Abstract:
A method and apparatus provide control of a neutral idle (NI) clutch to allow a vehicle with automatic engine start-stop functionality to utilize the NI state as a transitional shift state, either upon or just prior to engine shutdown, to minimize driveline disturbances. By controlling the NI state, the vehicle driveline is decoupled and torque multiplication is prevented upon engine restart. Execution of an algorithm unloads the engine upon shutdown, and unloads or partially loads the engine as a designated NI clutch reapplies during an engine restart event. The NI clutch may be a component of a multi-speed automatic transmission, e.g., a 6-speed or an 8-speed transmission, having a plurality of torque transfer mechanisms or clutches. One of these clutches is designated as the NI clutch, and this designated NI clutch may be selectively actuated to enter the NI state in conjunction with engine shut down/restart.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the shift control of a transmission having neutral idle functionality in a vehicle having an engine start-stop powertrain. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vehicle transmissions are designed to transmit torque from an engine to a set of drive wheels in order to propel the vehicle in a range of output speeds. The engine output shaft may be selectively connected to a transmission input shaft when engine propulsion is required. In a manual transmission, a clutch pedal may be depressed to allow a driver to shift gears and/or to place the transmission into a neutral state. In an automatic transmission, a hydrodynamic torque converter automatically provides this engine/transmission connection. 
         [0003]    A torque converter includes an impeller/pump, a turbine, and a stator. The torque converter is filled with oil. The pump, which may be bolted to a rotating engine flywheel to continuously rotate at engine speed, discharges the oil into the turbine. The turbine is connected to the transmission input shaft, and therefore rotation of the turbine ultimately causes a rotation of the coupled transmission input shaft. A stator redirects oil discharged from the turbine back into the pump. The use of a torque converter thus enables a variable fluid coupling effect to automatically occur between the engine and the transmission, thereby allowing the vehicle to slow to a stop without stalling, while also allowing required torque multiplication to occur at low vehicle output speeds. 
         [0004]    This variable slip capability allows the engine to continue to rotate when the vehicle is idling in certain transmission states or modes, e.g., in park (P), neutral (N), or in a drive state, i.e., a forward drive mode (D) or a reverse mode (R). In some transmission designs operating in a neutral (N) state during a drive detent position, i.e., when the vehicle reaches zero output speed at a standstill or when idling and the engine remains running, the transmission may be automatically shifted to a hydraulic neutral state referred to as neutral idle (NI). 
         [0005]    Certain vehicle powertrains such as hybrid electric vehicle (HEV) powertrains are able to selectively utilize different energy sources to optimize fuel efficiency. An HEV having a full hybrid powertrain can use either or both of an internal combustion engine and a high-voltage energy storage system (ESS) for propulsion. That is, a typical full HEV powertrain can be electrically-propelled immediately upon starting the HEV and during vehicle speeds up to a relatively low threshold speed. One or more high-voltage motor/generator units (MGU) may alternately draw power from and deliver power to the ESS as needed. Above the threshold speed, the engine can be restarted and engaged with the transmission to provide the required propulsive torque. 
         [0006]    The powertrain of a mild HEV lacks the capability of propelling the HEV via purely electrical means, but nevertheless retains certain key design features of the full hybrid powertrain, e.g., the capability of selectively shutting down or powering off the engine at idle. The capability of any HEV to selectively shut off and restart its engine when the vehicle is at a standstill, and/or when operating in a stabilized low-speed drive mode, is of particular fuel-saving benefit relative to conventional vehicle designs. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, a method and apparatus are provided herein for controlling a neutral idle (NI) clutch shift operation to allow a vehicle with automatic engine start-stop functionality to utilize the NI state as a transitional shift state. By using the NI state as a transitional shift state, either upon or prior to engine shutdown as well as upon restart of the engine, the vehicle driveline is decoupled to minimize driveline disturbances, and torque multiplication may be prevented upon engine restart. Shutting down an engine reduces fuel consumption, as noted above, however doing so may result in a temporary loss of oil pressure to the various clutches and gears of a transmission gear box. Some amount of oil pressure is required for optimal transmission control upon engine restart and vehicle launch, and therefore an auxiliary device, e.g., an auxiliary pump or a surge accumulator, may be used for this purpose without departing from the intended scope of the invention. 
