Abstract:
A powertrain includes an engine, a transmission, and one or more accessories, such as an alternator or an air conditioning compressor, all operating based on commands from a controller. The controller is programmed to coordinate commands to the transmission with commands to the accessories to mitigate the impacts of transmission state changes. The accessory drive torque may be adjusted to compensate for the torque required to change the speed of a transmission internal shaft. The accessory effective inertia may be adjusted to maintain a powertrain natural frequency so that active damping can be maintained throughout a transmission state change event.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to vehicle powertrain controls. More particularly, the present disclosure relates to a method of adjusting accessory torque to mitigate the impacts of transmission state changes. 
       BACKGROUND 
       [0002]    Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. 
         [0003]      FIG. 1  schematically illustrates a vehicle powertrain  10 . The flow of mechanical power is indicated by thick solid lines whereas dotted lines indicate the flow of control signals. Power is provided by engine  12 . Transmission  14  adjusts the speed and torque of the power to suit vehicle needs. Differential  16  divides the power between left and right drive wheels  18  and  20  while permitting slight speed differences as the vehicle turns a corner. Some of the engine power is diverted by front-end accessory drive  22  to drive accessories that are not directly related to propulsion. For example, power may be provided to an alternator  24  to generate electrical power and to and air conditioning compressor  26  to cool the passenger cabin. Controller  28  sends signals to the engine, transmission, and the accessories to coordinate their operation. Controller  28  may be a single microprocessor or may be multiple communicating micro-processors. 
         [0004]      FIG. 2  schematically illustrates a Dual Clutch Transmission (DCT)  14 . Input  30  is adapted for coupling the crankshaft of engine  12 , potentially via a damper assembly that reduces the transmission of engine pulsations. Ring gear  32  is fixedly coupled to differential  16 . First output pinion  34  is fixedly coupled to first layshaft  36  and meshes with ring gear  32 . Second output pinion  38  is fixedly coupled to second layshaft  40  and also meshes with ring gear  32 . First friction clutch  42  selectively couples input  30  to solid shaft  44 , while second friction clutch  46  selectively couples input  30  to hollow shaft  48  which is concentric with solid shaft  44 . 
         [0005]    Gears  50  and  52  are supported for rotation about first layshaft  36  and mesh with gears  54  and  56  respectively which are fixedly coupled to solid shaft  44 . Coupler  58  selectively couples gear  50  or  52  to first layshaft  36 . Gear  60  is supported for rotation about second layshaft  40  and meshes with gear  62  which is fixedly coupled to solid shaft  44 . Coupler  68  selectively couples gear  60  to second layshaft  40 . When couplers  58  or  68  have coupled one of gears  50 ,  52 , or  60  to the respective layshaft, a power flow path is established between solid shaft  44  and ring gear  32 . Each of these different power flow paths is associated with a different speed ratio. When clutch  42  is also engaged, a power flow path is established between input  30  and ring gear  32 . 
         [0006]    Gears  70  and  72  are supported for rotation about second layshaft  40  and mesh with gears  74  and  76  respectively which are fixedly coupled to hollow shaft  48 . Coupler  78  selectively couples gear  70  or  72  to second layshaft  40 . Gears  80  and  82  are supported for rotation about first layshaft  36  and mesh with gear  76  and  70  respectively. Coupler  84  selectively couples gear  80  or  82  to first layshaft  36 . When couplers  78  or  84  have coupled one of gears  70 ,  72 ,  80 , or  82  to the respective layshaft, a power flow path is established between hollow shaft  48  and ring gear  32 . When clutch  46  is also engaged, a power flow path is established between input  30  and ring gear  32 . The speed ratios associated with clutch  46  are interleaved with the speed ratios associated with clutch  42  such that clutch  42  is used to establish odd numbered gear ratios and clutch  46  is used to establish even numbered gear ratios and reverse. 
         [0007]    When a driver selects Drive with the vehicle stationary, coupler  58  is commanded to couple gear  52  to shaft  36  while clutch  46  is commanded to disengage. To launch the vehicle, clutch  42  is commanded to gradually engage. Similarly, when Reverse is selected with the vehicle stationary, coupler  84  is commanded couple gear  82  to shaft  36 . Then, clutch  46  is commanded to gradually engage to launch the vehicle. When cruising in an odd numbered gear, clutch  42  is engaged. To shift to an even numbered gear, clutch  46  is disengaged (if it was not already disengaged), and either coupler  78  or  84  pre-selects the destination power flow path. After the destination gear is pre-selected, clutch  42  is released and clutch  46  is engaged in a coordinated fashion to transfer power between the corresponding power flow paths and adjust the overall speed ratio. 
