Abstract:
A hill holding function of a hybrid electric vehicle is initiated when the powertrain control module determines that a hill holding condition exists. In the hill holding condition, the electro-hydraulic brakes will provide brake torque to each wheel and the powertrain control module will turn off the internal combustion engine. The system can also detect a two footer condition where the vehicle operator requests both acceleration torque and brake torque simultaneously. The hill holding function of the hybrid electric vehicle in a two footer situation will apply the electro-hydraulic brakes and turn off the internal combustion engine. When the operator requests acceleration in either the hill holding or the two footer condition, the electric brake controller will transition the release of the electro-hydraulic brakes and the powertrain control module will turn on the internal combustion engine.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a braking system for a hybrid electric vehicle and, more particularly, to a hill holding brake system for a hybrid electric vehicle.  
         [0003]     2. Background Art  
         [0004]     A conventional wheeled automotive vehicle includes an internal combustion engine powered by fossil fuels. The desire to reduce emissions and consumption of fossil fuels in an internal combustion engine vehicle is well established.  
         [0005]     An electric vehicle comprises a battery and an electrical generator motor powerplant system to provide torque to a set of wheels. However, electric vehicles have limited range, limited power capabilities, and require a substantial amount of time to recharge the battery. Additionally, electric vehicles require the development of an extensive infrastructure to recharge the battery and service the electric vehicles.  
         [0006]     A hybrid electric vehicle includes a conventional internal combustion engine powertrain and an electrical generator motor powerplant. Hybrid electric vehicles reduce emissions and consumption of fossil fuels. Hybrid electric vehicles address the limitations of the electric vehicle relating to battery life, vehicle range, vehicle performance, and vehicle infrastructure development requirements.  
         [0007]     Hybrid electric vehicles can have many different configurations. A limited storage requirement hybrid electric vehicle is one configuration having an internal combustion engine, providing tractive power to the wheels, in combination with an integrated starter generator motor powerplant, providing a small amount of tractive torque to the wheels. The integrated starter generator motor powerplant tractive torque is provided mainly as a boost. The integrated starter generator motor starts the internal combustion engine and charges the battery.  
         [0008]     Vehicle operators prefer hybrid electric vehicles that have braking and acceleration characteristics that are similar to a conventional internal combustion engine vehicle with an automatic transmission. Holding a vehicle on a hill when an operator&#39;s foot is taken off of the brake pedal, called hill holding, is a characteristic desired by vehicle operators. Hill holding in a conventional vehicle occurs when the vehicle is on a hill, the brake pedal and accelerator pedal are not actuated, and the automatic transmission is engaged. The vehicle powertrain delivers enough torque at idle from the internal combustion engine through the transmission to the wheels to hold the vehicle on a hill. The internal combustion engine in a hybrid electric vehicles wastes fuel and produces undesirable emissions to hold the vehicle on a hill because the internal combustion engine must continue to operate while the vehicle is stopped.  
         [0009]     The integrated starter generator motor in a hybrid electric vehicle is capable of delivering enough torque to hill hold. However, the use of the integrated starter generator motor in a hybrid electric vehicle wastes battery power and requires additional cooling during hill holding conditions.  
         [0010]     U.S. Pat. No. 6,321,144 to Crombez, for example, discloses a method and system for preventing roll back in an electric vehicle and a hybrid electric vehicle. The rotary electric traction motor disclosed in Crombez is capable of bi-directional operation and is not directly connected to the internal combustion engine.  
         [0011]     The present invention is directed to providing a robust hill holding system that reduces emissions, increases battery range, and reduces fuel consumption in a hybrid electric vehicle with an integrated starter generator motor.  
       SUMMARY OF INVENTION  
       [0012]     The present invention relates to a hill holding control method and system for a hybrid vehicle having an internal combustion engine, an integrated starter generator motor, and a electro-hydraulic brake system that provides hydraulic brake torque during a hill holding condition. The present invention improves the operating efficiency and driveability of the hybrid electric vehicle.  
