The Development of Petrol-Electric Systems.*
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By Percy Frost-Smith and W. A. Stevens.
We do not propose to review the whole history of petrol-electric systems, as this was most ably accomplished in a paper read by Messrs. Hart and Durtnall, in January, 1907, before this Society. We shall merely touch on the fringe of earlier systems, and trust that, in gently criticising them, we shall not be accused of too violently grinding our own axe. It is only necessary to ask you to look round and survey the present position of those companies who are running, or who have run, self-propelled vehicles, in the form of motorbuses, to explain why pepetrolelectric, or other form of vehicle, other than that now used, should be brought into existence. It is the motorbus question which chiefly concerns us at present. We shall not mention the names of some of those smaller companies who have already succumbed in their efforts to make motoromnibuses commercially possible, they, poor people, could not help " going under," but it is the larger companies whose present position makes us pause and wonder what is wrong. The appalling financial losses, in certain cases, point to something being radically in need of reform. We must not lightly condemn the constructors of the earlier vehicles supplied in loos, 1906, or 1907, neither must we lightly censure those poor overworked individuals, the responsible operators, but it is the system that we must condemn.
The reader claims some experience in the operation of the present, and generally accepted, types of motor omnibuses, viz., those having gear-driven systems, and it is by reason of this experience that he is emboldened to put forth his views, which will no doubt be sadly criticised in the subsequent discussion. It is obviously necessary that an entirely different type of vehicle to those now most generally used must be introduced, and we give our reasons as follows :— The general cost of running is much too high, which we will show in figures given below, and this high cost of operation is due to several causes, that may be summed up as follows :— (a) The abuse to which the vehicle can be subjected, by reason of bad driving and carelessness on the part of the drivers.—Here the authors must offer the opinion, that the scale of intelligence and the type of present driver will not, and cannot, be expected to become better, for many good reasons, and that we must take the man as he now exists, and design our vehicles to suit him.
(b) Bad driving by drivers, which now results in undue wear and tear on clutch gear, much damage to gear boxes, and to transmission gear.
(c) The present vehicle demands a very heavy night staff—about 1.5 men per vehicle, excluding washers, to prepare the mechanism for the following day's work, which object, even then, is not always achieved. Night staffs are, and always will be unsatisfactory ; the best class of labourer can, and does, get day jobs, and the wretched circumstances under which the night men have to work does not permit the best class of work to be produced, therefore, it only remains to reduce night labour to a minimum. This point we wish to urge as one of our most serious objections to the present systems.
(d) Maintenance should, perhaps, come first under the several heads, as it is governed by the efficiency of the drivers, by the work of the night staff, by the attitude taken by the licensing authorities, and by fair wear and tear.
Leaving bad drivers, night staff, and police obstructions, out of the question, and considering only the very best type of gear-driven bus now on the market—we are not in a position to name it—and also accepting the fact that this vehicle will, and does, run satisfactorily for twelve months or more, the time must yet come when parts will have to be renewed. Now add to replacements due only to fair wear and tear those due to bad driving, to careless work on the t Extracts from a paper read before the Society of. Road Traction Engineers, at the Hotel Cecil, Strand, WC,, on Monday, 28rd March, 1908, at 8 p.m., by Mr. P. rrost-Smith: Mr. George Pollard, M.I.Mech.E., in the chair.
night shift, and to labour and replacement of parts by reason of police demands, we have one reason for the present wreckage of London motorbus companies. We must here ask those gentlemen who are responsible for provincial motorbus or vehicle operation—if entering into the discussion to follow—to bear in mind that the conditions under which they are working, are entirely different to those obtaining in London, and due to the nature of the work, class of driver, and mileage done per annum. The reader can personally assure them that the two services do not bear comparison. The only items in which economy can be effected in motorbus operation are : depreciation, running costs, maintenance costs, oil and grease. The reader s directors have kindly given us permission to publish the following figures, relative to the working of some of their vehicles, which may appear high to some of those present, and many of you here to-night will scoff at them, but please do it slowly. Our colleagues will undoubtedly be indignant, but, gentlemen, let us assure you that they are true figures, and based upon proper calculations. The figures relate to vehicles which have been in operation for nearly three years, and they therefore give a fair indication of the maintenance necessary after the first year's working. It is quite easy to give figures that compare most favourably with these we give below, and, no doubt, such figures will be produced, or statements made, to the effect that vehicles can, and now do run, for at least twopence per mile less, but we want you to bear in mind that, if such figures are shown, or such statements made, they certainly will not relate to vehicles three years old, but more likely to comparatively new vehicles, which have not yet demanded the amount of ex. penditure which they will undoubtedly do later on. The financial position of the reader's company warrants the putting forward of these figures, for their results compare favourably with those of other of the London companies. We mention the foregoing fact in the hope that the figures given will be accepted as a fair basis of general operating• costs.
