the shift, both electric machines operate as motors and the first gear ratio is employed. At a given velocity above Vshift, the second gear ratio is employed, and M/G 2 begins to operate as a generator, while also slowing down its angular speed. Although the transmission mechanically has only four conventional gear ratios, the electric motors allow it to function as a continuously variable transmission. This variable ratio functions in addition to the torque multiplication of the planetary gears. A special automatic transmission (designated as a 2ML70 by General Motors) incorporates two 82 kW (110 hp peak) three phase permanent magnet motors, three planetary gear sets, and four selectively engaging friction clutches. This system amplifies the output of the electric motors similarly to the way in which a conventional transmission amplifies the torque of an internal combustion engine. It also, when required, permits transfer of more of the engine's torque to the wheels, making the transmission more efficient even without the electric motors in use. Finally, the whole system fits into the space of, and indeed appears as, a conventional Allison model 1000 automatic transmission. A 300 volt battery pack is housed elsewhere in the vehicle to store energy. How it works Operation of the 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 ML450 BlueHybrid. First Power Split Mode, also known as Input-Split: (clutches C1 engaged, C2+C3+C4 released) The first planetary gearset pair acts as a four-driveshaft power-split transmission, and the last planetary gearset reduces the rotational speed. The second motor-generator (MG-B) rotational speed is proportional to the output driveshaft. Second Power Split Mode, aka Compound-Split: (clutches C2 engaged, C1+C3+C4 released) The first planetary gearset pair acts as a four-driveshaft power-split transmission. No motor-generator is directly coupled with input or output. First Fixed Gear ratio: (clutches C1+C4 engaged, C2+C3 released) The first planetary gearset pair is maintained in a synchronous 1:1 fixed gear ratio by shunt C4 clutch action, and the last planetary gearset reduces the rotational speed. Both motor-generators are mutually synchronous, either both as motors, or both as generators. It's the midpoint of the first continuously variable range, when both motor-generators rotational speed join. Second Fixed Gear ratio: (clutches C1+C2 engaged, C3+C4 released) The last planetary gearset force the first pair into an intermediate fixed gear ratio. The rotational speed of both motor-generators are