Patent Description:
The present invention relates to a twin clutch system for a secondary axle of an all wheel drive vehicle and a hydraulic torque actuator with a dual action piston.

Hydraulic torque actuation is commonly used on a secondary axle on a front or rear wheel drive vehicle where all wheel drive is an option. This may be for both permanent active on demand systems (AOD) as well as disconnecting systems where the ring gear and pinion are stopped when the all wheel drive function is not needed. On a side mounted twin clutch axle, where independent torque control of left and right wheels is offered, each left and right mounted clutch has an actuator typically consisting of an independent piston where pressure modulation determines the thrust force and therefore the torque setting.

Reference is made to patent document <CIT>, which relates to a prior art four-wheel drive device for a vehicle having right and left clutch portions.

Reference is made to patent document <CIT>, which relates to a prior art hydraulic coupling for an auxiliary drive axle of a vehicle drivetrain.

Reference is made to patent document <CIT>, which relates to a prior art coupling device for coupling an input shaft to an output shaft of a vehicle.

Reference is made to patent document <CIT>, which relates to a prior art friction clutch having centrifugally operated valves.

Reference is made to patent document <CIT>, which relates to a prior art dry application clutch arrangement directed to marine environment usage with a propeller shaft.

Reference is made to patent document <CIT>, which relates to a prior art dual clutch device for connecting and disconnecting the transmission of power from an engine to first and second transmission input shafts.

Reference is made to patent <CIT>, which relates to a prior art power transmission device having left and right clutch pistons.

According to the invention there is provided a secondary drive unit for an all wheel drive vehicle having left and right secondary drive wheels which are selectively connected to a drive source through the secondary drive unit, the secondary drive unit housing a main shaft operatively coupled to the drive source. The secondary drive unit includes a secondary drive unit (SDU) housing, the housing defining a first section for receiving the main shaft, and the housing defining a second section for enclosing a twin clutch assembly, a left output shaft concentric with the main shaft, the left output shaft for transferring torque to the left secondary drive wheel; and a right output shaft coaxial with the left output shaft, the right output shaft for transferring torque to the right secondary drive wheel. The twin clutch assembly includes a clutch housing connected to the main shaft, a left clutch for selectively connecting the left output shaft and the clutch housing, a right clutch for selectively connecting the right output shaft and the clutch housing, and a rigid center plate extending from an inner wall of the clutch housing, the rigid center plate separating the left and right clutches. One of the right or left clutches further includes a dual action piston assembly. The dual action piston assembly includes a dual action piston, a cavity defined in a first inward facing wall of the SDU housing for receiving the dual action piston, and first and second ports defined in the SDU housing for passing fluid to activate the dual action piston assembly. The dual action piston and the cavity in the first inward facing wall define first and second chambers for receiving fluid through the first and second ports and the first chamber has a volume less than a volume of the second chamber.

In some embodiments, the second section of the SDU housing is located wholly on one side of the first section of the SDU housing. In some embodiments the right output shaft is piloted and supported by the left output shaft.

In some embodiments, the right clutch may include a set of right separator plates integrated into the clutch housing and interleaved with a set of right friction disks carried by the right output shaft. The left clutch may include a set of left separator plates integrated into the clutch housing and interleaved with a set of left friction disks carried by the left output shaft. The sets of left and right separator plates are separated by the center plate and the sets of left and right friction disks are separated by the center plate.

In some embodiments, as fluid is received through the first port for activation of the dual action piston assembly, fluid is drawn by the dual action piston assembly into the second port and second chamber.

In some embodiments the dual action piston has a generally H-shaped cross-section. In some embodiments the dual action piston has a generally stepped cross-section.

In some embodiments, the other of the right or left clutches further includes a non-dual action piston assembly, the non-dual action piston assembly includes a non-dual action piston, a cavity defined in a second inward facing wall of the SDU housing for receiving the non-dual action piston, and a third port defined in the SDU housing for passing fluid to activate the non-dual action piston assembly, the non-dual action piston and the cavity in the second inward facing wall defining a third chamber for receiving fluid through the third port.

