Fuel efficiency

Fuel efficiency(orfuel economy) is a form ofthermal efficiency,meaning theratioof effort to result of a process that convertschemicalpotential energycontained 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 plantsor industries dealing withcombustion,such asammoniaproduction during theHaber process.

In the context oftransport,fuel economy is theenergy efficiencyof a particular vehicle, given as aratioof distance traveled per unit offuelconsumed. It is dependent on several factors includingengine efficiency,transmissiondesign, andtiredesign. In most countries, using themetric system,fuel economy is stated as "fuel consumption" inlitersper 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 inmilespergallon(mpg), for example in the US and usually also in the UK (imperialgallon); 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 permilwere used inNorwayandSweden,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 forfreight,and passenger-specific efficiency (vehicle efficiency per passenger) for passenger vehicles.

Vehicle design

Fuel efficiency is dependent on many parameters of a vehicle, including itsengineparameters,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.

Hybrid vehiclesuse 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.

Fleet efficiency

Trucks' share of US vehicles produced, has tripled since 1975. Though vehicle fuel efficiency has increased within each category, the overall trend toward less efficient types of vehicles has offset some of the benefits of greater fuel economy and reduction in carbon dioxide emissions.Without the shift towards SUVs, energy use per unit distance could have fallen 30% more than it did from 2010 to 2022.

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.

Energy efficiency terminology

Energy efficiencyis 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 thedistancetravelled. 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:

• There are two different heat values for any hydrogen-containing fuel which can differ by several percent (see below).
• When comparing transportation energy costs, it must be remembered that akilowatt hourof electric energy may require an amount of fuel with heating value of 2 or 3 kilowatt hours to produce it.

Energy content of fuel

The specificenergy contentof 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 heatenergy 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 heatof 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%propaneand 40%butane)	25.5–28.7	~51			108–110
Ethanol	23.5	31.1	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	09.3	~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 consumptionfor more information.

Transportation

This section is an excerpt fromEnergy efficiency in transport.[edit]

Theenergy efficiency in transportis the useful travelleddistance,of passengers, goods or any type of load; divided by the totalenergyput into the transportpropulsionmeans. 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 energyorfood energy.Theenergy efficiencyis also occasionally known asenergy intensity.Theinverseof 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.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 efficiencyin transport largely varies by means of transport. Different types oftransportrange from some hundredkilojoulesper kilometre (kJ/km) for abicycleto 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).

Fuel efficiency of motor vehicles

This section is an excerpt fromFuel economy in automobiles.[edit]

Fuel consumption monitor from a 2006Honda Airwave.The displayed fuel economy is 18.1 km/L (5.5 L/100 km; 43 mpg_‑US).

ABriggs and Stratton Flyerfrom 1916. Originally an experiment in creating a fuel-saving automobile in the United States, the vehicle weighed only 135 lb (61.2 kg) and was an adaptation of a small gasoline engine originally designed to power a bicycle.

Thefuel economyof anautomobilerelates 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 fuelcan 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 energyin the fuel and thekinetic energyof the vehicle. Driver behavior can affect fuel economy; maneuvers such as sudden acceleration and heavybrakingwaste energy.

Electric carsdo not directly burn fuel, and so do not have fuel economy per se, but equivalence measures, such asmiles per gallon gasoline equivalenthave been created to attempt to compare them.

Driving technique

This section is an excerpt fromEnergy-efficient driving.[edit]

Energy-efficient drivingtechniques 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.Simple things such as keeping tires properly inflated, having a vehicle well-maintained and avoiding idling can dramatically improve fuel efficiency.Careful use of acceleration and deceleration and especially limiting use of high speeds helps efficiency. The use of multiple such techniques is called "hypermiling".

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.

Advanced technology

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.Railwaytrains can be powered using electricity, delivered through an additional running rail, overheadcatenarysystem or by on-board generators used indiesel-electriclocomotives 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 powerfor 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 aEurostartrain 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.

Hydrogen fuel cells

In the future,hydrogen carsmay 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 cellthat 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 byelectrolysisusing electricity from non-polluting sources such as solar, wind orhydroelectricityor nuclear. Commercialhydrogen productionuses 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,ToyotaandVolkswagen/Audi— came up with"Top Tier Detergent Gasoline Standard"togasolinebrands in the US and Canada that meet their minimum standards fordetergentcontentand 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 injectorandintake valve) known to reduce fuel economy and engine performance.

In microgravity

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 flamesin 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 flamesin microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.

See also

• Annual fuel utilization efficiency(AFUE)
• ACEA agreement
• Alternative propulsion
• Camless piston engine
• Carbon dioxide equivalent
• Corporate Average Fuel Economy(CAFE)
• EcoAuto(in Canada)
• Efficient energy use
• Emission standard
• Energy content of Biofuel
• Energy conservation
• Energy conversion efficiency
• Energy density
• FF layout
• Front-wheel drive
• Fuel economy in aircraft
• Fuel economy in automobiles
• Fuel economy maximising behaviors
• Fuel efficiency in transportation
• Gas-guzzler
• Heating value
• Jevons paradox
• Life cycle assessment
• Low-rolling resistance tires
• Miles per gallon gasoline equivalent
• Marine fuel management
• Twinjet
• Variable valve timing
• Unibody
• Automobile costs
• Vehicle metrics

External links

• US Government website on fuel economy
• UK DfT comparisons on road and rail
• NASA Offers a $1.5 Million Prize for a Fast and Fuel-Efficient AircraftArchived2016-03-03 at theWayback Machine
• Car Fuel Consumption Official Figures
• Spritmonitor.de "the most fuel efficient cars" - Database of thousands of (mostly German) car owners' actual fuel consumption figures(cf.Spritmonitor)
• Searchable fuel economy data from the EPA - United States Environmental Protection Agency
• penghemat bbm - Alat penghemat bbm
• Ny Times: A Road Test of Alternative Fuel Visions