24 October 2011
Just as the Chevy Volt uses an electric motor to propel you forward, it uses the same motor to help slow you down. When you either lift your foot from the accelerator or press on the brake pedal, the main drive motor temporarily operates as a generator, working to partially recharge the battery bank. In this mode, the motor produces a drag on the front wheels, slowing the car, though not to a complete stop. As the car slows, the effect of the regenerative braking fades out and the friction brakes take you the rest of the way to a full stop.
GM tuned the Volt’s control software so that the car drives like a snappy gas-powered car with an automatic transmission. If you want your Volt to drive more like a sports car with a manual transmission, do the following:
1. Take off and get up to a reasonable speed.
2. Shift the transmission selector to the Low setting. This engages electric motor braking when you’re coasting.
3. Press the dashboard Sport mode button. This will give you a bit more zip at higher RPMs.
4. Now take your foot off the gas pedal.
The Low setting makes the Volt’s regenerative braking considerably more aggressive, like driving a manual transmission car in second or third gear. You’ll slow down quickly without needing to tap the brake pedal, though you’ll still need to do that to come to a complete stop. Doesn’t that feel more like you’re driving a sports car?
How Well Does It Work?
Just how efficient is the Volt’s regenerative braking system, and how well does it work? You know the answer to that: it depends.
GM used the Environmental Protection Agency's City Driving Cycle Federal Test Procedure (FTP) to rate the Volt’s All Electric Range (AER). The maximum regenerative deceleration achieved during these tests was 0.315g, at which point the efficiency of the regenerative braking system was just under 75%. As the car slows down, the efficiency drops rapidly below 0.1g, at which point you’re increasingly reliant on the friction brakes. At higher speeds the effect of regenerative braking is outweighed by wind drag and tire friction.
The 75% figure represents the effectiveness of the regenerative braking system in slowing the forward motion of the car. Since the braking motor and electronics are ones that are driving the car—when braking the current flow is just reversed—the efficiency of the motor as a generator in refueling the battery bank is well over 90%.
So how much energy does regenerative braking put back into the system when you’re driving downhill with your foot off the accelerator? It depends on the kinetic energy of the car, plus the speed/wind drag, road grade, deceleration, and system controller’s algorithm. The basic answer is, “Not a lot—but it helps.” If you’ve just crested a small hill, you won’t get much back. If you’re driving down a long, steep mountain road, the regenerative payback can notably improve your mileage—not to mention prolong the life of your disk brakes.
If that isn’t a sufficient answer—and it isn’t—you can check the GM Volt Performance Simulation results based on the same inputs that GM used in their evaluations.
A nice feature for GM to add would be an adjustable regenerative braking system with a button on the dashboard where you could preset the desired stiffness of the system. That would enable you to choose a higher resistance when you’re going down a steep incline, just as you’d downshift to a lower gear in another car.
Still, the fact that you can recoup some of the energy that it took to get up to speed or up the mountain is one of those nice benefits that come with an electric vehicle.