Locomotive Engine Running and Management

by Angus Sinclair

Locomotives suffering from excessive back pressure are nearly always logy. The engine can not be urged into more than moderate speed under any circumstances; and all work is done at the expense of lavish waste of fuel, for a serious percentage of the steam-pressure on the right side of the piston is lost by pressure on the wrong side. It is like the useless labor a man has to do turning a grindstone with one crank, while a boy is holding back on the other side. The weight of obstruction done by the boy must be subtracted from the power exerted by the man to find the net useful energy exerted in turning the grindstone. In the same way, every pound of back pressure on a piston takes away a pound of useful work done by the steam on the other side.

EFFECT OF TOO MUCH INSIDE LAP.

Engines that have much inside lap to the valves are likely to suffer from back pressure when high speed is attempted. The inside lap delays the release of the steam; and, where the piston’s velocity is high the steam does not escape from the cylinder in time to prevent back pressure.

RUNNING INTO A HILL.

Most of engineers are familiar with the tendency of some engines to “run into a hill.” That is, so soon as a hill is struck, they suddenly slow down till a certain speed is reached, when they will keep going. This is generally produced by back pressure, its obstructing effect being reduced when the engine is moving slow.

COMPRESSION.

The necessity which requires lap to be put on a slide-valve to produce an early cut-off, in its turn causes compression, by the valve passing over the steam-port, and closing it entirely for a limited period towards the end of the return stroke. As the cylinder contains some steam which did not pass out while the exhaust-port was open, this is now squeezed into a diminishing space by the advancing piston. In cases where too much steam was left in the cylinders through contracted nozzles or other causes, or where, through mistaken designing of the valve-motion, the port is closed during a protracted period, the steam in the cylinder gets compressed above boiler tension, and loss of useful effect is the result. Under proper limits, the closing of the port before the end of the stroke, and the consequent compression of the steam remaining in the cylinder, have a useful effect on the working of the engine by providing an elastic cushion, which absorbs the momentum of the piston and its connections, leading the crank smoothly over the center. Where it can be so arranged, the amount of compression desirable for any engine is the degree that, along with the lead, will raise the pressure of the cylinder up to that of the boiler at the beginning of the stroke. When this can be regulated, the compression performs desirable service by cushioning the working-parts, thereby preventing pounding, and by filling up the clearance space and steam passages, by that means saving live steam. Compression probably does some economical service by reheating the cylinder, which has a tendency to get cooled down during the period of release, and by re-evaporating the water, which forms by condensation of steam in the cool cylinder.

Engines that are running fast require more cushioning than those that run slow, or at moderate speeds. The link-motion, by its peculiarity of hastening compression when the links are hooked up, tends to make compression a useful service in fast running.

DEFINITION OF AN ECCENTRIC.

The reciprocating motion which causes the valves to open and close the steam-ports at the proper periods, is, with most locomotives, imparted from eccentrics fastened upon the driving-axle. An eccentric is a circular plate, or disk, which is secured to the axle in such a position that it will turn round on an axis which is not in the center of the disk. The distance from the center of the disk to the point round which it revolves is called its eccentricity, and is half the throw of the eccentric. Thus, if the throw of an eccentric requires to be 5 inches, the distance between the center of the driving-axle and the center of the eccentric will be 2½ inches. The movement of an eccentric is the same as that of a crank of the same stroke, and the eccentric is preferred merely because it is more convenient for the purposes to which it is applied than a crank would be.