All wheel drive speed synchronization and connection

A vehicle driveline includes a power source, a transmission connected to the power source and including an output driveably connected to primary road wheels, a driveshaft, a synchronizer that opens and closes a drive connection between the output and the driveshaft, and a clutch that alternately opens and closes a drive connection between the driveshaft and secondary road wheels. A control executes synchronization and connection strategies using one of singular, overlapping and sequential activation of the driveline components that produce all-wheel-drive on-demand.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a motor vehicle driveline, particularly to a driveline in which primary wheels are connected continually to a power source and secondary wheels are connected selectively to the power source.

2. Description of the Prior Art

All-wheel-drive (AWD) systems tend to degrade vehicle fuel economy due to increased driveline parasitic losses even when AWD is not activated. These parasitic losses occur because some parts of the driveline continue to be driven by the engine and transmission, or the secondary drive wheels and their rotation cause a drag torque to be exerted on the driving element.

Driveline disconnect systems improve fuel economy by disconnecting as many of the driveline rotating parts as possible, as close to the transmission output and the secondary drive wheels as possible, when all-wheel-drive is not activated.

These disconnect systems provide a significant fuel economy benefit, but they pose challenges including (i) getting the driveline reconnected quickly when the AWD system must be activated, and (ii) maintaining system durability through many driveline disconnect/reconnect cycles. Meeting these challenges is complicated when some or all of the disconnect clutch or synchronizer designs are constrained by size and packaging limitations, or a desire to minimize drag. These constraints work against the characteristics required to provide fast and durable disconnect systems.

In the case of a front-wheel drive (FWD) based AWD vehicle, the initial phase of front disconnect and reengagement is performed using a synchronizer having a limited torque capacity, which limits the speed at which it is able to synchronize undriven drivetrain components elements by bringing them back to the same speed as the elements which are already rotating. The small size of the synchronizer also makes providing good durability under these circumstances a challenge.

SUMMARY OF THE INVENTION

A FWD-based AWD vehicle driveline includes a power source, a transmission connected to the power source, a primary driveline output continuously connected to primary road wheels and a secondary driveline output selectively connected to secondary road wheels. The secondary driveline consists of a driveshaft, a power transfer unit (PTU) with a synchronizer that opens and closes a drive connection between the PTU output and the driveshaft, and a rear drive unit (RDU) with a clutch that alternately opens and closes a drive connection between the driveshaft and secondary road wheels. A control executes synchronization and connection strategies using one of singular, overlapping and sequential activation of the driveline components that produce all-wheel-drive on-demand.

The driveline is controlled to produce all-wheel drive operation by using a clutch of a RDU to accelerate driveline components toward a speed of secondary wheels, disengaging the clutch (partially or completely), using a PTU synchronizer to accelerate the components toward a speed of a transmission output, engaging the synchronizer to transmit torque between the transmission output and the clutch, and reengaging the clutch to drive all wheels as needed.

The transmitted portion of the transmission output torque quickly accelerates those driveline components that are stationary when the AWD disconnects are in the disconnected state. Use of the RDU clutch to perform this function also reduces the demands on the PTU synchronizer, thereby increasing its service life.

To minimize to an acceptable level any loss of traction on the secondary wheels due to engagement of the RDU clutch, the AWD controller may receive constraints from a vehicle controller, such as a brake system controller, which limits the torque capacity of the RDU clutch46or limits the rate of change of RDU clutch torque capacity.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The driveline10ofFIG. 1includes a power source12, such as an internal combustion engine or an electric motor, and a transmission14that produces a variable ratio between the speed of its output16, which is continually driveably connected through a differential mechanism18to the primary road wheels20,22, and the speed of the transmission input, which is driveably connected to the power source.

The primary wheels20,22are connected continually to the engine through the transmission. The secondary wheels26,28are undriven road wheels, except that they are connected to the engine when AWD is operating.

A power transfer unit (PTU)24transmits power from the transmission output16selectively to the secondary road wheels26,28. A driveshaft30transmits rotating power from the PTU24to a rear drive unit (RDU)32.

The PTU24comprises a coupler34, such as a clutch or synchronizer, whose input is driveably connected to the transmission output16; a bevel ring gear36connected to the output of the PTU coupler34, and a bevel pinion gear38meshing with the bevel ring gear36and connected to driveshaft30. The PTU coupler34disconnects the rotating components of the PTU and driveline components downstream of the PTU from the transmission output16.

