A steam engine is a machine for transforming the pressure and expansive power of steam into mechanical energy. Since the steam is produced by the application of heat, the steam engine really transforms heat into mechanical energy. So great, however, are the losses in the burning of the coal, the expansion of the
steam, and the working of the engine, that the best modern steam engine does not utilize more than 17 per cent of the energy in the coal.
A simple form of steam engine is shown, partly in section, in Figs. 1 and 2. A is a steam pipe connecting a boiler with the steam chest B. The live steam passes from the steam chest through the port C into the left end of the cylinder, between the cylinder head D and the piston P. The pressure of this live steam forces the piston to the right, and drives the exhaust steam, on the other side of the piston, out of the port E under the slide valve V, and out of the exhaust port F, which leads either to a condensing chamber or to the open air.
When the piston has been forced over a part of its stroke, the slide valve will be moved by its rod r to the left, closing C, and the work done during the rest of the stroke will be due to the expansive power of the steam. By the time the piston has reached the end of its stroke to the right, the slide valve will be moved so far to the left that the live steam will now come into the right end of the cylinder through E, and the exhaust steam in the left end will go out of the exhaust port F, through the port C. The piston rod R is attached to a crank arm M on the main shaft S, and in this way the to-and-fro, or reciprocating, motion of the piston is changed to the rotary motion of the shaft. On the shaft are fixed one or two heavy flywheels W, the momentum of which serves to give steadiness to the engine; and one or more belt wheels, over which run the belts' by which the motion of the shaft is transmitted. to machinery.
In condensing engines the exhaust steam passes into a compartment containing water for condensing the steam. The condensation greatly reduces the back pressure which opposes the motion of the piston. In noncondensing engines, such as the locomotive, the exhaust steam passes into the open air - sometimes by way of the smokestack, in order to increase the draught through the fire box. The back pressure in noncondensing engines is the pressure of the atmosphere.
In compound engines, the steam gives up only part of its heat and expansive power in one cylinder; the exhaust steam from this escapes under pressure to a second cylinder, where it does more work. By thus using two or more cylinders in succession, a greater percentage of the energy of the steam can be utilized. A comparison of Stephenson's locomotive, the Rocket, built in 1829 (Fig. 3), with a modern type of locomotive (Fig. 4), shows the development that has taken place.