         [0008]    Maintaining oil pressure with an auxiliary device allows the transmission to remain in 1 st  gear in a conventional powertrain. However, a starter motor must crank against a stationary turbine and a locked gearbox, a situation which may produce cranking and combustion-related torsional transients along the driveline during engine restart. The present method and apparatus therefore enable the engine to shut down and restart in an unloaded or a partially-loaded state as set forth herein, depending on the particular auxiliary device that is used. 
         [0009]    Execution of the algorithm embodying the method by an onboard controller unloads the engine upon shutdown, and unloads or partially loads the engine as a designated NI clutch reapplies during an engine restart event. The vehicle includes a multi-speed automatic transmission, e.g., a 6-speed or an 8-speed transmission of the type disclosed herein, having a plurality of torque transfer mechanisms or clutches. One of these clutches may be designated as the NI clutch. This designated NI clutch may be selectively actuated to enter the NI state, and may also be used to launch the vehicle in 1 st  gear. 
         [0010]    As the NI state is entered, the unloaded engine shuts down. The algorithm commands a clutch pressure, which is either zero or a pre-learned return spring pressure depending on the particular oil-assist type or auxiliary device, if used, e.g., an auxiliary pump, a surge accumulator. Upon engine restart, the NI clutch may be held at a pre-learned return spring pressure or commanded via a fill pulse, again depending on the oil-assist type. Clutch reapply for vehicle launch begins at a predetermined point in the engine restart event. As the vehicle begins to move during launch, reapply of the NI clutch, which is also configured as the 1 st  gear clutch, continues until the NI clutch is locked and the vehicle moves. When a partially-loaded state is used, the NI clutch may begin slipping and pulling down turbine speed earlier in the process. 
         [0011]    In particular, a vehicle is provided that includes an engine, a plurality of clutches that are selectively engageable, alone or in combination with each other, to establish a plurality of forward drive modes, with one of these clutches being designated as the NI clutch. The engine is adapted to shut down at idle or when the vehicle is stationary to conserve idle fuel consumption. A controller includes an algorithm adapted for shifting the transmission into the NI state to at least partially unload the engine prior to the engine shut down and during the subsequent restart maneuver. 
         [0012]    A method is also provided for shifting a transmission of a vehicle into the NI state during an engine shut down and restart event, the transmission having a plurality of clutches that are selectively engageable, alone or in combination with each other, to establish a plurality of forward drive modes. The method includes determining the presence of a commanded engine shut down event using a controller, actuating a designated one of the clutches as an NI clutch using the controller to thereby enter the NI state prior to shutting down the engine to thereby at least partially unload the engine, and determining the presence of a commanded engine restart event using the controller. The method also includes starting the engine while the engine remains at least partially unloaded, and then actuating the NI clutch to thereby launch the vehicle. 
         [0013]    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 
         [0014]      FIG. 1  is a schematic illustration of a vehicle having an automatic transmission, engine start-stop functionality, and a neutral idle (NI) clutch shift control method or algorithm in accordance with the invention; 
           [0015]      FIG. 2A  is a lever diagram for one embodiment of a transmission usable with the vehicle shown in  FIG. 1 ; 
           [0016]      FIG. 2B  is a lever diagram for another embodiment of a transmission usable with the vehicle shown in  FIG. 1 ; 
           [0017]      FIG. 2C  is a lever diagram for yet another embodiment of a transmission usable with the vehicle shown in  FIG. 1 ; 
           [0018]      FIG. 3  is a graphical flow chart describing an algorithm suitable for executing the NI clutch shift control method of the invention; and 
           [0019]      FIG. 4  is a set of vehicle performance curves describing an NI clutch shift operation during engine start-stop cycling. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, the vehicle  10  shown in  FIG. 1  includes a controller (C)  26  having a neutral idle (NI) shift control algorithm  100 , as described below with reference to  FIGS. 3 and 4 . The controller  26  is adapted for executing the algorithm  100  to thereby control an NI shift event in conjunction with an engine shut down/restart or start-stop event. The NI state may be entered either during a coast-down maneuver from a forward drive mode while the vehicle  10  is still moving, or once the vehicle reaches a zero speed. Execution of algorithm  100  allows an engine (E)  12  to shut down and restart in a partially-loaded or a fully unloaded state, by controlling the shift operation of a designated NI clutch, assisted by the particular onboard oil-assist type described below. 
