Fuel efficiency
Encyclopedia
Fuel efficiency is a form of thermal efficiency
, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel
into kinetic energy
or 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 plant
s 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, and is given as a ratio
of distance travelled per unit of fuel
consumed. Fuel efficiency is expressed in mile
s per gallon
(mpg
) (prevalent in the USA and UK, using their respective definitions of a gallon) or kilometres per litre (km/L) (prevalent in the Netherlands and in several Latin America
n or Asian countries such as Brazil
, India
and Japan
). The reciprocal ratio, "fuel consumption", is usually expressed in liter
s per 100 kilometers (L/100 km) (common in Europe, Canada, New Zealand and Australia) or litres per mil (Norway
/Sweden
).
Variations on a vehicle's fuel efficiency include weight-specific efficiency for freight, and passenger-specific efficiency (vehicle efficiency / number of passengers).
parameters, aerodynamic drag
, weight, and rolling resistance
. There have been advances in all areas of vehicle design in recent decades.
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.
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:
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.
Converting from mpg or to L/100 km (or vice versa) involves the use of the reciprocal
function, which is not distributive. Therefore, the average of two fuel economy numbers gives different values if those units are used, because one of the functions is reciprocal, thus not linear. If two people calculate the fuel economy average of two groups of cars with different units, the group with better fuel economy may be one or the other. However, from the point of energy used as a shared method of measure, the result shall be the same in both the cases.
The formula for converting to miles per US gallon (3.785 L) from L/100 km is
, where 
is value of L/100 km. For miles per Imperial gallon (4.546 L) the formula is 
.
In Europe, the two standard measuring cycles for "litre/100 km" value are "urban" traffic with speeds up to 50 km/h from a cold start, and then "extra urban" travel at various speeds up to 120 km/h which follows the urban test. A combined figure is also quoted showing the total fuel consumed in divided by the total distance traveled in both tests. A reasonably modern European supermini
and many mid-size cars, including station wagons, may manage motorway travel at 5 L/100 km (47 mpg US/56 mpg imp) or 6.5 L/100 km in city traffic (36 mpg US/43 mpg imp), with carbon dioxide
emissions of around 140 g/km.
An average North America
n mid-size car
travels 21 mpg (US) (11 L/100 km) city, 27 mpg (US) (9 L/100 km) highway; a full-size
SUV usually travels 13 mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway. Pickup truck
s vary considerably; whereas a 4 cylinder-engined light pickup can achieve 28 mpg (8 L/100 km), a V8 full-size pickup with extended cabin only travels 13 mpg (US) (18 L/100 km) city and 15 mpg (US) (15 L/100 km) highway.
European-built cars are generally more fuel-efficient than American vehicles. While Europe has many higher efficiency diesel cars, European gasoline vehicles are on average also more efficient than gasoline-powered vehicles in the USA. Most European vehicles cited in the CSI study run on diesel engines, which tend to achieve greater fuel efficiency than gas engines. Selling those cars in the United States is difficult because of emission standards, notes Walter McManus, a fuel economy expert at the University of Michigan Transportation Research Institute. “For the most part, European diesels don’t meet U.S. emission standards,” McManus said in 2007. Another reason why many European models are not marketed in the United States is that labor unions object to having the big 3 import any new foreign built models regardless of fuel economy while laying off workers at home.
An interesting example of European cars' capabilities of fuel economy is the microcar
Smart Fortwo
cdi, which can achieve up to 3.4 l/100 km (69.2 mpg US) using a turbocharged
three-cylinder 41 bhp (30 kW) Diesel engine. The Fortwo is produced by Daimler AG and is currently only sold by one company in the United States. Furthermore, the current (and to date already 10 year old) world record in fuel economy of production cars is held by the Volkswagen Group
, with special production models (labeled "3L") of the Volkswagen Lupo and the Audi A2, consuming (NEDC ratified
) as little as 2.99 litre
s of diesel fuel per 100 kilometre
s (78 miles per US gallon
or 94 miles per Imperial
gallon).
Diesel engine
s generally achieve greater fuel efficiency than petrol (gasoline) engines. Passenger car diesel engines have energy efficiency
of up to 41% but more typically 30%,
and petrol engines of up to 37.3%, but more typically 20%. That is one of the reasons why diesels have better fuel efficiency than equivalent petrol cars. A common margin is 25% more miles per gallon for an efficient turbodiesel.
