Patent Publication Number: US-2016244039-A1

Title: Vehicular crawl mode deceleration control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/120,045 which was filed on Feb. 24, 2015, which is hereby incorporated by reference its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to vehicular crawl mode deceleration control. 
     BACKGROUND 
     In an automotive powertrain, a transmission gearbox is used to transfer input torque to the vehicle&#39;s drive axles at a desired gear ratio. The drivetrain of a vehicle may be configured as a two-wheel drive (2WD) or a four-wheel drive (4WD) system, with the latter system providing improved traction on slippery or off-road driving surfaces. A 4WD powertrain includes a multi-speed transfer case that is connected to the transmission output shaft. One of the power flow arrangements of a multi-speed transfer case provides for a high-range 2WD mode, while the other arrangement provides for separate high-range and low-range 4WD modes, i.e., 4WD-high and 4WD-low modes, respectively. 
     In a transfer case configured with a 4WD-low mode, substantially higher amounts of torque are generated at lower engine speeds relative to operation in a 4WD-high mode. As a result, a vehicle operating in a 4WD-low mode is able to execute what is generally known in the art as a crawl maneuver, wherein vehicle speed is limited and higher amounts of torque are delivered to the four corners of the vehicle as a driver applies the brakes and requests throttle. Crawl mode may be desirable in certain driving conditions such as when towing a trailer, launching a boat, negotiating a relatively steep incline, or driving on loose or rocky surfaces. However, the inclusion of the additional transfer case hardware that is necessary for establishing true 4WD-low mode functionality comes at a cost of additional curb weight, packaging space, and mechanical design complexity. 
     SUMMARY 
     A vehicle is disclosed herein that has a controller operable for simulating operation in a four-wheel drive (4WD)-low transfer case mode. The controller is programmed to selectively execute steps of a method in response to a requested crawl mode, and to thereby provide the benefit of more precise vehicle deceleration control relative to conventional approaches. The vehicle includes an electronic braking system in which a brake motor controls brake calipers disposed proximate to each of the road wheels of the vehicle, with the brake motor being responsive to a driver-requested braking signal applied to a brake pedal. The brake pedal is mechanically isolated from the brake motor and the brake calipers or other brake apply elements, i.e., the brake pedal is controlled by-wire as is well known in the art. Braking overlay signals are also selectively generated as needed by the controller during the crawl mode to provide additional vehicle deceleration at levels sufficient for mimicking 4WD-low driveline drag. 
     In an example embodiment, the vehicle includes an engine, an accelerator pedal, a transmission, a transfer case, a mode selection device, road wheels, an electronic brake assembly, and the controller noted above. The transfer case, which is connected to the transmission, is operable for establishing a predetermined transfer case mode such as 4WD-high or 2WD-high. The mode selection device receives a requested crawl mode of the vehicle. 
     The electronic brake assembly includes brake calipers or other brake apply elements disposed at each corner of the vehicle, or in other words, proximate a respective one of the road wheels. Each caliper is operable for braking a respective road wheel. A brake motor of the electronic brake assembly displaces fluid, with valves used to control brake pressure to the individual calipers as is known in the art, such that substantially equal amounts of brake pressure are applied across each drive axle. That is, for normal braking events the brake motor drives pressure to each corner of the vehicle with minimal valve control activity. 
     The controller is programmed to simulate a 4WD-low mode of the transfer case in response to the requested crawl mode from a predetermined transfer case mode, e.g., from 4WD-high or 2WD-high, decelerating the vehicle via automatic control of the electronic brake assembly, and limiting a gear state of the transmission, for instance to 1 st  or 2 nd  gear, while automatically applying smooth driveline drag via electronic braking control. Transmission gear limitation is intended to keep the vehicle in low gear to facilitate the deceleration control by limiting the number of gears needed for downshifting as the vehicle comes to a stop, and also when accelerating in crawl mode to help limit the top speed of the vehicle. 
     The controller may be optionally programmed to selectively disable auto-start/stop functionality of the engine during crawl mode. The controller may be programmed to engage an automatic “vehicle hold” mode via the electronic brake assembly after the vehicle has slowed to a stop so as to prevent the vehicle from rolling on an incline or creeping on a level surface, that is, to hold the vehicle stationary regardless of the apply state of a brake pedal so as to prevent rolling or creeping. 
