Hybrid powertrain with efficient electric-only mode

A hybrid powertrain comprising:

• an engine;
• an electrically-variable transmission having:
• an input member connected for rotation with the engine;
• an output member;
• a stationary member;
• a planetary gear set having a ring gear member, a sun gear member, and a carrier member rotatably supporting a plurality of pinion gears that intermesh with both the ring gear member and the sun gear member;
• first and second motor/generators;
• wherein the first motor/generator is operatively connected for rotation with the sun gear member;
• wherein the second motor/generator is operatively connected for rotation with the output member;
• a battery operatively connected to the first and second motor/generators for receiving power therefrom and delivering power thereto;
• wherein the battery is configured to be operatively connectable to an offboard power supply for recharging the battery;
• a one-way clutch engagable to interconnect the ring gear member and the output member;
• wherein the one-way clutch automatically engages when the ring gear member rotates in a forward direction and overruns when the ring gear member does not rotate;
• a second torque-transmitting mechanism selectively engagable by a controller to ground the ring gear member to the stationary member;
• a one-way brake clutch automatically engagable to ground the ring gear member to the stationary member;
• wherein the battery and the second motor/generator provide an electric-only operating mode when the one-way clutch, the second torque-transmitting mechanism and the one-way brake clutch are not engaged and the engine is off, the one-way clutch thus disconnecting the planetary gear set, the engine, and the first motor/generator from the output member during the electric-only operating mode;
• wherein the one-way brake clutch is automatically engaged when the battery and the first motor/generator are used to start the engine;
• wherein the engine, the battery, and the second motor/generator provide an input-split operating mode when the engine is on, the one-way clutch is engaged, and the one-way brake clutch and the second torque-transmitting mechanism are not engaged;
• and
• wherein the engine, the first motor/generator, the battery, and the second motor/generator provide a series hybrid operating mode when the engine is on, the one-way clutch is not engaged, the second torque-transmitting mechanism is engaged and the one-way brake clutch is not engaged.

A method of operating a hybrid electro-mechanical powertrain having an engine, a transmission with a first motor/generator, a second motor/generator, and a planetary gear set;

• wherein the engine and the motor/generators are each connectable to different members of the planetary gear set, the method comprising:
• providing a first engagable and disengagable torque-transmitting mechanism between the second motor/generator and the member of the planetary gear set connected with the second motor/generator;
• directing stored electrical power to the second motor/generator to drive a transmission output member to thereby establish an electric-only operating mode;
• wherein the first torque-transmitting mechanism is disengaged during said directing such that the second motor/generator and output member are isolated from the engine, the planetary gear set, and the first motor/generator, thereby eliminating parasitic drag;
• wherein the first torque-transmitting mechanism is a first one-way clutch that automatically overruns during the electric-only operating mode such that the second motor/generator and output member are automatically isolated from the engine, the planetary gear set, and the first motor/generator during the electric-only operating mode due to overrunning of the one-way clutch;
• powering the engine during said directing stored electrical power to the second motor/generator;
• and wherein the first torque-transmitting mechanism automatically engages during said powering the engine to thereby establish an input-split operating mode.