Patent Publication Number: US-8978375-B2

Title: Hydraulic transmission apparatus making quick positive clutching/declutching possible

Description:
FIELD OF THE INVENTION 
     The invention relates to hydraulic transmission apparatus for a vehicle, the apparatus including a pressurized fluid source and at least one hydraulic motor suitable for being fed with hydraulic fluid by the pressurized fluid source in order to drive at least one vehicle mover member; 
     the motor being connected to:
         a motor duct for feeding said motor with fluid, and a motor duct for discharging fluid from said motor, which motor ducts are suitable for being put into communication with cylinders arranged in a cylinder block and including pistons suitable for sliding in said cylinders; and   a casing duct connected to an internal space arranged inside a casing containing the cylinder block;   the motor being suitable for being operated in assisted mode, in which it generates drive torque or braking torque under the effect of a pressure difference between the motor ducts, and in “freewheel” mode or unassisted mode in which the motor does not generate any torque and the pistons are maintained in the retracted position inside the cylinders;   the apparatus further including an accumulator suitable for feeding both of the motor ducts during a stage of passing from the unassisted mode to the assisted mode in order to facilitate deployment of the pistons.       

     In unassisted mode, the motor is positively declutched; the pistons are maintained in the retracted position under the effect of the pressure prevailing in the internal space connected to the casing duct. Mechanical means such as springs or the like can be used in addition to the fluid pressure for making it easier to maintain the pistons in the retracted position. 
     BACKGROUND OF THE INVENTION 
     Such apparatus is, in particular, used in assistance on a vehicle, so as to make it possible, whenever necessary, to actuate the hydraulic motor as a supplementary or assistance motor, for moving the vehicle under difficult movement conditions, such as slippery ground, steep slope, etc. Typically, the hydraulic motor is arranged on an axle of the vehicle that is an axle on which the wheels are not driven wheels when the vehicle is in normal advance mode; thus, when the hydraulic transmission apparatus is activated, the vehicle has additional driven wheels. 
     A particularly important application of the invention relates to hydraulic assistance apparatus mounted on road vehicles such as heavy goods vehicles that can travel at some speed (over 50 kilometers per hour (km/h)). For such an application, the hydraulic assistance apparatus must satisfy certain additional constraints: firstly, it must guarantee full safety when the vehicle is traveling at high speeds, i.e. it must be almost impossible for the assistance motors to be triggered in untimely manner; secondly the apparatus must make it possible for the assistance motors to be switched on while the vehicle is advancing, and not merely when it is at a standstill or while it is traveling at very low speed (less than 5 km/h). 
     The accumulator is a hydraulic accumulator of the rechargeable spring or gas type that contains pressurized fluid and is capable of storing hydraulic energy in the form of hydrostatic energy and of delivering hydraulic energy using the stored hydrostatic energy. 
     In order to enable the hydraulic transmission apparatus to be relatively practical to use, it is necessary for it to be quite simple and quick to activate or to deactivate. 
     This constraint is particularly important when the assistance motor is a motor of the positively declutchable type that has radial pistons arranged to act on an undulating cam, it being possible for said radial pistons to be retracted into the cylinders in such manner as to enable the motor to be positively declutched. In such a motor, the need for rapid switch-over from unassisted mode to assisted mode and vice versa does not only concern the ergonomics of the apparatus; quick positive clutching/declutching of the motor is also necessary in order to avoid damaging the motor during positive clutching or declutching, by limiting any rattling and jolting that is caused by such operations. 
     In order to facilitate positive clutching of the motor, it is possible, in known manner, to use an accumulator. An accumulator constitutes a supplementary pressurized fluid source; the pressure that it delivers can be transmitted to the motor ducts in order to accelerate deployment of the pistons out of the cylinders, and thus in order to accelerate positive clutching. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the invention is to provide transmission apparatus of the type described in the introduction, and in which not only can positive clutching be performed quickly, but also the time taken by positive declutching is reduced, in order to protect the motor during this stage and in order to procure a system that is quick to activate and to deactivate. 
     This object is achieved by means of the fact that the accumulator is also suitable for feeding the casing duct so as to put said duct under pressure during a stage of passing from the assisted mode to the unassisted mode in order to facilitate retraction of the pistons. During this stage, the accumulator puts the casing duct at a pressure greater than the pressure of the motor ducts, in order to ensure that the pressure differential in the cylinders across the pistons is established in the direction that facilitates retraction of the pistons. 
     By means of the pressure imposed by the accumulator, the positive declutching is accelerated, thereby increasing the flexibility of use of the vehicle. The use of the accumulator for facilitating both deployment and retraction of the pistons makes it possible to avoid using a large booster pump, which would add considerably to the cost of the apparatus. 
     In an embodiment, the apparatus is advantageously arranged in such a manner as to enable the accumulator to be emptied via a constriction, and the accumulator is put into communication with the casing duct so as to make it possible for the pistons to be retracted during the emptying stage itself (i.e. while the fluid that the accumulator contains is being removed towards an unpressurized reservoir). Naturally, in this situation the connection between the casing duct and the accumulator is on the accumulator side of the constriction. 
     Due to the constriction, emptying the accumulator requires a certain length of time, during which the pressure in the vicinity of the accumulator firstly remains relatively high, before decreasing progressively. By means of this, putting the accumulator and the casing duct into communication with each other for facilitating retraction of the pistons can take place during the operation of emptying the accumulator. 
