Are multi-speed automatics better or might fewer gears be more?

By DPA | 13 September 2018


BERLIN: For many years, transmissions with a wide spread of gears or quick-shifting dual clutch transmission (DCT) were only offered in luxury and sports cars. For all other drivers, five gears was thought to be enough.

But these sophisticated transmissions are now finding their way into more mainstream Fords, Volkswagens and premium cars from Mercedes-Benz and BMW as makers seek to improve performance and efficiency.

Manufacturers like Germany's ZF are busy producing transmissions with an increasing number of gears and multispeed transmissions are considered to be good all-rounders with advantages for buyers.

A transmission with more gears allows for a greater spread between the ratios, enabling the optimal operation of the engine.

This gives the car more flexibility when it needs to accelerate and deliver peak power. The additional ratios can ensure the engine runs at its "sweet spot" - the optimal power band where it works best.

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The latest edition of the Ford Mustang sports coupe for instance can now be ordered with a 10-speed gearbox - something unheard of 20 years ago.

"The spreading of the ratios boosts acceleration and efficiency," says Ian Oldknow, who helps design gearboxes at the Ford Motor Company.

Improved efficiency means the car uses less fuel too. The Mustang's smart gearbox does not automatically select the next highest gear when accelerating but adjusts the ratio to suit road conditions.

This means it can change directly from 2nd to 4th gear using what the blue oval calls a Skip Shift or adaptive shift scheduling. The device is programmed to respond to particular parameters and is controlled electronically.

Without a gearbox, a combustion engine would be very sluggish at low revs and would not gain sufficient momentum to achieve high speeds, explains Karsten Stahl, who heads Munich University's gearbox construction research centre.

Stahl's view is shared by Marcus Sommer, project head at Mercedes-Benz automatic gearbox department. The Stuttgart-based company uses both double-clutch transmissions and more traditional torque converters with up to nine ratios in heavy SUVs. These use fluid in a casing to change gears.

Modern systems use planetary gears to provide smooth speed reduction and torque. "These offer the widest range of comfort when moving off and manoueuvring," says Bernd Vahlensieck, who heads driveline technology at ZF. Such modern transmissions are even more mechanically efficient than manual boxes where the driver selects the gears themself.

An 8-speed plug-in hybrid transmission.
An 8-speed plug-in hybrid transmission.


Modern automatics ensure seamless changes which are a far cry from the sometimes jerky low-speed shifts of older versions.

"More gears are technically possible but this would add weight and make gearbox architecture unnecessarily complicated," says Vahlensieck.

Of course electric cars are the next big thing and their motors do not need a multispeed gearbox at all.

An electric motor starts accelerating and developing torque at very low revolutions and over a broad range. Designers therefore select a single gear which represents a good compromise between acceleration and top speed.

More gears would not make an electric motor more efficient and widespread use of all-electric cars is set to render traditional gearboxes redundant. This is a classic case of simplicity versus complexity or put simply: Less can be more.

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