     The vehicle may include a door switch sensor and a seat belt switch sensor. In such an embodiment, the controller may be programmed to engage an electronic parking brake and release the electronic brake assembly when the sensors detect an open door/unlatched seat belt condition while in the vehicle hold mode. In a vehicle having an electronic range selection device, a park pawl may be used to lock the transmission into a park mode in such a condition. 
     The vehicle according to another example embodiment includes an engine, an accelerator pedal which controls a throttle level of the engine, a transmission operatively connected to the engine, and a transfer case operatively connected to the transmission that is operable for establishing a predetermined transfer case mode. The vehicle also includes a mode selection device operable for receiving a requested crawl mode of the vehicle while in the predetermined transfer case mode, a plurality of road wheels, and an electronic brake assembly. The electronic brake assembly includes a brake motor and a plurality of calipers in fluid communication with the brake motor, with each caliper disposed proximate a respective one of the road wheels and operable for braking the respective road wheel. 
     A controller of the same vehicle is programmed to execute the requested crawl mode in the predetermined transfer case mode by simulating a four-wheel drive-low mode of the transfer case, including controlling the brake motor and calipers to decelerate the vehicle and limiting a gear state of the transmission to 1 st  or 2 nd  gear. 
     A corresponding method is also disclosed. The method in a particular embodiment includes receiving a requested crawl mode from a predetermined transfer case mode using a mode selection device in a vehicle having a transfer case and an electronic brake assembly. The electronic brake assembly brake calipers in fluid communication with a brake motor, with each brake caliper disposed in proximity to and operable for braking a respective road wheel. The method also includes executing the requested crawl mode while in the predetermined transfer case mode, via a controller, including simulating a 4WD-low mode of the transfer case via control of the brake motor and brake calipers to decelerate the vehicle and limiting a gear state of the transmission to 1 st  or 2 nd  gear. 
     In another embodiment, a vehicle includes an engine having auto-start/stop functionality, an accelerator pedal which controls a throttle level of the engine, and a transmission operatively connected to the engine. The vehicle also includes a transfer case operatively connected to the transmission, and operable for establishing a predetermined transfer case mode, with the predetermined transfer case mode being one of a four-wheel drive high mode and a two-wheel drive high mode. Additionally, the vehicle includes a mode selection device operable for receiving a requested crawl mode of the vehicle while in the predetermined transfer case mode, a plurality of road wheels, an electronic brake assembly having a brake motor and a plurality of calipers in fluid communication with the brake motor, wherein each caliper is disposed proximate a respective one of the road wheels and is operable for braking the respective road wheel. 
     In this embodiment, a controller is programmed to execute the requested crawl mode in the predetermined transfer case mode by simulating a four-wheel drive-low mode of the transfer case, including controlling the brake motor and calipers to decelerate the vehicle, limiting a gear state of the transmission to 1 st  or 2 nd  gear, and disabling the auto-start/stop functionality, and engaging an automatic vehicle hold function via control of the electronic brake assembly after the vehicle has slowed to a stop to prevent the vehicle from rolling. 
     The above features and advantages, and other features and advantages, of the present disclosure are readily apparent from the following detailed description of some of the best modes and other embodiments for carrying out the disclosure, as defined in the appended claims, when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view illustration of an example vehicle executing a crawl mode driving maneuver as set forth herein. 
         FIG. 2  is a schematic illustration of an example vehicle having a controller programmed with vehicle crawl mode deceleration control logic mimicking four-wheel drive-low mode as set forth herein. 
         FIG. 3  is a flow chart depicting an example method for controlling vehicle deceleration during a crawl mode in the vehicle of  FIG. 1 . 
     
    
    
     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 vehicle  10  is shown in the process of executing an example crawl maneuver up an inclined surface  11 , with the vehicle  10  moving as indicated by arrow A up the inclined surface  11  in a limited gear state and with a limited speed. The vehicle  10  is shown as an example pickup truck without limiting the design to such an embodiment. For instance, the vehicle  10  may be alternatively embodied as a sport utility vehicle, a crossover vehicle, a sedan, a coupe, or any other style of vehicle having a controller (C)  50  programmed with steps of a method  100  as set forth herein. 
