The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 18 years. In this installment of the 2012 Behind the 10 Best Engines series, WardsAuto looks at development of the Infiniti M35h hybrid powertrain.

When Nissan engineers set out to design and develop their own gas hybrid-electric vehicle system a half-dozen years ago, the goal was to make it better and more desirable than any other on the market, or any foreseen to arrive by the time it would become available.

Based on observations from WardsAuto judges, who called its application in the Infiniti M35h luxury sedan a "spectacular technological achievement" when naming it one of their 2012 Ward's 10 Best Engines, it appears to have succeeded.

This is another instance in which an entire propulsion system, not just the engine, is honored as one of Ward's 10 Best Engines.

Infiniti, Nissan’s luxury division, touts the $50,000-plus M35h’s "unprecedented balance" of three important attributes: power, luxury and efficiency.

The power comes from a 302-hp 3.5L V-6 teamed with a 67-hp electric motor driving the rear wheels though an electronic 7-speed automatic transmission; the luxury from the M sedan's leather-lined cabin and extensive array of convenience and comfort features; the efficiency from Infiniti's innovative "Direct Response Hybrid System."

Uniquely, its gasoline engine (a version of the VQ V-6 that lived for 14 straight years on the Ward’s 10 Best Engines list) can be decoupled and shut down nearly any time when there is sufficient battery energy to power the car by electricity alone.

One judge reported cruising in EV-only for as much as a minute at a time even at freeway speeds during an extended road trip.

"At slower speeds the Infiniti system allows the engine to shut down more often, basically any time the accelerator is released,” another wrote. “By the end of the testing, the M35h was running in electric mode 16% of the time."

That resulted in an observed fuel-economy average of 30 mpg (7.8 L/100 km) over the two weeks WardsAuto had the car (compared with its official 27/32 mpg (8.7-7.4 L/100 km) city/highway mileage, which is deemed remarkable for a large, powerful sedan.

The Infiniti HEV propulsion system, designed by Nissan in Japan, incorporates technologies also developed for the Nissan Leaf battery electric vehicle – including the electric motor and the lithium-ion battery – plus several specific to the hybrid.

While development of the battery, motor and control systems began in the 1990s, design and development of the Infiniti hybrid luxury sedan began around 2006, according to Tomoaki Momose, powertrain engineering design manager.

The overriding priority was to provide outstanding fuel efficiency "while also living up to Infiniti’s brand image with highly responsive acceleration and performance," Momose says. And he believes this single-motor, dual-clutch system provides an optimal balance of both.

WardsAuto judges found the HEVs automatic transmission smooth and precise and its transitions between gas and electric modes seamless.

They also praised the car's excellent noise, vibration and harshness mitigation, its (very non-hybrid) exhaust rumble and its performance in Sport mode. “Decadently fast, a far cry from the poky hybrids we've come to know over the past decade," wrote one judge. The M35h’s other two operating modes are “eco” and “standard.”

Momose points out that one very significant challenge was the need to achieve luxury-car levels of smoothness, especially during transitions from gas to electric power. The solution was the team's development of very precise motor-control strategies. How precise?

Cruising in electric mode in seventh gear is possible, but mashing the accelerator will restart the V-6 instantly and require a downshift to fourth gear. Momose says four steps must take place in less than a second to ensure smooth, strong acceleration.

“First, the system increases motor torque so clutch No.2 can be slipped in to relieve vibration at engine start-up. Second, clutch No.1 engages and engine rpm begins to rise. Once firing occurs, rapid motor braking is applied so the engine doesn’t over-race.

“Third, for the downshift, with engine and motor rpm synchronized, motor torque is increased to push rpms to match fourth gear.

“Fourth, once the desired rpm level is reached, rapid braking of the motor is applied and clutch No.2 goes into full lockup. Then the motor assist comes back on again for acceleration," Momose says.

Because the electric motor and one clutch are packaged in the space typically occupied by the transmission's torque converter, the unit is essentially the same size as the conventional 7-speed automatic in the Infiniti M37 and M56.

Combined with the compact size of the 1.4-kWh, 346V Li-ion battery (located behind the rear seat), this design gives the M35h the same interior roominess as the standard M sedan, plus sufficient trunk space for two large suitcases or four fullsize golf bags.

Another design that helped achieve the M35h Hybrid’s needed smoothness and refinement was a simple but effective lightweight soft-mounting system for the electric motor. It consists of a plate on the front of the motor, three arms on the rear and two rubber O-rings.

This is the world’s first parallel hybrid system without a transmission torque converter, Momose says. And it offers better fuel economy than its competition in both city and highway driving despite its high-performance capability.

The system’s single electric motor has a significant weight advantage over a 2-motor arrangement, and its forward dry clutch between the gas engine and the electric motor can disengage the engine for higher efficiency.

Nissan’s Li-ion battery technology is lighter and has higher power density than the nickel-metal hydride batteries in most other HEVs, which provides higher motor power and more EV-mode range.

As in other HEVs, the motor doubles as both a propulsion unit and a generator that recovers energy otherwise lost during deceleration and braking. Its instant torque can boost acceleration at low speeds, and it powers the car by itself when the engine is decoupled.

Most competing hybrids, including those built by Toyota and Ford, use a parallel system with two motors and a planetary gear set, which is heavier and less energy efficient.

"That type of system cannot decouple the engine and motor," Momose says. "So the result is lower efficiency because of engine friction under EV drive conditions."

He adds that the Mercedes-Benz S400 HEV uses a parallel-hybrid system with a torque converter that cannot disengage between engine and motor, while BMW's 5-Series ActiveHybrid system does use a single-motor, 2-clutch, no-torque-converter system, but also (unlike the Infiniti) a starter motor.

Momose reveals that Nissan is working on a single-motor, two-clutch HEV system for front-wheel-drive cars that he expects will reach North America in 2013. Interestingly, it will mate a continuously variable transmission to a 2.5L supercharged 4-cyl.engine.

We're definitely looking forward to testing that one.