TRANSMISSION BY ELECTRICAL MEANS.
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I.—Objects, Advantages and Disadvantages of Electrical Transmissions.
BEFORE ENDEAVOURING to describe examples of the various systems of transmissisx which involve the use of electrical machinery, we must endeavour to indicate briefly the reasons why such Systems are designed, and also their inherent advantages and disadvantages as' compared with purely mechanical transmissions. The provision commonly made for varying the travelling speed corresponding to. a given engine'speed7 of course takes the form of sliding gears contained an a gearbox. It stands to reason that, with such a system, it is only possible to vary the relation of the engine speed to road speed in steps.
. The ideal naturally demands that the variation should take place gradually, so that, under any given condition of gradient, traffic, or road, exactly the correct gear ratio can be introduced. With sliding gears this is impracticable. Moreover, sliding gears, from a mechanical standpoint, fall short of perfection, since their engagement is more or less violent and sudden. With a -perfect gear the system of doubleclutching when changing speed would never be at all necessary. The changes would be effected by an anpractise a driver without any possible damage resultmg, and by some simple means involving no appreciable effort.
Some farms of mechanical gear involve no sudden engagement, but still have the disadvantage of operating only in steps. In some respects the nearest approach to the ideal is given by a purely friction drive, but these are practical difficultieswhich, up to the preeent, have prevented any friction drive from being accepted as satisfactory on heavy vehicles carrying considerable loads. In each purely mechanical system the friction drive figures to a certain extent, inasmuch as provision is made for slipping a friction clutch in. order to render the changes smoother.
• In no practical mechanical gear is it-possible to arrange for the engine to develop its full power and transmit nearly the whole of that .power to the road wheels while the wheels.are turning only very slowly. The reason for designing transmissions embodying electrical machinery is, then, to provide an-infinitelyvariable gear atileast %fairly efficient under all conditions, and enabling a very strong torque to be exerted when the road speed is extremely low and when the machine is starting up from a standstill. Such a gear, if satisfactory in practice, obviously has many advantages on passenger vehicles where the comfort of the occupants has to be considered. It renders possible gradual acceleration without the subjection of the mechanism of the vehicle to any sudden shock. This is evidently important, particularly in town omnibus services, where starting and stopping are very frequent.
Considerations of economy in transport point to the desirability of developing traffic unite capable of dealing with considerable loads without undue injury to the roads. Ability to provide for smooth acceleration from a standstill is very important in this cornicetion. Also, as was recently pointed out in an article in this journal, improved systems of road train may be most readily developed if power be ,transmitted electrically through cables to electric motors operating at least one pair of wheels of every vehicle in the train.
It is thus clear that there are many reasons why we should endeavour to use electrical machinery in transmission systems unless the accompanying disadvantages overweigh the very considerable potential advantages. One of the disadvantages is that the efficiency of an electrical transmission cannot be as high as that of a, direct drive by mechanical transmission. Thus, if most of the work of the vehicle can be carried out at a speed which enables the direct drive to be used, an electrical transmission is at a disadvantage. Another drawback is to be found in the difficulty of rendering electrical machinery efficient and free from breakdown and, at the same time, making it light in construction. On the whole, mechanical transmission has the advantage as regards weight. • Further, it is essential that any *electrical transmission shall continue to operate certainly for long periods without adjustment or skilled attention. Failing the possession of this quality,, we must call upon drivers to possess considerable knowledge of elee
tried principles and machines. This attainment is unusual, and if we expect a man to possess special and uncommon qualifications, we must evidently pay higher for his services.
These last considerations should not, however, be given tea much weight, since a well-designed electric motor can be made extremely reliable and almost completely • independent of skilled attention, other than that involved in the periodical overhaul to which every vehicle should be submitted. The difficulty of weight is more considerable, particularly if the system proposed involves the use of a secondary battery, which cannot be other than heavy. There are now in existence types of secondary battery which are extremely impervious to rough usage, but weight remains the,great objection against encouraging their incorporation in the system.
On the other hand, it is clear that if we have a secondary battery and suitable electric machinery, we can, when coasting down hill or when pulling up the vehicle for any reason, so arrange matters that, instead of merely wasting heat in braking, we utilize the momentum of the vehicle te drive a dynamo which in -turn charges a battery. . This is what is called the regenerative system, and, in principle there are strong reasons for its adoption, particularly in certain classes of service. Thus. Mr. Wimperis has worked out an example which serves to show that if a vehicle was worked upon a service which necessitated a stop every 200 yards, the average loss of energy due to braking would have the effect of nearly doubling the
total energy consumption per inile. .
If we could contrive to store this energy expended in braking, and use it again later on to assist in accelerating the vehicle or driving it up hill, we should evidently be effecting a very great saving. The only practicable method of so doing appears to involve the introduction of electrical machinery in the transmission, supplemented by a secondary battery. Reverting to the main point, which, unlike that just discussed, is invariably borne in mind whenever electrical transmission is proposed, it is quite clear that the ordinary mechanical change-speed gear gives results very far from ideal when used in combination with a petrol engine.
The engine itself, unfortunately, cannot be designed to develop full power over a complete range of. speeds. Within limits, the torque increases more or less in proportion to the decrease of speed, but below those limits it falls away very rapidly, and, when a certain point is reached in slowing up the engine, it ceases to develop power and stops altogether. Fig. 1 shows i the curve of ideal tractive effort which is, n fact, a rectangular hyperbola, with the reservation that it does not extend indefinitely upwards. The reason for this is that we reach a point beyond which it is useless to increase the tractive effort, because, by doing so, we should only cause the wheels to slip.
A second diagram, based on one prepared by Mr. Wimperis (Fig. 2), may, with advantage, be compared with Fig. 1. Here we have three curves showing the results of using three different gear ratios on a motor wagon. It, will be seen that the,consequence is to approximate more or less to the ideal curve, but it is quite clear that if we were to increase the number of gears the approximation would become closer and closer. If we were to carry this increase further and employ an infinitely-variable gear, we should get the perfect curve.
Fig. 3 shows, approximately, the curve of tractive effort obtained with the Tilling-Stevens system of electric transmission during tests made some six years ago. It will be seen that this curve approximates in character very nearly to the ideal curve in Fig. 1. We shall discuss later the principles of action of an electric motor, which make it possible for such results to be obtained. In the meanwhile, the point is that the results certainly can, be obtained by an electric transmission, and that this being so, electric trans mission is evidently deserving of very careful study with a view to its extended application to various types of traffic units.
On the other hand, we must bear in mind throughout that it is only worth while to aim at the ideal if, in so doing, we can avoid the introduction of failings which more than balance the advantages obtained. In the ease under consideration, the risk is that the overall efficiency of an electric system of transmission, averaged out under all service conditions, will be so low as to make the results compare badly with those obtained by the less ideal, but possibly more practical, system of mechanical transmission.