Abstract:
A powertrain in a vehicle includes an electro-mechanical transmission having a selectable one-way clutch mechanically-operatively coupled to an internal combustion engine and configured to selectively transmit mechanical power to an output member. A control method includes monitoring a vehicle speed, monitoring a transmission gear state, comparing the vehicle speed to a threshold low speed range, transitioning a selectable one-way clutch into an engaged mode when said vehicle speed is not in a forward direction in excess of the threshold low speed range, and maintaining the selectable one-way clutch in the engaged mode based upon the transmission gear state remaining in a first gear state and the vehicle speed remaining within said threshold low speed range, a neutral state, or a reverse state.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure is related to a transmission having a selectable one-way clutch. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    The present disclosure relates to a transmission within a vehicular powertrain having a selectable one-way clutch used to selectively transfer torque from an input member to an output member. One potential embodiment includes a selectable one-way clutch, utilized in combination with rotating input clutches that are slipped for engagement in forward and reverse. Clutch engagements can be performed in by a number of known methods. In a friction clutch engagement configuration, one or more clutches inside the transmission are slipped and gradually engaged to gradually transfer torque from the engine into the transmission for launch. In a frictionless clutch engagement configuration, clutch engagement occurs without clutch slip, requiring that the clutch be synchronized before engagement occurs. 
         [0004]    A transmission operates in one of a plurality of transmission gear states, defining the direction of torque applied to the output member and a gear ratio describing the relationship of torque applied to the input member to the output member. An exemplary transmission includes eight transmission gear states comprising six forward gears, a reverse gear, and a neutral gear state. 
         [0005]    A vehicle powertrain equipped with an automatic transmission can include a torque converter positioned between the engine and the transmission. In the alternative, a flywheel and isolator configuration can be used between the engine and the transmission. Some automatic transmissions include a low/reverse braking clutch and a one way clutch to provide reaction torque in both forward and reverse directions as input clutches are slipped for launching the vehicle without a torque converter. 
         [0006]    One-way clutches are used in numerous applications to selectively transmit power from a first rotatable driving member or input member such as an input race or first coupling plate, to a second independent driven member or output member such as an output race or second coupling plate. In application, the clutch transmits torque when the rotation of the driving member with respect to the driven member is in a first direction. When the relative rotational direction of the driving member with respect to the driven member is in an opposite or second direction, the clutch will release or decouple the driving member from the driven member. Once released, the driving member is free to rotate in the second opposite direction relative to the driven member. In this mode, the one-way clutch is free to overrun or “freewheel”. Freewheeling enables the vehicle to travel in a coasting movement, with the drivetrain decoupled from the engine, thereby eliminating losses associated with the drivetrain turning the engine. 
         [0007]    Engine braking is an operation whereby torque to the driving member is reduced, a coupled connection between the driving member and the driven member is maintained, and back-torque as a result of turning an engine with reduced power applied and pumping forces in the engine is transmitted through the connection with a result of slowing the driven member. Freewheeling of a one-way clutch decouples the driving member and the driven member inconsistent with operation of engine braking. Engine braking in a vehicle equipped with a one-way clutch operating as described above requires additional components, for example, an additional clutch to selectively couple the driving member and the driven member, to accomplish engine braking. 
         [0008]    In an exemplary front wheel drive, six speed automatic vehicle transmission, using a three gear set and five torque transmitting element (clutch) arrangement, one of the clutches, henceforth referred to as the low and reverse clutch, is only applied in manual low and in reverse. In first gear in the drive mode, the low and reverse clutch is disengaged. There is no relative motion between the input and output components of the low and reverse clutch because a reaction torque acting on a one-way clutch, to which the input components of the low and reverse clutch are connected, prevents rotation. However, in all other forward gears, second through sixth in the drive mode, there is no reaction torque acting on the one-way clutch as a result of one or more other clutches in the transmission being applied. Consequently, the one-way clutch freewheels and relative motion between the low and reverse clutch components is present. The relative speed between the input and output components of the low and reverse clutch increases with each successive gear change. 
