Patent Publication Number: US-8991341-B2

Title: Valve actuation mechanism and automotive vehicle comprising such a valve actuation mechanism

Description:
BACKGROUND AND SUMMARY 
     The invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle. The invention also concerns an automotive vehicle, such as a truck, equipped with such a valve actuation mechanism. 
     Automotive vehicles, such as trucks, often rely on an engine brake function to slow down in order, for example, to reduce wear of the friction brake pads and to prevent overheating of the friction brakes, particularly on downward slopes. It is known to perform engine brake by acting on the amount of gas present in the cylinders of the engine in two distinct phases. In a first phase, when the pistons are near a bottom dead center, one injects exhaust gases into the chambers of the cylinders so as to slow down the pistons when they move towards their high level. This is done by slightly opening at least a valve connected to an exhaust manifold, while exhaust gases are prevented to be expelled from the exhaust pipe and thereby at a certain pressure above atmospheric pressure. In the second phase, the gases which are compressed the piston are expelled from the chamber of the cylinder when the piston is at or near its top dead center position in order to prevent an acceleration of the piston under effect of volcanic expansion of compressed gas, this is done by slightly opening a valve so as to expel gases from the cylinder. In most cases, the valve (or valves) which is (are) opened for the engine brake function is (are) a main exhaust valve. An engine brake system is described in document WO 9009514. 
     To perform these engine brake valves movements, also called engine brake valves lifts, the engine comprises, for each cylinder, a rocker acting, on the valves to open and close them. The rocker is acted upon by a rotating cam which has at least one lift sector to cause the lifting (opening) of the valve. If the valve is also an exhaust or an intake valve, the corresponding cam will comprise a main valve lift sector and one or several auxiliary valve lift sectors (also called main valve lift bum  When engine brake is needed, a cam follower surface of the rocker is moved in close contact with a cam of a camshaft moving the rocker so that the brake movements of the valve are obtained, when the cam follower interacts with the auxiliary valve lift sectors. In normal operating conditions of the engine, the valves should not perform these movements and the roller of the rocker is kept slightly remote from the cam so that the cam follower does not interact with the auxiliary valve lift sectors. The distance or clearance between the roller and the earn ensures that only the larger main lift sector on the cam, dedicated to the main exhaust event, causes an opening of the exhaust valve, but not one or several smaller auxiliary lift sectors dedicated to the engine brake function. This clearance is suppressed when engine brake is needed, by moving an activation piston of the rocker to make a close contact between the roller and the cam, so that engine brake dedicated lift sectors on the cam also cause an opening of the valve. An engine brake system having such valve actuation mechanism is described in WO-91/08381 
     In the case of a system where two valves are to be actuated, the piston can be in contact with the valves through a valve bridge. 
     When the engine brake valve opening(s) have been performed, a reset function is preferably to be performed. In other words, the activation piston needs to be moved towards its initial position in order to ensure that the valves are closed early enough in order to prevent extended valve lift overlap. 
     Engine brake systems generally comprise a control valve to direct pressurized control fluid pressure in a chamber adjacent to the piston to move the activation piston from its initial position to its engine brake actuation position. The control valve controls whether or not the engine brake function is activated. This control valve lets pressurized, control fluid flow, at a pressure of for example 2 to 5 bars, towards each rocker as long as the engine brake function is needed, which typically lasts several seconds or tens of seconds during which the engine and the cam shaft may perform several hundreds or thousands of complete revolutions. In some systems, a check valve is provided to prevent any fluid flow out of the chamber. In some known systems, such as the one described in WO-91/08381, the check valve can nevertheless be forced to an open position, allowing the control fluid to escape the chamber when the engine brake is not needed. This is achieved when no control pressure is sent to the control valve. In known systems, there is only one control valve for several cylinders, so that it is not possible to use the control valve to empty the chamber to allow retraction of the piston, if such retraction is needed for a period of time inferior to one revolution of the camshaft. 
     It is known, for example from U.S. Pat. No. 6,253,730, to act on the check valve thanks to a stopper which is fixed to a housing, of the engine, so as to open the check valve and release fluid pressure in the chamber so that the piston may move towards its initial position, retracted. This technical solution is not applicable in the case of a so-called “single valve engine braking” where the additional valve lift opening are performed with only one of two exhaust vales is opened for performing engine braking. Indeed, the stopper has to be positioned with respect to the rocker so that it forces the check valve to an open position for a valve lift value superior to the additional valve lift value, but allows the check valve to close again at the same valve lift value when the valves are closing, allowing the actuation piston to be extended again, which delays the valve closing. 
     The aim of the invention is to provide a valve actuation mechanism in which the fluid pressure in the piston chamber can be reduced with satisfying time accuracy and relatively low forces. 
     To this end, the invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle, comprising rockers moved by a camshaft, each rocker being adapted to exert a valve opening force on at least a portion of a opening actuator of each cylinder, via an activation piston of the rocker movable with respect to the rocker under action of a fluid pressure raise in a chamber, from a first position to a second position, in which a cam follower of the rocker is adapted to read at least one auxiliary valve lift sector of a cam of the camshaft so as to perform an engine operating function, each rocker comprising a valve for releasing fluid from the chamber, wherein the valve actuation mechanism comprises, for each rocker, a stopper fast with a housing of the engine and adapted to exert, on a member of the rocker, a variable force for opening, the fluid releasing valve. 
