TRANSPORT TIPS FOR TRADESMEN.
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Particularly Addressed to Those Who are Replacing Horsed Vehiclesby Motors, or Contemplating So Doing.
THE TERM " horse-power " is a very much abused and, in some ways, a rather misleading expression. It originated out of an attempt by early engineers to compare the power of steam engines fairly with some other source of power, the capacity of which was fairly well known to the general public.
Horse-power and Its Origin.
It was estimated that a very powerful horse could do work equivalent to the lifting of 300 lb. through 100 ft. in a minute; in other words, it could do 33,000 ft.-lb. of work per minute. An engine which could do the same amount of work was, therefore, rated as being of one horse-power. As a matter of fact, the power of the horse was certainly overestimated, perhaps more or less intentionally in order to be on the safe side. Despite this fact, an engine of one horse-power cannot necessarily do everything that a horse can do. A horse can develop very much more than one horse-power for a brief period, as, for example, when starting a heavy load from a standstill. On the other hand, a horse, unaided by mechanical appliances, must do its work in its own way. It cannot lift a very small weight through a very long distance in a minute, because of its limitations in speed.
Brake Horse-power.
The proper way to compare the capabilities of vaiious engines is by reference to their brake horse power. This is the actual power developed by an engine at the selected speed, which is presumably that at which the best permanent results are obtainable. It is measured by making the engine turn a, wheel or brake drum, to the rim of which a brake is applied and so fitted up that the pull on the brake can be accurately measured. Now, if the brake is put on very hard, it may have the effect of slowing up the engine or of stopping it altogether, so that the full brake horse-power is not developed. To apply this fact to practice, we see that it is not enough to know the brake horse-power of a motor vehicle's engine, unless we know the road speed at which, the engine can exert that power. Suppose that a motor vehicle had only ope gear and this was • such that its engine would develop its full power, when the vehicle was travelling at 30 miles an hour, then, at any lower speed, the power developed would be smaller. If we had to tackle a stiff hill, we should have less power available just when full power is needed, hence the necessity for three or for four speeds.
A noteworthy point in this connection is that vehicles designed for different purposes axe differently geared. If a trader proposes to fit a van body on to an old touring car chassis, he may find that, if he runs the vehicle at reasonable speeds, his engine never gets a chance of developing its power on top gear, and it is necessary to change down to a lower gear an the slightest gradient. This is one of the difficulties about adapting the private car to trade purposes, the range of useful speeds of the trade vehicle being appreciably lower. Evidently, also, if we are satisfied with moderate speeds, we can do with an engine of considerably lower power. The ability of a vehicle to do the work required of it is not, therefore, determined solely by its power.
Power to Weight Ratio.
In practice, also, a vehicle has to move itself as its useful load. If the useful load is a fixed amount, then the lighter the vehicle the smaller is the total load to be moved and the smaller, also, is the horsepower required to do it at a given speed. Therefore, it does not follow that the machine with the biggest horse-power is the best value at a given price. The manufacturer may have had to provide more power because his chassis and mechanism are heavy and clumsy, or inferior materials have been used in their construction. In that case, the extra power is used up in doing the extra, work, and the fuel bill is unnecessarily high. A better-constructed, lighter vehicle of lower power may be able to do the same work in the same time at a much lower cost for fuel. It does not do, then, to. compare competitive machines on the basis of their price per horsepower. The question is rather what is the ratio of power to weight, the latter referring either to the vehicle alone or to the vehicle carrying its proper useful load. If the ratio of power to weight is considered and the gears are normal, the vehicle should be a good hill-climber and should score in respect of average speed, even if restricted to the same maximum speed as the heavier machine.
Rated Horse-power.
Motor owners who have no technical training may well be confused when they see it stated that, say, the engine of the X— 12 h.p. chassis develops 25 horse-power on the brake. The fact is that, as a matter of convenience and largely in connection with. taxation, it has become custoteary to estimate what is called the horse-power of an engine by the number and stroke of the cylinders instead of by an actual test. We have agreed that, if an engine has so many cylinders of such-and-such a bore, we will say that the engine is of such-and-such a horsepower. Meanwhile, improvements in engine design have been so rapid that, in almost every case, the power actually developed is much greater than that estimated by our formula. When we use the formula, it is quite possible to estimate that one engine—not very scientifically designed—is of twice the power of another, whereas, if tested, it would be found that tho second would give more power than the first. It will thus be seen that horse-power by what is called Treasury or R.A.C. rating is of little or no assistance in giving an idea of what the vehicle will do ; in fact, it may give an impression -which is altogether wrong and unfair to the makers.
Stated Horse-power.
Very often manufacturers designate a vehicle as being so many horse-power merely in order to give a sort of rough idea of its capabilities, but without the expression having any real meaning. The engine is, perhaps, a very efficient and powerful one for its size. The manufacturer does not like to quote its horse-power by formula, because, if he did so, people would think the machine insufficiently powerful. On the other hand, he does not like to quote it by the brake horse-power which the engine can actually develop, because this is large, and may give people the impression either that the engine will use a lot of fuel or else that it will be safe to give it much stiffer work than that for which the makers have designed the chassis. Consequently, the manufacturer compromises, and chooses a fancy figure somewhere between the two extremes. It is clear, then, that if, say, a car which is known as being of 20 h.p. is found to be just not quite up to the work, it is by no means safe to assume that one which is said to be at 25 h.p. will be powerful enough. The best way of determining the question is by a stiff road test. The next best way is by taking the advice of a qualified man who, when he has studied the design and knows the engine dimensions, the gear ratios, and so on, can tell pretty accurately what load a vehicle can be expected to carry satisfactorily in a given district with the roads of which he is familiar.