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The Petrol-Electric Drive for Motor Omnibuses.

21st March 1907, Page 20
21st March 1907
Page 20
Page 20, 21st March 1907 — The Petrol-Electric Drive for Motor Omnibuses.
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Which of the following most accurately describes the problem?

By Alfred Hay, D. Sc. , Ni. I. E. E.

Judging by the exhibits at the successful commercial motor show which has just come to an end at Olympia, the question of electric power transmission for motor omnibuses is engag ing a good deal of attention at the present time. In view of the interest which is being taken in the matter, it has been thought that an article dealing with this problem on broad, general lines would not be without interest to supporters of THE COMMERCIAL MOTOR." Details of the various systems exhibited have already been published in its pages, and an exhaustive list of the numerous petrol-electric drives proposed by different inventors will be found in the excellent paper which was read by Messrs. Hart and Durtna.11 before the Society of Motor Omnibus Engineers, in January last.* In what follows, the writer will only deal with systems not involving the use of storage batteries.

The first question which naturally presents itself to one's mind in comparing the usual purely mechanical drive with the electrical one, which involves the use of comparatively costly machinery, is as to the advantages to be derived from the adoption of this latter system. The answer to this question is not far to seek, and may be summed up in very few words —smoothness and noiselessness of operation. In this as in many other engineering problems, high first cost has to be balanced against lower cost of maintenance and increased

efficiency of operation, these latter furnishing the very raison d'être of the petrol-electric drive. Unless the petrol-electric

system can maintain its claims as to noiselessness, reduced wear and tear, reasonable average efficiency as compared with its purely mechanical rivals, it has no chance of surviving in the struggle for existence.

All petrol-electric systems may be divided into two classes: those in which the electric drive is merely used during the period of acceleration, or when hill-climbing ; and those in which it is used continuously. Generally speaking, systems of the latter class have the advantage of greater flexibility as regards conditions of operation, but achieve this at a sacrifice of efficiency.

Let us first consider systems employing the electric drive throughout. All the examples of such systems exhibited at Olympia use continuous current. So far as the writer is aware, no attempt has hitherto been made to use, any other form of current for the same purpose, although it is not at all unlikely that some such system may be evolved in the near future. As the standard method of operating an electric tramway line also makes use of continuous currents, it will be instructive to compare briefly the conditions of operation in the case of a tramway motor with those which obtain in the case of a motorbus motor. This comparison will serve to bring out several important characteristics of the petrol electric drive. In the case of an electric tramway line, the power is supplied at an approximately constant voltage. This fact necessitates the use of rheostatic control, both during the period of acceleration and when running on the level at speeds below either full or half speed. It is well known that where starting and stopping are frequent—as they generally are—the rheostatic method of speed control occasions heavy losses, and is responsible for a considerable amount of the fuel bill. In a motor omnibus, the conditions as regards supply of power are much more flexible, since the voltage may be varied at will. The significance of this will be fully understood when it is considered that the use of a variable voltage enables us to dispense with rheostatic control, and the heavy losses incidental to it. The entire absence of starting and speed regulating rheostats further simplifies and cheapens the equipment. From this point of view, therefore, the electrical system of a petrol-electric motorbus is greatly superior to that of an electric railway or tramway of the ordinary continuous-current type.

Three petrol-electric systems were exhibited at Olympia, viz. : (1) The British Thomson-Houston Company's system,

which has been developed by Mr. B. Hopps ; (2) that of Green wood and Batley, Limited ; and (3) that of W. A. Stevens, Limited, of Maidstone. Probably the clearest idea of the de

velopments represented by these three systems will be gained by considering (a) the features common to all; and (b) those peculiar to each.

The features common to the three systems under consideration are :— (1) The use of a shunt-wound or compound-wound generator coupled direct to the petrol engine. (21 The use of a variable voltage at the generator terminals, which may be obtained either by a suitable design of the generator, or by speed variation of the engine, or by a combination of both methods, (3) The use of simple series-wound motors.

(4) The use of series-parallel control pure and simple, and hence—

(5) The entire absence of starting and speed-regulating rheostats, and the use of a very simple, strong and substantially-built controller, with accessible and easily-renewable contacts.

(6) The absence of the clutch and gear-box required for a direct petrol-drive.

The differences among the systems considered consist mainly in the means whereby the voltage variation at the generator terminals is effected in the details of the series-parallel control, and in the methods of driving the road wheels.

As regards the variation of the generator voltage, this may in all systems be effected by varying the speed of the engine. It is well known, however, that for a shunt-wound generator there exists a certain critical speed below which the machine will refuse to build up its field. It is evident, therefore, that the generator must be suitably designed in order to enable it to maintain its excitation at the lowest engine-speed. The necessary Stability of the generator field is obtained by a different method in each of the systems under consideration. In the B.T.H. system, the generator is provided with a small, auxiliary, shunt-wound exciter, which is capable of maintaining its own field at the lowest speed, and which is connected in series with the shunt winding of the main generator. The auxiliary generator thus provides a minimum excitation, which prevents the main generator from losing its field at the lowest engine speed. In the Greenwood and Batley system, the compound-wound, generator is provided with a double shuntwinding: one of these windings is permanently across the generator terminals, and is sufficient to maintain a certain minimum generator voltage; the circuit of the cther winding is only closed when the main switch is at the "on" position. In the Stevens system, the necessary stability is obtained by the simple expedient of using a compound winding.

A special feature of the B.T.H. and Stevens systems is the automatic voltage control when the generator speed is constant. The B.T.H. generator is designed so as to give an extremely powerful armature reaction, and the forward brush lead is such that the voltage varies nearly inversely as the current; hence, the load on the engine will remain approximately constant for all speeds of the vehicle. The generator will even stand a short-circuit, The Stevens generator is provided with a double-wound armature and two commutators, and the armature windings may be connected either in series or in parallel. The characteristic of the generator has a sufficient droop to ensure a nearly constant output at constant speed. The series coil is merely intended to prevent the generator from losing its field at the lower speeds, but is too weak to exert any perceptible compounding affect. In view of the extending use of commutating poles, it is interesting to note that the Greenwood and Batley and the Stevens generators are both provided with such poles. The question of ventilation is always an extremely important one in the case of generators of this type. In the B.T.H. and Stevens systems, forced ventilation is used, a special Ian being mounted on the shaft of the Stevens generator, and fan blades being built into the armature spider of the B.T.H. generator. In the G. and B. generator, no forced ventilation is used, but the generator is left open at the end away from the commutator, a special shield protecting it from splashings.


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