GM Engineers Discuss the 2017 Chevy Bolt’s Powertrain

To date Chevrolet has accepted zero pre-orders for its 2017 Bolt EV, it makes no attempt to one-up fashionable German sedans, but it does have a few things going for it.

One is the purpose-built electric car will be for sale and in production at the end of this year – a year-and-a-half or more ahead of Tesla’s Model 3 – and will thus be the first 200-plus-mile EV priced from the mid 30s.

While the public and press are still dazzled after last Thursday’s Tesla reveal and nearly 300,000 paid reservations that piled in within a couple days after the event, yesterday General Motors in more quiet fashion spoke of the merits of its EV’s powertrain.

Greg Smith, Engineering Group Manager, Electrification, left, Tim Grewe, General Director, Electrification, center, and Stephen Poulos, Global Chief Engineer, Electrification, right, pose with a Chevrolet Bolt EV battery pack and drive unit in General Motors Global Battery Systems Laboratory at the GM Technical Center in Warren, Michigan, Tuesday, April 5, 2016. (Photo by Jeffrey Sauger for General Motors)

Greg Smith, Engineering Group Manager, Electrification, left, Tim Grewe, General Director, Electrification, center, and Stephen Poulos, Global Chief Engineer, Electrification, right, pose with a Chevrolet Bolt EV battery pack and drive unit in General Motors Global Battery Systems Laboratory at the GM Technical Center in Warren, Michigan, Tuesday, April 5, 2016. (Photo by Jeffrey Sauger for General Motors)

The occasion was a tech talk led by the automaker’s chief engineers at its Global Battery Systems Laboratory in the GM Technical Center in Warren, Michigan.

Although a couple journalists tried to lure them into comparing the Chevy compact crossover to the Model 3, they deftly ducked those and stayed on point.

Be that as it may be, it is long a matter of record that GM did build the Bolt at least in partial response to Tesla’s proposed $35,000 EV now known to have at least 215 miles range.

Headlines since last year also have included the term “Tesla killer” describing the Bolt to provoke reader clicks online, but GM this week was not going there, and let the facts speak for themselves.

Legacy

GM said independent market research had shown that 70 percent of would-be EV buyers indicated they would be sufficiently content with 200-miles range to the point that they could live with it in their one and only car.

Customers

The mission to build a 200-mile EV was spilled in 2013 by then-CEO Dan Akerson. At the time the news received an approving tweet from Elon Musk.

GM has since said it has built on lessons learned with the Chevy Volt, Spark EV and other experience garnered along the way.

The track record with the Volt’s powertrain has been especially stellar as noted last year by Larry Nitz, General Motors’ vice president of electrification.

Erick and daughter Erin Belmer with his 2012 Volt.

Erick and daughter Erin Belmer with his super-high mileage 2012 Volt.

“We’ve seen what I would call pharmaceutical levels of quality in cell production,” he said during a Volt ride and drive event. “Of the more than 20 million cells that have been produced for the first generation Chevy Volt, we’ve seen less than two problems per million cells produced.”

Volt batteries and drive units have been reported as durable. The highest-mileage Volt known now with over 300,000 total road miles and 105,000 EV miles and belonging to Erick Belmer of Ohio holds about as much energy in its T-shaped battery as it did when new.

As for the Spark EV, it is sold just in California, Oregon and Maryland but GM shoehorned an over-powered electric motor in it, and it too was a source of data and some hardware also for GM to build the Bolt.

Enter Bolt

Psumary

The Bolt EV at 164 inches long, 70 inches wide, and 63 inches high is still smallish, but larger than the tiny Spark EV and boasts outsized interior space utilization.

propulsion

Its 0-60 mph time may be in the six-second range, though GM is only saying below seven seconds at this point. Zero to 30 comes in a right-quick 2.9 seconds from the front-wheel driver.

Its range is to be more than 200 miles from its nominally 60-kilowatt-hour in-floor battery assembly. Propelling the EV is a single-motor drive unit developed in-house by GM.

Motor

The Spark EV uses an existing GM motor design which has a lower maximum speed of 4,500 rpm but higher torque output of 327 pounds-feet (444 Nm).

