Nissan’s Dual Clutch With a Difference
Infiniti, Nissan’s premium brand, has presented its first hybrid model, the M35 sports sedan. Powered by a 3.5 liter V-6 gasoline engine, the hybrid model drives its rear wheels through a dual clutch transmission – but it’s a different take on the DCTs used by other automakers.
Nissan’s engineers wanted a simple mechanical and electrical architecture that could later be used in front drive applications, too. It was thus decided to use a single electric motor-generator rather than the usual two or more employed on most premium hybrids. For this reason, Nissan engineers have labeled the system 1M2C – for one motor, two clutches.
Yet, in contrast to the basically manual gearbox technology of standard DCT setups, the Infiniti drives through a seven-speed planetary automatic, similar to that used on other premium Nissan-group models. In place of the torque converter, however, is the 50 kW electric motor-generator, a permanent magnet annular design. A clutch on either side of this motor allows a variety of different driving modes.
For conventional open-road driving, both clutches are closed and the V-6 engine – adapted to run on the Atkinson cycle to suit hybrid operation – drives directly through to the planetary transmission. The rear clutch, between the e-motor and the transmission, is a wet clutch allowing a degree of slip on take-up so as to smoothen its operation. In this medium-speed condition, the motor can also operate as a generator to top up the lithium ion battery.
For full acceleration the clutches both remain closed, but the e-motor adds its extra torque boost to that of the gasoline engine. Nissan claims that the absence of a torque converter provides a direct-feeling driveline that will appeal to sporty drivers. The motor control system incorporates rev-matching technology to ensure it is always synchronized.
When slowing down or braking, the forward clutch opens, removing engine drag from the equation and allowing the engine to be shut down if the control system so decides. The rear clutch, between the transmission and the e-motor, remains closed, spinning up the motor unit to provide regenerative energy to feed back into the battery.
Finally, the fourth mode is that of low-speed electric drive. Once again, the front clutch is opened to keep the gasoline engine and its frictional losses out of the picture, and the e-motor drives the transmission through the closed rear clutch.
Infiniti does not say whether the gasoline engine is always restarted by the traction motor-generator; the clutch strategy here could be more complicated. Electric power steering and an electrically powered air conditioning compressor ensure comfort is maintained even when the gasoline engine is shut down.
In EV mode the M35 is capable of up to 100 km/h, according to a report in Automotive Engineering. The same article reports Nissan engineer Owen Thunes as saying that the 1M2C principle is designed to be adaptable to smaller and lighter front-wheel drive platforms. Owens revealed that, for the M35 hybrid, the target was to reach the economy of a much smaller and lower-displacement 1.8 liter C-segment vehicle.
One of the challenges during the development of the vehicle, says Nissan, was to develop a battery and a battery management system capable of switching instantaneously from charge to discharge mode and back again. Infiniti claims that the M35 hybrid’s configuration allows it to be electrically powered for more of the time than conventional hybrids, engaging for as much as 50 percent of the time the car is in motion.
As the M35 hybrid is a 2012 model, no performance, economy, or CO2 emission figures have yet been issued. Nor has Nissan provided information in certain technical areas, such as how the clutches can operate to restart the gasoline engine from the e-motor under deceleration. It would be interesting, too, to know whether the vehicle’s control strategy could encourage the planetary transmission to downshift on the over-run, thus increasing the rotational speed of the motor-generator, boosting the amount of energy harvested, and providing a stronger feeling of "engine" braking for the driver.
All these questions, and more, will no doubt be answered once production-specification models appear and full technical documentation is made available.
Story Filed: 5/03/2010
By Tony Lewin, managing editor DCTfacts.com