         [0013]     According to one aspect of the invention, a hybrid electric vehicle is provided that includes an internal combustion engine that rotates in a single direction and is connected to an integrated starter generator motor. The integrated starter generator motor is provided for starting the internal combustion engine. The internal combustion engine and the integrated starter generator motor may selectively drive a set of wheels and provide brake torque at each driven wheel. A vehicle operator can selectively actuate the electro-hydraulic brake system. An electronic brake control system also controls the electro-hydraulic brake system and controls the level of electro-hydraulic brake torque applied to the wheels by the electro-hydraulic brake system. The electronic brake control system actuates the electro-hydraulic brake to hold the vehicle on a hill instead of using engine compression braking torque or integrated starter generator motor braking torque. A powertrain control module turns off the internal combustion engine while the electro-hydraulic brakes are applied during hill holding conditions.  
         [0014]     When a vehicle operator requests acceleration during the hill holding condition, the electronic brake control system reduces electro-hydraulic brake torque at the wheels and the powertrain control module turns on the internal combustion engine. A vehicle transmission is engaged for seamless acceleration following the hill holding brake application.  
         [0015]     Another aspect of the invention relates to the method of holding a hybrid electric vehicle on a hill. A vehicle roll-back state, a vehicle brake pedal actuation measurement, a powertrain pedal actuation measurement, and an internal combustion engine running state are monitored by the powertrain control module to determine if the hybrid electric vehicle is in a hill holding condition. In a hill holding condition the electronic brake controller actuates the electro-hydraulic brakes and the powertrain control module turns off the internal combustion engine.  
         [0016]     There are numerous benefits accruing to the method and system of the present invention. For example, the method and system: 
        1) improves the operating efficiency of the hybrid electric vehicle by not using the integrated starter generator motor to provide this function and eliminates the need for additional cooling for the integrated starter generator motor;     2) improves the drive-ability of the hybrid electric vehicle;     3) is capable of holding the hybrid electric vehicle on a hill of greater grade than present transmissions;     4) allows a manual transmission vehicle to emulate the hill holding function of a automatic transmission vehicle;     5) allows automatic transmissions a more efficient method of hill holding since the clutch pressure does not have to be maintained to provide hill holding; and     6) allows hill holding to be performed more efficiently than present day transmission hill holding systems by not using the transmission to provide this function saving energy by not keeping the clutch pressure on and eliminating the need for additional cooling for the transmission.        
 
         [0023]     These and other aspects of the invention will be apparent to one of ordinary skill in the art in view of the attached drawings and following detailed description of the preferred embodiment. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0024]      FIG. 1  is a schematic diagram illustrating a preferred hybrid electric vehicle configuration on which the method of the present invention can be implemented;  
         [0025]      FIG. 2  is a schematic diagram illustrating a representative torque controller for the hybrid electric vehicle&#39;s dual powerplants;  
         [0026]      FIG. 3  is a table of hybrid electric vehicle conditions which dictate hill holding strategies;  
         [0027]      FIG. 4  is a flowchart of vehicle transition when the engine is on and the operator desires to creep the vehicle forward;  
         [0028]      FIG. 5  is a flowchart of vehicle transition when the engine is off and the operator desires to creep the vehicle forward;  
         [0029]      FIG. 6  is a flowchart of vehicle transition for when the engine is off;  
         [0030]      FIG. 7  is a flowchart of vehicle transition for when the engine is on;  
         [0031]      FIG. 8  is a flowchart of vehicle transition for when the engine is on with no electro-hydraulic brakes and the operator desires to creep the vehicle forward;  
         [0032]      FIG. 9  is a flowchart of vehicle transition for when the engine is off with no electro-hydraulic brakes and the operator desires to creep the vehicle forward;  
         [0033]      FIG. 10  is a flowchart of vehicle transition for when the engine is off with no electro-hydraulic brakes; and  
         [0034]      FIG. 11  is a flowchart of vehicle transition for when the engine is on with no electro-hydraulic brakes. 