     According to further aspects of the invention which are advantageous but not compulsory, such a valve actuation mechanism may incorporate one or several of the following features:
         the variable force increases when the rocker rotates from a valve closing position to its valve opening position;   the stopper causes opening of the fluid releasing valve for a first position of the rocker and allows closing of the fluid releasing valve for a second position of the rocker, said second position being closer to the valve closing position of the rocker than said first position;   the stopper comprises elastic means which are stressed when the rocker travels from its valve closing position to its valve opening position;   the stopper comprises a spring adapted, when deformed, to exert a compression force on said member;   the stopper comprises a mobile contact element biased by the spring and adapted to cooperate with said member, the contact element and the spring are movable in translation with respect to a jacket in which the contact element and the main spring are housed, said jacket being fast with said engine housing.   the jacket comprises a stop element against which the contact element comes in abutment when the piston has to be moved from its second position to its first position;   the elastic means of the stopper have a variable stiffness;   the stopper comprises a main spring and an auxiliary spring, wherein, during a first portion of the rocker travel from a valve closing to a valve opining position, only the auxiliary spring is stressed, and wherein during a second portion of the rocker travel, the main spring is stressed;   the stopper is in permanent contact with the member of the rocker on which the force of the stopper is exerted;   prior to the exertion of the force of the stopper on the member of the rocker, the stopper is remote from the member by a clearance;       

     the force exerted by the stopper on said member is adapted to overcome a force keeping said valve in a closed position only when the piston has to be moved from its second position to its first position;
         for each rocker, the member on which the force of the stopper is exerted cooperates with a check valve adapted to allow fluid flow from a fluid feeding, circuit of the rocker to the chamber or to block fluid flow from the chamber to the fluid feeding circuit, said check valve forming the valve for releasing fluid from the chamber.   for each rocker, the member on which the force of the stopper is exerted cooperates with a reset valve, movable with respect to the rocker, between a first position, in which it blocks fluid flow between the chamber and the outside of the rocker, and a second position, in which it allows fluid flow between the chamber and the outside of the rocker, said reset valve forming the valve for releasing fluid from the chamber;   the fluid releasing valve is adapted to allow fluid flow from the chamber to the outside of the rocker, wherein the piston) comprises:   a first element housed in the bore and movable in translation with respect to the rocker,   and a central member housed in a portion of the first element and movable in translation with respect to the first element along a longitudinal axis of the piston,   wherein the fluid releasing valve is formed by a cooperation between the first element and the central member, and wherein the force of the stopper is exerted on the first element.   the valve for releasing fluid from the chamber is kept in its closed position by a fluid pressure force in a chamber fluidly connected to the piston chamber;   each rocker comprises a normally closed discharge valve which is opened by the fluid pressure in the chamber when such pressure exceeds a predetermined threshold to allow fluid flow out of the chamber, said discharge valve forming the valve for releasing fluid from the chamber, and wherein the member on which the force of the stopper is exerted is the piston;   the discharge valve is carried by the piston;   the valve for reducing fluid pressure in the chamber is biased towards its closed position by a spring;   the valve actuation mechanism is one of:   an exhaust valve actuation mechanism:   wherein the activation piston ( 95 ) activates an exhaust gases recirculation function when it is in its second position; or   wherein the activation piston ( 95 ) activates an engine brake function when it is in its second position; or   an intake valve actuation mechanism.       

     The invention also concerns an automotive vehicle, such as a truck, comprising a valve actuation mechanism as mentioned here-above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in correspondence with the annexed figures, as an illustrative example. In the annexed figures: 
         FIG. 1  is a side view of a valve actuation according to a first embodiment of the invention; 
         FIG. 2  is a sectional view, along plane II on  FIG. 1 , of a portion of the valve actuation mechanism of  FIG. 1 , in a first configuration; 
         FIGS. 3 and 4  are sectional views corresponding to the right part of  FIG. 2 , for a second and third configuration 
         FIGS. 5 and 6  are sectional views similar to  FIG. 2 , of a valve actuation mechanism according to a second and a third embodiments of the invention; 
         FIG. 7  is a perspective view of a rocker belonging to a valve actuation mechanism according to a fourth embodiment of the invention; 
         FIG. 8  is a sectional view along plane VIII on  FIG. 7 , of the valve actuation mechanism of  FIG. 7 ; 
         FIG. 9  is a schematic partial sectional view of a valve actuation mechanism according to a filth embodiment of the invention; 
         FIGS. 10 and 11  are schematic partial sectional views of a valve actuation mechanism according to a sixth embodiment of the invention, represented in two configurations 
     
    
    
     DETAILED DESCRIPTION 
     The valve actuation mechanism S of the invention, represented on  FIGS. 1 to 4 , comprises a camshaft  2  rotatable around a longitudinal axis X 2 . Camshaft  2  comprises several cams  22 , each being dedicated to moving the valves of one cylinder of an internal combustion engine E, of a non represented automotive vehicle, such as a truck, on which valve actuation mechanism S is integrated. Each cam has a earn profile which may comprise one or several “bumps”, i.e. valve lift sectors where the cam profile exhibits a bigger eccentricity with respect to axis X 2  than the base radius of the cam.  FIG. 1  shows a portion of valve actuation mechanism S corresponding to one cylinder of the engine. 
     In this embodiment, each cylinder of engine E, is equipped with two exhaust valves  4  and  5 . Valves  4  and  5  are biased towards their dosed position by respective springs  41  and  51 . Each valve  4  and  5  is movable in translation along an opening axis X 4  or X 5  so as to be opened, or lifted. More precisely, translation of valves  4  and  5  opens a passageway between the combustion chamber of the cylinder and an exhaust manifold. Valves  4  and  5  are connected to a valve bridge  7 , which forms a valve opening actuator, and which extends substantially perpendicular to axes X 4  and X 5 . In this embodiment, only one valve  4  is opened to perform the engine brake function. This technology called “single valve engine brake” permits to reduce forces excited on the valves, in order to improve the reliability of valve actuation mechanism S. The valve bridge  7  comprises a main portion  72 , which causes opening of valve  5 . Valve bridge  7  also comprises a slider block  71  which is movable with respect to main portion  72  of valve bridge  7  along opening axis X 4  of valve  4 . Slider block  71  is connected to valve  4  so as to be able to cause its opening. Consequently, valve  4  is also movable with respect to main portion  72  of valve bridge  7  along axis X 4 . 
