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A meaningful discussion of the subject of this paper is only possible within predefined limits. If all energy used is in the form of electricity generated by solar power or nuclear fusion, fuel composition is of no significance. However, since chemical fuels, particularly liquid chemical fuels, represent a convenient and inexpensive way of storing large amounts of energy in a small, light container, it is probable that they will be used for self-propelled vehicles at least for many years to come. The air pollution problems associated with their use are well on the way to being solved. The chemical fuels can be easily distributed and handled and they can be synthesized from available raw materials. This discussion will be limited to chemical fuels.Since the combustion products of fuels for oxidation processes must be capable of being handled by the biosphere without damaging it, the useful elemental compositions are limited. Elements whose oxidation products are irritating or toxic (for example sulfur) cannot be considered nor can those whose scarcity precludes their general use. Thus chemical fuels must probably be compounds of carbon, hydrogen, oxygen, and perhaps nitrogen, although inclusion of nitrogen compounds under some circumstances creates problems.Obviously, during the twentieth century the most widely used liquid fuels have been hydrocarbons, and these compounds have many advantages as chemical fuels. Their physical properties vary widely, making possible the tailoring of fuels to meet varying combustion requirements. Since they have been manufactured in quantity, the technology for their efficient distribution and use has been developed. Most importantly, their energy content per unit mass is higher than that of other eligible compounds. Figure 1 shows the net heat of combustion in Btu/# as a function of molecular weight for hydrocarbons, and some oxygen, nitrogen and sulfur compounds. As would be expected, as the oxygen content of a molecule increases the heat of combustion decreases.

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