System and method for improving regenerative braking in a rear-wheel-drive-based platform with four-wheel-drive capability

Abstract

Vehicle (10), comprising: a power source (12, 14) configured to provide a driving torque; a front axle (44); a first and a second front wheel (50) arranged on the front axle (44); a rear axle (32); a transfer case (24) configured to distribute the drive torque from the power source (12, 14) to the front axle (44) and the rear axle (32); a front differential (46) which couples the transfer case (24) and the front axle (44) together in a driving manner; a first separating device (52) arranged between the front differential (46) and the first front wheel (50) and configured to couple the front differential (46) and the first front wheel (50) in a driving manner when engaged; a second separating device (52) located between the front differential (46) and arranged at the second front wheel (50) and configured to drive the front differential (46) and the second front wheel (50) together when engaged; a clutch (42) arranged between the front axle (44) and the transfer case (24), wherein the clutch (42) has an engaged state and a disengaged state, wherein in the engaged state the clutch (42) couples the transfer case (24) and the front axle (44) in a driving manner; a regenerative braking system configured to deliver a regenerative braking torque to the rear axle (32) in response to a braking request; and a control device (20) configured to determine the permissibility of 4WD (four-wheel drive) operation based on current temperatures of various 4WD components and on the fact that the clutch (42) is in the disengaged state, to move the clutch (42) into the engaged state in order to couple the regenerative braking system with the front axle (44), and to engage the first and second disconnect devices (52) to deliver a regenerative braking torque to the front axle (44), further comprising disengaging the first and second disconnect devices (52) in response to the braking request falling below the associated threshold.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a propulsion system for a vehicle having a rear-wheel drive (RWD) based propulsion system with four-wheel drive (4WD) or all-wheel drive (AWD) functionality and a regenerative braking system. STATE OF THE ART

[0002] Two vehicle drivetrain configurations are predominantly found in the modern passenger car market: rear-wheel drive (RWD) and front-wheel drive (FWD). With additional devices, these two configurations can be configured to send power to all four wheels. Since traction at a particular wheel may be limited at certain times, the ability to send power to all four wheels improves maneuverability. However, these additional devices introduce additional reactive power losses, increasing fuel consumption even under conditions that don't require the additional capability.

[0003] In a typical RWD configuration, the internal combustion engine is mounted longitudinally within the vehicle so that the crankshaft axis is aligned with the direction of vehicle movement. A transmission mounted to the internal combustion engine drives a rear driveshaft at a speed that can be greater or less than the engine crankshaft speed, depending on the vehicle's current requirements. The rear driveshaft is connected to a rear axle, which changes the axis of rotation, reduces speed, and drives a left and right rear axle half-shaft, while allowing slight speed differences between the half-shafts when the vehicle corners. A RWD configuration can be adapted to also drive the front wheels by adding a transfer case between the transmission and the rear driveshaft.In addition to driving the rear driveshaft, the transfer case drives a front driveshaft, which in turn drives a front axle. Some transfer cases include a planetary gear set that splits torque between the front and rear driveshafts while allowing slight speed differences. Other transfer cases have an actively controlled on-demand clutch that drives the front driveshaft only under certain conditions, such as when a control device detects a loss of traction in the rear wheels.

[0004] In a typical FWD configuration, the internal combustion engine is mounted transversely in the vehicle so that the crankshaft axis is aligned with the wheel rotation axis. A transmission attached to the internal combustion engine drives a front differential at a speed appropriate for current vehicle requirements. The front differential is typically housed in a common housing along with the transmission gear. The front differential drives a left and right front axle while allowing slight speed differences between the axles when the vehicle corners. A FWD configuration is adapted to also drive the rear wheels by adding a power take-off unit (PTU) that drives a rear driveshaft at a speed proportional to the speed of the front differential.A rear drive unit (RDU) typically includes an on-demand clutch which, when engaged, drives a rear differential, which in turn drives a left and right rear axle.

[0005] DE 11 2010 002 242 T5 discloses a hybrid transfer case having first and second output shafts suitable for connection to a rear and front final drive, respectively, a first input shaft suitable for connection to a transmission and for selective connection to the first output shaft, and a second input shaft rotationally supported on the first input shaft and selectively connectable to the first output shaft. The transfer case may further include a planetary gear set, a transmission system, and an electric motor / generator. The planetary gear set may include a first member installed for rotation with the first input shaft, a second member installed for rotation with the second input shaft, and a third member. The transmission system may couple the second input shaft to the second output shaft, and the electric motor may selectively drive the third member.

