Page:International Library of Technology, Volume 93.djvu/85

 seldom formed in the gas-engine cylinder, but it is one of the principal constituents of water gas and producer gas.

17. In gas-engine practice, it is not customary to deal with the weights of the gases employed, since gases are invariably measured in cubic feet. To calculate the amount of air required for the complete combustion of any gas when its composition by volume is known, Table III and the formula for hydrocarbons afterwards given may be used.

18. To Find Volume of Air Required for Complete Combustion of Any Hydrocarbon. — All formulas for hydrocarbons may be expressed by the general formula CnHm, in which n represents the number of atoms of C, and m the number of atoms of H in one molecule of the hydrocarbon.

19. Every atom of C burning to CO, requires two atoms of oxygen. Every 2 atoms of H requires one atom of O. The number of atoms of O required for the complete combustion of any hydrocarbon would then be $$2n + {m \over 2}$$, or twice the number of atoms of C plus one-half the number of atoms of H contained in one molecule of the hydrocarbon.

20. According to chemical theory, each volume of the gas requires one-half as many volumes of O as there are atoms of O in the compound. The volume of O required for the complete combustion of one volume of the hydrocarbon is, then, equal to

$${2n + {m \over 2} \over 2} = n + {m \over 4} $$

Since the air contains 21 per cent., by volume, of oxygen, this quantity must be divided by .21 or multiplied by $1⁄.21$ = 4.76. The formula for the amount of air necessary for the complete combustion of one volume of any hydrocarbon is, then,

$$V = \left(n + {m \over 4}\right)4.76$$

in which V represents the volume of air required.