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Fuel efficiency (orfuel economy) is a form ofthermal efficiency, meaning theratio of effort to result of a process that convertschemicalpotential energy contained in a carrier (fuel) into kinetic energy orwork. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuousenergy profile. Non-transportation applications, such asindustry, benefit from increased fuel efficiency, especiallyfossil fuel power plants or industries dealing withcombustion, such asammonia production during theHaber process.
In the context oftransport, fuel economy is theenergy efficiency of a particular vehicle, given as aratio of distance traveled per unit offuel consumed. It is dependent on several factors includingengine efficiency,transmission design, andtire design. In most countries, using themetric system, fuel economy is stated as "fuel consumption" inliters per 100kilometers (L/100 km) or kilometers per liter (km/L or kmpl). In a number of countries still using other systems, fuel economy is expressed inmiles pergallon (mpg), for example in the US and usually also in the UK (imperial 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 permil were used inNorway andSweden, but both have aligned to the EU standard of L/100 km.[1]
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.[2] Weight-specific efficiency (efficiency per unit weight) may be stated forfreight, and passenger-specific efficiency (vehicle efficiency per passenger) for passenger vehicles.
Fuel efficiency is dependent on many parameters of a vehicle, including itsengine parameters,aerodynamic drag, weight, AC usage, fuel androlling resistance. There have been advances in all areas of vehicle design in recent decades. Fuel efficiency of vehicles can also be improved by careful maintenance and driving habits.[3]
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 thecar's electronics, allowing the engine to shut off and avoid prolongedidling.[4]
Fleet efficiency describes the average efficiency of a population of vehicles. Technological advances in efficiency may be offset by a change in buying habits with a propensity to heavier vehicles that are less fuel-efficient.[5]
Energy efficiency is similar to fuel efficiency but the input is usually in units of energy such asmegajoules (MJ),kilowatt-hours (kW·h), kilocalories (kcal) orBritish thermal units (BTU). The inverse of "energy efficiency" is "energy intensity", or the amount of input energy required for a unit of output such as MJ/passenger-km (of passenger transport), BTU/ton-mile or kJ/t-km (of freight transport), GJ/t (for production of steel and other materials), BTU/(kW·h) (for electricity generation), or litres/100 km (of vehicle travel). Litres per 100 km is also a measure of "energy intensity" where the input is measured by the amount of fuel and the output is measured by thedistance travelled. For example:Fuel economy in automobiles.
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:
The specificenergy content of a fuel is the heat energy obtained when a certain quantity is burned (such as a gallon, litre, kilogram). It is sometimes called theheat of combustion. There exists two different values ofspecific heat energy for the same batch of fuel. One is the high (or gross) heat of combustion and the other is the low (or net) heat of combustion. The high value is obtained when, after the combustion, the water in the exhaust is in liquid form. For the low value, the exhaust has all the water in vapor form (steam). Since water vapor gives up heat energy when it changes from vapor to liquid, the liquid water value is larger since it includes thelatent heat of vaporization of water. The difference between the high and low values is significant, about 8 or 9%. This accounts for most of the apparent discrepancy in the heat value of gasoline. In the U.S. (and the table) the high heat values have traditionally been used, but in many other countries, the low heat values are commonly used.
| Fuel type | MJ/L | MJ/kg | BTU/imp gal | BTU/US gal | Research octane number (RON) |
|---|---|---|---|---|---|
| Regulargasoline/petrol | 34.8 | ~47 | 150,100 | 125,000 | Min. 91 |
| Premiumgasoline/petrol | ~46 | Min. 95 | |||
| Autogas (LPG) (60%propane and 40%butane) | 25.5–28.7 | ~51 | 108–110 | ||
| Ethanol | 23.5 | 31.1[7] | 101,600 | 84,600 | 129 |
| Methanol | 17.9 | 19.9 | 77,600 | 64,600 | 123 |
| Gasohol (10% ethanol and 90% gasoline) | 33.7 | ~45 | 145,200 | 121,000 | 93/94 |
| E85 (85% ethanol and 15% gasoline) | 25.2 | ~33 | 108,878 | 90,660 | 100–105 |
| Diesel | 38.6 | ~48 | 166,600 | 138,700 | N/A (see cetane) |
| Biodiesel | 35.1 | 39.9 | 151,600 | 126,200 | N/A (see cetane) |
| Vegetable oil (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 | ~47 | 162,100 | 135,000 | N/A to turbine engines |
| Liquefied natural gas | 25.3 | ~55 | 109,000 | 90,800 | |
| Liquid hydrogen | 9.3 | ~130 | 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 inGibbs free energy, and is around 45.7 MJ/kg for gasoline.) The actual amount of mechanical work obtained from fuel (the inverse of thespecific 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. SeeBrake-specific fuel consumption for more information.[clarification needed]
Theenergy efficiency in transport is the useful travelleddistance, of passengers, goods or any type of load; divided by the totalenergy put into the transportpropulsion means. The energy input might be rendered in several different types depending on the type of propulsion, and normally such energy is presented inliquid fuels,electrical energy orfood energy.[9][10] Theenergy efficiency is also occasionally known asenergy intensity.[11] Theinverse of the energy efficiency in transport is the energy consumption in transport.