     
    
     DETAILED DESCRIPTION  
       [0035]     Referring now to the drawings figures, there is illustrated in  FIG. 1 a  representative configuration of a hybrid electric vehicle  10  having a internal combustion engine  12  coupled to an integrated starter generator motor  14 . The two powerplants are coupled by a clutch  16  to a transmission  18 , and a differential  20  to provide torque to a set of vehicle wheels  22 . A powertrain control module  24  controls the operating parameters of the internal combustion engine  12  and the integrated starter generator motor  14 . An electronic brake control system  26  controls a set of electro-hydraulic brakes  28 . Both controllers are shown as logical units, and can be embodied in one or more separate controller or computer-controlled devices. A vehicle communication data network  30  enables communications between the hybrid electric vehicle components, the electronic brake controller  26  and the powertrain control module  24  at suitable update rates.  
         [0036]     The powertrain control module  24  provides adaptive filtering that activates when the clutch  16  is engaged as the vehicle accelerates from a hill holding condition. The adaptive filtering could be continuously variable or could be provided by a sequence of individual filters.  
         [0037]     Referring now to  FIG. 2 , a diagram of the hybrid electric vehicle torque control system is provided. The system coordinates the powertrain control module  24  with the electronic brake controller  26  to provide hill holding in the hybrid electric vehicle.  
         [0038]     Inputs to the powertrain control module  24  include a gear selector switch  42  that provides the powertrain control module  24  with information relating to the position of the gear selector switch  42 . A powertrain torque sensor provides information relating to the amount of torque delivered by the powertrain  44  to the powertrain control module  24 . An accelerator pedal sensor  46  provides the powertrain control module  24  with information corresponding to the position of an accelerator pedal. A vehicle speed sensor  48  provides a value corresponding to the speed of the vehicle to the powertrain control module  24 . Values corresponding to a torque limit of the powertrain are obtained at 50 from a lookup tables and provided to the powertrain control module  24 .  
         [0039]     Inputs to the powertrain control module  24  from the electronic brake control system  26  include a master cylinder pressure  52  measured by a master cylinder pressure sensor. The electronic brake control system  26  sends a torque modification request at  54  to the powertrain control module  24 .  
         [0040]     Inputs to the electronic brake control system  26  from the powertrain control module  24  include the powertrain negative torque limit from a powertrain torque sensor at  56 . The powertrain control module  24  provides information from an accelerator position sensor at  58  to the electronic brake controller  26 . The vehicle speed sensors  48  and the gear selector switch  42  are analyzed by the powertrain control module to provide a vehicle direction at  60 . The powertrain control module  24  provides the electronic brake controller  26  with data representative of the powertrain torque request at  62 , the torque delivered at  64 , the final torque request at  66 , and the engine on/off state  68 .  
         [0041]     The powertrain control module, using inputs from the gear selector switch  42  and vehicle speed  48 , provides the electronic brake controller  26  with the PRDNE direction  70 .  
         [0042]     Inputs to the electronic brake controller  26  also include the wheel speed data at  72  from the wheel speed sensors. Brake pedal position sensors provide information relating to a brake pedal request at  74 . Brake pressure sensors provide an input for brake pressures at  76 .  
         [0043]     The electronic brake controller  26  provides a brake pressure required output at  78  that controls the electro-hydraulic brakes  28  of the vehicle.  
         [0044]     The data inputs and outputs of the powertrain control module  24  and the electronic brake controller  26  process in a vehicle level algorithm and a brake algorithm. The hybrid electric vehicle using the vehicle level algorithm and brake algorithm provides electro-hydraulic brake torque reduction by the electronic brake control system during driver commanded vehicle acceleration from a vehicle stopped on a hill.  
         [0045]     When the vehicle begins to accelerate under normal driving conditions, the vehicle algorithm reduces the electro-hydraulic brake torque using an adaptive filter that allows the powertrain system to start and provide the traction torque at the same rate. The adaptive filtering can be continuously variable or a sequence of individual filters. The different filters and control logic are determined by vehicle conditions, including environmental conditions, and conditions of vehicle components such as: the clutch, the transmission, the brakes, the engine, the motor, and the battery. Clutch filters may be dependent upon the clutch status, clutch wear, clutch temperature, clutch pressure, and clutch input and output speeds. Transmission filters may be dependent upon transmission current gear, transmission next gear, and transmission configuration. Brake filters may be dependent upon the driver brake pedal command, brake system fault conditions, brake fade/temperature conditions, and the ABS status. Engine and motor filters may be dependent upon master cylinder pressure, throttle angle, engine friction, engine speed, motor speed, motor efficiency, engine brake torque, engine emissions and engine fuel rate. Other miscellaneous filters and control logic may include such factors as the battery state of charge, energy storage device fault conditions, wheel slip conditions, driveline resonance and road surface conditions.  
