Abstract:
A method of smoothing hybrid vehicle engine shutdown. A powered and rotating electric machine is used to slow deceleration of an unpowered and rotating engine by transferring torque through a clutch from the machine to the unpowered engine. Prior to the machine being powered, torque may be transferred through the clutch from the unpowered and rotating engine to the unpowered machine to accelerate passage of the engine through a resonance frequency.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to a method of controlling an automotive engine and in particular to a method of smoothing engine shutdown. 
     Some automotive vehicle powertrains incorporate engine start/stop (ESS) systems to improve fuel economy. ESS shuts down an internal combustion engine under specified conditions when engine torque is not required and restarts the engine when torque is again required. For example, ESS may shutdown the engine of a vehicle after a driver brakes the vehicle to a stop and then restart the engine when the driver requests torque by depressing an accelerator pedal. ESS is commonly incorporated into hybrid electric powertrains. In general, the more conditions specified when ESS will shutdown the vehicle engine, the greater the improvement to fuel economy. 
     However, noise, vibration, and harshness may result from the engine shutdown as an engine speed falls through a resonance frequency of the engine. Harshness may also result from energy being transferred, as the engine speed decreases, through the engine mounts to the vehicle. 
     SUMMARY OF INVENTION 
     An embodiment contemplates a method of smoothing hybrid vehicle engine shutdown. An engine is powered and an electric machine is rotating while a clutch between the engine and machine is disengaged. The engine is unpowered while the machine remains rotating with the clutch disengaged. Deceleration of the unpowered engine is controlled by controlling the clutch to transfer torque from the rotating machine to the unpowered engine and decreasing rotation of the machine. 
     Another embodiment contemplates a method of smoothing hybrid vehicle engine shutdown. An engine is powered and rotating while a clutch between the engine and an unpowered and non-rotating electric machine is disengaged. While the engine is powered and rotating, and the machine unpowered and non-rotating, the engine is unpowered. The unpowered and non-rotating machine is then rotated by controlling the clutch to transfer torque from the unpowered and rotating engine to the machine. The rotating machine is powered when rotation of the engine and machine are each non-zero and within a predetermined speed range. Deceleration of the unpowered engine is controlled by decreasing rotation of the powered and rotating machine and controlling the clutch to transfer torque from the powered and rotating machine to the unpowered engine. 
     Another embodiment contemplates a method of smoothing hybrid vehicle engine shutdown. An engine is powered while a clutch interposed between the engine and a machine is disengaged. The engine is unpowered while the clutch remains disengaged. The machine is powered and the clutch controlled to transfer torque from the rotating machine to the unpowered engine to control deceleration of the unpowered engine while decreasing rotation of the machine. 
     An advantage of an embodiment is that noise, vibration, and harshness are reduced during the engine shutdown routine. This improves the driving experience for the vehicle driver. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of a hybrid electric powertrain. 
         FIG. 2  is a graph of engine speed and clutch torque during an engine shutdown routine. 
         FIG. 3  is a graph of engine speed and clutch torque during an engine shutdown routine. 
         FIG. 4  is a schematic view of a hybrid electric powertrain. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a hybrid electric powertrain  10  for an automotive vehicle  12 . The powertrain  10  shown in  FIG. 1  is exemplary and may be employed with front wheel drive, rear wheel drive, and all wheel drive vehicles. 
     The powertrain  10  includes an internal combustion engine  14  powering a crankshaft  16 . Interposed between the engine  14  and an electric machine  22 , which may be an electric motor or motor/generator, is a clutch  18 . When engaged, the clutch  18  connects the crankshaft  16  with an electric machine input  20  and transmits torque between the engine  14  and the machine  22 . In turn, the machine  22  transmits torque to a torque converter  26  through a torque converter input  24  and the torque converter  26  transmits torque to a transmission  30  through a transmission input  28 . There is a bypass clutch  27  that may be used to lock the torque converter  26 . The transmission  30  turns a driveshaft  32  which in turn drives a differential  34 . The differential  34  transmits torque to first and second axles  36  and  38 , respectively, which drive first and second wheels  40  and  42 , respectively. The machine  22  is electrically connected to a battery  44 . The engine  14  is secured to the vehicle  12  by a plurality of engine mounts  46 . Fuel is supplied to the engine  14  by a fuel supply  48 . The engine  14 , clutch  18 , and machine  22  may be controlled by one or more controllers  50 . The controller  50  may be comprised of various combinations of hardware and software as is known to those skilled in the art. 
       FIG. 2  will now be discussed with reference to  FIG. 1 .  FIG. 2  graphically illustrates a speed of the engine  14  and a torque load of the clutch  18  during an engine shutdown routine  100  for the internal combustion engine  14 . Typically, the engine shutdown routine  100  is used when the powertrain  10  is powering the first and second wheels  40  and  42 . 
     During a time period  102 , the engine  14  is powered and rotating at an idle speed  104  and the clutch  18  is disengaged. While the engine  14  is at the idle speed  104 , the clutch  18  carries a zero torque  108 . At a time point  106 , the engine  14  is unpowered by terminating the fuel supply  48  for the engine  14 . 
