FIRST APPLICATION OF LAP.
Lap was applied to the slide-valve in this country before its advantage as an element of economy was understood in Europe. As early as 1829, James of New York used lap on the valves of an engine used to run a steam-carriage; and in 1832 Mr. Charles W. Copeland put a lap-valve on a steamboat engine, and his father understood that its advantage was in providing for expansion of the steam. Within a decade after our first steam-operated railroad was opened, the lap-valve became a recognized feature of the American locomotive; but the cause of the saving of fuel, effected by its use, was not well comprehended. Many enlightened engineers attributed the saving to the early opening of the exhaust, brought about where outside lap was used, which they theorized reduced back pressure on the piston; and in that way they accounted for the enhanced economy resulting from the application of lap. It was not till Colburn applied the indicator to the locomotive, that the true cause of economy was demonstrated to be in the additional work taken from the steam by using it expansively.
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.
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.