     Valves  4  and  5  are partly represented on the figures, only their respective stems are visible. 
     For each cylinder, the transmission of movement between camshaft  2  and valve bridge  7  is performed by a rocker  9  rotatable with respect to a rocker shaft  91  defining a rocker rotation axis X 91 . Only one rocker  9  is represented on the figures. Each rocker  9  comprises a roller  93  which acts as a cam follower and cooperates with a cam  22 . Roller  93  is located on one side of rocker  9  which respect to shaft  91 . Each rocker  9  comprises, opposite to roller  93  with respect to shaft  91 , an activation piston  95  adapted to exert a valve opening force F 9  on the slider block  71  of valve bridge  7 , which is connected to valve  4 , for example merely by being in contact with the valve stem. 
     Rocker  9  further comprises a finger  121  substantially parallel to piston  95 , and centered on an axis X 121 . d 95  denotes the distance between axes X 91  and X 95 . d 121  denotes the distance between axes X 91  and X 121 . Distance d 121  is larger than distance d 95 . Piston  95  is arranged in rocker  9  so that it cooperates with slider block  71 , while finger  121  is adapted to cooperate with the main portion  72  of valve bridge  7 . It can be noted that the plane defined by the axes X 4 , X 5  of the valves is perpendicular to the rotation axis X 91  of the rocker  9 . Valve  5  is further away from the rocker rotation axis than valve  4 . 
     Rotation of camshaft  2  transmits, when the roller runs against a valve lift sector of the cam, a rotation movement R 1  to rocker  9  via roller  93 , this rotation movement inducing a translation movement of main portion  72  of valve bridge  7  and of slider block  71 , respectively due to finger  121  and to activation piston  95 , along an axis X 7  which is parallel to axes X 4  and X 5 . Cooperation between a main valve lift sector of cam  22  and roller  93 , on the one hand, and between piston  95  and slider block  71  and between finger  121  and main portion  72  of valve bridge  7 , on the other hand, generates exhaust openings of valves  4  and  5  during the corresponding operating phase of internal combustion engine E. The rocker has an alternate rotation movement and can therefore rotate between a valve closing position, and a valve opening position, depending on the cam profile. 
     In the shown embodiment, rocker shaft  91  is hollow and defines a duct  911  which houses fluid circuit coming from a non-shown fluid tank of valve actuation mechanism S. Rocker  9  comprises itself an internal fluid circuit which connects duct  911  to a piston chamber  101  of rocker  9 , partly delimited by piston  95 , via a check valve  97 . Activation piston  95  is housed in a bore  94  of rocker  9  and adapted to move with respect to chamber  101  along a translation axis X 95  corresponding, to a longitudinal axis of piston  95 . A duct U  27  partly-shown on  FIG. 2 , connects duct  911  to check valve  97 . A duct  913  fluidly connects check valve  97  to piston chamber  101 . 
     When the engine switches to engine brake mode, a non shown engine brake control valve delivers pressurized fluid to ducts  911  and  912 , which entails that pressurized fluid flows though check valve  97  in piston chamber  101 . The pressure raise in chamber  101  induces a translation movement of piston  95  outwardly with respect to rocker  9 , from a first position, in which piston  95  is entirely or partially pushed back into chamber  101  to a second position, in which piston  95  is partially moved out of piston chamber  101  until it comes in abutment against slider block  71 . Preferably, the control fluid is a substantially incompressible fluid such as oil. 
     Cam  22  comprises in this embodiment two auxiliary valve lift sectors which are adapted to cooperate with roller  93 . These sectors induce, when read by roller  93  of rocker  9 , two additional pivoting movements of rocker  9  on each turn of camshaft  2 . The auxiliary lift sectors are usually designed to cause only a limited lift of the valve, as they are not intended to allow a great flow of gases through the valve. Typically, the lift caused by the auxiliary valve lift sectors is less than 30 percent of the maximum valve lift value. These pivoting movements are transformed by piston  95  into two opening movements of valve  4  so as to perform an engine brake function at two precise moments during operation of engine E as described briefly above. The purpose and effects of these valve openings are well-known and will not be further described hereafter. According to an alternate embodiment, cam  22  comprises only one auxiliary valve lift sector fix performing only one opening of valve  4  on each turn of camshaft  2 , in addition to the main exhaust valve opening. 
     When piston  95  is in its first position, retracted, as shown on  FIG. 2 , roller  93  is offset with respect to the auxiliary valve lift sectors of cam  22  by an engine brake actuation clearance, so that when camshaft  2  rotates around axis X 2 , cam  22  does not come in contact with roller  93 , or piston  95  does not come in contact with slider block  71 . The clearance is such that the auxiliary valve lift sectors cannot cause the opening of valve  4 , because the rotation of the rocker induced by the auxiliary valve lift sectors is too limited to compensate for the clearance. To the contrary, a main valve lift sector causes a displacement of the rocker  9  around its axis which is sufficient to cause opening of both valves. 
