In steam engines a piston is alternately pushed forward and back in its cylinder by steam; and by means of the rod to which the piston is secured, motion is communicated to a crank and fly-wheel, and through these to the machinery to be driven: it is the same with pressure engines when used to move other machines, except that instead of the elastic vapor of water, a column of that liquid drives the pistons to and fro.
Note.—“The hydraulic engine of Huelgoat, in Brittany, is used to drain a mine; is single-acting, and acts directly to lift the piston of the pump. It makes five and a half strokes per minute, the stroke being a little more than eight feet in length. The piston-rod is 767 feet long, and it weighs 16 tons. The power of the engine is derived from a source at a height 370 feet above its own level.”—Knight.
In default of a natural head of sufficient pressure, the head is sometimes established in an accumulator of power; this is a body of water driven into a reservoir under heavy pressure, by forcing pumps worked by power. In cities where the water distribution is from elevated reservoirs, and in which the water supply is sufficiently abundant to justify the application of a portion of it to industrial uses, the water-engine or motor is recommended.
The following description of a water engine of world-wide adaptation will, if attentively studied, show the working of an approved type of this machine:
Ramsbottom’s hydraulic engine (Figs. [129 and 130]), is oscillating, and employs two cylinders b, l, operating one crank-shaft, a, by means of two cranks at right angles to each other. In one of the accompanying figures the channels of induction are marked j and are cast on the cylinders; the dotted circle c shows the position of the supply and discharge pipes; in the other figure these pipes are indicated by arrows. The two views are vertical cross-sections at right angles to each other, one being through the axis of the cylinders and the other through the middle post in which the inner trunnions of the cylinders are journaled. The apertures of induction are seen at h and those of eduction at i, and have the form of truncated circular sectors, whose center is the center of motion.
The induction and eduction spaces are divided by a sectoral partition; the apertures of admission and discharge on the sides of the cylinders are of similar construction. The surfaces of contact between the cylinders b, l and the support d are planed and polished and are made water-tight by the adjusting screws m m of the pivots. When the piston p is at the end of its course in either direction the cylinder and crank are vertical, and the valves all momentarily closed, the openings by which the channels j j communicate with the discharge and supply pipes presenting themselves exactly opposite the solid sectors which separate h from i.
In the next moment the flow of water will recommence, the cylinder discharging itself from the full side of the piston, and filling anew from the opposite side. Air chambers and relief-valves are used as a provision against counter-pressure and hydraulic shocks.
The Brotherhood three-cylinder reciprocating engine is an appliance for producing rotary motion by water-pressure.
The working parts of the Brotherhood three-cylinder hydraulic engine consist only of the three pistons and connecting rods, one crank and one rotating balanced valve and spindle which fits into the driver and is turned direct from the crank-pin; there are no glands, stuffing boxes, or oscillating joints.
It is shown by Figs. 131, 132. The three cylinders, A (made in one casting) are always open at their inner ends, and are attached to a central chamber, B. They contain three pistons, P, which transmit motion to the crank-pin through the rods, C. The water is admitted and exhausted by means of the circular disc valve, V, having a lignum-vitæ seat. The valve is rotated by the eccentric pin, E. A face view of this valve is shown above the steam chest. It has segmental ports which, in rotating, pass over apertures in the valve seat. There being no dead centers, the engine will start from all positions of the crank-pin, and a uniform motion of the shaft is produced without a flywheel.