The Great Chevy Volt Hybrid Mishap Explained
Of all the information spilling out of the Chevrolet Volt's national media launch in Detroit this week, none is more surprising to us tech heads than news that the Volt actually does, at times, power the drive wheels directly with the gasoline engine.
More specifically, at some speeds it contributes to the propulsion effort, but only once the battery pack has run down and the engine has turned on to run the generator. It never drives the wheels on its own, only as a hybrid, using both motor and engine.
How is this possible?
Now we know two interesting things that change our understanding of how the system works. First, the generator is actually a motor-generator that's tied not just to the gas engine but also to the other components, including the drive wheels. In lieu of a conventional transmission, there's a power-split device similar to the type in hybrids from Toyota and Ford. (There's some similarity with GM's 2 Mode system, too, but the latter uses a conventional transmission, and a power-split device essentially takes a conventional transmission's place.) The second thing is that the motor-generator and gas engine are both tied mechanically to the drive wheels.
Rather than a generator attached to an engine off on its own somewhere, the generator is actually a motor-generator tied into a planetary gearset along with the main drive motor, the gas engine and the wheels. Actually, the motor-generator can contribute to propelling the car along with the main "traction" motor. It's a method Volt engineers say is preferable to a single, high-rpm electric motor.
There are three clutches, as represented in the accompanying photo. One (called clutch No. 1) is between the ring gear and the case. Clutch No. 2 is between the motor-generator and the ring gear, and clutch No. 3 is between the motor-generator and the gas engine.
At speeds above 70 mph, the gas engine contributes its motive force to the wheels, and Volt powertrain engineer Pam Fletcher said it can also play a part at speeds as low as roughly 35 mph. The result is a performance boost and an efficiency improvement of 10 to 15 percent versus if the engine worked just to supply energy to the battery and electric motor.
The buzz around the internet — and at this event — suggests the world will soon come to an end because the Volt isn't what people thought it would be, that it's somehow a lesser vehicle. I don't see it. Once the engine starts, the point is efficiency.
I liken it to the fixation people have on electric-only driving in conventional hybrids (and their future plug-in versions). Driving for some number of miles on electric power alone might seem satisfying to drivers of, say, a Prius, but if that doesn't make it as efficient as it could be, what's the point?
It's a similar notion for the Volt, though the car is designed for something else entirely: to be gas-free for about 40 miles (roughly) of electric-only driving, not to get the lowest possible mpg rating in sustained driving, as conventional hybrids are. (To that end, the engineers estimate the Volt's good for 35 to 40 mpg in range-extending mode, once the engine starts up, and that's nothing to get excited about.) It seems to me the detractors would be more satisfied if the gas engine didn't power the wheels at all and the car got 25 mpg with the engine running.
This does raise the question of why folks at Chevrolet didn't release this information earlier. The answer, they said, was competitive concerns. They didn't want anyone to know exactly how the car operates too soon, especially given the long lead time. The details were relatively tightly held within the engineering group.
I'll keep digging into this and will report back here and in a coming review of the Volt.