Fig. 68. Calculating Efficiency.
Expansion Line.—These figures represent the expansion line. It is now necessary to get the mean effective pressure, which means that we must know what the average pressure of the gas is in each square inch from B to C.
Mean Effective Pressure.—This is obtained by adding together the figures given in the sketch, and the result is, 1530. As eleven pressures were required to produce this sum, it should be divided by that number, making the result 148, avoiding fractions, as we shall do in all the calculations.
The figures represent that the mean effective pressure of the gases on the piston is 148 pounds. If this is multiplied by the area of the piston, and this result by the stroke in feet and the number of power strokes per minute, we get what is called foot pounds.
Foot Pounds.—Assuming that the diameter of the piston is 5 inches, which, figure, if multiplied by 3.1416, will give its area as a little over 15-1/2 square inches. Let us assume the crank is 4 inches. This will give a power stroke of 8 inches.
To find out how many power strokes there are in a minute, we must know the revolutions, and this being taken at 800, and a power stroke at only every other revolution, would mean that we have 400 impulses, and each impulse traveled 8 inches, = 3200.
This represents inches, which must be converted into feet, so that we have 266 feet of power strokes per minute.
First multiply the mean effective pressure on the cylinder, that is 148 × 15-1/2, which equals 2294. Then, 2294 × 266, equals 610,204. This product represents foot pounds.
Work or Energy.—A foot pound is the amount of work or energy expended in raising a weight of one pound, through a distance of one foot. If 550 pounds should be raised one foot in one second of time it would represent one horse power of work accomplished. If 550 pounds should be raised one foot in one minute of time it would be equal to 550 × 60 = 33,000 foot pounds, and this would mean one horse power, or the work done in one minute of time.