         [0046]     When the acceleration is complete and the vehicle engages the clutch, the vehicle algorithm decreases the powertrain negative torque limit  62  with an adaptive filter such that the electro-hydraulic brake torque goes off at the same rate and the traction torque is increased. This method of control provides optimal drive-ability and energy savings allowing a seamless handoff between the electro-hydraulic brake torque and the powertrain torque.  
         [0047]     Referring now to  FIG. 3 , a table  90  of hybrid electric vehicle conditions that dictate the hill holding strategy is provided.  
         [0048]     The powertrain control module compares the vehicle speed  48  from the wheel speed sensor with a calibratable vehicle creep speed in column  92 . The vehicle creep speed is typically set at  6  miles per hour on a zero percent grade. The application of both the brake pedal and the accelerator pedal determines the existence of a two footer condition. During a two footer condition the vehicle can be on a grade in a forward or a reverse gear, the vehicle operator actuates both the accelerator pedal for a accelerator torque request and the brake pedal for a brake torque request, and the magnitude of the brake torque request is greater than the accelerator torque request.  
         [0049]     The results of the vehicle rollback determination is displayed in column  94 . Vehicle rollback occurs when the driver releases both the accelerator and brake pedals with the vehicle at a rest condition on a grade, while the vehicle is in a forward gear  82  and starts rolling backwards, or when the vehicle is in a reverse gear  82  and starts rolling forward. Column  96  displays the results of electronic brake controller comparison of the brake pedal actuation received by a brake pedal sensor with a calibratable predetermined force X, measured in pounds per square inch. Column  98  displays the powertrain control module comparison of an accelerator pedal actuation, measured by an accelerator pedal sensor, with a calibratable predetermined percentage value Z. Column  100  displays the internal combustion engine&#39;s running state from an engine sensor.  
         [0050]     Rows  102  and  134  result in the vehicle in a two footer condition with the engine on and the electro-hydraulic brakes applying at grade hold torque. The electro-hydraulic brakes applies at the wheel cylinders and is mechanically summed with the total torque request, which is applied additionally. The hill holding strategy maintains electro-hydraulic brakes at the grade hold torque amount, for example, the amount of torque needed to hold the vehicle on approximately  3 % grade. The hill holding strategy calculates the total torque request by summing the accelerator pedal torque request and brake pedal torque request.  
         [0051]     When the total torque request is greater than zero, the accelerator pedal torque request at the total torque request amount is added if the magnitude of total torque request is greater than the magnitude of grade hold torque. The hill holding strategy proceeds to the  FIG. 4  flowchart of the vehicle transition when the engine is on and operator desires to creep vehicle forward.  
         [0052]     When the total torque request is less than zero, additional vehicle friction brake torque is applied as follows.  
         [0053]     If the magnitude of brake pedal torque request is greater than the magnitude of grade hold torque, then the brake torque request applies at the brake pedal torque request minus the grade hold torque.  
         [0054]     If the magnitude of brake pedal torque request is less than the magnitude of grade hold torque, then no additional friction brake torque is applied and the hill holding strategy proceeds to the  FIG. 7  flowchart of the transition for when the engine is on.  
         [0055]     If total torque request equals zero, then the hill holding strategy proceeds to the  FIG. 7  flowchart of the transition for when the engine is on.  
         [0056]     Rows  104  and  136  result in the vehicle in a two footer condition with the engine off and the electro-hydraulic brakes applying at grade hold torque. The hill holding strategy continues to apply electro-hydraulic brakes at the grade hold torque amount and adds total torque request as follows.  