         [0009]    As is commonly known to those familiar with the art, disengaged multi-plate clutches, depending on various conditions, produce drag when there is relative motion between the input and the output members. One characteristic of a disengaged multi-plate clutch is that, as the relative speed between the input and output members increases, typically, so does the drag or spin loss. This spin loss contributes to decreased fuel economy. Since the low and reverse clutch is disengaged in all of the forward gears of the described exemplary six speed automatic transmission with this particular gear set and clutch arrangement, and since most of the transmission&#39;s operating mode is in the forward gear ratios, a measurable amount of the overall spin losses in the transmission can be attributed to the drag in the disengaged low and reverse clutch. 
         [0010]    A selectable one-way clutch (SOWC) is similar to a one-way clutch in basic operation. However, depending on the details of the design, an SOWC is capable of producing a mechanical connection between a driving and a driven member in one or both rotational directions. Also, depending on the design, an SOWC is capable of overrunning in one or both directions of rotation. Typically, a SOWC contains an externally controllable selection mechanism, that when moved to a second, and possibly even a third position, controls the operating modes of the device. An SOWC, replacing a one-way clutch and a low and reverse clutch configuration as described above, reduces the part complexity of the transmission, reduces weight and cost of the transmission, and reduces overall spin losses. 
         [0011]    The ability of the driving member of a SOWC to transmit torque to a driven member depends on whether or not the engaging torque transmitting elements of the SOWC, typically rollers, sprags, rockers, or struts, etc., are able to freely engage one or more cams, notches, recesses, or similar features in the adjacent member. Similarly, the ability of the driving member to freewheel in one or both directions with respect to the driven member also depends on whether or not the engaging elements are free to interact with the adjacent member. Typically, a rotatable retainer ring or selector plate is used to accomplish this task by allowing or restricting engagement of the torque transmitting elements between the driving and the driven member. Other similar clutch designs are known in the art and capable of use as one-way clutching devices, including synchronizers and dog clutches. A number of clutch designs capable of functioning as an SOWC are envisioned, and this disclosure is not intended to be limited to the particular exemplary embodiments described herein. 
         [0012]    Depending on the application, an SOWC can be designed to operate in two modes or three modes. This disclosure will focus on a two mode SOWC. In particular, this disclosure will reference an SOWC enabling a first, disengaged mode and a second, engaged mode. The disengaged mode includes operation of the SOWC as a one-way clutch, as described above, allowing transmission of torque in a forward direction and freewheeling in the reverse direction. The engaged mode includes locking the clutch, preventing relative rotation of the driving member in either direction. In the engaged mode, the clutch allows transmission of torque in forward and reverse directions. Selection between the two modes includes actuation of a selection mechanism, for example, rotating a retaining ring or selector plate, henceforth simply referred to as a selector plate, between annular positions. 
         [0013]    Use of an SOWC in a vehicular transmission, replacing a one-way clutch and a low and reverse clutch configuration, can increase transmission efficiency. 
       SUMMARY 
       [0014]    A powertrain in a vehicle includes an electro-mechanical transmission having a selectable one-way clutch mechanically-operatively coupled to an internal combustion engine and configured to selectively transmit mechanical power to an output member. A control method includes monitoring a vehicle speed, monitoring a transmission gear state, comparing the vehicle speed to a threshold low speed range, transitioning a selectable one-way clutch into an engaged mode when the vehicle speed is not in a forward direction in excess of the threshold low speed range, and maintaining the selectable one-way clutch in the engaged mode based upon the transmission gear state remaining in a first gear state and the vehicle speed remaining within the threshold low speed range, a neutral state, or a reverse state. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  schematically illustrates a stick diagram of an exemplary powertrain  10 , in accordance with the present disclosure; 
           [0017]      FIG. 2  is an exemplary clutching table, describing operation of various clutches in an exemplary transmission to accomplish various transmission gear states, in accordance with the present disclosure; 
           [0018]      FIG. 3  is a schematic partial cross-sectional view of an exemplary transmission utilizing an SOWC, in accordance with the present disclosure; 
           [0019]      FIGS. 4 and 5  illustrate in cross-section an SOWC selecting between one-way operation and fully-locked operation, in accordance with the present disclosure; 
           [0020]      FIG. 4  illustrates SOWC feature  100  with one engagement element in a down position and one engagement element in an up position, in accordance with the present disclosure;; and 
           [0021]      FIG. 5  illustrates SOWC feature  100  with both engagement elements in an up position, in accordance with the present disclosure;; 
           [0022]      FIG. 6  graphically illustrates an exemplary engine braking event as commanded by a manual mode command to first gear, in accordance with the present disclosure; and 
           [0023]      FIG. 7  illustrates a flowchart describing an exemplary process through which SOWC engagement and disengagement can be managed, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,  FIG. 1  schematically illustrates a stick diagram of an exemplary powertrain  10 , in accordance with the present disclosure. The powertrain includes an engine  12 , a planetary transmission  14 , and a conventional final drive mechanism  16 . 
