High Speed Automatic Cut Off Engines

What are termed high speed engines are those whose pistons run at a velocity of more than about 600 feet per minute, some making as high as 800 or 900 feet in regular work. High speed engines are usually provided with an automatic cut off, and a majority of them vary the point of cut off, by means of shifting the eccentric across the shaft, so as to reduce the eccentric throw, and therefore the valve travel. This causes the valve to cut off the steam earlier.

The eccentric, instead of being fixed upon the crank shaft, has an elongated bore, and is hung on an arm that is pivoted at its other end after the manner of a pendulum. This arm is called the eccentric hanger.

A wheel governor is usually employed to shift the eccentric across the shaft. In some cases, however, two valves are employed, one effecting the admission, the release, and the compression, and the other the cut off.

When two valves are employed, the lead, the point of cut off, the point of release, and the point of compression may be maintained equal for all points of cut off; whereas, when a single valve is employed, the lead, the point of release, and the compression will vary with the point of cut off, or, in other words, will be different for every different point of cut off.

The general principles upon which a wheel governor is constructed is, that two weights or weighted levers in moving outwards from the engine shaft, from the action of centrifugal force, move or rather shift the eccentric across the shaft, reducing its throw, and therefore by reducing the travel of the valve hasten the point of cut off and reduce the power of the engine.

In the governor of the Buckeye engine, the centrifugal force may be varied by increasing or diminishing the distance of the weights from the pivots of the arms on which they swing.

This is shown in Fig. 3382a, in which it is seen that the weights a are adjustable along the arms a, a. The points of attachment d, d of the springs to the weight arms are also adjustable.

When reversing is done, by shifting the eccentric across the shaft, the lead cannot be kept equal, but will, if the eccentric is swung from a pivot that is on the line of centres, when the crank is on a dead centre, be greater at the head end than at the crank end of the cylinder. The discrepancy may, however, be equalized by swinging the eccentric from a pivot that is not on the line of centres at a time the crank is on a dead centre.

But this equalization will only exist at some one point in the eccentric position, or in other words, if the eccentric is shifted across the crank shaft, simply to reverse the engine, and not to vary the point of cut-off, it will naturally be moved, in reversing the engine across the shaft, to a given and constant amount, and in this case, the pivot on which its hanger is hung may be so located with reference to the line of centres and the crank (the latter being on a dead centre when the point of suspension of the eccentric hanger is found) that the lead is equal for both the backward and forward gears.

But if the eccentric is shifted across the shaft to vary the point of cut off as well as to reverse the direction of engine revolution, the lead cannot be kept equal.

It is better, in this case, to so locate the point of eccentric hanger suspension as to let the lead be the most at the head end cylinder port, because the piston travels fastest at that end of the cylinder, and therefore requires more lead, in order to cushion the piston.

A construction for shifting the eccentric across the shaft is shown in Fig. 3383, in which d, d is a disc, having at b a pivot for the eccentric hanger. The amount the throw line of the eccentric must be shifted to reverse from full gear forwards to full gear backwards is from the line b x to line b x′, and the shifting is done by two racks f and j, having teeth at an angle of 45° to their lengths. f is fast to the eccentric, and j is carried in a sleeve that slides along the shaft, and sliding it moves the eccentric across the shaft by reason of the teeth of one rack being at a right angle to those of the other.

It is obvious that the eccentric may be moved around the shaft in place of across it, the distance its throw line requires to be moved being the same in either case.

To shift an eccentric so as to reverse the direction of engine revolution, all that is necessary is to place the crank on either dead centre and measure the amount of valve lead. Then loosen the eccentric from the crank shaft, and while the crank is stationary, move it around upon the shaft until it has opened the port full, and nearly closed it again, leaving it open to the same amount as it was before the eccentric was moved, or in other words, open to the amount of the lead.

Fig. 3384 represents a side elevation of a high speed wheel governor engine, designed and constructed by the Straight Line Engine Company of Syracuse, New York, the construction of the governor being shown in Fig. 3385, in which r is the eccentric rod, the eccentric being carried in a lever strap pivoted at a, and connected at b to two links c and d, the former of which connects to the spring e, and the latter to the weighted lever f. The centrifugal force generated by the weighted end of f endeavors to move the eccentric inwards, and thus reduce its throw, which reduces the valve travel and hastens the point of cut off.

On the other hand, the tension of the spring e acts to move the eccentric in the opposite direction, and maintain the full throw of the eccentric and maximum point of cut off. These two forces are so calculated in the design and proportion of the parts that under a maximum load the engine will run at its proper speed, while, if the load decreases, the action of f will hasten the point of cut off enough to allow for the decreased engine load, and thus keep the engine still going at the same speed.

Other novel and interesting details in the construction of this engine are as follows:

The two arms forming the frame are cast with and run in straight lines from the cylinder to the two main bearings, and rest upon these self-adjusting points of support.

There are two fly wheels, both between the main bearings, and one of which carries the governor so that the centre of the valve is brought in line with the centre of the eccentric.

In order to simplify the explanation, the mechanism has been separated into three separate sections. Figs. 3386 and 3387 show such of the details of the parts between the cylinder and crank as are peculiar to this engine. The cross head is of the slipper guide style, and the illustration, Fig. 3386, shows the simple method adopted for adjusting the guide to the proper height to maintain the alignment. Another feature peculiar to the straight line not mentioned above, that of making the cross head pin fast in the connecting rod, is used in this engine also, but in a somewhat different form. As will be seen by Fig. 3387, the pin is made much larger, and this allows of its being made of “steel casting” and cast hollow with cross bars at each end for centring. These pins are held in the rod by a binding screw which catches in a groove that is milled around one-fourth of its circumference. After the pin is placed in the rod and the binding bolt is inserted, the pin is prevented from working out endwise, and the binding bolt prevents it from turning; but when the binding bolt is slackened, the pin can be rotated one-fourth of a revolution. The scheme is as follows: After running the engine for a while, the engineer is instructed to slack the binding bolt, give the pin a quarter turn and bind it fast. By repeating this, the pin can be kept more nearly round, probably, than by any other plan. By referring again to Fig. 3386, it will be seen that the plan for taking up the wear in the cross head pin bearings is simply that of setting up the common half box, and the endurance of the arrangement, with the hardened and ground steel pin running in babbitt lined boxes of double the ordinary size and length, must be satisfactory.

The drop oil cups for lubricating the cross head pin are located so as to have the drop “picked” off just as the cross head completes its stroke at the cylinder end, and while it is travelling at its slowest speed. The oil, as it leaves the wearing surfaces of the pin, is conveyed to the lower slide.

Figs. 3388 and 3389 show the parts that connect the eccentric with the valve. The method of connecting the rod to the eccentric strap is convenient. The lower joint in the eccentric strap is set up tight, metal to metal, and the upper joint left open ⁄8 of an inch.