     By moving piston  95  to its second position, extended, as shown on  FIG. 4 , rocker pivots around the longitudinal axis X 91  of shaft  91 . Thus, the actuation clearance is suppressed and roller  93  comes into contact with the auxiliary valve lift sectors of cam  22 , while the activation piston s simultaneously in contact or quasi contact with the slider block  71 , allowing engine brake operations to be implemented when the roller  93  is acted upon by any one of the auxiliary valve lifts. 
     Normal exhaust openings of valves  4  and  5  during engine brake operations are implemented as follows. Piston  95  is first moved towards its second position, so that, when a rotation of rocker  9  along arrow R 1  starts, the system causes the opening, of only valve  4  when the cam follower reads the additional valve lift sectors. Those sectors do not cause opening of valve  5 . When the auxiliary valve lift sectors have been read by roller  93 , roller  93  begins to read a main valve lift sector  220 , inducing a rotation of rocker  9  sufficient to generate a contact between finger  121  and main portion  72 . From this moment on, the main portion  72  of valve bridge  7  is moved and opening of valve  5  begins, in parallel to the movement of valve  4 . 
     At a further rotation angle of rocker  9 , because piston  95  abuts against a non-shown stop of bore  94  which limits its motion outside rocker  9 , contact is lost between piston  95  and slider block  71 . From this moment on, main portion  72  cooperates with slider block  71  thanks to a stop  720  which cooperates with a shoulder  711  of slider block  71 . Slider block  71 , and also valve  4 , become integral in translational movement with main portion  72 , until the opening of valves  4  and  5  is complete. 
     When valves  4  and  5  return to their closed position, movement of bridge  7  is performed exactly in the opposite manner compared to the opening movement until contact is made again between piston  95  and slider block  71 . An elastic force is therefore exerted on piston  95  by spring  41  via slider block  71 , provoking a pressure raise in chamber  101 , which is closed at this moment. The fluid in chamber  101  blocks the motion of piston  95  towards its first position. Therefore, absent the invention, the valve  4  would close later than valve  5 . This would provoke extended valve overlapping, which reduces the efficiency of the engine brake function. 
     According to a variant of the invention, piston  95  may be adapted to activate or deactivate an internal, exhaust gases recirculation function. This function allows an exhaust valve opening during the intake stroke. By returning a controlled amount of exhaust gas to the combustion process, peak combustion temperatures are lowered. This will reduce the formation of Nitrogen oxides (NOx). 
     According to a non-shown embodiment of the invention, valve actuation mechanism S may be an intake valve actuation mechanism for moving two intake valves adapted to open passageway between the combustion chamber of the cylinder and an intake manifold. In this case, the activation piston may be adapted to activate or deactivate an intake function based on early or late Miller cycle (Atkinson) which are well known and not further described hereafter. 
     In the first embodiment of the invention represented on  FIGS. 1 to 4 , check valve  97  comprises a ball  970  which is kept, by a compression spring  972 , against a seat  974 . Bali  970 , spring  972  and seat  974  are arranged in a check valve chamber  976  realized in rocker  9 . Chamber  976  has a cylindrical form centered around a longitudinal axis X 97 . Chamber  976  is fluidly connected to piston chamber  101  via duct  913 . Ball  970  is movable along axis X 97  with respect to seat element  974 . Fluid pressure in the chamber  976 , and thus in chamber  101  tends to push the ball  970 , which acts as a plug member for the valve, on the valve seat  974 , thereby closing the valve. 
     Duct  911  of rocker shaft  91  is connected, via duct  912 , to a first chamber  915  realized in rocker  9 . First chamber  915  is connected to check valve chamber  976  through seat  974 . First chamber  915  is opposite the check valve chamber  976  with respect to the seat, so that fluid pressure in the first chamber  915  tends to push the ball away from the seat, thereby opening the check valve. A check valve actuation member  978  is housed in chamber  915 , also for forcing the opening of the valve. Actuation member  978  is movable with respect to chamber  915 , which has a cylindrical form, along axis X 97 . Actuation member  978  comprises an outer sleeve  9780 . Actuation member  978  further comprises a pushing pin extending along axis X 97  and adapted to make a contact with ball  970 . A further spring is provided to act on the actuation member  978  so as to push it in the direction in which it forces the ball  970  off the seat  974 , thereby forcing the opening of the check valve. When thud pressure is delivered to chamber  915  though duct  912 , which is controlled by the non shown engine brake control valve, the actuation member is pushed against the action of the spring, so as not to interfere anymore with the ball  970 , which can therefore open and close as a normal check valve, essentially based on the pressure differential on both sides of the valve. Actuation member  978  also comprises a central pin  9784  extending along axis X 97  opposite to pushing pin  9782 . Central pin  9784  extends in the vicinity of an end of chamber  915  which opens by a hole  917 , on the outside of rocker  9 . 
     According to the invention, a stopper  13  is provided which is fast with a housing of the engine F and adapted to exert, on a member of the rocker  9 , a variable force for opening the fluid releasing valve. 
     Preferably, the force exerted by the stopper  13  on said member is adapted to overcome a force keeping said valve in a closed position only when the piston  95  has to be moved from its second position to its first position. 
     Preferably, the variable force exerted by the stopper  13  increases when the rocker rotates from a valve closing position to its valve opening position. 
     In this embodiment, the stopper  3  is an elastic, stopper and the element of the stopper with which the stop  valve  97 , the check valve being the valve which performs the function of releasing fluid from the chamber  101 . Therefore, an elastic stopper  13  is adapted to cooperate, via actuation member  978 , with check valve  97 . Stopper  13  comprises a contact element, here in the form of a pushrod  131  extending along a longitudinal axis X 13  and having a pushing end  132 . Pushing end  132  is adapted to cooperate with central pin  9784 , through hole  917 . Stopper  13  is hidden on  FIG. 1  for the simplicity of the drawing. 
