576 HYDRAULICS AND ITS APPLICATIONS 



of an air vessel on the supply side of the engine materially improves the 

 smoothness of running, and by maintaining an approximately uniform 

 flow in the supply main, reduces frictional and shock losses and so 

 increases the efficiency of the motor. The necessary dimensions of this 

 air vessel depend on the number and arrangement of the cylinders ; on 

 the speed and dimensions of the engine ; and on the length of supply pipe. 

 If more than one cylinder is used, the cranks being placed at equal angles 

 round the shaft, the velocity of flow through the supply pipe is consider- 

 ably steadied, and indeed with the usual type of three-cylinder engine 

 having cranks at 120, becomes so nearly uniform that it becomes possible 

 to dispense with the air vessel. 



For a further consideration of the questions deciding the size of the 

 air vessel in any particular case reference should be made to Art. 168. 

 With the high pressures usually adopted in these engines it becomes 

 imperative either to provide some mechanical device for maintaining the 

 charge of air in the vessel, or to make the vessel of ample area to main- 

 tain its mean working level approximately constant over long periods of 

 working. Thus the necessary size of air vessel for the engine will in 

 general be slightly greater than that for the corresponding pump. 



Losses in the Hydraulic Engine. Port Areas. These losses are due, 

 partly to friction but more particularly to shock produced at sudden 

 changes of section, and are therefore approximately proportional to the 

 square of the velocity. To reduce these, all throttling is to be prevented as 

 far as possible, inlet and outlet ports are to be short, direct, and of ample 

 area, with easy curves and with few changes in sectional area or shape. 



Where, as in some instances, the inlet ports are emptied at the end 

 of each exhaust stroke, to be refilled before the commencement of the 

 next working stroke, this is productive of a direct loss of energy, since, 

 owing to the fact that the fluid is inexpansive, no work is done on the 

 piston until these ports are completely filled. This loss increases with 

 the area of the ports, while friction losses and those due to the loss of 

 the kinetic energy of entrance decrease as this area increases, so that in 

 any particular case there will be some one port area for which the total 

 loss is a minimum. In the case of a single acting engine : 

 The loss of energy per revolution due 1 



to water necessary to fill inlet ports of 



^ 7 * ^ = 2 ' 31 P I - a 



area a square feet and length I feet, at | 



a pressure p Ibs. per square inch 



?; 



Loss by friction per stroke = 62'4 L A X '( foot Ibs. 



2 g m 



