Storage Battery Systems for Motor Omnibuses.
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By Allred Hay, D.Sc., M.I.E.E.
In a previous article*, the writer briefly reviewed the ;Acta features of the more recent forms of petrol-electric lrive as exemplified by the exhibits at the recent Olympia !xhibition, confining himself, however, to those systems in vhich no use was made of secondary batteries. In th.t,! precot article, it is intended to consider briefly the part which s likely to be played by secondary batteries in connection
vith motor orrinibus-es. It isa efact that in spite q the enormous amount of work which has been done on :econdary cells—to, which the numerous contributions to the echnical Press and the records of the Patent Office bear unple testimony—the secondary cell of to-day is, in its ssential features, the same as it was in the days of Gaston ?lante, its original inventor. The Edison cell may, no loubt, be quoted as evidence to the contrary : but it must be emembered that this cell—the product of the most searching rivestigations and costly experiments—has as yet shown tself incapable of being considered a serious rival to the lead econdary cell. It may be inferred, therefore, that no reolutionary changes, either in the direction of reducing the eeight or cheapening the cost of secondary cells for a given :nergy capacity, are likely to take place in the immediate utu re.
From what has just been said, it must not be concluded hat secondary cells have undergone no important improve'lents since their introduction into electrical engineering iractice ; on the contrary, slow but steady progress has been aking place all aloneand great attention has been given
o those matters of along, which so frequently determine vhether a given invention shall be a commercial success or allure. Some of the troubles which formerly were so erlous—such as buckling of the plates and internal shortircuiting—are practically unknown in the best typesof nodern construction. But the experience gained with econdary cells has firmly established the important fact that _ secondary cell is an extremely delicate piece of apparatus, ensitive to the most minute disturbing influences. Absolute urity of the electrolyte, careful exclusion of all chemical gents likely to react unfavourably on the behaviour of the ell, and attention to the rules required to maintain the cell a a healthy working condition, are essential to suceess. The indiscriminate addition of water of doubtful quality to he electrolyte, heavy continuous discharges, and neglect to barge up immediately after a discharge have ruined many a attery. These points must be insisted on because the sueess or otherwise of the systems about to be tried, in which ecOndary batteries will play an important part, is sure to epend very largely on the skill, intelligence and technical itowledge of the driver.
All motor vehicles in which secondary batteries are used bipurposes of propulsion fall naturally into two groups : r) those in which the drive is entirely and purely electrical, mere being no petrol or other engine; and (2) those in which 'le battery is only an auxiliary source of power, the driving nder normal conditions being performed entirely by the ngine. The purely electric drive is, so far as the comfort. f the passengers .and passers-by is concerned, the ideal drive )1. a motor omnibus : its 'rapid yet smooth acceleration, dent running, and the absence of smell and vibration, are ?atures which tell strongly in its favour. The doubtful fealres of the system from the commercial standpoint are its ecessarily low efficiency and the cost of maintenance of the atteries; whether these will prove prohibitive or no exerience alone can decide, and the interesting and, it must e confessed, bold experiment which is about to be tried by me Electrobus Company On the streets of London will be ,atched with keen interest.. .
In the second group of systems, in which the battery is nly used as an auxiliary source of •power, the conditions of ,orking are entirely different: The best-known example of tis type of drive is the :Pieper or auto-mixte " system. Tnder normal Conditions, the drive is a direct one.; no gearox, with its attendant disadvantages, is used. Coupled. to le engine shaft is the arrnature-of a shunt-wound dynamo. rhich is connected across the battery terminals. The bat tery performs the following functions : (a) it serves to start the engine; (b) it equalises the load on the engine, so that the latter may be kept running under the most economical conditions ; and (c) it allows of "regenerative control " when running down a steep gradient, thereby considerably increas-ing the average efficiency of working in a hilly district. When the resistance to motion is such that the engine is running at normal load, the battery is kept simply " floating " across the dynamo terminals, neither receiving any charge, nor discharging. If the resistance to motion is insufficient to absorb the normal power of the engine, the excess power is used for charging the battery ; and if the normal power of the engine is insufficient to keep the vehicle running up a gradient, the battery is allowed to discharge, driving the dynamo as a motor. By levelling the peaks and Hing up the valleys of the load curve, the battery acts as a " buffer " between the engine and its load. A battery so used is known as a " buffer battery." It is very important to understand clearly that there is an essential difference between the working conditions of such a " buffer battery " and those of a battery used in a purely electric drive. In the latter case, we have a long continuous discharge, followed by a continuous charge at the charging station. But in a buffer battery, we have .a succession of brief intermittent charges and discharges, frequently following each other in rapid succession. Now for reasons which we cannot enter into here, such a rapid alternation of charges and discharges results in a much higher efficiency of working than can ever be reached with long continuous charges and discharges, and a "buffer battery " is in practice found to possess a much higher efficieney than a battery used in the ordinary way.
As compared with the purely electric drive, the bufferbattery, petrol-electric drive possesses the advantage of a very much higher efficiency ; on the other hand, it is inferior as regards noiselessness and absence of smell. The buffer action of the battery is, it must be noted, not entirely automatic—it must be controlled by hand, by suitably varying the excitation of the dynamo. In the hands of a negligent driver, the advantages claimed for the buffer-battery., system may become largely illusory.
In the opinion of the writer, the commercial success of any system in which secondary batteries are used for purposes of propulsion will be determined. very largely by the intelligence and Care displayed by the drivers, and it will be a wise policy which lays due stress on the proper training and general mental as well as physical fitness of the drivers.