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Petrol-Electric Transmission for Road Vehicles.

14th February 1907
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Page 6, 14th February 1907 — Petrol-Electric Transmission for Road Vehicles.
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Which of the following most accurately describes the problem?

Discussion on Messrs. Hart 8/ Durtnall's paper.

Mr. Douglas Mackenzie said there would be so many gentlemen desirous of expressing their views on the petrol-electric ssetem that he would not keep the meeting many minutes. He was extremely interested in the paper, and the only thing in connection with it he was going to ask the authors, was, that they should give some figures a. to cost. If modern ideas were to survive, it was going to be a question of "00 they pay?, ' not "are they ingenious or clever?" In this new system, the question was, "is the maintenance, cost going to be less than in the old systems?" The authors, probauly, had gone to some trouble in calculating the upkeep cost. Engineers knew that they could run the existing systems on their present buses at so much per mile for upkeep, maintenance, etc. He hoped the inventors of this system had been able to form some esti• mate of its cost of maintenance. It would be only a calculation, of course, because the vehicle had not, yet, been put upon the road under full load ; but, they were not dealing with something which was absolutely untried, and, therefore, there must be some data available to enable she authors to oalculate this. He hoped they would be able to give some idea ol the cost of petrol per mile. That, hitherto., had been the great fault of all petrolelectric systems. Ile had hod experience of one. They were using two gallons per mile. tLaughter. He hoped the HartDurtnall system would be able to improve upon that (hear, hear.) He understood that the Fischer vehicle gave a figure which came somewhere in that neighbourhood, if it did not, actually, reach it. There had been other petrol-electric vehicles, and they had had mane advaatages, but they simply ate, or drank, petrol. If the cost of petrol was a little more than under present mechanical systems, and if the cost of maintaining the other details was less, one might be able to look with calmness on the cost of the petrol; but if it was going to be anything like EC) expensive as under earlier petrol-electric systems, it

stand condemned on that score.

He was extremely glad to see that the authors had adopted the live axle. His early experiences were, alwaye, with live axles, in good-carrying vehicles, and he thought they would, always, be a mistake in such vehicles. They might have Local Government Board regulations. but the time would always come, sooner or later, when the 5-ton vehicle would be loaner: wanta 10-ton load, and so on, and, therefore, they could not be stirpsised. if an axle, occasionally, gave way. He once sent a van 'ts a firm, and told them it was a 35cwtevan, and that it was not to be loaded above that. A little time afterwards, he received a letter from the firm saying that they were extremely pleased with the vehicle, as it had carried a 1)-ton load to Shepperton. (Laughter.) in dealing wills omnibuses, however, the pbsition was reversed, for they had, there, a genuine and definite load which it was almost impossible to exceed. Except, perhaps, in a trial trip, when it was loaded with, say, pig iron, its pre-arranged load was, very rarely, exceeded. Even when coming home from Brighton with a party of tootballers, the omnibus would not be loaded up with more than a few extra hundredweight. The live axle ran with extreme silence, and, thereforee the authors, he thought, had acted very wisely in adopting it.

One point more, namely, that mentioned by Mr. Frost Smith as to ball bearings. He hoped that gentleman had reliable information, that the authors used ball bearings in their vehicles. Ball bearings were so absolutely reliable, that, plain bearings were quite out of it. In conclusion, he hoped that the authors would give the meeting some figures as to cost, as the whole thing turned upon that particular detail. (Applause.)

