Update on Nissan’s 1-motor, 2-clutch hybrid system
20 February 2014
Nissan’s 1-motor, 2-clutch parallel hybrid system (1M2CL), first introduced in 2010 for the rear-wheel drive Infiniti M35 Hybrid (earlier post), has since grown into its second-generation rear-wheel drive system as well as spawned a front-wheel drive variant with CVT (earlier post). The system and its offshoots are now applied in 4 Infiniti and Nissan hybrid vehicles—Q50, Q70, QX60 and Pathfinder.
At the recent SAE 2014 Hybrid & Electric Vehicle Technologies Symposium, Tomoaki Momose, manager of powertrain development from Nissan Technical Center North America, gave an update on the technical evolution of the core hybrid system.
2nd-generation 1M2CL rear-wheel drive. The 1M2CL system in the M35 Hybrid introduced in 2010 uses an original parallel one motor/two clutch design and features a decoupling 3.5-liter V6 engine; a 50 kW, 270 N·m (199 lb-ft) electric motor integrated into the 7-speed automatic transmission (7AT); and a high-output, fast charge/discharge 1.3 kWh Li-ion battery pack.
For the second-generation of the RWD 1M2CL system as applied in the Infiniti Q50 Hybrid, Nissan focused on improving all around performance. Main improvements to the RWD 1M2CL system included:
Reduced friction in the automatic transmission. Nissan engineers achieved a 6% reduction in friction via a change in clutch material and structure, as well as changing the operation of the electric sub-oil pump operation change. This boosts fuel economy.
Increased the maximum motor torque and battery power. Motor torque increased from 270 N·m in the first generation to 290 N·m. This enhances acceleration and smoothness, and also enables expanded EV driving.
Boosted battery power. The second generation sees a 15 kW increase in maximum battery power from 60 to 75 kW. Continuous power stayed the same at 50 kW. This also enhances acceleration and smoothness and extends EV driving.
Improved the response of the clutch-1 control system on engine start. Nissan engineers reduced the response time by 15%. The standby piston stroke for the start of clutch engagement is kept as short as possible. In addition, Nissan improved the hydraulic pressure circuit.
Nissan also reduced engine starting time by 20%. The new controls estimates the external disturbances via a disturbance observer and reduces the margin torque by 70% and redistributes torque for both clutches.
Combined, the second-generation controls reduce engine starting time by 35%.
Improved engine BSFC. The BSFC of the engine improves by up to 5%, primarily through improved tumble and squish gas flow enabled by a new tumble port and valve shape, as well as refined Atkinson valve timing.
|The performance improvements in the second-generation 1M2CL hybrid system in the Q50 Hybrid result in speedier acceleration (left) and fuel economy improvements (right). Source: Momose, SAE HEVT 2014. Click to enlarge. Click to enlarge.|
With the second-generation, all-electric range increased. The EV mode operation ratio increased from 47% EV, 53% EV in the Infiniti 2012 M35h to 51% EV, 49% HEV in the Q50 Hybrid in real world driving in San Francisco. Further, the speed range in which all-electric mode is available also increased.
|System architecture of the FWD 1M2CL system. Click to enlarge.|
1M2CL front-wheel drive. To support front-wheel drive vehicles, Nissan took the technologies of the original 1M2CL system and combined them with an XTRONIC CVT, working together with a compact lithium-ion battery and a 2.5-liter supercharged engine. This system is now applied in the QX60 and Pathfinder hybrids.
The compact clutch and motor design enable the installation of the 1M2CL system in the engine compartment of a FWD vehicle. The clutch and motor are installed co-axially in the space of the torque converter. A compact dry multi-plate clutch enables the overall powertrain length to be equivalent to that of a conventional vehicle.￼
Nissan faced three issues with adoption of a dry multi-plate clutch in the CVT, Momose said. While the advantage is low drag friction and packaging, Nissan found challenges with (1) uneven abrasion of facing surfaces; (2) drag friction by plates sticking; and (3) abrasion powder accumulation.
|Packaging of the FWD unit. Source: Momose, SAE HEVT 2014. Click to enlarge.|
Uneven abrasion was occurring due to contact pressure unbalance, affected by piston position and plate stiffness. Nissan devised a mechanism to provide uniform surface pressure, ensuring durability and torque capacity.
Drag friction was caused by contact between plates during the disengagement of Clutch 1. Nissan found that air flow optimization could reduce drag friction; engineers designed radial grooves and air holes in the plates to enable air flow assistance to the plates during disengagement.
With the design of the unit, the clutch is located inside the electric motor space. Abrasion powder accumulates by centrifugal force. Nissan designed the clutch shape with optimizing air flow to use the air to move the powder.
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