Attempting to follow the empirical constants evolved for the steam engine led the pioneer gas engine designers so far astray that one and all dropped them forthwith, and seemingly they, nor their successors have ever come together since. The indicated horse power of any engine is equal to 1-33,000 multiplied by the product of the mean effective pressure, the area of the piston, length of stroke in feet and number of power strokes per minute. In the case of steam engines this is usually expressed by the formula
H. P.= PLAN/33,000 in which:
P. Mean effective pressure in pounds per square inch.
L. Stroke in feet.
A. Piston area in square inches.
N. Number of power strokes per minute.
Taking into consideration actual working conditions and allowing for drop in pressure from various causes, the results of this formula will be found to fall within close limits of error. But with the explosive engine so many disturbing factors are present that it is almost impossible to predict from cylinder dimensions, speed and fuel, wbat power a given engine will actually develop. The most that can be done is to say what it should produce, granted that the numerous conditions of effective working are complied with. •
Assuming a given compression sufficiently short of the practical limit as not to involve any danger of spontaneous combustion, a speed that is neither unreasonably low nor excessively high, that the various parts have been correctly designed, that the mixture is good and ignition takes place at the proper point, that the resistance of the various ports and passages is not so great as to prevent the cylinder receiving an approximately full charge, a formula may be evolved which will, as above mentioned, show the best result theoretically that may be expected from an engine of given bore, stroke and speed.
By omitting the compression factor, the man who reads in a catalogue that the stroke and bore of a motor are of given dimensions, and B. P. M. are of specified number, may obtain approximate results by following this simple formula, the calculation in this instance giving the brake or actual horse power.
B. H. P. = D2 x L N/18,000 in which:
D. Piston diameter in inches or bore.
L. Stroke in inches.
N. Revolutions per minute.
The denominator varying according to the design of the engine and the character of the fuel.
By assuming a piston speed, this may be simplified to such an extent that, given the piston diameter, what any engine is capable of producing may be known with but little calculation.
General practice in four-cycle engines favors a piston speed of 500 feet per minute, in small stationary and marine engines up to 700 feet, and in automobile motors 800 to 1000 feet, so adopting 600 feet per minute as a mean, we have 2L x N / 12 = piston speed in ft. per minute, or = 600
L N = 3600
B.H.P.= D2 x 3,600 / 18,000=D2 / 5 in which D, L and N represent the same values as above. Thus with this piston speed the horse power per cylinder would equal approximately one-fifth of the square of the bore. In other words, an engine, with a 5 x 6 in. cylinder, should produce 5 h. p. at 600 R. P. M., but this Bomewhat lower than the speed of the average automobile motor. The denominator four will approximately express the difference represented by an additional one to two hundred revolutions per minute, will produce 6ih. p., and soon in proportion as speed increases.
For gasoline, E. W. Roberts, in the gas engine handbook, gives the following:
B.H.P. = D2 x L x N / 13.500 and this has been found to work out well in practice. But the engine builder who blindly follows any set formula is sure to be disappointed at the concrete result, and the majority of those who have started with a formula as a working basis, toiled over a long and weary road, full of vexatious obstructions being producing a motor that fulfilled expectations to any ex-teut. Formulae, generally speaking, represent maximum values obtainable under ideal conditions, and this is particularly the case where they have not been continually modified and readjusted as the result of experience extending over a number of years.-"Automobile Magazine. "