Mr. Wyatt wanted to know how this system compared with other petrol-electric systems, which were, already, on the market, or which were on the point of coming out. In adopting petrol-electric systems, they should aim at abolishing the geart ox, and employing a silent drive, and, also, at using a smaller engine than at present. On the top sneed, when the direct drive was in use, the loss on the Hart-Durtnall system would, he believed, be very sesail. That was, the less would he limited to a small power. It seemed to him probable that the loss in a continuous-current system would be smaller than that in which a differential was used. The continuous-cures.nt motor could be run at high speed, and, consequently, the weight could be economised, and, therefore, a generally higher efficiency be obtained. The same might be the case where they could run a smaller engine, and run it at its highest speed. One question he wished to put to Mr. Eturtnall. Was he convinced that the direct drive on top speed, with an electrical drive on lower speeds, gave an all-round higher efficiency than the system in which the differential was dispensed with, and an electrical transmission was employed at all speens? Mr. 1'. F. Carter (Associated Omnibus C:ornpany) said that the chief advantage claimed by this system of Messrs. Hart and Durtnall, was, that it mid away with the gear box. All engineers who handled petrol motor omnibuses knew full well that much expense and worry were caused by gear boxes, and by the in some makes more than by others. For one thing, the noise caused by them often led to the receipt of a police notice to stop the vehicle. All this kind of thing meant, to the companies, lust mileage, and that, of course, in its turn, meant lost money. As the chief advantage claimed for this system was the doing away with the gear box, be for one was anxious to know how it was likely to compare with geared chassis in respect to maintenance. He noticed that in this make the top drive was direct through the magnetic clutch, but, when a lot of mileage had to be done on lower speeds, he took it that that would be accomplished through the electric drive. He would, therefore, like to know if there was likely to be any heating tip of generator or motor, while so driving for any length of time, to cause any trouble. At the trial, he noticed the chassis was a well designed and strong one, and well sprung: An exceptionally good feature was the method adopted of fixing the motor and generating set on a sub-frame, which, in case of an overhaul being necessary, could be lowered out of the main frame, complete, and replaced by a spare set in a very short time. in conclusion, he thanked Messrs. Hart and Durtnall for their kindness in giving him an invitation to be present at the trial of the most interesting chassis at such an early stage. Mr. F. Broadbent said engineers -were rather surprised at the number of petrol-electric systems that had been devised, though a large number of them existed on paper only. Why did they want a petrol-electric system at all? To dispense with the noise, and the smell, which the petrol system, alone, gave them. The idea was to get the perfection of the electrical drive, joined to a purely rotary drive, and thus to eliminate the vibration which they must have with a rotary engine. There were two classes of systems: one in which a battery was used ; the other in which no battery was used. If they used a battery, they used it for the purpose of enabling them to start, and, thus, they could use a smaller engine than they would otherwise be able to do, and, in going down hill, they could ricuperate the cells. In any petrol-electric system, they must have the power to carry them up the steepest gradient. In the llart-Durtnall system, they did not get the electric drive on the wheels at the top speed. The system, it seemed to him, was not intended as an electric drive, but the set appeared to be used, really, -as a slipping Clutch. They had, of course, to start up, and to run on slow speeds electrically, and it was just the question of those slow speeds which would be the testing point of the system.

A good deal of criticism had been levelled at the polyphase system, and one speaker -claimed that, because the three-phase tramways had not come into general use, that was a sign they were not a success. The reason that the polyphase system had not come into use for tramways, was, that two trolley wires had to be used instead of one. He had just inspected a polyphase system, which was put up some years ago, and it was running as well, now, as it ever did, It was run by unskilled labour, and the manager of the chemical works where it was installed told him that he had never known anything give so much satisfaction as that system. That was one of the great features, the great advantage which the polyphase system had over the continuous-current system, that the motors would stand double and treble their normal loads without saying anything about it. (Hear, hear.) The Hart-Durtnall system had its advantages, but there were, notwithstanding, limits to it. He would gather, from the size of the motor they saw there last time, that it was not anything like a 40h.p. motor, but more like a 14 or 15h.p. motor. But it would have to run up hill, and, if the motor had to run up on top speed, he would give it about a ouarter-of-an•hour to burn out! (Laughter.)

