Abstract:
A method of regulating operation of a hybrid vehicle traveling on a surface having a grade includes determining a drive force of the hybrid vehicle, calculating a brake pressure value and determining whether a grade freeze condition exists based on the brake pressure value. The method further includes calculating a grade value of the surface based on the drive force when the freeze condition does not exist and regulating operation of the hybrid vehicle based on the grade value.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional application No. 60/815.151, filed on Jun. 20, 2006. The disclosure of the above application is incorporated herein by reference. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to hybrid vehicles, and more particularly to a system for determining a grade angle of a road that a hybrid vehicle is traveling on. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hybrid vehicles are driven by multiple powerplants including, but not limited to an internal combustion engine and an electric machine. The electric machine functions as a motor/generator. In a generator mode, the electric machine is driven by the engine to generate electrical energy used to power electrical loads or charge batteries. In a motor mode, the electric machine supplements the engine, providing drive torque to drive the vehicle drivetrain. 
         [0004]    During hybrid vehicle operation, the hybrid vehicle travels on various degrees of road grade, which is the road angle relative to horizontal. The degree of the road grade often influences driving behavior and vehicle operating parameters. Furthermore, it is desirable to be able to regulate certain vehicle operating conditions based on the road grade. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, the present invention provides a method of regulating operation of a hybrid vehicle traveling on a surface having a grade. The method includes determining a drive force of the hybrid vehicle, calculating a brake pressure value and determining whether a grade freeze condition exists based on the brake pressure value. The method further includes calculating a grade value of the surface based on the drive force when the freeze condition does not exist and regulating operation of the hybrid vehicle based on the grade value. 
         [0006]    In other features, the method further includes holding the grade value equal to a previous grade value when the freeze condition exits, Operation of the hybrid vehicle is regulated based on the grade value upon expiration of a predetermined time period after the freeze condition transitions from existing to not existing. 
         [0007]    In another feature, the method further includes indicating a brake on status when the brake pressure value exceeds a threshold value. The grade freeze condition exists when the brake on status is indicated. 
         [0008]    In another feature, the method further includes calculating a chassis braking force based on the brake pressure value and a vehicle speed, The grade value is further determined based on the chassis braking force. 
         [0009]    In another feature, the method further includes calculating the grade value as a tangent of a cosine of a quotient of a grade force and a product of a hybrid vehicle mass and a gravitational constant. 
         [0010]    In still another feature, the method further includes filtering the grade value. 
         [0011]    In yet other features, the method further includes monitoring a plurality of grade freeze conditions and indicating that the freeze condition does not exist when one of the plurality of freeze conditions is not true. The plurality of freeze conditions include at least one of a brake on condition, a shift in progress condition, a time since a gear shift being less than a respective threshold time condition a time since a range shift being less than a respective threshold time condition, a reduced vehicle speed condition, a time since a wheel slip was detected being less than a respective threshold time condition, a rate of throttle change condition, a time since a throttle change being less than a respective threshold time condition, a rate of braking change condition, a time since a braking change being less than a respective threshold time condition and a time since a fuel off event being less than a respective threshold time condition. 
         [0012]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0014]      FIG. 1  is a functional block diagram of an exemplary hybrid vehicle that is operated based on the road grade determination control of the present invention; 
           [0015]      FIG. 2  is a schematic illustration of an exemplay hybrid vehicle including forces acting thereon; 
           [0016]      FIG. 3  is a flowchart illustrating exemplary steps executed by the road grade determination control of the present invention; and 
           [0017]      FIG. 4  is a functional block diagram of exemplary modules that execute the road grade determination control of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. 
