It should be understood that fuel belongs to that class of substances that will not burn nor evolve energy under any temperature, pressure, or shock, without an outside supply of oxygen. This is the characteristic property of all fuels used with the internal combustion engine. Each element, such as carbon and hydrogen, in a compound fuel, develops a certain definite amount of heat during their complete combustion, and at the close of the process certain compounds are formed that represent the lowest chemical form of the compound. To restore the products of combustion to their original form as fuel would require an expenditure of energy equal to that given out in the combustion.
While all substances that are capable of oxydization or combustion can be made to liberate heat energy, it does not follow that all of them can be successfully used as fuels. A fuel suitable for the production of power must be cheap, accessible and of small bulk, and must burn rapidly. Such fuels must also be products of nature that require no expenditure of energy in their preparation or completion.
Fig. F-4. Fairbanks-Morse Producer Plant and Engine, Connected for Operation.
In practical work, the natural fuels are coal, mineral oils, natural gas, and wood, which are compounds of the elements carbon and hydrogen. When these fuels are burned to their lowest forms the products of combustion consist of carbon dioxide and water, the first being the result of the oxydization of carbon, and the latter a compound of oxygen and hydrogen. In solid fuels, such as coal, a portion of the compound consists of free carbon and the remainder of a compound of carbon and hydrogen known as a HYDROCARBON. In liquid fuels there is little, if any, free carbon, the greater proportion being in the form of a hydrocarbon compound. Natural gas is a hydrocarbon compound.
It should be noted that a definite amount of oxygen is required for the complete combustion of the fuel elements, and that a smaller amount of oxygen than that called for by the fuel element results in incomplete combustion, which produces a product of higher form than that produced by the complete reduction. The product of incomplete combustion represents a smaller evolution of heat than that of the complete process, but if reburned in a fresh supply of oxygen the sum of the second combustion together with that of the first will equal the heat of the complete oxydization. When pure carbon is incompletely burned the product is carbon monoxide (CO) instead of carbon dioxide (CO2).
Carbon completely burned to carbon dioxide produces 14,500 British thermal units per pound of carbon, while the incomplete combustion to carbon monoxide evolves only 4,452 British thermal units, or less than one-third of the heat produced by the complete combustion. Theoretically one pound of carbon requires 2.66 pounds of oxygen to burn it to carbon dioxide. On supplying additional oxygen, the carbon monoxide may be burned to carbon dioxide and the remainder of the heat may be recovered, or 10,048 British thermal units. When a hydrocarbon, either solid, liquid or gaseous is burned with insufficient oxygen, solid carbon is precipitated together with lower hydrocarbons, and tar. In an internal combustion engine the precipitated solid carbon is evident in the form of smoke.
Since the carbon and hydrogen elements of a fuel exist in many different proportions and conditions in coal and oil, different amounts of oxygen are required for the consumption of different fuels. It should also be borne in mind that a greater quantity of air is required for the combustion of a fuel than oxygen, as the air is greatly diluted by an inert gas, nitrogen, which will not support combustion. Because of the impossibility of obtaining perfectly homogenous mixtures of air and the fuel, a greater quantity of air is used in practice than is theoretically required.
In a steam engine the fuel can be used in any form, solid, liquid, or gaseous, but in an internal combustion, it must be in the form of a gas no matter what may have been the form of the primary fuel. Fortunately there is no fuel which may not be transformed into a gas by some process if not already in a gaseous state. The petroleum products are vaporized by either the heat of the atmosphere or by spraying them on a hot surface. Coal is converted into a gas by distilling it in a retort or by incomplete combustion. The heat energy developed by a gas when burning in the open air depends on its chemical combustion, but its mechanical equivalent in power when burned in the cylinder of the engine depends not only upon its composition but upon the conditions under which it is burned as stated in the chapter devoted to the subject of heat engines.