     Stopper  13  comprises a cylindrical housing jacket  134  which has an open upper end  1340  and a lower end  1342  which is fast with a housing E 1  of the engine E. Pushrod  131  is mounted in jacket  134  and is adapted to move translationally with respect to jacket  134  along axis X 13 . 
     In the vicinity of open end  1340 , jacket  134  comprises a stopper element  1344  which limits the translation of pushrod  131  along axis X 13  towards rocker  9 . Pushrod  131  also comprises a peripheral collar  1311 . A main compression spring  136  is mounted between peripheral collar  1311  and end  1342  so as to urge pushrod  131  against, stopper element  1344 . 
     In this embodiment, valve actuation mechanism S operates in the following way during an engine brake operation: prior to the rotation of rocker  9  from a valve closing position towards a valve opening position in the direction of arrow R 1 , a clearance C 1  separates central pin  9784  from pushing end  132  of pushrod  131 , as shown on  FIG. 2  or may be instead provided between actuation member  978  and bail  974 . In other words, in this embodiment, the clearance C 1  entails that, in the valve closing position of the rocker, the stopper does not exert a force on the fluid releasing valve, it can be noted that a control pressure is present in chamber  915  so that actuation member  978  does not interfere with ball  970 . When rotation of rocker  9  begins, due to the cam follower  93  cooperating with a main valve lift sector of the cam  22 , a contact is made between central pin  9784  and pushrod  131  as shown on  FIG. 3 . At this time, piston  95  has been moved to its second position and check valve  97  is closed due to the action of its bias spring  972  and of the pressure inside chamber  976 , both acting, on the hall  970 . Piston  95  is thereby prevented from going back into its first position under the action of a force F 7  exerted by valve bridge  7  and induced by springs  41  and  51 . 
     According to a non-shown variant, the clearance between central pin  9784  and pushing end  132  prior to the rotation of rocker  9  may be inexistent. Spring  136  may be designed to keep a permanent contact between central pin  9784  and pushing end  132 . 
     In the configuration of  FIGS. 2 and 3 , fluid pressure in chamber  976  exerts a force Fp on ball  970 , which urges ball  970  against seat element  974 . The contact between central pin  9784  and pushrod  131  induces a translation of pushrod  131  towards end  1342  and a subsequent deformation of main spring  136 . In this configuration, as the deformation of main spring  136  is relatively low, the compression force F 136  exerted by main spring  136  on pushrod  131  remains inferior to fluid pressure force Fp. The fluid pressure force Fp depends essentially on the force which is acting on activation piston  95 , i.e. the force of the return spring  41  of valve  4 . The fluid pressure in chamber  101  and in chamber  976  can be in the order of 20 bars. 
     When the rotation of rocker  9  goes further, pushrod  131  reaches a position, along axis X 13 , which induces an increased deformation of main spring  136  and an increased compression force F 136 . At this time, corresponding to a third configuration represented on  FIG. 4 , force F 136  exerted by main spring  136  becomes superior to fluid pressure Fp. Force F 136 , transmitted to ball  970  via actuation member  978 , then lifts ball  970  away from seat element  974 . Check valve  97  is opened and pressure in chamber  976  is therefore reduced because some fluid is released from the chamber through the check valve  97 . The pressure in chamber  101  can eventually fall to the value of the engine brake control pressure delivered by ducts  911  and  912 , which can for example be in the order of 3 bars. This allows piston  95  to be pushed back to its first position. This position of the pushrod can be associated to a corresponding position of the rocker  9  between its valve closing and opening position and to a corresponding timing within the opening/closing cycle of valves  4  and  5 . At said position and timing, which can be called the fluid release triggering position, the piston is moved from its second position to its first position, because said moment is not blocked anymore by the pressure in chamber  101 . 
     Preferably, check valve  97  is opened before contact, is made between piston  95  and slider block  71  so that the elastic force exerted by spring  41  on valve  4 , and transmitted to slider block  71 , overcomes the fluid pressure force Fp in piston chamber  101 . This allows to push back piston  95  towards its first position and to ensure valves  4  and  5  are substantially synchronized at closure. 
     The stiffness of main compression spring  136  is determined to obtain a pushing back of piston  95  in its first position at the time when valves  4  and  5  reach a lift value superior to the engine brake lift value, preferably close to maximal lift value of the valves  4  and  5 . Therefore the stiffness of main compression spring  136  is determined so that the deformation of main spring  136 , for such lift value of the valves, i.e. for the corresponding position of the rocker, and hence for the corresponding position of the rocker  9 , induces a compression force F 136  superior to the fluid pressure force Fp in chamber  976 . 
     During the rotation of rocker  9  in the opposite direction relative to rotation R 1 , the elastic means of stopper  9  induce an hysteresis effect on the opening/closing of the fluid releasing valve, which is here check valve  97 . Indeed after the rocker  9  has passed, on its way back to its valve closing position, the fluid release triggering position, the elastic means still exert a force on the relevant member of the rocker, here on the check valve  97 , and in this embodiment through pushrod  131  and actuation member  978 . Thereby, the fluid releasing valve, here check valve  97 , remains opened during most of the rotation of rocker  9  back to its initial position, as long as the force provided by the elastic means are sufficient to maintain the release valve open. This three tends to decrease as the rocker comes back to its valve closing position, but the force that would tend to close the fluid releasing valve is now limited. In the example of  FIGS. 2 to 4 , such force is essentially the force of spring  972  which pushes back the ball  970  towards the seat. In any case, it can be noted that the pressure in chamber  101  is then only the pressure delivered by ducts  911  and  912 , for example 3 bars. Therefore, the closing of the fluid releasing valve is allowed by the stopper at a position of the rocker, which can be called the fluid release inhibiting position, which is closer to the valve closing position of the rocker than the above mentioned fluid release triggering position. In other words, the stopper causes opening of the fluid releasing valve for a first position of the rocker and allows closing of the fluid releasing valve for a second position of the rocker, said second position being closer to the valve closing position of the rocker than said first position. 
