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12th August 1919, Page 20
12th August 1919
Page 20
Page 21
Page 20, 12th August 1919 — TRACTOR POWER LOSSES.
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Which of the following most accurately describes the problem?

What Happens to that Big Proportion of the Engine Power of a Farm Tractor which Does Not Show Itself by the Exertion of Draw-bar Pull.

IN VIEW of the coming trials of agrimotors and tractors at Lincoln, it is important again to refresh our memory on the subject of loss of power between engine and road wheels.

We 'are all thoroughly well aware that by no meang the whole pf the enginepower of a tractor is available for drawing the plough or other implement, and that the more we know about where the balance of the power is lost, the more capable shall we be of determining the relative merits of different types of tractor. Some of the power of the fuel is, of course, lost in the engine itself. It is impossible to convert into work the whole, of the heat that is generated when the fuel is burnt. Some of it is unavoidably lost in heating the cooling water, without which the cylinders would get so hot that the engine would refuse to work. Some again is lest in the exhaust gases, which dissipate quite a. large amount of heat into the air. Some is lost in the friction of the working parts of the engine itself.

The heat loss is being dealt with, by the way, in the design of the Still engine, which, it will be remembered, waTs described in a recent issue of The Commercial Motor. In that engine—the development of which, for tractor purposes; will rapidly follow upon its present application to marine work— of the total heat available for recovery about 15 per Cent is recovered.

The farmer cannot be expected to go into all the theories of engine design. What concerns him is the amount of work his tractor will do with a given expenditure of time and fuel, Some engines are more efficient than others, which means that they use a bigger percentage of their .fuel to really good advantage. Fuel economy is in some measure dependent on the adjustment of the carburetter and in part on the skill of the driver. All that the average tractor owner can do is to note the consumption oi fuel, and if this does not correspond fairly accurately with the estimates previously given by the.makers, to_go into the matter with a view to improvement. For the mornent we are merely taking the b.h.p. of the engine for granted and considering how much of this power is actually used for hauling the implement.

Efficiency of Transmission.

Some of it is wasted in the transmission. If the transmission is unduly complicated, the loss will be considerable. We want therefore a tractor of simple construction. Then, again, if the transmission is not absolutely well protected from weathex, dust and mud, a great deal of unnecessary friction will be caused. The parts will wear with rapidity and the power of the tractor to draw the implement will be reduced. There is left then the power which reaches the rims of the driving wheels. Some of this power is used to move the tractor itself and only a part is available to draw the implement. It will take more power to move a heavy tractor than a light one, particularly upon an. up grade. Again, it will take more power to move a. tractor on soft than on hard land, and it is particularly thi,s. last loss of power which I propose to discuss, with a view to trying to decide just how and where it occurs, because it is only by understa-ndin_g the real causes that any such loss can be minimized.

Power Loss on Soft Ground.

Moreover, if we know how the power is lost, we shall he able to estimate more or less whether any tractor submitted to us is likely to lose much or little power in the process of driving over the ground. We shall also be helped to a.rriveat the decision as to the right .sort of fittings to put on to the wheel's to secure adhesion. This is, of course, very important, because one way in which power may be wasted to any extent is by slipping of the driving wheels.

It is sometimes stated that when a tractor drives an soft ground, the loss of power is due to compresSing the soil. Undoubtedly, there is a close relation between the two, but the one is mat exactly the true cause of the other. Another theory is that most of the power is lost in pressing the spuds or spikes into the ground, turning the wheel over them, and pulling them out again. I do not know whether any accurate measurements have been taken to ascertain how much power goes in this way. If it were the main cause of loss, then so long as conditions made it possible to . dispense with spuds or spikes without slipping actually occurring, we should get very high efficiency, even on soft land. I do not think that this would be the case, and I suggest that the most accurate way of accounting for loss of power on soft land is by saying that it goes in a .continual effort to drive the tractor uphill. The meaning of this statement is illustrated in Fig. 1 (a) and (b). Fig. I (a) shows the wheel of a tractor driving up a gradient on hard land. For the moment, the wheel is twisting round the point (P), where it is in contact with the ground. In so doing it has to lift the axle of the tractor and power is expended in lifting the weight. In Fig. 1 (b) is shown a tractor wheel driving on aoft land. The dotted line indicates the depth to which the wheel sinks

in. For the moment the wheel is twisting about the point X. If the ground in front of X were quite hard, the wheel would presently reach the position shown by the dotted circle. If the ground is soft, what really happens is that the wheel does not actually rise, because whenever it attempts to do so, the ground sinks beneath it. All the same, the wheel is for the moment trying to twist round the point X and to raise the axle in so doing. This is tantamount to an endeavour to climb up a gradient along the lineA.X. From this we see that the movement in soft ground is much the same as movement uphill, and this gives us a fairly clear idea of why power is lost. It also shows that, provided always that we can get enough adhesion, the light tractor has the advantage _because it sinks in a smaller distance and the gradient up which the wheel has to try to move is therefore less severe.

Advantage of the Big Weeel.

We also see from Fig. 2 that, other things being equal, a tractor with a big wheel will lose less. power than one which has a comparatively small wheel, because in that figure the line A.B., which is the slope up which the bigger wheel tries to move, does not represent so steep an incline as the line A.C. Even if the smaller wheel were made so much wider that it would not sink in any further than the big wheel, this would remain true, though both would compress the soil to an equal depth. We see then, that a wheel of large diameter has advantages over a wheel of smaller diameter but greater width, and loses less power.

Reverting to the question of the effect of spuds or spikes, Fig. 3 gives some idea of why the presence of these may lead to loss of pow. In thi figure the length of the spikes is intentionally exaggerated. Three spikes are shown. Of these the first is entering the ground at A. The last is just leaving the ground at F. Now the distance from A to F is considerably longer than the distance B.C.E. measured round the wheel. From. this if stands to reason that each spud must actually move through the ground, while the wheel is turning over it, making holes somewhat of the shape indicated. Evidently we shall lose some power in vressing the spud straight down into the ground and some, in pushing if,backwards and compressing it while -making the holes. The holes really represent a sort of shp of the. spuds. So long as the ground holds the spud quite firmly and does not give to it, the whole of the horizontal movement of the spud is used in dragging the load behind the tractor ; but -when, the spud pushes the ground backwards there must be a loss of power. Obviously if we used thick, blunt spuds, we should waste quite a lot of power in compressing the ground below them. Consideration of this matter will show why narrow spuds are preferred, and also one of the reasons why spuds whicli reach across the whole width of the tyre are not recommended.

°These brief notes. are not intended to be a complete explanation of the losses of power between a tractor wheel and the ground, bat it is hoped that they are at least sufficient to give some readers a rather clearer • idea of what happens, and also that they may perhaps lead other readers to deal with some of the points raised in a more full and satisfactory manner. AGMMOT.


Locations: Lincoln

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