     The vehicle  10  includes a body  12 , doors  14  having door switch sensors (S 14 ) and seat belt switch sensors (S SB ), with the door switch sensors (S 14 ) and seat belt switch sensors (S SB ) respectively detecting a closed/latched state of the doors  14  and seat belts (not shown), as is known in the art. The vehicle  10  also includes a set of road wheels  16 , some or all of which may be powered as drive wheels depending on the embodiment. The vehicle  10  may be equipped with four-wheel drive (4WD)-high functionality, two-wheel drive (2WD)-high functionality, or true 4WD-low functionality without departing from the intended inventive scope. 
     As explained below with particular reference to  FIGS. 2 and 3 , the controller  50  is programmed to simulate or mimic the feel and performance of 4WD-low mode while executing a vehicular crawl maneuver. Such a maneuver is initiated via selection of crawl mode by a driver of the vehicle  10 . As used herein, the term “crawl mode” refers to a powertrain mode in which the vehicle  10  is allowed to move at a calibrated limited speed, e.g., about 10-20 KPH, without braking input on the part of the driver of the vehicle  10 , and with acceleration control still afforded to the driver as set forth below. Such a mode may be desirable while moving up steep terrain as shown in  FIG. 1  or while maneuvering a trailer, for instance when slowly backing down a ramp to launch a boat. 
     The controller  50  is therefore specially programmed with control logic embodying the method  100  which, upon its execution, simulates the 4WD-low mode when crawl mode is affirmatively selected. As explained below in detail with reference to  FIGS. 2 and 3 , when a driver releases throttle the controller  50  automatically commands driveline drag to be smoothly applied at the road wheels  16  via electronic braking. In crawl mode, the controller  50 , e.g., an engine control module (ECM) portion of the controller  50  in an example vehicular distributed control network, uses a unique throttle map with respect to pedal position in addition to altering a transmission shift pattern and top gear allowed, as set forth below. 
     In addition to this deceleration control functionality, the controller  50  selectively enters a vehicle hold mode when the vehicle  10  eventually comes to a stop in the crawl mode, such that the vehicle  10  remains stationary on an incline or a decline even when a brake pedal  13  as shown in  FIG. 2  is released. Using signals from the door switch sensors S 14  and/or seat belt switch sensors S SB , the controller  50  may also engage an electronic parking brake when one of the doors  14  is opened and/or a seat belt (not shown) is unlatched in the vehicle hold state. Under other drive conditions, the controller  50  may enable more aggressive corner braking, e.g., when executing a rock-crawling maneuver. The above-described functionality will now be described in further detail with reference to  FIGS. 2 and 3 . 
     Referring to  FIG. 2 , in a possible design the vehicle  10  of  FIG. 1  may include an internal combustion engine  18 , a transmission  20 , and the controller  50 . The engine  18  may be connected to the transmission  20  via an input clutch (not shown), e.g., a manual input clutch in the example of a manual transmission or a hydrodynamic torque converter in the example of an automatic transmission. Although omitted for illustrative simplicity, those of ordinary skill in the art will appreciate that the transmission  20  may include various planetary gear sets, clutches, brakes, a park pawl  21 , return springs, and hydraulic circuit components necessary for establishing a desired gear state and delivering output torque (arrow T O ) to the driveline. Additionally, a parking brake signal (arrow B P ) is commanded to engage an emergency or electronic parking brake as set forth below with reference to  FIG. 3 . 
     The vehicle  10  of  FIG. 1  may include a front drive shaft  22 , a front differential  24 , and a front drive axle  26  as shown, as well as a rear drive shaft  32 , a rear differential  34 , and a rear drive axle  36 . In such an embodiment, a transfer case  25  may be used to split power between the respective front and rear axles  26  and  36 , as indicated via arrows T F  and T R , respectively, with the transfer case  25  containing various drive chains, gear sets, clutches, and the like. 