For example, the current model Skoda Octavia, using Volkswagen engines, has a combined European fuel efficiency of 41.3 mpg for the 105 bhp petrol engine and 52.3 mpg for the 105 bhp — and heavier — diesel engine. The higher compression ratio is helpful in raising the energy efficiency, but diesel fuel also contains approximately 10% more energy per unit volume than gasoline which contributes to the reduced fuel consumption for a given power output.
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 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 the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. Experiments by NASA in microgravity reveal that diffusion flame
s 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. Premixed flame
s in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.
, LPG or biofuel
or electricity which creates various quantities of atmospheric pollution.
A kilogram of carbon, whether contained in petrol, diesel, kerosene, or any other hydrocarbon fuel in a vehicle, leads to approximately 3.6Kg of CO2 emissions. Due to the carbon content of gasoline, its combustion emits 2.3 Kg/l (19.4 lb/US gal) of CO2; since diesel fuel is more energy dense per unit volume, diesel emits 2.6 Kg/l (22.2 lb/US gal). This figure is only the CO2 emissions of the final fuel product and does not include additional CO2 emissions created during the drilling, pumping, transportation and refining steps required to produce the fuel. Additional measures to reduce overall emission includes improvements to the efficiency of air conditioners, lights and tires.
There is a growing community of enthusiasts known as hypermilers who develop and practice driving techniques to increase fuel efficiency and reduce consumption. Hypermilers have broken records of fuel efficiency, for example, achieving 109 miles per gallon in a Prius. In non-hybrid vehicles these techniques are also beneficial. Hypermiler Wayne Gerdes can get 59 MPG in a Honda Accord
and 30 MPG in an Acura MDX
.
The most efficient machines for converting energy to rotary motion are electric motors, as used in electric vehicles. However, electricity is not a primary energy source so the efficiency of the electricity production has also to be taken into account. Currently railway trains can be powered using electricity, delivered through an additional running rail, overhead catenary
system or by on-board generators used in diesel-electric locomotives as common on the UK rail network. Pollution produced from centralised generation of electricity is emitted at a distant power station, rather than "on site". 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 a Eurostar
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. This can be changed using more renewable sources
for electric generation.
In the future hydrogen cars
may be commercially available. Powered either through chemical reactions in a fuel cell
that create electricity to drive very efficient electrical motors or by directly burning hydrogen in a combustion engine (near identically to a natural 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 by electrolysis
using electricity from non-polluting sources such as solar, wind or hydroelectricity
or thermochemically by the use of the Thorium fuel cycle
in a molten salt reactor
.
In any process, it is vitally important to account for all of the energy used throughout the process. Thus, in addition to the energy cost of the electricity or hydrogen production, we must also account for transmission and/or storage losses to support large-scale use of such vehicles. For this reason the use of the idea "zero pollution" should be avoided.
Thermal efficiency
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...
, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
into kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
or work
Mechanical work
In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does 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
Industry
Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
, benefit from increased fuel efficiency, especially fossil fuel power plant
Fossil fuel power plant
A fossil-fuel power station is a power station that burns fossil fuels such as coal, natural gas or petroleum to produce electricity. Central station fossil-fuel power plants are designed on a large scale for continuous operation...
s or industries dealing with combustion, such as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
production during the Haber process
Haber process
The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen gas and hydrogen gas, over an enriched iron or ruthenium catalyst, which is used to industrially produce ammonia....
.
In the context of transport
Transport
Transport or transportation is the movement of people, cattle, animals and goods from one location to another. Modes of transport include air, rail, road, water, cable, pipeline, and space. The field can be divided into infrastructure, vehicles, and operations...
, fuel economy is the energy efficiency of a particular vehicle, and is given as a ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
of distance travelled per unit of fuel
Motor fuel
A motor fuel is a fuel that is used to provide power to motor vehicles.Currently, the majority of motor vehicles worldwide are powered by gasoline or diesel. Other energy sources include ethanol, biodiesel, propane, compressed natural gas , electric batteries charged from an external source, and...
consumed. Fuel efficiency is expressed in mile
Mile
A mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...
s per gallon
Gallon
The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...
