The present invention relates to a power transmission that is interposed between the internal combustion engine and the drive ratio selection transmission of a vehicle. A motor/generator is operatively connected to the power transmission, and the power transmission selectively adds and subtracts torque provided by the motor/generator to the torque provided by the internal combustion engine. The power transmission effects the operative interconnection between both the internal combustion engine and the motor/generator to the drive ratio selection transmission. The power transmission employs compounded first and second planetary gear sets. A common gear member serves not only as the ring gear for the first planetary gear set but also as the sun gear for the second planetary gear set. The power transmission provides operating advantages not possible with prior art mechanisms, and these advantages allow the internal combustion engine to operate with desirable engine emissions.