Chapter XLI.—Automatic Cut Off Engines

An automatic cut off engine is one in which the valve gear is so acted upon by the governor as to keep the speed of the engine uniform under variations of the load the engine drives, and notwithstanding variations in the boiler pressure. This it accomplishes by varying the point in the piston stroke at which the live steam is cut off. This is economical because it enables the engine to use the steam more expansively than is possible with engines having throttling governors, which govern the engine speed by wire drawing the steam.

There are two principal forms of automatic cut off engines, first, those in which the steam valve spindle or rod is released from the parts that move it to open for admission, while dash pots, weights, or springs close the valve to effect the cut off; and second, those in which the travel of the valve is varied so as to alter the point of cut off.

The first usually employ fly ball governors which actuate cams or stops to trip the valves for the steam cylinders. The second usually employ wheel governors or speed regulators, as they are sometimes termed.

The distinctive features in the action of the first, of which the Corliss engine is the most important, is that as two admission and two exhaust valves are used, therefore the amount of the valve lead, the point of exhaust and amount of the compression remain the same at whatever point in the piston stroke the cut off may occur; whereas in the second, the lead increases, the cut off occurs earlier, and the compression increases in proportion as the cut off occurs earlier in the piston stroke. In this class of engine the steam valve travels as quickly when opening the steam port for a short and early period of cut off as it does for a late one, hence the amount of steam port opening is as full, with reference to the piston motion, for an early as it is for a late point of cut off. In other words, there is the same amount of steam port opening for the first, second, third, and fourth inch of piston motion, let the point of cut off occur at whatever point in the piston motion it may. In engines which vary the point of cut off by reducing the travel of the slide valve, this is accomplished by using double ported valves or griddle valves.

Fig. 3378 represents the arrangement of the valves in a Corliss engine, v and v being the steam valves and v and v the exhaust valves. These valves, it will be seen, extend crossways of the cylinder and are circular. In the figure the valves are shown in the position they would occupy when the piston was at the crank end of the cylinder, as in the figure.

The principles of a Corliss valve gear will be understood from the following, which is derived from a book by the author of this work, and entitled Modern Steam Engines.

In 3379 and 3380 the valve gear (which is the distinctive feature of the engine) is represented with the parts in the position they occupy when the cut off occurs at half stroke, the piston having moved from the head end of the cylinder. In Figs. 3381 and 3382 the parts are shown in position with the crank on the dead centre and the piston at the crank end of the cylinder, valve v having opened its port to the amount of the lead.

VOL. II. THE CORLISS VALVE GEAR. PLATE XXXIV.

Referring to Fig. 3379, motion from the eccentric is imparted by the rod m to the wrist plate y, to which are connected the rods c, c′, for operating the admission valves, whose shafts are seen at s, s′, and the rods f, f′, for operating the exhaust valves, whose shafts are seen at t, t′.

The mechanism for the steam or admission valves may be divided into three elements: first, that for operating the valve to open the port for admission; second, that for closing the valve to effect the cut off; and third, that which determines the point in the stroke at which the cut off shall occur.

The first consists of the rod m, wrist plate y, and the rods c and c′, which operate the bell cranks r r, r′ r′ which are fast on the valve shafts s, s′. Upon the ends of bell cranks r r, r′ r′, are pivoted latch links u, u′, which have in them a recess for the latch blocks, of which one is seen at e (the rod r′ and its connection with the valve stem being shown broken away to expose e to view). During the admission the latch block abuts against the end y of the recess w and is tripped therefrom by the cam n′. The ends of arms g of the latch links abut against the hub of the arms d, d′ upon which are cams n, n′, and at a, a′ are springs for keeping the ends g of latch links u, u′ against the hubs and cams of d, d′.

