Chevrolet just released a bunch of new info about their upcoming electric car, the Volt, because their patent applications were just awarded so they’re now free to spill the beans! It’s a very interesting design, and also has some people up in arms.
Basically, the Volt has an electric engine, a battery that will give the car a 40 mile all-electric driving range, and a 300 mile “range extender” – a diesel generator to automatically start recharging the battery if you run them flat but are still driving.
(A diesel generator powering a battery and electric motor driving the wheels can be more efficient than a diesel motor driving the wheels, because a diesel motor has to be able to cope with a far wider range of load and revs, and much more horsepower is required, so it’s bigger and runs less efficiently. In contrast, a generator can simply sit in its most efficient band, and can also be smaller and less powerful and still output the same average energy, using less fuel to do so).
But we knew all that. Here’s interesting new stuff:
The two parts of the diesel generator (the diesel engine and the electric generator) are not permanently connected, but are connect by a clutch. This means the electric generator can be disengaged from the diesel engine, and used as an extra motor. (If you turn the shaft of an electric motor, it generates power, if you put in power, it turns the shaft – this is part of why electric cars own gasoline cars on efficiency, instead of dumping your power through the brakes, an electric engine can put it back into the fuel tank instead, and re-use it later).
When the car gets above 70mph, the electric engine is operating at high RPM. The engine is more efficient at a lower RPM, so (here’s part of the patented cleverness) the volt diverts some of the power from the electric engine into the electric generator, using it as a second engine, and those two engines are geared together such that their speeds combine (instead of their torque), and the result is high RPM, but at high efficiency.
Now the bit that has people up in arms: What happens if the diesel motor is using the generator to recharge the battery and you drive above 70mph? Isn’t the car trying to use the generator as a second motor at that speed? Yes. So what happens is that the diesel motor, which is turning the generator, continues to turn the generator when the generator gets coupled to the car’s electric engine, and then the same thing happens as when doing over 70mph on batteries – the generator’s speed gets added to the engine’s speed.
Except now, the generator’s speed is coming from a diesel motor. It’s an elegant drivetrain solution that achieves high efficiency under all modes of operation yet uses few parts to do it.
That’s the controversial bit – there is a drive mode in which the diesel motor has a mechanical connection the drivetrain. People feel that this means it is really just a hybrid when it has been billed as an electric vehicle with an onboard diesel generator for range-extension in case you want to do a road trip.
People feel that it’s not as ideologically pure any more.
I’d agree with them if the car required the diesel to go over 70mph, but it doesn’t (as far as we know). It drives at all speeds on pure electric, but if you’ve flattened your battery and the range-extender kicks in, the drivetrain is designed to use all available components in alternative configuration to give you the most bang for your buck.
If the Volt is as described, then it might not be ideologically pure, but something better - great engineering.
(I currently drive a 2007 Corvette. My plan was that the Volt would be my next car. It’s going to be HARD to drop down from a lightweight 400 horsepower car to a heavier car with 230 horsepower, you wonder how you ever drove anything else, but there is a lot about the Volt which is exciting in other ways.)