In some embodiments, the volume of the first chamber of the dual action piston assembly is less than a volume of the third chamber of the non-dual action piston assembly. In some embodiments, the first and second chambers of the dual action piston assembly have substantially the same pressure apply area as the third chamber of the non-dual action piston assembly.

The present application is directed to a twin clutch system for a secondary axle on a front or rear wheel drive vehicle which also supports all wheel drive. A dual action piston and methods for actuation of the twin clutch system are also provided. While described and illustrated with reference to the rear axle of a front wheel drive vehicle, the twin clutch system and dual action piston may be used for any secondary axle.

<FIG> illustrates an exemplary vehicle drive train assembly <NUM> for transferring torque to first or main set of wheels <NUM> and a second or secondary set of wheels <NUM> of a vehicle. The drive train assembly <NUM> includes a main or front driveline <NUM> and a secondary or rear driveline <NUM>. The front driveline <NUM> includes, among other components, an engine <NUM>, a transmission <NUM> and a power take off unit <NUM> (PTU). The PTU <NUM> includes an output <NUM> to transmit torque through a propeller shaft <NUM> to secondary drive unit and specifically a rear drive unit <NUM> (RDU) for driving the rear wheels <NUM>. The RDU <NUM> includes a twin clutch assembly according to the present disclosure. A controller (not shown) is in communication with the components in the front driveline <NUM> and rear driveline <NUM> and also in communication with one or more sensors located throughout the vehicle. The controller is also configured to control the hydraulic system for activating the pistons and twin clutch assemblies as described herein.

<FIG> illustrates a twin clutch assembly <NUM> as known in the prior art for a rear drive unit <NUM>. Torque flows from the pinion gear <NUM> to the ring gear <NUM> and then directly to the left and right clutch housings <NUM>, <NUM> since they are directly connected. Left and right pistons <NUM>, <NUM> control the apply pressure on the left and right clutch plates <NUM>, <NUM> thus independently controlling left and right output torque. The left and right pistons <NUM>, <NUM> can be made so that the clearance is large enough to separate the clutch plates <NUM>, <NUM> so that the left and right output shafts <NUM>, <NUM> and the respective clutch housings <NUM>, <NUM> become disconnected. In this example, provided that the front of the system is also disconnected, such as in the PTU, the pinion gear <NUM> and ring gear <NUM>, as well as the propeller shaft <NUM> will stop rotating. When the need to connect the driveline arises, the pistons <NUM>, and or <NUM> will modulate enough torque in a controlled manner so that the system is connected smoothly.

Separate left and right clutches pose some challenges, however, in terms of having two clutch housings, separate output shafts with their own bearing supports, and increased width and potentially weight. In addition, sufficient clearance between the clutch plates is needed to reduce parasitic loses during the disconnecting state and provide a low drag when the system is in a disconnected state. The response time, however, from when the clutch is completely open to closing the clearance and starting torque control may be longer than desirable for a dynamic, fast responding AWD system.

An RDU <NUM> and twin clutch assembly <NUM> according to one embodiment of the present disclosure are illustrated in <FIG>. The assembly <NUM> is contained within the housing <NUM> of the RDU <NUM> and is situated on one side of a drive source for the secondary axle. Specifically, the RDU housing <NUM> defines a first compartment or section <NUM> which receives the drive source. The RDU housing <NUM> defines a second compartment or section <NUM> which contains the twin clutch assembly <NUM>. In one embodiment, the drive source is a pinion gear <NUM> and pinion shaft <NUM> which are operatively coupled to the propeller shaft <NUM> and PTU <NUM> of the vehicle. The pinion gear <NUM> is coupled with a ring gear <NUM> having a main shaft <NUM> which is received in the section <NUM>. In the embodiment shown in <FIG>, the second section <NUM> and assembly <NUM> are located to the right of the first section <NUM> of the RDU housing <NUM>. In other embodiments (not shown), a symmetrical or mirror image version of the assembly <NUM> and second section <NUM> are located to the left of the drive source and first section <NUM>. In contrast to the twin clutch assembly <NUM> in <FIG>, which includes separate right and left clutch assemblies on each side of the drive source, the twin clutch assembly <NUM> according to the present disclosure is located on one side of the drive source.

