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Fuel efficiency

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Fuel efficiency sometimes means the same as thermal efficiency. This is the efficiency of converting energy contained in a 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.

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Energy-Efficiency terminology

"Energy efficiency" is similar to fuel efficiency but the input is usually 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 "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), GJ/tonne (for steel production), BTU/kwh (for electricity generation), or liters/100 km (of vehicle travel). This last term "liters/100km" is also a measure of "fuel economy" where the input is measured by the amount of fuel and the output is measured by the distance travelled. For example: Fuel economy in automobiles.

If one knows the heat value of a fuel, it's trivial to convert from fuel units (such as liters of gasoline) to energy units (such as MJ) and conversely. Except that there are two different heat values for the same fuel (see below) and for conversion from electricity to fuel energy, one may need to know how much heat energy from fossil fuel it took to generate the electricity used.

Energy content of fuel

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 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/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

Fuel economy is usually expressed in one of two ways:

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]).

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.

Fuel efficiency in microgravity

The energy output derived from fuel occurs during combustion. Ensuring a total, even combustion of fuel, as well as harnessable combustion at the appropriate moments, will have an impact on fuel 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 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. [SOFBAL-2 experiment results], National Aeronautics and Space Administration, April 2005.

Fuel efficiency in transportation

Japan. Independent transportation researcher David Lawyer attributes this difference to the fact that the losses in electricity generation may not have been taken into account for Japan. [Efficiency of Travel in the 20th Century, Appendix] and that Japanese trains have a larger number of passenger per car. [Efficiency of Travel in the 20th Century]

Transport mode Load factor (passengers/vehicle) J/m - vehicle J/m - passenger BTU per vehicle-mile BTU per passenger-mile Equivalent passenger-miles per gallon of gasoline
Automobiles 1.57 3 686 2 347 5 623 3 581 34.9
Personal trucks 1.72 4 574 2 659 6 978 4 057 30.8
Motorcycles 1.22 1 640 1 490 2 502 2 274 55.0
Transit Buses 9.1 24 579 2 705 37 492 4 127 30.3
Airlines 95.8 232 489 2 427 354 631 3 703 33.8
Intercity trains 14.0 44 454 3 166 67 810 4 830 25.9
Commuter trains 33.5 59 556 1 779 90 845 2 714 46.1

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