Energy Content

Every fuel can liberate a fixed amount of energy when it reacts completely with oxygen to form water. This energy content is measured experimentally and is quantified by a fuel’s higher heating value (HHV) and lower heating value (LHV). The difference between the HHV and the LHV is the “heat of vaporization” and represents the amount of energy required to vaporize a liquid fuel into a gaseous fuel, as well as the energy used to convert water to steam.

The higher and lower heating values of comparative fuels are indicated in Table 1-3.

All fuels burn only in a gaseous or vapour state. Fuels like hydrogen and methane are already gases at atmospheric conditions, whereas other fuels like gasoline or diesel that are liquids must convert to a vapour before they will burn. The characteristic that describes how easily these fuels can be converted to a vapour is the flashpoint. The flashpoint is defined as the temperature at which the fuel produces enough vapours to form an ignitable mixture with air at its surface.

If the temperature of the fuel is below its flashpoint, it can-not produce enough vapours to burn since its evaporation rate is too slow. Whenever a fuel is at or above its flashpoint, vapours are present. The flashpoint is not the temperature at which the fuel bursts into flames; that is the autoignition temperature.

Hydrogen has the highest energy to weight ratio, NASA has used it as a rocket fuel since the 1940’s.

NASA also uses hydrogen for it’s primary fuel while out in space, and for making drinking water, one pound of hydrogen when combined with oxygen will make nine pounds of pure distilled drinking water. Through the process, it will generate a significant amount of usable electricity as a by product. The Navy has been using electrolysers for their submarines to make oxygen for long missions; they would turn on the Diesel engines, and turn the sea water into hydrogen and oxygen, essentially opposite of what NASA is doing.