         [0021]    The engine  12  is controlled to provide start-stop functionality, also known as autostop/autostart capability, wherein the engine is selectively turned off at idle or at zero speed to conserve fuel as noted above. A starter motor  11  may be used to crank and restart the engine  12 . The engine  12  is selectively coupled to an automatic transmission (T)  14  via a hydrodynamic torque converter  16 . An output shaft  13  of the engine  12  rotates at an engine speed (N E ), and an input shaft  15  of the transmission  14  rotates at a turbine speed (N T ). Transfer of an input torque (T i ) to the transmission  14  thus occurs at a variable rate through the torque converter  16 . 
         [0022]    The transmission  14  also includes an output shaft  18  connected to a set of road wheels  24 . The output shaft  18  ultimately carries a transmission output torque (T o ) from various clutch and gear sets  17  of the transmission  14 , including a designated NI clutch as noted below with reference to  FIGS. 2A-C , to thereby propel the vehicle  10 . A differential (not shown) may be included in the design without departing from the intended scope of the invention. 
         [0023]    The clutch and gear sets  17  may be selectively actuated using electro-hydraulic controls powered by fluid from a main transmission pump (P)  33  at a line pressure (P L ). The pump  33  may be configured to draw fluid  37  from a sump  35 , with the fluid having a temperature (T Sump ). However, other non-fluidic actuating means or devices may also be used within the scope of the invention. Additionally, an optional auxiliary device (AUX)  33 A, e.g., an electrically-operated auxiliary fluid pump or a surge accumulator adapted for temporarily directing oil to the clutch and gear sets  17  when the engine  12  is restarted, may be used to ensure delivery of sufficient oil pressure to the transmission  14  during an engine-off state and upon engine restart. 
         [0024]    Still referring to  FIG. 1 , the transmission  14  may be configured as a multi-speed transmission, e.g., a 6-speed or an 8-speed transmission of the type set forth in  FIGS. 2A-2C  below, having NI state functionality. Transmission  14  has a designated NI clutch that can be automatically actuated to establish the NI state during a coast-down maneuver from a forward drive mode while the vehicle  10  is still moving, i.e., 1 st  gear or a higher forward drive gear, or upon the vehicle  10  reaching a zero speed, depending on the configuration of the vehicle. 
         [0025]    In a neutral idle (NI) state, the transmission  14  may be placed in a drive (D) mode while electro-hydraulic clutch pressure regulation valves (not shown) reduce the pressure on a designated NI clutch, thereby placing the transmission into a partially-loaded “hydraulic neutral” state as noted above. Data used by the algorithm  100  may reside within or may be accessible by the controller  26 , and may be sampled or processed thereby during other transmission states such as neutral (N) and park (P). 
         [0026]    Vehicle data that may be sampled in order to determine appropriate NI state entry conditions may include, but are not necessarily limited to: vehicle output speed (N O ), a value which may be measured by one or more sensors  39  shown separately in  FIG. 1  for clarity, but which could also be positioned as needed within the vehicle  10 , e.g., at or along the transmission output shaft  18  and/or at the road wheels  24 , etc; a throttle level (Th %) of a throttle input device such as an exemplary accelerator pedal  29 A; a braking level (B) such as pedal position/travel and/or a braking force applied to brake pedal  29 B; a PRNDL setting (S) of the transmission  14 ; a temperature (T Sump ) of the fluid  37  contained in or delivered from the sump  35 ; onboard diagnostics; etc. 