         [0019]    Referring now to  FIG. 1 , an exemplary hybrid vehicle  10  includes an engine  12  and an electric machine  14 , which drive a transmission  16 . Air is drawn into the engine  12  through a throttle  13 , whose position is regulated by a throttle actuator  15 . The air is mixed with fuel, and the air/fuel mixture is combusted within cylinders (not shown) to generate drive torque. The electric machine  14  supplements the engine  12  to produce drive torque to drive the transmission  16 . In this manner, fuel efficiency is increased and emissions are reduced. The engine  12  and electric machine  14  are coupled via a belt-alternator-starter (BAS) system  18 . More specifically, the electric machine  14  operates as a starter (i.e., motor) and an alternator (i.e., generator) and is coupled to the engine  12  through a belt and pulley system. The engine  12  and the electric machine  14  include pulleys  20 ,  22 , respectively, that are coupled for rotation by a belt  24 . The pulley  20  is coupled for rotation with a crankshaft  26  of the engine  12 . 
         [0020]    In one mode, the engine  12  drives the electric machine  14  to generate power used to recharge an energy storage device (ESD)  28 . In another mode, the electric machine  14  drives the engine  12  using energy from the ESD  28 . The ESD  28  can include, but is not limited to, a battery or a super-capacitor. Alternatively, the BAS system  18  can be replaced with a flywheel-alternator-starter (FAS) system (not shown), which includes an electric machine operably disposed between the engine and the transmission or a chain or gear system that is implemented between the electric machine  14  and the crankshaft  26 . 
         [0021]    The transmission  16  can include, but is not limited to, a manual transmission, an automatic transmission, a continuously variable transmission (CVT) and an automated manual transmission (AMT). Drive torque is transferred from the engine crankshaft  26  to the transmission  16  through a coupling device  30 . The coupling device  30  can include, but is not limited to, a friction clutch or a torque converter depending upon the type of transmission implemented. The transmission  16  multiplies the drive torque through one of a plurality of gear ratios to drive a driveshaft  32 . 
         [0022]    A control module  34  regulates operation of the vehicle  10 . The control module  34  controls fuel injection and spark to selectively activate and deactivate cylinders of the engine  12 . More specifically, when the vehicle  10  is at rest, none of the cylinders of the engine  12  are firing (i.e., are deactivated) and the engine  12  is stopped. During vehicle launch (i.e., acceleration from rest), the electric machine  14  drives the crankshaft to spin-up the engine  12  to an idle RPM and to initiate vehicle acceleration. During periods where low drive torque is needed to drive the vehicle, the engine cylinders do not fire and the valves can be deactivated. Drive torque is provided by the electric machine  14 . When deactivated, fuel and spark are cut-off to the cylinders of the engine  12 . Further, opening and closing cycles of the intake and exhaust valves can be prevented to inhibit air flow processing with the cylinders. 
         [0023]    An accelerator pedal  36  is provided. A pedal position sensor  36  is sensitive to a position of the accelerator pedal  36  and generates a pedal position signal based thereon. A brake pedal  40  is provided, and a brake pedal position sensor  42  is sensitive to a position of the brake pedal  40  and generates a pedal position signal based thereon. The control module  34  operates a brake system  43  based on the brake pedal position signal to adjust a pressure within the brake system, which in turn regulates a braking force of brakes (not shown). A brake position sensor  45  is provided in the brake system  43  and generates a brake pressure signal (BPS) corresponding to a brake fluid pressure downstream of a master cylinder (not shown). 
         [0024]    The road grade determination control of the present invention determines the grade angle (θ) of the road being traveled. The grade angle is expressed as a signed percent grade or tangent (θ), wherein a 100% grade is equal to a grade angle of 45° (i.e., 100%·tan(45°)=100%). For example, a 4° grade angle is equal to a 6.99% grade (i.e., tan(4°)=0.0699; 0.0699·100%=6.99% grade). Furthermore, a positive grade corresponds to an uphill grade and a negative grade corresponds to a downhll grade. 
         [0025]    Referring now to  FIG. 2 , exemplary forces acting on the vehicle are schematically illustrated. Accordingly, a general tractive effort equation is provided as; 
         [0000]      Σ F=ma=F   DRIVE   −F   GRADE   −F   AERO   −F   ROLL   −F   BRAKE    
       F DRIVE  is the driving force and is determined in accordance with the following relationship; 
       [0026]    
       
