The [[Enthalpy|enthalpy]] of combustion is the [[Heat|heat]] released when [[Combustion Process|combusting]] a fuel, therefore it is the difference between the enthalpies of [[Chemical Product|products]] and [[Chemical Reactant|reactants]]. $H_{RP}=H_P−H_R​$ $H_{RP}=\sum_Pn_e(\bar{h}_f^0+\Delta\bar{h})_e−\sum_Rn_i(\bar{h}_f^0+\Delta\bar{h})_i$ Similar to the enthalpy of combustion, the [[Internal Energy|internal energy]] of combustion is defined as the difference in product and reactant internal energies. $U_{RP}=U_P−U_R$ $U_{RP}=\sum_Pn_e(\bar{h}_f^0+\Delta\bar{h}−P\bar{\nu})_e−\sum_Rn_i(\bar{h}_f^0+\Delta\bar{h}−P\bar{\nu})_i$ For [[Ideal Gas|ideal gases]] we can utilize the ideal gas law: $U_{RP}=H_{RP}−\bar{R}T$ Heating value refers to the amount of heat transferred during combustion at a constant [[Temperature|temperature]]. The higher heating value is the heat transferred by the [[Liquid|liquid]] portion of product water. The lower heating value is the heat transferred by the [[Gas|gaseous]] portion of product water.