         [0025]    The planetary transmission  14  includes an input shaft  17  continuously connected with the engine  12 , a planetary gear arrangement  18 , and an output shaft  19  continuously connected with the final drive mechanism  16 . A torque converter may be positioned between the engine  12  and the input shaft  17 . If no torque converter is present, then an isolator would be positioned between the engine  12  and the input shaft  17 . The planetary gear arrangement  18  includes three planetary gear sets  20 ,  30  and  40 . 
         [0026]    The first planetary gear set  20  includes a sun gear member  22 , a ring gear member  24 , and a planet carrier assembly member  26 . The planet carrier assembly member  26  includes a plurality of pinion gears  27  rotatably mounted on a carrier member  29  and disposed in meshing relationship with both the sun gear member  22  and the ring gear member  24 . 
         [0027]    The second planetary gear set  30  includes a sun gear member  32 , a ring gear member  34 , and a planet carrier assembly member  36 . The planet carrier assembly member  36  includes a plurality of pinion gears  37  rotatably mounted on a carrier member  39  and disposed in meshing relationship with both the sun gear member  32  and the ring member  34 . 
         [0028]    The third planetary gear set  40  includes a sun gear member  42 , a ring gear member  44 , and a planet carrier assembly member  46 . The planet carrier assembly member  46  includes a plurality of pinion gears  47 ,  48  rotatably mounted on a carrier member  49 . The pinion gears  47  are disposed in meshing relationship with the sun gear member  42 , and the pinion gears  48  are disposed in meshing relationship with the ring gear member  44 . The ring gear member  34  is integral with the ring gear member  44 . The third planetary gear set  40  is a compound planetary gear set. 
         [0029]    The planetary gear arrangement also includes five torque transmitting mechanisms  50 ,  52 ,  54 ,  56  and  58 . The torque transmitting mechanisms  50 ,  54 ,  56  are rotating type torque transmitting mechanisms, commonly termed clutches. Clutches  50 ,  54 , and  56  are commonly multi-plate clutch devices, for example, in a friction launch configuration. The torque transmitting mechanism  52  is a stationary type torque transmitting mechanism, commonly termed brake or reaction clutch. The torque transmitting mechanism  58  is an SOWC. 
         [0030]    The input shaft  17  is continuously connected with the ring gear member  24 , and the output shaft  19  is continuously connected with the ring gear member  44 . The planet carrier assembly member  26  is selectively connectable with the sun gear member  42  through the clutch  50 . The sun gear member  32  is selectively connectable with the transmission housing  60  through the brake  52 . The planet carrier assembly member  26  is selectively connectable with the sun gear member  32  through the clutch  54 . The ring gear member  24  is selectively connectable with the planet carrier assembly member  46  through the clutch  56 . The planet carrier assembly member  36  is selectively connectable with the transmission housing  60  through the braking one-way clutch  58 . 
         [0031]      FIG. 2  is an exemplary clutching table, describing operation of various clutches in an exemplary transmission to accomplish various transmission gear states, in accordance with the present disclosure. The torque transmitting mechanisms  50 ,  52 ,  54 ,  56  and  58  are selectively engaged in combinations of two to provide six forward speed ratios and a reverse speed ratio between the input shaft  17  and the output shaft  19 . 
         [0032]    The reverse speed ratio is established with the engagement of the clutch  54  and SOWC  58 . The numerical value of this exemplary reverse speed ratio is 3.20. The clutch  54  is slipped for launching the vehicle in reverse. 
