No Quick Solution of Transmission Problems
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Says Professor Ewen M'Ewen, B.Sc. (Eng.), A.M.I.Mech.E., in a Paper Read to the I.Mech.E. AN excellent paper "Recent Developments in Automobile Transmissions," was read last Tuesday before the Automobile Division of the Institution of Mechanical Engineers, by Professor Ewen M'Ewen, B.Sc.(Eng.), A.M.I.Mech.E. It was limited to the gearbox and to devices replacing this.
The author points out that no universally satisfactory solution to the transmission problem has been found, nor is there any hope that it will quickly emerge. A true appreciation of what the transmission has to do will show the futility of expecting the same best answer for all types of service, and will permit intelligent selection.
The transmission has to serve three distinct purposes—to accelerate the vehicle from rest into top gear; enable the vehicle to overcome increased resistance due to gradients and other causes, and permit the prime mover to run at above its stalling speed when conditions of traffic, surface, etc., require it to operate at low revs, and part throttle.
It is commonly stated that if the ix. engine were capable of developing full torque at zero speed, like a steam engine, there would be no need for a transmission. This is obviously false for motor vehicles unless the torque increases with drop in speed. Even then it might be more economical to retain the transmission if the power tapers off rapidly with speed decreases.
The Various Requirements
The urban bus requires maximum acceleration from rest, top-speed running being of minor importance. Fuel economy shouli be best under high-loadfactor low-speed-acceleration conditions, for these represent the normal state governing the overall economy of such a vehicle. Bus acceleration must be smooth, for the average service speed is governed by the maximum possible acceleration, this, in turn, being limited
by its steadiness. The standing passenger can brace himself against a large, steady acceleration, but is thrown off his balance by much smaller jerks.
The powez-weight ratio of the urban bus is lower than that of the car, the standard of driving skill higher, and silence a prime consideration.
Long distance coach requirements differ from those of the bus. Acceleration from rest is relatively unimportant, but from half to full speed must be good. Hill-climbing performance is a more important function of coach transmission, whilst maximum fuel economy is requireo at high speed and a fairly high load factor
Urban delivery vehicles need a performance similar to that of the bus, but smoothness is less important, driving skill is lower, and initial cost lower.
Engine idling means so much fuel wasted that smail differences in transmission efficiency are unlikely materially to affect fuel economy.
Heavy commercial vehicles have a low power-weight ratio, but a high one of laden to tare weight Acceleration is not so important and the primary requirements of the transmission are to provide the necessary tractive effort and keep the engine near 'is power peak. Long periods are run in gears other than top. Driving skill is high, running cl 6 and initial cost must be low. Fuel economy is needed at high load factor and fairly low speeds. Maximum reliability is particularly important to the haulier.
Many think that an infinitely variable transmission is an end in itself and can compensate for serious losses in efficiency, high cost, and other drawbacks. The author shows, however, that even with a tra:ked cross-country vehicle, the gain in performance to be expected for speeds above five is but small. The law of diminishing returns applies, and for such a vehicle the overall average speeds, without considering limitations by traffic, etc., are in the ratio of 107f /105/100 for infinitely variable, seven-speed and five-speed transmissions respectively, assuming equal efficiencies. If the infinitely variable unit has, uniformly, 90 per cent. of the mechanical efficiency of the other two, the ratios become 100/105/100. The gain from such a transmission in a road vehicle may be even more trifling. This is on the basis of meeting tractive demand, but in a truck, bus and car, the requirements for smooth and rapid acceleration are progressively more important over this range of vehicles, and the infinitely variable transmission then increasingly scores.
Transmission Cannot Think The fundamental trouble with automatic transmissions is that they cannot think ahead. Given satisfactory gearchange, a good driver will always attain better performance than is given by an automatic transmission with the same ratios as the manual box. The field for the automatic will be settled by economic considerations. 'Here, the expensive car is the most fruitful field for the fully automatic, and the public-service vehicle for the semi-automatic, where the driver selects his gear or range and the transmission does the rest.
The common layshaft type of gearbox is still in an overwhelming majority, not only in respect of vehicles in service but when 'considering new production. Now that synchromesh is almost universal on cars, and with a tendency to higher power-weight ratios for buses and goods vehicles, gear-changing has lost its terrors. Any departure from this common type must therefore be produced cheaply and offer real gains before it can hope' to succeed. No transmission can produce power, although it may consume it unnecessarily, and no wonderful transmission can compensate for too small an engine or too heavy a vehicle.
The constant-load synchronizer is inferior in performance, but cheap to make. The baulked type will, however, probably spread as the patents expire. It may be that driver comfort will be put sufficiently far ahead of initial cost to justify at least synchromesh on the goods gearbox, in which case the cost of synchromesh components must be reduced to compete for export. Need for quietness in public transport is a primary factor in the popularity of the centre-bearing gearbox. It tends to be longer and heavier, but this is partly due to the use of the same basis of calculation in design. The increased rigidity and quietness may permit greater nominal loading for the same reliability.
In some designs of constant-mesh dog-change goods gearboxes the spigot bearing, a frequent source of trouble in the past, has been eliminated.
The fashion for over-drive gearboxes seems to be waning. What happened was that as neither engine nor vehicle was altered, the economic overall ratios were also unchanged, so that an overdrive gearbox simply involved a greater reduction in the final drive. Engine revs, per mile in any given gear were, therefore, unaffected, and the quietest gear has merely been changed from top to next-to-top, at the expense of an undesirable increase in propeller-shaft speed.
Points Seldom Appreciated
For orthodox transmission, high-alloy steels are no substitute for good workmanship. Gains from high physical properties are largely illusory in fatigue conditions within the mortal range.
Grinding is not a salvage operation for material spoilt by heat treatment or in the planning department. Better control of original forgings and more care in stress relieving, combined with the increasing use of die quenching, is producing work sufficiently free from thermal distortion to give quiet gearing if cut to a good standard. Grinding may do more harm than good. The reduction in dynamic loading due to increased accuracy is more than offset by the loss in endurance life at a given loading, as a result of the tensile stress in the case produced by its treatment.
The author then deals with, and gives illustrations of, many types of transmission, including the Wilson air-operated epicyclic, the synchro-self-shifting, the electro-magnetically clutched, the new five-speed Hobbs split drive, and the Brockhouse Turbo-transmitter.
Referring to future trends, the author says if the power-weight ratio were to improve in British buses, the claims of smooth acceleration would probably bring the torque converter to the fore.
For coaches, it is probable that the orthodox synchromesh gearbox will hold the field longest. In heavy vehicles the inevitable poor power-weight ratio makes the torque converter most unattractive.