Mr. E. G. E. Beaumont said that the paper, very completely, showed the different methods which different engineers had

adopted in endeavouring to obtain the possible advantages of the electrical transmission of power. There could be no doubt that several of the systems described provided for easy control of road speed, over a wide range, and that quieter running, than was now possible with the mechanical systems of transmission, would be obtained. A matter of primary importance was the extent to which electrical transmission could be expected to replace the change-speed gear, now in use. The different systems, described in the paper, might be divided into : those which employed combinations of dynamo and motors with ar without accumulators ; and those which employed accumulators with what had been, very aptly, called dy-namotors. With those systems which employed a continuous-current dynamo and series-wound motors, it was, practically, possible to obtain about three times the driving effort, or torque, at the road wheels, as compared with the effort delivered by the petrol engine, when it was running on top speed. On the other hand, it was possible, under disadvantageous circumstances, with the dynamotor-accumulator combination, to obtain about twice the maximum effort exerted by the engine. With the Hart-Durtnall system, a method of electrical transmission had been adopted which had been used with some success on electric railways. It had, however, to be remembered that, with the railway motor, the power supply was practically unlimited, whereas, with the road-vehicle motor, the power available was strictly limited. Bearing in mind what previous speakers had said with regard to certain well-know-n characteristics of the threephase motor, it seemed difficult to use it, satisfactorily, for road vehicle purposes or to obtain any advantage, so far as concerned increased torque at the road wheels.

Mr. Shrapnell Smith had, however, stated that the vehicle carried its full load and ran very well, and it was apparent that means had been found fur overcoming such difficulties as had been experienced.

Mr. Shrapnel' Smith here explained that the vehicle had been loaded, but not at the full load which it would have to carry for service in London.

Mr. Beaumont went on to join in the request of previous speakers for information as to the dimensions of the engine, which would enable those present to estimate its power. A question of some importance related to the amount of noise made by the motor when 'tinning up to normal speed. Some alternating-current machines gave forth very peculiar noises, when there was phase difference. With this system cost and weight would depend, to some extent, on the voltage selected. With those vehicles, in which engine, dynamotor and road wheels were positively connected, it seemed that the continuous effort of the dynamotor permitted the engine to run at a very reduced speed and, yet, to deliver a driving effort, or torque, not much less than that obtained at full speed. It seemed, indeed, that, with the electrical vehicle, less power was required, than with the petrol-propelled vehicle, to do similar work. There had been some reference to bearings and to the good results obtained from ball bearings, as compared with plain journals. This was, partly, because the ball bearings were able to digest the small particles of metal broken off the toothed wheels which considerably injured the surfaces of the plain bearings. Occasionally, forced lubrication, or pump circulation of oil, to the bearings had been adopted and, with the continuous, clean supply of oil, the difficulty referred to was avoided_ The Chairman said there was an enormous field for a drive on this system in tramcar and railcar work. It was, practically, impossible to design a railcar driven by an internal-combustion engine with any form of geared change speed. With an electric transmission, if it was successful, as he hoped it would be, there would be a large field in sparsely-populated districts, where there was hardly sufficient traffic to serve a heavy train, but sufficient to fill a small car ; also, on tramcar systems, that were called upon to deal with excessive loads at some periods of the day, when small cars could be run in relief.

Mr. W. P. Durtnall, who replied, spoke as follows :— We believe that all present will agree that we were not wrong in our statement, that our paper would, probably, bring forth a good discussion, and that the majority of the interest would, possibly, concentrate on our method of power transmission, because of the fact that it is not on paper only, like most other proposals, and, also, owing to our simple application of electricity for power transmission on motor vehicles. We beg to thank the various speakers, very heartily, for the honour they have done us in discussing our paper, and for their appreciative remarks.

