COMBUSTION. CLASSIFICATION OF FUELS. ACTION OF FUELS. FUEL SPECIFICATIONS. FILTERS. TANKS process of combustion which takes place in the cylinder of an internal combustion engine is simply a chemical reaction. In actuality the cylinder is merely a chem ist's retort wherein the atoms of hydrogen and carbon, which make up the body of the fuel charge, unite with the oxygen con tained in the air charge, forming oxides. The carbon in its union with oxygen forms either carbon monoxide (CO) or carbon dioxide (CO2). In this latter chemical reaction an atom of carbon unites with two atoms of oxygen forming one molecule of carbon dioxide; this combustion releases 14,600 B.t.u. Per pound of carbon and raises the remaining unburnt carbon to the incan descent point. Unless there is sufficient oxygen present to unite with this incandescent carbon, the latter unites with one of the oxygen atoms of the carbon dioxide causing the entire carbon oxide to assume the form of carbon monoxide (CO). Since the reaction is incomplete, the heat released by the forma tion of carbon monoxide from carbon and oxygen is less than that produced by the complete reaction, CO2. The value is approximately 4380 B.t.u., making evident the heat loss when the combustion is not complete. All Diesel cylinders are of ample volume to give sufficient oxygen for complete combustion. If the chemical reaction is not fully carried out, it is due to causes other than an insufficient supply of oxygen. In most instances the defect is traceable to poor atomization wherein the oil charge is not separated into particles of such minute dimensions that each carbon atom contacts with the required oxygen atoms. If the oil droplets entering the cylinder are of fairly large diameter, the oxygen is in direct contact with only the carbon at the surface of the droplet. The carbon atoms within the droplet must receive their oxygen from the carbon dioxide formed at the surface. For this reason the engine's efficiency depends on the degree of atomization of the fuel charge. The chemical reactions taking place probably follow this order: C + = CO2 • (1) C + = 2C0 (2) 2C0 + 02 = (3) The hydrogen atom of the oil molecule also unites with the oxygen, forming water or rather water vapor commonly called steam, the reaction being as follows: + = (4) This reaction generates 62,100 B.t.u. per pound of hydrogen. The equations do not refer to the actual weight of the carbon, oxygen and hydrogen but merely indicate the relation of the atoms. Since the atomic weights of the various substances
differ, it follows that the weight of each substance entering into the reaction depends on its atomic weight wherein a hydrogen atom has unity weight, carbon 12 and oxygen 16. Furthermore, where oxygen and hydrogen are not in combination with other gases, both oxygen and hydrogen have their atoms, or most minute particles, in groups of two or more. Equation (1) can then be written C = CO2 12 + 32 = 44 indicating that 12 parts by weight of carbon combining with 32 parts of oxygen form 44 parts of carbon dioxide; then, 1 pound of carbon requires or 2.67 pounds of oxygen to be converted into 3.67 pounds of CO2. Since a pound of air con tains pound of oxygen, there are 11.6 pounds of air re quired in the combustion of 1 pound of carbon. Air at 62° Fahrenheit has a volume of 13.14 cubic feet per pound; the 11.6 pounds then have a volume of 152.4 cubic feet. The hydrogen reaction (4) can be written as follows: + = 21120 4 + 32 = 36 where 1 pound of hydrogen requires 8 pounds of oxygen or 34.78 pounds of air.
While the petroleum oils contain hydrocarbons of a varied structure, the equation below, covering the ethylene series of hydrocarbons, outlines the process of equating the reactions taking place.
= + 28 + 96 = 88 + 36 where 28 pounds of ethylene require 96 pounds of oxygen to form 88 pounds of carbon dioxide and 36 pounds of water or steam. Since 23 per cent. of the air is oxygen, 417 pounds of air are required to consume the 28 pounds of of which 77 per cent., or 321 pounds, is nitrogen, which experiences no chemical reaction. Then, for perfect combustion, the percentage by weight of the exhaust products would be 1120 Carbon dioxide Water Nitrogen 20 8 72 These are the theoretical percentages and are quite different from those obtained on an actual Diesel engine test where the percentages obtained were as follows: N 0 Carbon dioxide Carbon monoxide Nitrogen Oxygen 7.2 .2 81.6 11 The percentage was not obtained in the test. This analysis would apparently indicate that an excessive amount of air was employed due to large cylinders or to an over-supply of injection air. The oxygen percentage could be reduced by shortening the period of injection valve opening. From these results, which were obtained under full-load conditions, the conclusion could be drawn that the engine able to carry a considerable overload.