Patent ID: 7610976

Claim:
A method of operating a hybrid vehicle powertrain having an engine, an input member operatively connectable with the engine, a motor/generator, an electric power storage device for interchanging electrical power with said motor/generator, a differential gear set operatively connected between the input member and the output member and having a first, a second and a third member, power flow from the engine and the motor/generator respectively to said output member being through different members of said differential gear set such that the transmission is an electrically variable transmission, comprising: slipping at least one torque-transmitting mechanism via a control unit operable to control engagement of said at least one torque-transmitting mechanism; wherein non-slipping engagement of said at least one torque-transmitting mechanism at least partially establishes a fixed speed ratio between said input member and said output member; wherein any other torque-transmitting mechanisms necessary to establish the fixed speed ratio are engaged; wherein said at least one torque-transmitting mechanism is selected from the group consisting of: a torque-transmitting mechanism that connects the input member with the differential gear set; a torque-transmitting mechanism that connects one of the members of the differential gear set with a stationary member where another of the members of the differential gear set is connected with the output member; and a torque-transmitting mechanism that connects any two members of the differential gear set together for common rotation; said slipping thereby decoupling a fixed relationship between engine torque and motor/generator torque or between engine torque and torque at said output member established by the differential gear set; and controlling the speed of the engine and the speed of the motor/generator to launch the vehicle.