         [0057]     When the total torque request is greater than zero, the vehicle acceleration pedal at total torque request amount is added if the magnitude of total torque request is greater than the magnitude of grade hold torque. The hill holding strategy proceeds to the  FIG. 5  flowchart of vehicle transition when the engine is off and the operator desires to creep the vehicle forward.  
         [0058]     When the total torque request is less than zero, the vehicle brake torque is added as follows.  
         [0059]     If the magnitude of the brake pedal torque request is greater than magnitude of grade hold torque, brake pedal torque request applies at brake pedal torque request minus the grade hold torque.  
         [0060]     If the magnitude of the brake pedal torque request is less than the magnitude of grade hold torque, no additional friction brake torque is applied. The hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0061]     If the total torque request equals zero, the hill holding strategy proceeds to the  FIG. 6  flowchart of the transition for when the engine is off.  
         [0062]     Rows  106  and  138  result in the engine on with a brake pedal torque request, and the electro-hydraulic brakes applying at grade hold torque. Additional brake pedal torque request is added as follows.  
         [0063]     When the magnitude of the brake pedal torque request is greater than magnitude of the grade hold torque, electro-hydraulic brakes apply at the grade hold torque adding the difference between the brake pedal torque request and grade hold torque.  
         [0064]     When the magnitude of the brake pedal torque request is less than the magnitude of grade hold torque, electro-hydraulic brakes apply at grade hold torque.  
         [0065]     Rows  108  and  140  result in the engine off with a brake pedal torque request, and electro-hydraulic brakes applying at the grade hold torque.  
         [0066]     The strategy continues with the electro-hydraulic brakes applying at the grade hold torque plus additional brake pedal torque request is added as follows.  
         [0067]     When the magnitude of the brake pedal torque request is greater than the magnitude of grade hold torque, electro-hydraulic brakes apply at grade hold torque amount adding the difference between the brake pedal torque request and the grade hold torque.  
         [0068]     When the magnitude of the brake pedal torque request is less than the magnitude of grade hold torque, the electro-hydraulic brakes apply at grade hold torque.  
         [0069]     When the magnitude of the brake pedal torque request is less than X psi, where X is predetermined and calibratable pressure measured in psi, the hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0070]     Row  110  results in the engine on with an accelerator pedal torque request, and electro-hydraulic brakes applying at grade hold torque. The electro-hydraulic brakes applies at the grade hold torque amount with the additional accelerator pedal torque request is added as follows.  
         [0071]     When the magnitude of the accelerator pedal torque request is greater than the magnitude of grade hold torque, the hill holding strategy proceeds to the  FIG. 4  flowchart of the vehicle transition when the engine is on and operator desires to creep vehicle forward.  
         [0072]     When the magnitude of the accelerator pedal torque request is less than magnitude of grade hold torque, the hill holding strategy proceeds to the  FIG. 7  flowchart of the vehicle transition for when the engine is on.  
         [0073]     Row  112  results in engine off with an accelerator pedal torque request, and electro-hydraulic brakes applying at grade hold torque. Additional accelerator pedal torque request is added as follows.  
         [0074]     When the magnitude of the accelerator pedal torque request is greater than the magnitude of grade hold torque, the hill holding strategy proceeds to the  FIG. 5  flowchart of the vehicle transition when the engine is off and the operator desires to creep the vehicle forward.  
         [0075]     When the magnitude of the accelerator pedal torque request is less than the magnitude of grade hold torque, the hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0076]     Row  114  results in the engine on and the electro-hydraulic brakes applying at grade hold torque. The hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0077]     Row  116  results in the engine off and the electro-hydraulic brakes applying at grade hold torque with the hill holding strategy waiting for a vehicle condition change.  
         [0078]     Rows  118  and  150  result in the vehicle engine on in the two footer condition. The total torque request applies as follows.  
         [0079]     When the total torque request is greater than zero, vehicle acceleration pedal applies at the total torque request amount. The hill holding strategy proceeds to the  FIG. 8  flowchart of the vehicle transition for when the engine is on with no electro-hydraulic brakes and the operator desires to creep the vehicle forward.  
