Automatic Transmission Trends
Page 67
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Extracts from papers presented at the A.S.A.E. symposium at Cranfield
0 N Tuesday and Wednesday The Advanced School of Automobile Engineering held a symposium on automatic transmissions. A total of 10 papers was read at the sympos:um, these being mainly concerned with small cars and public-service vehicles.
The symposium was divided up into four sessions, the first devoted to three papers, two dealing with small cars and one with p.s.vs. This was "Automatic Transmission Requirements for Public Service Vehicles" by C. H. Curt!s, London Transport Board.
The automatic transmission employed with an automatic overdrive for longin the Routemaster bus was described by distance working, Mr. Curtis, much of the detail covered by this paper having already been included in the article about the Routemaster automatic transmission which appeared in last week's issue of The Commercial Motor. It is important that the driver shoulli have complete confidence in the performance of an automatic transmission, under all conditions, said Mr. Curtis. In other words, it must perform according to the manner of normal drivin; technique, otherwise drivers might b reluctant to use it, and wish to revert to the normal method of gear selection; This means that the automatic contro must be sensitive to load and speed an must react positively and consistently to changes in these factors. Above all it must effect a smooth start from rest and provide for jerk-free changes of trans mission ratio without adversely affecting the acceleration of the vehicle.
London Transport believes that auto' matic gear changing can improve th passenger appeal of a public servic vehicle, quite apart from the obviou benefit to the driver in assisting his work i that 100,000,000 miles of operation o In his conclusions Mr. Curtis remarked that buses fitted with automatic gear change control have indicated a relatively high degree of reliability. The automatic system has been a new departure for London Transport arid it was desired to fit it in conjunction with a well-prove and reliable transmission system namely, the fluid flywheel and epicycli gearbox. This transmission system i • known to be the most efficient in Lando Transport service.
Other transmission systems has' naturally been examined and, in some cases, tested. These include the torque convertor, which is less efficient and somewhat noisy. On the other hand, it has a better " startability " factor than th conventional gearbox system. Reliabilit and ease of servicing still possibl demand a little more attention. Th second system now coming into th picture is the hydrostatic, but this agai suffers from the disadvantage of power efficiency. Little experience has been obtained but limited work seems to indicate that there is still a noise problem.
For the p.s.v., a maximum torque multiplication of 5: 1 is required, possibly Hydrostatic Developments
In the second session one of the two papers dealt with small cars, the other being "Hydrostatic Transmissions" by C. K. J. Price, Lucas Industrial Equipment Ltd. Starting his paper by dealing with the history of vehicle transmissions, Mr. Price said that the first really significant use of the hydrostatic principle of power transmission was in 1840, when a central hydraulic-power installation was used in London to supply winches and cranes at remote points. Developments in hydrostatic transmissions for vehicles were overtaken by the development of an acceptable form of multi-speed gearbox around 1910. After this early work the hydrostatic transmission was abandoned for vehicles due principally to the fact that the clutch and gearbox was a reasonably simple and efficient combination and could be produced at very low cost.
In the years between the Wars many systems of infinite-variable transmissions were tried. Most of these were unsuccessful, but a whole range of experiments had taken place both in Britain and in the United States and enough background had been obtained to show advantages in suitable types of vehicle. During the early 1950s the National Institute of Agricultural Engineering pioneered a tractor conversion to hydrostatic transmission and in order to investigate the use of a wheel-motor system of this type, Lucas, the author's company, converted a number of Ferguson tractors. Field trials over an extended period bad shown these to be very reliable.
After discussing the general requirements of a transmission and explaining the advantages of a hydrostatic system— variable speed and truly stepless, smooth changes, no tendency to creep at zero ratio settings, ability to accept extreme changes of load with negligible change of road speed, to act as a very effective brake and to allow a very considerable degree of flexibility of installation—the author went on to describe some significant applications. These included front loaders, excavators and fork-lift trucks, although it was said that in spite of hydrostatic transmission offering the full answer for these kinds of vehicles only
a small number had, in fact, been offered with it during the past few years. The reason for this was the question of first cost of the vehicle.
Considering the full transmission spectrum, Mr. Price said that road vehicles represented a series of special cases. The requirements were different in the cases of high-performance cars and commercial vehicles—with cars the greatest power requirement is normally taken by aerodynamic resistance, whereas in the cake of public-service vehicles and lorries the maximum power is normally required in climbing hills when fully laden. Diagrams were used to show the envelope of power output requirements for a 5-ton lorry and it was shown that, particularly on long-distance hauls, the potential fuel saving by using hydrostatic transmission could be between 10 and 20 per cent.
This saving could justify the expense of fitting hydrostatic transmission and in addition to this there was the reduced driver effort. Perhaps the most urgent need for automatic transmission, said Mr. Price, is for such vehicles as taxis and public-service vehicles which are perpetually in traffic, stopping and starting. Hydrostatic transmissions are attractive for these vehicles not so much on account of fuel economy as driving convenience and saving of transmission breakdowns; the reliability factor has its place in any assessment and tests have shown great reliability with hydrostatic transmission on tractors. Mr. Price said interest among vehicle manufacturers is keen.
Another advantage in the case of commercial vehicles was that by having an engine and hydrostatic pump located where most convenient and connected to hydrostatic wheel motors by pipes instead of a conventional engine location, transmission and rear axle, improved loading facilities could be incorporated on goods vehicles and the height of p.s.v.s could be reduced. There can be little doubt, said Mr. Price, that automatic transmission will become the rule for all types of vehicle and in the long term the trend will be to make them stepless; it might be argued, he said, that the future lies with hydrostatic transmission.
Possible developments of gas turbine engines for road vehicles were considered and it was suggested that a hydrostatic transmission would be ideal for use with these units.
Epicyclic Gearing
Both papers in the morning session on Wednesday were of some interest from a commercial vehicle aspect. In " Epicyclic Gearing in Vehicle Transmissions ", A. S. Lamburn and E. A. Whateley of Auto Transmissions Ltd. discussed the various types of epicyclie gears used in vehicle transmissions.