         [0027]    Still referring to  FIG. 1 , the engine  12  and torque converter  16  are in communication with the controller  26 , which is configured for storing and accessing the algorithm  100 . The algorithm  100  in turn is specially adapted to execute the method of the invention as described below with reference to  FIGS. 3 and 4 . The controller  26  may be configured as a microprocessor-based device having such common elements as a microprocessor or CPU, memory including but not limited to: read only memory (ROM), random access memory (RAM), electrically-erasable programmable read-only memory (EEPROM), etc., and circuitry including but not limited to: a high-speed clock (not shown), analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, a digital signal processor or DSP, and the necessary input/output (I/O) devices and other signal conditioning and/or buffer circuitry. However configured, the controller  26  is operable for executing at least the algorithm  100  of  FIG. 3  as needed to provide entry in an NI state during a coast-down maneuver from a forward drive mode. 
         [0028]    The controller  26  is adapted for receiving, reading and/or measuring, calculating, and recording or storing various required measurements, values, or figures including any required readings fully describing the engine speed (N E ), turbine speed (N T ), and the transmission output speed (N O ), such as via one or more speed sensors  39  having an output speed or speeds labeled generically as (N X ). The speed signals (N E ), (N O ) may be transmitted electrically via conductive wiring, although other transmitting means are also usable within the scope of the invention, for example radio frequency (RF) transmitters and receivers. 
         [0029]    The torque converter  16  includes a stator  30  between an impeller or pump  32  and a turbine  34 . An optional lockup torque converter clutch (TCC)  31  may also be used to selectively lock the pump  32  and turbine  34  above a threshold lockup speed. The pump  32  may be bolted or otherwise directly connected to the output shaft  13  to thereby rotate at engine speed (N E ). Within the torque converter  16 , the turbine  34  is driven by fluid  37  and is connected to the input shaft  15  of transmission  14 . Thus, a rotation of turbine  34  ultimately rotates the input shaft  15  at a turbine speed (N T ) less than or equal to engine speed (N E ). Viscous drag or friction losses occurring within the transmission  14  may reduce the turbine speed (N T ) to a level slightly less than engine speed (N E ) as shown in  FIG. 4 , and as understood by those of ordinary skill in the art. 
         [0030]    Referring to  FIG. 2A , the transmission  14  of  FIG. 1  is shown as a transmission  114  configured as a 6-speed front wheel drive transmission, which may be adapted for use as either a rear wheel drive (RWD) or a front wheel drive (FWD) transmission. Transmission  114  may include first and second gear sets  140  and  150 , respectively; braking clutches CB26, i.e., clutch  43 , and CBR1, i.e., clutch  136 ; and rotating clutches C35R, i.e., clutch  53 , and C1234, i.e., clutch  138 . 
         [0031]    In the 6-speed embodiment of  FIG. 2A , either of the following clutches noted above may be used as the designated NI clutch noted above in order to enter neutral idle (NI) from a forward drive mode or from a standstill: clutch CBR1, i.e., clutch  136 , and clutch C1234, i.e., clutch  138 . For clutch  136 , the NI state may be entered from as high as 1st gear; for clutch  138 , from as high as 4 th  gear. When using clutch  138 , a free-wheeling element (F 1 )  19  is used to prevent rotation with respect to node  156  of the second gear set  150 . 
         [0032]    The first gear set  140  may include nodes  142 ,  144 , and  146 , which in one possible embodiment may be a ring gear (R 1 ), a carrier member (PC 1 ), and a sun gear (S 1 ), respectively. The input shaft  15  may be directly connected to node  142 , and to an input side of clutch C456, i.e., clutch  51 . Node  144  may be connected to an input side of clutch C1234, i.e., clutch  138 , and to an input side of clutch C35R, i.e., clutch  53 . Node  146  is grounded to the stationary member  28 . As will be understood by those of ordinary skill in the art, as used in  FIGS. 2A-C  the term C1234, for example, refers to a clutch (C) used to establish each of 1 st , 2 nd ,3 rd , and 4 th  gear, i.e., the various forward drive modes that a clutch so labeled may be used to establish. Likewise, use of the letters B or R in the same clutch designation refers to a braking clutch and reverse gear, respectively. 
         [0033]    Second gear set  150  includes nodes  152 ,  154 ,  156 , and  157 , which may be respectively embodied as a sun gear (S 2 ), a ring gear (R 2 ), a carrier gear (PC 2 ), and another sun gear (s 2 A), respectively. Node  154  is directly connected to the transmission output shaft  18  and rotates at output speed (T out ). Node  156  is connected to an input side of clutch CBR1, i.e., clutch  136 , which is also connected to stationary member  28 . 
