Info Centre >> Fuel and Lube Oil Technical Manual >> 2. Fuel Oil - Characteristics >> 2.13 Specific Energy
Specific Energy of Fuel
The figure shows the relationship of the net specific energy of fuel, taking account of the density and sulphur content. On a world-wide basis, the density of residual fuel is typically in the range 975-990kg/m 3 and the sulphur level 2-4% m/m, hence the figure shows the variation in specific energy. For practical purposes, specific energy can be calculated from empirical equations. Of these there are two, one for the net specific energy (QN ) which is applicable for a diesel engine, and the other the gross specific energy (QG ) for boilers when all the water vapour is condensed out.
The figure shows the net specific energy taking account of variations in density and sulphur. Ash has a slight effect and may be accounted for by subtracting 0.02 MJ/kg for each 0.05% m/m of ash. Typically, the ash value is in the range 0.03-0.07% m/m.
The specific energy of a fuel expressed in MJ/kg depends on the composition. For residual fuel, the main constituents are carbon and hydrogen, both of which release energy on combustion. Sulphur also releases energy on combustion but to a lesser extent than carbon and hydrogen. The fuel density is mainly proportional to the ratio of carbon and hydrogen atoms in the fuel.