Setting engine torque to allow the slippage of a torque converter clutch to be maintained when the accelerator pedal is released in a hybrid vehicle.

Abstract

Power transmission system (10) for controlling a hybrid vehicle with a traction motor (30) between an internal combustion engine (20) and a gear-ratio automatic transmission (50) having a torque converter (40) with a bypass clutch (44), characterized by a control device (80) designed to control the engine torque (78) of the traction motor (30) in response to a reduced torque (76) of the internal combustion engine (20) as a result of releasing the accelerator pedal (92) in order to maintain non-zero slip via the bypass clutch (44) of the torque converter (40) while the bypass clutch (44) is in an engaged position, wherein the control device (80) is further designed to generate a relatively small amount of engine torque (78) to prevent the slip of the torque converter (40) from decreasing and reaching zero.

Description

TECHNICAL AREA The present invention relates to controlling the engine torque in a hybrid vehicle power transmission when changes in driver requirements are present. BACKGROUND A hybrid vehicle powertrain comprises an internal combustion engine and an electric motor, whereby the torque (or power) generated by the internal combustion engine and / or the electric motor can be transmitted to the vehicle's drive wheels via a transmission. A traction battery or other electrical power source connected to the motor supplies energy to the motor so that it generates engine torque. The motor can supply negative engine torque to the transmission (for example, during regenerative braking) and thereby act as a generator to store electrical energy in the battery. In a configuration sometimes referred to as a modular hybrid transmission (MHT configuration), the internal combustion engine can be connected to the transmission via a disconnect clutch. Alternatively, the engine can be connected to the transmission via a torque converter and a torque converter clutch. The internal combustion engine, disconnect clutch, engine, torque converter, and transmission are connected sequentially in series. Operating with the torque converter bypass clutch disengaged provides hydrodynamic coupling, which multiplies torque and dampens drivetrain disturbances, but also reduces the vehicle's energy efficiency. Engaging or locking the torque converter bypass clutch eliminates the hydrodynamic coupling and torque multiplication of the torque converter to reduce losses and improve energy efficiency, but can transmit torque disturbances from the internal combustion engine or motor through the drivetrain. It is desirable to control the position of the torque converter bypass clutch to operate in a disengaged position with reduced slippage. This can influence fuel efficiency.Modulating the torque converter bypass clutch to operate in a reduced-slip position near engagement reduces losses and contributes to improved energy efficiency, while also providing some damping of drivetrain disturbances. However, various vehicle operating conditions can cause the torque converter slip to approach zero, allowing the torque converter to transmit drivetrain torque disturbances, such as torque reversals associated with releasing the accelerator pedal and the vehicle coasting or braking. DE 10 2008 009 764 A1 discloses a hybrid vehicle drive system comprising a torque converter with a converter lock-up clutch and a control unit that adjusts the torque output of an electric energy converter device. US 2006 / 0 108 163 A1 relates to a hybrid vehicle comprising an internal combustion engine, a motor generator, a torque converter with a lock-up clutch, and a gear ratio change mechanism. SUMMARY Embodiments of the present invention relate to a control device and a control strategy for a hybrid electric vehicle comprising an internal combustion engine, an electric motor, a torque converter with a torque converter clutch, and a transmission. The control device and the control strategy adjust the engine torque so that controlled slip is maintained to dampen powertrain torque disturbances without disengaging the converter clutch during certain vehicle operating conditions. For example, the engine torque is increased to maintain the slip of the torque converter clutch after the driver releases the vehicle's accelerator pedal and the vehicle begins to coast. Typically, in such a situation, the torque converter clutch disengages to allow the torque converter to absorb drivetrain disturbances caused by releasing the accelerator pedal. Alternatively, the torque converter clutch is held in the engaged position, and the slip in the torque converter reaches zero as a result of releasing the accelerator pedal. In this case, the drivetrain disturbances are felt due to the direct torque path through the torque converter. In contrast to the typical operation that occurs when the accelerator pedal is released, the control