Locomotive Engine Running and Management

by Angus Sinclair

On one of the leading railroads in this country, an engineer was running a locomotive on a fast train where it was a hard matter making the card-time. A few minutes could be saved by passing a water-station; but this was done at serious risk, for the tender would nearly always be empty by the time the next water-station was reached. The master mechanic of the road determined to equip this engine with the Allen valve: and, after the change was made, there was no risk in passing the water-station; for there always was a good margin of water in the tank when the next watering-place was reached. The engine seemed to steam better, because the work was done with less steam; and there was a decided saving of fuel. The change made the engine smarter, and there seems to be no limit to the speed it can make. This valve can be applied to any locomotive with trifling expense. When an engine is designed specially for the Allen valve, the steam ports and bridges are usually made a little wider than for the ordinary valve. The only real difficulty in adopting the valve is getting the casting properly made, so that the supplementary port will not be too rough for the passage of steam, and the thin shell will be strong enough to stand the pressure.

INSIDE CLEARANCE.

For high-speed locomotives, where there is great necessity for getting rid of the exhaust steam quickly, the valves are sometimes cut away at the edges of the cavity, so that, when the valve is placed in the middle of the seat, it does not entirely cover the inside of either of the steam-ports. This is called inside clearance. In many instances inside clearance has been adopted in an effort to rectify mistakes made in designing the valve-motion, principally to overcome defects caused by deficiency of valve-travel. The fastest locomotives throughout the country do not require inside clearance, because their valve-motion is so designed that it is not necessary. Inside clearance induces premature release, and diminishes the period of expansion. Consequently inside clearance wastes steam, and ought to be avoided.

LEAD.

There are certain advantages gained in the working of a locomotive, by having the valves set so that the steam-port will be open a small distance for admission of steam, when the piston is at the beginning of the stroke. This opening is called lead. On the steam side of the valve the opening is called steam-lead: on the exhaust side it is called exhaust-lead. Lead is generally produced by advancing the eccentric on the shaft, its effect being to accelerate every event of the valve’s movement; viz., admission, cut-off, release, and compression. In the most perfectly constructed engines, there soon comes to be lost motion in the rod connections and in the boxes. The effect of this lost motion is to delay the movement of the valves; and, unless they are set with a lead opening, the stroke of the piston would in some instances be commenced before steam got into the cylinder. It is also found in practice, that this lost motion would cause a pounding at each change in the direction of the piston’s travel, unless there is the necessary cushion to bring the cranks smoothly over the centers. Without cushion, the change of direction of the piston’s travel is effected by a series of jerks that are hard on the working-parts. So long as the lead opening at the beginning of the stroke is not advanced enough to produce injurious counter pressure upon the piston, it improves the working of the engine by causing a prompt opening for steam admission at the beginning of the stroke. This is the time that a full steam-pressure is wanted in the cylinder, if economical working be a consideration. A judiciously arranged lead opening is therefore an advantage; since it increases the port opening at the proper time for admitting steam, tending to give nearly boiler pressure in the cylinder at the beginning of the stroke. With the shifting link-motion, the amount of lead opening increases as the links are hooked back towards the center notch; the magnitude of the increase, in most cases, being in direct proportion to the shortness of the eccentric-rods. A common lead opening in full gear with the shifting link is 1/16 inch, which often increases to ⅜ inch in the center notch. The tendency of wear and lost motion is to neutralize the lead, so that, when a locomotive motion gets worn, increasing the lead will generally improve the working of the engine.

OPERATION OF THE STEAM IN THE CYLINDERS.

As the work performed by a steam-engine is in direct proportion to the pressure exerted by the steam on the side of the piston which is pulling or pushing on the crank-pin, it is important that the steam should press only on one side of the piston at once. Hence, good engines have the valves operated so that, by the time a stroke is completed, the steam, which was pushing the piston, shall escape, and not obstruct the piston during the return stroke, and so neutralize the steam pressing upon the other side. When an engine is working properly, the steam is admitted alternately to each side of the piston; and its work is done against a pressure on the other side not much higher than that of the atmosphere.

BACK PRESSURE IN THE CYLINDERS.

When, from any cause, the steam is not permitted to escape promptly and freely from the cylinder at the end of the piston stroke, a pressure higher than that of the atmosphere remains in the cylinder, obstructing the piston during the return stroke, and causing what is known as back pressure. There is seldom trouble for want of sufficient opening to admit steam to the cylinders, for the pressure is so great that the steam rushes in through a very limited space; but, when the steam has expanded two or three times, its pressure is comparatively weak, and needs a wide opening to get out in the short time allowed. This is one reason why the exhaust-port is made larger than the admission-ports. Nearly all engines with short ports suffer more or less from back pressure, but the most fruitful cause of loss of power through this source is the use of extremely contracted exhaust nozzles. Were it not for the necessity of making a strong artificial draught in the smoke-stack, so that an intense heat shall be created in the fire-box, quite a saving of power, now lost by back pressure, would be effected by having the exhaust opening as large as the exhaust-pipe. This not being practicable with locomotives, engineers should endeavor to have their nozzles as large as possible consistent with steam-making.

Engines with very limited eccentric throw will often cause back pressure when hooked up, through the valve not opening the port wide enough for free exhaust.