WORKING COSTS Pence.
Day running .265
Night running -750 Maintenance 2-443
Lubricants .241 Grease .o8o
Paraffin .040
Vehicle lighting .105 Body upkeep .250 Drivers 1.308
Tires 1.750
Conductors .945 Petrol 1.325
Traffic expenses .255 Depreciation, insurance, rents, rates, taxes, water, gas, garage lighting, supervision, and establishment charges 2.180 Total 11.937
These figures, which extend over a period of three years, clearly show the hopelessness of going on as we are now doing. We, however, will later show figures, which we sincerely believe will be possible in the near future, but before going further we must not overlook the claims of others. "Steamers " do not appeal to us, they are extremely clever, and will, no doubt, be successful where slow moving and heavy vehicles are concerned, but for London traffic and motorbus work, No! Clarkson cannot yet claim to have been successful, and as regards the Darracq-Serpollet type—we shall see. We are of opinion that boilers, liquid fuel burners, thermostats, automatic fuel and water pumps, " red-hot " steam, with its consequent lubricating difficulties, and the necessity of a whole lot of funny little gadgets, will not allow of them ultimately proving practical or commercial. Hydraulic systems of transmission have not yet been made practicable, though we cannot but admire Mr. Hall for his dogged efforts in this direction. One hears of friction drives, but whether these will ever prove practicable, the future only will show.
We apologise for what might appear to be a lengthy explanation of to-day's difficulties, but we now go on to suggest a substitute for the present types, and one which might be run with profit by proprietors. This substitute is a petrol-electric combination, and by this we mean the selfcontained type of vehicle with an internal-combustion motor, direct-coupled to an electric generator, which shall supply electrical energy to eiectrical motors, directly or indirectly connected to the road wheels, by which means the clutch, gear box, and differential gear are dispensed with. The petrol-electric problem has, for some years past, received much attention and careful consideration, and many designers' names here deserve mention, viz. : Patten, of America, in 1890; the German system, of Belgium ; the Hart direct current system, in i9o3 ; the Fischer, in America; the Lohner-Porsche, in Austria; the Stevens double-ended motor; the Carolan ; the Krieger; Pieper ; Farrow ; Merekles Simplex; Kfittner ; Lehwess and Clark; British Thomson Houston; Greenwood and Batley; and last, but not least, the Hart and Durtnall, all of which types differ considerably in their application, and other details, to the system in which we dos interested.
We can only deal generally, and briefly, with the foregoing systems, which are broadly divided under three headings, viz. : the accumulator system ; the direct-current systems; and the alternating-current system. The prospects, we think, are of the worst for those interested in the accumulator-system, by which we mean, a motor, or motors, driven from a storage battery which is carried upon the vehicle. Traction battery sets have never yet met with success, and it is only necessary to look into the records of the Blackpool Tramways Company ; the Birmingham Central Tramways Company ; the London Motor Cab Company, and early American efforts in this direction, to condemn the system. Accumulators of the Plante type were as efficient fifteen years ago as they are to-day, and the only improvements shown are in mechanical details. The "perfect" battery has been promised at regular two-year intervals, by a famous American inventor, but we are still waiting. We are of opinion that for the systems which employ a direct current unit, in conjunction with secondary batteries, the prospects are also bad, chiefly on account of the heavy wear and tear on the battery. As regards the purely petrolelectric combination, we are of opinion that the electric unit applied heretofore, by a double reduction-gear, or by single motor on to a live axle, or to replace only the gear-box, and used only as a starter, does not warrant its existence. The Mercedes Simplex system, which is a direct application of electric motors, built into the road wheels, appears to us to be entirely wrong, owing to the very low speed, the limited size of the motors, and the necessarily low efficiency. We have mentioned the Mercedes Simplex system last, as the object they aim at is, in our opinion, the correct one, viz, the direct application of the electric motors to the road wheels, but, still, this must be done in a manner that shall be mechanical and efficient ; this we have endeavoured to do, and we hope successfully.
