HYDEAULIC ENGINES 573 



valve itself bears against a lignum-vitae seat. All pistons are packed with 

 " L " leathers, and the cylinders and all working parts are lined with 

 brass. 



The engine as thus constructed forms a compact and serviceable motor, 

 has no dead centre, and is built in sizes up to about 30 B.H.P. The 

 piston speed is about 30 feet per minute for all sizes, the engine shown in 

 Fig. 265 having cylinders 5J inches diameter X 4-inch stroke and 

 developing 13 B.H.P. at forty-seven revolutions per minute when supplied 

 with water at 700 Ibs. pressure. 



The full power efficiency, as measured on the brake horse power, varies 

 with the working pressure from about 52 per cent, with 60 Ibs. pressure, 

 to 65 per cent, at 1,050 Ibs. This is equivalent to an hydraulic efficiency 

 of from 61 per cent, to 76 per cent. These values are probably too low 

 rather than too high. 



All constant stroke engines, however, suffer from the fact that water 

 cannot be used expansively, and since, to avoid shock, it is necessary to 

 keep the cylinders full of water, the same energy is used whatever the load 

 on the engine, causing the efficiency to be low at light or variable loads. 

 It has been attempted to overcome this difficulty, but without great 

 success, by cutting off the pressure supply before the end of the stroke 

 and admitting low pressure water from an auxiliary supply for the rest 

 of the stroke. 



A device due to Meyer consists in providing an air chamber at each 

 nd of the (double acting) cylinder. Cut-off may then take place before 

 the' end of the stroke, the air afterwards expanding and driving the 

 piston. No gain in efficiency is to be anticipated from this method of 

 working. 



The more usual device is to reduce the stroke and piston displacement 

 to suit the varying demands for power. In the Brotherhood engine this 

 has been accomplished by the Hastie regulating device. 



As thus constructed, power is transmitted through a hollow crank shaft 

 to the driving pulley by means of a volute spring, an increase in the load 

 producing a further coiling of this spring. The inner end of the spring is 

 fixed to the hollow crank shaft, while the driving pulley carries a disc 

 keyed to a cam shaft working inside the crank shaft. Any variation in 

 the load then produces a relative rotation of the crank and cam shaft. A 

 cam keyed to the latter shaft utilizes this relative rotation to increase or 

 diminish the crank radius, and thus to adapt the volume of piston dis- 

 placement to the demand for power. This device is, however, seldom 

 now fitted to the engine. 