         [0034]    As noted above, either of clutches  136  and  138  may be utilized as the designated NI clutch without departing from the intended scope of the invention. When using clutch  138 , an optional free-wheeling mechanism (F 1 )  19  may be connected between stationary member  28  and node  156  to allow rotation with respect to node  156  in only one rotational direction. When using clutch  136  as the NI clutch, the free-wheeling mechanism  19  may be omitted. 
         [0035]    Referring to  FIG. 2B , the transmission  14  of  FIG. 1  is shown as a transmission  214  configured as another 6-speed front wheel drive transmission, which like the transmission of  FIG. 2A  may also be adapted for use as either a rear wheel drive (RWD) or a front wheel drive (FWD) transmission. Transmission  214  may include first, second, and third gear sets  240 ,  250 , and  260 , respectively; braking clutches CB26, i.e., clutch  243 , CBR1, i.e., clutch  236 , and CB1234, i.e., clutch  238 ; and rotating clutches C35R, i.e., clutch  253 , and C456, i.e., clutch  251 . 
         [0036]    In the 6-speed embodiment of  FIG. 2B , clutch CB1234, i.e., clutch  238 , may be used to enter neutral idle (NI) from a standstill or from a forward drive mode. When using clutch  238 , the free-wheeling element (F 1 )  19  may be used to prevent rotation with respect to node  254  of the second gear set  250 . 
         [0037]    First gear set  240  may include nodes  242 ,  244 , and  246 , which in one possible embodiment may be a ring gear (R 1 ), a carrier gear (PC 1 ), and a sun gear (S 1 ), respectively. The input shaft  15  may be selectively connected to nodes  244  and  246  via clutches  251  and  253 , respectively. Node  242  is directly connected to node  264  of the third gear set  260 . 
         [0038]    Second gear set  250  includes nodes  254 ,  256 , and  257 , which in one possible embodiment may be configured as a ring gear (R 2 ), a carrier gear (PC 2 ), and a sun gear (S 2 ), respectively. Node  257  is directly connected to the transmission input shaft  15 . Node  254  is connected to node  244  of the first gear set  240 . Free-wheeling element (F 1 )  19  connects to stationary member  28  to allow rotation with respect to node  254  in only one rotational direction. 
         [0039]    Third gear set  260  includes nodes  262 ,  264 , and  266 , which may be embodied as a ring gear (R 3 ), a carrier gear (PC 3 ), and a sun gear (S 3 ), respectively. Node  266  is selectively connected to stationary member  28  via a clutch CB1234, i.e., clutch  238 . Node  264  is connected to node  242  of the first gear set  240 , and to the output shaft  18  of transmission  14 . Node  262  is directly connected to node  256  of the second gear set  250 . 
         [0040]    Clutch  238 , i.e., CB1234, may be utilized as the NI clutch in this particular embodiment as noted above. When using clutch  238 , free-wheeling mechanism (F 1 )  19  may be connected between nodes  244  and  254  of gear sets  240  and  250 , respectively, to allow rotation with respect to node  254  in only one rotational direction. Clutch  236 , i.e., CBR1 can be used as the NI clutch if F 1  is omitted. 
         [0041]    Referring to  FIG. 2C , in yet another embodiment the transmission  14  shown in  FIG. 1  may be configured as an 8-speed transmission, either FWD or RWD, having a plurality of gear sets and clutches, i.e., the clutches and gears  17  of  FIG. 1 . Transmission  14  may include a first, second, third, and fourth gear sets  40 ,  50 ,  60 , and  70 , braking clutches CB12345R, i.e., clutch  41 , and CB1278R, i.e., clutch  36 ; and rotating clutches C13567, i.e., clutch  38 , C23468, i.e., clutch  58 , and C45678R, i.e., clutch  48 . 
         [0042]    In the 8-speed embodiment of  FIG. 2C , any of the following clutches noted above may be used to enter neutral idle (NI) from a standstill or from a forward drive mode: clutch CB1278R, i.e., clutch  36 ; braking clutch CB12345R, i.e., clutch  41 ; and clutch C13567, i.e., clutch  38 . For clutch  36 , the NI state may be entered from as high as 2 nd  gear; for clutch  41 , as high as 5 th  gear; and for clutch  38 , as high as 1 st  gear. 