Referring now to the valve mechanism at the head end only, suppose the piston to be at the head end of the cylinder, and latch block e will be seated in the recess provided in a to receive it, and as the bell crank moves, the latch block will be raised by the latch link, which is carried by a crank arm corresponding to that seen at x at the crank end of the cylinder, and as this crank arm is fast upon the valve spindle, the lifting of e will open the valve for admission. As soon, however, as the end g of the latch link meets the cam n′, the latch link will be moved so that the end y of its recess will leave contact with the latch block e and the dash pot will cause rod r′ to descend instantaneously and close the valve, thus effecting the cut off.

The period of admission, therefore, is determined by the amount of motion the latch link u′ is permitted to have before its end g meets the cam n′, which trips the latch link, and therefore frees e from the latch link recess.

The point at which the cut off will occur, therefore, is determined by the position of the cam n′, because if n′ is out of the way, the end g of the latch link will not meet it, the latch link will not disengage from the latch block e, and the cut off would be effected by the lap of the valve, and independently of the dash pot. As in Fig. 3379 the parts are shown in the positions they occupy at the instant the cut off is to occur, therefore the cam n′ has just tripped the latch link, and the end of e has just left contact with the end y of the recess w in the latch link u′.

The point in the stroke at which the tripping of u′ from e will occur and effect the cut off is determined by the governor, because d′ is connected to the governor through the rod g′. In proportion as the governor balls rise, d′ is moved from left to right, and the end of cam n′ meets g earlier, or, vice versa, in proportion as the governor balls fall, the arm d′ is moved to the left, g will meet the end of cam n′ later, and the point of cut off will be prolonged.

We now come to the means employed to close the valve quickly and without shock when the latch block is released from the latch link. Referring then to the crank end of the cylinder, the latch block for that valve is carried upon arm x, to which is attached the rod r from the dash pot piston (the arm corresponding to x, but at the head end being shown removed to expose the latch block to view). We may now turn again to the head end of the cylinder, rod r′ corresponding to rod r at the other end, and it is seen that r′ connects to a dash pot piston p′ having a stepped diameter, the lower half fitting into bore h′, and the upper half fitting into a bore h. The piston p′ fits the bore h′ and fills it when the rod r′ is at the bottom of the stroke, hence as p′ is raised there is a vacuum in h that acts to cause p′, and therefore r′ and x, to fall quickly and close the valve the instant the latch block is released from the latch link. To prevent the descent of rod r′ and piston p′ from ending in a blow, a cushion of air is given in h by the following construction:

At s and s′ are valves, threaded to screw and unscrew, the ends forming a valve for a seat entering h.

As the rod r′ and its piston p′ descend, the air in h finds exit through a hole at h until that hole is closed by the piston p′ covering it, after which the remaining air in h can only find exit through the opening left by the end of the valve s′, and this amount of opening is so regulated by the adjustment of s′ that a certain amount of air cushion is given, which prevents p′ from coming to rest with a blow. The head of valve s′ is milled or knurled, and a spring t′ fits, at its end, into the milled indentation, thus holding it in its adjusted position. The under surface of the upper part of p′ is covered by a leather disc, while the part that fits in h′ is kept air-tight by a leather-cupped packing.

The connection of the cam arms d and d′ with the governor is shown in Figs. 3381 and 3382, in which the parts are shown in the position they would occupy when the crank is on the dead centre and the piston at the crank end of the cylinder. The rod g′ connects the cam arm d′ with the upper end of lever a, which is connected to the governor and vibrates on its centre as the governor acts upon it.

Now suppose the speed to begin to diminish, and the governor balls to fall, and the direction in which a will move will be for its lower end to move to the right, thus moving d to the right and carrying its cam away from the end of the latch link, which will therefore continue to open the port for a longer period of admission. Or, referring to Fig. 3381, it is plain that, if the governor balls were to lower from a reduced governor speed, g′ would move to the left and cam n′ would be moved away from contact with the end g of the catch link, which, not being tripped, the admission would continue. On the other hand, suppose the governor balls to rise from an increase of governor speed, and d′ (Fig. 3379) would be moved to the right, and the cam n′ meeting g earlier, correspondingly hastening the cut off.

