Steam Engine

Steam-Engine is a machine for doing work by means of the elastic force of steam. In the earliest practicable form of engine - devised by Newcomen about 1712, and used to some extent for pumping water from mines - steam was admitted from a sort of overgrown kettle into a cylinder provided with a piston, which was arranged to be lifted by the weight of the pump-rod. Cold water was then injected into the cylinder, and the consequent condensation of the steam produced a partial vacuum under the piston, which was thus forced downwards by atmospheric pressure. The piston-rod and pump-rod were connected to opposite ends of a beam pivoted at its centre, so that the descent of the piston of the engine raised the piston of the pump. In 1759 James Watt began to improve this very inefficient mmngement. He provided a separate vessel in which the steam was condensed, thus saving the steam needed to heat the walls of the cylinder at each stroke, and applied the pressure of the steam to lift the piston. He further increased the power of his engine by admitting steam alternately to both sides of the piston, and provided an air-pump to remove any uncondensed gases from the condenser. He also introduced the crank for converting the reciprocating motion of the piston-rod into a rotary motion, and the centrifugal governor for regulating the supply of steam so as to give an approximately constant speed. The general arrangement of the essential parts of a modern steam-engine are shown in the figure. Steam is admitted into the valve-chest, which is a closed box: secured to one side of the cylinder, with both ends of which it communicates by means of two pipes or steam-ports S P. An exhaust-pipe E P is also provided, and these three openings in the side of the valve chest - which is here made perfectly flat and smooth - are uncovered or connected together by a sliding valve S V, which is an iron box open at one side, this open side fitting on the face of the valve-chest. The cylinder C is an iron tube carefully bored out and closed at each end by cylinder-covers; it is provided with a piston P, to which is secured a piston-rod P R, capable of sliding through a central hole in one cover. Leakage of steam past the piston is prevented by metallic rings fitting in a groove in the piston and forced outwards by springs, so as to be always in contact with the wall of the cylinder. The piston-rod works through a stuffing-box, which is a short tube packed with hemp or asbestos, and provided with a screw cover to compress this packing against the rod and prevent escape of steam. The arrangement of the connecting-rod and crank will be evident from the diagram. The motion of the slide-valve is produced by an eccentric-pulley E secured to the crank shaft. A ring fitting this pulley is attached to the eccentric-rod, which is jointed to the valve-rod. rhe latter passes through a stuffing-box in the valve-chest, and is fixed to the valve. When the various parts are in the position shown, steam which is supplied to the valve-chest can pass into the end A of the cylinder, while, the other end is in communication with the exhaust-pipe E P. The pressure in the A end will therefore be approximately the same as that in the boiler - say 80 lbs. per square inch - while that in the B end will either be the atmospheric pressure, or, if a condenser is used, will be, say, 12 1bs. per square inch less than that. These pressures will cause the piston to move from A to B, and the crank-shaft will be turned in the direction shown by the arrows. The eccentric turns with the crank, and it is evident from their re1ative positions that the slide-valve will now be moving in a direction opposite to that of the piston, and that the result of this will be that as the piston nears the end of its stroke both ports S P will be closed, thus shutting off the supply of steam. When the crank has moved 90° from the position shown, any pressure on the piston can of course produce no rotation; but the fly-wheel, which is always secured to the shaft, has sufficient momentum to continue the motion, and when the crank has turned a few degrees farther, the slide-valve will have moved sufficiently far to open the port connecting the B end of the cylirider with the steam-chest and to connect the A end with the exhaust. The piston willthen be forced towards the A end of the cylinder, and at the end of the stroke the steam will be again shut off, and the whole cycle of operations will be repeated. In order to obtain economy, it is necessary that the steam should be exhausted (either ito the air or into the condenser) at as nearly as possible the atmospheric pressure. The steam is therefore not admitted into the cylinder during the whole of the stroke, but is cut off when the piston has performed a portion of its movement, and this fraction of a cylinder-full of steam expands and gives up its energy during the rest of the stroke. The exhaust-port also must be closed rather before the end of the stroke, so that the compression of the steam behind the piston may cause it to be stopped gradually. If a heavy and rapidly-moving piston were stopped suddenly, an injurious jar would result. A simple slide-valve may be made to perform these somewhat complicated functions by being correctly proportioned; but a second valve, operated by a separate eccentric, is often added to enable the steam to be cut off more exactly at the proper moment. Other valve-motions have also been devised to enable the ports to be suddenly opened to their full extent, in order to avoid the loss of pressure which results from forcing the steam through a partly opened port. Very high steam pressures are now used for large engines; and as it is found impracticable, when working at 150 or 200 lbs. per square inch, to sufficiently expand the steam in one cylinder, compound engmes have been designed. In these a fraction - say one-half of a cylinder-full of steam - is admitted into a cylinder, and, after expansion, is exhausted, not into the air or the condenser, but into a second cylinder of larger diameter, where it is further expanded, and is in some cases exhausted into a third or even a fourth cylinder before passing into the condenser. In order to ascertain the power developed by a steam-engine it is necessary to know the mean pressure in the cylinder, which is of course quite different from that in the boiler. An indicator is used for this purpose, which consists of a cylinder provided with a piston, usually of half a square inch area. This is connected wlth the engine cylinder by means of a short pipe, and the pressure pushes up the indicator piston, against a spiral spring. If the force needed to stretch or compress the spring is known, the motion of the piston is a measure of the pressure. The piston moves a pencil by means of a system of multiplying levers, and draws a line on a piece of paper wrapped round a small drum. A string wound on this drum is connected to the piston-rod of the engine, so that a reciprocating rotary motion is given to the cylinder of paper which is similar to the motion of the piston. In this way the pencil draws a curve the axes of which are respectively proportional to the pressure on the piston and to the distance it has moved. The area of this curve is thus proportional to the work done on the piston, and from it it is easy to find the average pressure on the piston. If we multiply this average pressure (in pounds per square inch) by the area of the piston in square inches, we get the total mean pressure on the piston; and this, multiplied by the distance travelled by the piston (in feet) per revolution and by the number of revolutions per minute, gives us the number of foot-pounds of work done per minute; and this, divided by 33,000, is the indicated horsepower. This id, of course, somewhat greater than the actual available power, as some portion of it is used in overcoming the friction of the moving parts of the engine. The indicator diagram is also of great utility for showing whether the valves are or are not properly adjusted.