All About Engines

192 All About Engines boiler or steam pipes, and the cost of these, together with stop valves and other accessories, is saved. It is cleaner, and though coal and ashes have to be handled at the gas works, this can be done on a large scale which justifies employment of labour-saving machinery. The gas engine utilises from 25 per cent, to 30 per cent, of the heat obtainable from the fuel, while the steam engine converts less than 15 per cent, into useful work. But town gas is rather an expensive fuel, and for many years the engines were small, so that their use was in consequence limited. How it has developed during the last thirty years deserves a separate section. The Food of the Gas Engine The disadvantages of the early gas engines were their dependence upon a supply of town gas, the cost of this form of fuel, and the fact that the piston received only one impulse for two revolutions of the shaft—that is, for one-quarter of the time the piston drove the shaft, and for three-quarters of the time the shaft drove the piston. Consider the fuel ques- tion first. Coal gas is made by heating coal out of contact with air in closed retorts. The result is : (a) Gas. (fi) Ammoniacal liquor. (c) Tar. (d) Coke. In order to obtain gas of the highest illuminat- ing value the process of distillation cannot be carried