When the plug turns the port L serves to convey the live steam to the engine, while the other port K at the same time acts as the exhaust, and this condition is alternately reversed so that L acts as the discharge port.
Throttle Valves.—The throttle valves here illustrated are those used in connection with gasoline engines. The best known is the Butterfly valve, shown in [Fig. 82], and this is also used as a damper, for regulating the draft in furnaces and stoves.
This type is made in two forms, one in which the two wings of the valve are made to swing up or down in unison, and the other, as illustrated, where the disk A is in one piece, and turns with the spindle B to which it is fixed.
In [Fig. 83] the wing C is curved, so that by swinging it around the circle, the opening of the discharge pipe D is opened or closed.
Another design of throttle is represented in [Fig. 84]. One side of the pipe A has a lateral extension B, which is double, so as to receive therein a sliding plate C, which is easily controllable from the outside.
[Fig. 85] shows a form of double sliding plate, where the double lateral extensions project out in opposite directions, as at D, D, and within these extensions are sliding plates which are secured together in such a way that as one is pushed in the other also moves in, and thus acts in unison to close or to open the space between them. It is the most perfect form of throttle valve, as it causes the gases to open directly into the center of the outgoing pipe.
Blow-off Valves.—The illustration shows a type of valve which is used on steamboats and very largely on farm boilers throughout the country. The pipe A from the boiler has cast therewith, or otherwise attached, a collar B, which has a standard C projecting upwardly at one side, to the upper end of which is hinged a horizontal lever D, which has a weight at its other end.