Abstract:
A vehicle includes torque sources, a transmission, and a controller programmed to execute a method. In executing the associated method, the controller determines whether continuous output torque is required through a torque exchange. When continuous output torque is required, the controller synchronizes and fills the oncoming clutch, estimates capacity of the oncoming clutch, and expands a short-term torque capacity of the oncoming clutch during the torque exchange, doing so in response to a control objective having a threshold priority. Onset of the torque exchange delays until the short-term torque capacity is sufficient for receiving all torque load from the offgoing clutch without affecting output torque. The controller asynchronously controls the offgoing clutch and synchronously controls the oncoming clutch through the torque exchange, and loads the synchronous oncoming clutch via an expanding set of long-control torque capacity limits as a function of a simultaneously exhausting of the offgoing clutch load.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method and a system for exchanging torque from an asynchronous to a synchronous clutch in a hybrid electric vehicle. 
     BACKGROUND 
     A torque exchange between clutches of a conventional automatic vehicle transmission is closely controlled via a control module, e.g., a transmission control module (TCM). The control module of such a transmission commands offload of torque capacity of the particular clutch that is associated with a current speed ratio, i.e., the offgoing clutch, and simultaneously applies another clutch associated with a desired new speed ratio, i.e., the oncoming clutch. Torque from one or more sources, typically an internal combustion engine and/or one or more electric traction motors, is then exchanged from the offgoing clutch to the oncoming clutch in order to complete the shift. 
     The clutches of a transmission may be described in terms of the mode that is used to establish their control. Thus, the offgoing and oncoming clutches may be referred to as “synchronous clutches” in a typical synchronous shift. In an oncoming synchronous clutch, clutch pressure remains fully exhausted while the clutch is still slipping. Clutch pressure is applied only after the synchronous speed is attained. By way of contrast, for an offgoing asynchronous clutch, some amount of clutch pressure is applied to the clutch assembly even after the clutch slips. As a result, an asynchronous clutch is able to produce output torque while slipping. 
     A hybrid transmission lacks a fixed speed ratio. In other words offloading/oncoming of the clutches of a hybrid transmission are generally not required because of the speed ratio. Also, in a hybrid transmission one may transition from a gear state to a mode in which there is one offgoing clutch. Thus, not all shifts in a hybrid transmission have an offgoing-oncoming clutch combination. 
     SUMMARY 
     A hybrid electric vehicle is disclosed herein. The vehicle includes a controller, e.g., a hybrid control processor, and a plurality of fluid-actuated clutches. For a requested shift, one of the clutches is designated as the offgoing clutch, while another of the clutches is designated as the oncoming clutch, with both terms described above. The shift involves a torque exchange or “handoff” from the offgoing clutch to the oncoming clutch. The controller is programmed or otherwise configured to selectively execute steps of an associated method whenever a driver requires continuous output torque through the shift event, e.g., a shift through neutral. 
     When the present method is executed, the controller asynchronously controls the offgoing clutch, i.e., the offgoing asynchronous clutch, while the oncoming clutch is synchronously controlled. Thus, the oncoming clutch is referred to herein as the oncoming synchronous clutch. As part of the method, the controller also calculates and enforces short-term and long-term torque limits for the oncoming synchronous clutch. The torque limits are selectively enforced commencing at the end of a period of a phase of asynchronous offgoing clutch control, and continue until the start of a subsequent phase of synchronous torque control, as is explained in detail herein. Thus, selective execution of the present method may help to improve overall drive quality when a driver requests continuous output torque through the shift. 
     In particular, a hybrid electric vehicle is disclosed herein that includes a plurality of torque sources, a transmission, and a controller. The controller, which is in communication with the transmission and the torque sources, is configure, i.e., programmed in software and equipped in hardware, to determine whether continuous output torque is required through a duration of a torque exchange commanded via a requested shift. When the continuous output torque is required, the controller is operable to synchronize and fill the oncoming clutch, estimate a hydraulic capacity of the oncoming clutch, and temporarily expand a short-term torque capacity of the oncoming clutch during the duration of the torque exchange in response to a control objective having a threshold priority. The controller also delays onset of the torque exchange until the short-term torque capacity of the oncoming clutch is sufficient for receiving all torque load from the offgoing clutch without affecting the continuous output torque. Then, the controller asynchronously controls the offgoing clutch through the duration of the torque exchange, synchronously controls the oncoming clutch through the duration of the torque exchange, and loads the synchronous oncoming clutch via an expanding set of long-control torque capacity limits as a function of a simultaneously exhausting the offgoing clutch load. 
