Torque converter clutch capacity based on regenerative braking request

A vehicle includes an engine, a transmission, and an electric machine configured to provide drive torque and regenerative braking torque. The electric machine is selectively coupled to the engine via a clutch. The vehicle additionally includes a torque converter with an associated bypass clutch coupling the electric machine and transmission. The bypass clutch has an associated torque capacity. The vehicle further includes a controller. The controller is configured to, in response to a braking request indicative of an anticipated regenerative braking request, increase the torque capacity prior to the regenerative braking event by a quantity corresponding to an anticipated regenerative braking torque associated with the anticipated regenerative braking event.

TECHNICAL FIELD

This disclosure relates to control systems and methods for a vehicle including a hybrid powertrain with a torque converter having an associated bypass clutch arranged between a traction motor and a transmission.

BACKGROUND

Conventional automatic vehicles may include a transmission having a torque converter to provide a hydrodynamic coupling with torque multiplication. The hydrodynamic coupling allows the engine to continue running while connected to the transmission when the vehicle is stationary. In addition, the torque converter provides torque multiplication to assist vehicle launch and provides damping of driveline torque disturbances. The torque multiplication or torque ratio varies with the speed difference or slip between the torque converter input element (impeller) and output element (turbine). A torque converter clutch or bypass clutch may be provided to mechanically or frictionally couple the impeller and the turbine to eliminate the slip and associated losses to improve efficiency. The bypass clutch has an associated clutch capacity indicating a maximum torque transferrable by the torque converter without slipping.

Similarly, hybrid vehicle powertrains may include a transmission or gearbox having a torque converter arranged downstream of an electric machine, i.e. between the electric machine and vehicle wheels. The electric machine may be configured to provide either drive torque, which may conventionally be referred to as positive torque, or regenerative braking torque, which may be referred to as negative torque.

SUMMARY

A vehicle according to the present disclosure includes an engine, a transmission, and an electric machine configured to provide drive torque and regenerative braking torque. The electric machine is selectively coupled to the engine via a clutch. The vehicle additionally includes a torque converter with an associated bypass clutch coupling the electric machine and transmission. The bypass clutch has an associated torque capacity. The vehicle further includes a controller. The controller is configured to, in response to a braking request indicative of an anticipated regenerative braking request, increase the torque capacity prior to the regenerative braking event by a quantity corresponding to an anticipated regenerative braking torque associated with the anticipated regenerative braking event.

In various embodiments, the braking request may be initiated by a driver actuation of a brake pedal or a driver release of an accelerator pedal. In some embodiments, the controller is configured to increase the torque capacity at a rate based on a capacitization response rate of the bypass clutch.

A method of controlling a vehicle according to the present disclosure, wherein the vehicle has an engine, a traction motor, a clutch configured to selectively couple the engine and traction motor, and a torque converter coupling the motor to a gearbox, includes increasing a torque capacity of a bypass clutch associated with the torque converter. The torque capacity is increased in response to an anticipated application of regenerative braking torque associated with a braking request. The torque capacity is increased by a quantity corresponding to the anticipated regenerative braking torque. The method additionally includes, in response to the braking request, controlling the traction motor to provide regenerative braking torque after the torque capacity is increased.

A vehicle according to the present disclosure includes traction wheels, an electric machine configured to provide regenerative braking torque to the traction wheels, a clutch configured to selectively operatively couple the electric machine and traction wheels, and at least one controller. The controller is configured to, in response to an anticipated increase in magnitude of an input torque to the clutch based on a braking request, increase a torque capacity associated with the clutch prior to the increase in magnitude.

Embodiments according to the present disclosure provide a number of advantages. For example, embodiments disclosed herein provide a system and method for controlling a clutch downstream of an electric machine to ensure adequate clutch capacity prior to initiating a regenerative braking event. Furthermore, embodiments according to the present disclosure may reduce the use of vehicle friction brakes, extending the use life of various braking components.

The above and other advantages and features of the present disclosure will be apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.

DETAILED DESCRIPTION

Referring toFIG. 1, a schematic diagram of a hybrid electric vehicle (HEV)10is illustrated according to an embodiment of the present disclosure.FIG. 1illustrates representative relationships among the components. Physical placement and orientation of the components within the vehicle may vary. The HEV10includes a powertrain12. The powertrain12includes an engine14that drives a transmission16, which may be referred to as a modular hybrid transmission (MHT). As will be described in further detail below, transmission16includes an electric machine such as an electric motor/generator (M/G)18, an associated traction battery20, a torque converter22, and a multiple step-ratio automatic transmission, or gearbox24.