     In addition, the use of an accumulator can suffer from certain risks, in particular if the accumulator remains under pressure while the vehicle is being used, and more particularly for long periods and in unassisted mode, on the road. 
     In order to reduce this risk, in a preferred embodiment, the apparatus further includes an accumulator valve that has a port connected to the accumulator, and that has a first position in which it connects (or is suitable for connecting) said accumulator to a filling fluid source, and a second position in which it isolates said accumulator from said filling fluid source. 
     The accumulator valve procures considerable flexibility in managing the accumulator, in particular with a view to putting said accumulator under pressure only for the time for which it is necessary. The accumulator is then connected to the filling fluid source for a sufficient length of time, preferably just before its use stage. This applies, in particular at the time of passing to assisted mode, during which time the pressure of the accumulator is used to facilitate and to accelerate deployment of the pistons out of the cylinders. Conversely, while the vehicle is traveling at high speed or is at a standstill, the accumulator can remain empty. 
     In an embodiment, the apparatus includes a pressurization pump suitable for being connected to the accumulator while said motor ducts are being put under pressure. As a result, during the stage of putting the motor ducts under pressure, the motor ducts are then connected both to the above-indicated pump and to the accumulator. This makes it possible to accelerate filling the motor ducts with fluid and putting them under pressure on passing to assisted mode. 
     In addition, in general, in assistance apparatus such as the apparatus of the invention, since the assistance apparatus is used only occasionally, the problem arises of activating the hydraulic transmission apparatus: the means devoted to activation and deactivation must be relatively limited, and proportionate to the function of the apparatus, which is not more than providing assistance. 
     The solution usually chosen consists in providing a clutch system interposed between the internal combustion engine of the vehicle or of the machine and the main pump of the apparatus. Such a clutch system is quite costly, occupies a large volume, and requires a considerable amount of maintenance. 
     More particularly, an embodiment of such a solution is known for hydraulic transmission apparatus including a positively declutchable radial-piston hydraulic motor similar to the motor used in the apparatus of the invention. 
     In such apparatus, it is useful at all times to have at least some minimum pressure that can be applied to the insides of the internal spaces of the motors and thus that can maintain said motors in the positively declutched positions, with their pistons retracted into their cylinders. The technical solution adopted in such apparatus consists in making provision not only for the main pump to be declutchable by means of a clutch, but also for the internal combustion engine to actuate an auxiliary pump or booster pump of the apparatus continuously, so that fluid under some minimum pressure is delivered continuously, thereby guaranteeing that the assistance motors are maintained in positions in which they are positively declutched and thus safe. 
     Thus, in unassisted mode, since the main pump is declutched, there is no risk of it being damaged under the effect of insufficient pressure at its main orifices. 
     That technical solution still suffers from the drawback of using a clutch, but it does procure a certain amount of safety for the use of the positively declutchable hydraulic motors. However, the auxiliary pump operating continuously gives rise to non-negligible power consumption. In addition, the internal combustion engine has to have two outlets, one towards the clutch connected to the main pump, and the other towards the auxiliary pump. This gives rise to compactness problems, and requires the internal combustion engine to have a specific and complex design. 
     In order to remedy those drawbacks, a specific embodiment of the invention is proposed. This embodiment concerns the situation wherein the pressurized fluid source is a main pump, and the apparatus further includes an auxiliary pump that serves to maintain some minimum fluid pressure in at least one auxiliary duct, and wherein the main pump and the auxiliary pump are suitable for being actuated jointly by drive means. 
     Such drive means may be of any type, but in general they are constituted by an internal combustion engine, e.g. a diesel engine. A diesel engine is used in the following description in order to facilitate understanding. However, the description remains applicable regardless of the drive means actuating the pumps of the apparatus. 
     In such assistance apparatus, in known manner, the main pump may be a hydraulic pump having a variable delivery rate, thereby making it possible to adapt the speed of the motors as a function of need. 
     The specific embodiment proposed concerns hydraulic assistance apparatus including such a pump as the main pump, and more generally such apparatus in which the main pump is a pump that can be actuated without delivering, but that then requires some minimum pressure to be applied to its main orifices. If such pumps are actuated while pressure lower than the above-mentioned minimum pressure prevails in either of the main orifices of the pump, a risk of damaging the pump ensues. 
     In order to remove that risk, in the specific embodiment proposed, the assistance apparatus further includes:
         a “bypass” connection between a delivery orifice of the auxiliary pump and an unpressurized reservoir;   first constriction means arranged on the bypass connection and suitable for maintaining a pump protection pressure in a pump protection portion of the bypass connection; and   means for applying said pump protection pressure to main orifices of the main pump when said pump is actuated but is not delivering.       

     Advantageously, this arrangement makes it possible to operate the apparatus in an unassisted mode in which the above-mentioned risk is removed; power consumption is low; and this result is then obtained even though the apparatus does not include any clutch. 
     In unassisted mode, the main and auxiliary pumps are actuated continuously. However, the main pump is then not delivering, and so its power consumption remains tiny; in addition the consumption of the auxiliary pump also remains tiny, because the pressure at its delivery orifice is set at a relatively low pressure level that is merely sufficient to protect the main pump. Finally, the main pump is protected because a “pump protection pressure” is applied to its main orifices during the unassisted mode, this pressure naturally being chosen to be sufficient to guarantee that the pump is protected (but also to be as low as possible). 
     In an embodiment, the main pump is a rotary pump, and in particular a pump having a swashplate and axial pistons. 
