduces drag and increases fuel mileage by deflecting air away from the bottom of the car.
Small, delta-shaped roof mounted devices, just in front of the rear window, reduce drag by causing air to flow along the rear windshield and trunk lid, instead of flowing above them. Properly sized, shaped and positioned spoilers can reduce drag too.
Thinner and smaller wheels and low rolling resistance tires reduce road friction and air drag, increasing fuel mileage. Fully closed, concave rims are more aerodynamic than open rims.
Learn more about Vehicle Efficiency:
- Transportation 2050: More EVs, but Conventional Vehicles Will Still Dominate
- Can Fuel Efficiency Standards Be Met Cost-Effectively?
- The Reality Behind the Renewable Fuel Standard, the Economy, and the Environment
- Fact Sheet - Plug-in Electric Vehicles (2017)
- California vs. the EPA: A Potential Policy Standoff
- New EPA Reports: Automakers’ Performance Continues to Outpace Emissions Standards
- EESI Comments on the Midterm Evaluation of Corporate Average Fuel Economy (CAFE) Standards
- Protecting Public Health Through Cleaner Fuels and Lower Emissions
- Das Auto & Das EPA: What Did They Know, When Did They Know It?
- Fact Sheet - Vehicle Efficiency and Emissions Standards