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Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.
In the context of transport, fuel economy is the energy efficiency of a particular vehicle, given as a ratio of distance traveled per unit of Motor fuel consumed. It is dependent on several factors including engine efficiency, transmission design, and tire design. In most countries, using the metric system, fuel economy is stated as "fuel consumption" in per 100 kilometers (L/100 km) or kilometers per liter (km/L or kmpl). In a number of countries still using other systems, fuel economy is expressed in per gallon (mpg), for example in the US and usually also in the UK (Imperial units gallon); there is sometimes confusion as the imperial gallon is 20% larger than the US gallon so that mpg values are not directly comparable. Traditionally, litres per mil were used in Norway and Sweden, but both have aligned to the EU standard of L/100 km.
Fuel consumption is a more accurate measure of a vehicle's performance because it is a linear relationship while fuel economy leads to distortions in efficiency improvements. Weight-specific efficiency (efficiency per unit weight) may be stated for freight, and passenger-specific efficiency (vehicle efficiency per passenger) for passenger vehicles.
Hybrid vehicles use two or more power sources for propulsion. In many designs, a small combustion engine is combined with electric motors. Kinetic energy which would otherwise be lost to heat during braking is recaptured as electrical power to improve fuel efficiency. The larger batteries in these vehicles power the car's electronics, allowing the engine to shut off and avoid prolonged idling.
Given a heat value of a fuel, it would be trivial to convert from fuel units (such as litres of gasoline) to energy units (such as MJ) and conversely. But there are two problems with comparisons made using energy units:
| Regular gasoline/petrol | 34.8 | 150,100 | 125,000 | Min. 91 | |
| Premium gasoline/petrol | Min. 95 | ||||
| Autogas (LPG) (60% propane and 40% butane) | 25.5–28.7 | 108–110 | |||
| ethanol fuel | 23.5 | 31.1Calculated from heats of formation. Does not correspond exactly to the figure for MJ/L divided by density. | 101,600 | 84,600 | 129 |
| Methanol | 17.9 | 19.9 | 77,600 | 64,600 | 123 |
| Alcohol fuel (10% ethanol and 90% gasoline) | 33.7 | 145,200 | 121,000 | 93/94 | |
| E85 (85% ethanol and 15% gasoline) | 25.2 | 108,878 | 90,660 | 100–105 | |
| Diesel fuel | 38.6 | 166,600 | 138,700 | cetane number | |
| Biodiesel | 35.1 | 39.9 | 151,600 | 126,200 | cetane number |
| WVO (using 9.00 kcal/g) | 34.3 | 37.7 | 147,894 | 123,143 | |
| Aviation gasoline | 33.5 | 46.8 | 144,400 | 120,200 | 80-145 |
| Jet fuel, naphtha | 35.5 | 46.6 | 153,100 | 127,500 | N/A to turbine engines |
| Jet fuel, kerosene | 37.6 | 162,100 | 135,000 | N/A to turbine engines | |
| Liquefied natural gas | 25.3 | 109,000 | 90,800 | ||
| Liquid hydrogen | 9.3 | 40,467 | 33,696 |
Neither the gross heat of combustion nor the net heat of combustion gives the theoretical amount of mechanical energy (work) that can be obtained from the reaction. (This is given by the change in Gibbs free energy, and is around 45.7 MJ/kg for gasoline.) The actual amount of mechanical work obtained from fuel (the inverse of the specific fuel consumption) depends on the engine. A figure of 17.6 MJ/kg is possible with a gasoline engine, and 19.1 MJ/kg for a diesel engine. See Brake-specific fuel consumption for more information.
Because there are pollutants involved in the manufacture and destruction of a car and the production, transmission and storage of electricity and hydrogen, the label "zero pollution" applies only to the car's conversion of stored energy into movement.
In 2004, a consortium of major auto-makers — BMW, General Motors, Honda, Toyota and Volkswagen/Audi — came up with "Top Tier Detergent Gasoline Standard" to gasoline brands in the US and Canada that meet their minimum standards for detergent content Top Tier Gasoline and do not contain metallic additives. Top Tier gasoline contains higher levels of detergent additives in order to prevent the build-up of deposits (typically, on fuel injector and intake valve) known to reduce fuel economy and engine performance.
The common distribution of a flame under normal gravity conditions depends on convection, because soot tends to rise to the top of a flame, such as in a candle, making the flame yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes sphere, with a tendency to become more blue and more efficient. There are several possible explanations for this difference, of which the most likely one given is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs., National Aeronautics and Space Administration, April 2005. Experiments by NASA in microgravity reveal that in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions. LSP-1 experiment results, National Aeronautics and Space Administration, April 2005. in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer. SOFBAL-2 experiment results , National Aeronautics and Space Administration, April 2005.
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