The governor is driven by a belt from a pulley on the crank shaft to the pulley w, Fig. 3381, whose shaft conveys motion to the governor spindle through the medium of a pair of bevel pinions in which v represents (referring again to Fig. 3378) the steam or admission valve for the crank end port, and v that for the head end port, while v is the exhaust valve for the crank end, and v that for the head end of the cylinder. All four valves are shown in the positions they would occupy when the crank was on the dead centre and the piston at the crank end of the cylinder, hence the valve positions shown correspond to the positions the parts of the valve motion occupy in Fig. 3381.

The faces of the valves are obviously arcs of circles of which the axes of the shafts s, s′ are the respective centres. Valve v has opened its port to the amount of the lead, which in this class of engine varies usually from ⁄32 to about ⁄16 inch. As separate exhaust valves are employed, the point of release, and (as the same valve edge that effects the release also effects the compression) therefore that of the compression, may be regulated at will by adjusting the lengths of the rods f, f′, Fig. 3379, which have at one end a right and at the other a left hand screw, so that by turning back the check nuts and then revolving the rods their lengths will be altered.

Similarly the amount of admission lead may be adjusted by an adjustment of the lengths of rods c, c′, which also have right and left hand screws. Referring now to the admission valve v, it is seen that its operating rod c is at a right angle to bell crank r, r, hence the amount of valve motion will not be diminished to any appreciable extent by reason of the wrist plate end of rod c moving in an arc of a circle, and the point of attachment of rod c to the wrist plate is such that, during the admission, the valve practically gives as quick an opening as though rod c continued at a right angle to r. But, if we turn to valve v′, which has closed its port and covers it to the amount of the lap, we find that bell crank r′ and its operating rod c′ are in such positions with relation to the wrist plate, that the motion of the latter will have but little effect in moving the bell crank r′. This is an especial feature of the Corliss valve motion and is of importance for the following reasons:

The lap of the valve (which corresponds to the lap of a plain D slide valve) is usually, in this class of engine, such as to cut off the steam at about ⁄8 stroke, but the adjustment of the cam position is usually so made that, from the action of the governor, the latest point of cut off will occur when the piston has made ⁄8 of its stroke, the range of cut off being from this to an admission equal to the amount of the lead.

As the eccentric is fixed upon the shaft, the speed at which the valve opens the port for the admission is the same for all corresponding piston positions. Thus suppose the piston has moved an inch from the end of the stroke, and the valve speed will be the same, whether the cut off in that stroke is to occur at quarter stroke or half stroke, and as the valve continues to open the port until it is tripped, therefore, at the moment it is tripped, the direction of valve motion must be suddenly reversed.

As the duty of its reversal falls upon the dash pot, it is desirable to make this duty as light as possible, which is accomplished by the wrist motion, which acts to reduce the valve motion after the port is opened a certain amount for the admission.

We have, therefore, that during the earlier part of the admission, the port opening is quick because of the eccentric throw being a maximum, while during the later part of the port opening, this rapid motion is offset or modified by the wrist motion, thus lessening the duty of the dash pot and enabling it to promptly close the valve.

The range of governor action, so far as the governor itself is concerned, is obviously a constant amount, because a certain amount of rise and fall of the governor balls will move the cams a given amount. But the range of cut off may be varied as follows: At z, z′, are adjustment nuts, by means of which the lengths of rods g, g′ may be varied.

Lengthening rod g obviously moves arm d and its cam n further from the end of the latch link u, and therefore prolongs the admission period.

Shortening the rod g′ causes cam n′ to move around and away from the leg g of the latch link, and prolongs the admission.

The adjustment of the lengths of g and g′ may therefore be employed for two purposes; first, to prolong the point of cut off, and maintain the speed when the engine is overloaded, or to hasten the point of cut off for a given engine speed, and thus adjust the engine for a lighter load.