<Li> energy / mass of fuel </Li> <Li> energy / volume of the fuel </Li> <P> There are two kinds of heat of combustion, called higher and lower heating value, depending on how much the products are allowed to cool and whether compounds like H O are allowed to condense . The values are conventionally measured with a bomb calorimeter . They may also be calculated as the difference between the heat of formation ΔH of the products and reactants (though this approach is somewhat artificial since most heats of formation are calculated from measured heats of combustion). For a fuel of composition C H O N, the (higher) heat of combustion is 418 kJ / mol (c + 0.3 h--0.5 o) usually to a good approximation (± 3%), though it can be drastically wrong if o + n> c (for instance in the case of nitroglycerine (C O) this formula would predict a heat of combustion of 0). The value corresponds to an exothermic reaction (a negative change in enthalpy) because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in oxygen to the stronger bonds in carbon dioxide and water releases energy as heat . </P> <P> The quantity known as higher heating value (HHV) (or gross energy or upper heating value or gross calorific value (GCV) or higher calorific value (HCV)) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced . Such measurements often use a standard temperature of 15 ° C (59 ° F; 288 K). This is the same as the thermodynamic heat of combustion since the enthalpy change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is condensed to a liquid . The higher heating value takes into account the latent heat of vaporization of water in the combustion products, and is useful in calculating heating values for fuels where condensation of the reaction products is practical (e.g., in a gas - fired boiler used for space heat). In other words, HHV assumes all the water component is in liquid state at the end of combustion (in product of combustion) and that heat delivered at temperatures below 150 ° C (302 ° F) can be put to use </P>

Difference between higher and lower calorific value of fuel
find me the text answering this question