[0006] WO 2007 / 025 926 A1 discloses a control system for a braking system of a motor vehicle with a switchable all-wheel drive system, comprising several hydraulic, electronically controllable friction brakes, each assigned to a vehicle axle, and a generator that is permanently connected to a first vehicle axle on the power flow side and to the other, second vehicle axle via an electronically controllable all-wheel drive clutch. This is intended to ensure a particularly high regenerative power efficiency and high operational stability and reliability.

[0007] AT 009 756 U1 discloses a method and a device for controlling the hybrid drive of a motor vehicle with the components: Internal combustion engine (ICE), manual transmission (SG), at least one electric motor (EMi), at least one clutch (Kj) and an energy storage device (ES), and at least one driven axle (RA; VA). The described control system is intended to achieve maximum efficiency and service life of the components. PRESENTATION OF THE INVENTION

[0008] According to the invention, a vehicle according to claim 1 and a method according to claim 6 are proposed. Advantageous embodiments of the invention are specified in the dependent claims and the following description.

[0009] A vehicle according to the present disclosure includes a power source configured to provide drive torque, a front axle, a rear axle, and a transfer case configured to distribute drive torque from the power source to the front axle and the rear axle. The vehicle additionally includes a clutch disposed between the front axle and the transfer case. The clutch has an engaged state and a disengaged state. In the engaged state, the clutch drivingly couples the transfer case and the front axle. The vehicle also includes a regenerative braking system configured to provide regenerative braking torque to the rear axle in response to a braking request. The vehicle further includes a controller.The controller is configured to, in response to a braking request and the clutch being in the disengaged state, control the clutch to transition to the engaged state to couple the regenerative braking system to the front axle and deliver regenerative braking torque to the front axle.

[0010] In one embodiment, the vehicle additionally includes first and second front wheels disposed on the front axle, and a front differential drivingly coupling the transfer case and the front axle. In such an embodiment, a first wheel disconnect device is disposed between the front differential and the first wheel and is configured to driveably couple the front differential and the first wheel when engaged. A second wheel disconnect device is disposed between the front differential and the second wheel and is configured to driveably couple the front differential and the second wheel when engaged.In such an embodiment, the controller is further configured to control the first and second wheel separating devices to engage in response to the braking request and the first and second wheel separating devices being in a disengaged state.

[0011] In an additional embodiment, the regenerative brake comprises an electric machine. Another embodiment includes a traction battery electrically coupled to the regenerative brake. The traction battery has an associated state of charge. In such embodiments, the controller is configured to control the clutch to engage in response to the state of charge being below an associated threshold.

[0012] A method of controlling a vehicle according to the present disclosure, wherein the vehicle has a power source configured to provide drive torque to a rear axle, regenerative brakes configured to apply regenerative braking torque to the rear axle, and a switchable element configured to selectively driveably couple the power source to a front axle, includes engaging the switchable element. The switchable element is engaged in response to a braking request and the switchable element being disengaged. Engaging the switchable element allows regenerative braking torque to be applied to the front axle.

[0013] In one embodiment of the method, the switchable element comprises a clutch disposed between a transfer case and a front differential drivingly coupled to the front axle. In such an embodiment, engaging the switchable element comprises controlling the clutch to engage to driveably couple the transfer case and the front differential.

[0014] In another embodiment, the switchable element comprises a clutch disposed between a front wheel and a front differential drivingly coupled to the front axle. In such an embodiment, engaging the switchable element comprises controlling the clutch to engage to driveably couple the front differential and the front wheel.

[0015] In yet another embodiment, engagement of the switchable element occurs in response to the braking request exceeding an associated threshold.

[0016] Another embodiment further includes disengaging the switchable element in response to the braking request falling below the associated threshold.

[0017] A vehicle according to the present disclosure includes a front wheel, a rear wheel, and regenerative brakes configured to apply regenerative braking torque to the rear wheel in response to a braking request. The vehicle also includes a switchable element configured to selectively couple the regenerative brake to the front wheel to apply regenerative braking torque to the front wheel. The vehicle further includes a controller configured to control the switchable element to engage in response to a braking request and the switchable element being disengaged.