The twin clutch assembly <NUM> includes a common clutch housing <NUM>, a left clutch <NUM> and a right clutch <NUM>. The clutch housing <NUM> is connected to the main shaft <NUM>. A right output shaft <NUM> is piloted and supported by a left output shaft <NUM>. In other embodiments, the left output shaft <NUM> is piloted and supported by the right output shaft <NUM>. The left output shaft <NUM> is concentric and coupled within the main shaft <NUM>. Although not extended in <FIG>, the left output shaft <NUM> extends from the twin clutch assembly <NUM>, through the main shaft <NUM>, to the left rear wheel. When the clutch assembly <NUM> is connected to provide AWD, the right output shaft <NUM> transfers torque to the right rear wheel and the left output shaft <NUM> transfers torque to the left rear wheel. Depending on the amount of slip required between left and right clutch plates, differential motion is carried out by right and left outputs shafts <NUM>, <NUM> being piloted together with a bearing or bushing <NUM>. In one embodiment, the right and left outputs shafts <NUM>, <NUM> are piloted together with an "L" shaped sintered bearing.

Each clutch <NUM>, <NUM> includes respective sets of left and right separator plates, sets of left and right friction disks, and at least two piston assemblies for activating the clutch. Specifically, each of the left and right clutch assemblies includes respective sets of left and right separator plates <NUM>, <NUM> which are integrated into the clutch housing <NUM>. In one embodiment, the clutch housing <NUM> is splined to of the sets of left and right separator plates <NUM>, <NUM>. The left output shaft <NUM> carries a set of friction disks <NUM> for the left clutch <NUM> and the right output shaft <NUM> carries a set of friction disks <NUM> for the right clutch <NUM>. The left and right clutches <NUM>, <NUM> are actuated by respective pairs of left and right hydraulic piston assemblies, <NUM>, <NUM>. Thus, independent torque biasing is enabled.

The twin clutch assembly <NUM> includes a centre plate <NUM> which separates the set of left separator plates <NUM> and the set of right separator plates <NUM> in order to support independent left and right torque control. The centre plate <NUM> is stiff enough to carry the thrust load from the pistons <NUM>, <NUM>. Specifically, the centre plate <NUM> is stiff enough to react against the actuation force from either side of the clutch assembly <NUM> to avoid the plate <NUM> deflecting and compressing the clutch plates in the other side of the assembly <NUM>. In one embodiment, the centre plate <NUM> is rigidly fixed to the clutch housing <NUM>. In one embodiment, the left piston assembly <NUM> actuate the left clutch <NUM> via an external apply plate <NUM>. The external apply plate <NUM> may be mated to the side of the clutch housing <NUM> through windows or apertures (not shown) to squeeze the clutch plates inside. The right piston assembly <NUM> actuates the right clutch 314through one of the clutch plates, as described below.

In two wheel drive (2WD) mode, the friction disks <NUM>, <NUM> and separator plates <NUM>, <NUM> are spaced apart to cause a disconnect between the ring gear <NUM> and left and right output shafts <NUM>, <NUM>. In order to change the state back to a connected AWD drive mode, the ring gear <NUM> is synchronized to approximately the same relative speed as the rear wheels <NUM>. Torque is generated to overcome the inertial torque to spin the ring gear <NUM>, the pinion shaft <NUM> and the propeller shaft <NUM> to the same relative speed of the vehicle. This can be accomplished by engaging either one of the left and right clutches <NUM>, <NUM> at the same time or in serial fashion. The speed of synchronization will depend on how quickly the clearance gap is taken up by one or both of the left and right clutches <NUM>, <NUM> and the torque response of the actuator to produce torque in the clutches <NUM>, <NUM>. Once the driveline components are synchronized and the PTU <NUM> is connected, torque modulation can take place.

