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Quick guide to gearboxes


Technology’s providing a new take on the old debate about whether a manual gearbox or an automatic is the best.

It wasn’t all that long ago that automatics were for Americans and elderly people wearing hats. Everyone else stuck with the manual.

For sure, if you wanted the most power from your engine, going with a manual transmission has been the
best bet. About 96 percent of the energy that is put into a manual comes out of the other end. The manual’s also been excellent at contradicting itself.

It can offer good fuel economy, or in the wrong gear at high engine rpm, can help the engine suck fuel like there was an endless supply. It’s great for providing total driver control, but can also help cause the driver to lose control.

Torque interruption when gearshifting is a weak point. And shifting results in a jerky ride unless the driver’s careful – not fun for passengers and potentially stressful on the drivetrain.
The traditional automatic, on the other hand, offers good shift quality – sometimes all but imperceptible – thanks to its torque converter.

It’s easy to use, just drop it into drive and forget it. Trouble is, it uses more energy and more fuel, is complex and weighs more than an equivalent manual.

Today, computer controlled “smart” automatics that can be either left to do their thing, or work like a manual without a clutch, have blurred the arguments in favour of manual gearboxes.

Fuel economy is improving and in a few cases manufacturers are reporting automatic vehicles with slightly better consumption than the manual equivalent.

Even the worst autoboxes are more efficient than just a few years ago, both in their ability not to “suck out” engine power and in stretching that litre of fuel.

It’s still true to a point that automatics work best with larger-capacity engines, but today even little 1.3-litre units or below can zip along quite nicely under automatic control.

Manufacturers of new-breed conventional automatics have done a tremendous job and, like the internal combustion engine, there’s life in the old girl yet. But like the conventional engine, it’s newer technologies that may take their place in the future.

These include the continuously variable transmission (CVT); dual clutch transmissions (DCT) and the automated manual transmission (AMT).

The CVT uses a belt chain or toroidal shaped dish drive to provide an infinite number of gear ratios. It’s a transmission of relatively few components and works by varying the distance between the faces of the two main pulleys. A CVT keeps the engine in its optimum power range, increasing efficiency and fuel economy.
CVT is now well proven, although there have been problems. The transmissions must be correctly maintained at specified intervals and are sensitive to the correct fluid.

A DCT transmission is basically two manual gearboxes coupled together. Shifts are made by switching from one clutch on one gearbox to the clutch on the other. Shift quality equals a conventional automatic and it’s more efficient. Volkswagen and Audi are its main champions.

Though DCT isn’t the cheapest way to make an automatic gearbox, low cost is one attraction of AMTs, which use actuators to replace the clutch pedal and gear lever, retaining the efficiency of a manual – along with its torque interruptions and shift quality.

A transmission with great potential, but some years from production, is Zeroshift, which has been the subject of recent publicity in New Zealand. Zeroshift replaces the familiar gear synchromesh with an advanced dog engagement system.

Dog engagement is used in motorsport to allow fast shifts but has been unsuitable for road use as large spaces between the drive lugs or “dogs” create an uncomfortable “shunt.”

Zeroshift solves this problem by adding a second set of drive dogs each of which transmits torque in one or other opposing direction. The shift is instant and drive isn’t interrupted. The cost of production units is expected to be relatively low.

By Phil Hanson

Auto Trader New Zealand