         [0043]    The first gear set  40  may include nodes  42 ,  44 , and  46 , which may be a sun gear (S 1 ), a carrier (PC 1 ), and a ring gear (R 1 ), respectively. Node  46  maybe selectively connected to stationary member  28  via a clutch CB12345R, i.e., clutch  41 . Node  42  may be selectively connected to stationary member  28  via a clutch CB1278R, i.e., clutch  36 . Node  42  is also connected to a node  52  of second gear set  50 . Node  54  of gear set  50  is connected to an input side of a rotating clutch C13567, i.e., clutch  38 , as is the transmission input shaft  15  with input torque (T in ). Node  56  is connected to a third gear set  60  as explained below. 
         [0044]    The second gear set  50  may include nodes  52 ,  54 , and  56 , which may be a sun gear (S 2 ), carrier (PC 2 ), and ring gear (R 2 ), respectively. Node  52  maybe directly connected to node  42  of gear set  40 . Node  54  may be directly connected to the transmission input shaft  15 . 
         [0045]    The third gear set  60  may include nodes  62 ,  64 , and  66 , which may be a sun gear (S 3 ), carrier (PC 3 ), and ring gear (R 3 ), respectively. Node  66  may be directly connected to node  56  of the second gear set  50 , and selectively connected to node  54  by a clutch C23468, i.e., clutch  58 , and a clutch C13567, i.e., clutch  38 . 
         [0046]    The fourth gear set  70  may include nodes  72 ,  74 , and  76 , which may be a sun gear (S 4 ), a carrier gear (PC 4 ), and a ring gear (R 4 ), respectively. Node  76  is directly connected to node  44  via a member  45 . Node  74  is directly connected to the transmission output shaft  18 , and directly connected to node  64  of the third gear set  60  via a member  47 . Node  72  is selectively connected to node  62  via a clutch C45678R, i.e., clutch  48 . 
         [0047]    Referring to  FIG. 3  in conjunction with the vehicle  10  of  FIG. 1  and vehicle performance curves  75  of  FIG. 4 , the execution of the algorithm  100  utilizes the NI state in conjunction with engine on/off or start-stop cycling to minimize driveline disturbances.  FIG. 4  includes traces of engine speed (N E )(line  82 ), turbine speed (N T )(line  84 ), an engine run flag  85 , wherein a value of 1 represents an engine on/start state and a value of 0 represents an engine off/stop state, a brake on/off state  86 , and various traces describing the different command pressures for clutch control as represented by lines  87 ,  88 , and  94  and explained below. 
         [0048]    Algorithm  100  begins with step  102 , wherein the auxiliary system  33 A, if one is used, is turned on or made ready, and wherein a set of conditions (X) is examined to determine if the engine shutdown process may proceed. 
         [0049]    Conditions (X) may include, without being limited to, a determination that an NI state has commenced during engine shut down at approximately point  80  on the engine speed trace, i.e., line  82  of  FIG. 4 , that the vehicle is at a standstill at or before approximately point  91  of the same trace, the brake pedal  29 B of  FIG. 1  is applied as indicated by the brake on/off state, a previously-learned return spring pressure (P RS ) of line  88  in  FIG. 4  is recorded or available, and if so equipped, that the auxiliary system  33 A is on and is actively supplying oil pressure (P AUX )(line  87 ) to the clutch and gears  17 , including the designated NI clutch used to enter the NI state at engine shutdown. If conditions (X) are present, the algorithm  100  proceeds to step  104 , otherwise the algorithm exits. 
         [0050]    At step  104 , clutch pressure to the designated NI clutch, e.g., clutch  1234  of  FIG. 2A , is held at the previously-learned return spring pressure (P RS ), as represented by the level of line  88  of  FIG. 4 , during the active NI State, and then proceeds to step  106 . 
         [0051]    At step  106 , the engine  12  is automatically shut down. Engine run flag  85  may be set to zero at approximately point  91  of line  82  to indicate that engine shutdown has been completed. The brake pedal  29 B of  FIG. 1  is applied, as indicated by “1” state of the brake on/off state  86 . The algorithm  100  proceeds to step  108 . 