     With respect to braking of the vehicle  10 , the vehicle  10  utilizes an electronic brake assembly  35 , which as used herein refers to a brake motor M B  and individual brake calipers  37 , or any other suitable brake apply mechanism disposed proximate the wheels  16 . The brake motor M B  may be embodied as a solenoid device or other suitable motor design operatively displacing brake fluid to the corners of the vehicle  10 , for instance via valves, brake lines, and the like (not shown) as is well known in the art, to thereby control an engaged/released state of the calipers  37 . The use of the electronic brake assembly  35  maintains even deceleration at the corners of the vehicle  10 , or in other words applies substantially equal amounts of brake pressure across each axle of the vehicle  10  to prevent leading or pulling. 
     The electronic brake assembly  35  is responsive to a driver-requested braking signal (arrow B R ) as applied to the brake pedal  13 . However, unlike conventional vacuum-driven hydraulic braking systems in which a vacuum brake booster is used to reduce the amount of force a driver has to apply to the brake pedal  13 , the brake pedal  13  is isolated from the brake calipers  37  of the electronic brake assembly  35 , i.e., the connection between the brake pedal  13  and the brake motor M B  and calipers  37  is achieved solely by-wire via the controller  50  during normal operation of the vehicle  10 . The brake calipers  37  are used to slow rotation of the road wheels  16 , and thus use the brake motor M B  as an electronic actuator instead of using a hydraulic cylinder, with the process governed directly by the controller  50  instead of via a high-pressure brake master cylinder. 
     Thus, in electronic braking a driver applies a desired amount of pressure or travel to the brake pedal  13 , which is automatically detected via a brake pedal sensor S 13 , in order to command the driver-request braking signal (arrow B R ). As is known in the art, in the unlikely event an electronic braking system such as that shown schematically in  FIG. 2  loses power, mechanical backup braking capability is retained, such as by changing a valve position to enable the driver to manually brake the vehicle  10  to a stop. Otherwise, the brake pedal  13  remains mechanically isolated from the electronic brake assembly  35 , which is an advantage used by the controller  50  in providing a smooth braking feel during deceleration control and avoiding brake pedal pulsation disturbances while operating in crawl mode. 
     The vehicle  10  of  FIG. 2  also includes an accelerator pedal  15  operable for outputting a throttle request (arrow Th %) to the controller  50 , e.g., as determined via a throttle sensor (S 15 ), and a mode selection device  30  configured to receive a mode selection input signal (arrow  130 ). The mode selection device  30  may be embodied as a knob, button, or lever disposed in an interior of the vehicle  10 , or a touch-screen device, with the mode selection device  30  being any device operable for requesting execution of the crawl mode. Additionally, the door switch sensors S 14  and seat belt sensors S SB  noted briefly above with reference to  FIG. 1  are in communication with the controller  50  and operable for outputting a corresponding open/closed state signal (arrow  140 ,  141 ) to the controller  50  as part of the method  100 . 
     The controller  50  may be configured as a microprocessor-based computing device or devices each having memory (M) and a processor (P). While depicted as a single controller  50  in  FIG. 2  for illustrative simplicity, in practice the controller  50  may be embodied as multiple control modules, such as a body control module (BCM), an electronic braking control module (eBCM), a transmission control module (TCM), an engine control module (ECM), and the like, as is known in the art, with each control module in communication with the others via a controller area network (CAN) bus or other suitable communication channels. 
     The memory (M) includes a tangible, non-transitory memory device on which is recorded instructions embodying the method  100 , an example of which is shown in  FIG. 3  and explained below. In addition to the memory (M) and processor (P), the controller  50  may include additional circuitry including but not limited to a high-speed clock, analog-to-digital circuitry, digital-to-analog circuitry, a digital signal processor, and any necessary input/output devices and other signal conditioning and/or buffer circuitry. 