Several questions of considerable interest have been raised. As the Chairman at the last meeting stated, our bus chassis is built, and it behaves admirably; in fact, better than we, ourselves, anticipated. It is, of course, the first machine of this kind to be driven by self-contained alternating-current machinery. Mr. Shrapnel' Smith's kind remarks, as to the little,

series-wound, continuous-current exciter are quite plain and correct. It is quite another thing to have an exciter of 1,300 Watts normal capacity, and a continuous-current dynamo, and motor, of, say, 27,000 or 30,000 Watts capacity. In this small machine, we do not cut the iron and copper down to that extent which is necessary in order to get the weight down in mainpropulsion machinery. The exciter can be run for considerable periods, without undue heating, and its weight, about 1201b., is such a small portion of the total weight ; but you can quite follow that, as the exciter is working all the time the bus is running, either providing current for the generator field or magnetic clutch, it is most essential that the machine should work cool, and be just as reliable as an ordinary, stationary dynamo of similar capacity. The effect of possible sparking, with about 20or 30 amperes, is not dangerous, as the burning effect on the commutator, and brush, gear is, practically, nothing, in comparison to what it would be if the main-propulsion machinery were continuous-current, with a commutator, and brush gear, carrying, perhaps, 300 or 400 amperes. The fact that the exciter is series-wound follows good practice in large stationary plant, and this is most important, as it may have been observed how beautifully the control can be arranged for by its sectional winding, and, also, how extremely simple is our general arrangement. With one switch, we can change from electrical on to direct, mechanical drive, without flashing and sparking at the switch contacts. A point, which most of our critics have omitted to mention, is the effect of putting the power off, by our simple and reliable method, before the brake bands go on: or, when running in congested traffic, the power does not come on before the driver takes the band-brakes off' then, the power comes on so gently that no abrupt shocks can be experienced, and, as the voltage builds up, the bus starts and accelerates very quickly, as all electricians will, we feel certain, agree, quite independently of any further effort from the driver. It is very important, too, that the operation described is done without opening or closing any main circuit, and, for that reason alone, must be more robust than most contrivances previously used.

In reference to Mr. Shrapnell Smith's further remarks concerning the likelihood of the exciter going wrong, we might mention that series-wound dynamos are very substantial pieces of electric apparatus, and are used for many applications of electrical power ; and, although, probably, the most reliable piece of continuous-current apparatus, they are likely to go wrong just no more often than a crankshaft of a petrol engine is likely to go wrong. Should such a calamity occur, as the exciter going wrong, which, we can assure you, is very unlikely, the driver can use his electric-lighting cells, temporarily, in order to drive to the garage. It is quite obvious, as the exciter is, always, working against a set external resistance, which is provided by either the generator field or the magnetic-clutch winding, and the resistance to road traction does not affect it, that it is a reliable piece of apparatus. On the other hand, in the instance of applying continuous currents to any main-propulsion ma. chinery, the resistance in the external circuit is ever varying, which is not good for any continuous-current dynamo.

We value Mr. Carter's opinion on the running, and manipulation, of our bus, and are glad to receive such appreciation from a gentleman of his extensive experience. As he stated, the control is really good ; and, as a proof of its simplicity, it is a fact that the control lever was, originally, the ignition-control lever on our engine, for which purpose it is not required, now, for reasons which will be explained later. He saw how beauti ful the braking effect is, on descending a steep hill, and that it acts independently of any effort of the driver, the braking action coining on by itself, absolutely without shock to the vehicle; in fact, you can only tell that you are descending a hill by sense of sight. All engineers will support us in this one fact, that the wear and tear on brake bands and drums will, on our bus, be about 25 per cent. of what it is, at present, on geared buses, which, when running down hill, must resort to mechanical brakes, with the attendant trouble and, what is very important commercially, expense. In Birmingham, where a regenerative system of magnetic braking is used on the electric tramcars, it is stated that six-sevenths of the cost in repairs, due to wear and tear of wheels and brake blocks etc., was saved, and this was proved by comparative tests. We might mention that, although we carry two mechanically-operated brakes, they are used, only, when stopping, or for holding the car standing on a down grade.

