Balanced Valve.—One of the inventions for this purpose is a valve so balanced by the steam pressure that but little wear results. This has been the subject of many patents. Another type also largely used in engines is known as the oscillating valve, which is cylindrical or conical in its structure, and which revolves through less than a complete revolution in opening and closing the ports.
Rotary Valve.—The rotary valve, which constantly turns, is employed where low pressures are used, but it is not effectual with high pressures. This is also cylindrical in its structure, and has one or more ports through it, which coincide with the ports through the walls of the engine, as it turns, and thus opens the port for admitting live steam and closing the discharge port at the same time or at a later period in its rotation.
Engine Accessories.—While the steam engine is merely a device for utilizing the expansive force of steam, and thus push a cylinder back and forth, its successful operation, from the standpoint of economy, depends on a number of things, which are rarely ever heard of except by users and engineers.
Many of these devices are understood only by those who have given the matter thorough study and application. To the layman, or the ordinary user, they are, apparently, worth but little consideration. They are the things, however, which have more than doubled the value of the steam engine as a motor.
Efficiency of Engines.—When it is understood that with all the refinements referred to the actual efficiency of a steam engine is less than 30 per cent. some idea may be gained of the value which the various improvements have added to the motor.
Efficiency refers to the relative amount of power which is obtained from the burning fuel. For instance, in burning petroleum about 14,000 heat units are developed from each pound. If this is used to evaporate water, and the steam therefrom drives an engine, less than 4200 heat units are actually utilized, the remaining 9800 heat units being lost in the transformation from the fuel to power.
The value of considering and providing for condensation, compression, superheating, re-heating, compounding, and radiation, and to properly arrange the clearance spaces, the steam jackets, the valve adjustments, the sizes of the ports and passages, and the governor, all form parts of the knowledge which must be gained and utilized.
How Steam Acts in a Cylinder.—Reference has been made to the practice of cutting off steam before the piston has made a full stroke, and permitting the expansive power of the steam to drive the piston the rest of the way, needs some explanation.