     In an embodiment, the first constriction means are arranged to maintain a pump protection pressure less than 20 bars, and preferably less than 10 bars. By means of these particularly low pressures, the auxiliary pump in unassisted mode consumes little power even though it is being actuated. The pump protection pressure is at least the pressure that it suffices to apply to the main orifices of the main pump in order to enable said main pump to operate without being damaged. 
     In an embodiment, the drive means comprise an internal combustion engine having a power outlet to which the main and auxiliary pumps are coupled continuously, i.e. without any clutches. 
     In an embodiment, the apparatus includes a bypass valve interposed on said bypass connection and having:
         a first position, in which said bypass valve enables the auxiliary pump to deliver through the bypass connection; and   a second position, in which said bypass valve interrupts the flow through said bypass connection and enables fluid to pass towards said at least one auxiliary duct. This feature makes it possible to use the bypass valve as a trigger element for switching over between the assisted and the unassisted modes.       

     The bypass valve is then a general activation/deactivation valve for the assistance circuit. 
     It thus firstly has a first position corresponding to the unassisted mode of the apparatus, in which position minimum functions are ensured by the apparatus, in particular by means of the fact that the boost pressure (pressure at the delivery orifice of the auxiliary pump) or a pressure dependent on the boost pressure can be applied at different sensitive points of the apparatus, in particular the internal spaces of the above-mentioned motor, so as to guarantee that the motor is maintained in the positively declutched position. 
     The bypass valve also has a second position, corresponding to the assisted mode of the apparatus, in which position the booster pump is suitable for delivering pressurized fluid to various members of the apparatus via auxiliary ducts, in particular to the motors so as to facilitate positive clutching of the pistons (deployment of the pistons out of their cylinders). 
     When an accumulator valve is disposed on the filling and emptying line for filling and emptying the accumulator, and in one embodiment, the bypass valve and the accumulator valve are coupled together, i.e. when the bypass valve is in the first position, the accumulator valve is in the first position, and when the bypass valve is in the second position, the accumulator valve is in the second position. 
     The advantage of this arrangement is that it makes it possible, when the bypass valve is in the first position, to put the accumulator and the auxiliary duct under pressure, and thus to use the accumulator in assisted mode. Conversely, in the second position, use of the accumulator is not possible, and is therefore disabled in unassisted mode, thereby procuring safety in use of the apparatus. 
     It should be noted that this embodiment of hydraulic transmission apparatus may also be implemented in apparatus differing from apparatus of the invention merely by the fact that in such differing apparatus the accumulator is not provided for feeding the casing duct in order to put said casing duct at a pressure greater than the pressure in the motor ducts, during a stage of switching over from the assisted mode to the unassisted mode, in order to facilitate retraction of the pistons. 
     In an embodiment, in order to perform the coupling between the bypass valve and the accumulator, the accumulator valve has a hydraulic control chamber that is connected to a port of the bypass valve enabling the accumulator valve to be controlled by the bypass valve. Thus, the bypass valve is suitable for placing the accumulator in “pressurized” mode (the accumulator being connected to the auxiliary or booster pump) during the assisted mode, and for leaving the accumulator at zero pressure during the unassisted mode. 
     In an embodiment, the apparatus includes second constriction means arranged on the bypass connection, and suitable for maintaining a motor protection pressure in a motor protection portion of the bypass connection; and the motor protection portion (of the bypass connection) is suitable for being connected to said casing duct. 
     This feature is provided for the unassisted mode. In this mode, the fluid delivered by the auxiliary pump flows through the bypass connection. Under these conditions, the second constriction means maintain a “motor protection” pressure in a “pump protection connection” portion of said connection, which pressure is in general much lower than the pump protection pressure, and is sufficient to protect the motor(s). For example, for positively declutchable motors such as the above-described motors, the motor protection pressure is merely the pressure sufficient to maintain the pistons in the retracted position inside the cylinders, i.e. about 0.5 bars usually. 
     In an embodiment, the first constriction means include a first calibrated valve, calibrated to the pump protection pressure, and the second constriction means include a second calibrated valve, calibrated to the motor protection pressure, said first calibrated valve and said second calibrated valve being arranged in series on the bypass connection. 
     In an embodiment:
         said pressurized fluid source is a main pump;   the apparatus further includes an auxiliary pump serving to maintain some minimum fluid pressure in at least one auxiliary duct;   the apparatus includes two pump ducts connected to the main orifices of the main pump, and an activation valve having two upstream ports suitable for being connected to the two pump ducts, and two downstream ports suitable for being connected to the two motor ducts; and   said activation valve has a first position in which the two upstream ports are interconnected, and the two downstream ports are interconnected, and a second position in which the two upstream ports are connected to respective ones of the two downstream ports.       

     The activation valve is the valve that feeds the assistance motor(s) when they are activated, when it is in the second position, or isolates them from the pump ducts, when it is in the first position. 
     In an embodiment, the activation valve further includes a third upstream port suitable for being connected to the accumulator and/or to the auxiliary pump; when the activation valve is in the first position, the third upstream port is connected to the two downstream ports and is thus suitable for being connected to the two motor ducts; when the activation valve is in the second position, the third upstream port is isolated. 
     The connection established between the third upstream port and a pressurized fluid source thus makes it possible to impose a pressure in the motor ducts (e.g. a pressure dependent on the delivery pressure of the auxiliary pump), which is particularly effective for facilitating deployment or retraction of the pistons relative to the cylinders of the motor. 
     This connection may, in particular, be used for filling and emptying of the motor ducts. 