     The controller  50  of  FIG. 2  is programmed to selectively output brake control signals (arrow B X ) to each of the electronic brake assembly  35  as part of the method  100 , with the brake control signals (arrow B X ) being defined herein as the driver-requested braking signal (arrow B R ) applied to the brake pedal  13  in addition to an automatically-generated braking overlay value generated by the controller  50 . Braking control occurs in response to various vehicle parameters to produce drag on the driveline when a driver, via the mode selection device  30 , affirmatively requests the crawl mode. Such vehicle parameters may include the present vehicle speed (arrow N 10 ), e.g., as measured via a speed sensor (S 10 ) such as a transmission output speed sensor or individual wheel speed sensors, throttle level (arrow Th %), the driver-requested braking signal (arrow B R ), and road grade (arrow α), e.g., as determined via a low-G longitudinal accelerometer S n . Such a device may be a capacitive sensor outputting a voltage signal representing a tilt angle, with the change in degrees of tilt corresponding to a change in acceleration due to a changing component of gravity acting on the accelerometer S n . The controller  50  may also be programmed to selectively disable engine auto start/stop functionality via an engine control signal (arrow  17 ) as set forth below. 
     Referring to  FIG. 3 , an example embodiment is shown of the method  100 . As noted above, the method  100  allows for simulation of a 4WD-low mode via automatic imposition of driveline drag via braking control, along with automatic execution of a vehicle hold state when the brake pedal  13  of  FIG. 2  is released after a stop. Thus, when a driver requests crawl mode and applies or releases the accelerator pedal  15 , the controller  50  of  FIGS. 1 and 2  controls the electronic brake assembly  35  disposed at the corners of the vehicle  10  along with taking other control actions. 
     Beginning with step  102 , a driver of the vehicle  10  of  FIG. 1  selects the crawl mode using the mode selection device  30  of  FIG. 2 , with entry into the crawl mode permitted to occur from a predetermined transfer case mode such as 4WD-high or 2WD-high. In various embodiments, step  102  may include turning a mode selection knob or moving a mode selection lever to a corresponding “crawl mode” setting, or touching a corresponding icon or button on a touch screen device, so as to generate the mode selection input signal (arrow  130 ) and thereby signal to the controller  50  that the driver wishes to enter crawl mode. The method  100  proceeds to step  104  upon receipt of the mode selection input signal (arrow  130 ) by the controller  50 . 
     Step  104  entails ensuring the transfer case  25  of  FIG. 2  is in the predetermined transfer case mode. The predetermined mode depends upon the particular configuration of the transfer case  25 . For instance, in a 4WD embodiment step  104  may include ensuring that a 4WD-high mode or a 4WD-low mode are active, while in a 2WD embodiment step  104  may entail shifting to a 2WD-high mode. Method  100  proceeds to step  106  once the predetermined transfer case mode is verified. Step  104  is essentially redundant with step  102 , but may be used to ensure that the rest of method  100  proceeds only in crawl mode in the predetermined transfer case mode. 
     At step  106 , the controller  50  may optionally disable engine auto-stop/start functionality via the engine control signal (arrow  17 ) of  FIG. 2 . As is known in the art, auto-stop/start is an engine control function that reduces idle fuel consumption by shutting off the engine  18  when the vehicle  10  is idling at a standstill. Since auto-start is typically triggered by a driver removing pressure from the brake pedal  13  and applying pressure to the accelerator pedal  15 , such a function may interfere with control of the crawl mode in certain applications, and therefore the controller  50  may be programmed to temporarily disable auto-stop/start functionality in some embodiments. The method  100  then proceeds to step  108 . 
     Step  108  includes accessing a predetermined unique throttle map from memory (M) of the controller  50 , with the throttle map, as is known in the art, indexing commanded engine torque to a particular level of throttle or position/travel of the accelerator pedal  15 . Step  108  also includes accessing a predetermined shift strategy or gear shift pattern recorded as logic in the memory (M) of controller  50 , e.g., a transmission control module portion of the controller  50 . The shift strategy controls, for the duration of the crawl mode, gear shifts of the transmission  20  that are permitted during crawl mode, as well as the timing of such shifts. As part of this strategy the transmission  20  is permitted to be shifted in crawl mode only as high as a predetermined maximum allowable gear. For instance, in an example 8-speed transmission the maximum gear may be 1 st  gear, while a higher-speed transmission may use 1 st  or 2 nd  gear as the maximum gear. Collectively, the maximum gear, throttle map, and shift strategy govern the states and modes of the powertrain while in crawl mode, with a blending of brakes/throttle used to ensure optimal smoothness of the braking action. The method  100  then proceeds to step  110  as crawl mode initiates. 