     During a time period  110 , the clutch  18  is controlled to transfer torque between the unpowered engine  14  and the rotating machine  22  to transfer a torque  112  from the machine  22  to the engine  14 . As used herein, the term controlling the clutch means controlling the amount of slippage, and hence torque, transferred through the clutch. The torque  112  controls a deceleration of the engine  14  by altering a natural deceleration rate  114  (with the clutch disengaged) to a controlled deceleration rate  116 . The natural deceleration rate  114  is the rate at which rotation of the unpowered engine  14  slows to a stop once the engine  14  is unpowered and rotation of the engine  14  is slowing due to friction loss only when the clutch  18  is disengaged. The more torque  112  transmitted from the machine  22  to the engine  14 , the slower the engine  14  will decelerate. As illustrated, the controlled deceleration rate  116  is less than the natural deceleration rate  114 . Transferring torque from the machine  22  to the engine  14  extends a duration of the engine shutdown routine  100  and reduces torque transferred from the engine  14  through the engine mounts  46  to the vehicle  12 . Alternatively, the clutch  18  may disengage prior to the engine  14  reaching a stopped speed  118 . Upon disengagement of the clutch  18 , the engine  14  returns to decelerating at the natural rate  114 . 
     During a time period  120 , the shutdown routine  100  is complete. The engine  14  has slowed to the stopped speed  118 . While the engine  14  is at the stopped speed  118 , the clutch  18  is at a zero torque  122 . 
       FIG. 3  will now be discussed with reference to  FIG. 1 .  FIG. 3  graphically illustrates a speed of the engine  14  and a torque load of the clutch  18  during an engine shutdown routine  200  for the internal combustion engine  14 . The engine shutdown routine  200  is used when, at initiation of the shutdown routine  200 , the speed of the engine  14  is greater than a speed of the machine  22 . Typically, the engine shutdown routine  200  is used when the powertrain  10  is not powering the first and second wheels  40  and  42 . 
     During a time period  202 , the engine  14  is powered and rotating at an idle speed  204 , the machine  22  is unpowered and non-rotating, and the clutch  18  is disengaged. While the engine  14  is at the idle speed  204 , the clutch  18  carries a zero torque  208 . At a time point  206 , the engine  14  is unpowered by terminating the fuel supply  48 . 
     During a time period  210 , the clutch  18  is controlled to transfer torque between the unpowered engine  14  and the non-rotating machine  22  to transfer a first torque  212  from the engine  14  to the machine  22 . The torque  212  controls a first deceleration of the engine  14  by altering a first natural deceleration rate  214  (with the clutch remaining disengaged) to a first controlled deceleration rate  216 . The more torque  112  transmitted from the engine  14  to the machine  22 , the faster the engine  14  will decelerate. As illustrated, the controlled deceleration rate  216  is greater than the natural deceleration rate  214 . The transfer of torque  212  to the machine  22  produces an electric current stored in the battery  44 . The first controlled deceleration rate  216  passes the engine  14  more quickly through a resonance frequency than the first natural deceleration rate  214 . 
     During a time period  218 , the clutch  18  remains controlled to transfer torque when the machine  22  is powered to transfer a second torque  220  from the machine  22  to the engine  14 . The torque  220  controls a second deceleration of the engine  14  by altering a second natural deceleration rate  222  to a second controlled deceleration rate  224 . The more torque  220  transmitted from the machine  22  to the engine  14 , the slower the engine  14  will decelerate. As illustrated, the second controlled deceleration rate  224  is less than the second natural deceleration rate  222 . Transferring the torque  220  from the machine  22  to the engine  14 , in accordance with the shutdown routine  200 , extends a duration of the engine shutdown and reduces the torque transferred from the engine  14  through the engine mounts  46  to the vehicle  12 . Alternatively, the clutch  18  may disengage prior to the engine  14  reaching a stopped speed  230 . Upon disengagement of the clutch  18 , the engine  14  returns to decelerating at the natural rate  222 . 
     A time point  226  is the transition between the time period  210  to the time period  218 . At the time point  226 , the machine  22  changes from receiving the first torque  212  to transmitting the second torque  220 . At the time point  226 , neither the engine  14  nor the machine  22  are non-rotating. The time point  226  may be when a rotational speed of the engine  14  equals a rotational speed of the machine  22 . Alternatively, the time point  226  may be when the rotational speed of the engine  14  is within a predetermined range of the rotational speed of the machine  22 . For example, the predetermined range can be a range of speeds known to one skilled in the art at which the clutch  18  may be engaged without damage to the powertrain  10 , including the engine  14  and the machine  22 . By applying both the first torque  212  and the second torque  220 , the time period  218  may be less than the time period  110  when the idle speed  104  and the idle speed  204  are equal. 
     At a time period  228 , the shutdown routine  200  is complete and the engine  14  has slowed to the stopped speed  230 . While the engine  14  is at the stopped speed  230 , the clutch  18  has returned to a zero torque  232 . 
       FIG. 4  schematically illustrates a portion of a hybrid electric powertrain  310  for an automotive vehicle  312 . Since the powertrain in this embodiment is a modification of the embodiment of  FIG. 1 , like reference numerals designate corresponding parts in the drawings and detailed description thereof will be omitted. The powertrain  310  shown in  FIG. 4  is exemplary and may be employed with front wheel drive, rear wheel drive, and all wheel drive vehicles. 
     The shutdown routine  200  may also be used with the powertrain  310  including a belted integrated starter generator, in lieu of a machine, to transfer torque to and from the engine  314 . The shutdown routine  200  used with the powertrain  310  including the belted integrated starter generator does not require that the powertrain be powering the first and second wheels  340  and  342 . 
     While certain embodiments of the present invention have 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.