The RDU32includes a bevel pinion gear40, secured to driveshaft30; a bevel ring gear42, meshing with pinion40, a differential mechanism44, and a low-drag coupling46. The secondary wheels26,28are driven by halfshafts48,50though coupling46and differential44. Coupling46alternately connects and disconnects halfshafts48,50from the rotatable components of the RDU32.

FIG. 2illustrates details of the power path that connects the transmission output16continually to the halfshafts60,62for the primary wheels20,22through differential18, and to the PTU input shaft64, which is connected to bevel ring gear36.

A compound planetary differential180includes a sun gear72, secured through a spline74to axle shaft62; a carrier76, secured through a spline78to axle shaft60; a ring gear80, engaged with an pinion82formed on the transmission output shaft16; first planet pinions84supported on the carrier and meshing with the ring gear80; and second planet pinions85supported on the carrier76and meshing with the sun gear72and the first planet pinions84. One side of ring gear80is secured to a disc86and supported at a bearing88; the other side of ring gear80is secured to a disc90and supported at a bearing92. Disc90is formed with an internal spline93, which engages an external spline formed on a coupler sleeve94.

Disc90forms a cylinder96, which contains a piston98, actuated by pressurized hydraulic fluid carried to cylinder96through a passage100. A compression return spring102restores piston98to the disengaged position shown in theFIG. 2. Piston98is secured to coupler sleeve94such that they move along an axis103and rotate about the axis as a unit.

The volume104enclosed by piston98and spring retainer106forms a balance dam containing hydraulic fluid supplied from source of hydraulic lubricant108through a lube circuit, which includes passages110,112,114,116.

In operation, fluid from a source of line pressure is carried to a valve, which is controlled by a variable force solenoid. The valve opens and closes a connection between the line pressure source and passages126,128, which carry piston-actuating pressure to cylinder96depending on the state of the solenoid. When passages126and128are pressurized, piston98and coupler sleeve94move leftward, causing frictional contact at the conical surface between a member130and a synchronizing ring132. Member130is rotatably secured by spline134to PTU input shaft64. As the speed of member130is synchronized with the speed of ring gear80, the internal spline of coupler sleeve94engages the dog teeth on synchronizing ring132and the clutch teeth136on the radial outer surface of connecting member130, thereby driveably connecting ring gear80and PTU input shaft64.

Although the description refers to the speed of connecting member130being synchronized with the speed of ring gear80using a synchronizer, a connection between ring gear80and PTU input shaft64can be completed using a coupler, such as a clutch, instead of a synchronizer.

In the disconnected state, the RDU coupling46and PTU coupling34are open, causing the rotatable RDU components, driveshaft30, and rotatable PTU components to be disconnected from the secondary wheels26,28and halfshafts48,50.

In the connected state, the PTU coupler34is closed, causing driveshaft30to rotate with the primary wheels20,22and transmission output16. The RDU coupling46has a variable torque transmitting capacity, which may produce a fully engaged connection or slip between driveshaft30and the secondary wheels26,28, as required to produce AWD operation.

During synchronization of the secondary driveline, if the torque (and therefore the tractive effort) applied is too large, the resulting vehicle deceleration may be perceptible to the vehicle occupants, or the tractive force being exerted may cause loss of traction on a surface having a low coefficient of friction. Care must therefore be taken to avoid undesirable effects from these conditions.

To bring all components rapidly up to speed and then quickly perform the AWD on-demand function, the control relies on the state of the vehicle including its vehicle speed, the ambient temperature, and the temperature and parasitic drag level of the various rotating components, including effects due to component break-in or aging, which should be accelerated. Based on this information, the control will execute one of the three following synchronization and connection strategies.

When the vehicle is traveling at low speeds or otherwise requires a relatively low amount of power to synchronize the secondary driveline, the control uses only the PTU clutch34to accelerate the stationary components. In this case the PTU clutch34is a synchronizer, similar to that of a manual transmission, which transmits torque to driveshaft30, thereby accelerating the driveshaft and synchronizing its speed with the speed of the transmission output16. When the speed difference across the synchronizer34is sufficiently small, the synchronizer's sleeve engages the transmission output16allowing the synchronizer34to transmit transmission output torque to the RDU32through driveshaft30. The secondary wheels26,28are driven through coupling46, differential44and halfshafts48,50.

