Fuel Efficiency - Energy Content of Fuel

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.

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