Horsepower for the Spark EV’s motor is 105 kilowatts (140 horsepower) and the new Bolt EV has 150 kilowatts (200 horsepower).

Chevrolet Bolt EV drive unit. (Photo by Jeffrey Sauger for General Motors)

Chevrolet Bolt EV drive unit. (Photo by Jeffrey Sauger for General Motors)

Stephen Poulos, global chief engineer, Chevrolet Bolt EV propulsion system, said the Bolt’s new motor design has a higher max speed of 8,810 rpm, but lower torque at the motor of 273 pounds-feet (370 Nm).

It also has a higher fixed gear ratio of 7.05 versus 3.87 which allows higher final torque at the axle – approximately 1,844 pounds-feet (2,500 Nm) versus 1,261 pounds-feet (1,710 Nm).

The Bolt’s new motor uses bar-wound copper with “lots of copper fill” and end turns, and “takes a lot of experience and skill” to produce, but the automaker has become “pretty good at it” he said with deliberate understatement.

driveunit

That said, the design is not otherwise exotic or unusual but the motor did receive a lot of computer optimization. This followed proprietary GM software to yield weight efficiency versus motor cost across multiple typical motor operation rpm and torque load regions.

The Bolt’s motor reaches a peak efficiency of 97 percent under some conditions – very high.

Battery

The Bolt’s large in-floor battery uses LG Chem cells that have evolved beyond what the Volt gets with a chemistry providing a higher balance of energy than power in the power-to-energy ratio, and best suited for EVs, says GM.

Warranty: 8 years or 100,000 miles. Actually, said GM, it's designed to last for the life of the car – though they did not specify the precise mileage defining a car's lifespan.

Volumetrically, the Bolt battery is nearly double the size (2.05 times the size) as the Spark EV’s but has more than triple the capacity (60 kilowatt-hours versus 18.4 kwh for the Spark). This is because the pack energy density is 60-percent greater. Warranty: 8 years or 100,000 miles. Actually, said GM, it’s designed to last for the life of the car – though they did not specify the precise mileage defining a car’s lifespan.

Almost all of the energy is used in a full discharge cycle, and very little is left as a “buffer” as in the case of the Volt extended-range EV.

GM’s Greg Smith, engineering group manager, global EV battery packs avoided calling the battery “better” but said it is bleeding edge, he is “proud” of the total result, and it’s the best they have for the application.

The LG Chem cells are of NMC chemistry with extra nickel for improved heat tolerance, said Smith.

Further, the pack will be allowed to run a bit warmer than in the Volt. Temperatures in the pack change slowly, he said, due to the entire assembly’s 960 pound mass.

battery topology

Speaking also of the assembly, it is an integral member of the chassis. Smith said the tray – or bottom of the battery pack – is “extraordinarily stiff” and adds a substantial 28 percent to the body’s torsional rigidity.

Overall, the pack is “ridiculously rigid” and is made up of multiple layers.

Any crash forces during an accident shouldn’t get into the pack due to carefully positioned structural cross bars.

More to Come

Obviously the Bolt is more than an optimized and well-sorted powertrain.

From the outset the mission was to make a “super functional” vehicle, said Tim Grewe, general director, electrification.

Bolt_EV

The Bolt is also a marvel of space utilization with flat floor, good ingress and egress, and room for five people plus cargo.

Features include a 10.2 inch touchscreen with Chevy MyLink, gads of safety tech, and it represents an integrated holistic approach as General Motors’ first purpose-built EV since the EV1.

Beyond “regen on demand” and a paddle to control this, GM built in “one pedal drive” where the regen can bring the Bolt to a stop with no creep, so the driver may use just the accelerator in this mode.

Bolt - pre-production

Chevy Bolt is being readied for consumer orders starting at the end of this year. Tesla and Nissan are also due to have 200-mile EVs.

Total range should be within the ballpark of the Model 3, but GM’s Kevin Kelly said the EPA certifications are a “couple months” away and for now it’s only saying over 200 miles.

So stay tuned for that and other news GM will release as Chevrolet prepares to begin selling the 2017 Bolt EV through its nationwide dealer network late this year.