Abstract
Keywords - Transmission concept and design, Control system, Drag torque, Fuel efficiency, Start-stop functionality
In 2001, the 6-speed automatic transmission family by ZF was launched into the market. In the course of technical redesign in 2006, fuel consumption was reduced again by 3% for petrol and 6% for diesel engines. Shifting dynamics were raised to the level of dual clutch transmissions (1). To meet the further targets of fuel efficiency and CO2 reduction requirements, ZF made the decision to develop a new 8-speed automatic transmission family. The development targets are another increase in fuel economy by 6%, improved driving performance at the same time, retaining the shifting dynamics already achieved with the 6- speed 2nd generation.
A model range with four transmissions is defined that covers the entire torque range of petrol and diesel engines starting from 300 to 1,000 Nm. In this paper, the ZF 8HP70 transmission for engine torques up to 700 Nm is described. The 8-speed transmission is based on a gear set system with 5 shift elements and 4 planetary gear sets. The total gear spread is 7.05. The harmonic gear steps, the good gear set efficiency and the balanced distribution of speed and torque values are the precondition for a compact design and good transmission efficiency. This is supported by only two open shift elements in each gear. The package is comparable to the predecessor 6HP 2nd generation (6HP28), the weight could be reduced even further by a lightweight design. The transmission is equipped with a 3-line converter NW250 where the lock-up clutch is controlled by a separate line. For oil supply, an axially parallel vane-type pump was developed. Gear sets and shift elements are designed in a weight- and space saving way and can be adapted to the specific engine torque with various mountings. Due to stiffness, the transmission housing is a one-part design. The plastic oil pan was more or less taken over from the 6-speed transmission. The parking lock is based on the proven conus/pawl system. The control system of the new 8HP transmission family has been designed as a mechatronics solution and consists of a hydraulic control unit and an electronic module which contains the electronic transmission control unit, the entire sensor system, electric connections and transmission connector. The control system has been designed for shift-by-wire operation, the parking brake mechanism is operated electrically/ hydraulically. Mechanical shift operation is also a possible option.
The fuel savings of about 6% are a result of the good gear steps and overall ratio, the reduction of drag torque losses, a more efficient pump and an optimised lock-up strategy. The 8HP transmission family also offers the possibility to support an engine-start-stop function which turns off the engine when the vehicle stands still and thus avoids consumption at idling speed that leads to a further reduction of approx. 5% in the NEDC (New European Driving Cycle). The transmission can be realized with different start-up devices and all-wheel drive technologies (2). Micro, mild and full hybrid systems can be integrated without the need of further installation space. With the functions and technical data described, the 8HP70 represents a milestone in the automatic transmission world and, due to the good cost/benefit ratio, it will be a benchmark for other transmission systems in the passenger car sector.