used are more easily got at than ordinarily with engines of the "tandem" type. Piston-valves are used, and in this engine the steamchest is bored out and fitted with bushings which have supporting bars to prevent the valve catching upon the ports, When worn they can be withdrawn and new ones inserted, and a new valve introduced, without delay. Fig. 162 represents an automatic compound engine designed by Mr. F. H. Ball, especially for use in driving dynamo-electric machinery. The illustration represents engines using steam at 125 pounds pressure, and of 250 horse-power each. It was thought best to build these engines in the form of a double engine rather than the "tandem" type of compound, because it was believed that higher rotative speed could be successfully used where the work was distributed over two sets of crank-pins and journals of smaller sizes, rather than with the use of a single set of bearings of larger size, as in the case of a tandem engine developing the combined power of the double compound. An engine designed by Mr. Ide (Fig. 163) illustrates both the "tandem" form of compound high-speed engine, and some features of design of peculiar interest. This engine has its running parts covered in, to insure that the oil, which is freely supplied, may not be wasted or spattered about, to the injury of surrounding objects, while thus also obtaining thoroughness of lubrication approximating that of the "oil-bath." This gives, when fully effected, very great decrease in the wasted energy of internal friction of engine and corresponding increase of efficiency. The design is simple, inexpensive of construction, and embodies details of construction coming to be generally recognized as essential to high efficiency. The engine has a shaft-governor, controlled by a dash-pot, and thus enabled to regulate more closely. Its running parts are usually of steel. One of the dangers to which fast-running engines are peculiarly exposed is that of injury by the entrapping of water in the cylinder and the plunging of the piston against the mass of incompressible fluid which then fills the clear ance-spaces. In this engine, in addition to the relief-cocks or valves which are always fitted to such engines, a safeguard is introduced in the form of what engineers are ac FIG. 162.-"Cross" Compound Engine. customed to call the "breaking-piece," a part which is made purposely weaker than other portions of the machine, exposed to a common danger, so that this piece may go when danger arises. This piece is always one the replacement of which will give little trouble and make but little expense. Such a breaking-piece is made to form a part of the cylinder-head. This may be knocked out without injury to any important or costly part of the structure.' The single-acting multi-cylinder engine of Westinghouse, illustrated in the engraving, is a good typical representative of this class, and is one of the simplest devices of its kind. A single piston-valve, set horizontally above the two cylinders, distributes the steam and is regulated by a shaft-governor which properly varies its throw. The cranks are set opposite each other; the motions of the pistons are synchronous in opposite directions, and no receiver is needed. Both engines are single-acting, and high compression does away largely with the wastes due to considerable clearance. The cut-off in the high-pressure cylinder is effected by the lap of the valve. It has been found possible by this arrangement to bring down the consumption of steam to less than 20 pounds (9 kilos) per horse-power per hour when condensing, and below 25 pounds (11 kilos) when working non-condensing. In such single-acting engines it is usually intended that the rod shall never leave the crank-pin, in order that pounding may not occur. It is therefore evidently necessary that they should be so proportioned and speeded that the action of the inertia of their reciprocating parts shall not produce stresses, on turning the centre, in excess of the sum of weights and steam pressure. 1 The author planned an engine, about the year 1860, in which the whole cylinder head was made a safety-valve which could lift and discharge the water into the chamber behind it, the cover of the latter being bolted on, while the cylinder head was only held in place against a faced joint by steam-pressure. |