ZF's 8-speed automatic transmission 8HP has already conquered the passenger car world – it is providing sterling service in current full-size high-end luxury cars as well as in SUVs, premium compact cars, and high-performance sports cars. In the form of a new special version dubbed 8HP90S special vehicles can also benefit from its long-term qualities: These include fast, comfortable gearshifts, a high total spread, the very good power-to-weight ratio as well as flexible applications and outstanding efficiency.
The ZF engineers focused primarily on two criteria when designing the 8-speed automatic transmission for light special vehicles with four- or six-wheel drive: The 8HP90S must have the same outstanding basic characteristics as its passenger car counterpart and at the same time perform as effectively as possible the new commercial vehicle-specific tasks – regardless of whether this involves a fire truck, vehicles used in agriculture and forestry or municipal vehicles.
ZF solves this task with the 8HP90S whose basic transmission is designed for input torques of up to 1 000 Nm. Its eight gears provide a high total spread of 7.05 with very small gear steps, which also promotes power, comfort, and efficiency in equal measure. Nonetheless the 8HP90S remains a compact lightweight: It is less than 700 millimeters long and tips the scales at just 99 kilograms including oil. As such it saves valuable installation space and helps optimize the payload of special vehicles.
The technical prerequisite comes in the shape of a sophisticated, innovative gearset design with four planetary gearsets and five shift elements. Since there are always only two shift elements open per gear, inner drag losses in the transmission are greatly reduced. The torsion damper system in the torque converter also has a positive impact on shift characteristics, durability and, not least, fuel consumption: This system effectively decouples the torsional vibrations induced by the engine in the driveline. At the same time, the lock-up clutch can also be engaged faster. Overall these measures translate into outstandingly short response and shift times below the human threshold of perception. Double and direct multiple gearshifts are also possible. To ensure that the automatic transmission can also negotiate any off-road situation effortlessly, it has been designed for uphill or downhill gradients of up to 60 percent and 30-percent lateral inclination.
As a neutral interface, the 8HP90S comes at the output end with the cross cut commonly used in commercial vehicles to connect standardized propshafts. Added to which are unified connections: In terms of the mechanical components, this entails the SAE 4 engine interface; the SAE CAN J1939, J1939-14, or J2284 communications buses are available for the electronics. The actual hardware and software on the 8HP90S are fully integrated in the transmission, which minimizes the wiring required.
The control electronics on the 8HP90S provide even more benefits: The automatic transmission also comes as standard with the adaptive shifting strategy ASIS. This evaluates the personal driving style and the respective driving situation continually in order to select the best gear shifting point and gear at all times. Thanks to the flexible control, mechanical and pure shift-by-wire gearshift systems can be implemented.
However, the 8HP90S makes light special vehicles even more fuel-efficient thanks to other options – and, in turn, more environmentally friendly and also more efficient: A mild hybrid solution using a crankshaft starter generator can be easily implemented with this 8-speed automatic transmission, along with a start-stop system.