DEFINITIONS AND MECHANICAL PRINCIPLES 2j 



piston, and the pressure of the gases from the engine 

 cylinder acting on the piston of the indicator compresses 

 a calibrated spring above. The amount of pressure is re- 

 corded with a pencil point by a suitable mechanism on 

 the paper card. Thus if a diagram is obtained from an 

 engine at work, it not only permits a study of the engine 



in regard to the action of 

 valves, igniter, etc., but 

 also enables the amount of 

 work performed in the en- 

 gine cylinder to be calcu- 

 lated. 



FIG. 15 AN ACTUAL INDICATOR DIA- 



GRAM OBTAINED FROM A GAS EN- -T lg- 1 5 ShOWS an actual 



CINE, WITH THE SCALE OF THE diagram taken from a gas 



SPRING APPENDED 



engine. As the pressure 



varies throughout the stroke, an instrument like the 

 planimeter of Fig. 13 must be used to average the pres- 

 sure for the entire working stroke of the piston, and sub- 

 tract the pressure required in the preliminary and ex- 

 haust strokes. This average pressure is called the mean 

 effective pressure (M.E.P.). Knowing the distance the 

 engine piston travels a minute doing work, the area of 

 the surface on which the pressure acts, and the mean ef- 

 fective pressure, it is possible to calculate the rate of 

 work or the horse power. The horse power obtained in 

 this way is called the indicated horse power (I.H.P.), 

 and differs from the brake horse power (B.H.P.) by the 

 power required to overcome friction in the engine. The 

 ratio of the brake horse power to the indicated horse 

 power is called the mechanical efficiency of the engine. 



If P = Mean effective pressure, 

 L Length of stroke in feet, 

 A = Area of piston in square inches, 

 N = Number of working strokes a minute, 

 PLAN 

 33,ooo 



