Fuel Economy

Fuel economy in automobiles is the amount of fuel required to move the automobile over a given distance. While the fuel efficiency of petroleum engines has improved markedly in recent decades, this does not necessarily translate into better fuel economy, if larger and heavier vehicles are used.

FuelEconomy.com, fueleconomy.gov and fueleconomy.org are the same site provided by the US Department of Energy.


www.fueleconomy.gov helps consumers make informed fuel economy choices when purchasing a vehicle and helps them achieve the best fuel economy possible from the cars they own.
 

www.fueleconomy.gov is maintained jointly by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy and the U.S. Environmental Protection Agency. The site helps fulfill DOE and EPA’s responsibility under the Energy Policy Act (EPAct) of 1992 to provide accurate MPG information to consumers.

Car FuelEconomy
 

Ideally, a car traveling at a constant velocity on level ground in a vacuum with frictionless wheels could travel at any speed without consuming any energy beyond what is needed to get the car up to speed. With ideal regenerative braking, this energy could be completely recovered. In real-world conditions, energy is lost in a number of ways:


Engine efficiency, which varies with engine type, the mass of the automobile and its load, and engine speed (usually measured in RPM).


Aerodynamic drag force, which increases roughly by the square of the car's speed, but note that drag power goes by the cube of the car's speed.
 

Rolling friction.

Braking, although regenerative braking captures some of the energy that would otherwise be lost.
 

Losses in the transmission. (Manual transmissions can be up to 94% efficient whereas older automatic transmissions may be as low as 70% efficient.[26] Automatically controlled shifting of gearboxes that have the same internals as manual boxes will give the same efficiency as a pure manual gearbox plus the bonus of added intelligence selecting optimal shifting points
 

Air conditioning. The power required for the engine to turn the compressor decreases the fuel-efficiency, though only when in use. This may be offset by the reduced drag of the vehicle compared with driving with the windows down. The extra mass of the air conditioning system will cause a slight increase in fuel consumption.


Electrical systems. Headlights, battery charging, active suspension, circulating fans, defrosters, media systems, speakers, and other electronics can also significantly increase fuel consumption, as the energy to power these devices causes increased load on the alternator. Since alternators are commonly only 40-60% efficient, the added load from electronics on the engine can be as high as 3 horsepower at any speed including idle. In the FTP 75 cycle test, a 200 watt load on the alternator reduces fuel efficiency by 1.7 mpg. Headlights, for example, consume 110 watts on low and up to 240 watts on high. These electrical loads can cause much of the discrepancy between real world and EPA tests which only include the electrical loads required to run the engine and basic climate control.


Fuel-efficiency decreases from electrical loads are most pronounced at lower speeds because most electrical loads are constant while engine load increases with speed. So at a lower speed a higher proportion of engine horsepower is used by electrical loads. Hybrid cars see the greatest effect on fuel-efficiency from electrical loads because of this proportional effect.



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