         [0033]    The first forward speed ratio is established with the engagement of the clutch  50  and the SOWC  58 . The numerical value of this exemplary first forward speed ratio is 4.06. The clutch  50  is slipped for launching the vehicle in the forward direction. 
         [0034]    The second forward speed ratio is established with the engagement of the clutch  50  and the brake  52 . The numerical value of this exemplary second forward speed ratio is 2.37. 
         [0035]    The third forward speed ratio is established with the engagement of the brakes  50 ,  54 . The numerical value of this exemplary third forward speed ratio is 1.55. 
         [0036]    The fourth forward speed ratio is established with the engagement of the clutches  50 ,  56 . The numerical value of this exemplary fourth forward speed ratio is 1.16. 
         [0037]    The fifth forward speed ratio is established with the engagement of the clutches  54 ,  56 . The numerical value of this exemplary fifth forward speed ratio is 0.85. 
         [0038]    The sixth forward speed ratio is established with the engagement of the brake  52  and clutch  56 . The numerical value of this exemplary sixth forward speed ratio is 0.67. 
         [0039]    As set forth above, an exemplary engagement schedule or clutching schedule for torque transmitting mechanisms as illustrated in  FIG. 1  is shown in the truth table of  FIG. 2 . Also, the chart of  FIG. 2  describes the ratio steps that are attained utilizing sample tooth ratios (not shown). For example, the step ratio between the first and second forward speed ratios is 1.71, while the step ratio between the reverse and first forward ratio is −0.79. One having ordinary skill in the art will also appreciate that it can also be readily determined from the truth table of  FIG. 2  that all of the single step forward ratio interchanges are of the single transition variety, as are the double step forward ratio interchanges. 
         [0040]      FIG. 3  is a schematic partial cross-sectional view of an exemplary transmission utilizing an SOWC, in accordance with the present disclosure. As shown, the transmission  14  includes a transmission housing  60  enclosing planetary gear sets  30 ,  40  which rotate about a shaft  17 . The position of the SOWC  58  is shown in  FIG. 3 . The outer race  70  of the SOWC  58  is splined to the spline  72  on the transmission housing  60 . The inner race  74  of the SOWC  58  is splined to the planet carrier assembly member  36 . A snap ring  76  keeps the SOWC  58  in place. 
         [0041]    The SOWC  58  may be a controllable mechanical diode clutch or a selectable roller clutch design. The disclosure contemplates that other selectable (reversible) one-way clutches could be used as defined herein with the present disclosure. 
         [0042]    An exemplary method is provided for controlling the position of a selector plate in a two mode SOWC using a hydraulically actuated piston and return spring mechanism that is an integral part of a valve body casting in a front wheel drive transmission. 
         [0043]    In one exemplary arrangement of a front wheel drive automatic transmission, the valve body assembly is located to one side of the rotating torque elements and is aligned and secured to the transmission case with locating dowel pins and threaded fasteners. An additional cylindrical housing cast onto the inside surface of the inner portion of the valve body provides a bore containing a piston for the purpose of controlling the rotational movement of the selector plate, and hence, the operating modes of the SOWC. The centerline of the cylindrical bore feature is perpendicular to the axis of rotation of the transmission and is ideally located in the same plane as the selector plate&#39;s rotational arc. This enables a direct mechanical connection between the selector plate in the SOWC and the hydraulic control mechanism in the valve body. 
         [0044]    The piston is in direct contact with a movable actuator plate. A feature on the actuator plate, in this case, a slot, aligns with and engages a selector lever fitted to the end of the selector plate lever arm on the SOWC to form an articulating connection between the SOWC and the hydraulic control mechanism. Other connection methods such as a fork engaging notches in a plate could also be employed. Hard stops designed into the piston bore of the control mechanism limit the axial travel of the actuator plate. The stops serve a dual function. First, they make attachment of the valve body to the transmission easier by facilitating alignment of the slot in the actuator plate with the selector lever on the SOWC. Secondly, they prevent the selector plate in the SOWC from becoming the hard stop for the return spring force when the SOWC is in a forward mode. In the reverse mode, they prevent the selector plate from becoming the hard stop for the piston apply force, thereby minimizing premature wear or damage to the SOWC. 
