Hydraulic Engines or Motors

Hydraulic Engines or Motors are often conveniently used when water under a high pressure is obtainable, and where work is intermittently required, as in capstans, winches, &c.; they do not differ essentially from steam-engines. The water acts by difference of pressure—i.e. it is admitted at a high pressure at the beginning of the stroke, and exhausted at a low pressure at the end of the stroke, thus giving a reciprocating motion to the plunger. The velocity of the piston has to be kept low to avoid injurious shocks in suddenly bringing the column of water to rest; since they work under very much greater pressures than steam-engines (usual pressure 700 lb. per sq. in.), they can be much smaller. A common form is the three-cylinder single acting engine: in each cylinder works a plunger; water is admitted by valves behind the plungers and forces them out; at the conclusion of the out-stroke the pressure water-supply is cut off, and the exhaust valve opened, allowing the plunger to push the water out of the cylinder on the return stroke, and so on. There are two chambers in the framing, and one passage or port into the bottom of the cylinder; during the working of the engine the cylinder oscillates, and at the right time puts one or other of the two chambers in communication with the interior of the cylinder by means of this port; one chamber is open to exhaust-pipe, the other to supply-pipe. The plungers are connected to a three-throw crank. The great advantage of the single action is that shocks are avoided at the dead centres; the three cranks ensure a very uniform turning force on the crank shaft, and also enable the engine to start in any position.