     For example, in a single valve technology exhaust brake system where the reference exhaust valve  5 , would have a certain main lift value (the maximum displacement of the valve  5  when in its fully opened position compared to its fully closed position), the fluid releasing triggering position could be set between around 30% and 50% of the main lift value. The fluid releasing inhibiting position could be set at less then 10%, preferably less than 5% and ideally around 1 or 2 percent of the main lift value. 
     Because the fluid releasing valve is maintained in its open position, piston  95  cannot be moved towards its second position. 
     As previously noted, the check valve is constructed so that it is kept in its closed position by a fluid pressure force Fp in a chamber  976  fluidly connected to the piston chamber  101 . In other words, when the check valve is closed and when a pressure is present in chamber  101 , said pressure tends to maintain the reset valve in its closed position. Therefore, the variable force exerted by the stopper  13  needs to overcome the fluid pressure force to cause the opening of the check valve at the fluid release triggering position. To the contrary, such fluid pressure force does not exist, or to a limited extent when the rocker comes back to the valve closing position. Thereby the force which the variable force F 136  needs to overcome to maintain the check valve in its open position is much smaller than the force it needs to overcome to cause the opening of the check valve. Thus, the closing of the fluid releasing valve is allowed of the rocker, which can be called the fluid release inhibiting position, which is closer to the valve closing position of the rocker than the above mentioned fluid release triggering position. 
     In the following embodiments, elements similar to the first embodiment have the same references and work in the same way. 
     A second embodiment of the invention is represented on  FIG. 5 . In this embodiment, a jacket  134 , of an elastic stopper  13  fast with a housing E 1  of the engine E, comprises a central stopper sleeve  1346  which extends around axis X 13  in the interior of main spring  136 . Stopper sleeve  1346  comprises an abutment surface  1347  facing pushrod  131 . 
     In this embodiment, pushrod  131  comprises, opposite to pushing end  132 , an inner portion which defines an annular edge  1315 , which faces surface  1347 . 
     This embodiment operates in the following way: in a first phase, main spring  136  is deformed as in the first embodiment. Force F 136  therefore increases at a progressive rate. At the time check valve  97  must be opened, annular edge  1315  of pushrod  131  comes into abutment with abutment surface  1347  of jacket  134 . This induces the exertion of a large force on pushrod  131  and therefore on actuation member  978 , inducing the opening of check valve  97 . Piston  95  housed in a non-shown bore similar to bore  94 , can then be moved back in its first position. The position of abutment surface  1347  along axis X 13 , with respect to jacket  134  is determined to correspond to the rotation angle reached by rocker  9  at the moment when check valve  97  must be opened; i.e. at the fluid release triggering position. 
     A third embodiment of the invention is represented on  FIG. 6 . In this embodiment, an elastic stopper  13  fast with a housing E 1  of engine E comprises an auxiliary spring  138 , which extends along axis X 13  radially in the interior of main compression spring  136 . Auxiliary spring  138  extends from a base surface  1350  of jacket  134  and exerts a force F 138  on pushrod  131 . 
     This embodiment works in the following way: in the initial configuration of valve actuation mechanism S corresponding to  FIG. 2 . When the rocker is in a valve closing position, only auxiliary spring.  138  cooperates with pushrod  131 , which is not in contact with actuation member  978 . Main spring  136  is offset, along axis X 13 , by a clearance C 2 . When contact is made between pushing end  132  and actuation member  978 , deformation of auxiliary spring  138  begins and lasts until peripheral edge  1311  of pushrod  131  makes a contact with main spring  136 . The stiffness of auxiliary spring  38  is set to a value inferior to the stiffness value of main spring  136 . This implies that when cooperation between main spring,  36  and pushrod  131  begins, a force similar to force. F 136  is exerted on pushrod  131 . The stiffness of main spring  136  is set to a value implying that said force is directly superior to force Fp, allowing check valve  97 , which is housed in a non-shown bore similar to bore  94 , to be driven back to its first position. Clearance C 2  between main spring  136  and peripheral edge  1311  is set to a value allowing auxiliary spring  138  to be deformed until check valve  97  must be opened. 
     A fourth embodiment of the invention is represented on  FIGS. 7 and 8 . In this embodiment, each rocker  9  comprises a reset valve  99  housed in a chamber  999  of rocker  9 , fluidly connected to chamber  101  and adapted to reduce fluid pressure in chamber  101  by purging fluid via a non-shown discharge duct or to the outside of rocker  9 . Reset valve  99  is biased towards its closed position, with a ball  991  of reset valve being biased against a seat  995 , by a force F 993  exerted by a compression spring  993  along a longitudinal axis X 99  of reset valve  99 . More predominantly, reset valve  99  is also kept in its closed position by a fluid pressure force Fp exerted by fluid in chamber  999 . Said pressure reflects the pressure in chamber  10 , and in most cases is equal to the pressure in chamber  101 . In other words, when the reset valve is closed and when a pressure is present in chamber  101  said pressure tends to maintain the reset valve in its closed position. Reset valve is distinct from the check valve  97  as described in relation to the preceding embodiment, in that it is not provided between the chamber  101  and the control fluid source which can be formed by the ducts  911  and  912  of previous embodiments. Such check valve  97  may be present in this embodiment, although not described here. 