[0018] In one embodiment, the vehicle further includes a transfer case disposed between the front wheel and the rear wheel and configured to split torque between the front wheel and the rear wheel, wherein the switchable element includes a clutch configured, when engaged, to drivingly couple the transfer case and the front wheel. A first variation of this embodiment has a front differential configured to drive the front wheel and a drive shaft drivingly coupled to the front differential. In the first variation, the clutch is disposed between the transfer case and the drive shaft and configured, when engaged, to drivingly couple the transfer case and the drive shaft.A second variant has a front differential configured to drive the front wheel and a driveshaft that drivingly couples the front differential and the transfer case. In the second variant, the clutch is located between the front differential and the wheel and is configured to driveably couple the front differential and the wheel when engaged.

[0019] In additional embodiments, the controller is configured to control the switchable element to engage in further response to the braking request exceeding an associated threshold and / or in further response to the state of charge of a traction battery being below an associated threshold. In another embodiment, the regenerative brake comprises an electric machine.

[0020] Embodiments according to the present disclosure offer several advantages. For example, the vehicles disclosed herein can recover a greater amount of kinetic energy through regenerative braking compared to known vehicles. This, in turn, can increase fuel economy and reduce wear on friction brakes.

[0021] The above and other advantages and features of the present disclosure will become apparent from the following detailed description of the preferred embodiments when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a representative vehicle having a RWD-based platform with 4WD functionality according to an embodiment of the present disclosure; and Fig. 2 is a flowchart illustrating a representative method for controlling a vehicle according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0022] Referring to Fig. 1, a vehicle 10 according to the present invention is shown in schematic form. The vehicle 10 is a hybrid electric vehicle having a first power source comprising an internal combustion engine 12 and a second power source comprising an electric machine 14, which may alternatively be referred to as an internal combustion engine / generator. The electric machine 14 is electrically coupled to a traction battery 16. When operating as an internal combustion engine, the electric machine 14 is configured to convert stored electrical energy from the traction battery 16 into kinetic energy in the form of drive torque. When operating as a generator, the electric machine 14 is configured to convert kinetic energy into electrical energy for storage in the traction battery 16.The vehicle 10 also includes a transmission 18, which in various embodiments may include an automatic transmission, a manual transmission, or a continuously variable transmission (CVT).

[0023] The internal combustion engine 12, the electric machine 14, the traction battery 16, and the transmission 18 are all in communication with or under the control of at least one controller 20. Additionally, a plurality of sensors 22 are in communication with or under the control of the controller 20. The sensors 22 include, but are not limited to, a speedometer, a brake pedal sensor configured to detect a position of a brake pedal applied by a driver, and an accelerator pedal sensor configured to detect a position of an accelerator pedal applied by a driver.

[0024] Although the controller 20 is illustrated as a single unit, it may include a plurality of different control modules that may be used to control multiple vehicle systems according to overall vehicle control logic or software. The controller 20 may include an engine control module (ECM), a transmission control module (TCM), a vehicle system controller (VSC), a brake controller, and / or other suitable control devices. The controller 20 generally includes any number of microprocessors, ASICs, integrated circuits, memory (e.g., FLASH, ROM, RAM, EPROM, and / or EEPROM), and software code to coordinate the various control modules to perform a series of operations. The controller 20 may also include predetermined data or lookup tables based on calculations and test data stored in the memory.The control modules of the control device 20 communicate with each other and with other vehicle systems via one or more wired or wireless vehicle connections using common bus protocols (e.g., CAN and LIN).

[0025] The vehicle 10 is configured as a primarily RWD-based platform with 4WD functionality. The transmission 18 drivingly couples the internal combustion engine 12 and the electric motor 14 to a transfer case 24. The transfer case 24 is configured to distribute torque to a front output shaft 26 and a rear output shaft 28.

[0026] The rear output shaft 28 is drivingly coupled to a rear drive shaft 30, which in turn is drivingly coupled to a rear axle 32 via a rear differential 34. The rear axle 32 comprises two half shafts or side shafts 36, each coupled to a corresponding rear wheel 38.