         [0045]    An energy storage device, such as a compression or return spring, is contained in a guide housing also affixed to the inside surface of the inner portion of the valve body. The spring exerts a predetermined force on the actuator plate to bias it in a first position. Since the selector plate lever arm is engagingly connected to the actuator plate through the shift lever, the selector plate is held in the first angular position and the SOWC operates only in the disengaged mode as described above. The return spring force, however, must be sufficient to prevent viscous and frictional drag within the SOWC from unintentionally rotating the selector plate from its first angular position to the second angular position when the driving member is rotating in the second rotational, or freewheeling, direction. 
         [0046]    Because the actuator plate is in contact with the bottom face of the piston, the return spring force also biases the piston towards the upper end of the piston bore. Pressurized automatic transmission fluid from a dedicated channel in the valve body is directed through an inlet port to the upper end of the piston bore. Ideally, the hydraulic fluid channels in the inner valve body portion are routed so the dedicated SOWC control channel passes directly under the upper end of the piston bore thus providing a conveniently short flow path. 
         [0047]    When the transmission control algorithms command a mode change in the SOWC, pressurized hydraulic fluid enters the upper end of the piston bore via the dedicated channel. The piston moves to the bottom of the piston bore compressing the return spring and simultaneously moves the actuator plate to its second position. Since the selector plate lever arm is engagingly connected to the actuator plate through the shift lever, the selector plate is rotatably moved to the second angular position. The two mode SOWC thus operates in “reverse” mode, as described above. 
         [0048]    Several methods are known to implement an SOWC. Struts, rockers, rollers, or sprags are different features that can be utilized to selectively couple or decouple the members of the clutch.  FIGS. 4 and 5  illustrate in cross-section an SOWC selecting between one-way operation and fully-locked operation, in accordance with the present disclosure. SOWC feature  100  comprises a first member  105 , a second member  110 , a selector plate  115 , a selector plate actuation feature  120 , a first engagement element  130 , a second engagement element  135 , two return springs  140 , and two engagement features  150 . First member  105 , second member  110  and selector plate  115  can be rotatable features centered around a common axis of rotation (not shown). In the alternative, one of the members can be fixed, but still exposing a flat surface for the other member to rotate against. A preferable shape for members  105  and  110  are flat circular plates. Selector plate  115  is sandwiched between the two members and generally moves or remains fixed with second member  110 . Selector plate  115  includes selector plate actuation feature  120 . Selector plate  115  is movable relative to second member  110  by some small rotational angle, so as to provide calibrated movement of selector plate actuation feature  120 . Engaging elements  130  and  135 , illustrated as struts in this exemplary embodiment, are rotatingly located to second member  110 , are oriented substantially normally to the radius of the members, and provide the selective coupling and decoupling functions served by the SOWC. Each engaging element, when in an up position, fixedly fit against an engagement feature  150  in first member  105  and prevent first member  105  from rotating relative to second member  110  in one direction. The engagement elements are normally in an up position due to forces exerted by return springs  140 . Selector plate  115  can be actuated relative to the engaging elements, such that selector plate actuation feature  120  can be used to depress one of the engagement elements into a down position. The action of the engagement to stop relative rotation depends upon the geometry of the interacting features. In the present exemplary embodiment, if both engagement features are in an up position, then relative rotation of one member to the other is not possible. If one of the engagement features is in a down position, then the clutch can freewheel in the direction normally prohibited by the engagement feature now in the down position. 
         [0049]      FIG. 4  illustrates SOWC feature  100  with one engagement element in a down position and one engagement element in an up position. Engagement element  135  is in an up position and is fit against an engagement feature  150 . As a result, first member  105  cannot rotate to the left relative to second member  110 . However, engagement element  130  is in a down position. Engagement element  135  provides substantially zero resistance to first member  105  rotating to the right relative to second member  110 . When relative rotation occurs and first member  105  comes into contact with engagement element  135 , pressure upon the top, nearly horizontal surface of engagement element  135  creates a downward rotation of engagement element  135 . This ratcheting motion of engagement element  135  can continue as subsequent engagement features rotate past engagement element  135 . 
         [0050]      FIG. 5  illustrates SOWC feature  100  with both engagement elements in an up position. Selector plate  115  is actuated relative to the engagement elements such that neither engagement element is depressed. As a result, relative rotation of first member  105  and second member  110  is not possible. Each engagement element is fit against an engagement feature  150  and provides resistance against relative rotation in one direction. 