         [0080]     When the total torque request is less than zero, vehicle brake torque applies at the total torque request amount, the hill holding strategy proceeds to the  FIG. 11  flowchart of the vehicle transition for when the engine is on with no electro-hydraulic brakes.  
         [0081]     When the total torque request equals zero, apply nothing. The hill holding strategy proceeds to the  FIG. 11  flowchart of the vehicle transition for when the engine is on with no electro-hydraulic brakes.  
         [0082]     Row  120  results in the engine off in a two footer condition. The total torque request applies according to the following.  
         [0083]     When the total torque request is greater than zero, the vehicle acceleration applies at total torque request amount. The hill holding strategy proceeds to the  FIG. 9  flowchart of the vehicle transition for when the engine is off with no electro-hydraulic brakes and the operator desires to creep the vehicle forward.  
         [0084]     When the total torque request is less than zero, the vehicle brake torque applies at total torque request amount. The hill holding strategy proceeds to the  FIG. 10  flowchart of the vehicle transition for when the engine is off with no electro-hydraulic brakes.  
         [0085]     When the total torque request equals zero, apply nothing. The hill holding strategy proceeds to the  FIG. 10  flowchart of the vehicle transition for when the engine is off with no electro-hydraulic brakes.  
         [0086]     Rows  122  and  154  result in engine on and applying the brake pedal torque request.  
         [0087]     Rows  124  and  156  result in engine off and applying the brake pedal torque request. If the magnitude of brake pedal torque request is less than X psi, the hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0088]     Rows  126  and  158  results in engine on and applying the accelerator pedal torque request. If the vehicle speed is less than or equal to the creep speed then the hill holding strategy proceeds to the  FIG. 8  flowchart of the vehicle transition for when the engine is on with no electro-hydraulic brakes and the operator desires to creep the vehicle forward. If the vehicle speed is not less than the creep speed then the hill holding strategy proceeds to  FIG. 11  flowchart of the vehicle transition for when the engine is on with no electro-hydraulic brakes.  
         [0089]     Rows  128  and  160  result in the engine off and applying the accelerator pedal torque request. The hill holding strategy proceeds to the  FIG. 9  flowchart of the vehicle transition for when the engine is off with no electro-hydraulic brakes and the operator desires to creep the vehicle forward.  
         [0090]     Rows  130  and  162  results in the engine on and applying nothing.  
         [0091]     Row  132  results in engine off and the hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0092]     Row  142  results in engine on with an accelerator pedal torque request and the electro-hydraulic brakes applying at the grade hold torque. The accelerator pedal torque request is added as follows.  
         [0093]     When the magnitude of the accelerator pedal torque request is greater than or less than the magnitude of grade hold torque, the hill holding strategy proceeds to the  FIG. 4  flowchart of the vehicle transition when the engine is on and operator desires to creep vehicle forward.  
         [0094]     Row  144  results in engine off and electro-hydraulic brakes applying at the grade hold torque plus the accelerator pedal torque request.  
         [0095]     When the magnitude of the accelerator pedal torque request is greater than or less than the magnitude of grade hold torque, the hill holding strategy proceeds to the  FIG. 5  flowchart of vehicle transition when the engine is off and the operator desires to creep the vehicle forward.  
         [0096]     Row  146  results in the engine on and the electro-hydraulic brakes applying at grade hold torque.  
         [0097]     Row  148  results in engine off, electro-hydraulic brakes applying at grade hold torque, the hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0098]     Row  152  results in engine off in the two footer condition. The total torque request applies according to the following.  
         [0099]     When the total torque request is greater than or less than zero, vehicle acceleration applies at the total torque request amount. The hill holding strategy proceeds to the  FIG. 9  flowchart of the vehicle transition for when the engine is off with no electro-hydraulic brakes and the operator desires to creep the vehicle forward.  
         [0100]     When the total torque request equals zero, apply nothing. The hill holding strategy proceeds to the  FIG. 9  flowchart of transition for when the engine is off with no electro-hydraulic brakes and the operator desires to creep the vehicle forward.  
