[ad#post_ad]A significant focal point of the Chevrolet Volt launch debriefing that I attended focused on detailing the inner workings of the car’s electric drive unit. This has been dispersed across the Internet often inaccurately and to much controversy that’s rather unwarranted.
I was one of a handful of journalists that attended a presentation by Larry Nitz, GM’s executive director of EVs and hybrids, in which he fully explained the way the system works.
The system was first designed in mid-2007 at which point GM decided to build what up to then was only a concept Chevy Volt. The patent was applied for around that time, and only two weeks ago the company was advised the patent was awarded.
The key components are the 111 kw electric motor, the 55 kw electric motor/generator, and the 62 kw 1.4 L gasoline engine.
The core element is the large electric motor which always turns the driveshaft. The car is always electrically driven. The motor turns the sun gear of a planetary gearset which itself is then connected to the driveshaft through two sets of gears set at a 7 to 1 combined gear reduction ratio. In this state of driving the generator is used to only recapture kinetic energy during motor braking and coasting which is then fed back into the battery. It is grounded to the crankcase by one of three clutches.
The next drive state occurs when the car is still in EV mode, but reaches around 70 mph. At that point the 111 kw electric motor begins to spins too rapidly and loses efficiency, around 6500 rpm. To improve efficiency the system kicks in the smaller 55 kw electric motor to operate in parallel. GM thought a lot about this element and considered instead adding a second gear, but figured they could simply use the generator because at these speeds, “it’s not doing anything, ” said Nitz. “It’s just along for the ride.”
It does so by releasing a second clutch, disengaging the ring of the planetary gearset from its formerly fixed position against the case, which then causes coupling of the generator into the ring gear of the planetary. The parallel input from the smaller motor then allows the RPMs of the larger motor to be reduced, improving the overall efficiency of the system. By allowing the second motor to participate, engineers gained an additional 1 to 2 miles of electric range.
The third state of the system occurs when the battery state of charge drops to a 20 to 25% state of charge, and extended range or charge-sustaining operation commences. There is still a buffer in the battery used to handle the dynamic responses of the vehicle in this mode.
At low speeds, the gas engine comes on board and spins the generator motor simply to produce electricity sufficient to supplement the battery and supply the electric motor. The engine is locked to the generator through a third clutch, and the ring gear stays grounded to the crankcase. GM calls this a weak one motor series that is battery dominant with the engine in the background picking up the average amount of energy the vehicle needs.
The fourth and controversial state commences when the vehicle reaches speeds of 70 mph while in extended range mode.
As in EV mode the ring gear is decoupled from the case by the clutch and the smaller electric motor is once again allowed to operate in parallel with the large motor, increasing the system’s efficiency. The difference here is that the smaller motor is still being turned by the engine and not electricity. Thus the engine becomes coupled with both electric motors and all three work together to turn the driveshaft. Thus the gas engine participates in turning the wheels mechanically although indirectly. The generator is decoupled from the ring gear again when speeds drop back below 70 mph.
Thus although the engine generator can participate in mechanically driving the wheels it never does so directly or in isolation, at all points in time the large electric motor is the main driver of the wheels.
By adding this element, engineers were able to improve fuel efficiency by 10 to 15%.
There has been considerable blogospheric controversy over this as this appears to contradict GM’s previous statements that the engine never drives the wheels. Nitz said GM had to be coy about this element due to intellectual property reasons, and now that the patent has been awarded can finally be more transparent. He still says there is no solitary direct mechanical drive because to do so would require a clutch to the sun gear decoupling the 111 kw electric motor, something that doesn’t exist and doesn’t happen.
Does this element play a major role? It depends how often you drive over 70 mph and extended range mode.
See a video of Mr. Nitz’ whole presentation here.