Fuel efficiency

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Fuel efficiency relates the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. Often this applies specifically in a transportation vehicle, such as an automobile. Fuel economy relates to the amount of fuel required to move a vehicle over a given distance. While the fuel efficiency of petroleum engines has improved markedly in recent decades, this does not necessarily translate into fuel economy of cars. In the United States, tax laws and consumer preference has led to bigger and heavier cars, whereas European public policy has led to smaller, more economical cars. The average car sold in the European Union today can travel twice as far as its United States counterpart using the same fuel.

Other 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.

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Energy content of fuel

Fuel type     MJ/L     MJ/kg     BTU/imp gal     BTU/US gal     Research octane
number (RON)
Gasoline 29.0   45      150,000 125,000 91–98
LPG 22.16 34.39 114,660 95,475 115
Ethanol 19.59 30.40 101,360 84,400 129
Methanol 14.57 22.61 75,420 62,800 123
Gasohol (10% ethanol + 90% gasoline) 28.06 43.54 145,200 120,900 93/94
Diesel 40.9   63.47 176,000 147,000 N/A (see cetane)


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Fuel efficiency

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Image:Small efficient cars in the Netherlands.jpeg

Fuel efficiency is normally expressed in terms of power per unit of engine displacement, also known as specific output. It should be noted that despite common usage, "fuel efficiency" is not a synonym for "fuel economy" or "gas mileage". Modern fuel injected engines are much more efficient at producing power than their carbureted predecessors. For example, power output from Chrysler's 3.9 L LA V6 engine jumped from 125 hp (93 kW) to 180 hp (134 kW) in 1992 due to the addition of fuel injection and a free-flowing intake manifold.

However, improvements in fuel efficiency achieved over the last 20 years have not been translated into improvements in fuel economy — much of the savings have been offset by the use of heavier and less-aerodynamic body styles (especially SUVs and pickup trucks) and the use of more-powerful engines. For example, the 6.0 L Vortec V8 used in the Hummer H2 produces 53.6 hp (39 kW) per liter of displacement, which is more than double the 25.4 hp (19 kW) per liter produced by the original VW Beetle. However, the Hummer weighs more than four times as much as the original Beetle, has a much less-aerodynamic body, and uses a complex four wheel drive system, so the Beetle is able to travel three times farther than the Hummer on the same amount of fuel.

Naturally aspirated engines tend to be more fuel efficient than engines with forced induction (ex: turbocharged, supercharged).

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Fuel economy

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Fuel economy is usually expressed in one of two ways:

  1. As the amount of fuel used per unit distance; for example, litres per 100 kilometres (L/100 km). In this case, the lower the value, the more economic a vehicle is;
  2. As the distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L) or miles per gallon (MPG). In this case, the higher the value, the more economic a vehicle is.

The two European standard measuring scenarios for "L/100 km" value are autobahn (motorway -GB or freeway - US) travel at 90 km/h and rush hour city traffic. A reasonably modern European subcompact car may manage highway travel at 5 litres per 100 kilometres (47 mpg US) or 6.5 litres in city traffic (36 mpg US), with app. 140 grams of carbon dioxide emission per kilometre.

An average "car-shaped" US car achievs about 26 mpg (US) (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city; a large SUV usually gets 13 mpg (US) (18 L/100 km) city, 16 mpg (US) (15 L/100 km) highway. Pickup trucks vary considerably; whereas a light US pickup with a 4 cylinder engine produces circa 28 mpg (8 L/100 km), a full-size US pickup with extended cab with an 8 cylinder engine produces circa 13 mpg (US) (18 L/100 km) city, 15 mpg (US) (15 L/100 km) highway. An interesting example is the popular Smart ForTwo car which can achieve up to 4.0 L/100 km (70.6 mpg) using a three-cylinder engine with a turbocharger. The Smart is produced by DaimlerChrysler and is currently only sold by one company in the United States. ZAP, at zapworld.com.