         [0008]    Clutches  42  and  46  may be either dry or wet friction type clutches. One or more friction plates are fixedly coupled to one of the elements while a housing with a pressure plate and a reaction plate is fixedly coupled to the other element. The friction plates are between the pressure plate and the reaction plate. If there is more than one friction plate, they are separated by separator plates that are also fixedly coupled to the housing. When the clutch is fully disengaged, the reaction plate and the pressure plate are spaced apart such that the friction plate can rotate relative to the housing with minimal drag torque. To engage the clutch, an actuator causes a normal force that squeezes the friction plate(s) between the pressure plate and the reaction plate. The torque capacity of the clutch is proportional to the normal force and also proportional to the coefficient of friction. If the elements are rotating at different speeds, the clutch exerts torque on each element equal to the torque capacity in a direction tending to equalize the speeds. If the elements are at the same speed, then the clutch transfers as much torque as is applied up to the torque capacity. If the applied torque exceeds the torque capacity, then the clutch slips creating relative speed. 
       SUMMARY 
       [0009]    A vehicle includes an engine driven accessory, a transmission, and a controller. The accessory may be an alternator or a variable displacement pump such as an air conditioner compressor. The transmission has first and second gear states with equal speed ratios between an input shaft and an output shaft but different ratios between the input shaft and an internal shaft. For example, the transmission may be a dual clutch transmission and the first and second gear states may be different pre-select states. The controller is programmed to command the transmission to change state and to command the accessory to change its torque to offset the change in speed of the internal shaft. The controller may also change the accessory torque in preparation for commanding the transmission state change. 
         [0010]    A vehicle includes an engine driven accessory, a transmission, and a controller. The accessory has variable effective inertia. The transmission has first and second gear states with identical speed ratios but different effective inertias. The controller is programmed to command a transmission state change and command an accessory inertia change such that a powertrain natural frequency remains substantially constant. The controller may also command an accessory inertia change in preparation for commanding the transmission state change. The controller may be further programmed to dampen a vibration at the powertrain natural frequency throughout the transmission state change. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of a vehicle powertrain. 
           [0012]      FIG. 2  is a schematic diagram of a dual clutch transmission gearing arrangement. 
           [0013]      FIG. 3  is a graph illustrating the variation of alternator field current to compensate for a pre-select state change in the dual clutch transmission of  FIG. 2 . 
           [0014]      FIG. 4  is a flow chart of a method to adjust alternator field current and AC compressor displacement to mitigate transmission state changes. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0016]    Referring to the transmission schematic of  FIG. 2 , the transmission is prepared for launch in a forward direction by sliding coupler  58  to couple gear  52  to shaft  36 , thus pre-selecting 1st gear. Then, to start the vehicle moving, the torque capacity of clutch  42  is gradually increased. Couplers  68 ,  78 , and  84  and clutch  46  may be disengaged during this process. Shaft  48  and gears  70 ,  72 .  74 ,  76 ,  80 , and  82  all tend to remain stationary during this process because they have inertia and no torque acts upon them. In order to prepare for a shift into 2nd gear, gear  70  must be coupled to shaft  40  by sliding coupler  78 . Coupler  78  may be a synchronizer device that includes a blocker ring. The blocker ring rotates with shaft  40 . A sleeve exerts axial force on inclined surfaces of the blocker ring to push the blocker ring toward gear  70 . The blocker ring, in turn, transmits the axial force to gear  70  through a friction surface. This axial force results in a frictional torque tending to accelerate gear  70  toward the same speed as shaft  40  and the blocker ring. The torque on the blocker ring is transmitted back to the sleeve through the inclined surface, which resist the axial force. The angle of the inclined surfaces is set such that the increasing the axial force increases the resisting force to prevent further axial movement of the sleeve. When gear  70  reaches the same speed as shaft  40 , the torque suddenly drops. This drop in torque, in term, reduces the resisting force such that the sleeve can move further and engage dog teeth to positively couple gear  70  to shaft  40 . 
         [0017]    Of course, for gear  70  to accelerate to the speed of shaft  40 , many other components must also accelerate. Since gear  70  meshes with gear  74  and  82 , they must accelerate in proportion. Since the gear  74  is fixedly coupled to hollow shaft  48 , shaft  48  and all components that are fixedly coupled to it, including the disk of clutch  46 , must accelerate. Accelerating these components requires torque. Some of the power transmitted to ring gear  32  through the 1st gear power flow path must be diverted to shaft  40  to accelerate the components as opposed to being transmitted to the differential to propel the vehicle. If the power from the engine is constant, the torque transmitted to the vehicle wheels decreases suddenly when the pre-select operation starts and then increases suddenly when the pre-select event ends. These changes in propulsive torque may be noticeable by vehicle occupants, who may find them annoying. 
         [0018]    In some circumstances, it may be possible to accelerate the necessary components for a pre-selection event using the friction clutch. In the circumstance described above, for example, the clutch disk of clutch  46  must accelerate from near zero speed to a speed less than the speed of the clutch housing. Therefore, clutch  46  may be used to accelerate these components in the correct direction. (In some circumstances, the friction clutch would not accelerate the components in the desired direction.) Unlike a synchronizer with a blocker ring, there is no passive mechanism to stop the acceleration when the components reach the synchronized speed. Therefore, the clutch torque capacity must be actively controlled based on speed measurement feedback. Using one of the friction clutches  42  and  46  in this way is called a Clutch Before Synchronization (CBS) event. CBS events, like synchronization events using a synchronizer, may result in sudden changes in propulsive torque at the wheels which can annoy vehicle occupants. 