     The vehicle may include an electric motor and an internal combustion engine as the plurality of torque sources. In such a case, the controller is configured to offload the offgoing clutch using a combination of torque from the electric motor and the engine while maintaining the output torque at a continuous level. 
     A transmission assembly for the hybrid electric vehicle includes the gear set, the plurality of clutches, and the controller described above. An associated method for shifting the transmission is also disclosed that includes executing the functionality of the controller structure noted above. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a vehicle having an automatic transmission and a controller, the latter of which executes steps of the present method to thereby control an offgoing asynchronous-to-oncoming synchronous clutch torque exchange. 
         FIG. 2  is a flow chart describing an example method for executing an offgoing asynchronous-to-oncoming synchronous clutch torque exchange. 
         FIG. 2A  is another flow chart describing an optional embodiment for a portion of the method shown in  FIG. 2 . 
         FIG. 3  is an example time plot illustrating an application of the method shown in  FIGS. 2 and 2A , and illustrating enforcement of short-term and long-term torque limits on the oncoming synchronous clutch. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, and beginning with  FIG. 1 , an example hybrid electric vehicle  10  is shown that includes a controller (C)  50 . The controller  50  communicates with various components of an automatic transmission  12  via control signals (double headed arrow  11 ), e.g., signals transmitted and received over a controller area network (CAN) bus. The controller  50  is configured, via associated hardware and software elements as described herein, to selectively execute steps of a shift control method  100 , an example of which is described below with reference to  FIGS. 2 and 2A . 
     Execution of the method  100  allows the controller  50  to control a predetermined shift event, a term which is defined herein as a torque exchange or “handoff” occurring between an asynchronously-controlled offgoing clutch and a synchronously-controlled oncoming clutch when a driver of the vehicle  10  of  FIG. 1  requires continuous output torque through the commanded shift and immediately thereafter, for instance a power-on shift through neutral. 
     The present control approach is intended to improve drive quality by closely coordinating the torque offloading of a slipping clutch, i.e., the asynchronous offgoing clutch, with torque loading of a locked synchronous oncoming clutch. Most hybrid vehicle shift events, as noted above, are synchronous in nature. On rare occasions, however, an asynchronous shift may be required, with one such situation being when a driver requires continuous output torque through the shift. The torque exchange between the asynchronous and synchronous clutches is thus conducted under these circumstances in the manner set forth herein with reference to  FIGS. 2 ,  2 A, and  3 .  FIG. 3  in particular illustrates application of short-term (ST) and long-term (LT) torque limits, which are calculated and enforced by the controller  50 , as the asynchronous offgoing clutch exhausts. 
     Input torque to the transmission  12  of  FIG. 1  is provided via torque sources TS 1  and TS 2 , and optionally via another torque source TS 3  as shown in phantom. One or more of the torque sources TS 1 , TS 2 , and/or TS 3  may be a high-voltage electric traction motor, e.g., a polyphase electric machine rated for between 60 VDC and 300 VDC or more depending on the application. Another of the torque sources TS 1 , TS 2 , or TS 3  may be an internal combustion engine. An output shaft  25  of the torque source TS 3  may be selectively coupled to the torque source TS 1  via a rotating clutch C 3  as shown. Such an embodiment may allow for provision of electric assist to the torque source TS 3  by the torque source TS 1 . 
     The transmission  12  may include, in a simplified non-limiting embodiment, a single planetary gear set  20  having nodes  22 ,  24 , and  26 . In such an embodiment, the torque sources TS 1  and TS 3  may deliver input torque to node  22  via an input shaft  14  and another rotating clutch C 2 . Torque source TS 2  may be continuously connected to node  26  of the planetary gear set  20  via an interconnect member  16 , with the torque source TS 2  delivering motor torque as needed to node  26  of the transmission  12 . Node  22  may also be selectively connected to a stationary member  32  of the transmission  12  via a clutch C 1 , i.e., a braking clutch. Any of the clutches C 1 , C 2 , or C 3  may act as the oncoming or offgoing clutch for a given shift, as could any clutches used in other embodiments of the transmission  12 , as will be appreciated by those having ordinary skill in the art. 