The engine14and the M/G18are both drive sources for the HEV10. The engine14generally represents a power source that may include an internal combustion engine such as a gasoline, diesel, or natural gas powered engine, or a fuel cell. The engine14generates an engine power and corresponding engine torque that is supplied to the M/G18when a disconnect clutch26between the engine14and the M/G18is at least partially engaged. The M/G18may be implemented by any one of a plurality of types of electric machines. For example, M/G18may be a permanent magnet synchronous motor. Power electronics56condition direct current (DC) power provided by the battery20to the requirements of the M/G18, as will be described below. For example, power electronics may provide three phase alternating current (AC) to the M/G18.

When the disconnect clutch26is at least partially engaged, power flow from the engine14to the M/G18or from the M/G18to the engine14is possible. For example, the disconnect clutch26may be engaged and M/G18may operate as a generator to convert rotational energy provided by a crankshaft28and M/G shaft30into electrical energy to be stored in the battery20. The disconnect clutch26can also be disengaged to isolate the engine14from the remainder of the powertrain12such that the M/G18can act as the sole drive source for the HEV10. Shaft30extends through the M/G18. The M/G18is continuously drivably connected to the shaft30, whereas the engine14is drivably connected to the shaft30only when the disconnect clutch26is at least partially engaged.

The M/G18is connected to the torque converter22via shaft30. The torque converter22is therefore connected to the engine14when the disconnect clutch26is at least partially engaged. The torque converter22includes an impeller fixed to M/G shaft30and a turbine fixed to a transmission input shaft32. The torque converter22thus provides a hydraulic coupling between shaft30and transmission input shaft32. The torque converter22transmits power from the impeller to the turbine when the impeller rotates faster than the turbine. The magnitude of the turbine torque and impeller torque generally depend upon the relative speeds. When the ratio of impeller speed to turbine speed is sufficiently high, the turbine torque is a multiple of the impeller torque. A torque converter bypass clutch34may also be provided that, when engaged, frictionally or mechanically couples the impeller and the turbine of the torque converter22, permitting more efficient power transfer. The torque converter bypass clutch34may be operated as a launch clutch to provide smooth vehicle launch. In some applications, disconnect clutch26is generally referred to as an upstream clutch and torque converter bypass clutch34(which may be a launch clutch) is generally referred to as a downstream clutch. The torque converter bypass clutch34has an associated clutch capacity. The clutch capacity may be adjusted by increasing or decreasing a clamp load in the torque converter bypass clutch34, e.g. by increasing or decreasing a hydraulic pressure in the torque converter bypass clutch34, in response to a signal from an associated controller, such as a powertrain control unit (PCU)50.

The gearbox24may include gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes (not shown) to establish the desired multiple discrete or step drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the gear sets to control the ratio between a transmission output shaft36and the transmission input shaft32. The gearbox24is automatically shifted from one ratio to another based on various vehicle and ambient operating conditions by an associated controller, such as a powertrain control unit (PCU)50. The gearbox24then provides powertrain output torque to output shaft36.

It should be understood that the hydraulically controlled gearbox24used with a torque converter22is but one example of a gearbox or transmission arrangement; any multiple ratio gearbox that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present disclosure. For example, gearbox24may be implemented by an automated mechanical (or manual) transmission (AMT) that includes one or more servo motors to translate/rotate shift forks along a shift rail to select a desired gear ratio. As generally understood by those of ordinary skill in the art, an AMT may be used in applications with higher torque requirements, for example.

As shown in the representative embodiment ofFIG. 1, the output shaft36is connected to a differential40. The differential40drives a pair of wheels42via respective axles44connected to the differential40. The differential transmits approximately equal torque to each wheel42while permitting slight speed differences such as when the vehicle turns a corner. Different types of differentials or similar devices may be used to distribute torque from the powertrain to one or more wheels. In some applications, torque distribution may vary depending on the particular operating mode or condition, for example.