device and control strategy according to embodiments of the present invention hold the torque converter clutch in the engaged position and control the engine to generate a relatively small additional amount of engine torque, which is supplied to the torque converter to prevent the slip in the torque converter from decreasing and reaching zero. This ensures that the torque converter has sufficient slip to absorb drivetrain disturbances while simultaneously being held in or near the engaged position. The invention aims to eliminate or reduce the problems described above by controlling the position of the torque converter bypass clutch to operate in a reduced-slip position close to engagement. This reduces losses and contributes to improved energy efficiency, while also providing some damping of drivetrain disturbances. The problem is solved by a power transmission system according to claim 1. Further developments of the power transmission system are the subject of the dependent claims. According to one embodiment, a method is provided. The method comprises holding a bypass clutch of a torque converter in a position and applying motor torque from a motor to the torque converter, such that the slip of the torque converter does not otherwise decrease and reaches zero while the bypass clutch is held in the position. The maintained position of the bypass clutch can be engaged but not locked. Engine torque can be applied to the torque converter while the bypass clutch is held in the engaged position, rather than moving the bypass clutch from the engaged to a disengaged position to increase torque converter slip. The bypass clutch can be held in position when the accelerator pedal is released. The bypass clutch can be held in position when the torque from an internal combustion engine decreases. The bypass clutch can be moved from the engaged position to a disengaged position while the torque converter slip is above a threshold, instead of the bypass clutch being held in the engaged position and engine torque being applied. The bypass clutch can be moved from the engaged position to a disengaged position while the internal combustion engine torque level is above a threshold, instead of the bypass clutch being held in the engaged position and engine torque being applied. The bypass clutch can be moved from the engaged position to a disengaged position while the rate of change of the internal combustion engine torque is above a threshold, instead of the bypass clutch being held in the engaged position and engine torque being applied. According to one embodiment, a system is provided. The system includes a control device designed to keep a bypass clutch of a torque converter in an engaged position and to apply a torque from a motor to the torque converter, so that the slip of the torque converter would otherwise not reach zero while the bypass clutch is held in the engaged position. According to one embodiment, a hybrid-electric vehicle is provided. The vehicle comprises an internal combustion engine, an electric motor, a torque converter with a bypass clutch, a transmission, and a control unit. The control unit is designed to keep the bypass clutch engaged and to apply torque from the engine to the transmission via the torque converter, so that the slip of the torque converter would otherwise not reach zero while the bypass clutch is held in the engaged position. Additional tasks, features and advantages of embodiments of the present invention will be better understood with reference to the following detailed description in conjunction with the drawing, in which the same reference numerals denote corresponding parts. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows a block diagram of an example hybrid vehicle power transmission according to an embodiment of the present invention, and Fig. 2 shows a flowchart to describe the operation of a control strategy according to an embodiment of the present invention for adjusting the engine torque in order to allow the slip of the torque converter clutch to be maintained. DETAILED DESCRIPTION Detailed embodiments of the present invention are disclosed herein. However, it should be understood that the disclosed embodiments are intended to illustrate the invention only by way of example and that the present invention can be implemented in various alternative forms. The figures are not necessarily to scale. Some features may be exaggerated or minimized to show details of certain components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis to teach those skilled in the art how to use the present invention in various ways. Fig. 1 shows a block diagram of an exemplary power transmission system 10 for a hybrid electric vehicle according to an embodiment of the present invention. The power transmission system 10 comprises an internal combustion engine 20, an electric machine in the form of an electric motor / generator 30 (“motor”), a traction battery 36, a torque converter 40, and a multi-ratio