     The transmission  12  of  FIG. 1  also includes an output shaft  18 . The output shaft  18  ultimately conveys output torque (arrow T O ) from the transmission  12  to a set of drive wheels (not shown). The clutches C 1 , C 2 , C 3  can be selectively actuated via electro-hydraulic controls (not shown), including for instance a fluid pump, valves, fittings, hydraulic hoses, and the like. Such structure is well known in the art, and thus is omitted from  FIG. 1  for illustrative simplicity. 
     The controller  50  shown in  FIG. 1  is configured to execute associated process steps, i.e., is programmed in software and equipped in hardware, such that the controller  50  selectively executes code embodying the present method  100 . For instance, the controller  50  may execute, via a processor  52 , a set of computer code or instructions that is stored on tangible, non-transitory memory  54 . This occurs during a pre-determined shift of the transmission  12 , particularly when continuous output torque is required through the shift and immediately thereafter. The controller  50  may be configured as a microprocessor-based computer device having, as the memory  54 , any required read only memory (ROM), random access memory (RAM), electrically-programmable read-only memory (EPROM), etc. The controller  50  may also include logic circuitry including but not limited to proportional-integral-derivative (PID) control logic, a high-speed clock (not shown), analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, a digital signal processor, and the necessary input/output (I/O) devices and other signal conditioning and/or buffer circuitry. 
     The controller  50  of  FIG. 1  may be in communication with a throttle sensor  19  positioned with respect to an accelerator pedal  17 . The throttle sensor  19  measures a level of travel or apply pressure of the accelerator pedal  17 , and outputs a throttle signal (arrow Th %) corresponding to the measured level or travel or force of the accelerator pedal  17 . The throttle signal (arrow Th %) is received by a transceiver (not shown) of the controller  50 . 
     The controller  50  thereafter processes the received throttle signal (arrow Th %) to thereby determine a driver requested torque, and thus to determine when continuous output torque is desired through the shift. When a driver requires such continuous output torque, the ensuing torque exchange is closely coordinated to ensure a smooth transition from a designated asynchronous offgoing clutch to a designated synchronous oncoming clutch, either of which can be one of the clutches C 1 , C 2 , or C 3  described above, or other clutches in different embodiments of the vehicle  10 . The controller  50  may, as part of the present control approach, offload the offgoing clutch using a combination of torque from an electric motor and engine while maintaining output torque at a continuous level. This shift control functionality of the controller  50  of  FIG. 1  will now be explained with reference to the remaining Figures. 
     Referring to  FIG. 2 , the method  100  for executing an asynchronous-to-synchronous clutch shift begins at step  102 , wherein the controller  50  of  FIG. 1  determines whether such a shift is requested. As noted above, most shifts of a hybrid electric transmission are synchronously controlled. As a result, the oncoming and offgoing clutches are both synchronous clutches, with various control approaches being available to coordinate the torque exchange in this instance. Therefore, the method  100  may automatically default to step  118  whenever a synchronous shift is required at step  102 , i.e., whenever continuous output torque is not required through the impending shift. Otherwise, the method  100  proceeds to steps  104  (asynchronous control) and  105  (synchronous control). 
     At step  104 , the controller  50  of  FIG. 1  identifies which of the various clutches, e.g., clutches C 1 , C 2 , or C 3  of  FIG. 1 , or other clutches in different embodiments of the vehicle  10  shown in the same Figure, is to function as the designated offgoing clutch for the impending shift. Step  104  may include setting a flag in memory  54  of the controller  50  of  FIG. 1 , the setting of which triggers execution of the subsequent steps in the method  100  flowing from step  104 . The method  100  proceeds to step  106  after completion of step  104 . 
     Step  105  of the method  100  entails initiating synchronous control (INIT. SYNC) of the designated oncoming clutch for the shift. Step  105 , like step  104 , may entail setting a flag in memory  54  of the controller  50 , the setting of which triggers execution of subsequent steps in the method  100 . The method  100  proceeds to step  107  after completion of step  105 . 
