All About Engines

3*4 All About Engines piston upon which it acts, will give the total force driving the piston along. In one stroke the number of foot-pounds of work done will be given by the product of this force with the distance through which it acts, or the length of the stroke in feet. And if this result is multiplied by the number öf strokes per minute the result will be the number of foot- pounds of work done per minute. Calling the pressure P, the area A, the length of the stroke L, and the number of strokes per minute N, the work done per minute will be P x A x L x N, or, to put it in a form that is easily remembered, P.L.A.N. Lastly since I horse-power is 33,000 ft.-lb. per minute, the horse-power of the engine is : P.L.A.N. 33,000 Apply this now to an actual engine. The area of the piston and the length of the stroke can be measured, and the number of strokes per minute can be counted. But the pressure varies from the point of cut-off to the end of the stroke. What this pressure ought to be can be calculated from the known properties of steam. It is, however, impossible to look into the cylinder and to ascertain whether steam is admitted at exactly the right moment; if it passes freely through the ports without being throttled or “ wire-drawn,” and if it expands with loss of pressure by condensing in contact with the cold walls of the cylinder and the passages leading to it. We must, in fact, have some means of measur- ing the pressure of the steam behind the piston