automatic transmission 50. Internal combustion engine 20 and motor 30 are the power sources for the vehicle. Internal combustion engine 20 can be connected to motor 30 via a disconnect clutch 32, thus connecting internal combustion engine 20 and motor 30 in series. Motor 30 is connected to the torque converter 40. The torque converter 40 is connected to internal combustion engine 20 via motor 30 when internal combustion engine 20 is connected to motor 30 via the disconnect clutch 32. The transmission 50 is connected to the drive wheels 60 of the vehicle. The driving force exerted by internal combustion engine 20 and / or motor 30 is transmitted to the drive wheels 60 via the torque converter 40 and the transmission 50, thereby propelling the vehicle. The torque converter 40 comprises a pump impeller rotor attached to the output shaft 42 of the motor 30 and a turbine rotor attached to the input shaft of the transmission 50 (not shown). The turbine of the torque converter 40 can be hydrodynamically driven by the pump impeller of the torque converter 40. Accordingly, the torque converter 40 can provide a hydraulic coupling between the output shaft 42 of the motor 30 and the input shaft of the transmission 50. The torque converter 40 further comprises a torque converter clutch 44 (i.e., a bypass clutch 44). The converter clutch 44 is controllable between an engaged position (i.e., a locking position, an applied position, etc.) and a disengaged position (i.e., an unlocked position, etc.). In the engaged position, the converter clutch 44 couples the impeller and the turbine of the torque converter 40 by friction, thereby eliminating the hydraulic coupling and torque multiplication between these components. In the disengaged position, the converter clutch 44 allows hydraulic coupling between the impeller and the turbine of the torque converter 40. When the torque converter clutch 44 is disengaged, the hydraulic coupling between the pump impeller and the turbine of the torque converter 40 absorbs and dampens unacceptable vibrations and other disturbances in the power transmission. The source of these disturbances includes the torque exerted by the internal combustion engine 20 for propelling the vehicle. However, the vehicle's fuel efficiency is reduced when the converter clutch 44 is disengaged due to losses associated with the hydraulic coupling and the associated torque multiplication. Accordingly, it is desirable for the converter clutch 44 to be engaged whenever possible in order to increase the vehicle's fuel and energy efficiency. The torque converter clutch 44 can be controlled by the operation of a clutch valve 46, which can be implemented, for example, as a pulse-width modulated solenoid valve. Responding to a control signal, the clutch valve 46 pressurizes and vents the converter clutch 44 to engage and disengage the friction coupling between the pump impeller and the turbine. The operating pressure of the converter clutch 44 can be controlled so that the converter clutch 44 is neither fully engaged nor fully disengaged, but instead modulated to generate a variable amount of slip between the pump impeller and the turbine in the torque converter 40. The slip of the torque converter 40 corresponds to the speed difference between the pump impeller and the turbine of the torque converter 40. The slip of the torque converter 40 approaches zero as the converter clutch 44 approaches the fully engaged position.Conversely, the amount of slip of the torque converter 40 increases when the converter clutch 44 moves towards the disengaged position. When operated to generate a variable amount of slip, the torque converter 40 can be used to absorb vibrations (for example, when gear ratio changes are made, when the driver releases pressure from the accelerator pedal, etc.) by increasing the slip. This causes a greater proportion of the torque from the internal combustion engine to be transferred to the turbine of the torque converter 40 via hydrodynamic action from the pump impeller. The converter clutch 44 can be engaged further, thus increasing fuel efficiency without significantly affecting drivability, when the prevailing environmental and / or vehicle operating conditions are less likely to generate torque disturbances. However, as mentioned above, it is again desirable for the converter clutch 44 to be engaged whenever possible because of the fuel and / orThe vehicle's energy efficiency is increased when the torque converter clutch is engaged (44). As stated above, the internal combustion engine 20 can be connected to the motor 30 via the disconnect coupling 32. In particular, the internal combustion engine 20 has a shaft 22 which can be connected to an input shaft 24 of the motor 30 via the disconnect coupling 32. As further stated above, the output shaft 42 of the motor 30 is connected to the impeller of the torque converter 40. The turbine of the torque converter 40 is connected to the input shaft of the transmission 50. The