As electric vehicle (EV) powertrain design gathers pace, the debate continues about the necessity – and practicality – for transmissions with more than one or two gears.
In Rivoli, Italy, Oerlikon Graziano’s head of product development, Claudio Torrelli, believes the optimum number of ratios is four. But there is a need for a more subtle approach than a straightforward manual system, and it needs to achieve a level of smoothness of operation to complement an EV’s potentially linear performance.
To achieve this, the company recently revealed technical details of its clutchless multispeed transmission for EVs that uses the principles of dual-clutch transmissions (DCTs) to provide a four-speed system offering up to 15% improvement in vehicle efficiency.
The transmission, called e-DCT, incorporates a novel gearshifting concept based on Oerlikon Graziano’s experience in the development of DCTs for supercars, including the McLaren MP4-12C. It uses two motors in place of the twin clutches: One motor drives the input shaft that carries first and third gears; the other drives the shaft carrying second and fourth.
“Using two small motors instead of one large motor can give efficiency improvements, making this configuration a viable option for smaller cars,” explained Torrelli. He states that the e-DCT (for which a patent has been granted) can provide the fast gearshifts and minimal torque interruption of a DCT but within a much lighter, simpler, more cost-effective layout.
First application of the new transmission will be for a yet-unnamed high-performance EV for which high top speed and acceleration are required. Although the prototype is a four-speed system, the technology can be scaled to suit a wide range of different vehicle types, from supercars to small sedans.
Said Torrelli: “Having two motors allows seamless shifting between ratios without any torque interruption, giving exceptional driveline refinement, while the absence of clutches keeps driveline losses to a minimum.”
Under low loads and at high speed, the efficiency of electric motors can fall to some 60-70%, but multispeed transmissions allow electric motors to operate closer to their peak efficiency of 90% for more of the time. This helps to extend vehicle range, he explains.
And multispeed transmissions also reduce the compromise between top speed and acceleration. “They avoid the large ratio step of a two-speed transmission to improve the acceleration, top speed, and hill-climbing ability for a given motor size," noted Torrelli. "EV makers can also specify smaller motors to give the same performance with less battery drain, providing greater range.”
The e-DCT has been designed to provide compact packaging and application flexibility with the possibility of being configured to meet a wide range of installation and torque capacity requirements. It can also readily accept additional ratios.
The transmission is arranged in two stages with helical gears designed with splash lubrication and minimal numbers of meshes and bearings. A limited-slip differential is incorporated into the final drive, and electrohydraulic actuators for the shift mechanism are integrated into the assembly.
With no two sequential gears connected to the same motor, the system is able to preselect the next gear before the previous one has been disengaged, using the two motors to synchronize shaft speeds so that synchronizers are not needed.
Torrelli explains that during a power-on upshift from first gear with first and second both engaged, motor one is turning faster than motor two. At the initiation of the shift to third, the torque from motor one begins to reduce and that from motor two increases.
When motor one’s torque reaches zero, first gear is disengaged and motor two is doing all the driving. Motor one’s speed falls and once it synchronizes with the speed of third gear, third is preselected. Motor two’s speed continues to rise to a predetermined level at which the driving torque is handed back to motor one, which is now in third gear, and motor two’s torque drops to the preshift value.
The process carries on for as many ratio steps as are present in the transmission.
The Oerlikon Graziano design also allows the distribution of drive and recovered energy to be balanced between the two motors to enable the integration of regenerative braking, an important facet of EV efficiency.
The shift control system was developed by U.K. controls specialists, Vocis Driveline Controls, which is part owned by Oerlikon Graziano. In addition to a library of existing software algorithms, Vocis used its in-house TMS-20 rapid prototyping controller for development for the e-DCT project. In addition to direct inputs and outputs, the TMS-20 allows robust CAN communication with other controllers on the vehicle to ensure seamless shifting and intuitive control.
As the e-DCT is simpler than a standard DCT, software development could be based on a significantly reduced set of algorithms, explained Vocis Technical Director Richard Taylor.
“The engine interface is replaced by a motor interface while the clutch and synchronizer control algorithms are unnecessary and are deleted entirely,” he said. Software modules for driver strategy, shift sequencing, gear actuation, and safety functions complying with OBDII requirements were still required.
The way the transmission shares torque between the motors offers shift opportunities beyond that achievable with a DCT. “The shift strategy is proving so successful that test drivers find some shifts imperceptible and must refer to data logs to analyze each event,” Taylor said.
The low level software followed normal Vocis structure, established through extensive previous experience, he adds. “But only having one actuator and a reduced set of inputs allows a greater choice of cost-effective electronic hardware for production.”