Locomotive Engine Running and Management

by Angus Sinclair

ADVANTAGE OF CUTTING OFF SHORT.

When the links are notched close towards the center, the travel of the valves is so short that they close the steam-ports shortly after the beginning of the stroke, at six, nine, or twelve inches of the piston’s travel, as the case may be, permitting the steam to push the piston along the remainder of the stroke by its expansive power. Steam at a high pressure is as full of potential energy as a compressed spiral spring, and is equally ready to stretch itself out when the closing of the port imprisons it inside the cylinder; and, by this act of expanding, it exerts immense useful energy, which would escape into the smoke-stack unutilized if the cylinders were left in communication with the boiler till the release took place. Suppose, for instance, that a boiler pressure of ten tons is exerted upon the piston from the beginning to the middle of the stroke, and is then cut off. During the remainder of the stroke, the steam will continue to press upon the piston with a regularly diminishing force, till, at the end of the stroke, if release does not take place earlier, it will still have a pressure of five tons. The work performed by the steam during the latter part of the stroke is pure gain, due to its expansive principle. If the steam is cut off earlier, at a third or fourth of the piston travel, the gain will be correspondingly great. With the slide-valve link-motion used on locomotives, the steam can not be held to the end of the stroke; but the principle of expansion holds good during the period the steam is held in the cylinders after the cut-off.

The observing engineer of any experience does not require to have the advantages of working his engine expansively impressed upon his attention. His fuel-record has done that more eloquently than pen can write.

BOILER PRESSURE BEST FOR ECONOMICAL WORKING.

There is a close and constant relation between the boiler pressure carried, and the useful work obtained from expansion of steam. The higher the pressure, the greater elasticity the steam possesses. The tendency of modern steam engineering is, to employ intensely high boiler pressure, expanding the steam by means of excellent valve-gear in steam-jacketed cylinders, so that it is reduced to low tension before escaping into the atmosphere, or into the condenser, as the case may be. Wonderfully economical results have been obtained in this manner,—results which can never be approached in locomotive practice while the ordinary slide-valve is used. But, while we can not hope to rival the record of high-class automatic cut-off engines, their methods can teach us useful lessons.

It is advisable to keep the steam constantly close to the blowing-off point. During a day’s trip, considerably less water will be evaporated when a tension of 140 pounds is carried, than will be required with a pressure of 100 pounds or under. And, where less water is evaporated, a smaller quantity of fuel will be consumed in doing the work. Running with a low head of steam is a wasteful practice, for several good reasons. The comparatively light pressure upon the surface of the water allows the steam to pass over damp, or mixed with a light watery spray, which diminishes its energy; since the wet steam contains less expansive medium than dry steam. It requires nearly the same expenditure of fuel to evaporate water at the pressure of the atmosphere alone, that it does to make steam at the higher working tensions: consequently, the work obtained by the expansion of the high-pressed steam is clear gain over the results to be obtained by working at a low pressure. This is a very important principle in economical steam engineering. Engineers who are accustomed to making long runs between water-tanks, when every gallon is needed to carry them through, know that their sure method of getting over the dry division successfully, is to carry steam close to the popping-point, pull the throttle wide open, hug the links close to the center, and see that no loss occurs through the safety-valves.

RUNNING WITH LOW STEAM.

There are engineers who habitually carry merely sufficient steam to get them along on time, under the mistaken belief that they are working economically. John Brown runs steadily, and takes as good care of his engine as any man on the A. & B. road; but he dislikes to hear the steam escaping from the safety-valves, and prevents it from doing so by habitually using steam thirty pounds below the blowing-pressure. The consequence is, that he always makes a bad record on the coal-list, compared with the other passenger men.

THE THROTTLE-LEVER.

In the interest of economy, the throttle-lever should be kept wide open when practicable, and the speed regulated by the reverse-lever. Experiments with the indicator have demonstrated beyond a doubt, that running with the throttle-valve partly closed, wire-draws the steam before it reaches the cylinders, whereby the initial pressure is materially reduced, and its power for expansive work seriously diminished.