Why the Chevy Volt Should Drive Its Wheels with Gas
This opinion piece was contributed by Chris Ellis, CEO, HyKinesys.
The UK’s Telegraph caused a stir in June when it speculated that G.M. soon might turn the Chevy Volt into a hybrid by directly linking the engine to the wheels. HybridCars.com also reported that the 2011 model already has hardware, like a planetary gear set, which makes it possible for the Volt to work more like a hybrid than an electric car. Here’s the question that hasn’t been discussed: Why would G.M. ever want to use the engine to directly drive its wheels?
In a nutshell, the main reason is to make the car more efficient and economical when cruising at high speeds. It’s the same reason lock-up clutches were added to automatic transmissions decades ago—to improve efficiency at more than 50 mph in top gear.
How It Works Now
Let’s be clear. I’m not talking about when the Volt’s battery pack initially has more than 8 kilowatt-hours remaining of its total of 16 kilowatt-hours. As long as there’s enough juice in the battery, it operates just like a battery-only vehicle. The engine stays off until the battery drops below the trigger level. Only at that point does the engine start up and the Volt begins to operates as a charge-sustaining series hybrid. The control system doesn’t attempt to re-charge the pack much above the trigger level. Full re-charging must wait until the Volt is next plugged in.
When the engine is running, the principal flow of energy is from the gas tank via the engine and its generator to the electric motor connected to the differential and the front wheels. The battery is still involved, but only as an energy buffer during regenerative braking and strong acceleration.
How It Would Work
In most driving conditions, nothing would change—until the Volt’s (hypothetical at this point) direct drive mode would be called into action when the Volt reaches cruising speeds. At that point, the engine would be mechanically connected to the differential, bypassing the electrical pathway. The motor and battery pack will still be involved during strong acceleration and regenerative braking, but the engine will maintain cruising speed and provide mild acceleration by the most efficient means possible—that’s right, by putting power directly from the engine to the wheels.
When the engine is running, the Volt will operate as a series hybrid in the city but it will become a parallel hybrid above, say, 50 mph. Direct drive should be able to transfer energy from the engine at upwards of 95% efficiency at 70 mph.
Compare that level of efficiency with what happens with the current Volt at highway speeds. Although each element of the electrical pathway is relatively efficient, the losses start to multiply because there are four devices connected in series: the generator, two controllers and the electric motor.
Let’s assume the generator operates at 93% efficiency. It passes current to its controller, which is liquid-cooled because it also has losses. Let’s say this controller operates at 97% under these conditions. It passes current to the motor’s controller that also operates at 97% in passing current to the motor. Finally, the motor transforms electrical energy into mechanical energy at 92%.
Consequently the overall efficiency formula of the electrical pathway is something like this: 93% x 97% x 97% x 92% = 80.5%. Apologies for the theoretical math, but it’s important to understand how the losses multiply. You can see how a Volt with direct drive might need about 15% less fuel when traveling at 70 mph, compared to the current indirect drive.
A Bonus: Highway Acceleration
Adding the parallel option to the Volt not only promises reduced freeway fuel consumption but it also opens up the possibility of better acceleration. Early road tests suggest that the Volt’s initial acceleration is competitive, but there have been complaints that the Volt is sluggish over 50 mph, when the torque curve of the electric motor tails off.
With direct drive, the power of the engine will be added to that of the electric motor. In its simplest form, the additional power will be relatively low because the direct drive will be geared to deliver the highest possible mpg at around 70 mph. After all, the name of the game is efficiency and reduced use of petroleum, not more oomph on the highway or even “electricity at all costs.”
Making the Chevy Volt more of a plug-in parallel-series hybrid might be heresy to electric purists and Volt true believers. It also creates a slight challenge for G.M. marketers who have insisted that the Volt is a pure electric car (despite having a gas engine on board). But at the end of the day, G.M. executives and engineers hopefully will be guided by an ethos of maximum efficiency and good economics—and delivering the best possible ride using the least amount of energy.