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" (also referred to as MPG for short) 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.
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 [1]. 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:
- 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).
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 Transportation
- Humans:
- 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. [3]. 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 [4] (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:
- Note that intercity rail in the U.S. reports 3.17 MJ/passenger-km which is several times higher than reported from 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. [7] Note also that Japanese trains have a larger number of passengers per car.[8] It should also 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 are used on unelectrified US railway networks.
- 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 GJoules 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-enter 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.
The Center for Transportation Analysis of the United States Department of Energy claims the following average figures for the U.S.A. in 2002[9]:
| 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 |