In the majority of speed synchronization and AWD reconnect events the PTU24and RDU32are activated concurrently during the event. During some portions of the event the PTU24may be solely activated; during other portions, the RDU32may be solely activated; during other portions, both may be activated.

The sequenced activation of both elements allows for the most desirable method for controlling the relative motion of various components to obtain the lowest level of NVH.

The use of both the PTU synchronizer34and RDU clutch46concurrently, which typically occurs during the middle portion of the event, allows the larger torque capacity of the RDU to more quickly accelerate the components to be synchronized, and also has been shown by vehicle testing to allow, with careful shaping of the RDU torque profile, to provide the best NVH reconnect events over most conditions.

When the accelerating components reach the speed of the slower activating element, i.e. PTU24or RDU32, which is typically the RDU especially if the primary wheels20,22are rotating faster than the secondary wheels26,28, the slower element is at least partially released, as its contribution to speed synchronization is largely completed.

During the end stages of the event, especially if the primary wheels20,22are rotating faster than the secondary wheels26,28, an additional reason for reducing the torque capacity of the RDU coupling46is to prevent binding and allow the driveshaft30to come fully up to the speed of the primary wheels. This method also prevents potential overloading of PTU synchronizer34by the much higher torque capacity RDU coupling46.

With the PTU synchronizer34still engaged, driveshaft30continues to approach the speed of the primary wheels20,22. When the speed difference across synchronizer34is sufficiently small, the synchronizer's sleeve engages the transmission output16allowing the synchronizer34to transmit transmission output torque to the RDU32through driveshaft30. The secondary wheels26,28are driven through coupling46, differential44and halfshafts48,50.

Under some high drag conditions, such as when ambient temperature is low and/or vehicle speed is very high, it is difficult for the PTU synchronizer34, with its relatively low torque capacity compared to that of coupler46, to significantly contribute to synchronizing the driveline. Due to the extended length of the synchronization and disconnect events when ambient temperature is low, durability of the PTU synchronizer34may be adversely affected. In this case, it is desirable that RDU clutch46perform the bulk of the speed synchronization.

The RDU clutch46is sized to provide on-demand AWD operation by transmitting a substantial portion of the transmission output torque to the secondary drive wheels26,28. This is more than enough to quickly accelerate, even in a high drag condition, those components of driveline10that are stationary when the AWD disconnects. Use of the RDU clutch46to perform this function also reduces the demands on the PTU synchronizer34, thereby extending its useful life.

Using the RDU coupling46to synchronize the speed of stationary components to the speed of the secondary wheels26,28is not a complete solution for quick re-engagement of AWD operation. If, due to wheel slip, the speed of the primary wheels20,22is higher than the average secondary wheel speed, a speed difference exists across the PTU synchronizer34after the disconnected driveline components have been brought up to speed with wheels26,28.

For this reason the PTU synchronizer34must perform the final part of the synchronization and reconnect event. The RDU clutch46torque capacity is substantially reduced or eliminated, and with the PTU synchronizer now engaged, the driveshaft30continues to approach the speed of the primary wheels20,22. When the speed difference across synchronizer34is sufficiently small, the synchronizer's sleeve engages the transmission output16allowing the synchronizer34to transmit transmission output torque to the RDU32through driveshaft30. The secondary wheels26,28are driven through coupling46, differential44and halfshafts48,50.

With respect to all those synchronization and connection strategies which make use of the RDU clutch, as described in the immediately preceding sections of this document, it can readily be appreciated that the RDU clutch46uses vehicle kinetic energy to bring the previously disconnected driveline components up to speed. The RDU clutch46accesses this kinetic energy by applying a braking force through the secondary drive wheels26,28and their tractive effort on the road surface. If the braking torque and tractive effort is too large, the tractive force being exerted on wheels26,28may cause loss of traction on a low coefficient of friction road surface. To minimize to an acceptable level any such loss of traction on the secondary wheels due to engagement of RDU clutch46, the AWD controller may receive constraints from a vehicle controller, such as a brake system controller, which are used to limit the torque capacity of the RDU clutch46, or to limit the rate of change of RDU clutch torque capacity.

Referring now toFIG. 3, at step160of the method for engaging all-wheel drive operation of the vehicle driveline10clutch46is used to accelerate driveline components upstream of the clutch toward the speed of the secondary wheels26,28.

At step162clutch46is partially or completely disengaged.