The self contained clutch assembly <NUM> has a smaller overall size and enables an actuator system to be placed near the assembly <NUM>, resulting in shorter overall fluid porting to both left and right hydraulic piston assemblies. Thus, a faster responding AWD system may be achieved. In one embodiment, both clutches <NUM>, <NUM> could be utilized to start the synchronization process, depending on the torque required. In one embodiment (not shown), the left and right hydraulic piston assemblies <NUM>, <NUM> are provided with symmetrical geometries and cavity sizes and thus each piston has a similar response or performance. In other embodiments, since the torque level to synchronize the driveline is relatively small as compared to the capacity of the left and right clutches <NUM>, <NUM>, just one of the clutches is used to synchronize the driveline.

As illustrated in <FIG>, to further improve the synchronization and AWD connection response time, the right hydraulic piston assembly <NUM> is configured as a dual action piston according to one embodiment of the present disclosure. Since only one clutch may be needed to synchronize the driveline, the dual action piston assembly <NUM> is configured to engage the right clutch <NUM> or "synchronizing clutch" more quickly than the "non-synchronizing" left clutch <NUM>. The dual action piston assembly <NUM> includes a dual action piston <NUM> received or mounted a first cavity defined in a first inward facing wall <NUM> of the RDU housing <NUM>. The dual action piston <NUM> and the first cavity define two chambers, CH1 and CH2, for receiving fluid for actuating the piston assembly <NUM>. In the embodiment shown in <FIG>, the two chambers CH1 and CH2 are spaced apart linearly within the cavity for the dual action piston assembly <NUM> and are defined by the piston <NUM> having a generally "H" shaped cross-section. The non-dual action left piston assembly <NUM> includes a piston <NUM> received or mounted in a second cavity defined in a second inward facing wall <NUM> of the RDU housing <NUM>. The piston <NUM> and the second cavity define a third chamber CH3 for receiving fluid for actuating the piston assembly <NUM>. A bias means (not shown in <FIG>) is provided to return the pistons <NUM>, <NUM> to their retracted positions. Bearings (not shown in <FIG>) also may be placed between the piston assemblies <NUM>, <NUM> and the apply plate <NUM> and right separator plates <NUM>.

In one embodiment of the dual action piston assembly <NUM>, the overall volume of chamber CH1 is less than the volume of CH2 and it is also less than the volume of chamber CH3 for the left piston assembly <NUM>. As a result, as described below, the right piston <NUM> moves faster when the filling rate is the same for both piston assemblies. Since the reaction torque to synchronize the driveline is relatively small, only one clutch and the partial piston surface area of the dual action piston assembly <NUM> can be used with a faster response time as a result. The piston assembly <NUM> performs dual functions by quickly overcoming the clearance and synchronizing the right clutch <NUM> while at the same time chamber CH2 is filled. Chambers CH2 and CH3for piston assemblies <NUM>, <NUM> may be equally sized so that the left and right pistons <NUM>, <NUM> will push with the same force during primary torque control.

An RDU <NUM> and twin clutch assembly <NUM> according to another embodiment of the present disclosure is illustrated in <FIG>. The assembly <NUM> is similar to the assembly <NUM> shown in <FIG> but includes a different configuration of the RDU housing <NUM> and a different embodiment of a dual action piston assembly <NUM> for the right clutch <NUM>. A different embodiment of the non-dual action piston assembly <NUM> is also provided for the left clutch <NUM>.

The dual action piston assembly <NUM> includes a dual action piston <NUM> received or mounted a first cavity defined in the first inward facing wall <NUM> of the RDU housing <NUM>. The dual action piston <NUM> and the first cavity define two chambers, C1 and C2, for receiving fluid for actuating the dual action piston assembly <NUM>. In the embodiment shown in <FIG>, the two chambers C1 and C2 are staggered or spaced apart axially and radially within the cavity and are defined by the piston <NUM> having a stepped cross-section. The non-dual action left piston assembly <NUM> includes a piston <NUM> received or mounted in a second cavity defined in a second inward facing wall <NUM> of the RDU housing <NUM>. The piston <NUM> and the second cavity define a chamber C3 for receiving fluid for actuating the piston assembly <NUM>. The twin clutch assembly <NUM> includes bias means to return the pistons <NUM>, <NUM> to their retracted or disconnected positions. Example bias means are shown in <FIG> with right and left springs <NUM>, 420engaging the pistons <NUM>, <NUM> and portions of the RDU housing <NUM>. Bearings also may be placed between the piston assemblies <NUM>, <NUM> and the apply plate <NUM> and right separator plates <NUM>.

