Oscillating oscillating type of steam-engine was suggested by Trevithick, and is the best for ordinary paddle-wheel ves sels, as it is light and compact and has the fewest working parts. It will work with the cylinder in any position from horizontal to vertical, although best in the latter position. It does not, however, admit of very early cut off, and the trunnion is apt to leak. In Westland's oscillating engine, shown in Figure 5 (pi 84), while the crank rotates the cylinder oscillates on trunnions. There arc on top of the cylinder four valves, worked by a rock-shaft placed in the centre of the cylinder's length and having tappets which catch in bell-cranks moving the valves. Figure r represents the Mackintosh type. The steam is admitted through the trunnions or pivots upon which the cylinder oscillates. In this type the fly-wheel shaft is below and the cylinder oscillates above it; in Figure 4 (the Hicks) the fly-wheel is above; and in Figure 3 (p1. 85; Fi:vre's) the oscillating axes are on the bottom instead of at the centre of the cylinder. Figure 4 shows the Root oscillator.
Penn's Trunk Engine.---ru the trunk type of steam-engine (p1. 86, i) there is no piston-rod, the connecting-rod being hinged to a pin in the centre of the piston, which is surrounded by a cylindrical case or " trunk " concentric with the cylinder and continued out at its other end, so that there are the same effective piston-areas back and front. This is the lightest and most compact of all marine screw-engines; but the friction of the stuffing-boxes for the trunks is excessive, and the bear ings of the pin in the piston are liable to become heated, and are then dif ficult to cool, besides which the side-cylinder wear is very great, particu larly in running astern.
are those in which the shaft receives its rotary motion through the intermediation of a lever either above or at the side of the cylinders. Ordinarily the expression "beam-engine" refers to one in which the "walking-beam" or "working-beam" is above, the other types being generally spoken of as " side-lever " engines. Beam-engines are now but little used, except for marine purposes, although at one time they were the only type, and later on the most common one, for stationary engines; in fact, the beam was used in early locomotives. (See Vol. V., p.173.) aboard ship, a beam-engine requires a higher deck than is given to it in other countries than the United States.
Mc American Beam-engine for river-steamers was first designed in 1822 by Robert I,. Stevens. Its great advantage is its flexibility, per mitting long vessels to spring without crippling the engine. It works smoothly, and is economical and compact.
The Mississippi River steamboat constitutes a type of marine construc tion deserving special mention; but its peculiarities are so dependent upon the construction of the hull which it drives that its consideration will be deferred for the Volume on Marine Architecture. The North River and
Long Island Sound steamboats are in themselves a special variety to be found in no other country and in no other section of the United States.
The Side-lever Engine is of two kinds: (r) that in which the fulcrum is between the connecting-rod and the side-rods from the piston-rod cross heads, as in Figure 3 (p1. 86); and (2) that in which the fulcrum is at one end, as in Figure 2. The first is the true side-lever type, the second being often called the "grasshopper" engine. This type is especially adapted for marine purposes by reason of its simplicity, cheapness, capa bility of giving a long stroke in a shallow ship, comparative freedom from racking, and absence of a dead point even where there is but a single cylinder. It will run when in a state of repair under conditions which would disable any other type.
The Steefile Engine of Trevithick has the piston operating directly upon the crank; it is compact, light, and cheap, and has fewer working parts than the side-lever type, but requires a deep ship and needs two piston-rods, between which the shaft is placed. This engine, so modified as to lie horizontally, is well adapted for war-ships, in which the machinery must be low down in the hull. This modification is often known as the " return-connecting-rod" type.
The Engine, in very general use for marine pur poses, has the connecting-rod on the side of the crank-shaft opposite to the cylinder, and there are two piston-rods, one above and the other below the crank. This allows of very long stroke, but limits the diameter of the piston, makes packing the stuffing-boxes difficult, and allows of but very short eccentric-rods, unless they are placed on the same side as the connecting-rod.
The the theoretically perfect steam-engine the steam should be discharged at a pressure corresponding to absolute zero and at the temperature of the external air. The nearest approach to these points that can be obtained, consistent with other desirable features, the better. A device which would so condense the steam as to remove all pressure from the exhaust side of the piston would be very advantageous. The earliest attempt in this line effected this condensation in the cylinder itself by a jet of cold water, the unbalanced atmospheric pressure alone being the motive power (see p. 246). The use of the working cylinder as a condensing vessel having the disadvantage of cooling the entering steam before it had done any work, the employment of a separate con denser, due to Watt, has been found necessary; but the methods of effect ing condensation in a separate vessel are various. A jet from a plain tube or from a " rose " may be employed; or the steam-exhaust may be made to circulate against surfaces cooled by currents of cold water; and such surfaces may be either flat metal partitions or tubes containing the cool ing liquid.