In 2016, just two years into production of the C7 Corvette, the eight-generation Corvette E-Ray's electric drive unit was on the drawing board. The decision had already been made to build an all-wheel-drive version of America's Sports Car. With the McLaren P1, Porsche 918, and LaFerrari coming out, General Motors knew it was time to pull the trigger on electrifying the Corvette's next generation. The Chief Engineer of the C8, Josh Holder, explained at the launch of the E-Ray just how impressive the whole car is and how the team landed on turning the very first mid-engine Corvette into a hybrid supercar.
The E-Ray is special in the way it works, even for a hybrid. From the outset, the idea was simplicity. Other hybrid supercars tend to add electric motors around the transmission, at the wheels, all over the place. That just didn't make sense here. Likewise, driving all four wheels with just the internal combustion engine to achieve AWD didn't seem as practical. "Mechanical all-wheel drive in a mid-engine car is more complicated to execute. It certainly would've been heavier," Holder told me.
In comparison, adding an electric front axle and mid-mounted battery seemed like a much better option, for several reasons. "The fact we can use [the front drive unit] in a modular way, which is very different from most performance hybrids, to recapture that kinetic energy independent of what the internal combustion engine was doing" had big advantages in terms of utility and ease of production, he said. The configuration just made too much sense not to pursue.
Making all of this happen practically is a different story. The first thing that needed to be sorted out was the drive unit. It had to be extremely small, lightweight, and powerful. After many years of development, the final result is a three-phase radial flux motor mated to two stages of reduction gears, one of which is a planetary set. 160 horsepower is sent out of its compact and lightweight magnesium case through a tiny differential. Even the fasteners are all aluminum to keep the weight down.
"Everything doesn't fit by happenstance," Holder told me. GM has the in-house expertise to build fully electric and hybrid cars, and that's what it did. "We really didn't shop it around." Everything is produced in-house, with the exception of the battery cells.
The rectangular pouch-style lithium-ion cells are made by LG, but they're not like the units you may find in a conventional electric car. Instead of striking a careful balance between maximum output current and total capacity, the E-Ray's cells are heavily compromised toward providing as much instantaneous power whenever possible, at the cost of energy density.
The battery itself is 1.9 kWh, consisting of eight 10-cell modules. These 80 cells may be separated into the aforementioned modules for practical cooling, assembly, and charge management reasons, but they're all wired in series. In other words, the voltage of each cell gets added together, while the current remains the same.
With such a small quantity of cells, this arrangement is necessary to achieve high pack voltage, which you generally want in an electric car. Horsepower is just wattage, and wattage is just amps multiplied by volts. High current (amps) means high heat, and to prevent all the complications associated with that, many automakers go for higher voltage traction systems as opposed to higher current ones. The E-Ray's pack is rated at 300 volts nominal which is a little low for a full EV but plenty for a hybrid and unusually high for 1.9 kWh of total capacity.
The problem with this setup is that the total amperage that can be delivered by the pack is only as high as what a single cell can discharge. The math to 160 hp has to add up, and in the E-Ray's pack, it does so in an impressive way
Each individual cell can provide up to 525 amps in a burst but closer to 400 continuously in normal driving. "It depends on the loads of course, but we can hold those peaks for about two seconds, depending on temps and some other factors," Holder said. This is extreme. Fully charged, just three of these cells wired in series could easily jump-start a car. Each pouch, about the size of a piece of toast, can put out around two horsepower continuously, or more in a burst. The E-Ray has 80 of them. Do the math.
Cooling was a separate issue. Most full EVs, despite many lacking grilles, have heat exchangers to manage the temperature of the battery, the inverter, and other power electronics. The E-Ray, with its ultra-powerful and compact battery pack, has the same issue, but the C8 is already packed to the gills with radiators. The solution to this issue came from full-size trucks, where the combination of an AC condenser and a radiator—known as a "conrad"—turned out to be ideal. "It was just repurposed and packaged to fit in the same condenser space that all the other Corvettes share," said Holder.
Below, you'll notice the actual cooling channels in the heat exchanger get thicker towards the bottom. This is a separate water-glycol loop that cools almost everything to do with the electric drivetrain. The top portion with thinner channels is the AC condenser.
