Substituting in the formula,
| 40 × 1.5 × 113 × 600 | ||
| H. P. = | ————————— | = 123. |
| 33,000 |
The mean effective pressure may be found, approximately, for different conditions by means of the factors in the following [table of ratios], covering ordinary practice. The rule used is as follows: Multiply the absolute initial pressure by the factor corresponding to the clearance and cut-off as found from [Table II], and subtract the absolute back pressure from the result, assuming this to be 17 pounds for non-condensing engines, and 3 pounds for condensing.
Example 1:—A non-condensing engine having 3 per cent clearance, cuts off at 1⁄3 stroke; the initial pressure is 90 pounds gage. What is the M. E. P.?
The absolute initial pressure is 90 + 15 = 105 pounds. The factor for 3 per cent clearance and 1⁄3 cut-off, from [Table II], is 0.71. Applying the rule we have: (105 × 0.71) - 17 = 57.5 pounds per square inch.
Example 2:—A condensing engine has a clearance of 5 per cent. It is supplied with steam at 140 pounds gage pressure, and has a ratio of expansion of 6. What is the M. E. P.?
The absolute initial pressure is 140 + 15 = 155. The factor for a ratio of expansion of 6 (1⁄6 cut-off) and 5 per cent clearance is 0.5, which gives (155 × 0.5) - 3 = 74.5 pounds per square inch.
The power of an engine computed by the method just explained is called the indicated horsepower (I. H. P.), and gives the total power developed, including that required to overcome the friction of the engine itself. The delivered or brake horsepower (B. H. P.) is that delivered by the engine after deducting from the indicated horsepower the power required to operate the moving parts. The brake horsepower commonly varies from 80 to 90 per cent of the indicated horsepower at full load, depending upon the type and size of engine.
In proportioning an engine cylinder for any given horsepower, the designer usually has the following data, either given or assumed, for the special type of engine under consideration: Initial pressure, back pressure, clearance, cut-off, and piston speed.