(mpg
Mileage
-Motor Vehicles:* Fuel economy in automobiles * Distance travelled as measured by an odometer...
) (prevalent in the USA and UK, using their respective definitions of a gallon) or kilometres per litre (km/L) (prevalent in the Netherlands and in several Latin America
Latin America
Latin America is a region of the Americas where Romance languages – particularly Spanish and Portuguese, and variably French – are primarily spoken. Latin America has an area of approximately 21,069,500 km² , almost 3.9% of the Earth's surface or 14.1% of its land surface area...
n or Asian countries such as Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
). The reciprocal ratio, "fuel consumption", is usually expressed in liter
Litér
- External links :*...
s per 100 kilometers (L/100 km) (common in Europe, Canada, New Zealand and Australia) or litres per mil (Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
/Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
).
Variations on a vehicle's fuel efficiency include weight-specific efficiency for freight, and passenger-specific efficiency (vehicle efficiency / number of passengers).
Vehicle design
Fuel efficiency is dependent on many parameters of a vehicle, including its engineEngine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
parameters, aerodynamic drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
, weight, and rolling resistance
Rolling resistance
Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or tire rolls on a flat surface, in steady velocity straight line motion. It is caused mainly by the deformation of the object, the deformation of the surface, or...
. There have been advances in all areas of vehicle design in recent decades.
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.
Fleet efficiency
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, which are less efficient, all else being equal.Energy efficiency terminology
Energy efficiency is similar to fuel efficiency but the input is usually in units of energy such as British thermal units (BTU), megajoules (MJ), gigajoules (GJ), kilocalories (kcal), or kilowatt-hours (kW·h). 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 (of freight transport, for long/short/metric tons), GJ/t (for steel production), 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 the distanceDistance
Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical distance...
travelled. For example: Fuel economy in automobiles
Fuel economy in automobiles
Fuel usage in automobiles refers to the fuel efficiency relationship between distance traveled by an automobile and the amount of fuel consumed....
.
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 a kilowatt hour of 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 specific energy 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 the heat of combustion. There exists two different values of specific 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 the latent 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 below) 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 British thermal unit The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F... /imp gal Gallon The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry... |
BTU/US gal | Research octane number (RON) Octane rating Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating... |
|---|---|---|---|---|---|
| Regular gasoline Gasoline Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain... /petrol |
34.8 | ~47 | 150,100 | 125,000 | Min. 91 |
| Premium gasoline Gasoline Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain... /petrol |
~46 | Min. 95 | |||
| Autogas Autogas Autogas is the common name for liquefied petroleum gas when it is used as a fuel in internal combustion engines in vehicles as well as in stationary applications such as generators. It is a mixture of propane and butane.... (LPG) (60% propane Propane Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central... and 40% butane Butane Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or... ) |
25.5–28.7 | ~51 | 108–110 | ||
| Ethanol Ethanol fuel Ethanol fuel is ethanol , the same type of alcohol found in alcoholic beverages. It is most often used as a motor fuel, mainly as a biofuel additive for gasoline. World ethanol production for transport fuel tripled between 2000 and 2007 from 17 billion to more than 52 billion litres... |
23.5 | 31.1 | 101,600 | 84,600 | 129 |
| Methanol Methanol Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol... |
17.9 | 19.9 | 77,600 | 64,600 | 123 |
| Gasohol Alcohol fuel Although fossil fuels have become the dominant energy resource for the modern world, alcohol has been used as a fuel throughout history. The first four aliphatic alcohols are of interest as fuels because they can be synthesized chemically or biologically, and they have characteristics which allow... (10% ethanol and 90% gasoline) |
33.7 | ~45 | 145,200 | 121,000 | 93/94 |
| E85 E85 E85 is an abbreviation for an ethanol fuel blend of up to 85% denatured ethanol fuel and gasoline or other hydrocarbon by volume. E85 is commonly used by flex-fuel vehicles in the US, Canada, and Europe. Some of the benefits of E85 over conventional gasoline powered vehicles include the potential... (85% ethanol and 15% gasoline) |
33.1 | 44 | 108,878 | 90,660 | 100–105 |
| Diesel | 38.6 | ~48 | 166,600 | 138,700 | N/A (see cetane) Cetane number Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality.- Definition :... |
| BioDiesel Biodiesel Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol.... |
35.1 | 39.9 | 151,600 | 126,200 | N/A (see cetane) Cetane number Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality.- Definition :... |
| 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 Jet fuel Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification... , naphtha |
35.5 | 46.6 | 153,100 | 127,500 | N/A to turbine engines |
| Jet fuel Jet fuel Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification... , kerosene |
37.6 | ~47 | 162,100 | 135,000 | N/A to turbine engines |
| Liquefied natural gas Liquefied natural gas Liquefied natural gas or LNG is natural gas that has been converted temporarily to liquid form for ease of storage or transport.... |
25.3 | ~55 | 109,000 | 90,800 | |
| Liquid hydrogen Liquid hydrogen Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be... |
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 in Gibbs free energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
, 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
Brake specific fuel consumption
Brake Specific Fuel Consumption is a measure of fuel efficiency within a shaft reciprocating engine.It is the rate of fuel consumption divided by the power produced. It may also be thought of as power-specific fuel consumption, for this reason...