     At step  110 , the controller  50  determines if the accelerator pedal  15  of  FIG. 2  has been released while operating in crawl mode. Step  110  may include processing the throttle signal (arrow Th %) to determine if zero or a calibrated low non-zero amount of throttle is being requested. If so, the method  100  proceeds to step  112 . If more than the threshold zero or non-zero throttle is still being applied, the method  100  may proceed in the alternative to step  111 . 
     Step  111  includes executing a traction control system (TCS) “rock crawl” mode. In such a mode, the controller  50  uses traction control calibration biased towards aggressively applying brake torque on a slipping wheel  16  to allow more propulsion torque to reach the wheel(s) that are not slipping. The level of aggressiveness in applying the brakes is effective for maximum rock crawling capability, but may not be desirable to a driver during normal driving conditions when the wheels  16  are slipping. 
     As part of step  111  the controller  50  may reference a different version of the throttle map and shift strategy logic from memory (M) than that previously accessed at step  108 . The throttle map and shift strategy logic of step  111  are configured to optimize torque transfer to the corners of the vehicle  10 . The method  100  repeats steps  102 - 110  while in rock crawling mode. When the accelerator pedal  15  is released at step  110 , and if such a release is sustained for a calibrated duration to ensure that throttle release is not merely intermittent, the method  100  proceeds to step  112 . 
     At step  112  the controller  50  smoothly decelerates the vehicle  10  to a stop via control of the electronic brake assembly  35 , doing so as a calibrated function of the present gear state, vehicle speed, and road grade. The calibrated function may vary with the design of the vehicle  10  and the desired braking feel. Step  112  occurs via transmission of the brake control signals (arrow B X ) to the brake motor M B  and calipers  37 . As noted above with reference to  FIG. 2 , the brake control signals (arrow B X ) may include the driver-requested braking signal (arrow B R ). That is, a driver may still attempt to brake the vehicle  10  to a stop in crawl mode, and therefore the controller  50  generates as much of a braking overlay to the driver-requested braking signal (arrow B R ) as is needed to smoothly slow the vehicle  10  to a stop at a calibrated rate. 
     Because the braking system is electronic, and thus the brake pedal  13  is isolated from the electronic brake assemblies  35 , pulsations and other undesirable feedback to the driver through the brake pedal  13  during the braking process should be imperceptible to the driver. In other words, any automatically-generated braking control signals from the controller  50  in addition to those generated in response to the driver&#39;s own driver-requested braking signal (arrow B R ) should be smoothly applied and imperceptible to the driver, which is made possible largely due to the isolation of the brake pedal  13 . 
     In the event the powertrain of vehicle  10  is a hybrid powertrain, step  112  may also entail coordinated control of electronic braking elements of such a powertrain, e.g., motor torque delivered to the driveline. For instance, the controller  50  may temporarily prevent regenerative braking to minimize driveline torque disturbances in crawl mode. In such an embodiment, the controller  50  may communicate with a hybrid control module (HCM) and/or a motor control processor to ensure that power generation does not occur in creep mode, or is otherwise closely coordinated with creep mode if such function is to be retained. Alternatively, the controller  50  may coordinate the amount of regenerative braking that is used with the amount of electronic braking that is applied via the electronic brake assembly  35  so as to generate a desired amount of deceleration of the vehicle  10 . The method  100  proceeds to step  114  when the vehicle  10  has stopped. 
     Step  114  includes engaging the vehicle auto-hold mode or function noted briefly above. When the vehicle  10  stops, the vehicle  10  is prevented from moving forward or rolling back down the incline  11  of  FIG. 1  via operation of the electronic brake assembly  35 , such that the vehicle  10  is not allowed to roll on an incline. Likewise, on a level surface the hold function can be used to prevent creeping. This occurs even when the driver releases the brake pedal  13 , and is accomplished via automatic adjustment by the controller  50  of the brake control signals (arrow B X ). The method  100  proceeds to step  116  once the auto-hold mode is engaged. 