At step164synchronizer34is used to accelerate driveline components toward the speed of the transmission output16.

At step166synchronizer34is engaged to transmit torque between the transmission output16and clutch46.

At step168the torque capacity of the clutch46is increased, and is made to vary as required for all-wheel drive operation.

In order to further illustrate the sequence of actions which take place in the three synchronization and connection strategies that have been described above, reference is now made toFIG. 4, a diagram showing the various mutually exclusive states in which the control system can exist.

On initial power-up, the system is normally at step200, and enters the CONNECTED state, at step202, in which case the PTU has connected the transmission to the driveshaft and the RDU, and the system is capable of immediately performing the normal on-demand AWD function. The system will remain in this state until the condition ‘disconnect_required’ is true. This condition may be true if the vehicle operator has issued a manual disconnect command to maximize fuel economy or for other reasons, or if the auto_connect system has determined that conditions are appropriate to disconnect for the purpose of improving vehicle fuel economy. The auto_connect system monitors and models many current, past and predicted vehicle operating parameters and environmental conditions.

When the ‘disconnect_required’ condition is true, the system enters the DISCONNECTING state, at step204. In this state, the RDU clutch torque capacity is decreased in a manner which produces both acceptable NVH and speed of response, until the torque capacity is low enough to allow the PTU to be disengaged without component damage or driveline shock.

When vehicle operating parameters indicate that the PTU has been disconnected, the ‘disconnect_completed’ condition is true, and the system transitions to the DISCONNECTED state, at step206. It is possible, when in the disconnected state, that vehicle or environmental conditions do not permit the system to re-enter a connected state, even if the vehicle operator issues a manual connect command or if the auto_connect system has determined that conditions are appropriate to connect for the purpose of being prepared to quickly provide the on-demand AWD function. Under these circumstances, the system will remain in the DISCONNECTED state indefinitely.

If, however, vehicle and environmental conditions permit the system to enter the connected state when required by the manual or auto connect conditions, then the ‘connect_allowed’ condition is true, and the system transitions to the READY_TO_CONNECT state, at step208. If, while the control system is in the READY_TO_CONNECT state, vehicle or environmental conditions change so that the connect state can no longer be permitted, then the ‘connect_disallowed’ condition is true, and the system transitions back to the DISCONNECTED state at step206. While in the READY TO CONNECT state, the control system waits for either a manual connect command from the vehicle operator, or an auto connect command from the auto connect system. If either command is received, then the respective ‘auto_connect’ condition is true, at step212, or the ‘manual connect’ condition’ is true at step212, and the control system leaves the READY TO CONNECT STATE and uses one of the three previously described methods to synchronize and connect the driveline.

At this time, the control system evaluates vehicle and environmental conditions to determine whether which of three mutually exclusive conditions is true: The mutually exclusive conditions are:

First, ‘ptu_only_connect_preferred’ (see the above description of the synchronization and connection strategy labeled as ‘Singular Activation’ for a summary of conditions which could make this condition true); or

Second, ‘both_ptu_and_rdu_overlapping_connect_preferred’ (see the above description of the synchronization and connection strategy labeled as ‘Overlapping Activation’ for a summary of conditions which could make this condition true); or

Third, ‘both_ptu_and_rdu_sequential_connect_preferred’ (see the above description of the synchronization and connection strategy labeled as ‘Sequential Activation’ for a summary of conditions which could make this condition true).

If ‘ptu_only_connect_preferred’ is true, then the system transitions to the PTU_ONLY_SYNC state, at step214. While in this state, the control system performs the Singular Activation described earlier as synchronization and connection strategy number 1.

If ‘both_ptu_and_rdu_overlapping_connect_preferred’ is true, then the system transitions to the BOTH_OVERLAPPING_SYNC state, at step216. While in this state, the control system performs the Overlapping Activation described earlier as synchronization and connection strategy number 2.

If ‘both_ptu_rdu_sequential_connect_preferred’ is true, then the system transitions to the BOTH_SEQUENTIAL_SYNC state, at step218. While in this state, the control system performs the Sequential Activation described earlier as synchronization and connection strategy number 3.

In all of these states (steps214,216and218) the control system performs the appropriate synchronization and connection strategy until it determines that the connect action has completed. At that time, the ‘connect_completed’ condition is true, and the control system transitions to the ‘CONNECTED’ state at step202.