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A method is disclosed for enhancing reverse driving torque in a hybrid electric vehicle powertrain in which an electric motor is used for providing forward driving torque as well as reverse driving torque. An electric generator, functioning as a motor, supplies an added reverse driving torque using

A method is disclosed for enhancing reverse driving torque in a hybrid electric vehicle powertrain in which an electric motor is used for providing forward driving torque as well as reverse driving torque. An electric generator, functioning as a motor, supplies an added reverse driving torque using a gearset in which the engine provides a reverse driving reaction torque for the gearset.

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What is claimed: 1. A method for enhancing reverse driving torque in a hybrid electric vehicle powertrain having an engine, an electric motor, an electric generator, a battery; and gearing forming separate power flow paths during forward drive to vehicle traction wheels from the engine and the moto

What is claimed: 1. A method for enhancing reverse driving torque in a hybrid electric vehicle powertrain having an engine, an electric motor, an electric generator, a battery; and gearing forming separate power flow paths during forward drive to vehicle traction wheels from the engine and the motor, and from the electric motor and the generator, acting as a motor, during reverse drive, the electric generator and the battery being electrically coupled, and a controller for coordinating power delivery through the separate power flow paths during forward drive and during reverse drive, the motor operating in one direction during forward vehicle drive and in a direction opposite to the one direction during reverse vehicle drive, the method comprising the steps of: calculating a desired traction wheel torque in response to a driver demand for torque during reverse drive; determining a need to complement reverse driving torque of the motor; determining whether the battery has capacity to supply electric energy to the motor and to the generator acting as a motor to meet a driver demand for reverse driving torque; determining whether the engine should be on or off; determining an engine torque adder to traction wheel torque when the engine is not fueled; and adding the engine torque adder to reverse-driving torque whereby total reverse driving vehicle performance is enhanced. 2. The method set forth in claim 1 wherein the step of determining an engine torque adder is carried out as the engine is driven by the generator with the generator acting as a motor. 3. The method set forth in claim 2 wherein the engine, as it is driven by the generator, provides engine braking torque. 4. The method set forth in claim 3 wherein engine braking torque is maximized and engine speed is minimized to increase reverse driving torque. 5. The method set forth in claim 4 wherein engine braking torque is maximized and engine speed is commanded to be as small as possible to minimize power consumption. 6. The method set forth in claim 4 wherein engine braking torque is maximized and engine speed is minimized by increasing engine driven vehicle accessory loads. 7. The method set forth in claim 6 wherein the step of determining an engine torque adder to boost total reverse driving torque includes the step of determining a torque request that is a function of driver actuated accelerator pedal position; determining an engine-on torque adjustment limit based on table data in controller memory that is a function of battery power limit and vehicle speed; and clipping the torque request to a value that does not exceed a reverse drive torque limit whereby a traction wheel torque command is achieved for reverse drive. 8. A method for enhancing reverse driving torque in a split-power hybrid electric vehicle powertrain having an engine, an electric motor, an electric generator, a battery and a planetary gear unit with a ring gear, a planetary carrier and a sun gear; the engine being drivably connected to the carrier, the sun gear being drivably connected to the generator and the ring gear being drivably connected to vehicle traction wheels through torque transmitting gearing; and a controller for coordinating power distribution from the engine and from the battery; the motor operating in one direction during forward vehicle drive and in a direction opposite to the one direction during reverse vehicle drive, the method comprising the steps of: reducing fueling of the engine during reverse drive; determining a desired traction wheel torque in response to a driver demand for torque during reverse drive; determining a need to complement reverse driving torque of the motor; determining whether the battery has capacity to supply electric energy to the motor and to the generator acting as a motor to meet a driver demand for reverse driving torque; determining an engine state indicating whether the engine should be on or off determining an engine torque adder to traction wheel torque when wheel torque is less than a torque limit; and adding the engine torque adder to reverse-driving wheel torque whereby reverse driving vehicle performance is enhanced. 9. The method set forth in claim 8 wherein the step of determining an engine torque adder to traction wheel torque is carried out as the engine is driven by the generator with the generator acting as a motor. 10. The method set forth in claim 9 wherein the engine provides engine braking torque as the engine is driven by the generator. 11. The method set forth in claim 10 wherein engine braking torque is maximized and engine speed is commanded to be as small as possible to increase reverse driving torque. 12. The method set forth in claim 10 wherein engine braking torque is maximized and engine speed is commanded to be as small as possible to minimize power consumption. 13. The method set forth in claim 12 wherein engine braking torque is maximized with the engine defueled by increasing engine driven vehicle accessory loads. 14. A method for enhancing reverse driving torque in a split power hybrid electric vehicle powertrain having a throttle controlled engine, an electric motor, an electric generator, a battery and a planetary gear unit with a ring gear, a planetary carrier and a sun gear; the engine being drivably connected to the carrier, the sun gear being drivably connected to the generator and the ring gear being drivably connected to vehicle traction wheels through torque transmitting gearing; and a controller for coordinating power distribution from the engine and from the battery, during forward drive; the motor operating in one direction during forward vehicle drive and in a direction opposite to the one direction during reverse vehicle drive; the method comprising the steps of: controlling fueling of the engine during reverse drive in response to a command by the controller when the controller receives a driver reverse drive power request; and operating the generator as a motor during reverse vehicle drive as the motor drives the ring gear in the opposite direction and the generator drives the sun gear in the one direction whereby the engine is driven in the one direction. 15. The method set forth in claim 14 wherein the step of operating the generator as a motor during reverse drive is accompanied by an engine torque that results in a tendency for rotation of the ring gear in the opposite direction during reverse vehicle drive whereby ring gear torque augments motor torque. 16. The method set forth in claim 15 wherein braking torque of the engine is increased on a command by the controller in response to vehicle operating variables by adjusting the engine throttle as the engine is driven by the generator acting as a motor, whereby ring gear torque is increased. 17. The method set forth in claim 16 wherein braking torque of the engine is increased by adding vehicle accessory loads on the engine. 18. The method set forth in claim 17 wherein the ring gear torque is increased by adjusting engine operating parameters. 19. A method for enhancing reverse driving torque in a split power hybrid electric vehicle powertrain having a throttle controlled engine, an electric motor, an electric generator, a battery and a planetary gear unit with a ring gear, a planetary carrier and a sun gear; the engine being drivably connected to the carder, the sun gear being drivably connected to the generator and the ring gear being drivably connected to vehicle traction wheels through torque transmitting gearing; and a controller for coordinating power distribution from the engine and from the battery, the generator and the motor during forward drive; the motor operating in one direction during forward vehicle drive and in a direction opposite to the one direction during reverse vehicle drive; the method comprising the steps of: controlling fueling of the engine during reverse drive in response to a command by the controller when the controller receives a driver reverse drive power request; operating the generator as a motor during reverse vehicle drive as the motor drives the ring gear in the opposite direction and the generator drives the sun gear in the one direction whereby the engine is driven in the one direction; determining a battery power limit at repetitive time intervals by the controller and determining a threshold power limit at repetitive time intervals by the controller; computing an effective reverse power required in response to a driver request for power during reverse vehicle drive; allowing a ring gear torque boost if the driver request for power is less than the threshold power limit; and disallowing a ring gear torque boost if a driver request for power is greater than the threshold power limit. 20. The method set forth in claim 19 wherein the step of determining a threshold power limit includes the step of determining a hysteresis value for a reverse drive battery power limit whereby an actual threshold power limit is modified to prevent undesired frequent transfers to and from a reverse drive torque boost state.