Second, because the steam is wire drawn during the time that the slide valve is closing the port to effect the cut off.
Third, because the live steam in the port and passage at the time the cut off occurs gives out some power during the period of expansion.
Fourth, because there is some condensation of the steam in the cylinder after the point of cut off, and there is no means of finding by calculation how much loss there may be from this cause.
During the live steam period there is also loss from condensation in the cylinder, but this is made up for by steam from the steam chest.
Fifth, the loss from condensation after the cut off has occurred will vary with the speed of the engine, and is greater in proportion as the piston speed is less, because there is more time for the condensation to occur in.
Sixth, there is some pressure on the piston between the time that the exhaust begins and the piston ends its stroke.
Seventh, because the compression absorbs some of the piston power.
Assuming the average pressure on the piston to be known, however, we may calculate the horse power as follows:
Example.—What is the horse power of an engine whose piston is 20 inches in diameter, and stroke 30, the revolutions per minute being 120, and the average pressure on the piston 60 lbs. per square inch?
| Diameter of piston | 20 | |||
| Diameter of piston | 20 | |||
| Diameter of piston squared | 400 | |||
| .7854 | ||||
| 400 | ||||
| Area of piston = | 314.1600 | ( these two ciphers neglected.) | ||
| 60 | average steam pressure. | |||
| lbs. pressure on piston | 18849.60 | (this cipher neglected.) | ||
| 30 | length of stroke in inches. | |||
| 565488.0 | inch lbs. per stroke. | |||
| 2 | two piston strokes per revolution. | |||
| 12 ) | 1130976 | inch lbs. per revolution. | ||
| 94248 | foot lbs. per revolution. | |||
| 120 | revolutions per minute. | |||
| 1884960 | ||||
| 94248 | ||||
| 11309760 | foot lbs. per minute. | |||