HYDRAULIC ENGINE, an engine which is driven by the power or pressure of water seeking a lower level. The simplest form of engine utilizing water power is the water wheel. The overshot wheel acts partly by the impact of the stream flowing in the sluice, partly by the weight of the water in the buckets ; the breast wheel gets its power almost wholly from the rush of the water, but to a small de gree by the weight of water in the curved • buckets; the undershot wheel is acted upon purely by the force of the moving stream, and thus is the more purely hydraulic of the three_ (See WATERWHEELS). The turbine wheels are highly developed hydraulic engines. They utilize the power of falling water in one of three ways: as axial turbines, in which the water is supplied and discharged in a current parallel to the axis; as outward flow turbines, in which the water enters and is discharged in currents radiating away from the axis; as in ward flow turbines, in which the water enters and is discharged in currents converging radically toward the axis. (See TURBINES). The well-known hydraulic ram (q.v.) is another simple form of hydraulic engine, but is waste ful of power even in the most improved forms.
The rotary forms of hydraulic engines such as the turbines, Pelton wheels, etc., have proved difficult of adaptation to many forms of work on account of the high speed at which they run, and the necessity of gearing them down to the usable speed, which renders them less flexible. But a more potent objection is that they vary extremely in efficiency under vary ing speeds. These difficulties led to the in vention of the reciprocating hydraulic engine, which has been found dependable and very steady in operation, although it must run always at the same speed, using the same power, no matter what its load: it is therefore of commendable efficiency (about 65 per cent) only when running under a full load. The
reciprocating engine which has found most favor is the Brotherhood hydraulic engine, or modifications of it. This engine has three cylinders set at angles of 120 degrees with each other, the three piston rods working on the same crankpin. Each cylinder has a single port which becomes alternately an inlet and an outlet, the cylinders being all the time full of water, that on one side of the piston having a greater pressure than that on the other. The opening and closing of the ports and of the inlet and outlet valves is accomplished by a rotary valve mechanism carried by the crank shaft. The piston speed of these engines is about 30 feet per minute for all sizes, and they are built up to 30 horse power. They are for use with water pressures (in pipes) ranging from 60 pounds per square inch up to 1,000 pounds per square inch. Another type of reciprocating engine is the Riggs hydraulic engine. In this, the crankpin is fixed, and the cylinders, fas tened 120 degrees apart upon a disc, revolve with the disc around. Consult Butler, E., Pumping and Hydraulic (London 1913) ; Dunkerley, S., (Vol. 1, °Hydraulic Machinery )) London 1907) ; Gelpke, V., and Van Cleve, A. II., (Hydraulic Turbines' (New York 1911) ; Gibson, A. H., and Its Applications> (New York 1915).