) 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
Brake specific fuel consumption
Brake Specific Fuel Consumption is a measure of fuel efficiency within a shaft reciprocating engine.It is the rate of fuel consumption divided by the power produced. It may also be thought of as power-specific fuel consumption, for this reason...
for more information.
Fuel efficiency of vehicles
The fuel efficiency of vehicles can be expressed in more ways:- Fuel consumption is the amount of fuel used per unit distance; for example, litreLitrepic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
s per 100 kilometreKilometreThe kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
s (L/100 km). In this case, the lower the value, the more economic a vehicle is (the less fuel it needs to travel a certain distance); this is the measure generally used across Europe (except the UK and Denmark - see below), New Zealand, Australia and Canada. Also in Uruguay, Paraguay, Guatemala, Colombia, Japan, China, and Madagascar., as also in post-Soviet space.
- Fuel economy is the distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L) or mileMileA mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...
s per gallonGallonThe gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...
(MPG), where 1 MPG (imperial) = 0.354013 km/L. In this case, the higher the value, the more economic a vehicle is (the more distance it can travel with a certain volume of fuel). This measure is popular in the USA and the UK (mpg), but in Europe, India, Japan and Latin America the metric unit km/L is used instead.
Converting from mpg or to L/100 km (or vice versa) involves the use of the reciprocal
Multiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
function, which is not distributive. Therefore, the average of two fuel economy numbers gives different values if those units are used, because one of the functions is reciprocal, thus not linear. If two people calculate the fuel economy average of two groups of cars with different units, the group with better fuel economy may be one or the other. However, from the point of energy used as a shared method of measure, the result shall be the same in both the cases.
The formula for converting to miles per US gallon (3.785 L) from L/100 km is
, where is value of L/100 km. For miles per Imperial gallon (4.546 L) the formula is .In Europe, the two standard measuring cycles for "litre/100 km" value are "urban" traffic with speeds up to 50 km/h from a cold start, and then "extra urban" travel at various speeds up to 120 km/h which follows the urban test. A combined figure is also quoted showing the total fuel consumed in divided by the total distance traveled in both tests. A reasonably modern European supermini
Supermini car
A supermini is a British term that describes automobiles larger than a city car but smaller than a small family car. This car class is also known as the B-segment across Europe, and as subcompact in North America....
and many mid-size cars, including station wagons, may manage motorway travel at 5 L/100 km (47 mpg US/56 mpg imp) or 6.5 L/100 km in city traffic (36 mpg US/43 mpg imp), with carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
emissions of around 140 g/km.
An average North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n mid-size car
Mid-size car
A mid-size car is the North American/Australian standard for an automobile with a size equal to or greater than that of a compact...
travels 21 mpg (US) (11 L/100 km) city, 27 mpg (US) (9 L/100 km) highway; a full-size
Full-size car
A full-size car is a marketing term used in North America for an automobile larger than a mid-size car. In the United States, the EPA uses "large car" to denote full-size cars....
SUV usually travels 13 mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway. Pickup truck
Pickup truck
A pickup truck is a light motor vehicle with an open-top rear cargo area .-Definition:...
s vary considerably; whereas a 4 cylinder-engined light pickup can achieve 28 mpg (8 L/100 km), a V8 full-size pickup with extended cabin only travels 13 mpg (US) (18 L/100 km) city and 15 mpg (US) (15 L/100 km) highway.