The powertrain12further includes an associated powertrain control unit (PCU)50. While illustrated as one controller, the PCU50may be part of a larger control system and may be controlled by various other controllers throughout the vehicle10, such as a vehicle system controller (VSC). It should therefore be understood that the powertrain control unit50and one or more other controllers can collectively be referred to as a “controller” that controls various actuators in response to signals from various sensors to control functions such as starting/stopping engine14, operating M/G18to provide wheel torque or charge battery20, select or schedule transmission shifts, etc. Controller50may include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer-readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the engine or vehicle.

The controller communicates with various engine/vehicle sensors and actuators via an input/output (I/O) interface that may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process particular signals before being supplied to the CPU. As generally illustrated in the representative embodiment ofFIG. 1, PCU50may communicate signals to and/or from engine14, disconnect clutch26, M/G18, launch clutch34, transmission gearbox24, and power electronics56. Although not explicitly illustrated, those of ordinary skill in the art will recognize various functions or components that may be controlled by PCU50within each of the subsystems identified above. Representative examples of parameters, systems, and/or components that may be directly or indirectly actuated using control logic executed by the controller include fuel injection timing, rate, and duration, throttle valve position, spark plug ignition timing (for spark-ignition engines), intake/exhaust valve timing and duration, front-end accessory drive (FEAD) components such as an alternator, air conditioning compressor, battery charging, regenerative braking, M/G operation, clutch pressures for disconnect clutch26, launch clutch34, and transmission gearbox24, and the like. Sensors communicating input through the I/O interface may be used to indicate turbocharger boost pressure, crankshaft position (PIP), engine rotational speed (RPM), wheel speeds (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT), intake manifold pressure (MAP), accelerator pedal position (PPS), ignition switch position (IGN), throttle valve position (TP), air temperature (TMP), exhaust gas oxygen (EGO) or other exhaust gas component concentration or presence, intake air flow (MAF), transmission gear, ratio, or mode, transmission oil temperature (TOT), transmission turbine speed (TS), torque converter bypass clutch34status (TCC), deceleration or shift mode (MDE), for example.

An accelerator pedal52is used by the driver of the vehicle to provide a demanded torque, power, or drive command to propel the vehicle. In general, depressing and releasing the pedal52generates an accelerator pedal position signal that may be interpreted by the controller50as a demand for increased power or decreased power, respectively. Based at least upon input from the pedal, the controller50commands torque from the engine14and/or the M/G18. The controller50also controls the timing of gear shifts within the gearbox24, as well as engagement or disengagement of the disconnect clutch26and the torque converter bypass clutch34. Like the disconnect clutch26, the torque converter bypass clutch34can be modulated across a range between the engaged and disengaged positions. This produces a variable slip in the torque converter22in addition to the variable slip produced by the hydrodynamic coupling between the impeller and the turbine. Alternatively, the torque converter bypass clutch34may be operated as locked or open without using a modulated operating mode depending on the particular application.

To drive the vehicle with the engine14, the disconnect clutch26is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch26to the M/G18, and then from the M/G18through the torque converter22and gearbox24. The M/G18may assist the engine14by providing additional power to turn the shaft30. This operation mode may be referred to as a “hybrid mode” or an “electric assist mode.”

To drive the vehicle with the M/G18as the sole power source, the power flow remains the same except the disconnect clutch26isolates the engine14from the remainder of the powertrain12. Combustion in the engine14may be disabled or otherwise OFF during this time to conserve fuel. The traction battery20transmits stored electrical energy through wiring54to power electronics56that may include an inverter, for example. The power electronics56convert DC voltage from the battery20into AC voltage to be used by the M/G18. The PCU50commands the power electronics56to convert voltage from the battery20to an AC voltage provided to the M/G18to provide positive or negative torque to the shaft30. This operation mode may be referred to as an “electric only” operation mode.

In any mode of operation, the M/G18may act as a motor and provide a driving force for the powertrain12. Alternatively, the M/G18may act as a generator and convert kinetic energy from the powertrain12into electric energy to be stored in the battery20. The M/G18may act as a generator while the engine14is providing propulsion power for the vehicle10, for example.

A brake pedal53is used by the driver of the vehicle to provide a demand for braking or negative torque to slow the vehicle. In general, depressing and releasing the brake pedal53generates a brake pedal position signal that may be interpreted by the controller50as a demand for increased braking or decreased braking, respectively. Based at least upon input from the pedal, the controller50commands braking torque from vehicle brakes (not illustrated). The vehicle brakes generally include friction brakes. The M/G18may additionally act as a generator to provide regenerative braking, in which rotational energy from spinning wheels42is transferred back through the gearbox24and is converted into electrical energy for storage in the battery20.