     In an embodiment, the apparatus includes a pilot valve having an outlet port connected to a hydraulic control chamber controlling the position of the activation valve, and an inlet port that is suitable for being connected to a pilot fluid source via said bypass valve, and said pilot valve is suitable for putting said pilot fluid source and said control chamber into communication with each other or for isolating them from each other. 
     This arrangement offers the advantage of thus having the bypass valve and the first pilot valve in series, in the control line of the activation valve. Since it is necessary for two valves to be in specific positions in order to enable the pilot valve to be activated, the risk of accidental failure and thus of untimely and undesired activation of the assistance motor is reduced to the minimum. Naturally, the bypass and pilot valves should be arranged so that their respective default positions correspond to the positions in which no instruction for going into a second position or activation position is transmitted to the activation valve. 
     An object of the invention is also to provide a transmission system for a vehicle, including a main transmission suitable for driving vehicle mover members, and an auxiliary transmission making it possible to deliver supplementary drive torque when circumstances so require, which system has relatively low power consumption, and has its auxiliary transmission implemented simply and in a manner requiring little maintenance. 
     This object is achieved by means of the fact that the auxiliary transmission is constituted by apparatus as defined above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be well understood and its advantages appear more clearly on reading the following detailed description of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings, in which: 
         FIG. 1  is a generic diagram of a vehicle on which apparatus of the invention is mounted; 
         FIG. 2  is a timing diagram showing the various stages in implementing the apparatus of  FIG. 1 , as a function of time; 
         FIGS. 3 to 5  are diagrams showing the apparatus of  FIG. 1  in unassisted mode: 
         FIG. 3  is a diagram showing the apparatus of  FIG. 1  in freewheel mode and during the step of retracting the pistons; 
         FIG. 4  is a diagram showing the apparatus of  FIG. 1 , during the step of deploying the pistons; and 
         FIG. 5  is a diagram showing the apparatus of  FIG. 1 , in assisted mode; and 
         FIGS. 6A and 6B  are diagrammatic axial section views of the activation valve of the apparatus of  FIG. 1 . 
     
    
    
     MODE DETAILED DESCRIPTION 
       FIG. 1  shows a vehicle  10  on which apparatus  20  of the invention is mounted. 
     The vehicle  10  is a vehicle having four wheels, namely two rear wheels  12 A &amp;  12 B and two front wheels  14 A &amp;  14 B. The drive for the vehicle is delivered mainly by a main transmission  16 . Said main transmission includes a diesel engine  18  (“drive means” in the meaning of the invention) that is connected to the rear wheels  12 A &amp;  12 B and that drives the vehicle in a normal advance mode of said vehicle. 
     In addition, in order to ensure that the vehicle is propelled even under difficult road conditions (sloping or downhill road, slippery road, etc.), the vehicle also has an auxiliary transmission  20 . Said auxiliary transmission serves to drive the two wheels  14 A and  14 B that are not driven wheels while the vehicle is in normal advance mode. Thus, by means of the auxiliary transmission  20 , the vehicle has an assisted mode in which all four wheels  12 A,  12 B,  14 A and  14 B are driven wheels. 
     The auxiliary transmission  20  is coupled to a shaft  21  that is the power outlet of the engine  18 , from which it takes the power that it transmits to the wheels  14 A &amp;  14 B when it is activated. 
     The auxiliary transmission  20  is constituted by hydraulic transmission apparatus  22  that transmits a fraction of the power from the engine  18  to the wheels  14 A &amp;  14 B, and that also performs various additional functions: activation/deactivation of the auxiliary transmission, making the members of the auxiliary transmission safe during the deactivated mode, etc. 
     For driving the wheels  14 A,  14 B, the hydraulic transmission apparatus  22  firstly includes two pressurized fluid sources: a main pump  24 , and an auxiliary pump  25  for maintaining some minimum fluid pressure in various auxiliary ducts of the apparatus. 
     The main pump  24  is a reversible pump having a variable delivery rate and a swashplate. 
     Both the main pump  24  and the auxiliary pump  25  are connected to the outlet shaft  21  of the engine  18  directly, i.e. without any clutches. They can thus be actuated jointly by the engine  18 , i.e. both pumps can be driven at the same time. 
     The main pump  24  serves to feed pressurized fluid to two hydraulic motors  26 A,  26 B that are coupled to respective ones of the two wheels  14 A,  14 B. For this purpose, the pump  24  has two pump ducts  28 A,  28 B connected to respective ones of its main orifices  24 A,  24 B. 
     Said orifices can be put into communication with respective ones of two motor ducts  30 ,  32 . Each of said motor ducts has a first portion  301 ,  321  suitable for being connected to a pump duct, and a second portion in which it splits into two branches  302 A,  302 B,  322 A,  322 B connected to respective ones of feed and discharge enclosures of the motors  26 A and  26 B. 
     In a manner known per se, the motors  26 A,  26 B are hydraulic motors having radial pistons, each of which motors comprises a cylinder block in which cylinders are arranged that contain pistons. The pistons can be positively declutched to take up a retracted position in which they are retracted into the cylinders and in which they do not deliver any torque, or they can be positively clutched to take up a deployed position, in which they (partially) extend from the cylinders and come to bear on an undulating cam that transforms their radial forces into drive torque. Such motors are, for example, described in French Patent No. 2 504 987. 
     Each of the motors  26 A,  26 B has an outlet shaft coupled to a respective one of wheels  14 A &amp;  14 B. Under the effect of the pressure difference imposed by the main pump between the pump ducts, and thus between the motor ducts, in assisted mode, the motors  26 A and  26 B deliver drive torque (or braking torque) that enables them to drive the wheels  14 A &amp;  14 B. 
