The fuel consumption is an equivalent measure for cars sold outside the United States, typically measured in litres per 100 km traveled in general, the fuel consumption and miles per gallon would be reciprocals with appropriate conversion factors, but because different countries use different driving cycles to measure fuel consumption, fuel economy and fuel consumption are not always directly comparable. The MPG of a given vehicle starts with the thermal efficiency of the fuel and engine, less all of the above elements of friction. Cars sold in the United States are advertised by their measured overall efficiency (fuel economy) in miles per gallon (mpg). Overall efficiency is measured and reported, typically by government testing, through operating the vehicle in a standardized driving cycle designed to replicate typical use, while providing a consistent basis for comparison between vehicles. See table below translating retail electricity costs for a GGE in BTU. Additional losses will affect the overall efficiency, similar to a conventional internal combustion car, including rolling resistance, aerodynamic drag, accessory power, climate control, and drivetrain losses. An electric motor typically has internal friction only at the main axle bearings. Generally speaking, an electrical motor is far more efficient than an internal combustion engine at converting the stored potential energy into useful work in an electric vehicle, traction motor efficiency can approach 90%, as there is minimal waste heat coming off the motor parts, and zero heat cast off by the coolant radiator and out of the exhaust. Next is the conversion of stored energy to distance traveled under power. In battery or electric vehicles, calculating the vehicle's overall efficiency of useful work begins with the charge–discharge rate of the battery pack, generally 80% to 90%. The vehicle's external styling affects its aerodynamic drag, which is another dynamic loss that must be considered for overall efficiency. Other dynamic losses can be caused by friction outside the motor/engine, including loads from the generator / alternator, power steering pump, A/C compressor, transmission, transfer case (if four-wheel-drive), differential(s) and universal joints, plus rolling resistance of the pneumatic tires. In addition, friction inside the engine happens along the cylinder walls, crankshaft rod bearings and main bearings, camshaft bearings, drive chains or gears, plus other miscellaneous and minor bearing surfaces. ![]() Thermal efficiency is affected by both friction and heat losses for internal combustion engines, some of the stored energy is lost as heat through the exhaust or cooling system. The overall efficiency of converting a unit of fuel to useful work (rotation of the driving wheels) includes consideration of thermal efficiency along with dynamic losses that are inherent and specific to the design of a given vehicle. ![]() Forced induction for an internal combustion engine using supercharger or turbocharger may also affect the optimum fuel–air mix and thermal efficiency. ![]() This can be a manual adjustment using tools and test instruments or done automatically in computer-controlled fuel injected and multi-fuel vehicles. Each fuel–engine combination requires adjusting the mix of air and fuel. In general, an engine is designed to run on a single fuel source and substituting one fuel for another may affect the thermal efficiency. A diesel cycle engine can be as much as 40% to 50% efficient at converting fuel into work, where a typical automotive gasoline engine's efficiency is about 25% to 30%. The measure of this is engine efficiency, often called thermal efficiency in the case of internal combustion engines. To start, only a fraction of the stored energy of a given fuel (measured in BTU or kW-hr) can be converted to useful work by the vehicle's engine. Main article: Fuel economy in automobiles In the context of GGE, a real world measure of overall efficiency is the fuel economy or fuel consumption advertised by motor vehicle manufacturers. This is measured by the car's overall efficiency. Using GGE to as a measure compare the stored energy of various fuels for use in an internal combustion engine is only one input for consumers, who typically are interested in the annual cost of driving a vehicle, which requires considering the amount of useful work that can be extracted from a given fuel. ![]() CNG sold at filling stations in the US is priced in dollars per GGE. It can be measured by its volume in standard cubic feet (ft³) at atmospheric conditions, by its weight in pounds (lb), or by its energy content in joules (J), British thermal units (BTU), or kilowatt-hours (kW In 1994, the US National Institute of Standards and Technology (NIST) defined "gasoline gallon equivalent (GGE) 5.660 pounds of natural gas." Compressed natural gas (CNG), for example, is a gas rather than a liquid.
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