Most Disruptive Powertrain Gets Even Better

Ward’s 10 Best Engines competition has recognized outstanding powertrains for 22 years. This installment of the 2016 Behind the 10 Best Engines series looks at development of GM’s second-generation EREV.

The Chevrolet Volt story began nearly a decade ago at the concept’s late-2006 press introduction by former GM Product Development Vice Chairman Bob Lutz.

Publicly unveiled a month later at Detroit’s 2007 North American International Auto Show, the intriguing concept previewed the world’s first production extended-range electric vehicle.

Unlike any other EV or hybrid of the time, it would be powered solely by its battery for the first 40 miles (61 km) or so until it was depleted. Then, during a seamless transition, electricity created by a generator driven by a small gasoline engine under the hood would power the car’s drive motors.

It was an emissions-free battery-electric vehicle that could morph into a fuel-efficient gas-powered compact once its 16 kWh of battery energy was gone. The idea was to eliminate EV range anxiety, the sweaty-palm fear of running out of juice before running out of trip.

Soon after the production Volt hit the streets four years later, it was named one of the 2011 Wards 10 Best Engines (or in this case, “propulsion system”). WardsAuto editors said it was “a car that revolutionizes mobility” and “a magnificent achievement that marries the best attributes of all-electric vehicles with those of fun-to-drive, fuel-efficient gasoline cars.”

So it is no surprise the vastly improved ‘16 Gen II Volt easily earns a 2016 10 Best trophy.

“General Motors has taken what already is the most innovative and disruptive propulsion system in the auto industry and made it lighter, stronger and a lot more fun to drive,” says WardsAuto editor Drew Winter.

“The second-gen Volt is considerably more responsive and torquey,” adds editor Bob Gritzinger, who (like fellow editor Christie Schweinsberg) is a satisfied Gen I Volt owner.

The ‘16 Volt is much more attractive inside and out with more user-friendly controls and room for three rear passengers (although it’s tight in the middle).

It’s also lighter, quicker, more agile and more fun to drive. Its new propulsion system includes a 1.5L Atkinson-cycle I-4, two electric motors, two planetary gear sets, three clutches and a new-design lighter, lower, more energy-dense lithium-ion battery pack good for 18.4 kWh (vs. the 2015’s 17.1 kWh).

Its EPA-official EV-only range is 53 miles (85 km), 40% more than the previous 38 miles (61 km). This writer and a co-driver achieved an even better 58 miles (93 km) on twisty two-lanes before the engine quietly started to continue our drive. Its EPA combined economy in range-extending mode is up from 37 mpg (6.4 L/100 km) to 42 mpg (5.6 L/100 km).

Chevrolet’s electrified vehicle engineers achieved all this by rethinking and redesigning both the car (based on GM’s new ‘16 compact Chevy Cruze architecture) and its entire Voltec powertrain, from the 100-lb. (45-kg) lighter 2-motor drive unit to the direct-injected 1.5L range-extender engine to the control wizardry that (once the battery is depleted) continually finds the most efficient blend of electric and gasoline torque.

Gen I Volt customers provided excellent feedback. “Our generation-one customers were very vocal on what they liked and what else they wanted,” says GM General Director of Electrification Tim Grewe. “Everyone wanted more EV range and more fun-to-drive factor.”

Beyond those were the usual next-generation-vehicle objectives, including lower mass, lower cost and better packaging, both inside the cabin and under the hood. “Those goals were clearly identified, then we sent every group off to determine how well they could meet them to make the car better in every way,” Grewe relates, “starting with the powertrain architecture.”

Using Both Electric Motors Simultaneously

One significant improvement was the ability to use both motors at any time, as motor or generator, in EV or range-extended mode, which enabled a 60-lb. (27-kg) lighter drive unit. Another was mounting the power electronics directly on top of the drive unit, which got rid of the Gen I’s big, heavy, expensive orange connecting cables and another 40 lbs. (18 kg).

More mass reduction came from eliminating one of the Gen I’s two drive unit oil pumps, says Jason Ditman, Global Chief engineer, Hybrid and Electric Drive Unit. “In the first-generation gearbox we used a high-voltage electric pump when driving electrically and a mechanical pump in extended range mode. But we had high enough reliability to go with only the electric pump, which saved a significant amount of mass and a lot of real estate inside the gearbox," Ditman says. "We also implemented a chain system to transfer torque from the engine to the output drive.”

And the 40% improvement in EV range? “Like mass, which comes off by the 10th of a gram,” Grewe says, “range goes up with every detail. Our motors are more efficient, our battery discharge is more efficient, our inverter efficiency is higher and our drive-unit spin losses lower. Yet we also improved fun-to-drive, with 30% better launch and low-end acceleration, because that’s a very pleasing feel.

“One significant challenge was ensuring the drive-unit-mounted integrated power electronics could survive significant powertrain vibration, especially if the engine misfires. Another was the redesigned battery pack, which is 30 lb. (14 kg) lighter and 20% more energy-dense with 10% more power.

“The battery has to meet life requirements as well as range and environmental requirements,” Grewe says, “which involves cell chemistry, the way you mount it, its interconnection and its cooling.”

Besides 20% more power (101 hp) and 10% more torque (103 lb.-ft. [140 Nm]) vs. the previous 1.4L, the new 1.5L boasts a high 12.5:1 compression ratio and cooled exhaust-gas recirculation. And, unlike the Gen I, it burns regular gasoline.

“This engine was specifically designed with high compression and cooled EGR to be most efficient (in the rpm range) where the drive unit and the motors are most efficient,” Grewe says. While it shares many parts with the new 1.5L I-4 in other GM products, its combustion process and calibration are specific to the Volt.

The Gen II Volt drive unit essentially is the same one used in the ‘16 Malibu Hybrid, Ditman says. The only five parts that are different are the transmission damper (because it’s tuned to the Malibu’s 1.8L engine), the A-motor’s rotor and stator (it changes back from a ferrite to a rare-earth-based motor), the B-motor’s stator and a small change in gearing.

There was early controversy around the Gen I system because the range-extender engine could help power the wheels under some conditions, yet that capability is enhanced with this one.

“It is most efficient at all times,” Grewe explains. “A real-time optimizer senses driver, road load and environmental demands and continually searches about 100 times a second for the most efficient operating points, which are sometimes in the parallel mode. I think the world has gotten over that being controversial.”

And impressive as this Gen II system is, Gen III surely will be better. “It is still a highly evolving technology,” Grewe confirms, “and at some point you have to go to production. But the beauty of this is that every component’s technology is improving, and as the components improve, along with the integration and controls, there is a lot of opportunity to re-optimize.”