     An activation valve  34  is interposed between the pump ducts  28 A,  28 B and the motor ducts  30 ,  32 . Said activation valve has four “upstream” ports A, B, C, D, two “downstream” ports E and F, two positions I &amp; II, and two hydraulic control chambers  34 A &amp;  34 B. In this text the terms “upstream” and “downstream” as applied to the ports of a valve designate, in general, the most frequent direction of flow of the fluid, or of transmission of an instruction, without this excluding other operating modes. 
     Ports A and D are connected to pump duct  28 A. Port C is connected to pump duct  28 B. Ports E and F are connected to motor ducts  30  and  32 . Port B is connected to a pressure control valve  36 . 
     The activation valve  34  also has a return spring that tends to maintain it in its first position I. 
     In the first position I, port A is isolated, port B is connected to ports E and F, and ports C and D are interconnected. 
     In the second position II, ports B and D are isolated, ports A &amp; E are interconnected and ports C &amp; F are interconnected (ports A &amp; D remain interconnected and connected to pump duct  28 A). 
     Thus, in the first position I, pump ducts  28 A &amp;  28 B are interconnected (bypass position); the pump is then set to a delivery rate of zero. In addition, the motor ducts are interconnected, and their pressure is the pressure that is imposed on them by the pressure control valve  36 , in a manner described below. 
     Conversely, in the second position II, the motor ducts are connected to the pump ducts and they feed the motors  26 A &amp;  26 B so that they drive the wheels  14 A &amp;  14 B, which constitutes the assisted mode of the apparatus. 
     In addition, each of the motors  26 A &amp;  26 B has a casing  38 A,  38 B containing its cylinder block. An internal space is provided inside the casing, and is connected to a duct for casing duct leak return (references  40 A &amp;  40 B) connected to an unpressurized reservoir in a manner described in detail below. 
     The delivery orifice of the auxiliary pump  25  is connected to a boost duct  41 . 
     The boost duct  41  is connected to the pump ducts  28 A,  28 B via non-return or “check” valves  42 A,  42 B. This connection makes it possible to ensure that the pressure in the pump ducts remains at all times at the level of the boost pressure (pressure at the delivery orifice of the pump  25 ). 
     In addition, the duct  41  is connected to an unpressurized reservoir  46  that is at atmospheric pressure, via a pressure limiter  44  that prevents any excessive rise in pressure in the duct  41 . 
     Similarly, the pump ducts  28 A,  28 B are connected to the duct  41  via pressure limiters  48 A,  48 B, also in order to avoid any excessive pressure. 
     A “bypass” solenoid valve  50  is disposed on the boost duct. Said solenoid valve has two upstream ports A and B, two downstream ports C and D, and two positions I and II. 
     Port A is connected to an unpressurized reservoir  52  (which may the same as the reservoir  46 ). Port B is connected to an end of the boost duct  41 . Port C is connected to a “transmission” duct  54 , the purpose of which is described in more detail below. Port D is connected to a “bypass” duct  56 . 
     The bypass valve  50  also has a return spring that tends to maintain it in its first position I. 
     In the first position I, ports A and C are interconnected, and ports B and D are interconnected. 
     In the second position II, ports A and D are interconnected, and ports B and C are interconnected. 
     The first position I is a default position for the bypass valve  50  and is the “deactivated” position, corresponding to the “unassisted” mode (normal advance mode). In this position, the fluid coming from the booster pump is directed towards the boost duct  56 , and the transmission duct  54  is maintained at zero pressure (which, in this document, means atmospheric pressure). 
     The second position II of the bypass valve is the “activated” mode of the transmission apparatus  22 , corresponding to the “assisted” mode for the vehicle. In this second position, the bypass valve directs the flow from the booster pump towards the transmission duct  54 , which is an auxiliary duct of the apparatus  22 . 
     The bypass duct  56  has three portions, namely an upstream portion  561 , a middle portion  562 , and a downstream portion  563 . 
     The upstream portion  561  and the middle portion  562  are interconnected via a pressure limiter  58  and via a check, valve  60  that are mounted in parallel. The check valve is mounted in the direction that prevents the fluid from flowing towards the middle portion  562 . 
     The pressure limiter  58  is controlled by its upstream pressure, and makes it possible to maintain a minimum pressure in its upstream portion  561  that is chosen to be equal to 10 bars. 
     The middle portion  562  and the downstream portion  563  are interconnected via a calibrated valve  62 . Said calibrated valve guarantees that a minimum pressure is maintained in the middle portion that is chosen to be equal to 0.5 bars. 
     The boost duct  41  associated with the bypass duct  56  form a “bypass” connection  64 . Said bypass connection thus interconnects the delivery orifice of the auxiliary pump  25  and the unpressurized reservoir  52 . 
     As explained above, the boost duct  41  is connected to the pump ducts  28 A and  28 B (via the valves  42 A,  42 B). 
     Thus in assisted mode (valve  50  in position I), the pressure of 10 bars (more precisely 10.5 bars) maintained by the pressure limiter  58  in the upstream portion of the bypass connection (uniting the boost duct  41  and the upstream portion  561  of the bypass duct) applies in the pump ducts; as a result, the pump is protected. The pressure maintained by the limiter  58  is thus a “pump protection” pressure, and the upstream portion of the bypass link is thus a “pump protection” portion. 