Everybody will agree that, if there is an authority on motorbus construction and running, it is Mr. Clarkson, whose fine steam system has gone so far to prove how, if the torque is ap plied gradually, and evenly, at the tires, the life of rubber is greatly increased, and how, further, the comfort of passengers

has to be studied. His remarks are perfectly correct : our system of power transmission will go a long way to make the petrol engine as flexible as the steam motor, the control being just as simple as turning the regulating wheel of one of his steam buses ; but, of course, we do not have the boiler, pumps, etc. As to his remarks re figures which were left out, we knew that

that point would be brought up during the discussion. Efficiency tests were taken, with a 42h.p., continuous-current, stationary motor, with a known efficiency of 81 per cent., as prime mover. The efficiency data of the electrical transmission, including exciting current, are :—

When running on slow .speed, such as climbing a hill at 4.65 m.p.h.; with the prime mover developing 32.8b.h.p. at 800 r.p.m., the polyphase, induction motor, with a water-cooled brake-pulley, gave 21.8b.h.p. at 310 r.p.m., with a pull, on the brake-pulley, of 4001b. The efficiency was 66.5 per cent., and represented a reduction in speed of 61.2 per cent. After stopping, and anchoring, the rotor shaft, with a chain attached to a spring gauge at the end of a 48-inch lever, the gauge registered 1081b., or 432 pound-feet starting torque.

When running on top electrical speed, i.e., a vehicle-speed of 10.5 m.p.h., for operations such as starting, or stopping, in crowded traffic, the prime mover gave 36.9b.h.p. at 805 r.p.m., the polyphase, induction motor, with a water-cooled brakepulley, gave 25.6b.h.p. at 700 r.p.m., with a pull, on the brake pulley, of 2541b. The efficiency was 69.5 per cent., and represented a reduction in speed of 12.6 per cent. With rotor anchored, the starting torque was 461b. on a 48-inch lever, or 184 pound feet.

When running at 12 m.p.h., on direct drive, the efficiency, including exciting requirements, which are about 450 Watts when transmitting full power, was 97 per cent.

As regards fuel consumption, we are sorry to say tests have not been carried out yet, although the trials will start in a few days ; but, considering that we are on, practically speaking, direct drive nearly all the time we were running, we anticipate a very high efficiency as regards fuel, especially as the ignition is advanced and retarded on a special, centrifugal, spring governor, which keeps the ignition point within six or seven per cent, of variation. We are of the opinion that the regulation of the ignition should be out of the driver's hands ; in fact, it should be locked up after it is adjusted correctly. As regards gas control, the engine is governed by a centrifugal, mechanical governor, which, should the speed exceed about 850 r.p.m., cuts the gas off. At normal times the gas is controlled by a special solenoid

arrangement, as illustrated, and you will observe that the windings are cut up into as many sections as the field of the little, series-wound, continuous-current exciter. You will, possibly, notice that this solenoid has a high-resistance, shunt winding ; in fact, it is in shunt with the field winding of the little, serieswound, continuous-current exciter. It will be noticed that, as the driver moves his handle round the contacts, the current rises in the coils of the little, series-wound, continuous-current, exciter field-coils, and the voltage across these coils rises : the consequence is, that the core is drawn up, and the throttle is opened. Should the driver, suddenly, put his brakes on, the exciter field would he bridged over, and no voltage would be in the solenoid. The core would fall down, and the throttle, immediately, would close, the engine, then, running round, light, at any pre-arranged speed, which can, of course, be fixed by means of a set screw. It will be observed, that the engine cannot race up in speed, should the load be, suddenly, released, which we consider is an advantagein this method of gas control, and one which will tend to economy of petrol.

We recognise, in Mr. Bernard Hopps, an expert who has had considerable practical experience in petrol-electric transmission, on road vehicles with continuousscurrent apparatus. We, therefore, take his remarks as coming from a gentleman who has studied this, as he says, most important and far-reaching subject. We agree with him that the most serious defect in the present-day motorbus is the noise, and it would, indeed, he very interesting to know what the financial loss has been to the operating companies, caused through this alone. Petrol-electric power transmission, properly applied, offers the solution to this problem, but we must draw attention to the fact that it.is very necessary to have other means than worm gear for reducing aria increasing the speed when required. Some sort of electro-mag• netic gear, such as series-parallel control, or doubling the supply voltage, or, for instance, our pole-changing method, combined with an increase of the voltage.

