Changing your ways

Variable transmission is the way to go. ALASTAIR SLOANE explains how it works.

The motoring world is full of acronyms: ABS for anti-lock brakes, DSC for dynamic stabiliser control, ICE for in-car entertainment and ATC for automatic temperature control.

Then there's EBD for electronic brake distribution, ETC for electronic traction control, ACE for active cornering enhancement, and so on.

Basically they are electronic systems designed to keep people comfortable - ICE and ATC to cool and entertain them, the others to stop them and the vehicle from turning turtle or going backwards through a hedge.

There is another acronym which has been around for a while but isn't in widespread use.

It is CVT, standing for continuously variable transmission. It won't blow cold air around the cabin or stop the brakes locking up, but it will change the way you drive.

CVT is a fancy-pants gearbox, overseen by electronics and pretty much confined in production form - in the past at least - to small-capacity engines. Subaru used it in the Justy. Honda has it in the HR-V and Logo. Nissan used it in the Micra city car, which wasn't sold here.

Now Nissan New Zealand has put what it calls its Hyper CVT unit in its 2-litre Primera station wagon. The system also has a six-speed manual mode called M6.

Hyper CVT has been used in the Primera in Japan for months. Reports say it works particularly well and without the sensation under throttle of slip, which was apparent in the belt system of some early CVT units.

But first, CVT itself. The self-shifting system was developed in the Netherlands in the 60s and fitted to the Dutch-built Volvo 340 and DAF small cars.

CVT differs from a conventional automatic in that it has no specific gear ratios. Instead of four or five mechanical gears, CVT uses an opposing pair of split conical pulleys, linked by a steel belt.

One pulley receives the torque generated by the engine and the other transmits drive torque to the wheels. The system varies the width of each split pulley, allowing the steel belt to revolve further up or down the conical pulley, thus providing infinitely variable gearing.

The effect is to have acceleration that is both efficient and unusual. The car will seamlessly speed from, say, 60 km/h to 100 km/h without much of an increase in revs. This also improves fuel economy by about 20 per cent, says Nissan.

In a conventional step-system automatic, say the supporters of CVT, engine torque is lost when the transmission shifts up a gear.

But engineers say the downside of CVT is that its steel belt system is subject to slippage and its designers have yet to match the mechanical system's lock-up torque converter, which provides a direct mechanical link at cruising speeds and minimises the efficiency lost through hydraulic slip.

However, Nissan says it has developed a hydraulic system to generate the higher pressures that are applied to the pulleys that squeeze the steel belt.

This system applies more pressure when the 110kW engine is at full power, but reduces the squeezing force on the belt when less power is been used.

But regardless of the pros and cons, CVT is seen by almost all carmakers as the transmission of the future.

A Honda Formula One team engineer a few years ago said CVT could ultimately be developed to handle the enormous power and torque of race cars.

Nissan is happy for the moment with its Primera station wagon, which has already attracted interest from fleet markets.

CVT has the usual automatic modes, drive, park, reverse and neutral. Gone are the gears 3, 2 and 1.

In urban and city driving, CVT can be used like an automatic, without the need to make gear changes.

But on the open road, for instance, the driver can the shift across the gate and operate the gearbox sequentially, pushing the shift forwards to go up the range and backwards to go down.

The manual mode automatically returns to first gear if the Primera is stopped. It will not let the driver over-rev the engine in any gear.

The six preset manual ratios and the standard automatic settings are shown in the instrument cluster.

The Primera CVT wagon has the same specifications as the five-speed manual model unveiled last month.