         
           
             
               F 
               DRIVE 
             
             = 
             
               
                 T 
                 4 
               
               
                 t 
                 TIRE 
               
             
           
         
       
     
         [0000]    where: T d =axle torque; and 
         [0027]    r TIRE =tire rolling radius. 
       F GRADE  is the grade force and is determine in accordance with the following relationship; 
       [0028]        F   GRADE   =mg  sin(θ) 
         [0000]    where: m=the vehicle mass (e.g., assume nominal mass); and 
         [0029]    g=gravitational constant (i.e., 9.81 m/s 2 ). 
       F AERO  is the aerodynamic force and is determined in accordance with the following relationship; 
       [0030]    
       
         
           
             
               F 
               AERO 
             
             = 
             
               
                 
                   c 
                   d 
                 
                  
                 A 
                  
                 
                     
                 
                  
                 ρ 
                  
                 
                     
                 
                  
                 
                   V 
                   VEM 
                   2 
                 
               
               2 
             
           
         
       
     
         [0000]    where: C d =vehicle aerodynamic drag coefficient; 
         [0031]    A=frontal area of the vehicle, 
         [0032]    ρ=air density (e.g., function of barometer pressure and air temperature); and 
         [0033]    V VEH =vehicle speed (kph). 
         [0000]    F ROLL  is the rolling resistance of the tires and is determined in accordance with the following relationship: 
         [0000]        F   ROLL   =c,mg  cos(θ) 
         [0000]    where: c r =the vehicle rolling resistance coefficient: and 
         [0034]    cos θ=1 for drivable road grade angles. 
         [0000]    F BRAKE  is the chassis braking system force and is determine as a function of the BPS. More specifically, F BRAKE  is determined based on the following relationship: 
         [0000]    
       
         
           
             
               F 
               BRAKE 
             
             = 
             
               
                 T 
                 BRAKE 
               
               
                 r 
                 TIRE 
               
             
           
         
       
     
       T BRAKE  is a brake torque calibration value that is determined from a look-up table based on V VEH  and BPS. 
       [0035]    In the above-described relationships sin(θ) is the only unknown. Accordingly, a real-time estimate of the road grade can be performed based on the following relationship: 
         [0000]      θ=tan(sin −1    X ) 
         [0000]    where: X=F GRADE /(m*g)
   F GRADE  is initially determined based on the following relationship:   
 
         [0000]    
       
      
       F 
       GRADE 
       =F 
       DRIVE 
       −F 
       ROLL 
       −F 
       AERO 
       −F 
       BRAKE 
       −ma  
      
     
         [0000]    where: a is the vehicle acceleration.
   θ is then determined in accordance with the following relationship:   
 
         [0000]    
       
         
           
             θ 
             = 
             
               tan 
                
               
                 ( 
                 
                   
                     sin 
                     
                       - 
                       1 
                     
                   
                    
                   
                     ( 
                     
                       
                         F 
                         GRADE 
                       
                       
                         m 
                         · 
                         g 
                       
                     
                     ) 
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    θ can be filtered to provide a filtered θ (θ FILT ). θ FILT  can be determined as a running average of θ based on the following relationship: 
         [0000]    
       
         
           
             
               θ 
               FILT 
             
             = 
             
               ( 
               
                 
                   
                     θ 
                     1 
                   
                   + 
                   
                     θ 
                     2 
                   
                   + 
                   
                     θ 
                     3 
                   
                   + 
                   … 
                   + 
                   
                     θ 
                     n 
                   
                 
                 n 
               
               ) 
             
           
         
       
     
