III.

Modern Steam Engines

The operation of a typical modern steam engine is depicted in Figures 1a-d, which show a steam engine's cycle of operation. In Fig. 1a, when the piston is at the left end of the cylinder, steam admitted to the valve chest flows through the port into the cylinder at the left-hand side of the piston. The position of the slide valve also allows the used steam at the right-hand end of the cylinder to escape through the exhaust port. The piston's motion drives a flywheel, which in turn drives the rod that controls the slide valve. The relative positions of the piston and the slide valve are governed by the relative positions of where the crankshaft and the slide valve rod are fastened to the flywheel.

In the second position, shown in Fig. 1b, the steam at the left side of the cylinder has expanded and moved the piston to the centre point of the cylinder. At the same time, the valve has moved to the closed position so that the cylinder is entirely sealed and neither the steam in the cylinder nor the steam in the valve chest can escape.

As the piston continues to move towards the right under the pressure of the expanding steam, as shown in Fig. 1c, the port at the left end of the cylinder is connected to the exhaust by the valve, and, at the same time, the valve chest, which contains steam, is connected to the right end of the cylinder. In this position, the engine is prepared for the second stroke of its double-action cycle. Finally, in the fourth position (Fig. 1d), the valve again covers the ports from both ends of the cylinder, and the piston moves towards the left, driven by the expansion of steam at the right end of the cylinder.

The type of valve illustrated in the figure is the simple slide valve, which is the basis for most valves used on modern steam engines. Such valves have the advantage of being reversible; their position relative to the piston can be varied by varying the position of the eccentric that drives them, as shown in Fig. 2. Moving the eccentric through 180° makes it possible to reverse the direction of rotation of the engine.

The slide valve, however, has a number of drawbacks, the most important of these being the friction caused by steam pressure on the back of the valve. To avoid the wear caused by this pressure, steam-engine valves are frequently made in a cylindrical form entirely enclosing the piston, so that the pressure is equal all around the valve and friction is minimized. The development of this type of valve is attributed to the American inventor and manufacturer George Henry Corliss. In other types of slide valves, the moving portion of the valve is designed so that steam does not press directly on the back of the valve.

The linkage between the piston of the engine and the valve supplying steam to the engine is very important in determining the power and efficiency of an engine. By varying the point in the engine cycle at which steam is admitted to the cylinder, it is possible to vary the amount of compression and expansion in the cylinder and hence to vary the power output of the engine. A number of different types of valve gears for linking the piston to the valve have been developed that permit not only reversing of the engine, but also a range of control of the admission time and cut-off of the steam. Valve gears are of particular importance in steam locomotives in which the effort required from the engine varies widely; the effort is at a maximum when the locomotive is starting and less when it is running at full speed.

An important adjunct to all types of reciprocating steam engines is the flywheel, which is driven by the piston crank. Because of its inertia, the flywheel, usually a heavy metal casting, makes continuous the individual surges of power of the steam expanding within the cylinder, and permits the engine to provide a uniform flow of power.

In single-cylinder steam engines, the engine can stop when the piston is at one end of the cylinder or the other. If the cylinder is in this position, the engine is said to be on dead centre and is impossible to start. To eliminate the dead-centre points, steam engines are frequently equipped with two or more coupled cylinders, arranged in such a way that no matter what the position of the pistons, the engine is able to start. The simplest way of coupling two cylinders in an engine is to arrange the two cranks on the flywheel as shown in Fig. 3. For better balance, it is also possible to use a three-cylinder engine in which the various cranks are set at an angle of 120°. The coupling of engines not only eliminates difficulties in starting but also produces a power plant that operates more reliably.

The cylinder of a compound engine, unlike that of a single-cylinder engine of the ordinary type, can be kept close to a uniform temperature, which makes the engine more efficient.

Further improvement in the design of steam engines is afforded by the uniflow engine, which uses the piston itself as a valve and in which all portions of the cylinder remain at approximately the same temperature when the engine is operating. In the uniflow engine, steam moves in only one direction while entering the cylinder of the engine, expanding, and then leaving the cylinder. This unidirectional flow is accomplished by employing two sets of inlet ports at either end of the cylinder, together with a single set of outlet ports in the cylinder wall at the centre. The flow of steam into the two sets of inlet ports is controlled by separate valves. The inherent advantages of the uniflow system are such that engines of this type were usually chosen for use in large installations, although the initial cost of the engines is considerably higher than that of conventional steam engines. One virtue of the uniflow engine is that it permits the efficient use of high-pressure steam in a single cylinder engine without the necessity of compounding.