     The middle portion  562  of the bypass duct  56  is connected via a duct  66  to the casing ducts  40 A and  40 B. Thus, the pressure in the casing ducts remains at all times no more than 0.5 bars (calibration pressure of the valve  62 ). However, in transient manner, the pressure can increase to a greater extent in said middle portion  562  since the maximum flow rate that the valve  62  can remove is relatively limited. This property is used in the circuit in a manner that is described in detail below. 
     The activation valve  34  is controlled by a first pilot valve  68 . 
     Said first pilot valve is a solenoid valve having two upstream ports A &amp; B, and two downstream ports C &amp; D, and two positions I &amp; II. 
     Port A is connected to the transmission duct  54 . It can be understood that said transmission duct transmits a boost pressure that the first pilot valve  68  can use to control the activation valve. Port B is connected to a removal duct  70  that is itself connected to the unpressurized reservoir  52 . Port C is connected to the hydraulic chamber  34 B of the valve  34 , the increase in the pressure of which chamber tends to cause the activation valve  34  to go into its first position (unassisted mode); port D is connected to the other chamber  34 A of the valve  34 , the increase in the pressure of which chamber tends, conversely, to cause the valve  34  to go into its second position (assisted mode). 
     The first pilot valve  68  also has a return spring that tends to maintain it in the first position I. 
     In the first position I, ports A and C are interconnected, and ports B and D are interconnected; and, in the second position II, ports A and D are interconnected, and ports B and C are interconnected. 
     Since the first pilot value  68  is situated downstream from the bypass valve  50 , it is active only when the bypass valve is in position II, and thus only when the boost pressure is prevailing in the transmission duct  54 . 
     Under these conditions, action on the valve  68  makes it possible to cause the activation valve to go into position I or into position II, depending on whether the pilot valve is itself placed in position I or in position II. When the first pilot valve  68  is in position I, the boost pressure is transmitted into the chamber  34 A, and zero pressure is maintained in the chamber  34 B, so that the activation valve  34  is placed in position I (unassisted mode); and vice versa. 
     The apparatus  22  also has a second pilot valve  72  that controls the pressure control valve  36 . 
     Firstly, the arrangement of the hydraulic valve  36  is specified below. This valve has two upstream ports A and B, one downstream port C, and a hydraulic control chamber  361 . Port C of the valve  36  is connected to port B of the activation valve  34 . 
     The valve  36  has two positions I and II. 
     In the first position I, the ports A and C are interconnected, and port B is isolated. In the second position II, ports B and C are interconnected, and port A is isolated. 
     The valve  36  also has a return spring that tends to maintain it in the first position I. 
     The second pilot valve  72  is a solenoid valve having two upstream ports A and B, and one downstream port C. 
     Ports A and B are connected respectively to the transmission duct  54  and to the removal duct  70 . Port C is connected to the hydraulic control chamber  361  of the pressure control valve  36 . 
     The second pilot valve  72  also has a return spring that tends to maintain it in the first position I. 
     In its first position I, the second pilot valve  72  interconnects ports B and C, A remaining isolated. In the second position II, the second pilot valve  72  interconnects A and C, B remaining isolated. 
     Like for the first pilot valve, the valves  36  and  72  are active only during the assistance stages, i.e., in this example, when the bypass valve  50  is placed in position II. The boost pressure then prevails in the transmission duct  54  (connected to port A of the valve  72 ), while zero pressure prevails in the removal duct  70  (connected to port B of the valve  72 ). 
     Under these conditions, the valve  72  makes it possible to apply either the boost pressure or a zero pressure to the hydraulic control chamber  361  of the valve  36 , depending on whether it is placed in its first or in its second position. The pressure in the chamber  361  constrains the valve  36  to take up its first position I if the valve  72  is in its first position I, or to take up its second position if the valve  72  is in its second position II. 
     This arrangement thus makes it possible to select the pressure that it is desired to apply to port B of the activation valve  34 . When said activation valve is in its first position I, the pressure in port B is transmitted to the motor ducts  30 ,  32  (conversely, when the activation valve is in the second position II, port B is isolated). 
     Finally, the apparatus  22  has an additional pressurized fluid source, namely a fluid accumulator  74 , operation of which is regulated by an accumulator valve  76 , disposed on an accumulator duct that connects the accumulator  74  to the remainder of the apparatus. 
     The accumulator valve  76  has an upstream port A, two downstream ports B and C, and a hydraulic control chamber  761 . Port A is connected to the accumulator. Port B is connected to the middle portion  562  of the bypass duct  56 , via the duct  66 . Port C is connected to the transmission duct  54 . 
     The accumulator valve  76  can take up a first position I, in which the valves A and C are interconnected, and the port B is isolated; and a second position II, in which the ports A and B are interconnected, and the port C is isolated. 
     The accumulator valve  76  has a return spring that tends to urge it to return to its first position I. 
     The control chamber  761  of the accumulator valve  76  is connected to the upstream portion  561  of the bypass duct  56 . As a result of this connection:
         If the bypass valve  50  is in position I (unassisted mode), the boost pressure (the value of which in this mode is set at a “pump protection” value) applies in the hydraulic chamber  761  and thus, the accumulator valve is placed in position II, in which the accumulator is connected to the middle portion of the bypass connection, maintained at a pressure of 0.5 bars (motor protection pressure). Thus, the accumulator is not really put under pressure and is in no way dangerous;   Conversely, if the bypass valve  50  is in position II (assisted mode), zero pressure is applied to the hydraulic chamber  761 . The accumulator valve is placed in position I, i.e. the accumulator is connected to the transmission duct  54 .       