         [0000]    where: n is the average number of calculation loops. 
         [0038]    The road grade determination control of the present invention monitors a plurality of conditions, described in further detail below, and selectively sets a grade force freeze flag (FLAG FRZ ) based thereon. The plurality of conditions corresponds to those conditions that will result in abrupt changes in the vehicle operating parameters that would significantly effect the determination of the road grade. If one of the conditions is true, θ is frozen at the last determined value. Once FLAG FRZ  is set (e.g., equal to 1 indicating the θ should be frozen), an unfreeze timer (t UNFRZ ) is initiated. FLAG FRZ  is unset (e.g., set equal to 0) upon t UNFRZ  achieving a threshold time (t THR ). In this manner, θ remains frozen for only t THR . 
         [0039]    The plurality of conditions include, but are not limited to, whether the brake is on, whether a shift is in progress, the time since a gear shift, the time since a range shift, V VEH , the time since a wheel slip was detected, a positive/negative rate of throttle change, a time since the positive/negative rate of throttle change, a positive/negative rate of braking change, a time since the positive/negative rate of braking change, and a time since a fuel off event. FLAG FRZ  is set if the brake pressure is greater than a threshold brake pressure, if a gear shift is in progress, if the time since the last gear shift is less than a threshold time, if the time since a range change (e.g., change between one of Park (P), Neutral (N), Reverse (R) and Drive (D)), if V VEH  is less than a threshold V VEH  (V THR ) or if the time since a wheel slip, which can be monitored using traditionally provided ABS sensors, is less than a respective threshold time. The throttle position is also monitored and FLAG FRZ  is set if a positive or negative rate of change of the throttle position is greater than a respective threshold. FLAG FRZ  is also set if the time since the positive or negative rate of change exceeded its respective threshold exceeds a respective threshold time. Finally, FLAG FRZ  is set is the time since a fuel off event (e.g., transitioning into a hybrid engine off (HEOff mode) is less than a respective threshold time. 
         [0040]    The control module  34  regulates operation of the vehicle based on the road grade. For example, if the road grade exceeds a threshold road grade, the control module  34  will not turn off the engine  12  and enter the HEOff mode, even if the other vehicle operating parameters indicate that the HEOff mode is appropriate. In this manner, the engine  12  remains active on steep road grades. Alternatively, the control module  34  can selectively activate hill-hold devices (e.g., the brakes, redundant transmission clutches and/or a parking pawl) based on the road grade value. Furthermore, the control module  34  can regulate the gear ratio of the transmission  16  based on the road grade value. For example, for steeper downhill road grades, a lower gear ratio is selected, particularly if the vehicle is being operated in a cruise control mode. Also, the idle speed of the engine  12  can be adjusted based on the road grade to inhibit vehicle rollback. For example, the engine idle speed can be proportional to the road grade. 
         [0041]    Referring now to  FIG. 3 , exemplary steps executed by the road grade determination control will be described in detail. In step  300 , control determines the forces (e.g., F DRIVE , F ROLL , F GRADE , F AERO  and F BRAKE ) as discussed in detail above. In step  304 , control calculates θ based on the forces. Control filters θ in step  306  and determines the road grade in step  308 . 
         [0042]    In step  310 , control determines whether any of the freeze conditions is true. If none of the freeze conditions is true, control sets FLAG FRZ  equal to 0 in step  312 , and control ends. If one or more of the freeze conditions is true, control sets FLAG FRZ  equal to 1 in step  314 . In step  316 , control initiates t UNFRZ , Control determines whether t UNFRZ  is equal to t THR  in step  318 . If t UNFRZ  is not equal to t THR , control increment t UNFRZ  in step  320  and loops back to step  318 . If t UNFRZ  is equal to t THR , control resumes road grade determination in step  322  and control ends. 
         [0043]    Referring now to  FIG. 4 , exemplary modules that execute the road grade determination control will be described in detail. The exemplary module include a T d  determining module  400 , an F DRIVE  determining module  402 , an F GRADE  determining module  404 , F AERO  determining module  406 , an F ROLL  determining module  408 , an F BRAKE  determining module  410 , a road grade determining module  412  and a vehicle control module  414 . 
         [0044]    The T d  determining module  400  determines T d  based on engine operating parameters including, but not limited to, RPM, MAP and TPS. The F DRIVE  determining module  402  determines F DRIVE  based on T d  and other vehicle parameters (e.g., r TIRE ). Such vehicle parameters can be stored in memory, or can be determined. In the case of r TIRE , for example, r TIRE  can be determined using a tire pressure sensing routine, for example. The F GRADE  and F ROLL  determining modules  404 ,  408  determine F GRADE  and F ROLL , respectively, as described above. 
         [0045]    The F AERO  determining module  406  determines F AERO  based on vehicle operating parameters and environmental parameters including, but not limited to, V VEH , a barometric pressure (P BARO ) and an air temperature (T AIR ). The F BRAKE  determining module  410  determine F BRAKE  based on BSP and V VEH . The road grade determining module  412  determines the road grade based on the various forces, as described in detail above. The road grade determining module  412  also monitors various inputs (e.g., BPS, TPS, V VEH , shift status, fuel off and the like) that correspond to the road grade freeze conditions. The vehicle control module  414  regulates operation of the vehicle based on the road grade. 
         [0046]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.