     A contact element, such as a pushrod  131 , of an elastic stopper  13  fast with a housing E 1  of the engine E, may exert, from outside of the rocker, a force F 136  on the ball  911  to open the valve, by lifting the ball  991  from the seat  995 , against, the action of the compression spring  993 . When force F 136  becomes superior to forces F 993  and Fp, ball  991  is lifted away from seat  995 , allowing fluid to flow outside rocker  9  through a hole  997  directly following seat  995  along the fluid stream direction. Piston  95  housed in bore  94  can then be moved back to its first position. 
     Therefore, the variable force exerted by the stopper needs to overcome the fluid pressure force to cause the opening of the check valve at the fluid release triggering position. To the contrary, such fluid pressure force does not exist, or to a limited extent when the rocker comes back to the valve closing position. Thereby the force which the variable force F 136  needs to overcome to maintain the reset valve in its open position is much smaller than the force needs to overcome to cause the opening of the reset valve. Thus, the closing of the fluid releasing valve is allowed by the stopper at a position of the rocker, which can be called the fluid release inhibiting position, which is closer to the valve closing position of the rocker than the above mentioned fluid release triggering position. 
     In the two following embodiment elements similar to the second embodiment have the same references and work in the same way. 
     A fifth embodiment of the invention is represented on  FIG. 9 . In this embodiment, each rocker  9  comprises a discharge valve  103 , which can be a safety valve known per se, and which, in this embodiment is carried by the piston, for example by being housed in a hollow portion  950  of piston  95  housed in bore  94 . Discharge valve is a normally closed valve which is opened by the fluid pressure in the chamber  101  when such pressure exceeds a predetermined threshold to allow fluid flow out of the chamber  101 . The discharge valve  103  forms the valve for releasing fluid from the chamber  101 . As an example, discharge valve  103  shown on  FIG. 9  is kept in sealing contact with a seat  952  of piston  95  by a compression spring  1035  exerting a force F 1035 . Seat  952  extends around a hole  954  which fluidly connects chamber  101  with a hollow portion  950  of piston  95 . Piston  95  comprises two bleed passages  956  which fluidly connect hollow portion  950  with the outside of piston  95  and rocker  9 . 
     In this embodiment, an elastic stopper  13  fast with a housing E 1  of engine E cooperates, for example via a contact element similar to pushrod  131 , with a surface  958  of piston  95 . Discharge valve  103  is movable with respect to seat  952  along axis X 95 . 
     This embodiment works in the same manner as in the previous embodiments. When contact is made between pushrod  131  and surface  958 , main compression spring  136  is first deformed until compression force F 136  becomes superior to fluid pressure force Fp exerted by fluid in chamber  101  on piston  95 . At this time, as pushrod  131  stops movement of piston  95  along axis X 95 , fluid pressure force Fp is then exerted on discharge valve  103  through hole  954 . When fluid pressure force Fp becomes superior to compression force F 1035  exerted by spring  1035  on discharge valve  103 , discharge valve  103  opens. As discharge valve  103  is not anymore in sealing contact with seat  952 , fluid is purged from chamber  101  to hollow portion  950  and then outside of piston  95 . Thus, piston  95  can be pushed back in its first position. In this case, the exertion of force F 136  permits to overcome force F 1035  to open discharge valve  103 , without the stopper acting, directly on the discharge valve, only due to the increase of pressure in chamber  101  created by force F 136  exerted on the piston. 
     In a variant of this embodiment, instead of being carried by the piston, the discharge valve could be carried by the main body of the rocker, as long as it can release, fluid out of the chamber  101  when pressure in chamber  101  exceeds a certain threshold due to the force exerted by the stopper on the activation piston. 
     A sixth embodiment of the invention is represented on  FIGS. 10 and 11  in which the exhaust valves and the valve opening actuator are not shown. 
     Valve actuation mechanism S also comprises a stopper  13 , which comprises elastic means  136  which are stressed when the rocker travels from its valve closing position to its valve opening position. The stopper  13  may have a fork-shaped contact element  135 , for example with a half-circular shape extending between two parallel fingers. The contact element  135  is connected to the engine housing E 1  by elastic means which are here embodied as a compression spring  136 . The part of the engine E housing E 1  to which the stopper  13  is attached is preferably the cylinder head, but could be an other part rigidly connected to the cylinder head or to the crankcase. 
     In this embodiment, activation piston  95  comprises a first element  9501  which has a hollow portion  9502  and comprises a tubular peripheral wall  9503  parallel to axis X 95 . A plane circular wall  9507  extends perpendicularly to axis X 95  from an end of peripheral wall  9503  on the side of piston chamber  101 . Plane wall  9507  comprises a central hole  9509  aligned with axis X 95 . Central hole  9509  forms a fluid passageway between chamber  101  and hollow portion  9502  of first element  9501 . 
     First element  9501  is mounted within a corresponding cylinder bore  94  created in the rocker  9  in the continuation of the chamber  101  and having the same axis X 95  and first element is adapted to move in translation with respect to rocker  9  along axis X 95 . 
     Piston  95  further comprises a central member  9551  housed in hollow portion  9502  of first element  9501  and movable in translation with respect to first element  9501 , and subsequently with respect to rocker  9 , along axis X 95 . Hollow portion  9502  is defined as the inside of the tubular peripheral wall  9503 . Central member  9551  comprises two bleed passages  959  adapted to let fluid flow from hollow portion  9502  of first element  9501  to the outside of rocker  9 . Central member  9551  may comprise only one bleed passage  959 . 
