Transmission specialist Oerlikon Graziano has spotted a way to plug a "gap" in the transmission market. It is the gap suffered by most automated manual transmissions (AMT) caused by torque interruption between shifts.
AMT technology may save weight and power losses by obviating the need for a torque converter or the cost of using a double clutch transmission (DCT) solution. But torque interruption effects as gears shift are invariably a problem that requires varying degrees of footwork finesse by a driver. Many AMTs—particularly early versions with less sophisticated control software—have the ability to irritate driver and passengers. Anyone who has travelled in a car with an AMT would appreciate the luxury that torque infill could provide.
And that is what a new Graziano transmission, now in development, has been designed to achieve for hybrid systems. Claudio Torrelli, Graziano’s Head of Product Development, believes his company’s system would be particularly apposite for hybrid supercars: “It fills in the missing torque by drawing from the vehicle’s electric traction motor for the duration of the shift event,” he told AEI.
The Graziano system is a lighter, more package-friendly solution than a DCT, Torelli believes, helping to offset the inevitable weight and package issues that hybrid powertrains bring. He also claims a potential improvement of up to 4% fuel economy compared to a wet-clutch DCT when based on like-for-like simulations, thanks to lower internal losses and the elimination of hydraulic cooling requirements.
Current thinking in supercar circles favors the DCT as the optimum solution for smooth gearshifting combined with good efficiency, promising the refinement of an automatic with the fuel economy and driver involvement of a manual. Torrelli accepts that premise, noting that Graziano produces such systems. But he says that as performance car manufacturers embrace hybrid technology in the search for lower emissions and higher performance, AMT with torque infill may offer a more suitable solution for hybrids.
“Our experience in both AMTs and DCTs gives us the impartiality to select the most appropriate technology for a particular application," he explained. "There is no one-size-fits-all solution for all market sectors.”
The trick to enabling torque infill
At the recent CTI [Commission for Technology and Innovation (Innovative Transmissions, Hybrids and Electric Drives)] symposium and expo in Berlin, the company revealed technical details of its new AMT. The basic architecture is a 6-speed, two-shaft configuration coupled to an electric motor through a two-speed epicyclic geartrain and connected to the combustion engine via a conventional clutch.
The two-shaft layout provides good installation flexibility to support different potential hybrid arrangements, while the two-speed epicyclic enables more-effective torque infilling during hard acceleration or at higher road speeds than a single-speed could achieve, explains Torrelli.
He adds: “The water-cooled electric motor provides 120 kW (30 s peak rating) and 200 N·m with a maximum speed of 14,000 rpm. In order to improve the package and optimize the total weight of the system, the electric motor is embedded into the transmission casing. The system is configured to permit a number of alternative power modes by allowing the traction motor to drive independently of the engine.”
The epicyclic assembly combines their outputs to enable the electric motor to contribute additional torque for acceleration, to smooth out gearshifts, or to operate in fully electric mode with the engine cut.
“The engine can also back-drive the electric motor to charge the battery pack,” Torelli noted.
The new transmission uses the ISR (Independent Shift Rod) system for gear-change actuation, which the company introduced on the ultra-fast shift for the Lamborghini Aventador. This integrates each shift valve into its corresponding shift rod, eliminating the cross-gate movement of a conventional “H” gate.
As a result, the system can begin to move the rod for the next gear while still withdrawing the previous one, allowing the shift to be accomplished faster. The hydraulic module that operates the transmission consists of a power pack with controllers for hydraulic line pressure, clutch and electronic limited slip diff (eLSD), where fitted. Built up from proprietary parts, the design priority for the entire hydraulic system was the optimization of shift time.
Next step: demonstrator vehicle
The control system, including software, was produced by Vocis Driveline Controls (part-owned by Oerlikon Graziano). It uses an in-house rapid prototyping controller, the TMS-20, with modules for individual functions based on its Siena software. Simultaneous engineering of the mechanical architecture and the software allowed the team to take best advantage of the inherent dynamic behavior of the system.
Control systems are often designed after the gearbox hardware development is frozen, and must operate within the constraints of an existing mechanical layout. After modeling component elasticities and inertias, the system responses were simulated and optimum strategies for shift execution identified, eliminating much of the compromise inherent in this area.
The development program for the new transmission is now in the prototype procurement stage, with testing scheduled for coming months and a demonstration vehicle constructed by the fall.
Oerlikon Graziano claims that the new transmission provides the best of all worlds. By using the ISR system for gear-change actuation, the driver can still access track-day levels of excitement when required. During less extreme use, high levels of shift refinement are provided by taking advantage of the torque available from the electric motor to eliminate torque interruption.