Engine Efficiency

In the internal combustion engine, air and fuel are mixed to form a combustible mixture that is ignited and releases energy in the form of heat. The amount of heat released depends on a number of factors. While the amount of fuel trapped in the cylinder is the primary determinant of the energy content of the trapped air/fuel mixture and thus the total amount of heat that can be released, a number of secondary factors are also important. These secondary factors include details about the fuel composition such as the type of elements contained in the fuel and the nature of the bonds joining the elements together.For engines, the net energy released from combustion is typically represented by the lower heating value (LHV) of the fuel since the water produced by combustion is assumed to remain in the vapor state. Figure 2 shows the LHV of a range of fuels that could be used in an internal combustion engine versus their stoichiometric air fuel ratio. Notice that for hydrocarbon fuels, the LHVs are very similar and considerably higher than for fuels containing oxygen. Oxygenated functional groups contribute less net energy during combustion while contributing significantly to the fuel’s mass and volume.