         [0051]    It will be appreciated that the members of an SOWC are likely to have a plurality of features like SOWC feature  100 , each actuated similarly to allow or prevent rotation in either direction, with the total torque transmitted through the SOWC distributed between the SOWC features. Similar SOWC features are known in the art for a rocker mechanism with a pair of engagement elements located at distal ends of the rocker, capable similarly of preventing or enabling relative rotation, in combination with engagement features on an opposing member, based upon rocking actuation of the rocker. Rollers or sprags can alternatively be used in members located one radially inside the other, with a gap between the members. The rollers or sprags can be actuated to interact within the gap to selectively couple the members in one or bother directions of rotation. 
         [0052]    Application of an SOWC, as described above, to an automatic transmission can reduce parts and increase fuel efficiency. Yet, when the SOWC needs to be engaged, both ends of the SOWC need to spin at the same speeds. Slip across the clutch must substantially equal zero before the clutch is engaged. 
         [0053]    When not engaged, the SOWC acts like a regular one-way clutch, being locked in first gear while free-wheeling in other gears. When engaged, the SOWC is locked in both directions, which can provides reverse gear and first gear engine braking. Once the SOWC is in an engaged mode, it can remain engaged so long as the speed and gear settings allow. An exemplary SOWC can remain in an engaged mode continuously through operation in parked, reverse, neutral, and first gear operation. For example, when a transmission shifts from park to reverse or to neutral, the SOWC stays engaged to prepare the reverse gear. If the transmission stays in the reverse gear, the SOWC remains engaged. When a driver shifts the transmission into drive, the SOWC can stay engaged until the vehicle picks up speed in excess of a threshold low speed range. Once disengaged, the SOWC remains disengaged to act as a regular one-way clutch so long as the vehicle remains above the threshold low speed range. A method to control the scheduling of engagement of an SOWC is disclosed. 
         [0054]    Shifting into the engaged mode requires conditions of zero slip. Zero slip in the SOWC enabling a shift to the engaged mode can occur under various circumstances. One condition exists when the powertrain begins in a parked state with the transmission gearset in a static or zero speed condition. Under these conditions and in an indication that the engine is about to be started, the SOWC can be engaged immediately because slip of SOWC is zero. 
         [0055]    Under operation in the disengaged mode, the SOWC can switch to the engaged mode once the vehicle speed drops into the threshold low speed range, allowing application of torque in the reverse direction consistent with engine braking. As mentioned above, shifting into engaged mode requires a condition of zero slip in the clutch. Speed of the driving and driven members must be matched in order to enable a zero slip shift into the engaged mode. The speed of the driven member is dictated by the drivetrain and cannot be abruptly changed without affecting vehicle movement and drivability. Instead, a method is disclosed to match speeds in the SOWC by changing engine speed, for example, by applying electronic throttle control (ETC). A target engine speed is calculated by multiplying transmission output speed with the gear ratio of the destination first gear. Slip is tracked in the SOWC and a feedback error compensation value is applied to guarantee SOWC slip reaching zero. As soon as the engine reaches the target and the slip reaches zero, the SOWC is engaged and the ETC is dropped immediately. 
         [0056]    As described above, selection of the engaged mode is enabled only when the vehicle speed is within a threshold low speed range. If a driver shifts from drive back to neutral when vehicle speed is above the threshold low speed range, the SOWC stays disengaged until the speed falls below the threshold. 
         [0057]    A rolling garage shift from drive to reverse is only allowed in first gear and when the vehicle speed is below a threshold creep speed, a speed at or below an upper limit of the threshold low speed range. If the vehicle speed is higher than the threshold creep speed, the transmission shifts to neutral and the SOWC remains disengaged until vehicle speed returns below the threshold creep speed. Once the vehicle speed drops below the threshold creep speed, then the SOWC can be engaged, and then the reverse clutch is enabled. 
