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/imp gal | BTU/US gal | Research octane number (RON) |
|---|---|---|---|---|---|
| Regular gasoline/petrol | 34.8 | ~47 | 150,100 | 125,000 | Min. 91 |
| Premium gasoline/petrol | ~46 | Min. 95 | |||
| Autogas (LPG) (60% propane and 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) | 33.1 | 44 | 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 | 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, 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.
Read more about this topic: Fuel Efficiency
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