transmission 50 has several discrete gear ratios. The transmission 50 has an output shaft 54 ​​which is connected to a differential 56. Drive wheels 60 are connected to the differential 56 via their respective wheel axles 66. With this arrangement, the transmission 50 transmits a power transmission output torque 68 to the drive wheels 60. The internal combustion engine 20 is a primary power source for the power transmission system 10. The internal combustion engine 20 is a combustion engine of the type powered by gasoline, diesel, or natural gas. The internal combustion engine 20 generates internal combustion engine power and corresponding internal combustion engine torque 76, which is supplied to the engine 30 when the internal combustion engine 20 and the engine 30 are connected by the disconnect clutch 32. The power output of the internal combustion engine corresponds to the product of the torque 76 and the rotational speed of the internal combustion engine 20. To propel the vehicle with the internal combustion engine 20, at least a portion of the torque 76 of the internal combustion engine 20 is transmitted via the disconnect clutch 32 to the engine 30 and then from the engine 30 via the torque converter 40 to the transmission 50. The traction battery 36 is a secondary power source for the power transmission system 10. The motor 30 is connected to the battery 36 by wires 53. Depending on the vehicle's operating mode, the motor 30 either converts electrical energy stored in the battery 36 into motor power with a motor torque 78 or sends a corresponding amount of electrical power to the battery 36. The motor power is the product of the motor torque 78 and the motor speed of the motor 30. To propel the vehicle with the motor 20, motor torque 78 is also sent to the transmission 50 via the torque converter 40. When the motor 30 generates electrical power for storage in the battery 36, it receives power either from the internal combustion engine 20 in a drive mode or from the vehicle's inertia when the motor 30 acts as a brake in a regenerative braking mode. As described, the internal combustion engine 20, the disconnect clutch 32, the motor 30, the torque converter 40, and the transmission 50 can be connected sequentially in series, as shown in Fig. 1. The power transmission system 10 represents a modular hybrid transmission configuration (“MHT configuration”), wherein the internal combustion engine 20 is connected to the motor 30 via the disconnect clutch 32, while the motor 30 is connected to the transmission 50 via the torque converter 40. Depending on whether the disconnect clutch 32 is engaged or disengaged, the input torques 76 and 78 transmitted to the gearbox 50 are determined. For example, if the disconnect clutch 32 is disengaged, only the engine torque 78 is supplied to the gearbox 50. If the disconnect clutch is engaged, both the internal combustion engine torque 76 and the engine torque 78 are supplied to the gearbox 50. If only the internal combustion engine torque 76 is desired for the gearbox 50, the disconnect clutch 32 is engaged, but the engine 30 is not operated, so that only the internal combustion engine torque 76 is supplied to the gearbox 50. The transmission 50 comprises planetary gear sets (not shown) that are selectively engaged in different gear ratios by the selective engagement of friction elements (not shown) to produce the desired multiple discrete drive ratios. The friction elements are controlled by a shift scheme that engages and disengages specific elements of the planetary gear sets to control the ratio between the transmission output and input. The transmission 50 automatically shifts from one ratio to another based on the vehicle's requirements. The transmission 50 then supplies the power transmission output torque 68 to the output shaft 54, which ultimately drives the drive wheels 60. The kinetic details of the transmission 50 can be implemented by a wide range of gear arrangements.The gearbox 50 is an example of a gearbox arrangement for use with embodiments of the present invention. Any gearbox with multiple ratios that accepts an input torque or input torques from an internal combustion engine and / or a motor and then provides torque at the different ratios of an output shaft is acceptable for use with embodiments of the present invention. The power transmission system 10 further comprises a power transmission control unit 80, which forms a vehicle system control unit. An accelerator pedal 92 is used by the vehicle's driver to provide a desired torque or drive command for propelling the vehicle. Generally, depressing the pedal 92 generates an accelerator pedal position signal, which can be interpreted as a request for increased power. Similarly, the less the driver depresses the pedal 92, the corresponding pedal position signal is generally interpreted as a request for decreased vehicle power. Completely releasing the pedal 92 may allow the vehicle to coast, or it may be interpreted