         [0058]    When the vehicle speed is within the threshold low speed range, the SOWC can be selected between the disengaged mode and the engaged mode depending upon the operation of the vehicle. Transmission control logic, processing information regarding vehicle operator intention, including input from accelerator and brake pedal positions and vehicle speed, selectively commands SOWC mode based upon desired activation of engine braking or desired vehicle coasting. For example, if vehicle speed is below the threshold low speed range, and the vehicle operator abruptly releases the gas pedal, an intention to slow the vehicle can be assumed. Accordingly, transmission control logic can command engine braking through the SOWC in response to an operator releasing the gas paddle when the vehicle is within the threshold low speed range. In one disclosed embodiment, in forward powered operation, the SOWC can only remain engaged in first gear. However, it is envisioned that in different gearset configurations, the SOWC can remain engaged to allow engine braking in additional gears, for example, in second gear, and the disclosure is not intended to be limited to the particular embodiments expressly described herein. 
         [0059]    A vehicle operator, in addition to providing inputs through an accelerator pedal and a brake pedal, can indicate a desire to initiate engine braking by commanding the transmission in manual mode to the first gear. When a downshift to first gear is commanded and the vehicle speed is within the threshold low speed range, transmission logic initiates commands to enable engine braking. As described above, the SOWC remains in a disengaged mode until vehicle speed is within the threshold low speed range. Once the engaged mode is enabled, transmission logic commands that slip across the SOWC be reduced to zero, then the SOWC is engaged, and then engine braking can occur. 
         [0060]      FIG. 6  graphically illustrates an exemplary engine braking event as commanded by a manual mode command to first gear, in accordance with the present disclosure. A number of factors relevant to the operation of the vehicular powertrain through a braking event are plotted against a common timescale. A commanded gear described a requested transmission gear state. In the exemplary graph, the commanded gear begins in a second gear and is commanded at a time to a first gear, consistent with an operator manually selecting the transmission operate in a first gear through an exemplary PRNDL selector. A command pressure for a second gear enabling clutch, for example, a friction launch clutch, is reduced from an initial operational level to zero. Required clutch reactive torque, describing reactive torque that was initially transmitted through all applied clutches in the second gear, simultaneously drops and progresses through a transition period during the shift. As described above, a condition of zero slip must be achieved to shift an SOWC to an engaged mode. SOWC slip begins at some initial non-zero value. Through the transition period, engine speed changes, for example through application of ETC, to some target speed enabling a zero slip shift. Accordingly, slip is reduced through the transition period to substantially zero and the SOWC can subsequently be engaged. At the end of the transition period, pressure in a first gear enabling clutch, for example, another friction launch clutch, is transitioned from zero to some operational level. At some duration after the pressure in the first gear enabling clutch is commanded to the operational level, reactive torque is applied through the first gear enabling clutch, and vehicle speed and engine speed are reduced through engine braking. 
         [0061]    The engaged mode is intended for use in low speeds. When an operator aggressively launches the vehicle with gas paddle input when the SOWC is engaged, the SOWC immediately disengages. This disengagement clears the transmission to accelerate the vehicle through successive gears outside of the intended slow operation of the engaged mode. 
         [0062]    Engine control such as ETC discussed above is controlled by an engine control module (ECM) as part of an overall control system. Similarly, transmission control is accomplished within a transmission control module (TCM) as part of the overall control system. In overall operation, the control system is operable to synthesize operator inputs, ambient conditions, engine operating parameters, and combustion performance measurements, and execute algorithms to control various actuators to achieve targets for control parameters, including such parameters as fuel economy, emissions, performance, and drivability. The control system is operably connected to a plurality of devices through which an operator controls or directs operation of the engine. Exemplary operator inputs include an accelerator pedal, a brake pedal, transmission gear selector, and vehicle speed cruise control when the engine is employed in a vehicle. The control system may communicate with other controllers, sensors, and actuators via a local area network (‘LAN’) bus or control area network (‘CAN’) (not shown) which preferably allows for structured communication of control parameters and commands between various controllers. 
