The Truth is Out: Chevy Volt's Wheels Sometimes Powered Partially by Combustion Engine [Updated]

· · 8 years ago

It's been a long, arduous and somewhat controversial road to the truth, but as of this past weekend there can be no more arguments about whether or not the Chevrolet Volt's range extending engine is mechanically connected to its wheels. The venerable Motor Trend got its hands on the Volt for a multiple-day test drive and provided the most in-depth look at the mysterious transmission to date.

What did they find? Above 70 miles per hour in range extending mode, the Volt's 1.4 liter, 4-cylinder engine gets meshed directly to the wheels through the transmission which, reportedly, increases the efficiency of high speed driving by 10-15 percent. The patent application which surfaced last month turns out to be completely accurate.

PluginCars.com was able to chat more about this with Andrew Farah, the Volt Chief Engineer, and according to him the 70 mph cutoff is not accurate. According to Farah, the Volt gets feedback from the road, driver demands and state of battery charge, and then, using an efficiency map, chooses what preset combinations of motive forces (large motor, small motor, combustion engine) it should be using at any given point in time.

As most of us know, the Volt operates as an electric car for about the first 40 miles after a full charge and after that the Volt's gas engine kicks on and runs a generator which delivers electricity to the batteries and power the electric motor to provide another 300 or so miles of range. As of today, this relatively simple interaction has gotten a bit more complicated to explain.

The Volt's drivetrain is similar to the Prius's in many aspects, namely that it has a planetary gearset, two electric motors (a main 149 horsepower one and the smaller generator which can also act as an electric motor) and an engine. Here's how it differs from the Prius in very fundamental ways:

When the battery is fully charged:
  • The Volt uses its 149 horsepower main motor under most conditions.
  • At higher speeds, when the battery still has enough charge, the Volt can mesh both its 149 horsepower main motor and its smaller generator/motor combo to the wheels for it to reach its maximum 101 mph top speed and bring the rpms of the large motor down for increased efficiency.
When the battery is depleted (after about 40 miles, AKA range extending mode):
  • The Volt's engine turns on and gets clutched to the generator to deliver electricity to power the main 149 horsepower motor which drives the wheels.
  • At higher speeds, the main 149 horsepower electric motor is spinning too fast to remain efficient and the planetary gearset meshes the gas engine with the secondary electric motor (the generator) so that both are working to drive the wheels directly.

It's important to note that the Volt's engine never directly powers the wheels completely on its own and only directly powers the wheels at speeds above 70 mph in range extending mode after the battery is drained. In addition, according to Andrew Farah, the engine cannot drive the Volt's wheels in any capacity without electricity flowing to the motor to provide the engine a force to work against, which means that the Volt cannot, at any point in time, move without using electricity.

[10/12/2010, 3:00 PM Pacific Time: Post was updated to include information from Volt Chief Engineer, Andrew Farah]

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