The news that ZF has released a nine-speed gearbox for front-wheel-drive vehicles does not come as a surprise. The trend towards more ratios has shown no sign of dying because car manufacturers will accept any design that improves fuel consumption. The extra complication that so many ratios bring in its wake is swept under the carpet.
The new gearbox has the remarkable ratio spread (from first to top gear) of 9,84, compared to something like 6,0 for most other transmissions. This implies that the extra ratios have not only been used to shorten the gaps between ratios but also make two very high top ratios available under cruising conditions.
The interior arrangement of the gear sets incorporates some very clever solutions to the space problems usually faced when a gearbox is designed for front-wheel drive. The four gear sets are not strung out in a line, but are nested, to safe space. The six shifting elements utilize dog-clutches instead of the traditional multi-plate clutches, to reduce drag torque.
The gearbox is coupled to the engine via a torque converter, because Asian and American clients prefer the smooth take-up and creeping ability of such a drive. Its design incorporates a multi-level torque damper and a lock-up clutch that comes into operation soon after pull-away.
ZF calls the complete unit a construction kit. This means it can be adapted for four-wheel drive by adding a transfer case. It can also cope with a stop-start configuration that needs a quick change into first gear as well as fitment to a hybrid vehicle. In this case the torque converter is replaced by an electric motor.
The transmission controls are highly adaptable to cope with the differing OE requirements. For example, shifting down more than one gear at a time and staying longer in any particular gear are all part of the multi-level control configuration. The optimum ratio for fuel economy is automatically selected at any cruising speed.
The new box is claimed to offer fuel consumption improvements of up to 16 per cent compared to six-speed gearboxes. The many available ratios imply that a very high ratio (low numerically) can be used for cruising, since an instantaneous change-down is always available. For example, at 120 km/h a typical six-speed box in top gear would require the engine to run at 2 600 r/min, but the nine-speeder would allow the engine to run at 1 900 r/min.
The gearbox, called the 9HP, is available in two model ranges, to cope with maximum torques of 280 or 480 N.m.
