57. In all types of two-cycle engines thus far described, it should be noted that the work of the expanding gas is done on one side of the piston only, the other side being used for the air pump. The disadvantage of this method is that the parts required for the air pump must be strong enough to resist the pressure of the explosion, as the same parts are used for the working end of the piston. The engine may, however, be made double-acting, but it is then necessary to provide a separate air pump. But the work to be done by the pump is comparatively light, and the total weight of the double-acting engine, together with the air pump, is therefore not nearly so great as that of two single-acting engines of the same bore and stroke.
58. Koerting Engine. — In Fig. 22 is shown the main cylinder of the Koerting two-cycle engine, which operates with an independent pump, or, in fact, two independent pumps. This type of engine is adapted for using blast-furnace gas, which is very weak in heating power, and hence a little more gas than air is required. Gas and air are supplied by two independent pumps, and are forced into the cylinder through valves s, s'; exhaust from either end of the cylinder takes place through the piston-opened port e. The piston p is made very long, being only a little shorter than the stroke of the engine, which is double-acting. Work is done on each end of the piston alternately; hence, there are two explosions to every revolution. As shown in the figure, the crank end of the cylinder is exhausting, the crank-pin being on the inner dead center. Valve s' is open and the fresh air is coming in, followed by a mixture of air and gas. The fresh air is allowed to enter first, in order to clear the cylinder of burned gas, a cleaning process known as scavenging. The valves s, s' are open during a small portion of the stroke only. The valves and the cylinder are entirely surrounded with a water-jacket j. The piston p is made hollow, and a stream of water is constantly driven through the hollow piston rod r and around the inner surface of the piston, returning through the piston rod by