European-built cars are generally more fuel-efficient than American vehicles. While Europe has many higher efficiency diesel cars, European gasoline vehicles are on average also more efficient than gasoline-powered vehicles in the USA. Most European vehicles cited in the CSI study run on diesel engines, which tend to achieve greater fuel efficiency than gas engines. Selling those cars in the United States is difficult because of emission standards, notes Walter McManus, a fuel economy expert at the University of Michigan Transportation Research Institute. “For the most part, European diesels don’t meet U.S. emission standards,” McManus said in 2007. Another reason why many European models are not marketed in the United States is that labor unions object to having the big 3 import any new foreign built models regardless of fuel economy while laying off workers at home.
An interesting example of European cars' capabilities of fuel economy is the microcar
Microcar
A microcar is the smallest automobile classification usually applied to standard small car . Such small cars were generally referred to as cyclecars until the 1940s. More recent models are also called bubblecars due to their egg-shaped appearance.-Definition:The definition of a microcar has varied...
Smart Fortwo
Smart Fortwo
The Smart Fortwo is a rear-engined two-seater city car manufactured by Smart GmbH, introduced at the 1998 Paris Motor Show as the Smart City Coupé, and currently in its second generation...
cdi, which can achieve up to 3.4 l/100 km (69.2 mpg US) using a turbocharged
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
three-cylinder 41 bhp (30 kW) Diesel engine. The Fortwo is produced by Daimler AG and is currently only sold by one company in the United States. Furthermore, the current (and to date already 10 year old) world record in fuel economy of production cars is held by the Volkswagen Group
Volkswagen Group
Volkswagen Group is a German multinational automobile manufacturing group. , Volkswagen was ranked as the world’s third largest motor vehicle manufacturer and Europe's largest....
, with special production models (labeled "3L") of the Volkswagen Lupo and the Audi A2, consuming (NEDC ratified
New European Driving Cycle
The New European Driving Cycle is a driving cycle consisting of four repeated ECE-15 driving cycles and an Extra-Urban driving cycle . The NEDC is supposed to represent the typical usage of a car in Europe, and is used, among other things, to assess the emission levels of car engines...
) as little as 2.99 litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
s of diesel fuel per 100 kilometre
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
s (78 miles per US gallon
Gallon
The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...
or 94 miles per Imperial
Imperial unit
The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire...
gallon).
Diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s generally achieve greater fuel efficiency than petrol (gasoline) engines. Passenger car diesel engines have energy efficiency
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...
of up to 41% but more typically 30%,
and petrol engines of up to 37.3%, but more typically 20%. That is one of the reasons why diesels have better fuel efficiency than equivalent petrol cars. A common margin is 25% more miles per gallon for an efficient turbodiesel.
For example, the current model Skoda Octavia, using Volkswagen engines, has a combined European fuel efficiency of 41.3 mpg for the 105 bhp petrol engine and 52.3 mpg for the 105 bhp — and heavier — diesel engine. The higher compression ratio is helpful in raising the energy efficiency, but diesel fuel also contains approximately 10% more energy per unit volume than gasoline which contributes to the reduced fuel consumption for a given power output.
Fuel efficiency in microgravity
How fuel combusts affects how much energy is produced. The National Aeronautics and Space Administration (NASA) has investigated fuel consumption in microgravity.The common distribution of a flame under normal gravity conditions depends on convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, 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
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
, convection no longer occurs, and the flame becomes spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
, 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. Experiments by NASA in microgravity reveal that diffusion flame
Diffusion flame
In combustion, a diffusion flame is a flame in which the oxidizer combines with the fuel by diffusion. As a result, the flame speed is limited by the rate of diffusion. Diffusion flames tend to burn slower and to produce more soot than premixed flames because there may not be sufficient oxidizer...
s 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. Premixed flame
Premixed flame
A premixed flame is a flame in which the oxidizer has been mixed with the fuel before it reaches the flame front. This creates a thin flame front as all of the reactants are readily available...
s in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.
