As I understand it, at that speed, the energy lost by using the engine to recharge the battery is greater than simply using it to drive the wheels directly. Makes sense to this non-engineer.
That said, I wouldn't buy one if you (or the federal government) paid me.
10 to 15 percent assist. This is an electric vehicle with a bonus. That’s what GM has consistently maintained.
At any speed, there is energy lost in the energy conversion from mechanical-to-electrical power and then back to mechanical.
In engineering, we have this thing called “the Second Law of Thermodynamics” which stated in layman’s terms says “You can’t break even.”
The three laws of Thermo are (in layman’s terms):
1. You can’t win.
2. You can’t break even.
3. You can’t quit the game.
(and people wonder why we engineers get so weary when we hear of the next brilliant energy saving scheme...)
1 means that you can’t make an energy process wherein you get more energy out than you put in. No free lunches, period.
2 means that there’s a loss every time you convert energy from one form to another. The losses escape from the system typically by heat.
3 means that you can’t set up a different set of ground rules somewhere else in the universe, that your perpetual motion machine would work if you could just get off this whole nasty planet, etc. The universe works the same way things work here. Death, taxes and thermo. They all suck.
The engineers at Chevy probably discovered that with all-electric drive that their MPG at highway speeds was not what they needed and also that their range started to suffer quickly. So they decided to divert some power from the engine to the drivetrain directly to hit their MPG numbers.
I don’t begrudge the GM engineers their solution, only that GM execs and their PR machine lied to the public. It isn’t “all electric drive” and it isn’t a “series drive.” It is a selectively parallel drive, much like existing hybrids. There’s no shame in that unless they were making other claims - which they were.
And I wouldn’t buy one either. There’s far better economy to be had in buying a diesel Audi sedan than buying a Volt.
This epicyclic drive is pretty innovative. The prius drive train never locks any of the elements meaning that there is a ratio of at all times between the 3 elements sun planets and ring, at some point the ice engine must start spinning its part or the smaller motor gen overspeeds. The volt drives its epicyclic differently and has 3 clutches to stop and start different elements at different speeds. at all times the primary electric drive motor must supply torque, however above 70 mph the primary motor starts to over speed so the second motor is clutched in and together with the sun and ring gears they lower the speed of the primary motor to an acceptable level. when the battery is depleted a second clutch closes and the engine is coupled to the smaller motor/generator providing torque to it as well as the ring gear which previously was driven solely by the smaller motor/gen now the smaller motor consumes torque to generate electricity for sustaining the battery pack level and simultaneously to power the primary electric motor. The deficit of previously supplied torque from the smaller motor/gen is now mechanical supplied via the clutch and ring gear this is genius. The third clutch is to disconnect the smaller motor/gen from the ring gear at low speeds and allow the ice to turn it as a true generator in pure series hybrid mode again genius. This is a series electric + mechanical parallel+ electric cvt system the best of all the current hybrid modes. Should be a winner technically.