Abstract:
A method of regulating a crankshaft position in a hybrid electric vehicle includes deactivating cylinders of an internal combustion engine, driving a crankshaft of the internal combustion engine using an electric machine, and determining a target crankshaft position when a rotational speed of the crankshaft crosses a first threshold. The crankshaft is driven towards the target crankshaft position at a nudge rotational speed, and rotation of the crankshaft is braked using the electric machine when a brake crankshaft position is achieved at the target rotational speed. Rotation of the crankshaft is arrested at the target position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/816,684, filed on Jun. 27, 2006. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to hybrid vehicles, and more particularly to crankshaft stop positioning control system for a hybrid vehicle. 
     BACKGROUND OF THE INVENTION 
     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. 
     In hybrid electric vehicles, the engine is often stopped and started to improve fuel economy. Acceleration and deceleration of the engine crankshaft should be controlled during engine stop and starts to reduce driveline oscillations and other vibration inducing events that diminish the vehicle drivability. The crankshaft should also be parked in a desirable rotational position to further improve the stop-start characteristics of the engine. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a method of regulating a crankshaft position in a hybrid electric vehicle. The method includes deactivating cylinders of an internal combustion engine, driving a crankshaft of the internal combustion engine using an electric machine, and determining a target crankshaft position when a rotational speed of the crankshaft crosses a first threshold. The crankshaft is driven towards the target crankshaft position at a nudge rotational speed, and rotation of the crankshaft is braked using the electric machine when a brake crankshaft position is achieved at the target rotational speed. Rotation of the crankshaft is arrested at the target position. 
     In one feature, the step of driving the crankshaft includes driving the crankshaft at a lash rotational speed to minimize driveline lash in the hybrid electric vehicle. 
     In another feature, the method further includes determining a braking torque of the electric machine based on a difference between an actual crankshaft position and the target crankshaft position during the step of braking. 
     In another feature, the method further includes adjusting a torque of the electric machine to provide a slight motoring of the crankshaft when the actual crankshaft position is approximately equal to the target crankshaft position. 
     In another feature, the method further includes adjusting a torque of the electric machine to provide a slight motoring of the crankshaft when the rotational speed of the crankshaft is approximately equal to zero. 
     In still another feature, the method further includes ramping a torque of the electric machine to zero when the actual crankshaft position is equal to the target crankshaft position. 
     In yet other features, the method further includes determining respective rotational speeds of the crankshaft and the electric machine, and relaxing a throttle actuator of the internal combustion engine when the respective rotational speeds each equal zero. A timer is initiated when the respective rotational speeds both equal zero. The step of relaxing is executed upon the timer achieving a threshold time. 
     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 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of an exemplary hybrid vehicle that is operated based on the crankshaft stop positioning control of the present invention; 
         FIG. 2  is a graph illustrating exemplary vehicle operating parameter traces during a crankshaft stop positioning cycle in accordance with the present invention; and 
         FIG. 3  is a flowchart illustrating exemplary steps executed by the crankshaft stop positioning control of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     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 . 
     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 . 
     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 . 
     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. Further, opening and closing cycles of the intake and exhaust valves can be prevented to inhibit air flow processing with the cylinders. 
     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. 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). 
     An EM position sensor  44  is responsive to the rotational position of a rotor of the electric machine  14  and a rotational speed of the electric machine  14  (RPM EM ) is determined based thereon. Similarly, an engine position sensor  45  is responsive to the rotational position of the crankshaft  26  and a rotational speed of the engine  12  (RPM ENG ) is determined based thereon. The control module  34  operates the vehicle  10  based on the pedal position signals generated by the pedal position sensors  38 ,  42  and the signals generated by the position sensors  44 ,  45 , as described in further detail below. 
     The crankshaft stop positioning (CSP) control of the present invention includes a virtual encoder that calculates the crankshaft&#39;s angular position (θ ENG ) even at low speeds including zero RPM. The CSP control also determines a desired or target stop position (θ TRG ) and executes a multi-plateau EM control routine that controls rotation of the crankshaft  26  at a plurality of levels and parks the crankshaft at θ TRG  using the virtual encoder as its input. 
     Positioning the engine at θ TRG  is based on use of position and speed information from the engine position sensor  45 , which can be provided as a 58× toothed wheel crankshaft sensor and/or a 4× camshaft sensor, and the EM position sensor  44 , which can be provided as a resolver of the electric machine  14 . The engine position information is used to generate a signal that represents the position of cylinder #1 (i.e., the first cylinder in the firing order) on a 720 degree scale (i.e., two crankshaft revolutions of 360 degrees each for one complete engine cycle in which all cylinders go thru a complete cycle of intake, compression, ignition and exhaust). Top-dead-center (TDC) of cylinder #1 during the compression stroke represents zero degrees. 
