We need these two ways of expressing the heating value of fuels because the combustion of some hydrogen-rich fuels releases water that is subsequently evaporated in the combustion chamber. In other words, the process of evaporating water “soaks up” some of the heat released by fuel combustion. That heat, known as the “latent heat of vaporization,” is temporarily lost and therefore does not contribute to the work done by the combustion process. As a result, the formation and vaporization of …