Space fiction has become part of some auto companies’ description of hybrids. Finding space for an electric motor, battery and all the controls that go with them while still providing sufficient 5-passenger cabin room and trunk/luggage capacity is not easy. And that means the hybrid version of some sedans suffers in useful cargo space.
But at Oerlikon Graziano, Claudio Torrelli, Head of Product Development, says far better packaging can be achieved. And to demonstrate it, the company recently fitted a Mercedes-Benz SLS AMG with its new OGeco transmission that integrates a 120-kW (161-hp) electric motor within the same space as a conventional transmission. The integration saw the electric motor installed deep in the gearbox and changing of the layout of the gearbox itself.
The OGeco transmission is a 2-shaft configuration with one secondary shaft, whereas the dual-clutch (DCT) is a 3-shaft with two secondary shafts.
To achieve the compact solution, Oerlikon Graziano worked with technical specialist, Vocis, which is responsible for the control unit and control software. “We chose a 6-speed AMT (automated manual transmission) arrangement instead of the more commonly used DCT because the motor can provide torque infill during gearshifts, significantly reducing the torque interruption and overcoming the traditional weakness of an AMT," Torrelli explained.
Mechanical simplification, such as the deletion of any mechanical reverse assembly because the motor provides this function, is another benefit of this approach, he said.
The key to hybrid functionality is linking the motor to both the input (engine) and output (wheels) side of the transmission. “This is achieved through a 2-speed epicyclic gear set that enables the OGeco to provide torque infill, electric boost, regeneration, electric drive and engine cranking, as required," Torrelli noted. "It also enables more effective torque infill during hard acceleration and at higher road speeds than could be achieved through a single-speed arrangement.”
It was deemed essential by Oerlikon Graziano to show the potential of the system via a specially built technology demonstrator vehicle, converting the 435-kW (583-hp) V8 SLS AMG to hybrid specification by replacing the standard Mercedes 7-speed DCT with a prototype OGeco unit.
The car was selected because Oerlikon Graziano has considerable experience of high torque rear transaxles of similar capacity such as those supplied to Aston Martin, explained Torrelli. The standard DCT also provided a relevant benchmark for gearshift quality and speed.
Coupling a 120-kW/200-N·m (147-lb·ft) electric motor to the existing V8 engine provided an obvious and significant performance increase, but Torrelli believes that the fuel economy benefit and the improved gearshift control are equally important.
“In direct comparisons, fuel economy is 6% better over the EU combined cycle because of the higher efficiency of the OGeco transmission, even without the hybrid functionality,” he stated.
The efficiency gain stems from reduced hydraulic energy losses, achieved because the OGeco uses on-demand gear actuation and does not require a clutch to be held closed for prolonged periods. The additional fuel economy gain available from hybrid operation depends on the energy capacity of the battery pack; in this respect the proof-of-concept demonstrator car is not representative of a production installation, which would have markedly greater capacity.
The first priority for the SLS demonstrator was to achieve representative gearshift characteristics that showed how vehicle “personality” could be influenced by the shift mode selected: from extreme feel in aggressive sport mode to high refinement at other times. Current DCT solutions do not always satisfy both these requirements. However, according to Torrelli, OEMs’ reactions following drive appraisals in the OGeco SLS have been very encouraging
“Once a driver has adjusted to the extra performance and is able to focus instead on the shift quality, their reaction is very positive," he reported. "They find the shift comfort, especially around town, is comparable to a DCT.
“Under maximum acceleration the situation is different, as some torque interruption is part of the thrill of driving a high performance sports car: in fact OEMs tell us that a DCT which is too smooth-shifting in ‘sport’ mode, must be recalibrated to make the interruption more apparent!”
While such characteristics may be part of the driving experience of a supercar, for less performance demanding hybrid applications with downsized engines (which have peak power output similar to that of the electric motor) the capability of that motor to provide effective torque infill even under full power shifts is higher to meet customer expectations regarding shift refinement.
Torrelli stressed that Oerlikon Graziano is working towards this aim by promoting OGeco as a cost-effective approach to hybridization that minimizes the impact on vehicle layout and is compatible with front-, mid-, or rear-mounted engines. To support future production applications all the control software for OGeco has been developed in line with the functional safety standards of ISO26262.