In this embodiment, the volume of chamber C1 is less than the volume of C2 and only one clutch and the partial piston surface area of the dual action piston assembly <NUM> can be used for synchronization with a faster response time as a result. Collectively, chambers C1 and C2 have the same pressure apply area as the non-dual action left piston assembly <NUM> with chamber C3 so that the left and right pistons <NUM>, <NUM> will push with the same force during primary torque control.

The sizes and geometries of the chambers for each piston assembly <NUM>, <NUM>, <NUM>, <NUM> may vary and the ratio of the chamber sizes, orifice sizes, porting diameters, valve clearances and pump flow rates are configured to maximize piston stroke response. While the dual action piston assembly is shown for the right clutch in <FIG> and <FIG>, in other embodiments, the left piston assembly may be configured as a synchronizing, dual action piston and the right piston assembly may be a non-synchronizing, non-dual action piston. In other embodiments, both the left and right piston assemblies may be configured as a synchronizing, dual action piston. It will also be appreciated that the dual action piston assemblies according to the present disclosure also may be used in other disconnecting clutch systems, such as the assembly shown in <FIG>.

The operation of the twin clutch assembly <NUM> will be described first with reference to the illustration of the piston assemblies <NUM>, <NUM>, hydraulic control system <NUM> and hydraulic schematic in <FIG> and <FIG>. Starting from a disconnected state, a controller (not shown) activates an electric motor <NUM> in the hydraulic control system <NUM>. The controller may be part of the vehicle controller or a standalone system. The motor <NUM> spins an electric pump <NUM> to generate hydraulic pressure. In one embodiment, the motor <NUM> operates at a fixed speed resulting in the pump <NUM> providing a fixed flow rate. The pump <NUM> forces hydraulic fluid from a reservoir <NUM> through a one-way check valve CV1 into a linear solenoid valve LSV1 which is also controlled electronically by the controller. Hydraulic fluid flows from the valve LSV1 through a dedicated port P1 into the chamber C1 of the right or dual action piston assembly <NUM>. Chamber C1 is filled with fluid under pressure and as a result, the piston <NUM> moves more rapidly since a smaller cavity, relative to chamber C3 for piston assembly <NUM>, is being filled. The force applied by the piston <NUM> to the outer separator plate of the right clutch <NUM> is sufficient to synchronize the driveline components. In other words, since the reaction torque to synchronize the driveline is relatively small, only the right clutch <NUM> and a partial piston surface area of the dual action piston assembly <NUM> are used at this stage. This results in a faster response time for synchronizing and ultimately engaging the driveline components for AWD operation.

While the dual action piston <NUM> moves towards the right separator plates <NUM> (to the left in <FIG>), a negative pressure or vacuum is created in chamber C2 which causes chamber C2 to be filled as it draws fluid from the reservoir <NUM> through a one way check valve CV2 and input port P2. Although a second input port P3 IN is provided for chamber C2, this port is configured with a smaller orifice. Thus, as the chamber C1 is being filled, limited fluid flows into chamber C2 through port P3 IN. The piston cavity ratio, C1 vs. C2, orifice size, porting diameters, valve clearances and pump flow rates are configured to ensure that the piston stroke response is maximized. Chambers C1 and C2 and the piston <NUM> are configured such that once the piston <NUM> has moved through its complete stroke, to a contact point or kiss point (clutch clearance take up) of the right clutch <NUM>, both chambers C1 and C2 are filled and are at equal pressure. At the kiss point, equal pressure is supplied to chambers C1, C2 through ports P1 and P3 IN, respectively. Although the orifice of P3 IN is restricted, it is sufficiently sized to enable pressure control in chamber C2 for torque modulation. Pressure relief from chamber C2 is provided by a one way check valve CV3 through port P3 EX to a drain port of the LSV1. Pressure relief from chamber C1 is provided through a drain port of the LSV1.

