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An automatic choice for heavy traffic

16th January 1982
Page 36
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Page 36, 16th January 1982 — An automatic choice for heavy traffic
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RESEARCH into driver fatigue shows that on average a driver changes gear three times every kilometer or five times a mile. In heavy traffic, the incidence of gear changing is likely to double or even treble and this contributes greatly to other stress factors which are said to increase the respiratory and pulse rates.

Vehicles equipped with automatic transmissions, on the other hand, were shown to demand much less physical effort, and were less tiring to the driver.

Who is more deserving to benefit from such a laboursaving device than the van driver/salesman involved in multi

Below: Ford's C3 gearbox is similar in overall design to the boxes used by Bedford and Leyland

drop work but whose driving is of secondary importance to his main function? In the light commercial field, all three of Britain's major vehicle manufacturers provide automatic transmissions as options or to special order on their panel van ranges. They are similar to the ones fitted to luxury cars.

Ford, still dominating the panel van market, employs its C3 automatic transmission produced in Bordeaux. It is claimed to be smaller, lighter and at least as efficient as any comparable unit currently available and is designed to match the power output of Ford engines ranging from 1.3 to three litres.

Bedford offers the GM automatic transmission, manufactured and built by the Opel fac tory at Strasbourg, in Germany, to suit the Cheyenne and CF models.

The Leyland Freight Rover company uses a proprietary unit made by Borg Warner, at Kenfig, West Glamorgan, the largest independent manufacturer of automatic transmissions.

While vehicle manufacturers claim increased benefits in durability, luxury and safety, objections to their installation include the price, service complications and reliability.

The initial cost adds about 10 per cent to the list price, but wi be partly recoverable in tft eventual trade-in price.

Mechanical operation, thougl different, is no more compl cated than that of the standar manual box, but its technicall superior design gives it bett€ protection against abuse, whic in the long term should make more reliable.

Although produced in di ferent parts of Europe thes three manufacturers' automati transmissions are similar in dE sign and operation. They contai e common basic components lich include a torque cverter, gearbox, control unit, vernor and pump.

fhe torque converter, consist

of impeller, stator and turle, acts as a fluid coupling beeen engine and gearbox ich not only transmits the tore of the engine but can also iltiply it at lower revs. As the oeller is rotated by the engine,, auses the special type of oil to N outward from its centre and :o the vanes of the turbine.

forque multiplication is lieved as the oil is forced bards and recirculated back /ards the impeller and defted through the stator, so accrating the flow.

,s both the speed of the imler and the turbine are inased, the stator also begins to olve, thus gradually demi nishing the multiplication effect to the point where all three components assume a similar speed. The throughput to the gearbox input shaft is at a ratio of 1 to 1.

Unlike a manual gearbox where the ratios are selected by engaging different sets of gear wheels into mesh, the gears in these boxes stay constantly in mesh. Changes in„ratio are made by locking parts of the gearset. In turn, this induces other parts to rotate at different speeds.

Three sets of planetary gears driven by a pair of central sun gears are contained within an outer ring gear. Symmetry in design affords a better balance and is suited to the heavy loadings involved, while fewer moving parts are subjected to less wear.

Changes in drive ratio are controlled hydraulically and depend upon the engine load and the road speed of the vehicle. Inbuilt clutch and brake bands ensure a smooth transition from one to the other.

Engine load is recognised in the throttle position via carburettor manifold vacuum depression. Vehicle speed acts upon a centrifugal governor which regulates the oil pressure of the control unit.

Imbalance of the two opposing pressures determine the direction of the change or whether the same ratio is maintained.

Operation from behind the steering wheel of the Ford, Bedford and Leyland models is similar though not identical.

The selector lever for the Borg Warner box has a total of six positions. P for park provides a mechanical lock to hold the vehicle stationary, independently of the park brake.

Application of the foot brake before moving to drive position can, however, be a wise move when starting up on a cold morning with the choke out.

Both N for neutral and R for reverse need little more explanation. D gives fully automated gear changing while 2 provides for immediate change down to intermediate ratio from top gear, but when starting from rest limits selection to first and second. Position 1 restricts gearing to low but is protected against selection at too fast a road speed.

Manual override is most useful for overtaking manoeuvres where the gear needs to be held for a longer period than normal to obtain extra acceleration.

Reversely, engine braking is available for long down-hill descents.

The manual control is additional to the kick-down facility also operated' in drive when extra acceleration is required.

In city traffic, a driver may have to change gear and operate the clutch as many as 220 times in an hour, so for the driver who spends much of his time in conjested traffic, the benefits of the automatic should be most apparent.

With a smoother rate of progress in often taxing conditions, fuel consumption for the automatic is likely to come out ahead. While manual boxed models are inevitably driven in the lower gears in slow moving traffic, the automatic will change into top at quite a low speed under a light throttle.

The smoothness of the power take-up creates fewer demands on other major components such as the engine, final drive and tyres, and there is no clutch to worry about.

Borg Warner claims that servicing and parts replacement costs are comparable with those of manuals.

Perhaps the strongest point in favour of changing to automatic transmission is the example of the operators who already use them. About 95 per tent of automatic sales are Said to be replacements to those who have been converted already.

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