A. On double acting engines, multiply the stroke in inches by two and by the number of revolutions per minute and divide by 12.
Q. Why multiply the stroke in inches by 2?
A. Because in double acting engines there are two working strokes to each revolution.
Q. Why do you divide by 12?
A. To reduce the inches to feet.
Q. How is the “piston’s speed” of a single acting engine found?
A. Multiply the stroke in inches by the revolutions per minute and divide by 12.
Q. What is the horse-power developed by an engine, say 12 × 24 inch, running 125 revolutions per minute, with 40 pounds mean effective pressure?
A. Area = 12 × 12 × .7854 = 113.0976 sq. ins.
- Piston speed = 24 × 2 × 125 ÷ 12 = 500 feet per minute.
- M. E. P. = 40 lbs.
- Then
- (113.0976 × 500 × 40)
- ————————— = 68.544 H. P.
- 33000