     At step  116  the controller  50  may determine whether a door  14  of  FIG. 1  is open and a seat belt (not shown) is unlatched, such as via processing of the open/closed state signals (arrows  140 ,  141 ) from the respective door switch sensors S 14  and seat belt switch sensor S SB . A purpose of step  116  is to ensure that the vehicle  10  is prevented from rolling back down the incline  11  of  FIG. 1  if the driver exits the vehicle  10  while still in auto-hold, e.g., to attend to a trailer or boat launch action. Detection of an open door  14 , and/or detection of an unlatched seat belt is therefore used to determine if the auto-hold state engaged at step  114  is likely to be sustained for an extended period of time. If the auto-hold state is maintained for more than a calibrated duration, e.g., as determined via the open door  14  and/or via expiration of a timer of the controller  50 , the method  100  proceeds to step  117 . Otherwise, if the door  14  remains closed and/or the timer noted above does not expire, the method  100  may proceed in the alternative to step  118 . 
     Step  117  entails transmitting, via the controller  50  of  FIG. 2 , the parking brake signal (arrow B P  of  FIG. 1 ) to thereby command setting or engagement of the parking brake, e.g., an electronically-actuated parking brake of the type known in the art. This action allows the electronic brake assembly  35  to be released and braking of the vehicle  10  to occur via the emergency parking brake, that is, a direct engagement of the brake calipers  37  via a mechanical linkage (not shown) or engagement of the park pawl  21 , thereby reducing the load on the electronic brake assembly  35  during a sustained holding of the auto-hold function. The vehicle  10  can remain in this state as long as necessary, with the method  100  thereafter proceeding to step  118 . 
     At step  118 , the controller  50  may release the electronic parking brake set at step  117 , or ensure that the parking brake remains released if step  118  is arrived at from step  116 . Step  118  may be performed by transmitting the parking brake signal (arrow B P  of  FIG. 1 ) to command release of the parking brake. The electronic parking brake will release if the driver uses the accelerator pedal  15  to command motion of the vehicle  10 . The method  100  then proceeds to step  110 . 
     Using the controller  50  and method  100  described above, and using available electronic braking functionality, 4WD-low mode may be mimicked in a vehicle powertrain. As described above, when in crawl mode the vehicle  10  of  FIGS. 1 and 2  will shift the transfer case  25  into 4WD-high, AWD, 2WD-high, or 4WD-low transfer case mode, apply a unique throttle map and transmission shift strategy, and limit the top transmission gear state that is allowed. Additionally, the controller  50  may selectively disable engine auto-start/stop and enable rock crawling in 4WD-high. As part of the described strategy, the controller  50  performs lift throttle deceleration when the driver releases the accelerator pedal  15  so as to brake the vehicle  10  to a stop, and thereafter engages the automatic vehicle hold function. In this manner, the ability is afforded of controlling vehicle deceleration in the same precise manner as is available with 4WD-low transfer cases when the vehicle  10  is in 4WD-high, 2WD-high, or 4WD-auto/AWD, thereby potentially foregoing the use of 4WD-low transfer case components and their associated weight, complexity, and packaging space requirements. 
     As used herein with respect to any disclosed values or ranges, the term “about” indicates that the stated numerical value allows for slight imprecision, e.g., reasonably close to the value or nearly, such as ±10 percent of the stated values or ranges. If the imprecision provided by the term “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range. 
     The detailed description and the drawings or figures are supportive and descriptive of the disclosure, but the scope of the disclosure is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed disclosure have been described in detail, various alternative designs and embodiments exist for practicing the disclosure defined in the appended claims. Furthermore, the embodiments shown in the drawings or the characteristics of various embodiments mentioned in the present description are not necessarily to be understood as embodiments independent of each other. Rather, it is possible that each of the characteristics described in one of the examples of an embodiment can be combined with one or a plurality of other desired characteristics from other embodiments, resulting in other embodiments not described in words or by reference to the drawings. Accordingly, such other embodiments fall within the framework of the scope of the appended claims.