Horse Power. MECHANICAL AND ANIMAL COMPARED.

Progress, Volume II, Issue 6, 1 April 1907, Page 227

Horse Power. MECHANICAL AND ANIMAL COMPARED.

By A. J. McKinney. (In Autocar)

Physical and Mechanical. Early this season a motor cyclist was overheard asking an experienced fuend whether he meant to say that " three horses were needed to hold back his 3 h.p. bicycle ? " There has been a good deal of talk to this effect among people who use both horses and motor vehicles. It does seem strange to the average individual that there should be such a marked difference in the performances of animals and machines. A motorist who lives in a hilly district near London finds that a 12 h.p. motor car is but little better than his pair of horses as far as the saving of time on the three miles journey to the railway station is concerned. Of course down hill it is another thing, but he, as well as the motor cyclist just mentioned, have asked the same question How can the difference in the two types of vehicles be explained "> It is not in motoT vehicles only that this question arises, but it is found to be the same throughout all types of machinery. For example, a church in Surrey, not far from the Thames, has a very fine organ which was blown by three men. The addition of a new pedal stop made this work just too much for men, and a small water motor was suggested. The churchwardens were much surprised when the experts told them that a hydraulic engine of 2J h.p. would be needed, and this when fitted was none too much power. A starting test between a single cylinder 8 h.p. De. Dion motor car and a pair of horses attached to a landaulet showed that the carriage and pair got away the quicker, and crossed a line ten yards distant well in front of the car, though the car was rated at four times the horses. These are very wide differences in the respective performances of the animal and mechanical powers, and cannot fail to impress even a superficial observer and make him ask the reason of it. Two answers at once present themselves. (1) Either the basis of the powers in question cannot be the same, i.e., the unit called one horse-power in each case is different from the other ; or (2) the machine does not develop what it ought to do, or perhaps the horse is exceeding the mark. But the basis is the same, for Watt used a strong dray horse as the subject of his experiments, and as he allowed an increase of fifty per cent, upon the work actually done by it, it is not probable that any horse would ever much exceed this standard, so that the answer cannot be that many horess are far more powerful than the 33,000 foot lbs. per minute which is the basis of the mechanical horse-power. Watt found that the horse could raise a weight from a deep well at such a rate that the work done represented 22,000 lbs. raised through a height of one foot in each minute. Having allowed fifty per cent, more, he took the total 33.000 foot lbs. per minute to represent the mechanical equivalent of one horsepower, and this is adopted as such for British machines. The French unit is slightly smaller, but the difference is so little that it would not be

noticeable, so that it is no solution of the question to say that the cause is that the French standard of horse-power has been used.

What is the Difference ? The real answer to the question is given by two things • (1) The difference in the respective conditions, and (2) the difference in the character of the two types of power in question. As to (1), the work done by the horse represented the actual effort, the tractive force at the ground ; but in the case of the motor car the conditions are vastly different, and, in fact, so much so that there is no fair comparison between them. There are two kinds of horse-power as applied to a motor which may be noticed here. There is the actual power which is being developed within the cylinder. This is the l.h p , which is given on the indicator diagram while the engine is working, and is above the actual output, as allowance must be made for internal factional losses. The second, and the one which concerns the question, is the b h.p. This is found by calculating by means of a rope and weight the power developed at the fly-wheel, and from the method of operation is called brake horse power. Under very good conditions this may be taken as about 9-10 of the indicated horse-power, and each of these is measured by Watt's standard But this is only at the beginning of things, for the question is the difference of the power between the horse and that at the road wheels of the motor vehicle. It is transmitted through the changes of speed, the differential, and the live axle or side chains as the case may be. Now, all of these were said to be practically neghgable, but they are not, and to-day people are waking up to the fact. Professor Spooner gives a reasonable estimate of the loss caused by friction in the transmission gear as follows — Fifteen per cent, from crank shaft to propeller shaft , twenty per cent, from propeller to cross shaft ; and fifteen per cent, from this shaft to the road wheels through the chains. In the case of an engine of 10 b.h p., there would only be at the road wheels 7 2 h.p. An experiment in point was made with a four cylinder car with side chains on a good road. What wind there was was favourable, so this may be taken as a neghgable quantity A down grade of one in fourteen was selected, and the car was driven at a speed of fifteen miles an hour, and then the clutch was thrown out and the time noticed The car was allowed to run down the decline for exactly one minute when the speed had fallen to five miles an hour, when the clutch was let in again. This drop m the speed of ten miles an hour m one minute was entirely owing to the friction of the transmission gear, as care was taken that the clutch was right out and that the brakes were quite off. Now, allowing 60ft)s per ton for tractice force, and taking into consideration the total weight of the car and passengers, which was just 20 cwt., a force of 4.09 h p. was exerted by internal resistance due to the transmission gear. This agrees very well with the estimate given above. Here there is a strong reason why low powered motor vehicles are apparently so inferior to the same numerical power developed by animals or men It is useless and misleading to reckon animal horse power from the road while motor power is calculated from the engine only As well might it be done from the horses' stomach based upon his food (fuel) capacity ; vet the case is exactly parallel to motor car procedure.

Reserve Power. But there is still another factor which enters into the question, and which will help to give the answer. When the performances of horses and motor vehicles are compared it will be noticed that their power is exerted very differently. As has been remarked, the horse can get away far more easily, but it cannot maintain the high speed of the motor. The motor, on the other hand, is unable to make a speedy start unless its horse-power is well up in the double figures which will make the comparison useless. The motor, in fact, has no reserve of power to fall back on when starting, as the horse has. All animate creatures have the power of " effort " which distinguishes them from machines. A paper was read some years ago at the meeting of the Institution of Mechanical Engineers, in which it was said that a particular horse was able for a short time to develop a tractive effort equal to nearly six horse-power when hauling waggons. An engine is not capable of taking an overload, whereas a horse is capable of working with an enormous overload for a comparatively long time. This faculty gives the horse a great advantage over the engine in this respect. The horse, too, can use his weight at an increased advantage, though this must have been partly taken into account by Watt when experimenting in this direction There may be something, too, in the way the power is applied in each case some maintaining that an external pull above the centre of the wheels is far more efficient than when the power is transmitted by a turning effort through the spokes and rim of the road wheels.

However this may be, there are two things which go a long way towards answering the questions as to the reason of the difference observed between animal and mechanical horse-power. One is that it is the h.p. given off at the road wheels which must be dealt with and not the engine power, and the other is the radical difference in the nature and performance of these respective sources of energy