Simpler Gear -changing and Greater Comfort
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Two 'Trends in Heavy Goods and Passenger Vehicles Forecast by Mr. A G. Wilson
. .
T RENDS in heavy passenger and goods vehicles would be towards
. simplified gear changing and greater comfort, even at the expense of higher first cost. Increased performance, high speeds and less wear and tear could result from improvement in the transmission system, said Mr. A. Gordon Wilson in a paper, " Transmission De v el o p ments for Public Service and Heavy Goods Vehicles," presented to the Institution of Mechanical Engineers in London on .Tuesday.
. An important consideration in the economy of operation was the performance and acceleration of a vehicle. Improved performance could bring about savings in journey time which could more than offset increased costs in terms of first cost or fuel consumption, whilst giving a better service to the customer. It was even possible to operate a service with fewer vehicles by modest gains in average speed.
Cost versus Performance
In the operation of goods vehicles, themain determining factor in the adoption of improved transmissions would be whether the additional first cost could be offset by lower cost per ton-mile, in which maintenance, availability and fuel consumption would all be counted. as well as any advantage in terms of the work that could be done in a given time.
In passenger vehicle operation, additional first cost and heavier fuel consumption resulted in increased costs which had to he passed on to the customer. The operator's proBlem was to determine the extent to which this could be tolerated, without loss of custom, rather than to provide a lower standard of service and comfort at cheaper fares.
Transmission faults which gave rise to criticism were noise, jerking when starting or in gear-changing. and vibration, whereas silence and smoothness attracted custom.
The introduction of a fluid coupling into the transmission system could plsy an important part in two ways. It allowed a perfectly smooth start to be made and it eliminated the transmission of torsional vibrations which caused rattles and roughness.
The price which had to be paid for this was the fuel loss by slip in the coupling, which varied from about 3 to 6 per cent., depending upon circumstances and care both in maintenance and driving technique. On the other hand, it allowed a quick
change transmission to tie used, which improved performance.
Similar conditions applied to a torque converter, but the fuel loss was somewhat greater. ' There were no noticeable alterations in engine speed during acceleration and, consequently, no possible surges in engine speed, such as were associated with gearchanges, the only counterpart to gearchanging being the transfer from converter drive to direct drive. •
More Noise
Against this had to be set the.somewhat increased noise when starting from rest, because the engine speed was higher at start than with either a fluid coupling or a normal clutch. Fuel consumption was high and the rate of engine wear was greater.
A gear transmission was inherently more efficient than a torque. converter, but the gear-changes involved a definite change in the relationship of engine speed to vehicle speed. Smoothness of gear-changing was hard to guarantee under all conditions of driving and, although planetary gears could be made to engage smoothly, they also could cause a jerk if incorrectly set and handled.
In planetary transmissions with pneumatic or hydraulic means for operation, the rate of engagement of brake bands and clutches could be controlled, and much success in the provision of smooth gear-changes had followed the development and study of ways of controlling their engagement. Automatic means for selecting and changing gear relieved the driver and saved the mechanism from abuse. They were found frequently to reduce fuel consumption.
Power Loss
The use of a stepped gear behind a torque converter to widen the range was becoming common practice in American design. The smoothness in operation resulting from this combination was attractive, but it was gained at the expense of the steady loss in power which was associated with all torque converters so far built.
Once it was agreed that automatic or semi-automatic transmissions were acceptable, the way was open for all sorts of new approaches to the problem of control. The driver's feet alternated between brake and accelerator, the left foot.working the clutch and sometimes the dip-switch. Both hands were normally on the steering wheel, but in addition they must either signal Manually or by indicators. operate the horn, work
the gear lever and perform occasional duties, such as turning on lights, windscreen wiper, and so on.
Elimination of the clutch pedal freed the driver's left foot, but the tendency with two-pedal control rernained for the right foot to work both accelerator and brake pedals.
In Britain, urban buses were normally fitted with four-speed gearboxes with ratios of I to 1 to about 44to 1, and started mostly in second gear, and passed through third to top. These gearchanges went on all day, with perhaps eight stops per mile, and everything pointed to automatic control.
This control had, however, to provide a means for choosing whether to start in first or second gear and with preferably a limiting device to hold a lower gear on a greasy road, also, of course. a reverse ratio.
In the United. States nearly all urban buses were. fitted with torque converters of the Lysholm-Smith type, with an automatic means for engaging the direct-drive clutch at a speed of approximately 20 m.p.h. . For urban conditions, said Mr. Wilson, automatic gear-changing would have to become universal before long.
More Down-changes
On long-distance coaches the number of gear-changes was fewer, but the relative number of down-changes to up-changes was higher than in an urban bus. The probable number of gears was five, the top being a direct drive with possibly a low ratio of 7+ to 1 or thereabouts. Alternatively, a four-speed gearbox with a bottom gear of about 5 to 1 or...51. to 1, and an overdrive of about 0.7 to 1, could be suitable.
In the near future, the trend in coach design would be more towards semiautomatic than fully-automatic transmissions, but the introduction of fully automatic devices would follow.
The adverse power-to-weight ratio in heavy lorries gave rise to more frequent gear-changes, and truck drivers might be unlikely to accept readily an automatic stepped gear, being accustomed to deciding the timing of gear changes.
Torque converters were in growing demand in the United States for use in short-distance trucks, but the power losses were great and this was resulting in a search for a system which provided lower power losses in countries where fuel was more expensive and heavily taxed.
Engine braking was an essential safeguard, and provision had to be made for it. A number of problems arose and could be described roughly as being the converse of the transmission of power to the wheels of a vehicle.
The engine exerted a braking effort when driven, which was approximately proportional to its speed. Ideally, therefore, a variable-speed mechanism which could, at the will of the driver, cause the engine to be driven at any number of revolutions up to its safe maximum, irrespective of vehicle speed. was what was needed.