Creating smoother-shifting, more energy efficient, and lighter weight transmissions for electrified vehicles is the impetus behind a new system called MSYS. Compared with single-speed EV and hybrid transmissions, MSYS provides a 40% weight saving and potentially 10-15% lower vehicle energy consumption, according to Alex Tylee-Birdsall, Technical Director at DSD System Design, the U.K.-based engineering consultancy that unveiled the system at the 2013 International CTI Symposium in Berlin.
He said the key to MSYS is a fundamental re-thinking of synchronizer architecture, "which permits downsizing of the motor and, by being in the optimum gear ratio more often, extends the time the motor spends operating at high efficiency by reducing the demand on motor power electronics."
Improving the efficiency of the link between a traction motor and road wheels is critical for increasing an EV's range, driveability, and overall customer appeal, Tylee-Birdsall noted. With MSYS, the object is to develop a transmission scaled to fit vehicles of various sizes and motor power outputs.
Unleashing the dogs
The secret behind the MSYS design is in how it separates the two basic synchronizer functions of friction and latching to enhance the friction capacity.
“DSD has been able to create a powershift system that provides an uninterrupted flow of torque to the wheels yet uses no energy to hold the transmission in gear," Tylee-Birdsall said. He noted that the increased friction capacity allows the powershift cone clutch to "temporarily drive the vehicle during shifts while latching in gear is achieved by dog clutches with self-locking teeth, similar to a conventional synchronizer.”
Necessary friction levels are achieved by using multi-cone clutch assemblies, with forced lubrication via variable flow through the clutch packs. Flow variation has been designed to permit good thermal management at times of high slip, with maximum efficiency at times of lower demand.
By using proven technologies in what Tylee-Birdsall describes as a unique arrangement (synchronizer-derived cone clutch and dog clutches), DSD has been able to seek patent protection for the system while minimizing the technical risks during development and enabling systems suppliers to make use of their existing expertise.
Fully automated, the three-speed MSYS transmission (as it is scalable, the number of ratios can be varied depending on application) is operated by shift actuators in a similar way to an automated manual transmission. But unlike a typical AMT, it isn't hampered by sudden torque interruption that can make for rough and sometimes even violent gear changes.
The project is now in its development stage with tests under way of a company-built prototype. Tylee-Birdsall is enthusiastic about the results.
“The cone clutches have successfully completed characterization testing, which shows good linear engagement properties (torque versus applied pressure)," he reported. "This capability enables clutch shifting that is an essential part of the MSYS design. The clutch and transmission development is ongoing to improve thermal capabilities and installation in a demonstrator vehicle which will be completed in the next two months, allowing calibration to commence.”
He emphasized that MSYS has been designed to obviate the disadvantages inherent in most other transmission options. Besides the AMT's inherent torque-interruption issues, a DCT "consumes energy continually to maintain clutches in the open or closed position (depending on clutch type)," and planetary-type automatics "add complexity, cost and drag."
By comparison, the actuators in the MSYS transmission are only energized during the shifts; the friction clutches allow torque infill for smooth shifting and the efficiency, weight, package space, and bill of material is lower than that of an automatic, Tylee-Birdsall explained.
The choice of suitable ratio steps for the MSYS transmission formed an integral part of DSD’s strategy for seamless shifting. Previous experience had shown that a threshold exists at ratio steps of 1.5:1. “Below this value a shift event can be made to feel imperceptible when sufficiently accurate control of the power shift actuators is exercised,” he said.
Rapid, smooth shifting
The use of separate friction and dog clutches for each gear enables the MSYS transmission to powershift using three distinct modes: standard, optimized, and fast shift. The standard mode can be used for all shifts because it takes up the drive torque through the friction clutch before unlatching the dog clutch, shifts clutch-to-clutch into the next gear, then latches the next dog clutch once torque handover is complete.
During power-on upshifts and power-off downshifts (the most common conditions), an optimized shift mode can be employed which overlaps some aspects of the shift sequence to reduce both shift duration and unlatching forces. By taking the optimized mode to the extreme, a fast shift option is also available for these shift cases, providing a “sport” feel if required. The fast shift is effectively achieved by shifting directly from the cone clutch to the dog clutch.
“The HEV and EV market is changing rapidly, particularly with the introduction of BMW’s i3, and this is putting a new emphasis on the driveline,” observed Tylee-Birdsall. He says DSD's turnkey module is designed for OEMs who want to enter the segment but may not have the technological capabilities to do so effectively.
MSYS is scheduled to be ready for series production during 4Q16, to support what DSD is confident will be the growing demand from vehicle manufacturers needing to comply with increasingly tight CO2 emissions regulations. A family of MSYS powertrains with various power and torque outputs is planned.