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~ two indicator diagrams are required, 
| _the pressures on the front and back 
CHAPTER XIX 
PISTON AND CRANK EFFORT DIAGRAMS 
271. Piston Effort Diagrams.—An engine or machine worked by 
fluid pressure has usually a piston or ram which receives reciprocating 
motion in a cylinder. When the piston or ram is single-acting, the 
pressure of the fluid introduced into the cylinder causes the piston or ram 
to move outwards, and the return stroke is usually performed either by 
the energy stored up in a fly-wheel, or by the pressure of the fluid in 
another cylinder, through intermediate mechanism. In this case the 
effort on the piston or ram at any instant is simply the force exerted on 
it by the fluid in the cylinder at that instant. When the piston is 
double-acting, the fluid is admitted into the cylinder on opposite sides of 
‘the piston alternately, and after doing its work it is allowed to escape. 
In this case the effort on the piston at any instant is the difference 
between the forces exerted by the fluid on the opposite sides of the 
piston at that instant. 
When the fluid used is water, the pressure which it exerts is practi- 
cally constant throughout the stroke, and the effort is therefore constant, 
and the effort diagram is a rectangle whose length represents the length 
of the stroke of the piston or ram. 
In heat engines and in machines worked by compressed air the 
pressure of the fluid is generally variable throughout the stroke. In 
such cases the actual effort on the piston is obtained from indicator 
diagrams, which are simply the records 
of self-registering _ pressure - gauges, 
which show the pressure of the fluid 
at every point of the stroke of the 
n. If the engine is double-acting ty 
one for each side of the piston. The 
indicator diagram shows the intensity 
of the pressure of the fluid, generally 
in Ibs. per square inch. 
Let p, and p, be the intensities of 
of the piston respectively at any in- 
stant, and let a, and a, be the effective 
areas of the front and back of the 
piston respectively, then the effort on 
the piston at the instant considered 
iS P,4,—Pyt, If a,=a,=a, then the effort is a(p, — P2). In double- 
acting engines a, is not generally equal to a, on account of the presence 
of the piston-rod on one side. 
Fig. 488. 
313 