as a request for slight deceleration of the internal combustion engine / motor, depending on the specific application and implementation. The control unit 80 divides the overall drive / power command between an internal combustion engine signal 100 (representing the amount of torque 76 from the internal combustion engine to be supplied to the transmission 50 by the internal combustion engine 20, operating at a corresponding speed) and an engine torque signal 98 (representing the amount of torque 78 from the engine to be supplied to the transmission 50 by the engine 30, operating at a corresponding speed). Again, the internal combustion engine 20 generates the torque 76 and the engine 30 generates the engine torque 78 for the transmission 50 to propel the vehicle. This torque 76 from the internal combustion engine and this engine torque 78 for propelling the vehicle are "positive" torques. However, both the internal combustion engine 20 and the engine 30 can generate "negative" torques for the transmission 50 to brake the vehicle. The control unit 80 is further designed to control the clutch valve 46, which in turn controls the operation of the torque converter clutch 44 of the torque converter 40. The control unit 80 controls the operation of the torque converter 40 so that the converter clutch 44 is modulated over a range between the engaged and disengaged positions to generate a variable amount of slip in the torque converter 40. Again, the slip of the torque converter 40 corresponds to the difference between the input speed and the output speed of the torque converter 40. The output speed approaches the input speed as the converter clutch 44 approaches the engaged position, so that the slip is zero when the converter clutch 44 is in the fully engaged position.Conversely, the output speed deviates from the input speed as the converter clutch 44 approaches the disengaged position, thus increasing the amount of slip. The amount of slip can be measured or estimated using one or more appropriate sensors and operating parameters. For example, the motor current or a corresponding motor output speed sensor can be used to determine the speed of the pump impeller. A turbine speed sensor can be used to determine the transmission input speed. Similarly, a transmission output shaft speed sensor or one or more wheel speed sensors can be used to derive the transmission input speed using the current gear ratio.Here, one or more sensors can be collectively referred to as a slip sensor 48, which is designed to detect the slip of the torque converter 40 and to provide slip-indicating information to the control unit 80. With reference to Fig. 2 and further reference to Fig. 1, a flowchart 200 is now shown, which describes the operation of a control strategy according to an embodiment of the present invention for adjusting the torque 78 of the motor 30 in order to make it possible to maintain the slip of the torque converter clutch 44 of the torque converter 40. According to one embodiment, the control strategy is effective when the vehicle is propelled by the torque 76 from the internal combustion engine 20, as specified in block 202. The engine torque 78 from the motor 30 can optionally be provided to supplement the torque 76 of the internal combustion engine to propel the vehicle during this time. In any case, the total input torque (i.e., the torque 76 of the internal combustion engine and, if present, the engine torque 78) for propelling the vehicle corresponds to the total drive command based on the position of the accelerator pedal 92. As stated above, it is desirable for the torque converter clutch 44 to be located as close as possible to its engaged position to increase fuel efficiency. Accordingly, after the driver has depressed the accelerator pedal 92 for a period of time and the vehicle has reached a sufficient speed, the control unit 80 engages the torque converter clutch 44 so that it is located near its engaged position, as specified in block 204. Therefore, the slip of the torque converter 40 is relatively small, thus providing acceptable drivability because disturbances in the internal combustion engine's torque and the like are likely to be minimal during this time. At some point after blocks 202 and 204, the driver releases the accelerator pedal 92, as specified in block 206. The driver can release the accelerator pedal 92 to allow the vehicle to coast or to allow the vehicle to decelerate. In either case, when the driver releases the accelerator pedal 92, the overall drive command is reduced, and the torque 76 from the internal combustion engine 20 is reduced accordingly. The reduction in the internal combustion engine's torque 76 causes the slip of the torque converter 40 to decrease and reach zero if the torque converter clutch 44 remains near its engaged position, as specified in block 204, without any adjustment. A slip-free condition can lead to the transmission of undesirable torque disturbances to the vehicle