On show for the first time in the UK is a new 6-speed automatic transmission for rear wheel drive cars. The new unit, known as the ZF 6HP-32, is the latest in a growing range of 6-speed automatic transmissions from ZF that are setting the benchmark in automatic applications.

The key benefits of having a sixth gear are greater fuel economy, which comes from the extra 'long' ratio, and a quieter, smoother ride. The lower gears can also be adjusted to give greater, initial acceleration.

ZF first revealed a 6-speed automatic car transmission when it showed
the 6HP-26, a unit able to handle an input torque of up to 600 Nm, at
the 1999 Frankfurt Show. The new ZF 6HP-32 can handle up to 750 Nm input torque and caters especially for the needs of the latest diesel engine developments.

Surprisingly this new automatic transmission, in spite of housing an
extra ratio, is both lighter and smaller than the 5-speed version it is expected to supersede. The key to these advantages is in the use of a Lepelletier gear set which enables the 6th gear to be incorporated with ease, whilst saving 13% in weight compared to the comparable 5-speed unit. This also results in an improved power to weight ratio.

Mechatronics

The new 6-speed transmission also incorporates a significant development known as the "mechatronics" control module. This design step sees the integration of the electronic system with the hydraulic shift unit, sensors and actuators into a single module. It is a development which points the way forward for all future automatic transmissions offering technical and production advantages to vehicle manufacturers. Both electronics and hydraulics can be housed together within the transmission. This arrangement reduces the number of interfaces between mechanics, hydraulics, actuators, sensors and electronics. All these components are specifically matched such that the signals and the pressures which activate gear changes are achieved with minimum tolerances and the gearshifts are carried out very precisely. Not only is that good for performance and fuel consumption, it also helps to deliver a more comfortable ride.

Wide control capability

The electronics which control the transmission system are based on a new Adaptive Shift Strategy, known as ASIS. This system receives a constant flow of data relating to the demands being made on the transmission from both driving conditions and driver behaviour. Comprehensive evaluation of this information evokes a complex, functional response by the control system.

The outcome is the optimisation of the transmission shifting points as the control system responds to varying driver behaviour or road
conditions. For example, it can adapt swiftly to variations of driving style ranging from a high performance sports approach to one which is more relaxed and economical. It also reacts to the demands on the vehicle caused by traffic conditions or the load being carried by the car.

Tick-over mode

An additional feature of the electronic control system is the Stand-by Control or SBC capability. Conventionally, the converter would run
'against' the engine with the vehicle just ticking over at rest (with
the driver's foot on the brake pedal to avoid 'creep'). The converter in ZF's new 6-speed automatic is disconnected from the power train at rest so that any residual load is minimised, improving fuel consumption.

Production

After 10 years of producing highly successful 5-speed automatic
transmissions, which account for 80% of automatic car transmissions
currently fitted, ZF expects the new 6-speed automatics to take over
from the heavier and bulkier 5 speed units in a relatively short period of time.

In the 6-speed range, ZF offers solutions for various drive and engine configurations including all-wheel drive. Torque inputs are expected to range from 300Nm to 750Nm.