At the SAE 2013 Hybrid and Electric Vehicle Technology Symposium in Anaheim this week, Owen Thunes, Senior Project Engineer, Zero Emissions Research Group at Nissan, provided an updated overview of the company’s development efforts on the upcoming FF (Front-engine, Front-wheel drive) hybrid system.
As background, in 2010, Nissan introduced the FR-Hybrid system, a front-engine, rear-wheel drive one-motor, two clutch powertrain offered in the Infiniti M. (Earlier post.) The FF-Hybrid is the front-wheel drive successor to FR-Hybrid. Like the FR-Hybrid, FF-Hybrid also uses Nissan’s one-motor, two-clutch configuration, which Nissan calls Intelligent Dual Clutch control.
Previewed in the Green Program 2016 announcement in 2011 (earlier post) and outlined further in an advanced technology briefing in Oppama in October 2012, the FF-Hybrid system is intended to be adaptable to vehicles of any size, and support applications including all-wheel drive and plug-in hybrid configurations.
There are four elements when in place that Nissan thinks will encourage wider market adoption of hybrids, Thunes said: affordable cost; meaningful fuel economy gains; a rational evolution from conventional powertrains to hybrid powertrains; and preserving cabin space and cargo area.
For several reasons, including compatibility with conventional powertrains, the ability to choose a transmission type, and better highway fuel economy, Nissan decided to focus on a parallel hybrid system—an evolution of a conventional powertrain—and then address the development of the fuel economy in city driving to approach to levels of a series parallel hybrid system such as Toyota uses, Thunes noted.
Further, within the parallel hybrid system, there is another significant benefit—the motor sizing possibility. In a series parallel system, motor power and engine power are somewhat scaled together, resulting in two electric machines—sometimes three—which are relatively large compared with the battery output power capability. In a parallel system, the engine power and the motor power are completely independent—it’s a design choice.—Owen Thunes
The FF Hybrid system that will be launched soon for front-wheel drive and all-wheel drive is built with three main technologies: a downsized internal combustion engine; a new highly efficient CVT, and a compact Li-ion battery. The system is more compact and lighter than before.
Engine. A new 2.5-liter, 4-cylinder supercharged engine replaces the 3.5-liter V6. Based on the third-generation QR25 platform which was introduced to the new Altima last year, the engine features a Roots-type supercharger, and has output and torque equivalent to the 3.5L V6. Torque exceeds the 3.5L v6 at the low to middle engine speed areas except at very very low speeds; the e-motor assist provides low-speed power, filling the gap.
There is an efficiency gain due to downsizing at low load and middle speed driving while still retaining abundant torque in the high load driving. Overall fuel economy is improved in a wide range.—Owen Thunes
CVT. CVT. The Nissan CVT is a core component of the new FF Hybrid system. With the new CVT with wider ratio coverage and the one-motor, two-clutch system, the hybrid can achieve a 40% friction reduction, which puts it on the level of a wet dual clutch transmission for efficiency, Thunes said.
A CVT has an advantage on engine operating points over a stepped gear transmission, Thunes said. The new CVT improves engine operating ability by 3% in highway mode, and by 3-5% in all modes compared to a dual clutch transmission.
It also offers an advantage with the operation of the electric motor, improving the motor/generator efficiency by about 3% compared to a fixed gear ratio. It also enables continuous regeneration by having a continuously variable transmission.
To install the hybrid system in the engine compartment of a traditional front-wheel drive vehicle, the overall powertrain length has to be set to be the equivalent of a conventional vehicle; the goal is to reuse as many of the parts as possible to keep cost down.
To do this, Nissan developed an integrated compact motor-clutch configuration which allows for the engine to be disengaged and for the vehicle to be powered by the motor through the CVT. Physically, the motor and clutch occupy the space of the conventional vehicle’s torque converter. It’s embedded within the transmission. This allows the use of the conventional vehicle transmission to reduce the cost.—Owen Thunes
The motor also functions as a generator, conveying deceleration force from the CVT to the battery. One clutch is between the gasoline engine and the input to the transmission, the other between the motor and the CVT.
When both clutches are engaged, the engine and electric motor connect directly to the CVT, allowing the electric motor to augment torque from the engine as required. When the gasoline engine isn’t needed, the clutch between the engine and motor disengages and the engine shuts down leaving the electric motor powered by the Li-ion battery to drive the CVT.
The only thing that makes it possible to handle all the functions with one motor (including restarting the engine as the vehicle drives down the road), Thunes noted, was the “amazing development over the last 20 to 30 years” of controls engineering. The controls work the wet clutch in the transmission and the dry clutch between the engine and the transmission together to phase power or slip. Smoothly starting the engine, for example, is a matter of finely balancing the clutches, Thunes said.
Design flexibility. The one-motor, two-clutch with CVT forms the core of the system. This can then be combined with various engines and various sizes of battery pack, depending on the vehicle. An SUV might use the system combined with the 2.5L supercharged engine and a smaller pack to not encroach on cabin space; a sedan or compact might use a 2.0L engine and a larger pack to extend electric driving range. The key point, said Thunes, is that the system is scalable to different applications and is designed to suit.