This technical deep dive builds on the concepts expressed in part 1. If you haven’t yet read it, start there.
Extended-Range Drive Modes
When the internal-combustion engine comes into play, things get a bit more hairy. The three modes – CS1, CS2, and CS3 – use the same clutches as the electric drive modes, plus the ratcheting one-way clutch and the ICE, to power the wheels. It gets even more confusing when you learn that CS1 and CS3 – the system’s low extended-range and high extended-range modes, respectively – are technically continuously-variable transmission modes.
More on that later.
In any case, all three of these modes see the ICE activate and interact with the smaller MGA motor via its one-way clutch. In CS1, the ICE turns the ring gear of MGA’s planetary set; the planetary gears send torque through to the wheels, where the central sun gear turns MGA, so that the motor acts as a generator. That generated power is sent to the battery pack, which transfers some amount to power MGB.
So the reason that CS1 is considered a type of CVT mode is because through mapping the power generated/power used by the electric motors, GM’s engineers can keep the ICE at or around peak efficiency.
CS2 is a fixed ratio extended-range, meaning that it is not a CVT-like mode. It is engaged at moderate torque or speed. Both Clutch 1 and Clutch 2 are engaged in this mode; Clutch 1 locks the sun gear of MGA to the outer ring gear of MGB, while Clutch 2 locks the ring gear of MGB to the case. Thus, MGA is not being used in either a regenerative nor a productive capacity in CS2.
This is the simplest mode by far; the ICE turns the outer ring gear of MGA’s gear set, sending torque through the planetary gears directly to the drive shaft. At the same time, MGB is also active, providing supplementary torque through its planetary gears to the wheels.
The high extended-range mode – or CS3 – comes on only at higher speeds (or before, if torque demand is particularly small). This mode leaves Clutch 1 engaged, connecting MGA’s sun gear to MGB’s ring gear. Clutch 2 is disengaged, allowing MGB’s ring gear to turn; the bigger motor acts as the generator in this mode, taking some of the output from the ICE.
The mindblowingly brilliant part: MGA is active only to turn the ring gear of the bigger MGB. In that way, MGA is left to act as a gear ratio controller of sorts, manipulating how much of the ICE’s torque is passed through its planetary gear set to the drive shaft, and how much is siphoned off for regeneration.
If you’ve made it this far, you deserve a round of applause; many lesser men and women have tried – and failed – to understand the complexity of GM’s Voltec drivetrain. It’s an exceedingly brilliant bit of engineering, and beside perhaps powering a second-generation Cadillac ELR, we hope it goes on to do great things.