Fuel efficiency
Fuel efficiency sometimes means the same as
thermal efficiency or
fuel economy. This is the efficiency of converting energy contained ina carrier
fuel to
kinetic energy or
work. But fuel efficiency can also mean the output one gets for a unit amount of fuel input such as "
miles per gallon" for an
automobile. Here, vehicle-miles is the output, but for transportation, output can also be measured in terms of passenger-miles or ton-miles (of freight). While the
thermal efficiency of
petroleum engines has improved in recent decades, this does not necessarily translate into
fuel economy of
cars, as people in
developed countries tend to buy bigger and heavier cars. 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.
"Energy efficiency" is similar to fuel efficiency but the input isusually in units of energy such as BTU (British Thermal Units), MJ(MegaJoules), GJ (GigaJoules), kcal (kilo-calories), or kwh(kilowatt-hours). The inverse of "Energy efficiency" is "Energyintensity", or the amount of input energy required for a unit ofoutput such as MJ/passenger-km (of passenger transport), BTU/ton-mile(of freight transport), GJ/tonne (for steel production), BTU/kwh (forelectricity generation), or liters/100 km (of vehicle travel). Thislast term "liters/100km" is also a measure of "fuel economy" where theinput is measured by the amount of fuel and the output is measured bythe
distance travelled. For example:
Fuel economy in automobiles.
If one knows the heat value of a fuel, it's trivial to convert fromfuel units (such as liters of gasoline) to energy units (such as MJ)and conversely. Except that there are two different heat values forthe same fuel (see below) and for conversion from electricity to fuelenergy, one may need to know how much heat energy from fossil fuel ittook to generate the electricity used.
The specific energy content of a fuel is the heat energy that is obtained by burning a specific quantity of it (like a gallon, liter, kilogram, etc.). It's 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 high 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. See
[Appendix B, Trans. Energy Data Book]. In the U.S. (and thetable 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/imp gal | BTU/US gal | Research octane
number (RON) | | Gasoline | 32.90 | 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) |
Fuel economy is usually expressed in one of two ways:
*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 (the less fuel it needs to travel a certain distance);
*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 more distance it can travel with a certain volume of fuel).
Converting from mpg or km/L 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. 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.
In Europe, the two standard measuring cycles for "L/100 km" value are
motorway travel at 90 km/h and rush hour city traffic. A reasonably modern European
supermini may manage
motorway travel at 5 L/100 km (47 mpg US) or 6.5 L/100 km in city traffic (36 mpg US), with
carbon dioxide emissions of around 140 g/km.
An average
North American
mid-size car travels 27 mpg (US) (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city; a
full-size SUV usually travels 13 mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway.
Pickup trucks 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.
An interesting example of fuel economy is the popular
microcar Smart ForTwo, which can achieve up to 4.0 L/100 km (70.6 mpg) using a
turbocharged three-cylinder engine. The Smart is produced by
DaimlerChrysler and is currently only sold by one company in the United States (see external link
ZAP). The current record in fuel evonomy of production cars is held by
Volkswagen, with a special production model of the
Volkswagen Lupo (the
Lupo 3L) that can consume as little as 3
litres per 100
kilometres (78 miles per
US gallon or 94 miles per
Imperial gallon).
Diesel engines often achieve greater fuel efficiency than petrol (gasoline) engines: 50% of all cars sold in the EU are now diesel vehicles. This can also 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.
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 efficiency. 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.
[ CFM-1 experiment results, National Aeronautics and Space Administration, April 2005.] 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 aseries 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.
[SOFBAL-2 experiment results, National Aeronautics and Space Administration, April 2005.]*
Humans (see
Human-powered transport):
** walking or running one kilometre requires approximately 70
kcal or 330
kJ of
food energy [[1]]. This equates to about 1 l/100 km or 235 mpg in
gasoline energy terms.
**
cycling requires about 120 kJ/km
*
Airplanes: passenger airplanes averaged 4.8 l/100 km per passenger (1.4 MJ/passenger-km) (49 passenger-miles per gallon) in 1998. Efficiencies around 3 l/100 km per passenger are reached by some carriers.
[IATA - Fuel efficiency, IATA]. Note that on average 20% of seats are left unoccupied. However, airliner exhaust is more dangerous compared to pollution from land transportation, since the jet exhaust is directly spewed into the stratosphere, where NOx is especially active in ozone layer destruction.
*
Ships: the
RMS Queen Elizabeth 2 gets 49.5 feet per gallon
[[2], Cunard Line] (25,000 l/100 km or 13 l/100 km 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:
** Freight: the
AAR claims an energy efficiency of over 400 ton-miles per gallon of diesel fuel in 2004
[Railroads: Building a Cleaner Environment, Association of American Railroads] (0.588 l/100 km per tonne or 235 J/km-kg)
** Passengers: the
East Japan Railway Company claims for 2004 an energy intensity of 20.6 MJ/car-km, or about 0.35 MJ/passenger-km
[Environmental Goals and Results, JR-East Sustainability Report 2005] * Note that intercity rail in the U.S. reports 3.17 MJ/passenger-km which is several times higher than reported fromJapan. Independent transportation researcher David Lawyer attributesthis difference to the fact that the losses in electricity generationmay not have been taken into account for Japan.
[[http://www.lafn.org/~dave/trans/energy/fuel-eff-20th-3.html#japan_Fuel Efficiency of Travel in the 20th Century, Appendix]] andthat Japanese trains have a larger number of passenger per car.
[[http://www.lafn.org/~dave/trans/energy/fuel-eff-20th-2.html#_japanFuel Efficiency of Travel in the 20th Century]]It should be noted that modern electric trains, like the
shinkansen use
regenerative braking, returning current into the
catenary while they come to a halt. This method results in very significant energy savings, but is impossible to emulate with
diesel locomotives, which used on unelectrified US railway networks.
* the Center for Transportation Analysis of the
DOE claims the following average figures for the U.S.A. in 2002
[Passenger Travel and Energy Use, 2002, Center for Transportation Analysis, Oak Ridge National Laboratory]:
| Transport mode | Load factor(passengers/vehicle) | J/m - vehicle | J/m - passenger | BTU per vehicle-mile | BTU per passenger-mile | Equivalent passenger-milesper 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 G
Joules of energy, or about 100,000 l/100 km 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/100 km or about 0.5 mpg.
*
Emission standard*
Energy conservation*
Fuel economy in automobiles*
Tips on improving fuel efficiency*
How to increase auto fuel efficiency*
In-depth advice to help imcrease fuel efficiency*
US Government website on fuel economy
