Mechanical Horse-Power.*
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What It Is, and How It Is Measurz.d.
Indicated Horse-power.
The modern method of rating steam engines is by " in
dicated horse-power " The indicator is a mechanical contrivance consisting, essentially. : (i) of a small springloaded piston whose underside is in communication with one end or other of one cylinder of the engine to be indicated; and (2) of a spring-controlled drum, oscillated about its axis by means of a cord attached to some reciprocating part of that section of the engine corresponding to the cylinder under observation. The indicator piston carries a pencil which traces a line upon a paper wrapped around the drum, and the combined motions of the pencil and of the drum cause a diagram to be traced out which represents the action of the steam on the one side of the engine piston. As steam engines are double-acting, the indicator has to be placed in communication with both sides of the piston, and a diagram obtained from each. To an expert this indicator card or diagram conveys much other information, but, so far as indicated horse-power is concerned, the sole function of time diagram is to enable the actual mean pressure on the piston to be measured the indicator card does not give the horse-power of the engine right away. This has to be calculated in much the same way that Watt did for his nominal horse-power, only, in this case, the mean pressure is actually measured instead of being assumed, and, instead of Watt's rule for pisfon speed, which assumed a constant for a given engine, the revolutions of the engine are taken at the same time as the diagram, and from the former the piston speed is exactly obtained.
The rule for finding the indicated horse-power of a steam engine is as follows :— I.H.P. • — 33,000 Where P = the mean of the mean pressures on both sides of piston, in pounds per sq. inch.
„ A area of piston in square inches. „ L =. length of stroke in feet. „ N number of revolutions per minute.
It is very important, for the proper understanding of horse-power, to realise how this formula is built up, since practically all methods of computation and measurement are based upon it. By itself the formula may convey no meaning ; in its construction, however, it is full of interest.
Now, if we examine this formula in detail, we see that, since P is the average pressure of the steam (or of the gas in an internal-combustion engine), and A is the area of the piston upon which that pressure acts, P x A is the force which causes the piston to move and the engine to work, and it is measured in pounds. P is the pressure per sq. inch. and A the number of sq. inches, therefore P x A is, as stated, simply a load in pounds. Then I., being the length of the stroke, gives the distance through which the force acts. This is measured in feet, and the product of the force multiplied by the distance 1(P x A) x LJ gives the amount of work done in foot-pounds during one stroke of the piston. (Compare this with Watt's experiment with a horse.) A steam engine being double-acting the 'force being applied on each side of the piston alternately on the " go " and return strokes), it is necessary to double this amount to obtain the work done on the piston during a complete revolution of the engine. This amount of work will, therefore, be represented by 2 [(P x A) x Li ; but, in order to find the horsepower, we require to know the amount of work done in one minute; we must, therefore, multiply again by the number of revolutions which the engine makes in a minute. Our formula, then, becomes 2 UP x A) x Li x N, and if we divide this by the amount of work (33,000 foot-pounds) which lioulton and Watt deduced, and which is now universally
accepted as the equivalent of one horse-power, the result will be the number of horse-power developed by the engine.
As an aid to memory, the order of the letters is sometimes transposed and the upper portion of the formula is written 2 PLAN, but the important part to remember is that the tions per minute at which the engine is running is an essential piston is measured in inches, whilst the stroke is measured in feet. It is, also, important to note that the number of revolutions per minute at which the engine is running is an essential factor in determining the horse-power : the same engine will develop different powers at different steeds.
I.H.P. of an Internal-Combustion Engine.
In the case of steam engines, the speed of revolution is nearly always low as compared with that to which we are accustomed with motors, and, consequently, difficulties arise in applying indicators to the latter which do not occur with the former.2 Special indicators are made for internalcombustion engines, and diagrams are frequently taken, bu:: chiefly in experimental practice ; it is not usual to determine the power of a motor engine, either in use or (except, perhaps, in the case of the first of a type) in the factory, by means of indicator diagrams. Nevertheless, when the power is so calculated, the principle is the same as in the case of steam engines, and the remarks in the preceding section apply, in general, equally to motor engines, but with certain modifications in the formula.
All usual types of motors are single-acting (i.e., they take pressure on one side of the piston only), therefore the "2" in the above formula at once comes out. In the case of twostroke motors, in which an explosion or impulse occurs once in every revolution, that is the only modification. Fourstroke motors, however, receive an impulse once in every two revolutions only, and, therefore, instead of multiplying by 2, as in the steam formula, it becomes necessary to divide by 2, so that for a four-stroke motor the formula stands
i.H.p. = P.A.L.N.or
33,000 ' 2 X 33,000 It is necessary to remember that this applies to one cylinder only, and that the result must be multiplied by the number of cylinders to obtain the h.p. Of the motor.
The Fallacy of Indicated Horse-power.
It will have been observed that indicated horse-power depends upon the mean pressure of steam or gas in the cylinder ; therefore the power measured is that of the piston, so to speak, not of the engine as a whole. If one takes a new motor which is fairly stiff, a considerable force will be found necessary to turn it by hand by means of the starting handle. It is conceivable that in such a motor there might be a considerable pressure of exploded gas on the piston which would be no more than sufficient to just revolve the shaft. If that motor were running, an indicator computation would show that it was developing an appreciable amount of power, which might be as much as two or three horse-power, yet one could stop the motor easily by the mere pressure of a hand on the 11y-wheel, thus showing that, though a certain measure of horse-power may be indicated, none is available for practical purposes-. In eliect, a portion of the indicated horse-power of any engine is absorbed in overcoming its own internal resistances, and the amount varies considerably with different engines. Thus the indicated horse-power does not give the measure of the horse-power actually being developed by the engine as a whole and available at the fly-wheel or clutch. Still, in the case of steam engines at least, the indicator method remains the most convenient and generally satisfactorv means for obtaining the horse-power.