It is perfectly correct that much less time is required to train drivers.

As Mr. Parker said, Mr. Hopps has struck the nail right on the head. The matter of low cost of maintenance is what, in our opinion, will make petrol-electric transmission popular, when once the operating engineers grasp the situation. The matter of tires, alone, will go a long way. It will be very evident that, if any of you wilt study our arrangement, one of the special features is that there cannot be any shock, or snatch, at starting, or when changing the electro-magnetic gear ratio. When changing the speed, a bridge contact goes across the field of the exciter and, before the change-over switch leaves the contacts, the current is off and the switch is opened without any voltage being on. On the switch making contact on the opposite side, the bridge piece allows the field of the exciter to build up, and no shock can be felt ; only a very gentle, but evenly-applied, retardation, or acceleration, accoraing to whether the change in electro-magnetic gear is lowered, or raised. That is a most important point. As regards frequent starting and stopping, it is a fact that the motorbus problem is similar, in that particular, to electric railways and tramways. Rapid and smooth acceleration is of very great importance, in order to maintain a fast schedule, and that without exceeding the maximum speed allowed. We agree that the question of acceleration has, as yet, not received much attention from operating engineers, but, when petrol-electric transmission is installed, owing to the low maintenance, and the series-wound, continuous-current exciter behaving itself as it should do, operating engineers will then have time to go further into the matter of acceleration. Most of the gentlemen here, and, especially, those who have seen how beautifully, and rapidly, a polyphase induction motor will run into phase, will appreciate that, as regards acceleration, the HartDurtnall system will have no trouble in proving its superior qualities, particularly as the speed of acceleration is not in the driver's hands. It is our opinion that, owing to the simplicity of control of our system, the large body of experienced roadmen, the ordinary horse-bus driver, will be the men who will fill the places of drivers on petrol-electric buses, when there are more on the road.

We, certainly, do support the alternating current system, but we do not. condemn the continuous-current system, as we have one ourselves. On looking at the connections of our continuouscurrent system, it will be apparent to electrical engineers how simple the control is, and, also, that we use series-wound, continuous-current apparatus. That is, however, only on brewers' lorries, millers' vans, heavy, railway, delivery vans, pantechnicon vans, and, in fact, any class of vehicle which does not exceed four, or five, miles per hour in speed. In these vehicles, the speed being lower, the road shocks are very much lighter, and the effect of making the brushes on the commutator chatter, and jump off, and, thus, cause considerable sparking, and wear-andtear, is, in that case, not so important as it is on, say, a motorbus travelling at 12, or 14, miles an hour. The only piece of electrical apparatus that will stand this class of work, with all the vibration, satisfactorily, is the polyphase, induction motor.

We were glad to hear Mr. Hopps say that he did not champion the continuous-current system to the exclusion of the alternating. current system, because I think you will all agree that the ad. vantage on the side of the polyphase induction motor would far outweigh any sort of argumen; which will, eventually, be put up, when the alternating-current system is put on its merits. We must take exception, however, to his remarks regarding the design of our 40h.p. continuous-current dynamo and motor, on which our first experiments were carried out, some four years ago. We can only add that they were designed, especially for the purpose, by one of the finest designers in Europe, and we are certain of this one thing, that it was not the design of the dynamo-motor that was at fault, but the very nature of the continuous-current machine for this high-speed, passenger work. Mr. Hopps said that the design of a satisfactory, automobile, continuous-current motor was a problem. We agree with him: we have not seen a good one yet ; however, we have given up the idea of continuous-current for this motorbus, and other high speed vehicles, and, by adopting the alternating-current system, we are not limited by area of brushes and by commutator troubles. We agree with him that a large experience i3 necessary, but we cannot agree with him that it is not possible to follow every day experience on stationary machines, which have worked so satisfactorily and have been so well tried.


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