structure.However, a small, non-zero slip amount in the torque converter 40 provides some damping, so that no severe vibrations are transmitted to the vehicle cabin. As explained below, the control strategy employs a setting technique in which the slip of the torque converter 40 does not reach zero during the reduction of the internal combustion engine's torque 76, with the converter clutch 44 being near its engaged position, as shown in Block 204. Typically, when the reduction of the internal combustion engine's torque 76 occurs upon the driver's release of the accelerator pedal 92, the converter clutch 44 is controlled to move towards its disengaged position to induce slip in the torque converter 40. This slip is generated to absorb powertrain disturbances caused by the reduction and / or rate of change of the internal combustion engine's torque 76 resulting from the release of the accelerator pedal 92, and also to prevent the transmission of combustion pulses from the internal combustion engine when the accelerator pedal 92 is depressed again.For example, powertrain disturbances can be caused by combustion pulses from the internal combustion engine 20 that occur during normal driving, and also by the change in the torque 76 of the internal combustion engine as a result of releasing the accelerator pedal 92. In contrast to typical operation, the control strategy keeps the torque converter clutch 44 near its engaged position, as shown in block 204, and controls the motor 30 to generate a relatively small amount of engine torque 78 to prevent the slip of the torque converter 40 from decreasing and reaching zero, as shown in block 208. That is, instead of controlling the converter clutch 44 to move from its engaged position to its disengaged position, as shown in block 204, in order to generate slip in the torque converter 40 according to typical operation, the motor 30 is controlled to generate an increased component of the engine torque 78 to prevent the slip of the torque converter 40 from decreasing and reaching zero.This relatively small amount of the additional torque generated by motor 30 is applied by motor 30 to torque converter 40 and can be a positive or a negative input torque, depending on what the situation requires. The criteria for controlling the motor 30 to maintain the engagement of the torque converter clutch 44 in block 208 according to one embodiment of the present invention include the following. The torque level and rate of change of the torque 76 from the internal combustion engine 20 are monitored. If specific threshold values ​​are exceeded and the amount of slip in the torque converter 40 falls below a specific value, the motor 30 is actuated to add a small amount of motor torque to prevent the slip of the torque converter 40 from reaching zero. Accordingly, the motor 30 is used to keep the torque converter clutch 44 in a mode (i.e., close to its engaged position as shown in Block 204), enabling the converter clutch 44 to maintain fuel efficiency while smoothing out variations in the internal combustion engine's torque through the generated additional engine torque component. An input / output control loop can be added to the internal combustion engine's control loop to control the amount of torque output by the motor 30. The following are generally disclosed: A. A method for controlling a hybrid vehicle with a traction motor between an internal combustion engine and a gear-ratio automatic transmission having a torque converter with a bypass clutch, comprising the following steps: engaging the torque converter bypass clutch to a position providing non-zero slip, and controlling the engine torque to maintain the engagement and non-zero slip in response to a reduction in driver-requested torque. B. The method of A, wherein the engagement comprises: modulating a bypass clutch pressure control signal to partially engage the bypass clutch. C. The method of B, wherein reducing the driver-requested torque corresponds to a change in the accelerator pedal position. D.Method A, further comprising: controlling the bypass clutch so that, in response to the release of the accelerator pedal, it remains partially engaged with a non-zero slip. E. Method A, further comprising: controlling the engine torque to maintain a non-zero slip in the bypass clutch in response to a decrease in the internal combustion engine torque. F. Method A, further comprising: disengaging the bypass clutch when the slip exceeds an associated threshold. G. Method A, further comprising: disengaging the bypass clutch when the internal combustion engine torque exceeds a corresponding threshold. H.Method according to A, further comprising: disengagement of the bypass clutch in response to the rate of change of the internal combustion engine torque exceeding a corresponding threshold. I. System for controlling a hybrid vehicle with a traction motor between an internal combustion engine and a gear-ratio automatic transmission, comprising a torque