     At step  106 , the controller  50  commands offgoing clutch torque at a level that corresponds to the requested axle torque, which may be determined by the controller  50  as a function of the received throttle signal (arrow Th %) shown in  FIG. 1 . Step  106  may entail calculating a corresponding offgoing clutch torque command, accessing a calibrated lookup table, processing the requested torque through a model, and/or any other suitable approach. The method  100  proceeds to step  108  upon completion of step  106 . 
     At step  107 , the controller  50  predicts when synchronous speed will be attained, e.g., via clutch speed measurement or calculation of the clutches, and also monitors the clutch slip speed and fill rate. While this is occurring, the method  100  proceeds to step  108 . 
     Step  108  entails determining, via the controller  50 , whether the oncoming synchronous clutch has reached synchronous speed, e.g., via direct speed measurement or indirect methods such as speed calculation. Steps  106  and  107  are repeated if synchronous speed has not yet been reached. Prior to this point, the synchronous and asynchronous control portions of the method  100  run independently of each other. The controller  50  proceeds to steps  109  and  110  once synchronous speed has been reached. 
     At step  109 , the controller  50  of  FIG. 1  executes oncoming synchronous clutch control which occurs in three steps. Referring briefly to  FIG. 2A , at step  109 A the controller  50  of  FIG. 1  ramps a torque command to the oncoming clutch (T ONC ) at a calibrated rate. Alternatively, this step may entail ramping a pressure command in a similar manner, with torque and position being related, for instance, via a calibrated torque-to-pressure table. This step results in an increase in estimated torque capacity, e.g., as estimated by the controller  50  as a function of the torque command (T ONC ), whether via lookup table, modeling, formula, or otherwise. 
     Step  109 B follows immediately after step  109 A. Here, the controller  50  calculates positive (+) and negative (−) short-term (ST) torque capacity limits for the oncoming clutch. These short-term limits may be bounded by the estimated torque from step  109 A in the manner described below with reference to  FIG. 3 . Such limits are enforced by the controller  50  during the torque exchange phase of the commanded shift. While the short-term torque capacity limits used herein are not preferred in ordinary operation, the controller  50  selectively enables their use as a torque reserve on an as needed basis, for instance when a transient output torque spike or bump is necessary to protect hardware, e.g., a hybrid motor, battery power constraints, etc., which are of a higher priority than output torque, requires access to the output torque reserve provided by the ST torque capacity limits. 
     Step  109 C follows step  109 B. In this step, the controller  50  of  FIG. 1  calculates long-term (LT) torque capacity limits for the synchronous oncoming clutch. These long-term torque capacity limits are shown along with the short-term limits in FIG.  3 . The LT torque capacity limits provide preferred torque boundaries or recommended limits, which ultimately merge with the short-term limits at the completion of the torque exchange. Step  109 C may entail subtracting a torque capacity of the synchronous clutch clutch, which is the equivalent of the current asynchronous load from output torque, from the short-term torque capacity limits (ST ONC ) determined at step  109 B. The synchronous and the asynchronous clutches have different transmission ratios associated with them when calculating the effect on output torque, i.e., To=K×clutch load. The method  100  proceeds to step  111  of  FIG. 2  after completing steps  109 A-C of  FIG. 2A . 
     At step  110  of  FIG. 2 , the controller  50  limits the torque command (T OFG ) to the asynchronous offgoing clutch used in the present shift control event. Step  110  may entail temporarily preventing the torque command to the asynchronous offgoing clutch from increasing in magnitude until the synchronous oncoming clutch is ready to handle the entire clutch load for the shift. The method  100  then proceeds to step  112 . 
     Step  111 , which is arrived at from steps  109  and  114 , entails determining whether the asynchronous offgoing clutch has fully exhausted. This may entail determining when a modeled capacity of the offgoing asynchronous clutch indicates is fully exhausted. If the asynchronous offgoing clutch has not yet exhausted, the method  100  proceeds to step  114  and  109  for the asynchronous and synchronous clutches, respectively. The method  100  otherwise proceeds to step  116 . 
     Step  112  involves making a comparison, via the controller  50 , of two calculated absolute value torque limits: (i) the short-term (ST) clutch torque capacity for the synchronous oncoming clutch (T ONC,ST ), and (ii) an estimated torque for the offgoing clutch (T OFG,EST ), with the latter value optionally multiplied by a calibrated gain (K). In this instance, K=K OFG /K ONC , which provides an output torque equivalent of the asynchronous offgoing clutch per unit of synchronous oncoming clutch torque. A truth test for the following mathematical relationship, where ∥ represents absolute value, may be programmed into memory  54  of controller  50  shown in  FIG. 1  and evaluated by the processor  52 :
 
