Petrol-Electric Transmission for Road Vehicles.
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Discussion on Messrs. Hart 8/ Durtnall's paper.
(Continued from page 474.)
The adjourned discussion, on the paper by Messrs.. Hart and Durtnall, before the Society of Motor Omnibus Engineers, was continued, at the Hotel Cecil, Strand, W.C., on Monday night last, when Mr. F. C. A. Coventry (Great Western. Railway), Chairman of the Society, presided over a large attendance. Mr. Coventry, before calling upon Mr. Frost Smith to open the evening's proceedings, stated that it afforded him great pleasure to be able to announce that the Society now included another chief engineer in its membership, as Mr. George l'ollard, a member of the Council, had been. appointed to that position with the London Road Car Company, Limited. This announcement was received with acclamation.
Mr. Percy Frost Smith (Thomas Tilling, Limited), said that, as an engineer closely connected with the running and maintenance of motor omnibuses, he was specially interested in the valuable, clever, and interesting paper by Messrs. Hart and Durtnall. The cost of upkeep of the transmission apparatus of the vehicles now in use was so serious, and the general designs so defective, from a public-service point of view, that it made the hydraulic, or electric, or any absolutely quiet system of power transmission, a question of the first importance. He would give his reasons why a different principle of system must be evolved. In the first place, the cost of maintenance of the various systems of transmission gear of the sliding type was unduly expensive, particularly when one considered their attendant troubles. The chief cause of this expense was due to the abuse the wearing parts were subjected to by the average driver. This was an evil, and a great evil. Drivers had, to-day, to be too highly trained, for the best results, under present systems, to be, generally, obtainable. He had only to draw attention to the deterioration of clutches, their withdrawal rods, levers, and joints; how gears were damaged, by reason of clutch-stops not being arranged for, or not being rightly designed, or adjusted ; hew clutches could be rightly or wrongly adjusted, and what poor results badly-adjusted clutches, either fierce or slipping, yielded.
The average driver badly misused the whole transmission arrangement; hence the necessity for removing it, and for supply. ing something a man could not misuse. Even in the best. designed gear box, and with the best use, the bearings, either ball, roller or plain, had, from time to time, to be renewed. Gears wore or were stripped, for the reasons just given. Shafts wore, change-speed forks broke, and, sometimes, a stray bolt got into the box, or might come out of a gear ring, and resulted in the gear box being burst. English manufacturers, unfortunately, had a tendency to copy Continental designs, with the result that, by the time their chassis were put on the market, most of the features in them were obsolete, and the firm from whom they had obtained their concessions remained well ahead of the home manufacturers in all respects. It was, therefore, pleasing that, at last, somebody had come forward with a vehicle of English design, embodying many good features, and involving a new principle of transmission mechanism. Taking, briefly, three or four of the most favoured types of motor vehicles now in London, none of them could, honestly, be called satisfactory, and all of them were, equally, subject to abuse by drivers, due to their—the drivers'—ignorance, carelessness, and, sometimes, maliciousness.
Now, they came to the licensing authorities. These gentlemen were showing much disfavour to present designs; it is true, more to some models than others. If any manufacturers were present that night, he sympathised with, them, for, whether they admitted it or not, they were having a shockingly rough time. All new vehicles were having much trouble to pass the Noise Committee, by reason of alleged humming gears, noisy engines, noisy chains, or noisy final gear drives. The attitude taken by this Committee was not, in all cases, in his opinion, justifiable : he felt he must congratulate those gentlemen upon the highlysensitive aural organs they possessed. The fact remained, they would not pass new buses. What price the old ones? He, personally, had had much trouble with police stop notices, in common with his colleagues. When buses were stopped, it was a big job to get them re-passed. Sometimes, the vehicles had been noisy, and it had caused much time and money to get them back again to that pitch of perfection required by the police. What u as the solution? Why, to eliminate all noisy features, or features that would, say, after six months' work, become noisy, or, again, features that cost much money to keep quiet. It appeared to him that Messrs. Hart and Durtnall were on the right track. It could, and it would, be done. He had tried to show, from the foregoing, all he had found to be unsatisfactory in existing motor-omnibus designs, and that was not a little. The Hart-Durtnall, or °the:. petrol-electric systems—maybe a petrol-hydraulic system—was bound to come. He liked the look of the Hart-Durtnall scheme, and he was more than sorry that he was unable to avail himself of their kind invitation to have a test run of the vehicle. He assumed that the engine was, at least it could be made, as good and as reliable as, let him say, a 28h.p. afilnes-Daimler engine, and that it would, of course, if not now, be automatically lubricated. Now, he began to glow with joy. He found that the clutch, or what we knew and understood as a clutch, was missing: thank heavens, at last their leathers and nerves were spared! They came, at once, to the generator, with a rotor mounted on substantial ball bearings. He had better explain, because there were unkind people, that he did not hold a brief for the HartDurtnall system, neither was he interested, except as a possible operator. Ile did hold a brief for ball bearings. He had had much trouble with this type of bearing, and, he did contend that, with the experience of the past two years behind them, plain or other form of bearings had been rendered obsolete, though it must be borne in mind that the application of ball bearings, particularly where thrusts occurred, must be good. He should like to know whether it would be possible to burn out the rotor. The necessity of introducing an exciter was, of course, a weakness, particularly in a system where direct-current machines were reviled. What assurance could Messrs. Hart and Durtnall give them with regard to this feature? He only knew of magnetic clutches in machine-shop practice, in which department they were extremely useful, and he, personally, thought the amount of power used in magnetising this clutch, when on direct drive, was negligible. The motor, of course, was much the same in design as the generator, but he should like to know in what proportion the torque compared, in an alternating-current motor, with that of a direct-current motor. He had been assured, by electricians, that the starting torque was very much greater in a system such as that of Stevens—who, by the way, was using, now, only one direct-current motor—than it was possible to obtain in any alternating-current three-phase motor. Was this so?
