Locomotive Engine Running and Management

by Angus Sinclair

THE ALLEN VALVE.

Fig. 8.

An improvement on the plain D slide-valve has been effected in a simple and ingenious manner in the Allen valve, which is receiving considerable favor for high-speed locomotives. This valve is shown in [Fig. 8]. The valve has a supplementary steam-passage, A, A, cast above the exhaust cavity. The valve and seat are so arranged, that, so soon as the outside edge of the valve begins to uncover the steam-port at B, the supplementary passage begins receiving steam at C; and this gives a double opening for the admission of steam to the port when the travel is short. As the travel of the valve is always short when an engine is running at high speed, the advantage of this double opening is very great; for it has the effect of admitting the steam promptly at the beginning of the stroke, and maintaining a full pressure on the piston till the point of cut-off.