All About Engines

Power and Its Measurement 323 sometimes the ropes only are used, as in Fig. 180. The flywheel is gripped by attaching one pair of rope ends to a spring balance and the other pair to a weight or another spring balance. The grip on the fly- wheel is measured by the difference in the readings of the spring balances or the balance and the weight. When the engine is working this difference is greatly decreased, and the decrease gives the force which the engine is overcoming at the rim of the flywheel. Using the same letters as before—P for the force in pounds and r for the radius of the brake circle in feet, the horse-power is given by the formula : 2 7F rn P 33,000 Brakes of this kind are called absorption dynamo- meters, because they absorb the energy given out by the engine. This energy is converted into heat by friction, and if the test is to last for more than a few minutes, or the engine is more than a few horse- power, the flywheel must be water-cooled. For this purpose the inside of the rim is in the form of a channel. Water is continually poured in through a small pipe, carried round by centrifugal force, and scooped out by a wider pipe with a specially shaped end. If this were not done the brake blocks or ropes would soon catch fire. Special precautions are desir- able with four-stroke gas and oil engines, which have flywheels with very heavy rims. It is difficult to cast these without strain being set up while they are cooling in the mould, and the heating in a brake