One of our first considerations was the general design of the chassis. Unlike other manufacturers, we have employed a comparatively small engine developing 3oh.p. according to the R.A.C. rating and this has a bore of tomm., and a stroke of otomm. rating, engine is of the Saurer design, modified, and manufactured, by J. and E. Hall, Limited, of Dartford, and it is identical in all respects to the engine used by them in the gear-driven vehicle which gained the highest award in the heavy-vehicle trials inaugurated by the R.A.C. We have nothing but praise for this compact little engine; it is economical in its fuel consumption, and particularly so as regards lubricating oil, due to the clever forced-feed lubrication system, and it is, moreover, a clean and simple motor. Having secured the foregoing, the application of the electric motors to the road wheels next demanded our attention, and determined to apply them direct, so as to avoid double reduction gears. We hit upon the design of back axle, now known as the S. B. and S., which, after 6,000 miles' running, has realised our optimistic anticipations regarding it. This axle controls the position of the electric motors, and allows of them being heavy, robust, and efficient, and to run at a normal speed of200 revs.; they are directly coupled to our patented worm-driven road wheels by a 12 : I ratio worm gear.
[The details of the construction of this vehicle were illustrated and described in " THE COMMERCIAL MOTOR" on the 16th of January last, and certain other details will be found in the present issue in our Olympia Show report.—Eo.1 Speaking from an electrical point of view, the transmission is similar to that of the ordinary electric car, the engine and dynamo taking the place of the storage battery, with the important advantage of giving an output exactly in accordance with the demands of the electric motors, without the necessity of breaking the electrical circuit, as the effect of throttling the engine down to a slow speed causes the dynamo to lose its excitation, which it immediately recovers on the engine speed being accelerated, owing to the action of the inter-poles. The speed of the engine is controlled by a small throttle pedal, operated by the right foot, and this pedal in its normal, or top, position throttles the engine down below the speed required to excite the dynamo, the latter only exciting when the speed of the engine is increased by opening the throttle. On level roads, and very slight gradients, the car will run with a stiff field on the dynamo and a slow engine speed. For fast running and hill climbing, however, a weaker field is necessary to enable the engine to get away with the load and give its maximum horse-power, thus providing the electrical equivalent of a universal gear.
Having given a general description of the system employed, it will, we trust, be of interest to discuss the conditions under which the petrol-electric drive will produce the greatest efficiency. if the petrol engine had the characteristics of the steam engine, without its disadvantages, giving a heavy torque at slow speeds and a smaller torque at high speeds, the product of torque, and speed, through a large range being practically a constant quantity, or, if a highly efficient universal gear of low first cost, and low cost of upkeep could be produced, the petrol-electric system would have no claim to existence, but, in the absence of the two conditions we have specified, the ideal omnibus has still to he evolved, and we claim that the continuous-current petrolelectric system, with a suitable gear as the final transmission, from the electric motors to the road wheels, is the nearest approach to the ideal at present attained. In one of the figures, the characteristic of the Hanford petrol engine is compared with the characteristic of the interpolar dynamo which it drives by means of a flexible coupling. The curve (A) represents the engine power read on the vertical divisions (or ordinates) as b.h.p. and the curve (B) the output of the dynamo in kilowatts, read directly on the vertical scale, the distance between the two curves as compared with the total engine power at any speed above 6so shows the efficiency of the conversion. To this end the scale of the engine b.h.p. is brought to odd vertical values, having the same ratio to the parallel values as the theoretical electrical horse-power has to the kilowatts. A dotted line (C) on engine characteristic indicates that the engine must be throttled down below the speed of soo to allow the dynamo to lose its excitation, thereby obviating the necessity of breaking the main switch in stopping the vehicle in traffic. It is not
possible to show the characteristic of the dynamo under all conditions of demand for current at varying voltages ; the curve, however, shows the average efficiency under ordinary running conditions. Speaking generally, the electrical transmission should be so designed that by its characteristics the lack of flexibility of the petrol engine is so corrected that the necessity for the clutch, the gear-box and differential gear are obviated. It should have the highest possible efficiency in its average work, and, in addition, retain a good efficiency on hills. The electrical equipment throughout (dynamo, motors, and controller) should be constructed with special reference to the arduous conditions of motorbus work, and should be as light as possible consistent with durability and high efficiency.
