Controls and methods to calculate clutch torque to include seal friction

A hybrid vehicle includes an engine, an electric machine, a disconnect clutch configured to selectively couple the engine to the electric machine, and a controller. The controller is programmed to, in response to a request to start the engine with the electric machine, command a target pressure to the disconnect clutch that depends on a seal friction derived from a measured line pressure of the disconnect clutch and a rate change of the measured line pressure.

TECHNICAL FIELD

The present disclosure relates to controlling electrified vehicle powertrains and more specifically to controlling an engine disconnect clutch.

BACKGROUND

A hybrid-electric powertrain includes an engine and an electric machine. The torque (or power) produced by the engine and/or the electric machine can be transferred through a transmission to the driven wheels to propel the vehicle. A traction battery supplies energy to the electric machine. The hybrid powertrain is also capable of performing regenerative braking where the electric machine brakes the vehicle by converting mechanical power into electrical power to recharge the battery.

SUMMARY

According to one embodiment, a hybrid vehicle includes an engine, an electric machine, a disconnect clutch configured to selectively couple the engine to the electric machine, and a controller. The controller is programmed to, in response to a request to start the engine with the electric machine, command a target pressure to the disconnect clutch that depends on a seal friction derived from a measured line pressure of the disconnect clutch and a rate change of the measured line pressure.

According to another embodiment, a hybrid vehicle includes an engine, an electric machine, a disconnect clutch configured to selectively couple the engine and electric machine, and a controller. The controller is programmed to command a target pressure to the disconnect clutch that depends on a seal friction derived from a rate of change of line pressure to the disconnect clutch during a start of the engine with the electric machine.

According yet another embodiment, a method of starting an engine with an electric machine includes, in response to a request to start an engine with an electric machine, commanding a target pressure to a disconnect clutch, that couples the engine and the electric machine, that depends on a seal friction derived from a measured line pressure of the disconnect clutch and a rate change of the measured line pressure.

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, the 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 engine14, M/G18, torque converter22, and the automatic transmission16are connected sequentially in series, as illustrated inFIG.1. For simplicity, the M/G18may be referred to as a motor.

The engine14and the M/G18are both drive sources for the HEV10and may be referred to as actuators. The engine14generally represents a power source that may include an internal-combustion engine such as a gasoline, diesel, or natural gas powered engine. 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 electronics condition 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. When the disconnect clutch26is locked (fully engaged), the crankshaft28is fixed to the shaft30.

The engine14is started by the M/G18rather than using a separate starter motor. To do so, the disconnect clutch26partially engages to transfer torque from the M/G18to the engine14. The M/G18may be required to ramp up in torque to fulfill driver demands while also starting the engine14. The disconnect clutch26can then be fully engaged once the engine speed is brought up to the speed of the M/G18.

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 impeller23fixed to M/G shaft30and a turbine25fixed to a transmission input shaft32. The torque converter22provides a hydraulic coupling between shaft30and transmission input shaft32. The torque converter22transmits power from the impeller23to the turbine25when 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. Alternatively, or in combination, a launch clutch similar to disconnect clutch26may be provided between the M/G18and gearbox24for applications that do not include a torque converter22or a torque converter bypass clutch34. In some applications, disconnect clutch26is generally referred to as an upstream clutch and the launch clutch34(which may be a torque converter bypass clutch) is generally referred to as a downstream clutch.

The gearbox24may include gear sets, such as planetary gear sets, that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes to establish the desired multiple discrete or step drive ratios. For simplicity, the gear ratios may be referred to as gears, i.e., first gear, second gear, etc. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the gear sets to control the speed and torque ratios between a transmission output shaft36and the transmission input shaft32. The gearbox24may have six speeds including first through sixth gears. In this example, sixth gear may be referred to as top gear. First gear has the lowest speed ratio and the highest torque ratio between the input shaft32and the output shaft36, and top gear has the highest speed ratio and the lowest torque ratio. 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). 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 one or more controller50such as a powertrain control unit (PCU), an engine control module (ECM), and a motor control unit (MCU). While illustrated as one controller, the controller50may 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 controller50and 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, 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 vehicle.

The controller communicates with various 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, controller50may 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 controller50within 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. This may be referred to as driver-demanded torque. 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. When the engine14alone provides the torque necessary to propel the vehicle, this operation mode may be referred to as the “engine mode,” “engine-only mode,” or “mechanical mode.”

The M/G18may assist the engine14by providing additional power to turn the shaft30. This operation mode may be referred to as a “hybrid mode,” an “engine-motor 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 controller50commands the power electronics56to convert voltage from the battery20to an AC voltage provided to the M/G18to provide positive torque (drive torque) or negative torque (regenerative braking) to the shaft30. This operation mode may be referred to as an “electric only mode,” “EV (electric vehicle) mode,” or “motor 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. The M/G18may additionally act as a generator during times of regenerative braking in which rotational energy from spinning wheels42is transferred back through the gearbox24and is converted into electrical energy for storage in the battery20. The M/G18may be referred to as providing negative torque when acting as a generator.