         [0052]    At step  108 , the algorithm  100  commands NI clutch pressure to a predetermined level, represented as P X  in  FIG. 4 . This level (P X ) depends on the particular oil-assist type used within the transmission  14 . That is, if an auxiliary system  33 A in the form of an auxiliary pump is available, it may be commanded to provide oil pressure to the designated NI clutch at a pre-learned return spring pressure (P RS ), i.e., line  88  of  FIG. 4 . If a surge accumulator is used, or if no oil-assist is provided during the engine off state, the NI clutch may be commanded to zero pressure as indicated by line  187 . The algorithm  100  then proceeds to step  110 . 
         [0053]    At step  110 , another set of conditions (Y) is examined to determine if a subsequent restart of the engine  12  may commence. For example, conditions (Y) may include a driver taking a foot off of the brake pedal  29 B of  FIG. 1 , moving a PRNDL lever out of drive, making a determination of whether an onboard device requires air conditioning or heat, etc. If equipped with an optional surge accumulator, the accumulator may begin to supply oil to the designated NI clutch, as indicated by the pulse  89  of line  187  in  FIG. 4 . The algorithm  100  then proceeds to step  112 . 
         [0054]    At step  112 , the engine  12  is cranked and started, e.g., using the starter motor  11 , or using a belt alternator starter (BAS) system if so equipped. The turbine  34  of the torque converter  16  begins to rotate in conjunction with the engine output shaft  13 . If equipped with a surge accumulator or no oil-assist mechanism at all other than the pump  33 , a fill pulse may be optionally commanded to the designated NI clutch. Otherwise, the return spring pressure (P RS ) of line  88  may be commanded to begin to fill the NI clutch, which is also configured as the 1 st  gear clutch. If equipped with an auxiliary pump, the auxiliary pump could be commanded on so that a hydraulic or other clutch control system (not shown) ultimately fed by the pump  33  may command the return spring pressure (P RS ) of line  88  while the engine is off, and holding the return spring pressure (P RS ) during engine crank, until the main pump  33  takes over. Algorithm  100  proceeds to step  114 . 
         [0055]    At step  114 , another set of conditions (Z) is examined to determine if the NI clutch may be actuated. For example, conditions (Z) may include passing a calibrated engine speed threshold, or an event in which the engine run flag  85  transitions from a value of 0 to a value of 1, i.e., when engine speed reaches approximately point  81 . The algorithm  100  proceeds to step  116  if conditions (Z) are satisfied, otherwise the algorithm proceeds to step  118 . 
         [0056]    At step  116 , NI clutch control is executed to prevent turbine speed (N T ) represented by line  84  of  FIG. 4  from rising beyond a calibrated range of engine speed (N E )(line  82 ). Vehicle  10  begins to launch, and the algorithm  100  proceeds to step  120 . 
         [0057]    At step  118 , an alternate or default shift sequence may be executed when conditions (Z) of step  114  are not satisfied. For example, a state other than NI may be entered for launch of the vehicle  10 , such as a command to maximum holding pressure resulting in a loaded start. The algorithm then proceeds to step  120 . 
         [0058]    At step  120 , the designated NI clutch is commanded to full pressure, i.e., rising from the level of point  92  to a maximum holding pressure level of line  94 . When using clutch C1234 of  FIG. 2A  as the designated NI clutch, for example, this pressure level can be used to complete a shift to 1 st  gear, and the vehicle  10  will begin to move forward in 1 st  gear. The algorithm  100  is then finished upon launch, with overall shift control authority thereafter provided by a top-level transmission shift control algorithm (not shown). 
         [0059]    Accordingly, execution of the algorithm  100  using the controller  26  allows the engine  12  to shut down and restart in an unloaded or a partially loaded state by using the NI state as a transitional shift state. Execution of the algorithm  100  may provide an optimal driveline feel during engine restart and shutdown, and may provide a reduced rate of idle fuel consumption, e.g., zero when the engine  12  is off, in city driving or in other stop-and-go traffic conditions conducive to engine start-stop cycling. 
         [0060]    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.