Fig. 67.—Petrol Engine Brake.
Example:—An engine being tested by a brake applied to the flywheel as shown in the sketch (Fig. [67]) exerts a pull of 50 lbs. at a speed of 2,000 revolutions per minute. If the length of brake arm is 30 inches, calculate the brake horse-power developed.
Work done per minute = 50 × 6·28 × 30/12 × 2000 ft. lbs.
B.H.P. = (50 × 6·28 × 30/12 × 2000)/33,000 = 47·5
Rated Horse-Power.—For taxation purposes the Treasury makes use of a formula for the rating of petrol engines according to their probable horse-power. This formula is based on a certain speed of the piston which was regarded as a limiting value some years ago (when the formula was first proposed) and on the attainment of a certain effective pressure in the cylinder.
Horse-power from the Treasury formula = 0·4 d2n.
Where d = diameter of cylinder in inches,
n = number of cylinders.
With modern engines much greater horse-power is obtained, and a near approximation to the true output is obtained by using what is now known as the Joint Committee’s formula.
Brake Horse-Power = 0·46 n (d + s) (d - 1·18)
Where d = diameter of cylinder in inches.
s = length of piston’s stroke in inches.
This formula is only to be used in an attempt to predict the probable maximum horse-power which any engine will give. It must not be confused with the ordinary brake horse-power formula.
Example:—Find the probable maximum horse-power of an engine having four cylinders each 3 in. bore and a piston stroke of 4 in. What would be its horse-power for taxation purposes?