It should be understood that the schematic illustrated inFIG.1is merely exemplary and is not intended to be limiting. 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 the M/G18may be provided between the torque converter22and the gearbox24. Other configurations are contemplated without deviating from the scope of the present disclosure.

In hybrid vehicles, such as vehicle10depicted inFIG.1, the driveline torque is the summation of the engine torque and the motor torque less any slipping losses at the clutches, such as the disconnect clutch, and other losses. When the engine is pulled down (turned OFF) due to various requests, driveline torque is solely provided by the motor. The engine may be started (pulled up) based on various driver requests to continue providing the propulsive torque required to meet the driver-demanded wheel torques. When the engine and the motor are connected, the requested torque may be based on the driver demand and the engine torque in order to maintain battery state of charge and improve fuel economy.

Referring toFIG.2, the disconnect clutch26may be hydraulically actuated, e.g., a wet clutch. The disconnect connect clutch26may include a housing60that supports a plurality of separator plates62that are rotationally fixed to the housing and a plurality of friction plates64that are interleaved with the separator plates62. The friction plates64are rotatable relative to the housing. The collection of separator plates62and friction plates64may be referred to as a clutch pack66. The housing60is rotationally fixed to the engine or the M/G and the friction plates64are rotationally fixed to the other of the engine and the M/G. In the example embodiment, the housing60is rotationally fixed relative to the crankshaft of the engine and the friction plates64are fixed relative to the rotor shaft of the M/G. This is just an example, and any type of disconnect clutch may be used.

The clutch26is engaged and disengaged by flowing pressurized fluid to a hydraulic piston68. The piston68includes a portion70that contacts the clutch pack66. Movement of the piston towards the clutch pack (referred to as stroke) frictionally engages the separator plates and the friction plates to engage the clutch. Movement of the piston away from the clutch pack (referred to as de-stroke) disengages the clutch. The piston68includes an associated apply chamber72that is in fluid communication with a fluid source. Flowing pressured fluid into the chamber72strokes the piston68. Depressurizing fluid in the chamber72allows a return spring74to de-stroke the piston68. The piston68also includes associated seals76,77, and78. The seals may engage between the piston68and portions of the housing60.

Referring toFIG.3, a hydraulic clutch, such as the disconnect clutch26, may be controlled by a hydraulic system100that operates the piston68to control clutch torque capacity. The hydraulic system100is configured to deliver pressured fluid (oil) to the piston apply chamber72. The system100may be a branch of the hydraulic system of the transmission. The system100includes one or more lines, hoses, etc. that connect between the clutch26and a fluid source, e.g., a valve body. Fluid within the line102is pressurized by a pump or other device. In the simplified example, the line102connects to a solenoid106, e.g., an electrohydraulic valve, or other device capable of controlling an outlet pressure, flow rate, velocity, etc. to line104. The line104connects the clutch26to the solenoid106. During operation of the clutch26, the solenoid106is controlled to vary the fluid pressure, flow rate, etc. sent to the piston chamber72to achieve a desired clutch torque capacity. A pressure sensor108is configured to sense a measured pressure of the fluid within line104. The pressure sensor108is in electric communication with the controller and is configured to output data indicative of the measured pressure in line104. The pressure sensor108is upstream of the piston chamber72; therefore, a pressure difference may exist between the measured pressure at the sensor108and the pressure inside the piston chamber72. The vehicle may use modeling to infer the piston apply chamber pressure (or piston pressure) based on the measured pressure from sensor108. This inferred pressure is sometimes still referred to as a measured pressure as it is based on readings from a pressure sensor.

The torque capacity of a hydraulic clutch is controlled by commanding fluid to the associated piston to increase and decrease pressure applied to the clutch plates. A mathematical relationship between pressure of the hydraulic actuator and the torque capacity of the clutch may be referred to as a clutch transfer function. Such a transfer function may be utilized to control the clutch torque capacity. For example, the transfer function may be utilized to determine the clutch pressure command based on a desired torque capacity of the clutch.FIG.4illustrates an example clutch transfer function120. As can be seen, the transfer function120has hysteresis behavior resulting in a single pressure producing two different clutch torque capacities. This is due to the differences in torque observed for a given pressure during stroking and de-stroking the piston. The lower branch122models the behavior during stroking of the piston, whereas the upper branch124models the behavior during de-stroking of the piston.

In order to accurately model the torque transfer function, many clutch variables must be considered. An often-overlooked variable is the friction between the piston seal and the housing. This may be referred to as seal friction. The seal friction can be quite high and failure to consider it can result in an error as high as 10%. Inclusion of seal friction improves both the clutch pressure command based on the desired torque capacity and the actual clutch torque capacity calculation.