| T   ONC     ,ST     |&gt;K|T   OFG     ,EST   |
 
If this particular relationship holds true, then the method  100  proceeds to step  114 . Otherwise, the controller  50  repeats step  110 .
 
     At step  114 , the controller  50  of  FIG. 1  ramps the torque command (T OFG ) of the asynchronous offgoing clutch to zero. This step is performed at a calibrated rate. Once the torque command reaches zero, the clutch is commanded to the “exhaust completely position”, i.e., via control of a variable force control solenoid valve or other control solenoid feeding the clutch. The method  100  then proceeds to step  111  which is described above. 
     Step  116 , which may be arrived at from step  111  only upon determination of successful exhaustion of the asynchronous offgoing clutch, e.g., by observing an estimated or modeled torque capacity of the offgoing clutch in logic, entails terminating the asynchronous offgoing clutch control that was originally instituted at step  104 . Control of the synchronous oncoming clutch thereafter may continue in the default manner, e.g., via PID-based feedback, model-based feedforward torque, and/or position controls, while still adhering to the long-term torque capacity limits depicted in  FIG. 3 . The shift, and thus the method  100 , are complete. 
     At step  118 , the controller  50 , having earlier determined at step  102  that an asynchronous-to-synchronous shift is not presently required, executes default control over the impending shift. Such control may take many forms, with synchronous shift control of both the offgoing and oncoming clutches being well known in the art and outside of the scope of the present control approach. The method  100  is thus finished at step  118 . 
     Referring to  FIG. 3 , various vehicle control parameters are depicted to further illustrate the control method  100  described above. The amplitudes (A) of the parameters are plotted with respect to the vertical axis, while time (t) is plotted separately on a pair of horizontal axes to illustrate the control timelines for the synchronous oncoming clutch and the asynchronous offgoing clutch. 
     The asynchronous phase (I) of the present clutch control method  100  begins upon commencement of a requested shift at t 0  and ends at t 2 . Phase I is immediately followed by a torque exchange phase (II) between t 2  and t 3 , wherein clutch torque load, i.e., clutch capacity, is offloaded from the asynchronous offgoing clutch to the synchronous oncoming clutch. This offloading occurs within the applied constraints of the long-term and short-term torque capacity limits described above. A synchronous torque application phase (III) commences at t 3 , whereupon the asynchronous offgoing clutch is fully exhausted and offloaded. 
     Slip of the offgoing clutch, which is represented in  FIG. 3  as trace ω OFG , is held constant until t 2  before being gradually ramped to zero during the synchronous torque application phase (Phase III). Slip of the oncoming clutch, trace ω ONC , is synchronized at a calibrated profile, e.g., an S curve. The torque request (trace T R ) corresponding to the received throttle signal (arrow Th %) remains positive and substantially level throughout Phases I-III of the shift, although this trajectory is merely representative. That is, temporary spikes in throttle may be experienced over this duration and automatically accounted for via selective resort to the short-term torque capacity limits described below. Likewise, transmission output torque (T O ) remains constant until the completion of Phase II of the shift. The output torque (T O ) begins to ramp up after the torque exchange phase (Phase II) of the shift is complete at t 2 . Trace T O,ST  of  FIG. 3  illustrates a typical output torque profile in the absence of the present method  100  and enforced long-term torque capacity limits. 
     Clutch torque commands issued by the controller  50 , e.g., hydraulic pressure commands to the clutches, or the allowed actual clutch load by the engine and motors (e.g., TS 1-3  in  FIG. 1 ) involved in a torque exchange, are represented in  FIG. 3  as traces T ONC  and T OFG  for the synchronous oncoming and asynchronous offgoing clutches, respectively. The asynchronous offgoing clutch is plotted in an opposite torque direction relative to the oncoming clutch, i.e., with the offgoing clutch torque being negative in  FIG. 3  and thus “rising” toward zero as viewed in  FIG. 3 . The modeled or estimated torque of the asynchronous offgoing clutch is represented as trace T OFG,EST . The actual clutch command is shown as trace T OFG , which is at an amplitude of A 2 −. This amplitude is determined by the energy capacity and the control slip of the offgoing clutch. Offgoing torque is ramped down to zero soon after entry into Phase II, a process which occurs at a calibrated ramp rate as noted above. 
     In a normal synchronous hybrid shift, i.e., one in which an asynchronous-to-synchronous shift is not commanded at step  102  of  FIG. 2 , the oncoming torque command (T ONC ) would typically ramp upward to an amplitude of A 1 + beginning at about t 1  of  FIG. 3 , i.e., when the oncoming clutch is synched and the clutch is filled, and would thereafter step to its maximum before about t 2  near the end of Phase I. The oncoming torque command would be maintained through Phases II and III as shown. When an asynchronous-to-synchronous shift is commanded, the controller  50  of  FIG. 1  manages the synchronously-applied oncoming clutch such that the torque sources, for instance TS 1  and TS 2  of  FIG. 1 , effectively perform the torque transfer function simultaneously with an exhausting of the asynchronous offgoing clutch. 
     As a key part of the present control approach, the controller  50  enforces short-term and long-term torque limits on the synchronous oncoming clutch as noted above with reference to  FIG. 2 . The short-term (ST) limits are represented in  FIG. 3  as traces T ONC,ST  beginning shortly before t 2 . In this example, t 2  is when |Sync clutch ST limit|&gt;=|(K Async /K Sync )×Async clutch load |, i.e., with “∥” again representing absolute values, or in other words, when the synchronous clutch is ready to take the entire load from the asynchronous clutch in one control loop without affecting output torque (To). In the above relationship, K Async  and K Sync  describe constants. That is, after the synchronous clutch is applied, in the torque equation corresponding to that transmission state the output torque T O  can be expressed as:
 