     As a result of the connection between the port D of the bypass valve and the hydraulic control chamber  761 , the accumulator valve and the bypass valve are coupled together, and the position of the bypass valve imposes its position on the accumulator valve. 
     Operation of the hydraulic transmission apparatus  22  is described below. 
     The hydraulic transmission apparatus  22  is used by complying with certain stages marked mainly by activation or deactivation actions performed on the various valves. The sequence of going from the unassisted mode to the assisted mode, and vice versa, is shown in  FIG. 2 . Certain significant steps in this sequence are shown in  FIGS. 3 to 5 . 
     Sequencing of the various steps is controlled automatically by an electronic control unit (ECU) that is not shown, on the basis of the assistance activation request (or of the assistance deactivation request) by the driver of the vehicle. The ECU is also suitable for taking certain protective measures for protecting the hydraulic transmission apparatus  22 , while certain actions are being taken by the driver, such as gear-changing or braking, as described below. 
       FIG. 2  shows, in particular, and as a function of time, the positions taken up by the first bypass valve  50 , and by the first and second pilot valves  21  and  23 . Underneath those curves lies a curve showing the variations in the cylinder capacity D 24  of the main pump  24 . 
     Time is plotted along the abscissa, with, in particular, significant times t 0  to t 11 . The timing diagram shows a fictitious scenario for use of the apparatus  22 , in which scenario, starting from the unassisted mode, at a time to, an assistance activation instruction is issued by the driver of the vehicle. An assistance preparation first stage takes place from t 0  to t 4 . As from time t 4 , assistance is active until time t 7 . However, the scenario in  FIG. 2  makes provision for a momentary assistance declutching stage from time t 5  to t 6 . Finally, at a time t 7 , an assistance disengagement request is made by the driver, and the hydraulic assistance apparatus goes from the assisted mode to the unassisted mode from t 7  to t 11 . 
     Unassisted Mode ( FIG. 3 ) 
     In this mode, the bypass valve is in position I, and the fluid delivered by the booster pump  25  flows through the bypass connection  64  to return to the reservoir  52 , thereby following a relatively short circuit, without using the various valves of the apparatus  22  (except for the bypass valve  50 ). The accumulator valve  76  is in position II. The accumulator empties via the valve  62  of the bypass connection. The activation valve  34 , the pilot valves  68  and  72 , and the pressure control valve  36  are in position I. The pump protection pressure prevails in the upstream portion of the bypass connection  64 , and therefore in the pump ducts, thereby making it possible to protect the main pump  24 . Said main pump is being driven by the engine  18 , but is not delivering. 
     Since the pump protection pressure is low (10 bars), the power consumption by the auxiliary pump or booster pump  25  remains very low. 
     The casing ducts  40 A and  40 B are connected to the middle portion  562  of the bypass duct, in which a pressure of 0.5 bars prevails. This pressure is sufficient to guarantee that the pistons remain in the retracted positions inside the cylinders (and is chosen for the purpose). 
     Pass to Assisted Mode 
     Activate the Bypass Valve 
     At time t 0 , the driver issues an assistance instruction for putting the transmission of the vehicle in assisted mode, i.e. for activating the auxiliary transmission  20 . 
     After a brief interval, at time t 1 , the ECU activates the bypass valve  50  and places it in position II. 
     The accumulator valve  76  reacts immediately and goes from position II to position I. The accumulator is thus connected to the booster pump via the boost duct and via the transmission duct  54 . It fills and rapidly reaches the pressure delivered by the booster pump. Conversely, the casing ducts  40 A,  40 B empty into the reservoir  52 , via the valve  60  and via the valve  50 . 
     Synchronize the Pump and Deploy the Pistons ( FIG. 4 ) 
     The accumulator is full at time t 2 . The ECU then, at time t 3 , triggers synchronization of the main pump  24 , by progressively increasing the cylinder capacity of the pump, to a target value at which the cylinder capacity is stabilized. This target value is determined by the ECU in such a manner that the pump drives the motors  26 A and  26 B at a speed of rotation equal to the speed of rotation of the wheels of the vehicle. The instant t 3  is determined by the ECU either after a set lapse of time after t 2  (0.5 seconds), or by a pressure reached on a pressure switch. 
     The pistons, which were previously retracted into the cylinders, are then deployed from their cylinders at time t 4 , and placed in the working position, i.e. in contact with the cam. This operation is triggered by activating the second pilot valve  72  that passes to position II. As a result, the boost pressure applies in the hydraulic chamber  361 , thereby causing the pressure control valve  36  to pass to position II. As a result, the pressure applied in the port B of the activation valve  34 , and therefore in the motor ducts  30 ,  32  rises, and reaches the boost pressure. 
     Under the effect of this pressure, and since, conversely, the casing ducts  40 A,  40 B remain at zero pressure, the pistons extend from their cylinders, thereby enabling the rotors of the motors to turn at the same speed as the wheels, the delivery rate of the pump adapting to said speed. At this stage, the motors do not, however, deliver any torque. 
     Simultaneously with the pistons being deployed out of the cylinders, the casing ducts  40 A,  40 B that are connected to the middle portion  562  of the bypass duct  56  remove the fluid from the internal spaces of the motors  26 A,  26 B via the check valve  60 , towards the reservoir  52 . 
