EVENTS OF THE STROKE IN REVERSED MOTION.
Supposing an engine to be running ahead, and the necessity arises for stopping suddenly, and the reverse-lever is pulled into the back notch. When the crank-pin is on the forward center, and therefore the piston at the forward end of the cylinder, about to begin its backward stroke, the valve has the forward port open a distance equal to the amount of lead, as in [Fig. 10]. But, as the back-up eccentric has control of the valve, the latter is being pushed forward; and it closes the forward port just as the piston begins to move back. This shuts off all communication with the forward end of the cylinder; and the receding piston creates a vacuum behind it, just as a pump-plunger does under similar circumstances. At this time the back end of the cylinder is open to the exhaust, and the piston pushes out the air freely to the atmosphere. By the time the piston travels about two inches, the valve gets to its middle position; and, immediately after passing that point, it opens the forward end of the cylinder to the exhaust, and closes the back port. When this event happens, the vacuum in the forward end of the cylinder gets filled with hot gases, that rush in from the smoke-box; and the receding piston keeps drawing air into the cylinder in this way during the remainder of the stroke, and air from that quarter seldom gets in without bringing a sprinkling of cinders. The back steam-port is closed only during about two inches of the stroke, while the lap of the valve is traveling over it. About the time the piston reaches four inches of its travel, the back steam-port is open to the steam-chest, and the piston forces the air through the steam-pipes into the boiler during the remainder of the stroke. The forward stroke is merely a repetition of the backward stroke described.
When it is necessary to reverse a locomotive, it is a better plan to hook the lever clear back than to have it a notch or two past the center, as some men persist in doing, under the mistaken belief that they are in some way saving their engine from harsh usage. When the link is reversed full, the cylinders are merely turned into air-pumps. When the links are put near the center, the travel of the valve is reduced; and the periods when the piston is creating a vacuum in one end of the cylinder, and compressing the air in the other, are prolonged. The result is, that, when the exhaust is opened in the first case, the gases rush in violently from the smoke-box, carrying a heavy load of cinders: in the other case, the piston compresses the air in the cylinder so high that it jerks the valve away from its seat in trying to find outlet. This causes the clattering noise in the steam-chest, so well known in cases where engines are run without steam while the reverse-lever is near the center.
A locomotive with the piston-packing in bad order will not hold well running in reverse-motion. Some kinds of piston-packing do not seem to act properly when the engine is reversed, especially at low speed. Where a valve has much inside lap, there will be a vacuum in one end of the cylinder, and compressed air in the other end. With piston packing that requires pressure to expand it, the void at one end of the cylinder may neutralize the pressure at the other by drawing the air through the piston. This would be most liable to happen where the lever was kept near the center.