converter with a bypass clutch, comprising: a control device designed to control the engine torque of the traction motor to maintain non-zero slip via the torque converter bypass clutch while the bypass clutch is in an engaged position. J. System according to I, wherein: the control device increases the engine torque output to maintain non-zero slip in response to a reduced driver torque request. K.System according to I, wherein: the control device is designed to hold the bypass clutch in the engaged position in response to a reduced torque of the internal combustion engine. L. System according to I, wherein: the control device is further designed to move the bypass clutch from an engaged position to a disengaged position in response to the slip exceeding a corresponding threshold. M. System according to I, wherein: the control device is further designed to move the bypass clutch from an engaged position to a disengaged position in response to the torque of the internal combustion engine exceeding a corresponding threshold. N.System according to I, wherein: the control device is further configured to move the bypass clutch from an engaged position to a disengaged position in response to the rate of change of the internal combustion engine torque exceeding a corresponding threshold. O. Hybrid electric vehicle comprising: an internal combustion engine, an electric motor, a torque converter with a bypass clutch, a transmission, and a control device configured to maintain the bypass clutch in an engaged position and to apply engine torque from the electric motor in order to maintain non-zero slip of the bypass clutch in response to a reduced torque demand. P.Vehicle according to O, wherein: the control device is further configured to hold the bypass clutch in the engaged position with non-zero slip in response to a reduced torque of the internal combustion engine. Q. Vehicle according to O, wherein: the control device is further configured to move the bypass clutch from the engaged position to a disengaged position in response to the slip exceeding a corresponding threshold. R. Vehicle according to O, wherein: the control device is further configured to move the bypass clutch from the engaged position to a disengaged position in response to the torque of the internal combustion engine exceeding a corresponding threshold. S.Vehicle according to O, wherein: the control device is further configured to move the bypass clutch from the engaged position to a disengaged position while the rate of change of the internal combustion engine's torque is above a corresponding threshold value. Vehicle according to O, which further comprises: an accelerator pedal, wherein the control device is further configured to hold the bypass clutch in the engaged position with non-zero slip when the accelerator pedal is released.

Claims

Power transmission system (10) for controlling a hybrid vehicle with a traction motor (30) between an internal combustion engine (20) and a gear-ratio automatic transmission (50) having a torque converter (40) with a bypass clutch (44), characterized by a control device (80) designed to control the engine torque (78) of the traction motor (30) in response to a reduced torque (76) of the internal combustion engine (20) as a result of releasing the accelerator pedal (92) in order to maintain non-zero slip via the bypass clutch (44) of the torque converter (40) while the bypass clutch (44) is in an engaged position, wherein the control device (80) is further designed to generate a relatively small amount of engine torque (78) to prevent the slip of the torque converter (40) from decreasing and reaching zero. Power transmission system (10) according to claim 1, wherein: the control device (80) increases the motor torque output in order to maintain non-zero slip in response to a reduced driver torque demand. Power transmission system (10) according to claim 1, wherein: the control device (80) is designed to keep the bypass clutch (44) in the engaged position in response to a reduced torque (76) of the internal combustion engine (20). Power transmission system (10) according to claim 1, wherein: the control device (80) is further designed to move the bypass clutch (44) from an engaged position to a disengaged position in response to the slip exceeding a corresponding threshold value. Power transmission system (10) according to claim 1, wherein: the control device (80) is further designed to move the bypass clutch (44) from an engaged position to a disengaged position in response to the fact that the torque (76) of the internal combustion engine (20) exceeds a corresponding threshold value. Power transmission system (10) according to claim 1, wherein: the control device (80) is further designed to move the bypass clutch (44) from an engaged position to a disengaged position in response to the fact that the rate of change of the torque (76) of the internal combustion engine (20) exceeds a corresponding threshold value.

Images