[0027] The front output shaft 26 is selectively coupled to a front drive shaft 40 by means of an on-demand clutch 42. When the clutch 42 is engaged, the front output shaft 26 is drivingly coupled to the front drive shaft 40. When the clutch 42 is disengaged, the front drive shaft 40 is decoupled from the front output shaft 26. In an alternative embodiment, the clutch 42 is disposed within the transfer case 24 and configured to decouple the front output shaft 26 from other components of the transfer case 24 when disengaged.

[0028] The front drive shaft 40 is drivingly coupled to a front axle 44 via a front differential 46. The front axle 44 includes two half shafts or sideshafts 48, each selectively coupled to a corresponding front wheel 50 via a corresponding wheel separator 52. Each wheel separator 52 has an engaged state and a disengaged state. When a wheel separator 52 is engaged, the associated half shaft 48 is drivingly coupled to the associated front wheel 50. When a wheel separator 52 is disengaged, the associated half shaft 48 is decoupled from the associated front wheel 50. The wheel separators 52 and the clutch 42 are also in communication with, or under the control of, the controller 20.

[0029] The controller 20 may disengage the clutch 42 and / or the wheel separators 52 to decouple the associated components and operate in RWD mode. Disengaging the front wheels 50 and / or the front driveshaft 40 reduces reactive power losses caused by reducing the amount of driven and / or rotating components. In particular, disengaging the clutch 42 and the wheel separators 52 allows the front wheels 50 to rotate freely without requiring rotation of the front axle 44, the front differential 46, or the front driveshaft 40. In other embodiments, additional components disposed between the clutch 42 and the wheel separators 52 may also rotate freely. Reactive power losses may therefore be reduced in RWD mode, increasing fuel economy.

[0030] Controller 20 may engage clutch 42 and wheel disconnect devices 52 to driveably couple engine 12 and electric machine 14 to front wheels 50 and operate in a 4WD mode. This mode may provide increased traction and / or stability compared to RWD mode. Controller 20 may include logic configured to operate in the 4WD state in response to at least one 4WD condition being detected by sensors 22.

[0031] The vehicle 10 also includes rear friction brakes 54 configured to apply friction braking torque to the rear wheels 38, and front friction brakes 56 configured to apply friction braking torque to the front wheels 50. The rear friction brakes 54 and the front friction brakes 56 are in communication with or under the control of the controller 20. The controller 20 is configured, in response to a braking request, such as that detected by a brake pedal sensor 22, to coordinate the rear friction brakes 54, the front friction brakes 56, and the electric machine 14 to provide friction braking torque and regenerative braking torque, respectively, that meets the braking request.

[0032] Previously known FWD / 4WD or RWD / 4WD vehicles are generally configured so that when regenerative braking is active, they decouple the non-driven axle and reduce reactive power losses while simultaneously applying regenerative braking torque to the driven axle. Applying braking torque exclusively to the rear axle in a RWD-based system is generally less stable than applying braking torque to both the front and rear wheels. Therefore, friction brakes are generally activated in such systems to apply friction braking torque to the non-driven axle. However, during a braking event, dynamic weight transfer may cause the majority of vehicle braking to be applied to the front wheels. For example, during a typical braking event, 70% of braking may occur at the front wheels.In a rear-wheel-drive (RWD)-based system, this can result in most of the braking being performed as friction braking at the front wheels rather than regenerative braking at the rear wheels, limiting the amount of energy that can be harvested at the rear axle without causing vehicle instability. Friction brakes dissipate kinetic energy as heat, thereby reducing the maximum amount of kinetic energy that can be recovered. Consequently, such systems cannot recover the maximum available energy.

[0033] Other known systems may include a separate combustion engine / generator for each axle or wheel. While such systems allow for increased regenerative capacity compared to decoupling the non-driven axle, they also involve increased complexity due to the increased number of components.

[0034] Referring to Fig.2, a method for controlling a vehicle according to the present disclosure is illustrated in flowchart form. The algorithm begins at block 60. At block 62, the vehicle is in motion, with the transmission 18 in DRIVE, the speed above an entry threshold, and the accelerator pedal depressed, as illustrated at 62. The entry threshold may be calibrated based on various factors, including (but not limited to) system reactive losses, drivability due to front axle locking at low speeds, and electric machine torque characteristics over a range of vehicle speeds.

