Control system for hybrid vehicle

1. A control system for a hybrid vehicle comprising:

an engine;

a first motor that controls a speed of the engine;

a second motor that applies a brake torque to drive wheels when regenerating energy; and

an electric storage device that is electrically connected to the first motor and the second motor,

wherein the control system is configured to decelerate the hybrid vehicle by controlling the second motor to generate the brake torque while controlling the first motor to reduce a speed of the engine,

the control system comprises a controller that controls the engine,

the controller is configured to select a control mode of the engine from:

a low-power mode in which the hybrid vehicle is decelerated in response to a decelerating operation by reducing a torque and a power of the engine at a predetermined rate while generating the brake torque by the second motor; and

a high-power mode in which the hybrid vehicle is decelerated in response to the decelerating operation by reducing the torque and the power of the engine at a rate slower than the predetermined rate of the low-power mode while generating the brake torque by the second motor.

2. The control system as claimed in claim 1 , wherein the high-power mode includes:

a first control mode in which the hybrid vehicle is decelerated while reducing the torque and the speed of the engine such that an operating point of the engine is shifted along an optimum fuel economy curve; and

a second control mode in which the hybrid vehicle is decelerated while reducing the torque of the engine at a same rate as in the first control mode, reducing the speed of the engine at a faster rate than in the first control mode, and reducing the power of the engine at a slower rate than in the low-power mode, so as to reduce emission.

3. The control system as claimed in claim 2 , wherein the controller is further configured to select the control mode of the engine based on a state of charge level of the electric storage device or an acceptable value of an input power to the electric storage device when decelerating the vehicle.

4. The control system as claimed in claim 3 , wherein the controller is further configured to

select the first control mode when the state of charge level of the electric storage device is lower than a first threshold level,

select the second control mode when the state of charge level of the electric storage device is higher than the first threshold level but lower than a second threshold level, and

select the low-power mode when the state of charge level of the electric storage device is higher than the second threshold level.

5. The control system as claimed in claim 3 , wherein the controller is further configured to

select the first control mode when the acceptable value of the input power to the electric storage device is greater than a first acceptable value,

select the second control mode when the acceptable value of the input power to the electric storage device is smaller than the first acceptable value but greater than a second acceptable value, and

select the low-power mode when the acceptable value of the input power to the electric storage device is smaller than the second acceptable value.

6. The control system as claimed in claim 3 , wherein the controller is further configured to

determine whether the input power to the electric storage device given that the engine is controlled in the first control mode is smaller than a current acceptable value of the input power to the electric storage device,

determine whether the input power to the electric storage device given that the engine is controlled in the second control mode is smaller than the current acceptable value of the input power to the electric storage device,

select the first control mode when the input power to the electric storage device given that the engine is controlled in the first control mode is smaller than the current acceptable value of the input power to the electric storage device,

select the second control mode when the input power to the electric storage device given that the engine is controlled in the first control mode is greater than the current acceptable value of the input power to the electric storage device, but smaller than the input power to the electric storage device given that the engine is controlled in the second control mode, and

select the low-power mode when the input power to the electric storage device given that the engine is controlled in the second control mode is greater than the current acceptable value of the input power to the electric storage device.

7. The control system as claimed in claim 3 , wherein the controller is further configured to

select the control mode of the engine based on a required deceleration including a shift range, an operating amount of a brake pedal, an operating speed of a predetermined pedal in a one-pedal mode, and a road grade with respect to a flat road,

select the first control mode or the second control mode when a predetermined condition of the required deceleration is satisfied, and

select the low-power mode when the predetermined condition of the required deceleration is not satisfied.

8. The control system as claimed in claim 1 , wherein the controller is further configured to determine whether the vehicle can be decelerated in the high-power mode in response to the decelerating operation using an equation of motion in which the torque of the second motor and the torque of the engine are employed as parameters.

9. The control system as claimed in claim 1 , wherein

the hybrid vehicle further comprises a power split mechanism that distributes an output power of the engine to an output member and to the first motor,

the power split mechanism includes a planetary gear unit having an input element connected to the engine, a reaction element connected to the first motor, and an output element connected to the output member, and

the second motor is connected to a power transmission path between the drive wheels and the output member.