All of this technology would work OK without advanced software controls, but once again GM has the know-how to do some seriously cool stuff with this car. When the E-Ray is on a race track, for instance, the battery could quickly be expended as high power is consistently demanded from it. For this reason, the car has Charge+ mode, which is actually very simple.
It starts by using regenerative braking whenever possible. "We call it free beer," Holder said. "Any time you can get it, take it." This is especially easy for the E-Ray since its battery cells can absorb so much power. The cells can discharge 525 amps in a burst, sure, but they can also charge at 400 amps for a short time as well. That means the car can absorb as much as 120 kW during braking, almost as much as many high-speed EV chargers can provide. Sometimes on surfaces besides dry asphalt, this amount of regen is theoretically enough to lock the front wheels, although Holder says there are advanced traction estimation systems to prevent that.
Charge+ also limits how quickly the battery can discharge by effectively metering the available current. In combination with more energy from regen, longer stints can be had. This is best illustrated in the below graph. You can also see how the car keeps a reserve of power so that it will always have all-wheel-drive capability.
There's still more, though. All new Corvettes have active fuel management, also known as cylinder deactivation. The car can run on four, six, or eight cylinders depending on the load to save gas. Thanks to the electric motor, the amount of time spent with less than eight cylinders firing can now be greatly extended. "We call it AFM extension," Holder said. "We can keep it in AFM longer using the electric motor to boost... Hilly roads are a good example of watching that happen." This means the E-Ray with its 345-mm rear tires and draggy downforce-producing accoutrements has the same combined fuel economy as a regular Stingray (19 mpg), despite having 655 hp versus the base car's 495.
Adding It All Up
Modularity was the name of the game here. The E-Ray's hybrid system is extremely compact and containerized. Skeptics of hybrid cars will typically say that they're complicated and more difficult to repair, but from the beginning, the E-Ray was designed to eliminate that concern. All of the batteries and power electronics are housed in one unit that can be dropped right out of the bottom of the car. "[It's] all one module, in fact, they're in one composite-encased housing," Holder said. "You disconnect cooling lines, [the] phase cables, and some electrical communication connections, and just drop it out of the bottom."
It really seems like GM figured this whole hybrid supercar thing out. It makes me curious why other automakers seem to insist that more motors, more battery, and more complexity are necessary. Think BMW i8 or Acura NSX. Is all of that stuff really worth it? This thing, in comparison, just works. The peak power figure is just the output of the two systems added together. It absolutely hauls and is a riot to drive. The associated weight—the E-Ray weighs just 260 pounds more than a comparable Z06—is also more than acceptable. At a time when the pure ICE-powered BMW M2 somehow weighs more than this thing, I think we can accept that hybrid cars don't have to be heavy, and this one really isn't.
The only thing really missing for me from this entire equation is more control. I touched on it in my review, but when you get two drivetrains like this in a car, the magic isn't just having them, it's controlling how they work and interact together. If I wanted to drive as far as possible in all-electric mode, I should be able to limit the peaks in terms of battery current discharge myself. Listen, I get this thing isn't my electric go-kart, but I know enough about how inverters work to understand that this kind of control is very possible.
Electric drivetrains, especially when combined with a great combustion engine, are super cool. There are words like "flux" and "phase" flying around everywhere—it's worth trying to get people enthusiastic about that. If a driver can't change their own gears, you might as well let them play around with some current levels within safe limits. Put a phase cable temp gauge on the dash so I know how the current is actually heating the windings in the motor. Give me more control over the charge and discharge curve. A way to apply regen myself would be nice, too. These kinds of things are well within the capabilities of the $100 inverters I use. Surely it's possible for GM to make that kind of thing a reality.
Nickpicky complaints aside, though, the E-Ray is easily the most impressive Corvette ever built from a technical perspective. To boil the car down to its 2.5-second zero-to-60-mph time (a new record for any factory Corvette) is a disservice to the project as a whole, which is why I avoided mentioning it. The thing is nearly a decade in development and it shows. It proves that hybrid performance cars just don't have to be conceptually complicated. I hope to see many more vehicles like it in the future, not just from GM, but from a variety of automakers. This is a winning configuration.
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