By DUMB IRON.
ALTHOUGH “Horse Power” is perhaps the most used of technical terms, it is curious how few laymen have any conception of what this unit of power really is. In cold technical language, ” power ” is the rate at which a machine is capable of doing work, and “work ” is the effect of force moving through a distance. “Force,” we are told, is that which moves or tends to move a body.
Let us consider force. Suppose that we attempt to lift a weight against the action of gravity, by means of a rope and pulley as illustrated in Pig. 1. We shall be applying a force to the rope, but we shall be doing no work until we have succeeded in lifting the weight off the ground. If the object to be lifted weighs 55 lbs., and we succeed in lifting it 10 feet, we have done 55 x 10 =550 foot lbs. of work. A “foot pound” is the British standard of work and it is the product of the force in pounds, and the distance through which the force operates in feet. It should be understood that if we were to lift 550 lbs. through I foot, or 5 lbs. through iro feet, the resultant amount of work would be the same.
It is now seen that work, force and power are very different things, although the terms are often thought to be synonymous. Power, being the rate of performing work, involves a time factor which cannot be ignored. It is not enough to determine the amount of work done and leave it at that ; for it is quite as possible for a 2 h.p. motor to haul a ton as for a ro h.p. engine to do so—the problem is simply one of gearing. The difference between the two engines lies in the fact that the larger one would be able to pull the weight five times as great a distance as the small, in the same time.
The Origin of H.P.
When James Watt was building his first steam engines about a century and a half ago, he found diffi
culty in stating to his clients the amount of work his engines were capable of doing in a given time. So, after observing the performances of dray horses, he invented “the horse power,” and took as his unit 33,000 foot lbs. per minute (or 550 per second). On glancing through a motor cycle catalogue we may be confronted with a statement such as follows :—
” 31 h.p., Ungine 4.98 A.C.U. H.P. Gives 8 brakehorse-power at 3,000 revs, per minute.”
The nominal rating at 31 h.p. is an arbitrary one, and is a relic of the days when it represented the. full power of an engine of 85 mm. bore, and 88 mm. stroke. Nowadays engines of this size are capable of twice this horse-power on full throttle, and some makers have departed from the time honoured convention in rating. The Auto-Cycle Union have, with a laudable desire for standardisation, brought out a rating of their own. In this case the engine is rated on the volume swept by its piston, and roo c.c. are taken as being equivalent to r horse-power. In reality the A.C.U. horse-power is a measure of the engine’s size and not its power. In the car world the H.P. rating is even more confusing, for the Treasury horse-power has but little relation to the actual brake h.p. of the engine. The Treasury rating formula is B2xN Rating H.P. = 2.5
li=l3ore in inches. N=No. of Cylinders. B 2 X N Or when millimetres are used 1613
B =Bore in millimetres. N=No. of Cylinders.
This formula was evolved about 1906 by the Royal Automobile Club, and as a result of many tests it was found that in those days a good engine developed a mean effective pressure of 90 lbs. per square inch, a mechanical efficiency of 75% and a piston speed of 1,000 feet per minute. Owing to heavy reciprocating parts and poor induction systems it was found that an increase in stroke length merely brought down the speed in revolutions, so that in those days a long stroke engine showed no advance over a short stroke engine.
Those sceptics who contend that the last decades have brought only detail improvement in autocar engines will be interested to note that it is claimed that one of the 200 mile race Supercharged Darracq engines has reached over zoo B.H.P. This engine is rated at 11.9 H.P. R.A.C. rating and 14.97 A.C.U. rating.
Power Measuring Appliances.
The actual power given at the shaft is, of course, the Brake-Horse-Power, so called because it is measured on a “brake ” or “dynamometer.” In principle all dynamometers measure power by absorbing it, and the
three means of absorption in common use are friction, water and electricity. The most elementary type of dynamometer in common use is the PRONY, shown in rig. 2. It consists of an externally contracting brake which envelops a watercooled flywheel. To the brake shoes is attached a yard arm, which supports the weight W. When the flywheel rotates, it tends to lift the weight, and, as we know the distance the weight is lifted per minute, we therefore
know the number of foot pounds of energy absorbed per minute. By dividing this number by 33,000 we have the brake-horse-power of the engine. In spite of the simplicity of the PRONY brake, its use is limited, because it is apt to overload the crankshaft bearings, its readings are rendered unsteady by variations in temperature and it steams and clatters. These shortcomings have induced manufacturers to use other methods of measuring power. The most popular dynamometer to-day is the FROUDE hydraulic absorption brake. As in the Prony brake, the engine tends to lift a weight at the end of an arm, and, basically, the calculations for both types of machines are the same. The Fronde, however, absorbs the engine power by means of water which is circulated in a number of pockets in the casing and in the rotor. This circulating water sets up a vortex action which provides the hydraulic resistance and simultaneously the means of carrying away the heat developed by the destruction of power. All Fronde dynamometers have lever arms of such a length that the power may be calculated by a simple formula. This is as follows :— Wx N
where W.Load on arm. N=Revs. per minute. K=A constant.