Generally, the forces acting on the clutch pack are the piston force minus the return spring force±the seal friction as shown below in equation 1, where p is the piston chamber pressure, Apis the area of the piston, FRSis the force of the return spring, and FSFis the force of the seal friction. FSFis negative during stroke and is positive during de-stroke. (When piston pressure is not available, a model may be used to infer the pressure from measured pressure at sensor108.)
Fnet=pAp−FRS−FSF(Eq. 1)

The seal friction can be represented as a function of pressure using equation 2, where p=clutch pressure and Toil=oil temperature. The piston displacement x indicates the initial transition between static friction and dynamic friction. The piston velocity dx/dt represents the directional dependence as well as the sensitivity of seal friction to the sliding velocity.

Measuring the displacement x and the velocity dx/dt are difficult. To avoid this impracticality, the seal friction model may be approximated as shown in the following equations. In equation 3, dp/dt (the change in pressure) represents the effect of dx/dt and the direction of piston motion x, g is a function indicating that the seal friction offset is a function of p, dp/dt and T, k1and k2are calibration parameters for online control processes that may be defined as a function of oil temperature (Toil). The output of equation 3 may be used as an offset (OffsetSF) in the below equations to determine an accurate clutch torque for a given pressure. The OffsetSFmay be in units of pressure or force. From equation 3, it can be seen that the seal friction offset is based on at least the pressure, the rate of change of pressure, and the oil temperature.

There are a number of variations to the above equation that may be used to represent the directional and pressure-dependent effects of FSFas a linear function or a non-linear function. One example is to simplify equation 3 to represent pressure dependent seal friction is as shown in equation 4. Again, the output of equation 4 may be used to determine OffsetSF.

The net force may be used for calculating the clutch torque capacity of the disconnect clutch based on a commanded pressure. Equation 5 illustrates the relationship between clutch pressure (p) and torque capacity (T) while accounting for forces/pressures of the return spring and the seal friction. In equation 5, μ is the coefficient of friction and Reffis the effective clutch radius.
T=μFnetReff=μReff(pAp−FRS±FSF)  (Eq. 5)

The constants Apand Reffcan be represented by a gain and the spring force and seal friction can be represented as offsets, e.g., OffsetSFand OffsetRS. These offsets may be added together to get a combined offset (Offsetcombined) to simplify the calculations as shown in equation 6. Equation 6 may be used by the controller to determine what pressure to command for a desired torque capacity of the clutch.

Referring toFIG.5, the seal friction changes during the engagement of the clutch. Actuating the clutch from the fully disengaged state to the fully engaged is shown as having four phases inFIG.5. The first stage is filling of the piston apply chamber. During this phase, the clutch capacity is nominal and the fluid being sent to the clutch is filling the piston apply chamber. Once the piston chamber is filled, additional fluid entering into the chamber increases the pressure on the piston and once the pressure overcomes the return spring force and the static seal friction, the piston begins the stroking phase where the piston begins to move towards the clutch pack. The seal friction is zero at the beginning of the stroking phase when the piston is stationary, and quickly builds to static level as indicated by the mini-peak at (a). At the beginning of the stroke phase, the seal deforms without sliding and the seal friction is large due to the higher static coefficient of friction. Once the piston starts sliding over the seal after overcoming the static friction (a), the friction drops corresponding to the low piston pressure as a result of the lower dynamic coefficient of friction. During the stroking phase, the seal friction (b) rises in proportion to the pressure and the sliding velocity, which may be equal to the piston sliding velocity. Near the end of the stroke phase, the seal friction (c) starts dropping due to reduced piston motion and drops to zero at (d) as the piston stops once fully stroked. By the end of this stage, the clutch torque capacity is enough for cranking the engine. As the engine is fired, it starts generating torque. To provide a smooth transition from electric-mode to engine-mode, the piston is partially de-stroked by dropping the piston pressure as shown in the de-stroking phase. During the de-stroking phase, the seal friction (e) reverses in direction indicated by the negative sign as the piston is moving in the opposite direction. The seal friction is generally low during the de-stroking phase compared to the stroking phase due to decreasing clutch pressure. A locking phase follows the de-stroking phase and is used to secure the clutch in the fully engaged position when the engine speed reaches the level of the motor speed. To prevent slip, a high pressure is applied. Seal friction (f) is low during the locking phase as piston movement is limited.

As discussed above, the engine may be started using the M/G. To start the engine this way, the disconnect clutch is gradually engaged to pull up the engine. During starting of the engine, a control module calculates a series of desired clutch torque capacities that accelerate the crankshaft from rest and slowly synchronize the speeds between the crankshaft and the rotor shaft, at which point, the clutch may be locked. The following controls focus on determining the appropriate pressures to command to the clutch in order to achieve the desired clutch torque capacities. The controls account for seal friction to improve accuracy.