         [0101]     Row  164  results in the engine off and nothing applied. The hill holding strategy proceeds to the  FIG. 6  flowchart of the vehicle transition for when the engine is off.  
         [0102]     Referring to  FIG. 4 , flowchart of the vehicle transition when the engine is on and operator desires to creep vehicle forward is provided.  
         [0103]     In step  170 , during each frame interval the following events occur. The forward clutch is applied. Next, a crankshaft torque sensor computes a traction torque at the crankshaft. The crankshaft torque can also be determined by sensing various engine factors such as speed, throttle angle, and fuel rate. Corrections to these input factors should be made if these factors were modified to perform additional functions that do not normally translate to drag on the drivetrain, for example some of the load of the engine may be diverted to supply the auxiliary load or charge or discharge the energy storage subsystem and may reflect as a higher throttle angle. The engagement factor is determined by sensing various clutch factors, such as pressure, to obtain the knowledge of what percentage of the torque applied to the clutch can be obtained at the clutch output. The gear ratio from the engine to the wheel is computed by knowledge of the present gear. The traction torque at the crankshaft computation, the engagement factor computation and the gear ratio from the engine to the wheel computation are a function of the driver accelerator pedal command and may be preprogrammed in an accelerator pedal map. The traction torque at the crankshaft computation, the engagement factor computation and the gear ratio from the engine to the wheel computation compute the desired traction torque at each wheel.  
         [0104]     Next in step  172 , wheel traction torque desired is summed with the wheel brake torque desired to compute the total wheel torque delivered.  
         [0105]     In decision step  174 , the total wheel torque delivered is compared to zero.  
         [0106]     If the total wheel torque delivered is greater then zero, then the flowchart continues to step  176 . In step  176 , the electro-hydraulic brakes torque is computed as the difference between the grade hold torque and the total wheel torque delivered.  
         [0107]     If the total wheel torque delivered is less than or equal to zero, then the flowchart continues to decision step  178 . In step  178 , the magnitude of the brake pedal torque request is compared with the magnitude of the grade hold torque.  
         [0108]     When the magnitude of the brake pedal torque request is greater than the magnitude of the grade hold torque, the flowchart proceeds to step  180  where wheel brake torque desired is calculated by summing the electro-hydraulic brakes torque at grade hold torque and the brake pedal torque request minus the grade hold torque.  
         [0109]     When the magnitude of the brake pedal torque request is less than or equal to the magnitude of the grade hold torque, the flowchart proceeds to step  182  where wheel brake torque desired is set to the electro-hydraulic brakes torque at grade hold torque.  
         [0110]     Steps  176 ,  180  and  182  then proceed to step  184  where the total wheel torque delivered is computed as the difference between the wheel traction torque desired and the wheel brake torque desired.  
         [0111]     Next in step  186  the electro-hydraulic brake torque is compared to zero. If the electro-hydraulic brake torque is equal to zero, then the hill holding strategy returns to the analysis in table of  FIG. 3 . If the electro-hydraulic brake torque is not equal to zero the analysis returns to the top of the flowchart to step  170 .  
         [0112]     Referring to  FIG. 5 , a flowchart of vehicle transition when the engine is off and the operator desires to creep the vehicle forward is provided. During each frame interval the powertrain control module starts the engine in step  200 . Step  202  shows that the hill holding strategy proceeds to the  FIG. 4  flowchart of the vehicle transition when the engine is on and the operator desires to creep the vehicle forward.  
         [0113]     Referring to  FIG. 6 , a flowchart of the vehicle transition for when the engine is off is provided. During each frame interval the powertrain control module starts the engine in step  204 . Step  206  shows that the hill holding strategy proceeds to the  FIG. 7  flowchart of the vehicle transition for when the engine is on.  
         [0114]     Referring to  FIG. 7 , a flowchart of the vehicle transition for when the engine is on is provided. In step  210 , the desired traction torque at each wheel is zero. The wheel brake torque desired is equal to the electro-hydraulic brakes torque. The flowchart continues to decision step  212  where the magnitude of the brake pedal torque request and the magnitude of the grade hold torque are compared to each other. The comparison is used to compute the amount of electro-hydraulic brake torque to be delivered at each wheel by the hydraulic system.  