<FIG> is a graph illustrating pressures in the chambers C1 and C2 during this process. The upper line <NUM> of the graph shows the primary piston pressure for chamber C1. The secondary piston pressure for chamber C2 is indicated by the dashed lower line <NUM> of the graph and this shows the negative pressure initially created in chamber C2. The time t denotes the time for the dual action piston <NUM> to engage the right clutch <NUM> and take up or overcome the clearance between the friction disks <NUM> and separator plates <NUM>. The pressure in chamber C1 is initially larger in order to quickly overcome or take up the clearance between the clutch plates in the right clutch <NUM>. Pressure is reduced as the right clutch <NUM> nears the kiss point. After that stage, once the driveline components are synchronized, torque modulation is achieved by controlling the pressures in chambers C1 and C2 through ports P1 and P3 IN.

In one embodiment, once the right clutch <NUM> has provided enough torque to synchronize the driveline components, the controller activates the left piston assembly <NUM> through the control of linear solenoid valve LSV2. Specifically, the pump <NUM> forces hydraulic fluid from the reservoir <NUM> through a one way check valve CV4 into LSV2. Hydraulic fluid flows from the valve LSV2 through a dedicated port P4 into the chamber C3 of the left or non-dual action piston assembly <NUM>. Pressure relief from chamber C3 is provided through a drain port of the valve LSV1. Once both left and right piston assemblies <NUM>, <NUM> are at their stand by or kiss point positions, and once the driveline components are synchronized and the PTU is connected, torque modulation can take place. In one embodiment, some amount of engagement overlap between the left clutch <NUM> clearances being overcome, the synchronization of the propeller shaft <NUM>, and the connection in the PTU <NUM> can be realized in order to improve the total system response. <FIG> illustrates an example time line and such overlap for one embodiment. As shown, the synchronization time for the rotation of the propeller shaft <NUM> may start after the right clutch <NUM> reaches the kiss point and may overlap with the activation of the left clutch <NUM>. The PTU engagement also may overlap with the synchronization for the rotation of the propeller shaft <NUM> and the activation of the left clutch <NUM> to its clearance point.

For the twin clutch assembly <NUM> to return to a disconnected state, the controller operates the linear solenoid valves LSV1 and LSV2 to drain or bleed out fluid from chambers C1, C2 and C3 through ports P1, P3 EX and P4, respectively. The left and right pistons <NUM>, <NUM> retract back to an open or disconnected position. In one embodiment, the left and right pistons <NUM>, <NUM> are urged back to the open or disconnected position by a bias means such as the springs <NUM>, <NUM>, as illustrated in <FIG>.

Alternative synchronization and engagement sequences may be implemented depending on the arrangement of the dual action piston assembly and the desired performance of the twin clutch assembly. The operation of the embodiment of the clutch assembly <NUM> of <FIG> is described with reference to the illustration of the piston assemblies <NUM>, <NUM> and hydraulic control system <NUM> in <FIG> and <FIG>. Again, the apply principle is accomplished in two stages. The first is to move the right clutch <NUM> to the kiss point and synchronization and the second is torque modulation by using the respective filled chambers. <FIG> represents a starting point from a disconnected state and <FIG> represents the state of the piston assemblies <NUM>, <NUM> and hydraulic control system <NUM> at the kiss point.

