Explanation of the famous new Allison Transmission (AHS-2), or Two-Mode Hybrid, from the Global Hybrid Cooperation. This transmission is mounted on the BMW ActiveHybrid and the Mercedes-Benz BlueHybrid.

First Power Split Mode:
aka Input-Split, or in french "sortie couplée"
The first planetary gearset pair act like a four-driveshaft power-split transmission, and the last planetary gearset reduce the rotational speed (clutches C1 engaged, C2+C3+C4 released). The second motor-generator rotational speed (MG-B) is proportional to the output driveshaft.

Second Power Split Mode:
aka Compound-Split
The first planetary gearset pair act like a four-driveshaft power-split transmission (clutches C2 engaged, C1+C3+C4 released). No motor-generator is directly coupled with input or output.

First Fixed Gear ratio:
The first planetary gearset pair are maintained in a synchronous 1:1 fixed gear ratio by shunt C4 clutch action, and the last planetary gearset reduce the rotational speed. By the way, both motor-generators are mutually synchronous, either all like motor, or all like generator (clutches C1+C4 engaged, C2+C3 released). It's the midpoint of the first continuously variable range, when both motor-generator rotational speed meet.

Second Fixed Gear ratio:
The last planetary gearset force the first pair into a intermediate fixed gear ratio (clutches C1+C2 engaged, C3+C4 released). The rotational speed of both motor-generators are very asymmetric (9:1), excluding effective usage of the first one (MG-A). It's the boundary between both continuously variable ranges. The two-modes switching occurs at this point, when the third planetary gearset ring gear reach zero rotational speed, and all the planetary gearset carriers gears reach the same rotational speed together.

Third Fixed Gear ratio:
The first planetary gearset pair are in a synchronous 1:1 fixed gear ratio by C4 clutch action, and, by the way, both motor-generators are mutually synchronous, either all like motor, or all like generator (clutches C2+C4 engaged, C1+C3 released). It's the midpoint of the second continuously variable range, when both motor-generator rotational speed meet.

Fourth Fixed Gear ratio:
The third C3 clutch block the first planetary gearset sun gear, and, by the way, the second motor-generator (MG-B) still off-line (clutches C2+C3 engaged, C1+C4 released).