Locomotive Engine Running and Management

by Angus Sinclair

When an engine is run with the lever in the center notch, the supply of steam is admitted by the lead-opening alone. In full gear the eccentric, whose rod-end is in line with the rocker-pin, exerts almost exclusive control over the valve movement; but, as the link-block gets hooked towards the center, it comes to some extent under the influence of both eccentrics.

A thoughtful examination of [Fig. 12] will throw light on the reason why the proper position of a slipped eccentric can be determined by the other eccentric when the engine is on the center. In the cut, the crank-pin is represented on the forward center; and in that position the eccentric centers are both an equal distance in advance of the main shaft center. It will be evident now that the valve must occupy practically the same position for forward or back gear, as each of the eccentric-rods reaches the same distance forward. Putting the motion in back gear would bring the backup eccentric-rod pin to the position now occupied by the pin belonging to the forward eccentric-rod.

VALVE-MOTION OF A FAST PASSENGER LOCOMOTIVE.

Reducing the travel of the valve by drawing the reverse-lever towards the center of the quadrant, and consequently the link-block towards the middle of the link-slot, not only hastens the steam cut-off, but it accelerates in a like degree every other event of steam distribution throughout the stroke. To explain this point, let us take the motion of a well-designed engine in actual service, which has done good economical work on fast train running. The valve-travel is five inches, lap one inch, no inside lap, lead in full gear 1/16 inch, point of suspension 9/16 inch back of center of link.

EFFECT OF CHANGING VALVE-TRAVEL.

When this engine is working in full gear, the steam will be freely admitted behind the piston till about eighteen inches of the stroke, when cut-off takes place; and the release or exhaust opening will begin at about twenty-two inches of the stroke, giving four inches for expansion of steam. Now, if the links of this engine are hooked up so that the cut-off takes place at six inches of the stroke, the steam will be released at sixteen inches of the stroke; and at that point compression will begin at the other end of the cylinder.

WEAK POINTS OF THE LINK-MOTION.

This attribute which the link-motion possesses, of accelerating the release and compression along with the cut-off, is very detrimental to the economical operating of locomotives that run slow. High-speed engines need the pre-release to give time for the escape of the steam before the beginning of the return stroke; and the compression is economically utilized in receiving the heavy blow from the fast-moving, reciprocating parts, whose direction of motion has to be suddenly changed at the end of each stroke, and in helping to raise the pressure promptly in the cylinder at the beginning of the stroke. A locomotive, on the other hand, that does most of its work with a low-piston speed, would not suffer from back pressure if the steam were permitted to follow the piston close to the end of the stroke; and a very short period of compression would suffice. If the engine, whose motion we have been considering, instead of releasing at sixteen inches, could allow the steam to follow the piston to twenty-two inches of the stroke, after cutting off at six inches, a very substantial gain of power would ensue. And this would be well supplemented by avoiding loss of power, did compression not begin till within two inches of the return stroke.

WHY DECREASING THE VALVE-TRAVEL INCREASES THE PERIOD OF EXPANSION.

Increase of expansion follows reduced valve-travel, from a similar cause to that which produces expansion when lap is added to the edge of a slide-valve. When the valve is made with the face merely long enough to cover the steam-ports, there can be no expansion of the steam; for, so soon as the valve ceases to admit steam, it opens the steam-port to the exhaust. When lap is added, however, the steam is inclosed in the cylinder, without egress for the time that it takes the lap to travel over the steam-port. An arrangement of motion which will make the valve travel quickly over the port, has a tendency to shorten the period for expansion; while making the valve travel slowly over the port, has the opposite effect, and protracts expansion. A valve with, say, five inches travel, has a comparatively long journey to make during the stroke of the piston; and the lap-edges will pass quickly over the steam-ports,—much more quickly than they will when the travel is reduced to three inches. In a case of this kind, there is more than the mere reduction of travel to be considered. Suppose the valve has one inch lap at each end. When it stands on the middle of the seat, it has a reciprocating motion of two and one-half inches at each side of that point to make. At the beginning of the stroke, it has been drawn aside one inch (we will ignore the lead), but still has one and one-half inch to travel before it begins to return. On the other hand, when the travel is reduced to three inches, the valve has only one and one-half inch to travel away from the center; and, one inch being moved to draw the lap over the port, there only remains one-half inch for the valve to move before it must begin returning. This entails an early cut-off; for the valve must pass over the ports with its slow motion, and be ready to open the port on the other end, before the return stroke. Thus a travel of five inches draws the outside edge of the valve one and one-half inch away from the outside of the steam-ports, three inches travel only draws it one-half inch away, and a greater reduction of travel decreases the opening in like proportion.