The admission and exhaust of steam, for each end of the cylinder, may be controlled by separate valves, but very commonly a single moving piece, called a slide-valve, serves to control the events for both ends. The slide-valve was invented by William Murdock, an assistant of Watt. A common form of it is illustrated in section in fig. 5, which also shows the cylinder, with the piston, and the steam ports and passages leading to each end of the cylinder. The face on which the valve slides is a plane surface on one side of the cylinder with three ports or openings which extend across the greater part of the cylinder's width. The central opening is the exhaust port, through which the steam escapes after doing its duty in the cylinder. The others, which are narrower, lead to the two ends. The valve itself is a box-shaped cover, sliding on the face, and contained in the valve-chest, or chamber to which steam is admitted from the boiler. When the valve moves a sufficient distance to either side of the middle position steam is admitted to one end of the cylinder, past the outer edge. Similarly steam escapes from the other end of the cylinder, through the cavity of the valve, to the exhaust port. The valve takes its motion from an eccentric on the engine shaft, which is set more than 9o° ahead of the crank, so that the valve has already begun to uncover the port on one side when the piston is at the corresponding dead point, with the result that steam passes from the steam chest above the valve into the space behind the piston. Figure 6 shows a slide-valve in its middle posi tion, and illustrates the internal lap i and the external lap e which the valve must have to make expansive working possible. At the beginning of the stroke it has already passed its middle position by an amount at least equal to e, so that steam may enter. It con tinues to move further, opening the port more widely, and then begins to return, while the piston is still advancing in the cylinder. At a particular point in its return the outer edge of the valve closes the port ; this determines the instant of cut-off. The piston continues to advance, with expansion of the steam confined in the cylinder, while the valve continues to move back, until the inner edge of the valve begins to un cover the port, allowing steam from the cylinder to escape to the exhaust channel. This point, which depends on the internal lap i, determines the instant of re lease. The port then opens more widely to exhaust, and remains open during most of the back stroke of the piston, until the valve, again moving in the origi nal direction, brings its inner edge again over the port and compres sion begins. Similar events for the other end of the cylinder are determined by the other side of the valve. The positions of the piston when the several events occur depend on the amounts of the laps e and i, and of the "angu lar advance" which is the excess over 9o° in the angle by which the eccentric stands ahead of the crank. If there were no laps and no angular advance steam would be admitted during the whole for ward stroke and exhausted during the whole backward stroke. By giving the valve laps and angular advance expansive working be comes possible. But the slide-valve is not well adapted to effect a cut-off early in the stroke, and when much expansion is desired, a device called a separate expansion valve is added, or valves of a different type are used to control the distribution of the steam.
Fig. 7 shows Meyer's expan sion valve, which consists of two blocks sliding on the back of a slide-valve. It is caused to reciprocate by an eccentric, generally set directly opposite the crank, so that it moves to the left while the piston moves to the right, and vice versa. The admission of steam ceases when the relative displacement of the expansion blocks over the slide-valve amounts to the distance 1, and this can be arranged to happen early in the piston's stroke without affecting the release or the compression.
In this example the rod which carries the two blocks is fitted with right- and left-handed screws, so that by turning the rod the distance between the blocks may be adjusted, and the distance / consequently altered. This allows the valve to be set to give an earlier or later cut-off : when 1 is increased by bringing the blocks nearer together, the cut-off comes later in the stroke. The adjust
ment may be made while the engine is running. Another way to change the cut-off with a valve of the Meyer type is to alter its travel. Increasing the travel of the expansion valve, while 1 re mains constant, makes the cut-off come later.
In many engines the events are controlled by using, at each end of the cylinder, separate valves for admission and exhaust (fig. 8). Sometimes these are drop valves which are "double beat" in the sense that the valve when it drops into its seat closes simultaneously two passages. These are so arranged that the steam pressure tending to close or open the valve remains bal anced, and no large force has to be exerted. Such valves are often worked by cams and levers from a lay shaft alongside the cylinder, driven from the main shaft so as to turn in unison. In many examples the valves giving admission are caused to close by a trip ping device ; that is to say they spring suddenly into the closed position, at the instant when cut-off is desired, through the action of a trigger which brings a closing spring into play; and the trip ping device is connected with the governor of the engine in such a manner that the cut-off comes earlier or later when the speed exceeds or falls short of the normal. In this way the engine auto matically adjusts the amount of power it develops to suit changes in the demand made upon it by the mechanism which it drives.
Fig. 5 shows a slide-valve form usual in locomotives and small engines but in large sizes the objection is that the unbalanced pressure of steam on the back of the valve would cause much friction and wear in the movement of the valve over the working face on which it slides. In large engines, such as those of steamships, it is usual to relieve the pressure of the valve on the working face by fitting the back of the valve with what is called a relief frame, extending between the valve and the steam-chest cover, the effect of which is to prevent the steam of the steam-chest from having access to the whole back surface of the valve. Another device, often resorted to, is to sub stitute for the flat form of slide-valve a piston form in which the effective sliding portions of the valve extend all round the surface of a cylinder and slide over cylindrical fixtures in the valve chest, round which the steam-ports extend.
The slide-valve is a particularly conveni ent means of controlling the admission of steam when, as in loco motives or marine engines, etc., the engine's direction of motion has frequently to be reversed. It is only necessary to provide means by which the valve may be actuated by either of two eccen trics, one set at the angle suitable for forward running and the other set at the angle suitable for backward running.
In Stephenson's link motion which is an early and familiar device for this purpose, the engine shaft carries two eccentrics, one of which is set ahead of the crank at a suitable angle for one direction of running, and the other at a suitable angle for the other direction. Their rods are connected to the ends of a link which gives its name to the contrivance. The link is a slotted bar curved to a circular arc, and capable of being moved up or down, being suspended from a rod which can be raised or lowered by means of the hand lever above. The valve rod ends in a block which slides within the link. This device allows either the forward eccentric or the backward eccentric to determine the motion of the valve, and so allows the engine to be reversed. But it does more, for if the link be set to an intermediate position, in which it takes its motion chiefly from one eccentric but partly from the other, the valve receives a motion virtually the same as that which it would get from a single eccentric of shorter travel and greater angular advance. The effect is to give a distribution of steam in which the cut-off is hastened and the expansion and compression are increased. Thus besides allowing of "full forward" and "full backward gear" the link may be "notched up" to give a much shorter admission of steam along with more compression, a matter of practical value in the running of a locomotive, where frequent large variations in the demand for driving effort have to be met.