Vehicle efficiency and transportation pollution
Fuel efficiency directly affects emissions causing pollution by affecting the amount of fuel used. However, it also depends on the fuel source used to drive the vehicle concerned. Cars can, for example, run on a number of fuel types other than gasoline, such as natural gasNatural gas vehicle
thumb|right|200px|2009 [[Honda Civic GX]] hooked up to Phill refueling system.A natural gas vehicle or NGV is an alternative fuel vehicle that uses compressed natural gas or liquefied natural gas as a clean alternative to other fossil fuels...
, LPG or biofuel
Biofuel
Biofuel is a type of fuel whose energy is derived from biological carbon fixation. Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases...
or electricity which creates various quantities of atmospheric pollution.
A kilogram of carbon, whether contained in petrol, diesel, kerosene, or any other hydrocarbon fuel in a vehicle, leads to approximately 3.6Kg of CO2 emissions. Due to the carbon content of gasoline, its combustion emits 2.3 Kg/l (19.4 lb/US gal) of CO2; since diesel fuel is more energy dense per unit volume, diesel emits 2.6 Kg/l (22.2 lb/US gal). This figure is only the CO2 emissions of the final fuel product and does not include additional CO2 emissions created during the drilling, pumping, transportation and refining steps required to produce the fuel. Additional measures to reduce overall emission includes improvements to the efficiency of air conditioners, lights and tires.
Driving technique
Many tips are available from various government and local sources to help drivers improve their fuel efficiency.There is a growing community of enthusiasts known as hypermilers who develop and practice driving techniques to increase fuel efficiency and reduce consumption. Hypermilers have broken records of fuel efficiency, for example, achieving 109 miles per gallon in a Prius. In non-hybrid vehicles these techniques are also beneficial. Hypermiler Wayne Gerdes can get 59 MPG in a Honda Accord
Honda Accord
The Honda Accord is a series of compact, mid-size and full-size automobiles manufactured by Honda since 1976, and sold in a majority of automotive markets throughout the world....
and 30 MPG in an Acura MDX
Acura MDX
The Acura MDX, or Honda MDX as known in Japan and Australia , is a mid-size luxury crossover SUV produced by the Japanese automaker Acura since the 2001 model year. The MDX was the first crossover SUV to have third-row seating. It is related to the Honda Pilot. The alphanumeric moniker stands for...
.
The most efficient machines for converting energy to rotary motion are electric motors, as used in electric vehicles. However, electricity is not a primary energy source so the efficiency of the electricity production has also to be taken into account. Currently railway trains can be powered using electricity, delivered through an additional running rail, overhead catenary
Overhead lines
Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point...
system or by on-board generators used in diesel-electric locomotives as common on the UK rail network. Pollution produced from centralised generation of electricity is emitted at a distant power station, rather than "on site". 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 a Eurostar
Eurostar
Eurostar is a high-speed railway service connecting London with Paris and Brussels. All its trains traverse the Channel Tunnel between England and France, owned and operated separately by Eurotunnel....
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. This can be changed using more renewable sources
Renewable energy
Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...
for electric generation.
In the future hydrogen cars
Hydrogen vehicle
A hydrogen vehicle is a vehicle that uses hydrogen as its onboard fuel for motive power. Hydrogen vehicles include hydrogen fueled space rockets, as well as automobiles and other transportation vehicles...
may be commercially available. Powered either through chemical reactions in a fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
that create electricity to drive very efficient electrical motors or by directly burning hydrogen in a combustion engine (near identically to a natural gas vehicle
Natural gas vehicle
thumb|right|200px|2009 [[Honda Civic GX]] hooked up to Phill refueling system.A natural gas vehicle or NGV is an alternative fuel vehicle that uses compressed natural gas or liquefied natural gas as a clean alternative to other fossil fuels...
, 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 by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
using electricity from non-polluting sources such as solar, wind or hydroelectricity
Hydroelectricity
Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy...
or thermochemically by the use of the Thorium fuel cycle
Thorium fuel cycle
The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts...
in a molten salt reactor
Molten salt reactor
A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...
.
In any process, it is vitally important to account for all of the energy used throughout the process. Thus, in addition to the energy cost of the electricity or hydrogen production, we must also account for transmission and/or storage losses to support large-scale use of such vehicles. For this reason the use of the idea "zero pollution" should be avoided.
See also
|
|
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 Aircraft
- Spritmonitor.de "the most fuel efficient cars" - Database of thousands of (mostly German) car owners' actual fuel consumption figures
- Independent testing proves fuel savings of innovative engine