         [0019]    To mitigate these output torque variations, the input torque may be varied to compensate for the power diverted to overcome component inertia. However, internal combustion engine  12  may not be capable of changing its torque output sufficiently rapidly to compensate for these events. Some accessories, such as alternator  24  and/or AC compressor  26  may be capable of rapidly changing the load they impose on engine  12 . If the load imposed by an accessory is reduced as the pre-select event starts and then increased as the pre-select events ends, the output torque remains nearly constant. For some pre-select events, components must slow down as opposed to speeding up. For these events, the accessory load would be increased as the pre-select event begins and decreased as the pre-select event ends. 
         [0020]    Variation of alternator load to mitigate a pre-select torque disturbance is illustrated in  FIG. 3 . An alternator is typically controlled by setting a field current to achieve a nominal bus voltage as shown at  90 . The load can be decreased by commanding a lower field current  92  between the time the pre-select event begins at  94  and the time the pre-select events at  96 . To avoid excessive variation in bus voltage, a capacitor may be added to the bus to stabilize the voltage. To further reduce the variation the controller may prepare for the pre-select event by gradually increasing the field current before the pre-select event at  98  and gradually reducing the field current after the event at  100  such that the average field current is equal to the current required for the nominal bus voltage. Since the variations in torque at  98  and  100  are gradual, vehicle occupants are unlikely to notice. 
         [0021]    The load imposed by an AC compressor is dependent upon the displacement of the compressor. When an AC compressor has a variable displacement, the controller can command the displacement in a similar fashion to that illustrated in  FIG. 3  for field current of an alternator. Varying the displacement of an AC compressor, or other variable displacement engine driven pump, may provide a greater range of load adjustment than varying the field current of an alternator. The air temperature change due to a short-term adjustment in AC compressor displacement will not be noticeable to vehicle occupants. 
         [0022]    The natural frequency of a powertrain system changes depending upon the state of engagement of clutches and couplers. The system may have one natural frequency when a particular clutch is fully released, a second natural frequency when the clutch is slipping, and a third natural frequency when the clutch is fully engaged. Similarly, the natural frequency may change when a coupler is engaged or released during a pre-selection event. Any sudden change of shaft torque may start an oscillation at a powertrain natural frequency. One technique used to mitigate powertrain oscillations is active damping. A controller measures an oscillating speed or torque and commands an actuator to exert an oscillating torque at the same frequency with a phase difference such that the commanded torque reduces the oscillation. The actuator may be, for example, a slipping clutch or an alternator field current. One limitation of this active damping technique is that the frequency of the oscillation must be constant. If the natural frequency of the powertrain changes abruptly, the active damping must be suspended until the controller can re-adjust to the new frequency. 
         [0023]    In some instances, the change in natural frequency may be avoided by commanding a compensating change in the displacement of the AC compressor or other engine driven variable displacement pump. Since the fluid being pumped has mass, there is an equivalent rotational moment of inertia at the pump. The equivalent rotational moment of inertia is proportional to the pump displacement per revolution. Varying the pump displacement, therefore, varies the system natural frequency. In some cases, the controller may be able to compute a change in the pump displacement that impacts the natural frequency by the same amount as the change in pre-select state, but in the opposite direction. By commanding the displacement to change by this amount at the same time that the pre-select state change occurs, the natural frequency remains constant and active damping can continue uninterrupted throughout the event. 
         [0024]      FIG. 4  is a flow chart illustrating a process that uses a combination of AC compressor displacement change and alternator field current change to avoid sudden changes in powertrain natural frequency and output torque. Various aspects of this method may be used in isolation without implementing all aspects. The process begins when a pre-select event is scheduled. At  110 , the controller calculates the amount that the AC compressor displacement must be changed in order to maintain a constant natural frequency. This may include an amount that it must be changed at the beginning of the pre-select event and the amount that it must be changed at the end of the pre-select event. The amounts for various pre-select events may be computed or measured in advance such that these amounts may be calculated by the controller at  110  using a table look-up. At  112 , the controller calculates the amount that the field current must be changed at the beginning of the pre-select event and at the end of the pre-select event in order to maintain a constant output torque. The amount includes compensation for the torque that is diverted to overcoming inertia and also the amount that compensates for the AC displacement changes calculated at  110 . At  114 , active damping control using a slipping clutch is initiated if it was not already occurring. Active damping control continues throughout the remaining steps. At  116  and  118 , the controller may command a gradual change in AC compressor displacement and field current in preparation for the changes calculated at  110  and  112  respectively. For example, if the AC compressor torque is to be reduced to maintain the natural frequency, it may be gradually increased at  116  to make room for the reduction. Because the changes are gradual, the vehicle occupants do not notice and active damping control may continue. When the pre-select event actually begins, via coupler actuation, coupler release, or a CBS event, the changes calculated at  110  and  112  are executed at  120  and  122  respectively. When the event completes, do to coupler engagement, disengagement, or completion of the CBS event, the commands are changed at  124  and  126  by the second amounts calculated at  110  and  112  respectively. Then, at  128  and  130 , the AC compressor command and the field current command are gradually returned to their nominal values. 
         [0025]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.