7/12/11

Overview: Z.F. 9HP automatic transmission and electric axle drive

Z.F.’s established eight-speed 8HP epicyclic automatic is an accomplished unit. It’s quick and responsive, and it achieves high levels of efficiency, despite using a torque converter and suffering the usual pumping losses associated with this type of box. Its accomplishments are not least as a result of having plenty of ratios available to chase the engine’s torque curve. Fitted to a wide range of BMW models, amongst others, it manages to return slightly superior fuel economy results to six-speed manual derivatives, with their more widely-spaced ratios and narrower spread.

But the 8HP is designed specifically for longitudinal installation. Now Z.F. has introduced a nine-speed automatic for transverse installation and capable of driving the front wheels or all four. Like the 8HP it is an epicyclic unit with a torque-converter. Its standard ratio spread is 9.84 (divide the numeric value of the lowest ratio by that of the highest), which compares with 7.07 for the 8HP unit fitted to the BMW 116d. Z.F. estimates a 16 per cent. efficiency gain over a six-speed automatic.

Z.F. 9HP automatic transmission for transverse installation.

Like the 8HP, the 9HP can be hybridised by the relatively simple expedient of replacing the torque-converter with what Z.F. calls a ‘hybrid module’ — a motor-generator — in the bell-housing. The Company’s current module, which may or may not be used in the new transmission, generates a short-term peak torque of 550Nm and 54PS.

The electric axle drive does not need a combustion engine. It was designed for small and medium-sized passenger cars and consists of an electric motor, a compact single-speed reduction gearbox, power electronics and control software. Peak outputs are 122PS and a distinctly substantial 1400Nm. The complete axle drive weighs 43kg. It is a concept study that will be further developed for volume production.

For mini- and micro-cars, the electric twist-beam is installed close to the wheel and integrates the drivetrain into the semi-independent rear suspension assembly. A compact transmission/electric motor unit is installed on each rear wheel. The system’s peak power output is 81PS.

The design of the electric twist-beam drive installation leaves space in the middle of the vehicle. The connection points of the axle to the body are the same as in a conventional vehicle, so it should be possible to integrate the assembly into existing vehicle architectures almost as an off-the-shelf component.

Light-weight progress

On another front, Z.F. has introduced a couple of mass-saving technologies.

Perhaps a composite brake pedal seems like a drop in a bucket when it comes to reducing the kerb mass of a vehicle, but reducing the mass of components is an important part of making progress in this area. Z.F.’s light-weight brake pedal is produced using endless-fibre reinforced composite materials and injection molding; it weighs about half as much as a steel component for a similar load capacity. The Company also claims that the plastic pedal can be made in fewer process steps than a conventional item. The materials used by Z.F. for the pedal can also be recycled.

Another, rather larger mass-reduction exercise by Z.F., which in this case is still at the theoretical stage, is the Company’s transverse leaf-spring rear suspension assembly using a GRP spring. The spring in Z.F.’s design is part of the wheel location geometry and apparently provides a modicum of passive steering. The set-up is simple, and apparently weighs around 10-12 per cent. less than what the Company rather vaguely describes as a ‘conventional steel rear axle’. Comparisons made by Z.F. between the simplicity of its transverse leaf arrangement and the relative complexity of multi-link designs seem specious, though, as the raison d’être of multi-link location is to separate forces acting in different planes, something which perforce requires the use of multiple separate links.

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