     The engine position sensor  45  can not be used to determine the RPM and position below a low value of RPM (e.g., approximately 100 RPM) or in the reverse direction. Rotation of the crankshaft in the reverse direction can be detected by the control module  34 , but the rotation appears the same as the rotation in the forward direction. The EM position sensor  44  can determine the EM position and RPM EM  down to zero RPM. Because the electric machine  14  and the engine  12  are coupled by a belt of fixed ratio, RPM EM  and the EM position can be used to determine RPM ENG  and engine position below the point where engine position sensor  45  no longer detects rotation, as well as allow rotation of the crankshaft in the reverse direction to appear to the control module  34  as rotation different than rotation in the forward direction. 
     The EM position sensor  44 , unlike the engine position sensor  45 , has bi-directional rotation sensing capability (i.e., reverse, or rock-back rotation of the crankshaft is detectable). When the EM position sensor detects movement in the reverse direction and the engine position sensor  45  signal is still able to detect movement, the delta of this movement is determined, but the detected movement deltas are subtracted rather than added to the current θ ENG  value. Also, this reverse detection capability can be used to update the final stop position of the crankshaft  26  when the engine position sensor  45  stops generating a signal and engine movement is still present. A delta of EM position on a scale of 0 to 360 degrees, rather than a delta of running crank position sensor pulses, can be determined and based on the ratio of the belt, a delta of θ ENG  can be determined and this amount can be added or subtracted from the θ ENG  value depending on the detection of a forward or a reverse direction. 
     The CSP control of the present invention implements the engine position sensor  45  to determine θ ENG  of cylinder #1 on the 720 degree scale until the engine position sensor  45  no longer generates a usable signal. If the EM position sensor  44  detects the reverse direction at RPMs where the engine position sensor  45  signal is still usable, this is taken into account when determining θ ENG  from the engine position sensor  45 , as described above. When the engine position sensor  45  no longer generates a usable signal (e.g., at very low engine speed), the EM position sensor signal can be used to continue to determine θ ENG . At low engine speeds (e.g., less than 100 RPM), where the engine position sensor  45  can not be used to determine engine speed, RPM EM  is monitored as it approaches 0 RPM, instead of EPM ENG . When θ ENG  approaches θ TRG , the EM is braked which stops movement of the engine  12  until RPM EM  is 0 RPM, as described in further detail below. 
     Referring now to  FIG. 2 , the CSP control will be described in further detail. Once HEOff is commanded the CSP control determines θ TRG . More specifically, when the engine is ready to be stopped, the CSP control executes the multi-plateau electric machine control. During the first plateau, the electric machine  14  is used to control the crankshaft speed to RPM ML  (e.g. 500 RPM ENG ). RPM ML  is the speed used to draw down manifold absolute pressure (MAP) and to take up driveline lash. Accordingly, the first plateau phase minimizes driveline lash to improve the following engine restart smoothness. The RPM ENG  drop to RPM ML  is also performed with fuel off and the throttle closed, thereby reducing MAP. Because the cylinder pressures are reduced during the first plateau phase, the compression disturbance and electric machine motoring torque during the ensuing engine stop is also reduced. The throttle remains closed until the engine  12  is stopped, thereby trapping the vacuum. In this manner, the amount of engine rock back during crankshaft parking is minimized. Control during the first plateau also allows for the same starting conditions of the engine and MGU for control from speed #1 to speed #2 to speed #3 which allows for consistency across HEOffs. 
     θ TRG  is calculated between the first and second plateaus, speed #1 and speed #2, respectively. More specifically, the θ TRG  calculation is initiated when RPM ENG  drops below a threshold RPM (RPM INIT ) (e.g., 750 RPM). This is based on the current θ ENG  and can be one of a plurality of locations (e.g., 4 locations for a 4 cylinder engine), to provide one stop position per quadrant. For example, for an exemplary 4 cylinder engine, if θ TRG  is determined to be 85 degrees before TDC (BTDC) of any cylinder in its compression stroke, and given that 0 degrees is TDC of cylinder #1 compression, 180 degrees is TDC of cylinder #3 compression, 360 degrees is TDC of cylinder #4 compression, and 540 degrees is TDC of cylinder #2 compression (i.e., firing order is 1-3-4-2), then the stop positions would be 635 degrees, 95 degrees, 275 degrees, 455 degrees, respectively. 
     In this manner, the engine stopping is effectively delayed by one cylinder event (i.e., 180 degrees) or until the next quadrant after determining θ TRG . For example, if θ ENG  is near 300 degrees when RPM ENG  crosses RPM INIT , θ TRG  would be that for the next cylinder in compression, or 455 degrees. Furthermore, the window of the initial cylinder to determine the next compression cylinder is limited to the first 160 degrees (e.g., a calibratable value) of the initial cylinder&#39;s compression stroke. For example, if the compression stroke of the initial cylinder in which RPM INIT  is crossed ranges from 180 to 360 degrees, RPM INIT  must have been crossed between 180 and 340 degrees for a θ TRG  of 455 degrees to be selected. 