Energy efficiency in transport is often described in terms offuel consumption, fuel consumption being the reciprocal of fuel economy.[10] Nonetheless, fuel consumption is linked with a means of propulsion which usesliquid fuels, whilst energy efficiency is applicable to any sort of propulsion. To avoid said confusion, and to be able to compare the energy efficiency in any type of vehicle, experts tend to measure the energy in theInternational System of Units, i.e.,joules.
Therefore, in the International System of Units, the energy efficiency in transport is measured in terms of metre per joule, or m/J, while the energy consumption in transport is measured in terms of joules per metre, or J/m. The more efficient the vehicle, the more metres it covers with one joule (more efficiency), or the fewer joules it uses to travel over one metre (less consumption). Theenergy efficiency in transport largely varies by means of transport. Different types oftransport range from some hundredkilojoules per kilometre (kJ/km) for abicycle to tens of megajoules per kilometre (MJ/km) for ahelicopter.
Via type of fuel used and rate of fuel consumption, energy efficiency is also often related to operating cost ($/km) and environmental emissions (e.g. CO2/km).
Thefuel economy of anautomobile relates to the distance traveled by a vehicle and the amount offuel consumed. Consumption can be expressed in terms of the volume of fuel to travel a distance, or the distance traveled per unit volume of fuel consumed. Since fuel consumption of vehicles is a significant factor in air pollution, and since the importation ofmotor fuel can be a large part of a nation'sforeign trade, many countries impose requirements for fuel economy.
Different methods are used to approximate the actual performance of the vehicle. The energy in fuel is required to overcome various losses (wind resistance,tire drag, and others) encountered while propelling the vehicle, and in providing power to vehicle systems such as ignition or air conditioning. Various strategies can be employed to reduce losses at each of the conversions between thechemical energy in the fuel and thekinetic energy of the vehicle. Driver behavior can affect fuel economy; maneuvers such as sudden acceleration and heavybraking waste energy.
Electric cars do not directly burn fuel, and so do not have fuel economy per se, but equivalence measures, such asmiles per gallon gasoline equivalent have been created to attempt to compare them.
Energy-efficient driving techniques are used by drivers who wish to reduce their fuel consumption, and thus maximize fuel efficiency. Many drivers have the potential to improve their fuel efficiency significantly.[13] Simple things such as keeping tires properly inflated, having a vehicle well-maintained and avoiding idling can dramatically improve fuel efficiency.[14] Careful use of acceleration and deceleration and especially limiting use of high speeds helps efficiency. The use of multiple such techniques is called "hypermiling".[15]
Simple fuel-efficiency techniques can result in reduction in fuel consumption without resorting to radical fuel-saving techniques that can be unlawful and dangerous, such as tailgating larger vehicles.The most efficient machines for converting energy to rotary motion are electric motors, as used inelectric vehicles. However, electricity is not a primary energy source so the efficiency of the electricity production has also to be taken into account.Railway trains can be powered using electricity, delivered through an additional running rail, overheadcatenary system or by on-board generators used indiesel-electric locomotives as common on the US and UK rail networks. Pollution produced from centralised generation of electricity is emitted at a distant power station, rather than "on site". Pollution can be reduced by using more railway electrification andlow carbon power for electricity. Some railways, such as the French SNCF and Swiss federal railways derive most, if not 100% of their power, from hydroelectric or nuclear power stations, therefore atmospheric pollution from their rail networks is very low. This was reflected in a study by AEA Technology between aEurostar train and airline journeys between London and Paris, which showed the trains on average emitting 10 times less CO2, per passenger, than planes, helped in part by French nuclear generation.[16]
In the future,hydrogen cars may be commercially available. Toyota is test-marketing vehicles powered by hydrogen fuel cells in southern California, where a series of hydrogen fueling stations has been established. Powered either through chemical reactions in afuel cell that create electricity to drive very efficient electrical motors or by directly burning hydrogen in a combustion engine (near identically to anatural gas vehicle, and similarly compatible with both natural gas and gasoline); these vehicles promise to have near-zero pollution from the tailpipe (exhaust pipe). Potentially the atmospheric pollution could be minimal, provided the hydrogen is made byelectrolysis using electricity from non-polluting sources such as solar, wind orhydroelectricity or nuclear. Commercialhydrogen production uses fossil fuels and produces more carbon dioxide than hydrogen.
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 andVolkswagen/Audi — came up with"Top Tier Detergent Gasoline Standard" togasoline brands in the US and Canada that meet their minimum standards fordetergent content[17] 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, onfuel injector andintake valve) known to reduce fuel economy and engine performance.[18]
How fuel combusts affects how much energy is produced. TheNational Aeronautics and Space Administration (NASA) has investigated fuel consumption inmicrogravity.
The common distribution of a flame under normal gravity conditions depends onconvection, because soot tends to rise to the top of a flame, such as in a candle, making the flame yellow. In microgravity orzero gravity, such as an environment inouter space, convection no longer occurs, and the flame becomesspherical, 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 thatdiffusion 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.LSP-1 experiment results, National Aeronautics and Space Administration, April 2005.Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.[19]
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