ADVANTAGES OF THE ALLEN VALVE.
With an ordinary valve cutting off at six inches, and having five inches eccentric throw, the port opening seldom exceeds ⅜ inch. It is a hard matter getting the full pressure of steam through such a small opening in the instant given for admission. If an Allen valve is used with that motion, the opening will be double, making ¾ inch, which makes an important difference. The practical effect of a change of this kind is that an engine will take a train along, cutting off at six inches with the Allen valve, when, with the ordinary valve, the links would have to be dropped to eight or nine inches. The valve can be designed to work on any valve-seat, but the dimensions given in [Fig. 8] are those that have been found most satisfactory with our large passenger engines. In designing an Allen valve for an old seat, it is sometimes advisable to widen the steam-ports a quarter of an inch or more, by chamfering off the outside edges that amount. Care must be taken to prevent the valve from traveling so far as to put the supplementary port over the exhaust-port, for that would allow live steam to pass through. The proper dimensions can best be schemed out on paper before making the required change on the seat.
In very carefully conducted experiments made on the Boston and Albany Railroad, to compare the performance of the Allen valve with an engine equipped with a common valve, it was found that the Allen valve effected a fuel saving of seven per cent.