[0035] At operation 64, it is determined whether the accelerator pedal has been released. If the determination is negative, control remains at operation 64. Thus, the algorithm remains at operation 64 until the accelerator pedal has been released. If the determination is positive, control continues with operation 66.

[0036] At operation 66, it is determined whether the vehicle is in 4WD mode, e.g., whether the clutch 42 and the wheel disconnect devices 52 are engaged. If the determination is affirmative, control continues to block 68. At block 68, the vehicle is maintained in 4WD mode, and an amount of regenerative braking torque is applied by releasing the pedal. Additionally, the clutch 42 and / or the wheel disconnect devices 52 may be controlled to vary the amount of torque transferred to the front wheels based on factors including, but not limited to, the amount of energy available for harvesting, the regenerative capacity of the electric machine 14 and / or the traction battery 16, vehicle stability conditions, noise, vibration, harshness (NVH), and drivability. Control then continues to operation 70.

[0037] At operation 70, it is determined whether braking is requested, e.g., whether a brake pedal is depressed. If the determination is affirmative, control continues to block 72. At block 72, an AWD_regen condition is enabled. If the AWD_regen condition is active, the vehicle is maintained in 4WD mode, and regenerative braking torque and friction braking torque are balanced to brake the vehicle and recover kinetic energy via regenerative braking. Because the vehicle is in 4WD mode, electric machine 14 applies regenerative braking torque to both rear axle 32 and front axle 44. Controller may also control friction brakes 54 and 56 to provide friction braking torque if electric machine 14 is unable to meet the braking request solely using regenerative braking torque. Control then continues to operation 74.

[0038] At operation 74, it is determined whether the vehicle speed has decreased below the input threshold. If the determination is negative, control continues with operation 76.

[0039] At operation 76, it is determined whether the brake has been released. If the determination is positive, control returns to operation 66. If the determination is negative, control returns to block 72. Thus, the vehicle is maintained in 4WD mode, and regenerative braking torque and friction braking torque are applied until the brake is released.

[0040] Returning to operation 74, if the determination is affirmative, control continues to block 80. At block 80, the AWD_regen condition ends. Although the AWD_regen condition has ended, the vehicle may still be maintained in 4WD mode due to other conditions, such as traction or stability requirements. The algorithm then ends at block 82.

[0041] Returning to operation 70, control continues to operation 78 if the determination is negative. At operation 78, it is determined whether the accelerator pedal has been pressed again. If the determination is negative, control returns to block 68. Thus, regenerative braking torque is applied by releasing the pedal until braking is requested or the accelerator pedal is pressed again. If the determination is positive, control continues to block 80.

[0042] Returning to operation 66, if the determination is negative, control continues to block 84. At block 84, the vehicle is maintained in RWD mode. Control then continues to operation 86.

[0043] At operation 86, it is determined whether braking is requested, e.g., whether a brake pedal is depressed. If the determination is negative, control proceeds to operation 88.

[0044] At operation 88, it is determined whether the accelerator pedal has been depressed. If the determination is negative, control returns to block 84. If the determination is positive, control returns to operation 64.

[0045] Returning to operation 86, if the determination is affirmative, control continues to operation 90. At operation 90, a determination is made as to whether 4WD operation is permissible based on durability factors, such as current temperatures of various 4WD components. If the determination is negative, control returns to block 84. If the determination is affirmative, control continues to operation 92.

[0046] At operation 92, it is determined whether regenerative braking is permissible. This determination may be made based on a variety of factors, including, but not limited to, the current charge status of the traction battery, limitations on charging the traction battery, the presence and magnitude of wheel slip, and other stability considerations. If the determination is negative, control returns to block 84. If the determination is positive, control continues to block 94.

[0047] At block 94, the AWD_regen condition is activated. Consequently, the clutch 42 and the wheel disconnect devices 52 are engaged to operate the vehicle in 4WD mode. Regenerative braking torque and friction braking torque are also balanced to brake the vehicle and recover kinetic energy via regenerative braking. Because the vehicle is in 4WD mode, the electric machine 14 applies regenerative braking torque to both the rear axle 32 and the front axle 44. Control may also control the friction brakes 54 and 56 to provide friction braking torque if the electric machine 14 is unable to meet the braking request solely using regenerative braking torque. Control then continues with operation 96.