     The second plateau (speed #2) is a crankshaft nudge phase (NP), during which the crankshaft  26  is motored or nudged toward a desired position in the next quadrant using the electric machine  14 . During the second plateau, the electric machine  14  controls the crankshaft speed to RPM NP , which is the nudge speed to move the crankshaft position into the next quadrant described above (i.e., to prevent the engine  12  from stopping too early). In between RPM ML  and RPM NP , the CSP control calculates θ TRG , as described above, and the electric machine  14  controls the deceleration rate of the crankshaft  26  to reduce vibration. If the crankshaft  26  is allowed to dwell in its resonance band (e.g. approximately 300 RPM), vibration can be felt by the vehicle occupants. Likewise, if the crankshaft  26  is decelerated too abruptly, powertrain mount rocking can also lead to occupant disturbance. RPM NP  is a calibratable value and is chosen to be sufficiently high such that it enables the electric machine  14  to operate robustly enough to motor the crankshaft to the next quadrant but low enough such that it is below RPM INIT  and after θ TRG  has been selected. Control of the electric machine between the first and second plateaus allows for the events of calculating θ TRG . 
     The crankshaft  26  is motored at RPM NP  until a braking crankshaft position (θ BRK ) is achieved, where θ BRK  is defined as the calibratable position delta (θ Δ ) before the desired stop position θ TRG  (i.e., θ BRK =θ TRG −θ Δ ). The third plateau is the final stop speed of zero RPM. More specifically, once θ BRK  has been achieved, the electric machine  14  is switched to generator mode to retard the crankshaft speed and to control both RPM EM  and RPM ENG  down to zero RPM. In the generator mode, the brake torque of the electric machine  14  (T EM ) is used to brake rotation of the crankshaft. T EM  is determined as a function of the crankshaft position away from θ TRG  (i.e., θ Δ ). When zero RPM is detected, the CSP control monitors a reverse rotation flag from the EM position sensor  44 . When near zero RPM and at or near θ TRG , the electric machine braking can be calibrated to be a slight motoring before gently ramping out the electric machine torque completely. In this manner, rock back minimization is ensured. Reverse rotation is undesirable, because extra energy and time is required to reverse the backwards rotation if an engine restart is commanded. 
     The engine  12  is considered parked once there is no motion detected by both the engine position sensor  45  and the EM position sensor  44  for a threshold stop time (t STOP ). Once the engine  12  is deemed parked, the throttle  13  is held closed until the MAP leaks back up to the barometric pressure (P BARO ). It is desirable to maximize the time of low MAP, because the subsequent engine restart can be performed more smoothly and with less power. Once P BARO  has been achieved, the throttle actuator  15  is relaxed back to its rest position, thereby conserving electrical energy. 
     The above described three plateau EM control for engine stopping is applicable when entering the routine from a deceleration fuel cut-off situation. If the engine  12  has been idling (i.e., fuel on), the CSP control is effectively a four plateau EM control, where the fueled engine speed and MAP are first stabilized before fuel is cut and RPM ENG  is dropped to RPM ML . 
     Referring now to  FIG. 3 , exemplary steps executed by the CSP control will be described in detail. In step  300 , control determines whether to initiate HEOff. If HEOff is not to be initiated, control loops back. If HEOff is to be initiated, control uses the electric machine  14  to control RPM ENG  to achieve RPM ML  in step  304 . In step  306 , control determines whether RPM ENG  is equal to RPM ML . If RPM ENG  is not equal to RPM ML , control loops back to step  304 . If RPM ENG  is equal to RPM ML , control continues in step  308 . 
     In step  308 , control uses the EM to control RPM ENG  towards RPM NP . Control determines whether RPM ENG  is less than RPM INIT  in step  310 . If RPM ENG  is not less than RPM INIT , control loops back to step  308 . If RPM ENG  is less than RPM INIT , control determines θ TRG  in step  312 . In step  314 , control determines whether RPM ENG  is equal to RPM NP . If RPM ENG  is not equal to RPM NP , control loops back. If RPM ENG  is equal to RPM NP , control continues in step  316 . 
     In step  316 , control motors the crankshaft at RPM NP  using the EM. Control determines whether θ ENG  is equal to θ BRK  in step  318 . If θ ENG  is not equal to θ BRK , control loops back to step  316 . If θ ENG  is equal to θ BRK , control operates the electric machine  14  as a generator to brake the rotation of the crankshaft  26  in step  320 . In step  322 , control determines whether RPM ENG  and RPM EM  are both equal to zero. If either RPM ENG  or RPM EM  is not equal to zero, control loops back to step  320 . If both RPM ENG  and RPM EM  are equal to zero, control continues in step  324 . 
     In step  324 , control starts a timer t. Control determines whether t is equal to t STOP  in step  326 . If t is not equal to t STOP , control continues in step  328 . If t is equal to t STOP , control continues in step  330 . In step  328 , control increments t and loops back to step  326 . In step  330 , control relaxes the throttle actuator  15  and control ends. 
     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.