lock, through which the fuel, preferably pea-sized anthracite or coke, is fed to the fire. An air lock is necessary to prevent the air from entering while the fuel is being introduced. At the bottom of the generator is a fire-door g for working the fire, and an ash door h for removing ashes. The ash-pit has a water seal, to prevent the escape of gas or the entrance of air. At each charging stroke of the engine, a quantity of gas is sucked out of the generator and passes through the evaporator b. This device performs the double function of generating steam and cooling the gases, and resembles in form a small tubular boiler. The gases, in passing down through the tubes of this boiler or evaporator, give up part of their heat to the water that surrounds the tubes, furnishing sufficient steam to supply the generator a. The water first passes through the water-jacket of the engine, where it takes up considerable heat, so that it is readily changed to steam, in the evaporator. As the steam passes to the generator, it draws a definite amount of air with it around the inlet pipe at i, and thus supplies the oxygen necessary for the partial combustion of sufficient coal to maintain the necessary temperatures in the generator. By this arrangement, it is obvious that the heat taken from the hot gas on its way to the engine, as well as that taken from the heated cylinder walls, is returned to the generator in the steam and air with which it is supplied.
74. The gases generated from ordinary fuels, especially from soft coal, are not fit to be used directly in the cylinder of an engine, because of entrained impurities. The amount and nature of these impurities vary widely with the kind of fuel used. The impurities to be removed from the gas generated from a ton of fuel are, for anthracite, from 1 to 2 pounds of ammonia, traces of sulphur, and from 5 to 10 pounds of tar, and for bituminous coal, from 4 to 5 pounds of ammonia, sulphur varying from traces to 5 per cent., and from 10 to 12 gallons of tar.
75. In the case of suction-producer plants using anthracite and carbonized fuels, such as charcoal and coke, the