         [0063]    The control system is operably connected to the engine and the transmission, and functions to acquire parametric data from sensors, and control a variety of actuators of the engine over appropriate interfaces. The control system receives an engine torque command, and generates a desired torque output, based upon the operator inputs. Exemplary engine operating parameters that are sensed by control system using the aforementioned sensors include engine coolant temperature, crankshaft rotational speed (‘RPM’) and position, manifold absolute pressure, ambient air flow and temperature, and ambient air pressure. A sensor capable of monitoring crankshaft rotational position can be utilized to monitor or determine a progression of the engine and various cylinders through various stages of a combustion cycle. Combustion performance measurements may comprise measured and inferred combustion parameters, including air/fuel ratio, location of peak combustion pressure, among others. Various speed sensors are located throughout the transmission to measure and estimate speeds of different members within the transmission. 
         [0064]    The control system preferably comprises a general-purpose digital computer generally including a microprocessor or central processing unit, read only memory (ROM), random access memory (RAM), electrically programmable read only memory (EPROM), high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (I/O) and appropriate signal conditioning and buffer circuitry. Each controller has a set of control algorithms, comprising resident program instructions and calibrations stored in ROM and executed to provide the respective functions of each computer. 
         [0065]    Algorithms for engine control may be executed during preset loop. Algorithms stored in the non-volatile memory devices are executed by the central processing unit and are operable to monitor inputs from the sensing devices and execute control and diagnostic routines to control operation of the engine, using preset calibrations. Loop cycles may be executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine operation. Alternatively, algorithms may be executed in response to occurrence of an event or interrupt request. 
         [0066]      FIG. 7  illustrates a flowchart describing an exemplary process through which SOWC engagement and disengagement can be managed, in accordance with the present disclosure. Process  400  begins in block  402 . At block  404 , the SOWC locked status is determined. If the SOWC is not locked, then the process proceeds to block  406 . If the SOWC is locked, then the process proceeds to block  424 . At block  406 , a determination is made whether the vehicle is in park and being started. If these conditions are met, then the process proceeds to block  422 , and the SOWC is locked. If these conditions are not met, then the process proceeds to block  408 . At block  408 , a determination is made whether the vehicle is in reverse. If this condition is met, then the process proceeds to block  422 , and the SOWC is locked. If this condition is not met, then the process proceeds to block  410 . At block  410 , a determination is made whether the vehicle is moving forward at less than a threshold creep speed and the transmission is indicated for a shift to reverse. If these conditions are met, then the process proceeds to block  420 , and the clutch is synchronized in preparation for being locked. If these conditions are not met, then the process proceeds to block  412 . At block  412 , a determination is made whether the vehicle is in neutral. If this condition is met, then the process proceeds to block  418 . If this condition is not met, then the process proceeds to block  414 . At block  414 , a determination is made whether the vehicle is in first gear. If this condition is met, then the process proceeds to block  416 . If this condition is not met, then the process returns to block  402 . At block  416 , a determination is made whether the operator input indicates desired engine braking. If this condition is met, then the process proceeds to block  418 . If this condition is not met, then the process returns to block  402 . At block  418 , a determination is made whether the vehicle is within the threshold low speed range. If this condition is met, then the process proceeds to block  420 , and the clutch is synchronized in preparation for being locked. If this condition is not met, then the process returns to block  402 . Block  420  includes actions to synchronize the SOWC, in accordance with methods described herein. Block  422  includes locking the SOWC, in accordance with methods described herein. At block  424 , a determination is made whether the vehicle is within the threshold low speed range. If this condition is met, then the process proceeds to block  426 . If this condition is not met, then the process proceeds to block  432 , wherein the SOWC is disengaged, in accordance with methods described herein. At block  426 , a determination is made whether the vehicle is in a forward gear other than the first gear. If this condition is not met, then the process proceeds to block  428 . If this condition is met, then the process proceeds to block  432 , wherein the SOWC is disengaged, in accordance with methods described herein. At block  428 , a determination is made whether the vehicle is being aggressively launched. If this condition is not met, then the process proceeds to block  430 . If this condition is met, then the process proceeds to block  432 , wherein the SOWC is disengaged, in accordance with methods described herein. At block  430 , a determination is made whether operator input indicates desire vehicle coasting into a higher/different gear state. If this condition is not met, then the process returns to block  402 . If this condition is met, then the process proceeds to block  432 , wherein the SOWC is disengaged, in accordance with methods described herein. As described in process  400  by return lines to block  402 , the process is intended to be repeated continually or in a loop in cycles according to the particulars of the control module or modules utilized. 
         [0067]    The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.