It should be understood that the schematic illustrated inFIG. 1is merely exemplary and is not intended to be limited. Other configurations are contemplated that utilize selective engagement of both an engine and a motor to transmit through the transmission. For example, the M/G18may be offset from the crankshaft28, and/or an additional motor may be provided to start the engine14. Other configurations are contemplated without deviating from the scope of the present disclosure.

Known methods for controlling the torque converter22involve adjusting the clutch capacity of the torque converter bypass clutch34in response to a change in input torque to the impeller of the torque converter22. The input torque may be a sum of an engine torque, an electric machine torque, and/or other driveline torques. In response to an increase in input torque the clutch capacity is increased, and in response to a decrease in input torque the clutch capacity is decreased. In conventional (i.e. non-hybrid) powertrains this control is advantageous, as large changes in input torque result in clutch slip while the clutch capacity is adjusted, serving to filter transient changes in the driveline.

However, in a hybrid powertrain having a torque converter arranged downstream of an electric machine, as in the embodiment illustrated inFIG. 1, it may be undesirable in some circumstances to modify the clutch capacity only after the input torque has changed. During a regenerative braking event, the M/G18applies negative torque to the input of the transmission. During such an event, slippage in the torque converter bypass clutch34may result in a decrease of the input speed to the torque converter22. In response to such a speed decrease, the speed of the transmission pump also decreases, which may result in additional clutch opening or slipping in the transmission. In addition, if the torque converter bypass clutch does not have adequate clutch capacity, friction brakes may be applied to provide braking torque to satisfy the braking request while the torque converter bypass clutch is capacitized.

Referring toFIG. 2, a method of controlling a vehicle according to the present disclosure is illustrated in flowchart form. The algorithm begins at block60. A determination is made of whether a brake request is present, as illustrated at operation62. The brake request may be, for example, in response to a driver actuation of a brake pedal or a driver release of an accelerator pedal (“lift pedal braking”), as illustrated at block63. If yes, then a regenerative braking request is calculated as a braking torque as illustrated at block64. If no, the regenerative braking request is set to 0, as illustrated at block66.

In either case, a torque converter bypass torque capacity requirement is calculated, as illustrated at block68. A torque capacity margin is subsequently added to the calculated requirement, as illustrated at block70. As a nonlimiting example, a margin of approximately 20% of the calculated torque requirement may be provided. A first rate limit is subsequently applied, as illustrated at block72, to result in a rate limited torque requirement based on the regenerative braking request. This rate limit is based on a capacitization response rate of the torque converter bypass clutch. The first rate limit is implemented to ensure that the commanded clutch capacity does not change more rapidly than the torque converter bypass clutch is physically able to capacitize. The resulting rate limited torque requirement based on the regenerative braking request is subsequently added to the conventional clutch capacity calculation to obtain a combined torque requirement, as illustrated at block74.

The torque converter bypass clutch is then commanded to capacitize to the combined torque requirement, as illustrated at block76. Because the torque capacity calculations illustrated in blocks68-74are performed in response to the regenerative braking request itself, the capacitization may be initiated prior to the actual application of the regenerative braking torque. Thus, when the electric machine is commanded to provide regenerative braking torque, the torque converter bypass clutch capacity may already be increased accordingly.

The actual torque converter capacity is then estimated, as illustrated at block78. A second rate limit is then applied, as illustrated at block80. The second rate limit is based on capabilities of the powertrain and regenerative braking system. The algorithm subsequently returns to operation62.

Variations on the above system and method are, of course, possible. As an example, the method may be used in conjunction with not only a torque converter bypass clutch, but also other devices that can slip between an electric machine and traction wheels.

As may be seen, the above system and method provide various advantages. The torque converter bypass clutch capacity is increased to provide adequate capacity for a regenerative braking event prior to the initiation of the braking event. Thus, potentially detrimental reductions in input speed to the torque converter may be avoided. Furthermore, because the torque converter bypass clutch has adequate capacity prior to activation of the regenerative brakes, it may not be necessary to engage friction brakes at the beginning of the braking event as in prior art systems, thus reducing wear on the friction brakes.