Referring toFIG.6, a flowchart200of an algorithm for controlling clutch pressures is shown. The controls200may be executed in response to a request to start the engine with the electric machine. At operation202, the controller receives the previous clutch pressure commanded to the clutch. At operation204, the controller determines a current pressure of the clutch. The current pressure may be the estimated or inferred pressure inside the piston chamber. This pressure may be inferred or estimated based on a measured pressure from a sensor, e.g., sensor108. At operation206, the controller determines the direction of piston movement, i.e., stroke or de-stroke. The direction may be determined by comparing the previous clutch pressure of operation202and the current clutch pressure of operation204. That is, the direction of piston motion may be determined by taking the derivative of the pressure with respect to time. If the pressure is increasing, this indicates stroke; if the pressure is decreasing, this indicates de-stroke.

At operation208, the controller determines the seal friction offset. This may be calculated using the above-described equations. At operation210, the controller determines the return spring offset. These offsets are combined, e.g., added or subtracted, to determine a combined offset at operation212. At operation214, the controller determines the gain (see equation 6) to apply in determining the commanded pressure. The gain is determined based on the constants from Eq. 5, i.e. gain=μReffAp. It is then adjusted during the calibration phase based on variations observed under different use cases and operating conditions.

At operation216, the controller receives a desired clutch torque capacity for the disconnect clutch. Using equation 6, for example, or other methodology, the controller determines the clutch pressure to command to deliver the desired clutch torque capacity at operation218.

At operation220, the controller commands the pressure to the disconnect clutch. For example, the commanded pressure may be sent to a control module associated with controlling the solenoid106. This control module may convert the commanded pressure into a voltage or current signal that is sent to the solenoid to open or close the valve as desired.

While not shown, the after pressure is commanded, the controller can measure the actual pressure and calculate the true clutch capacity with inclusion of seal friction. The controller can also generate a leading torque signal.

The controls200may also be used to control the decoupling of the engine from the M/G, i.e., during disengagement of the clutch. The calculations are similar, but during decoupling, the return spring has a positive sign as it is aiding movement of the piston rather than providing a resistance. The gains and other calibrations may differ to account for the differences between engagement and disengagement of the clutch. See the example clutch transfer function above.

FIG.7illustrates an alternative control strategy250. The flowchart250is a slightly different approach thanFIG.6to calculate the seal friction offset. InFIG.6, the controls directly defined offset as a function of (p, dp/dt, and Toil) and explained how the seal friction is used for determining the command pressure based on desired torque capacity. However, the pressure commanded may not always be exactly achieved. The actual measured pressure may be slightly different from the command pressure. Therefore, the controllers can use this measured pressure and determine the true capacity of the clutch (so it can compare this to the desired torque and accordingly adapt the command pressure in the next time instant in case of variation between the two using a technique called adaptive controls).FIG.7describes the method for calculating the seal friction force as a function of p, dp/dt, and Toilinstead of calculating the seal friction offset. Then, this seal friction force is used along with return spring force, and piston force to calculate the net force using force balance. Once the net force is available, a Coulomb model may be used to calculate clutch torque. Separately, the controller may calculate the clutch capacity from just the gain and offset with a linear transfer function relation that is calibrated as a part of the strategy. The controller may compare the difference between the torque calculated using seal friction and the torque using linear transfer function (which does not include seal friction). The difference in the two gives the seal friction offset. So, based on this, the controller updates the offset used in the strategy. With this updated offset, the true clutch capacity can be calculated using the calibrated gain and pressure value (instantaneous or lead to get Tc and T_lead respectively).

Control begins at operation252where the controller determines a commanded pressure. At operation254, the controller determines a leading pressure response and instantaneous piston pressure. The leading signal is calculated based on the combination of 1storder delay applied to the command signal and lead-lag control. The piston pressure is determined using a model based on sensor pressure and estimated flow rate. At operation256, the controller determines the direction of piston motion e.g., stroke or de-stroke. At operation258, the controller determines the force of the seal friction. At operation260, the controller determines the net force acting on the clutch pack using equation 1, for example. Once the net force is known, the controller determines a first reference clutch torque capacity while accounting for seal friction at operation262. In operation264, the controller determines a second reference clutch torque capacity based on linear transfer function, which does not include the seal friction contribution. The second reference torque capacity is subtracted from the first reference torque capacity to determine the offset of the seal friction at operation266. At operation268, the offset value, which is based only on the return spring force and other factors not including the seal friction, is updated to include seal friction offset (this is the combined offset). At operation270, a true torque capacity is calculated using the updated offset of operation268, the clutch gain and the piston pressure.