Diesel engines often produce greater fuel efficiency than petrol (gasoline) engines: 50% of all cars sold in the EU are now diesel vehicles. This can aslo be attributed to the fact that diesel has 17.6% more energy per unit volume than petrol, and due to economic factors in certain areas, offers more energy for the money.

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Fuel efficiency in microgravity

The energy output derived from fuel occurs during combustion. Ensuring a total, even combustion of fuel, as well as harnessable combustion at the appropriate moments, will have an impact on fuel effciency. Recent research by the National Aeronautics and Space Administration (NASA) has gained possible insights to increasing fuel efficiency if fuel consumption takes place in microgravity. This probably does not apply to vehicles so much as industry where the benefit from the increased fuel efficiency will outweigh the initial cost of operating in a microgravity environment.

The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, such as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, 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 that the cause is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. <ref> CFM-1 experiment results, National Aeronautics and Space Administration, April 2005.</ref> Experiments by NASA in microgravity reveal that diffusion flames 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. <ref>LSP-1 experiment results, National Aeronautics and Space Administration, April 2005.</ref> Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer. <ref>SOFBAL-2 experiment results, National Aeronautics and Space Administration, April 2005.</ref>

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Fuel efficiency in transportation

  • Humans (see Human-powered transport):
    • walking or running one kilometre requires approximately 70 kcal or 330 kJ of food energy <ref name=brianmac>[1]</ref>. This equates to about 1 l/100km or 235 mpg in gasoline energy terms.
    • cycling requires about 120 kJ/km <ref name=brianmac />
  • Airplanes: passenger airplanes averaged 4.8 l/100km per passenger (1.4 MJ/passenger-km) (49 passenger-miles per gallon) in 1998. Efficiencies around 3 l/100km per passenger are reached by some carriers <ref>IATA - Fuel efficiency, IATA</ref>. Note that on average 20% of seats are left unoccupied.
  • Ships: the RMS Queen Elizabeth 2 gets 49.5 feet per gallon <ref>[2], Cunard Line</ref> (25,000 l/100km or 13 l/100km per passenger (3.8 MJ/passenger-km)). Note that about 40% of the power produced by the ship engines is used for propulsion, the rest being used to generate electricity for heating, lighting, and other passenger comforts.
  • Trains:
  • the Center for Transportation Analysis of the DOE claims the following average figures for the U.S.A. in 2002 <ref>Passenger Travel and Energy Use, 2002, Center for Transportation Analysis, Oak Ridge National Laboratory</ref>:
Transport mode Load factor

(passengers/vehicle)

J/m - vehicle J/m - passenger BTU per vehicle-mile BTU per passenger-mile Equivalent passenger-miles

per gallon of gasoline

Automobiles 1.57 3 686 2 347 5 623 3 581 34.9
Personal trucks 1.72 4 574 2 659 6 978 4 057 30.8
Motorcycles 1.22 1 640 1 490 2 502 2 274 55.0
Transit Buses 9.1 24 579 2 705 37 492 4 127 30.3
Airlines 95.8 232 489 2 427 354 631 3 703 33.8
Intercity trains 14.0 44 454 3 166 67 810 4 830 25.9
Commuter trains 33.5 59 556 1 779 90 845 2 714 46.1
  • Rockets:
    • The NASA space shuttle consumes 1,000,000 kg of solid fuel and 2,000,000 litres of liquid fuel over 8.5 minutes to take the 100,000 kg vehicle (including the 25,000 kg payload) to an altitude of 111 km and an orbital speed of 30,000 km/h. This amounts to about 3,300 GJoules of energy, or about 100,000 l/100km or 12 feet per gallon of gasoline. It's worth noting that a rocket can, in theory, re-entry on any place on Earth, giving it a best-case "ground" distance of 20,000 km. This would amount to 500 l/100km or about 0.5 mpg.
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See also

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References

<references/>

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External links

Sites and pages commissioned by various governments' institutions

Other sites

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