 T   O   =K   ONC   ·T   ONC   +K   OFG   ·T   OFG  
 
     Likewise, the long-term (LT) limits are represented as traces T ONC, LT(MAX)  and T ONC,LT(MIN) . These short-term and long-term limits eventually merge just prior to entry into Phase III as shown, i.e., the synchronous torque application phase of the shift. The short-term and long-term limit merger always coincides with the moment in time at which the estimated torque of the asynchronous offgoing clutch (T OFG, EST ) reaches zero. 
     The controller  50  of  FIG. 1  may calculate the short-term limits for the oncoming clutch as follows:
 
 T   ONC,ST   =T   OFG,EST   *K  
 
wherein K is the torque ratio of the oncoming and offgoing clutches
 
               (       K   ONC       K   OFG       )     ,         
i.e., the proportion of the output torque (T O ) load on the synchronous oncoming (ONC) clutch relative to the asynchronous offgoing (OFG) clutch. The torque exchange is delayed until the synchronous oncoming clutch is able to handle the entire offgoing torque load without adversely decreasing the output torque (T O ), as noted above. The controller  50  does not use separate delay logic, but rather delay occurs naturally as the LT limits for the synchronous clutch are +/−0 until t 2 .
 
     The controller  50  of  FIG. 1  may calculate the long-term limits for the oncoming clutch as follows:
 
 T   ONC,LT   Max =Max(0,( T   ONC,ST   Max   −|K×T   OFG,EST |))
 
 T   ONC,LT   Min =Min(0,( T   ONC,ST   Min   −|K×T   OFG,EST |)).
 
When one substitutes 0 for the offgoing estimated torque (T OFG,EST ), the result is that the Min/Max LT limits equal the Min/Max ST limits, i.e., the limits converge exactly when the estimated torque/load goes to zero.
 
     When confined within the LT limits (or riding them), the present approach allows for a shaping of a smooth output torque profile during a torque exchange. In some instances it may be necessary to go beyond the LT limits into the ST limits, e.g., in order to protect/limited by hardware as noted above. The choice as to whether or not to access these limits may be made with an eye to higher priorities such as preserving hardware, and thus the short term reserve may be tapped into only selectively, that is, on an as-needed basis. Most of the time, the controller  50  of  FIG. 1  may simply operate within the LT limits. 
     By using the method  100  shown in  FIGS. 2 and 2A  in a hybrid electric vehicle, such as of the type shown in  FIG. 1 , selective enactment of an asynchronous offgoing-to-synchronous oncoming clutch shift is enabled for predetermined shifts. In this approach, as best shown in  FIG. 3 , the asynchronous offgoing clutch is offloaded while the synchronous oncoming clutch is simultaneously loaded. The enforcement of the short-term and long-term torque command limits described above commencing at or near the end of a period of asynchronous offgoing clutch control (Phase I), and continuing until the start of the synchronous torque application phase (Phase III) at t 3  of  FIG. 3 , i.e., when the offgoing asynchronous clutch is exhausted, may help to improve overall drive quality during shift events in which a driver requests continuous output torque through the duration of the shift. Likewise, drive quality may be improved via the control of slip, with the offgoing asynchronous clutch slip held constant in phase I, and then allowed to follow its target speed in phase II, albeit at a slower rate. 
     While the best modes for carrying out the 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 within the scope of the appended claims.