The final drive, on to a worm-driven live axle, was good. Twelve months ago, he was antagonistic to live axles, but he had been converted. A fine example was the Dennis axle. This gave unqualified satisfaction on heavy vehicles, the reason being, in his opinion, due to three causes : the accuracy with which the worm and worm wheel were cut, and the care taken as to the shape of the tooth, lead, pitch, etc., and, last but not least, to the admirable application of ball load-carrying, and thrust, bearings. The weight, also, was carried, not on the live axle itself, but on the casing surrounding it, the wheel being driven from a dog secured on the outer end of the live shaft. Taking the Hart-Durtnall scheme as a whole, if workable, it could not fail to be silent, and to retain that silence, over a considerable period, under ordinary working conditions, and little or no trouble should be caused by the licensing authorities on account of noise. The upkeep should be small. He could not quite see where any heavy expense was going to arise. The tram-like, and, generally, fool-proof control commended itself, very strongly, to his mind. He, always, did harp on electrictramcar control, and that was because he contended that it would be impossible to obtain, in quantities, the quality of men to run, properly, the type of vehicle now in use. There were
such men, but they were exceptions. If the Hart-Durtnall scheme was right, the driver problem was settled, and he was prepared to sacrifice a little weight, and efficiency, if he could cnly run the vehicles silently, and at a profit. He had gone the "whole hog" on petrol-electric schemes, and be hoped he had explained the reasons. (Applause.)
Mr. Thomas H. Parker, M.I.Mech.E., A.I.E.E., said that Mr. Bernard Hopps was fortunate in being able to secure an advance copy of the paper, and a very excellent defence of the
vlirect.eurrent motor was the result; in fact, there was, as he
the speaker) said at the time, very little to be added to his 'remarks, because the direct-current motor was in such univer9a1 ruse that it was well able to hold its own. Like Mr. Clarkson, he looked in vain for the justification of the title of the paper : he could find no comparison with purely mechanical vehicles, except with some supposed to be about 400 years old. (Laughter.) No figures were quoted to give one an opportunity to find ,a. basis for discussion. With almost lightning rapidity, those who were present a fortnight earlier had been asked to study no liess than 18 different petrol-electric, and four ancient, systems of transmission, a series which was sufficiently terrible to over-awe any would-'be orator. He had, since that evening, had 'time to read tire paper rather carefully.
He found itt to state that the engine gave 40h,p. at 800 rev°. lution-s per minute, and that the vehicle, on top speed, ran at ;12 miles per Lour. They were left in doubt as to whose formula was used in deciding the power of the engine ; but, perhaps, the 'authors, like himself, would not know which formula to choose ,amongst so many, and they had, presumably, taken the maker's published figure. They were, also, left in doubt whether or not Ythe miles were measured by one of the London Omnibus Cornpany's drivers. If the figures given were to be taken seriously, 'then the engine speed, at the average London driver's computation of the legal limit, would be about 1,200 r.p.m., and the horse-power pro rata about 60. All knew that rotative speed had a direct relationship with weight and power in things mechanical. So it had with things electrical. They knew, also, that an engine rated at 40h.p., with a normal speed of 800 r.p.m., was ample for omnibus purposes in London, even with.the very terrible and unsatisfactory mechanical, change-speed gears now in use.
Reverting to the authors' system, it would appear that, in 'order to keep the weight of the elecrical outfit within reasonable limits, they had been forced to use a high gear-ratio between the engine shaft and road wheels. In practice, it would, therefore, be found, owing to the excessive engine speed, that this ratio would have to be considerably reduced, with the natural xesult of a proportionate increase in the size and weight of both :the dynamo and motor. It must not be imagined that, because the engine was rated at 40h.p. at 800 r.p.m., the electrical plant coupled thereto was capable of dealing, efficiently, with anything like this power, and it would he interesting to know the cornbined efficiency of such a plant dealing even with only 20h.p. at that speed. Take the figures given by Mr. F. C. Blake. Here they had something tangible to serve as a guide. An engine, weighing 2081b., delivering 14h.p. at 995 r.p.m., into a continuous-current dynamo, weighing 3361b., which yields, at its terminals, 11.5h.p., thus giving an efficiency of 82 per cent., or a figure which must be considered extremely high for the -weight involved. A motor to transmit this power at anything up to 70 per cent. efficiency, at a like speed, would weigh, approximately, the same ; therefore, for a combined efficiency of .57 per cent, in transmitting 14h.p., i.e., to give 8h.p. on the motor shaft, one got, without the engine or an exciting dynamo, a weight equal to 6721b. If the speed of this plant were reduced to 800 r.p.m., then the weight (to give the same efficiency) would have to be increased to about 9001b. Taking this as a basis for 14h.p., it was obvious that a plant for transmitting 40h.p., with the same efficiency, would not be less than 2,6001b. for direct-current, and considerably more for alternating-current, plant. It would appear, therefore, a little extravagant to use such a high-powered engine, or, on the other hand, it was, perhaps, more extravagant to add, to the already overburdened
omnibus, nearly half-a-ton of inefficient electrical apparatus, for use only during acceleration, or .while climbing gradients.