     Advantageously, during this stage, the accumulator  74  is put in line at the same time as the booster pump  25  so as to deliver the fluid and so as to fill the motor ducts  30  and  32 . The accumulator thus facilitates and accelerates filling the motor ducts and bringing them up to pressure, and thus makes it possible for the motors  26 A,  26 B to be positively clutched quickly. 
     Activate the Assistance Motors ( FIG. 5 ) 
     Either slightly after the step of synchronizing the pump, or simultaneously as shown in  FIG. 2 , the assistance motors are activated by placing the first pilot valve  68  in the second position II. This causes the activation valve  34  to go to position II and the respective pressures of the pump ducts  28 A,  28 B thus apply in the motor ducts  30 ,  32 . Under the effect of the pressure difference between these ducts, the motors  26 A and  26 B are thus caused to operate and they drive the wheels  14 A and  14 B which thus become driven wheels. 
     The delivery rate of the pump is then progressively increased, as the speed of the vehicle increases. 
     During the assistance stage, the accumulator  74  is connected to the transmission duct  54  and remains subjected to the boost pressure. 
     Temporarily Deactivate the Assistance 
     If, during an assistance stage, it is necessary, for example, to brake or to change gear (on the motors  26 A,  26 B), the assistance apparatus  22  is deactivated temporarily during this stage. 
     This operation is performed (see  FIG. 2 ) from times t 5  to t 6 , during which period the second pilot valve  68  is placed temporarily in position I (deactivated position). As a result, during this period, the activation valve  34  also returns to position I, and thus the pressure balances out between the motor ducts. The motors  26 A and  26 B then, temporarily, no longer deliver any drive torque or any braking torque. 
     Return to Unassisted Mode 
     The return to unassisted mode takes place as follows: 
     At an instant t 7 , the driver of the vehicle transmits a return-to-unassisted-mode instruction to the ECU. Said ECU progressively reduces the delivery rate of the main pump  24 , in order to reduce the assistance pressure to the minimum (to 30 bars) so as to reduce any jolts in the assistance motors. 
     Then, after a predetermined lapse of time, the motors  26 A,  26 B are deactivated by causing the first pilot valve  68  to go back to position I, thereby causing the activation valve  34  to go to position I. The cylinder capacity of the pump is then maintained constant for a short lapse of time (from times t 8  to t 9 ), and then the reduction in the delivery rate of the pump is resumed from times t 9  to t 10 , whereupon the pump returns to zero delivery rate. 
     At this time t 10 , the second pilot valve  72  is deactivated, which enables the fluid contained in the motor ducts to be removed towards the reservoir  52 , and enables the pressure in them to be caused to drop. 
     Then, the bypass valve  50  itself is deactivated at time t 11 . The valves are then in the positions shown by  FIG. 3 . However, initially the flows of fluid are totally different from the unassisted mode, in particular because the accumulator is initially under pressure (at the boost pressure). 
     The fluid delivered by the booster pump is, once again, directed towards the bypass duct  56 . In addition, deactivation of the valve  50  causes the accumulator valve  76  to pass immediately to position II. The accumulator is then put into communication firstly with the casing ducts  40 A,  40 B via the duct  66 , and secondly with the reservoir  52 , via the calibrated valve  62 . 
     The flow-rate of fluid that can pass through said valve  62  is, however, limited. Thus, when the accumulator  74  is put into communication with the duct  66 , most of the fluid that it contains is removed towards the reservoir  52 , but during the initial moments after the accumulator  74  and the duct  66  have been put into communication with each other, a non-negligible pressure is established in these ducts. This pressure is transmitted to the internal spaces of the motors  26 A,  26 B via the casing ducts  40 A,  40 B. Thus, the accumulator being put into communication makes it possible to accelerate retraction of the pistons into the cylinders. 
     At the end of these operations, the vehicle finds itself in unassisted mode, and the wheels  14 A and  14 B can turn freely, as in the initial situation. 
     It should be noted that it is also possible firstly to deactivate the bypass valve  50 , and then to deactivate the second pilot valve  72 . The choice of such a sequence facilitates rapid retraction of the pistons. 
     The arrangement of the valve  34  is described below with reference to  FIGS. 6A and 6B . 
     This valve has a body  100  inside which a substantially cylindrical bore  102  is arranged along an axis X. A slide  104  that is substantially in the shape of a cylindrical segment is slidably received in the bore. Six circumferential grooves G 1 -G 6  are also formed in the bore; two wide circumferential grooves H 1 -H 2  are formed in the outside surface of the slide. 
     The valve  34  has four upstream ports A, B, C, and D, two of which (A and D) coincide in practice because they are interconnected via an internal duct  106 . Port A-D is connected to the grooves G 1  and G 6 ; port B is connected to groove G 2  via an internal duct, and port C is connected to groove G 4  via an internal duct. 
     The valve  34  has two downstream ports E and F. Port E is connected to groove G 5  via an internal duct, and port F is connected to groove G 3  via an internal duct. The slide is designed to take up two positions I and II. 
     In position I, i.e. in the unassisted position ( FIG. 6A ), the slide  104  is positioned on the same side as groove G 1  (rather than on the same side as groove G 6  along the axis X). In this position, groove H 1  puts grooves G 1  and G 2  into communication with each other, and groove H 2  puts grooves G 3 , G 4 , and G 5  into communication with one another. 
     In position II, i.e. in the assisted mode ( FIG. 6B ), the slide  104  is positioned on the same side as the groove G 6 . In this position, groove H 1  puts grooves G 2  and G 3  into communication with each other; groove H 2  puts grooves G 5  and G 6  into communication with each other.