[0048] At operation 96, it is determined whether the brake has been released. If the determination is affirmative, control returns to operation 66. If the determination is negative, control continues to operation 98. At operation 98, it is determined whether the accelerator pedal has been pressed again. If the determination is affirmative, control continues to block 80. If the determination is negative, control returns to block 94.

[0049] Although the foregoing has been discussed in conjunction with a selectable RWD / 4WD platform, other embodiments may be implemented in conjunction with a selectable AWD platform or a platform in which the front axle is the primary driven axle.

[0050] In an additional embodiment, the wheel separators 52 are omitted. In such an embodiment, only the clutch 42 is engaged to activate the 4WD mode. In another embodiment, only one wheel separator 52 is provided on the differential 46. In such an embodiment, separating a front wheel allows high-drag components of the differential 46 to stop spinning rapidly.

[0051] In yet another embodiment, a calibratable braking threshold is provided, and operations 70 and 96 are only satisfied when the braking request exceeds the calibratable threshold. Thus, light braking requests can be satisfied by applying regenerative braking torque only to the rear axle 32, while higher-order braking requests can be satisfied in AWD mode. The threshold can be calibrated to avoid rear wheel slippage.

[0052] In another embodiment, the control logic may determine at various times whether locking is occurring. If the determination is positive, AWD mode may be reduced or interrupted until the locking subsides.

[0053] As can be seen from the various embodiments, the present invention provides a system and method in which regenerative braking torque is applied to both the front and rear axles during a braking event. This can increase the amount of kinetic energy recovered through regenerative braking compared to known vehicles and consequently increase fuel economy.

Claims

[1] Vehicle (10), comprising: a power source (12, 14) configured to provide drive torque; a front axle (44); a first and a second front wheel (50) arranged on the front axle (44); a rear axle (32); a transfer case (24) configured to distribute the drive torque from the power source (12, 14) to the front axle (44) and the rear axle (32); a front differential (46) which drivingly couples the transfer case (24) and the front axle (44); a first disconnect device (52) disposed between the front differential (46) and the first front wheel (50) and configured to driveably couple the front differential (46) and the first front wheel (50) when engaged; a second separating device (52) arranged between the front differential (46) and the second front wheel (50) and is configured such that, when engaged, it drivingly couples the front differential (46) and the second front wheel (50) to one another; a clutch (42) disposed between the front axle (44) and the transfer case (24), the clutch (42) having an engaged state and a disengaged state, the clutch (42) in the engaged state drivingly coupling the transfer case (24) and the front axle (44) to one another; a regenerative braking system configured to provide regenerative braking torque to the rear axle (32) in response to a braking request; and a controller (20) configured to, in response to the braking request exceeding an associated threshold, determine permissibility of 4WD operation based on current temperatures of various 4WD components and the clutch (42) being in the disengaged state, control the clutch (42) to change to the engaged state to couple the regenerative braking system to the front axle (44), and control the first and second disconnect devices (52) to engage to provide regenerative braking torque to the front axle (44), further comprising disengaging the first and second disconnect devices (52) in response to the braking request falling below the associated threshold. [2] The vehicle (10) of claim 1, further comprising the transfer case (24) disposed between the front axle (44) and the rear axle (32) and configured to distribute torque to a front output shaft (26) and a rear output shaft (28). [3] The vehicle (10) of claim 2, further comprising the front differential (46) configured to drive the front axle (44) and a drive shaft (40) drivingly coupled to the front differential (46), wherein the clutch (42) is disposed between the transfer case (24) and the drive shaft (40) and is configured to driveably couple the transfer case (24) and the drive shaft (40) when engaged. [4] The vehicle (10) of claim 1, wherein the controller (20) is configured to control the clutch (42) to engage in further response to a state of charge of a traction battery (16) being below an associated threshold. [5] The vehicle of claim 1, wherein the regenerative brake comprises an electric machine. [6] Method for controlling a vehicle (10) according to claim 1. [7] The method of claim 6, wherein the vehicle (10) includes the clutch (42) disposed between the transfer case (24) and the front differential (46) drivingly coupled to the front axle (44), and wherein engaging a switchable element comprises controlling the clutch (42) to engage to driveably couple the transfer case (24) and the front differential (46) together.

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