     Central member  9551  comprises a pin  9559  having a form corresponding to the form of central hole  9509 . Pin  9559  extends from a planar annular surface  9561  adapted to come in abutment against a portion of plane wall  9507 , which acts as a stop, under action of a traction force F 9563  exerted by a spring  9563  arranged between first element  9501  and central member  9551 . The cooperation between pin  9559  and surface  991  forms a fluid releasing valve  105 . 
     Piston  95  has a pushing surface  963  realized on a pin  964  which extends from a surface  961  of central member  9551  for cooperation with a valve opening actuator such as valve bridge  7  or more particularly, in the case of single valve brake technology as described above, with a slider block of a valve bridge. 
     Contact element  135  of stopper  13  is adapted to cooperate with an annular outer edge  9513  of first element  9501 , located on the outside of rocker  9 , without interfering with the central member  95551 . 
     Valve actuation meclranism S orks&#39;i h ollo i way: when rocker  9  is in a position corresponding to the closed state of valves  4  and  5 , a clearance C 1  separates edge  9513  from contact element  135  of stopper  13 . Prior to the engine brake valve openings, piston  95  is moved to its second position thanks to a fluid pressure raise in chamber  101 . 
     Once the two engine brake valve openings have been realized, thanks to a rotation R 1  of rocker  9 , a main exhaust opening of valves  4  and  5  is to be performed. Therefore, during the opening of valves  4  and  5 , piston  95  must be pushed back to its first position. When rotation R 1  of rocker  9  approaches its maximal angular value, contact is made between edge  9513  and fingers  136  of fork stopper  13 . At this moment, the exertion of a force F 136  by stopper  13  on first element  9501  begins. 
     The exertion of force F 136  on edge  9513 , which increases as the rocker travels towards its valve opening position induces a movement of first element  9501  along axis X 95  with respect to central member  955  under action of fluid pressure force Fp exerted on pin  9559 . 
     Planar annular surface  9561  therefore becomes remote from plane wall  9507 , as shown on  FIG. 4 , causing fluid releasing valve  105  to open and provoking fluid flow inside hollow portion  9502  of first element  9501 . Fluid is purged outside rocker  9  via bleed passages  959  which are realized in base portion  9557  of central member  9551 . Central member  9551  is moved towards chamber  101  under action of spring  9563 , until a contact is made again between surface  9561  and wall  9507 . Piston  95  as a whole is then pushed in its first position under action of valve opening actuator, which exerts a force F 7  on central member  9551  induced by the springs which return the exhaust valves to their closed positions. 
     In other words, during a movement of the rocker  9  towards the opening of the valves  4  and  5  corresponding to a main exhaust event, the stopper will progressively block the movement of first element  9501  with respect to the engine casing. Due to the fact that the rocker continues its movement towards the valve bridge  7 , the pressure in the main chamber, acting on the pin  9559  causes the central member  9551  to continue the movement in the direction of the valve bridge. Therefore, there is a tendency for the central member  9551  and the first element  9501  to separate, and when the pin  9559  escapes of hole  9509 , the control fluid contained in chamber  101  can be discharged though the central hole  9509  and then through bleed passages  959 . 
     In a non-represented embodiment of the invention, applicable to all those embodiments having elastic means, the elastic means can be realized with a variable stiffness. This can be done by providing a variable pitch between the coils of a compression spring  136 . The pitch between the coils of compression spring  136  is determined so that the force increase needed to overcome the force which keeps check valve  97 , reset valve  99  or discharge valve  103  in closed position is obtained with no point of inflexion, in order to reduce the force variations exerted on the various parts of valve actuation mechanism S and particularly on the valves. For example, in the embodiment of  FIGS. 1 to 4 , compression spring  136  can have a relatively low pitch between its coils in the vicinity of pushrod  131 , and an increasing pitch towards end  1342 , so that the deformation of compression spring  136  induces an increase of compression force F 136  according to a parabolic profile. 
     According to a non-shown embodiment of the invention valve actuation mechanism S may apply to a single exhaust valve system, in which each rocker is adapted to move only one valve. In this case, the valve actuation mechanism does not comprise any bridge, the single valve being moved via an intermediate part adapted to cooperate with piston  95 . 
     According, to a non-shown embodiment, piston  95  is adapted to exert valve opening effort F 9  on the whole of valve bridge  7 . Both valves  4  and  5  are connected to valve bridge  7  so that they are opened or closed simultaneously. 
     In all the above embodiments, the position of the stopper with respect to the engine housing can be set so that it interferes with the relevant member of the rocker at a given position of the rocker between its valve closing, and valve opening positions. Therefore, the position of the stopper with respect to the housing and with respect to the rocker is one of the parameters which defines the fluid release triggering position of the rocker, which should correspond to the timing at which the activation piston has to be moved from its second position to its first position in the valve opening and closing cycle. The position of the stopper can be made adjustable for fine-tuning of the timing at which the activation piston is effectively moved from its second position to its first position. 
     Also, in case the stopper comprises elastic means, such means can take various forms. In the example shown, a compression spring is used and is stressed in compression when the rocker travels from the valve closing position to the valve opening position of the rocker. But other types of springs could be used, such as tension springs or torsion springs, which are then to be stressed respectively in traction or in torsion when the rocker travels from the valve closing position to the valve opening position of the rocker Fine tuning of the fluid release triggering position and/or of the fluid release inhibiting position can be altered by providing some adjustability of the pre-stressing of the elastic means. 
     The technical features of the various embodiments and variants described here above can be combined in the scope of the invention. Particularly, the features of the embodiments of  FIGS. 5 and 6  may apply to the embodiments of  FIGS. 7 to 11 .