Driving the Volt

The electric propulsion system for GM's new plug-in hybrid gives a silent yet powerful ride.

As part of a marketing plan for its Volt plug-in hybrid, which is slated to go on sale by the end of 2010, GM has been taking the unusual step of allowing journalists to drive test vehicles. The test cars, called "mules," have the same basic propulsion system that the Volt will have, but not the same body. Here's what it feels like to be behind the wheel.

Volt tester: GM engineers have incorporated the propulsion system for the GM Volt into one of their other vehicles for testing. Credit: Technology Review

Multimedia   TR’s energy editor takes the Volt for a drive.

The Volt will be electrically powered, its wheels driven exclusively by an electric motor. For the first 40 miles, that motor will be powered using energy stored in a large battery pack. After that, an onboard gasoline- or ethanol-fueled generator with provide the electricity (with the battery acting as a buffer to improve the generator's efficiency).

The test drive was meant to show off only the electric drive system, not the car's handling (since it does not have the Volt body and chassis), nor the performance of the car once the generator kicks in (since GM prefers to emphasize the electric-only portion of the driving experience). Unlike other plug-in hybrid designs, in which a gas engine is connected to the wheels and used to supplement the electric motor, the Volt gets all of its acceleration from the electric motor.

The key distinctive feature of electric drive is the instantaneous response of the motor. Electric motors deliver their maximum amount of torque right away, whereas conventional internal combustion engines have to work up to it. As a result, the car accelerates faster. Frank Weber, the GM executive in charge of the Volt program as well as vehicles that will use the Volt's underlying propulsion technology worldwide, says that this means the car feels as though it's powered by a 250-horsepower engine, even though the motor is only rated at about 150 horsepower. The car certainly felt more powerful than a typical compact car. Accelerating from 40 to 80 kilometers per hour seemed effortless. And because there is only one gear, the acceleration is smooth.

Story continues below

The engineers have designed the control system to mimic conventional cars in several ways. Taking your foot off the accelerator in a conventional car causes the vehicle to slow down quickly due to engine braking, a phenomenon that drivers are used to and count on to slow down when approaching a car on the freeway, for example. If you cut off power to an electric motor, it can still spin freely, so the car doesn't slow down much. So the engineers have programmed the control system to start using the motor to recharge the battery when the driver lifts up on the accelerator, which slows the car down. They've also included a setting that increases the amount of this recharging, which slows the car down faster. In the future, these settings might be user-configurable, although Weber says that the amount of control would probably be limited to a few presets.

The engineers also decided to program in a small amount of vehicle creep. When conventional cars are stopped, a driver can edge forward by releasing the brake without depressing the accelerator. The same thing is programmed to happen in the Volt propulsion system. This was done not just to make the car feel like a conventional car, but also to give drivers some feedback that the car is on. Because the motor is silent, it would be easy for a driver to get out of the car without realizing that it is on.