         [0115]     When the magnitude of the brake pedal torque request is greater than magnitude of grade hold torque, the flowchart goes to step  214 . In step  214 , the wheel brake torque desired equals electro-hydraulic brake torque at grade hold torque plus the difference between the brake pedal torque request and the grade hold torque.  
         [0116]     When the magnitude of the brake pedal torque request is less than or equal to the magnitude of grade hold torque, the flowchart goes to step  216 . No additional friction brake torque is applied. The wheel brake torque desired equals electro-hydraulic brake torque at grade hold torque.  
         [0117]     Steps  214  and  216  continue to step  218  where the total wheel torque delivered is computed by adding the wheel traction torque desired with the wheel brake torque desired.  
         [0118]     The flowchart continues with decision step  220  where the electro-hydraulic brake torque is compared to zero. If the electro-hydraulic brake torque is equal to zero the flowchart goes back to the initial analysis in  FIG. 3  table as shown in step  222 . If the electro-hydraulic brake torque is not equal to zero the system returns to step  210  of the  FIG. 7  flowchart.  
         [0119]     Referring to  FIG. 8  flowchart of the vehicle transition when the engine is on with no electro-hydraulic brakes and operator desires to creep vehicle forward.  
         [0120]     In step  230 , during each frame interval the following events occur. The forward clutch applies. Next, a crankshaft torque sensor, or other sensing methods enable the computation of the traction torque at the crankshaft. Clutch sensors enable computation of the engagement factor. Knowledge of present gear enables computation of the gear ratio from the engine to the wheel. The traction torque at the crankshaft computation, the engagement factor computation and the gear ratio from the engine to the wheel computation are a function of the accelerator pedal command and may be preprogrammed in an accelerator pedal map. The traction torque at the crankshaft computation, the engagement factor computation and the gear ratio from the engine to the wheel computation are used to compute the desired traction torque at each wheel.  
         [0121]     Next in step  232 , wheel traction torque desired is summed with the wheel brake torque desire to compute the total wheel torque delivered.  
         [0122]     Next in step  234 , the clutch is determined to be fully engaged or not. If the clutch is determined to be fully engaged, then the flowchart strategy returns to the analysis in table  90  of  FIG. 3 . If the electro-hydraulic brake torque is not equal to zero the analysis returns to step  230  of the flowchart of the vehicle transition when the engine is on with no electro hydraulic brakes and operator desires to creep vehicle forward.  
         [0123]     Referring to  FIG. 9 , a flowchart of transition for when the engine is off with no electro-hydraulic brakes and the operator desires to creep the vehicle forward is provided.  
         [0124]     In step  240 , the powertrain control module turns the engine on. The flowchart continues to step  242 , where the hill holding strategy proceeds to the  FIG. 8  flowchart of the vehicle transition when the engine is on with no electro-hydraulic brakes and the operator desires to creep the vehicle forward.  
         [0125]      FIG. 10  flowchart of transition for when the engine is off with no electro-hydraulic brakes. In step  244 , the powertrain control module turns the engine on. The flowchart continues to step  246 , where the hill holding strategy proceeds to the  FIG. 11  flowchart of the vehicle transition when the engine is on with no electro-hydraulic brakes.  
         [0126]     Referring to  FIG. 11 , a flowchart of transition when the engine is on with no electro-hydraulic brakes is provided.  
         [0127]     In step  260 , the wheel traction torque desired at each wheel is zero.  
         [0128]     In step  262 , the amount of electro-hydraulic brake torque to be delivered at each wheel by the hydraulic system is zero and wheel brake torque desired equals brake pedal torque request.  
         [0129]     In step  264 , the total wheel torque delivered equals wheel traction torque desired plus the wheel brake torque desired.  
         [0130]     The flowchart continues with decision step  266  where the decision on clutch engagement is made. If the clutch is engaged the flowchart goes back to the initial analysis in  FIG. 3  table  90 . If the clutch is not fully engaged the flowchart proceeds to step  260  of the  FIG. 11  flowchart.  
         [0131]     While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.