Electric Cars Take Dual Clutch Transmission Lessons
It has long been the secret fear of transmission manufacturers that a wholesale shift away from combustion-engined cars to electric vehicles would be bad for business. Rather than the sophisticated and pricey multispeed gearboxes demanded for diesel and gasoline models, electric cars could get by with a simple off-the-shelf, single-speed transmission, valued at just a few dozen Euros. And the fact that the electric motor produces its maximum torque from zero rpm means that there would be no need for a clutch or torque converter assembly, either.
At least, that’s the way it started. Almost without exception, all the first-generation electric cars such as the Nissan Leaf, Mitsubishi MiEV and smart electric drive had single speed drives. The smart’s was, in fact, the combustion car’s six-speed unit locked into a single ratio. Even the Tesla Roadster, with its dazzling performance, went over to a single speed drive after the motor’s huge torque proved too strong for early two-speed models, which were retrofitted with the fixed unit.
But now, with electric cars fast moving into the mainstream of the auto business and anxious marketing departments eager to promote superior range on a full charge, attention has shifted toward getting the most effective − and least compromised – performance out of each battery charge. The ratio chosen for a single speed motor drive is inevitably a compromise: on the one hand, low enough to provide sufficient torque at the wheels for a fully-laden hill start, perhaps with a trailer in tow and, at the other end of the scale, high enough to provide an acceptable maximum speed on the highway. The problem is especially acute in electric light commercial vehicles, where the payload is high in relation to the vehicle’s weight and power.
And, while most studies show that single- and two-speed EVs use broadly similar amounts of energy over the statutory drive cycle, the consensus is that the two speed will give greater range in general driving thanks to its taller top gear. The lower first gear provides a double benefit, improving gradeability and allowing the use of a smaller, and therefore cheaper and easier to package, electric motor.
High-profile electric car designer Gordon Murray concedes that multiple speeds are unnecessary for his T27 city commuter car. There is an emerging consensus, though, that for faster out-of-town EVs, two or more gear ratios do provide major benefits to acceleration, top speed and driving range. Vocis Driveline Controls is now demonstrating a clever system in an electric minibus that is able to shift gears without a conventional clutch.
Dual Clutch Architecture
More recently, Vocis has announced an even more ingenious four-speed transmission for EVs, this time mimicking the architecture of dual clutch transmissions used on ICE vehicles – though again, avoiding the use of clutches.
The driveline is radically different in its configuration. It uses twin electric motors, each feeding one end of an input shaft divided in the middle. In its internal layout it is much like a dual clutch gearbox, but without the clutches; the final drive is taken off of the center of the output shaft.
The transmission’s operation is much like a DCT, too, with the next ratio pre-engaged in preparation for the next shift. The motors are able to speed up or slow down the gear pairs so as to eliminate shift shock and torque interrupt.
The concept’s ingenuity goes even further. The energy absorbed by one motor when slowing down one gear pair is fed back to the next gear pair via the second motor to speed it up. The smaller electric motors are helpful in terms of packaging, too, making for a flat assembly that could fit below the floor.
“The concept is eminently scalable,” says Mike Everitt, managing director of Vocis. “It’s set for four speeds at the moment, but it could easily be three or six.”
The launch application for the transmission is the French Furtive eGT supercar built by Exagon motors, based at the Magny Cours circuit in central France. Powered by two Siemens electric motors of 125 kW each, it boasts peak torque of 480 Nm; it is thus impressive not only in its looks and specification, but in its performance, too. Top speed is regulated at 250 km/h, though 287 km/h is technically possible. 100 km/h is reached from rest in 3.5 seconds, and maximum range on the standard battery, incorporated into the floor structure, is 402 km. The optional range extender motor increases this to 807 km – enough, observes the company, to go from Paris to Nice on a single charge.
Genuine Dual Clutch
Getrag has broken new ground with a modular electric drive concept that pairs a single electric motor with a two-speed gearbox fitted with dual wet clutches. The arrangement allows ratios to be shifted under load without any torque interrupt. Electro-hydraulic actuating elements control the clutches to make the shifts in advance, with sufficient overlap.
Labeled the 2eDCT600, the new module can be configured for either axially parallel or coaxial drivetrain assemblies. And, as it incorporates modular parts from many other Getrag products, it can be scaled up or down to suit the application.
Maximum input torque is 500 Nm, with a peak output figure of 6,000 Nm. Both clutch cooling and gear set lubrication are electrically controlled, with the option of integrated cooling of the traction motor, too. Dry weight is quoted at 42 kg, and a park lock can also be specified.
Getrag says that, compared to known one-speed electric drives for hybrid or purely electric vehicles, the use of multiple gears offers better starting performance and an increased maximum speed. “Depending on the configuration of the powertrain, considerable energy savings can be achieved; these savings result in an extended range of up to 10% in electric mode,” says the company.
Single Clutch, Manual Transmission
That is the unusual arrangement for an unusual and highly innovative car: the lightweight +E electric sports car that, perhaps surprisingly, comes from ultra-traditional British sportscar maker Morgan.
In response to the brief to produce a simple, lightweight, rear-drive electric sportscar, Morgan has taken as its basis the Aero Supersports chassis and packaged an axial flux electric motor in the transmission tunnel ahead of the clutch and a standard five-speed manual gearbox.
“The emphasis is on fun,” says Neil Cheeseman, engineering program manager for driveline supplier Zytek. “As it’s an electric motor, you don’t need to use the clutch for starting or stopping, but you do dip it to shift gears.”
A further advantage of this arrangement, says Cheeseman, is that the driveline does not have to be dimensioned to cope with the high loadings of “idiot starts” performed by over-zealous drivers. “Maximum torque is from zero rpm, so you don’t need to rev it and sidestep the clutch for a fast start,” he notes wryly.
Story Filed: 11/29/2011
By Tony Lewin, managing editor DCTfacts.com