All About Engines

How a Modern Engine Works 21 A body or a machine which, as in these examples, is capable of doing work, is said to possess energy. Energy due to position, as in the case of the weight, is called potential energy, and energy due to motion, as in the case of the grindstone, the mangle, or the flywheel, is called kinetic energy. The energy stored up in the gas may, for the present, remain unclassified. We shall need these ideas later. Now work may be done quickly, or it may be done slowly. A thousand ft.-lb. may be done in an hour, or it may be done in a minute, and the rate of doing this amount of work is sixty times greater in the second case than it is in the first. To measure the rate at which work is done we need a unit, and that chosen is 33,000 ft.-lb. per minute. It is called a horse-power, though it would require a pretty strong horse to raise nearly 15 tons one foot high in one minute or 1 lb. 33,000 feet high in the same interval of time. If, therefore, the number of ft.-lb. of work done in a minute is divided by 33,000 it gives the rate in horse-power. How, then, is the horse-power of an engine calculated ? Consider the piston in the cylinder with the steam pressing on it, and let the average pressure per square inch throughout the stroke be represented by P. If the piston is A square inches in area the total force acting on it will be P x A. In one stroke of length L feet, the work done will be P x A x L ft.-lb., and in N strokes P x A x L x N ft.-lb., or arranging the letters so that they are easily re- membered, P.L.A.N. Lastly, since 1 horse-power is