Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(148) The Cylinder and Slide Valve.

The steam engine cylinder consists essentially of a smoothly bored iron casting in which a plunger called the “piston” slides to and fro, the cylinder acting not only as a container for the steam acting on the piston but as a guide and support as well. Needless to say, the contact or fit between the piston and cylinder walls must be as perfect as possible, tight enough to prevent steam passing the piston, and free enough to allow the piston to slide without unnecessary friction. The reciprocating piston is connected to the crank through a connecting rod by which the pressure on the piston is communicated to the crank arm.

The pressure exerted on the crank pin by the piston depends on the area of the piston (in square inches) and the pressure of the steam on each square inch of the area. With a given steam pressure, the greater the area, the greater the force tending to turn the crank. As power is the rate or distance through which the force acts in a unit of time it is obvious that the power developed by the engine is equal (in foot pounds) to the force in pounds multiplied by the velocity of the piston in feet per minute. Since there are 33,000 foot pound minutes in a horse-power, the power developed by such a cylinder is equal to the force multiplied by the piston velocity, divided by 33,000.

As the cylinder is necessarily limited in length it is evident that the piston cannot travel in one direction continuously but must be reversed in direction when it travels the length of the cylinder bore thereby traveling the next distance in the opposite direction. This reversal of the piston is accomplished by admitting the steam in one end of the cylinder and then into the other, this causing the steam to act on the opposite sides of the piston alternately. To establish a difference of pressure on the two piston forces, the steam pressure is relieved on one side while the steam acts on the other.

A typical cylinder furnished with the ordinary steam tractor is shown by Fig. 133, in which T is the cylinder, P the piston and R is the piston rod. When the steam in the cylinder end E acts in the direction shown by arrow E, the piston pulls the rod R in the direction shown by arrow S, the pressure in the cylinder end D being relieved to atmospheric at this time. The steam is admitted and relieved by the valve L which slides back and forth on its seat actuated by the valve rod VR.

In the position shown, the valve L is moving to the left as shown by arrow O. The edge of the valve N is just opening the steam port G through which the cylinder end F is placed in communication with the steam filled valve chest A. Steam at boiler pressure fills the space A, which flows into E past N and through G when the valve opens and establishes pressure against P, which, through the piston and connecting rods turns the crank.

The steam is exhausted from the cylinder end D, through the port F, through the exhaust port U, and out of the exhaust pipe X. As will be seen from the figure, the inside valve edge Y has moved to the left so that the port F is fully opened. When the piston reaches the left hand end of the cylinder, the valve L moves to the right so that the end of the cylinder E is connected to the exhaust port V through the cylinder port G, thus allowing the steam in the space E to pass out of the exhaust pipe X. A further movement of the valve to the right causes the left edge Z of the valve to uncover the cylinder port F which allows the steam to flow into the cylinder space D and push the piston to the right. This motion is carried on continuously, the valve moving in a fixed relation to the piston, and admits the steam and releases it first on one side of the piston and then on the other. The valve shown is known as a “D” valve and is one of a variety of valves furnished with steam engines, which, however perform exactly the same functions as the valve shown.

An “eccentric” which is really a form of crank, drives the valve to and fro, the eccentric being fastened to the crankshaft. The full pressure of the steam forces the D valve down on its seat, and as the valve is of considerable size, this pressure causes much friction and power loss. In some engines a “balanced” valve is used in which the pressure on the valve is balanced by an equal pressure that acts on the under side of the valve face. Balanced or unbalanced, the function of the slide is to alternate the flow of steam in the two ends of the cylinder.

Steam is prevented from passing the piston into the opposite end of the cylinder by elastic rings placed in grooves on the piston which are known as “piston rings.” Being thin and elastic these rings instantly conform with any irregularity of the piston bore and effectually stop the flow of steam past them. At the point where the reciprocating piston rod R passes through the cylinder, a steam tight joint is made by the “stuffing box” or gland H. The space between the inner walls of the stuffing box and the piston rod are either filled with some description of fibrous packing or a metallic packing that fits around the rod in the same manner that the piston rings fit in the bore of the cylinder. The packing is arranged around the valve rod VR in the same manner.

As the piston, piston rod, and valve slide on their respective surfaces with considerable pressure it is absolutely necessary that these parts receive ample lubrication. In practically all engines the oil is taken into the cylinder with the steam in the form of drops, the oil being measured out by a sight feed lubricator that is tapped into the steam supply pipe. In this device, the oil from the lubricator reservoir is fed through a regulating needle valve, drop by drop, up through a gauge glass so that the engineer can tell the amount of oil that he is feeding. The body of the lubricator is filled with condensed water up to the level of the outlet through which the oil passes into the cylinder, and the entire lubricator, reservoir and all is under boiler pressure at all points. The oil regulating valve is placed at the bottom of the lubricator, and as oil is lighter than water, it floats up from the valve to the level of the outlet, through the gauge glass, and from the outlet level floats out into the steam pipe and mixes with the steam. By floating the oil in this manner, the engineer can see every drop that is fed.