The architecture of Nissan/Infiniti’s new parallel one motor/two clutch system. Source: Nissan. Click to enlarge.

The first clutch is installed between the naturally-aspirated 3.5-liter V6 and the electric motor. This clutch allows the full decoupling of the V6 when the systems is in either electric or power regeneration modes, thereby reducing mechanical drag and boosting the efficiency of the electric motor. The second clutch transmits the drive force.

Nissan opted for this particular design, Thunes said, after a thorough modeling comparison of a range of hybrid systems including series-parallel (power split) and parallel architectures, with a particular focus on fuel consumption and acceleration, but also drivetrain loss, packaging and cost potential.

The scope of Nissan’s study of hybrid architectures. Source: Nissan. Click to enlarge.

Nissan found that on a V8 5.6L system, two variants of the parallel system, a 7AT 1Motor-2Clutch, and a 7AT 2Motor-2Clutch, outperformed all the other systems for fuel consumption on the US combined cycle. With a 3.5L and CVT-based (continuously variable transmission) system, the 1Motor-2Clutch and 2Motor-2Clutch types bested the power split systems in terms of acceleration, and performed as well in fuel consumption.

Likewise, in a 2.5L and CVT case, the CVT + 1Motor-2Clutch and 2Motor-2Clutch systems outperformed the power split systems in terms of fuel consumption and acceleration. In a 1.5L-CVT case, the CVT+1Motor-2Clutch and 2Motor-2Clutch systems outperformed all the others in the terms of fuel consumption.

The study also concluded that the 1 motor parallel system had the most attractive cost structure.

To be able actually to implement the 1 motor/1 clutch system and achieve the performance and cost breakthroughs, Nissan needed to develop an integrated control technology to regulate battery, inverter, motor, clutches, engine and transmissions simultaneously. A high-output Li-ion battery capable of switching between charging and discharging very rapidly was required, as well as high-speed and precise motor control to allow a smooth connection between the two clutches.

The ability to connect and disconnect the engine offers a number of benefits, Thunes said.

• At low- and mid-speed, frequent electric driving can be realized without engine friction. In a three-month US-based test program that incorporated a large range of use, the M35 Hybrid was able to drive in electric-only mode for as much as 50% of the time.

• At mid- and high-speed, the engine can be used in the efficient operating range by generating ower with the motor and the high final gear ratio. The system is highly efficient because the clutch transmits the engine torque directly.

• In deceleration (regeneration), disconnecting the engine eliminates engine friction.

Layout of the system components. Source: Nissan. Click to enlarge.

The M35 Hybrid’s transmission is a more sophisticated version of Infiniti’s 7-speed Adaptive Shift Control (ASC) automatic transmission. One difference over the ASC transmission used in other Infiniti models is that there is no torque converter, and therefore none of the losses that torque converters cause.

The lack of a torque converter also enhances responsiveness for a sportier-feeling drive. The throttle can be “blipped” just as in a manual transmission car, while acceleration is seamless thanks to motor-assisted gear-shifting that ensures ratio changes are completely smooth.

Apart from its fast charge/discharge response, the Li-on battery pack is built with laminated cells, enhancing its cooling performance. Temperature stability is further improved by the use of manganese cathodes. An end result of this is exceptional battery reliability, Infiniti says.

The technology, which previewed in the Essence concept coupé at the Geneva Motor Show in 2009, has been given the name Infiniti Direct Response Hybrid.

The M35 Hybrid will be the first Nissan hybrid offered in the US, Europe and Japan markets. The vehicle is due to arrive in Western Europe from spring 2011, depending on market. Power outputs and performance data will be confirmed at a later date.