Ile would be extremely pleased if, when the bus was completed, and after a prolonged test with a full complement of passengers, his doubts were proved to be unfounded, and the authors' hopes to be fulfilled.
He had not raised a single point against electrical transmission, either with direct or alternating current. Each had its sphere of usefulness, and both were °little short of human in action. He thought all must agree that, for the purposes of London traffic, and in order to have a little power in reserve, it was necessary to have an engine that would develop not much less than 40h.p. at a speed not exceeding 800 r.p.m. Now, if any electrical firm in the country were asked to make a polyphase electrical outfit of generator, exciter, and motor, to trans. mit 40h.p., with a combined efficiency of, say, 75 per cent, at this speed, under guarantee that the outfit should not exceed 18cwt., they would, probably, keep their opinion of the enquirers to themselves. The standard weight quoted for such plant was 4,200lb. (Laughter.) The authors' outfit for this power was said to be less than lOcwt., and it was upon this point that most electrical engineers would place the finger of doubt : his finger was, already, on it, In the face of these difficulties, which were well known to all electrical engineers, the authors had left the beaten track, and had, with great perseverance and ingenuity, succeeded in producing a chassis which, under light loads, had been running successfully for some time. They would all await, with keen interest, the result of this great advance in electrical design, and of a few days' test of the complete bus in ordinary, passenger service on, say, the leinchley route, which, he thought, would offer a not too severe test. Within a short space of time, they had been promised liquid air, compressed air, two or three hydraulic systems, paraffin gas and carburetters, suction gas, various modifications of steam ntant the " Aato-rnixte," electrobnses, magnetic change-speed gear, and, now, petrol-electric by polyphase currents, etc., etc. Each one, as it came along, made a lump rise in one's throat, and led one to rush about to get the latest particulars, so as not to appear Ignorant when questions were asked about the latest development. The worthy Editor of the Society's official jo'lli-nal must often say, from amongst it all, "Where am I?" (Laughter.) Fortunately, novel ideas had little effect upon steady.going British engineers, who say : "What is is 1 what comes, tomorrow, will do when it comes well tried." He had seen some of the latest of these extremely interesting and novel things, during the past few months : they bore, indirectly, upon the problem before those present, and would, no doubt, sooner or later, be added to the list, and be the subject of papers and discussion by the Society. He might mention three of them. First, a novel and ingenious, mechanical, and automatic method of changing gear, invented by Captain the Hon. F. W. Stanley, a son of the Earl of Derby. He would not dwell on this, as he believed it would be described in the technical papers. Second, a modification of the Walker metal-to-metal plate clutch, in which any number of the driven plates were held, or allowed to go free, at the will of the driver, no slip taking pace, as in rho Hele-Shaw clutch, when the engaging bolts were withdrawn. After the load had been, gradually, brought on to the engine, by the engaging bolts picking up each and all the driven plates separately, they finally engaged with the fly-wheel, and the drive was independent of the clutch. When the clutch was withdrawn, the bolts left the plates in such a form, spirally, that it was impossible to snap in the clutch ; it must go in, and bring the load on to the engine, gradually. Any degree of slip could be maintained, as the lubrication was of a perfect nature. This clutch seemed to fill a great want, but the inventor was not, yet, in a position to put it on the market. Third, the Von Pittler hydraulic transmission. He had had the pleasure of seeing what was going on in this " magic " circle, and he could, with certainty, predict quite a revolution in rotary pumps and engines. He did not think he had seen anything, mechanically, more perfect than Von Pittler's variable pump, which worked equally well as a motor, the combination of pump and motor giving infinite variations, in quantity and pressure, at a constant speed. He was inclined to think, that both electrical transmission for motor vehicles, and steam turbines for men of war, etc., would be heavily handicapped in meeting this their latest enemy.
All this discussion and promise, unfortunately, did not ease the immediate responsibility of the motor-omnibus engineers, who, meantime, had got to make the best of the vehicles in their charge. The most severe problems in engineering, were converting old to new, and bad to good. In conclusion, he wished to congratulate them on the way they had tackled these problems, and on the enormous improvement they had brought about in the general running of the vehicles at present in service. He had an excellent opportunity of judging nearly all the bus services, from his office window, and he hoped, soon, to be able to compare the present, acknowledged makeshifts with the only possible and perfect solution—the Hart-Durtnall.
(To be continued.)