Accessibility is a most important consideration, and applies especially to the commutators and brush gear, and all parts should be so mounted as to admit of easy removal. Taking these conditions in detail, the construction of the dynamo has a very essential bearing upon the successful development of the system. In common with the electric motors, it must run absolutely sparkless under all conditions of load. Its characteristic must be such that it will generate, approximately, constant watts, the volts, and amperes, varying in inverse ratio, according to the requirements of the series-wound motors, which, as will be seen later, demand current in almost direct ratio to the torque demanded from them by the road conditions. As the power of the petrol engine is limited, the volts must necessarily fall to keep the load on the engine constant. To this end the dynamo must have a falling characteristic similar to that of a shunt-wound generator, but, owing to its special duties, a simple shunt field winding would be unsuitable, as the armature reaction, with a heavy main current, and a weak field—conditions occurring when the vehicle is negotiating a steep hill—would cause heavy sparking, and', as a result of the armature reaction, the machine would lose its excitation at a time when its power is most required. The dynamo we have adoptedis of the inter-polar type. In the transverse section, the inter-poles are clearly shown between the larger main poles ; their object is automatically to correct the effect of armature reaction. They are excited by a heavy winding, in series with the armature, which winding is so proportioned that the inter-poles enable the main poles to resist the distorting effect on their magnetic field which is set up by the react:on of the armature. The heavier the current through the armature, the heavier the current through the inter-poles. By their use a fixed brush position is possible under any condition of load. A diagram of a shunt-wound dynamo without inter-poles is shown, and the effect of the armature reaction is illustrated by the distortion of the lines of magnetic force between the field magnets and the armature. The latter, when generating a heavy current, tends to set up magnetic induction at right-angles to that of the pole pieces, and distortion is the resultant effect of the two forces. One effect of armature reaction is to render the position of sparkless commutation unstable, owing to the varying loads on the armature. With a welldesigned shunt-wound dynamo, generating current at a practically ()constant voltage, this action may be neglected, but with a dynamo with a field strength dropping to a third of the normal, and an armature taking heavy over
loads at the time the field strength is at its lowest, the sparking would be excessive, and quickly ruin the commutator. A similar dynamo with the addition of inter-poles is shown, with the effect of the corrective field set up by the inter-poles, and such a machine would be free from sparking under all conditions of load.
The controller is arranged to give two forward speeds, a neutral, and a reverse. By the inter-locking action of the foot pedal, which operates the main switch, it is impossible to move the controller—operated by a side lever in the same way as the ordinary gear-changing device—until the electrical circuit between the dynamo and controller is broken, thus ensuring that the controller is absolutely sparkless under any conditions, and at the same time rendering it impossible for the driver, through carelessness, or intent, to injure the electrical equipment in any way. The current is taken from the controller to the two series-wound electric motors bolted to the outer sides of the frames, with which their shafts are parallel. These motors are of larger construction than those usually used on petrol-electric vehicles, in order to obtain high efficiency on heavy gradients. It is obvious that the motors must be of a powerful type, as they are required to give out their maximum power when running slowly and giving a heavy torque : these conditions do not apply to the dynamo which runs at its highest speed when giving its maximum output. A diagram of one of the authors' series-wound motors is shown below.