Specifically in <FIG>, starting from a disconnected state, the controller activates an electric motor <NUM> and controls linear solenoid valves LSV1, LSV2 in the hydraulic control system <NUM>. The motor <NUM> spins an electric pump <NUM> to generate hydraulic pressure. The pump <NUM> forces hydraulic fluid from a reservoir <NUM> through valves LSV1 and LSV2 and ports P1 and P3 to fill chambers CH1 and CH3. Port P2A for chamber CH2 is blocked during this process by the piston <NUM>. As the dual action piston <NUM> moves towards the right separator plates <NUM> (to the left in <FIG>), a negative pressure or vacuum is created in chamber CH2 which causes chamber CH2 to be filled as it draws fluid from the reservoir <NUM> through a one way check valve CV1 and port P2B. Although all three chambers CH1, CH2, CH3 are being filled in this embodiment, the right piston <NUM> moves more quickly than the left piston <NUM> since the size of CH1 is less than the size of CH3. Thus, the right piston <NUM> moves the right clutch <NUM> to the kiss point to start synchronization.

As shown in <FIG>, as the right piston <NUM> completes its stroke, port P2A is opened and chamber CH2 is filled completely. Also as port P2A opens, an axial valve <NUM> in the chamber CH1 closes port P1 and port P4 is opened. As a result, at the kiss point, fluid from chamber CH1 discharges back into the reservoir <NUM>. The one way check valve CV1 prevents fluid from backing out of chamber CH2 through port P5. Thus, during torque modulation, fluid in chamber CH1 is discharged to the reservoir <NUM> through P4 so that the left and right piston assemblies <NUM>, <NUM> will push with the same force during primary torque control.

Either the left or the right piston assembly can be a dual action piston and initially made to move faster than the other with the same supply pressure to both right and left sides. Therefore the synchronization (from an open piston to the kiss point or standby mode) can happen more quickly without any additional moving parts. One piston assembly is made to have a dual role by adding two cavities with which the piston is controlled. The dual action piston assembly <NUM> also fills chamber CH1 under pressure and chamber CH2 under vacuum due to the moving piston so that both chambers are filling under the leftward movement of the piston <NUM>. Since chambers CH2 and CH3 are of equal size, and are controlled and configured to fill at relatively the same rate, this ensures that primary piston pressure for primary torque control is available at the same time.

Claim 1:
A secondary drive unit (<NUM>) for an all wheel drive vehicle having left and right secondary drive wheels (<NUM>) which are selectively connected to a drive source through the secondary drive unit, the secondary drive unit (<NUM>) housing a main shaft (<NUM>) operatively coupled to the drive source, the secondary drive unit (<NUM>) comprising:
a secondary drive unit (SDU) housing (<NUM>), the housing (<NUM>) defining a first section (<NUM>) for receiving the main shaft (<NUM>), and the housing (<NUM>) defining a second section (<NUM>) for enclosing a twin clutch assembly (<NUM>),
a left output shaft (<NUM>) concentric with the main shaft (<NUM>), the left output shaft (<NUM>) for transferring torque to the left secondary drive wheel (<NUM>);
a right output shaft (<NUM>) coaxial with the left output shaft (<NUM>), the right output shaft (<NUM>) for transferring torque to the right secondary drive wheel (<NUM>);
the twin clutch assembly (<NUM>) having:
a clutch housing (<NUM>) connected to the main shaft (<NUM>),
a left clutch (<NUM>) for selectively connecting the left output shaft (<NUM>) and the clutch housing (<NUM>),
a right clutch (<NUM>) for selectively connecting the right output shaft (<NUM>) and the clutch housing (<NUM>), and
a rigid center plate (<NUM>) extending from an inner wall of the clutch housing (<NUM>), the rigid center plate (<NUM>) separating the left and right clutches (<NUM>, <NUM>),
characterized in that:
one of the right or left clutches (<NUM>, <NUM>) further comprises a dual action piston assembly (<NUM>), the dual action piston assembly (<NUM>) comprising:
a dual action piston (<NUM>),
a cavity defined in a first inward facing wall (<NUM>) of the SDU housing (<NUM>) for receiving the dual action piston (<NUM>), and
first and second ports defined in the SDU housing (<NUM>) for passing fluid to activate the dual action piston assembly (<NUM>),
the dual action piston (<NUM>) and the cavity in the first inward facing wall (<NUM>) defining first and second chambers (CH1, CH2) for receiving fluid through the first and second ports, the first chamber (CH1) having a volume less than a volume of the second chamber (CH2).