ZF 8HP AT Transmission Moves to Next Generation
This year, ZF’s ubiquitous 8HP automatic transmission family, which has received worldwide acclaim for its refined performance and high efficiency, moves into a second generation with further improvements said to reduce fuel consumption by a further 3 per cent compared to the first generation eight-speed transmission. The 8HP has become a favourite of premium car manufacturers and customers, including Jaguar Land Rover, Audi, BMW, Bentley Rolls-Royce and many more.
The 8HP is designed for longitudinal drivelines and is available either for rear-wheel or four-wheel drive. Hybridized versions are also available. The transmission consists of four gear sets with five shift elements. It has an integrated mechatronic transmission control unit to deliver response times that ZF claims are faster than the threshold of human perception.
ZF 8HP second generation 8-speed automatic transmission.
Core features of the design are a wide ratio spread — without individual ratios having to be widely spaced (thanks to the number of them), low internal drag, neutral idle control disconnecting the transmission from the engine at idle, and a high power-to-weight ratio. On that note, the 8HP 70 typically fitted to Jaguar, Range Rover and BMW models can handle 700 Nm torque but weighs only 87 kg. The 8HP also makes maximum use of torque converter lock-up to minimize slip. Engine and transmission ECUs can be functionally integrated by the OEM to maximize efficiency and ensure “right gear, right time.”
The second generation transmission moves further down the path of CO2 reduction, not just by what it does internally but also by what it enables the engine driving it to achieve. The 8HP could already be optionally equipped with hydraulic impulse storage (HIS), which means hydraulic pressure of the internal actuating system can be maintained when the engine (and transmission) stop revolving. This, in turn, enables the use of stop-start, the transmission being fully primed and active the instant the engine fires. On this latest generation, the start-stop function has been improved so that the engine can be stopped immediately. Previously, it took up to 1.5 seconds.
Now, the transmissions have been further optimized for downsizing and also down speeding, two key powertrain fuel consumption reduction strategies in use by OEMs. Support for down speeding and the steady movement of peak torque further down the rev range has been provided by improvements in vibration damper design. ZF says that reducing the rpm across all gear ratios by 50 is worth an improvement of 1 per cent in fuel consumption. The improvement in damper technology also means that torque converter lock-up can take place much earlier in the rev range without introducing vibration and harshness.
Improvements in the shift element clutch pack design have led to more effective separation when the packs are open, reducing drag and energy loss by more than two thirds. Losses are further reduced by a clutch which opens during deceleration and when the vehicle is stationary. This not only means the vehicle does not have to be braked against the driveline, but also enables coasting with the engine stopped if the OEM desires, at speeds of up to 99 mph (160 km/h). A new axial vane pump, which also appeared in the 9HP, has enabled internal hydraulic operating pressure to be reduced from 5 to 3.5 bar, reducing energy consumption.
Performance is enhanced, too. In common with the transverse nine-speed 9HP gearbox first introduced in 2013 by Land Rover and Chrysler, the transmission control unit now supports nested downshifts, making it possible to “skip shift” from eighth to second gear in one operation. The 8HP is also equipped with ASIS (Adaptive Shift Strategy), which mirrors the driver’s mood within seconds, sharpening up response during quicker driving then slipping seamlessly into a more economical regime when the driver resumes a more relaxed approach. The Curve function prevents shifting when cornering and the Fast-Off function measures the rate of throttle release, anticipates further requests by the driver for high power (such as when contemplating an overtake), then holds the gear, if necessary. If the driver requests a downshift when the vehicle is traveling too fast, the transmission will remember the request and make the shift when the speed drops to an appropriate level.
Hybrized and AWD versions are also available.
There are four variants of the 8HP: the 8HP 30, 8HP 45, 8HP 70 and 8HP 90, covering both rear-wheel drive and four-wheel drive, and input torques from 300 Nm to 1,000 Nm, the latter with the 8HP 90 four-wheel drive. An electrified version is available to deliver a virtually “off the shelf” parallel hybrid (at least, in terms of transmissions), equipped with an electric motor in place of the torque converter, and occupying virtually the same package space. That the 8HP has been so widely adopted by premium large car and SUV makers is no surprise since, at its inception, it already delivered an 11 per cent improvement in fuel efficiency compared to its predecessor, the six-speed 6HP. The 8HP family is also supplied as a transmission “kit,” with full vehicle integration support provided for OEMs choosing to use it. OEMs like Jaguar Land Rover have come back for more, equipping four-cylinder variants of the new Jaguar XE sports sedan with the 8HP 70’s smaller sister, the 8HP 45.
SOURCE: DRIVELINE NEWS
