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

A valve actuation mechanism for an internal combustion engine includes 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 includes a valve for releasing fluid from the chamber, wherein the valve actuation mechanism includes, 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.

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 bumWhen 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; orwherein the activation piston (95) activates an engine brake function when it is in its second position; oran intake valve actuation mechanism.

The invention also concerns an automotive vehicle, such as a truck, comprising a valve actuation mechanism as mentioned here-above.

DETAILED DESCRIPTION

The valve actuation mechanism S of the invention, represented onFIGS. 1 to 4, comprises a camshaft2rotatable around a longitudinal axis X2. Camshaft2comprises several cams22, 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 X2than the base radius of the cam.FIG. 1shows 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 valves4and5. Valves4and5are biased towards their dosed position by respective springs41and51. Each valve4and5is movable in translation along an opening axis X4or X5so as to be opened, or lifted. More precisely, translation of valves4and5opens a passageway between the combustion chamber of the cylinder and an exhaust manifold. Valves4and5are connected to a valve bridge7, which forms a valve opening actuator, and which extends substantially perpendicular to axes X4and X5. In this embodiment, only one valve4is 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 bridge7comprises a main portion72, which causes opening of valve5. Valve bridge7also comprises a slider block71which is movable with respect to main portion72of valve bridge7along opening axis X4of valve4. Slider block71is connected to valve4so as to be able to cause its opening. Consequently, valve4is also movable with respect to main portion72of valve bridge7along axis X4.

Valves4and5are partly represented on the figures, only their respective stems are visible.

For each cylinder, the transmission of movement between camshaft2and valve bridge7is performed by a rocker9rotatable with respect to a rocker shaft91defining a rocker rotation axis X91. Only one rocker9is represented on the figures. Each rocker9comprises a roller93which acts as a cam follower and cooperates with a cam22. Roller93is located on one side of rocker9which respect to shaft91. Each rocker9comprises, opposite to roller93with respect to shaft91, an activation piston95adapted to exert a valve opening force F9on the slider block71of valve bridge7, which is connected to valve4, for example merely by being in contact with the valve stem.

Rocker9further comprises a finger121substantially parallel to piston95, and centered on an axis X121. d95denotes the distance between axes X91and X95. d121denotes the distance between axes X91and X121. Distance d121is larger than distance d95. Piston95is arranged in rocker9so that it cooperates with slider block71, while finger121is adapted to cooperate with the main portion72of valve bridge7. It can be noted that the plane defined by the axes X4, X5of the valves is perpendicular to the rotation axis X91of the rocker9. Valve5is further away from the rocker rotation axis than valve4.

Rotation of camshaft2transmits, when the roller runs against a valve lift sector of the cam, a rotation movement R1to rocker9via roller93, this rotation movement inducing a translation movement of main portion72of valve bridge7and of slider block71, respectively due to finger121and to activation piston95, along an axis X7which is parallel to axes X4and X5. Cooperation between a main valve lift sector of cam22and roller93, on the one hand, and between piston95and slider block71and between finger121and main portion72of valve bridge7, on the other hand, generates exhaust openings of valves4and5during 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 shaft91is hollow and defines a duct911which houses fluid circuit coming from a non-shown fluid tank of valve actuation mechanism S. Rocker9comprises itself an internal fluid circuit which connects duct911to a piston chamber101of rocker9, partly delimited by piston95, via a check valve97. Activation piston95is housed in a bore94of rocker9and adapted to move with respect to chamber101along a translation axis X95corresponding, to a longitudinal axis of piston95. A duct U27partly-shown onFIG. 2, connects duct911to check valve97. A duct913fluidly connects check valve97to piston chamber101.

When the engine switches to engine brake mode, a non shown engine brake control valve delivers pressurized fluid to ducts911and912, which entails that pressurized fluid flows though check valve97in piston chamber101. The pressure raise in chamber101induces a translation movement of piston95outwardly with respect to rocker9, from a first position, in which piston95is entirely or partially pushed back into chamber101to a second position, in which piston95is partially moved out of piston chamber101until it comes in abutment against slider block71. Preferably, the control fluid is a substantially incompressible fluid such as oil.

Cam22comprises in this embodiment two auxiliary valve lift sectors which are adapted to cooperate with roller93. These sectors induce, when read by roller93of rocker9, two additional pivoting movements of rocker9on each turn of camshaft2. 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 piston95into two opening movements of valve4so 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, cam22comprises only one auxiliary valve lift sector fix performing only one opening of valve4on each turn of camshaft2, in addition to the main exhaust valve opening.

When piston95is in its first position, retracted, as shown onFIG. 2, roller93is offset with respect to the auxiliary valve lift sectors of cam22by an engine brake actuation clearance, so that when camshaft2rotates around axis X2, cam22does not come in contact with roller93, or piston95does not come in contact with slider block71. The clearance is such that the auxiliary valve lift sectors cannot cause the opening of valve4, 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 rocker9around its axis which is sufficient to cause opening of both valves.

By moving piston95to its second position, extended, as shown onFIG. 4, rocker pivots around the longitudinal axis X91of shaft91. Thus, the actuation clearance is suppressed and roller93comes into contact with the auxiliary valve lift sectors of cam22, while the activation piston s simultaneously in contact or quasi contact with the slider block71, allowing engine brake operations to be implemented when the roller93is acted upon by any one of the auxiliary valve lifts.

Normal exhaust openings of valves4and5during engine brake operations are implemented as follows. Piston95is first moved towards its second position, so that, when a rotation of rocker9along arrow R1starts, the system causes the opening, of only valve4when the cam follower reads the additional valve lift sectors. Those sectors do not cause opening of valve5. When the auxiliary valve lift sectors have been read by roller93, roller93begins to read a main valve lift sector220, inducing a rotation of rocker9sufficient to generate a contact between finger121and main portion72. From this moment on, the main portion72of valve bridge7is moved and opening of valve5begins, in parallel to the movement of valve4.

At a further rotation angle of rocker9, because piston95abuts against a non-shown stop of bore94which limits its motion outside rocker9, contact is lost between piston95and slider block71. From this moment on, main portion72cooperates with slider block71thanks to a stop720which cooperates with a shoulder711of slider block71. Slider block71, and also valve4, become integral in translational movement with main portion72, until the opening of valves4and5is complete.

When valves4and5return to their closed position, movement of bridge7is performed exactly in the opposite manner compared to the opening movement until contact is made again between piston95and slider block71. An elastic force is therefore exerted on piston95by spring41via slider block71, provoking a pressure raise in chamber101, which is closed at this moment. The fluid in chamber101blocks the motion of piston95towards its first position. Therefore, absent the invention, the valve4would close later than valve5. This would provoke extended valve overlapping, which reduces the efficiency of the engine brake function.

According to a variant of the invention, piston95may 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 onFIGS. 1 to 4, check valve97comprises a ball970which is kept, by a compression spring972, against a seat974. Bali970, spring972and seat974are arranged in a check valve chamber976realized in rocker9. Chamber976has a cylindrical form centered around a longitudinal axis X97. Chamber976is fluidly connected to piston chamber101via duct913. Ball970is movable along axis X97with respect to seat element974. Fluid pressure in the chamber976, and thus in chamber101tends to push the ball970, which acts as a plug member for the valve, on the valve seat974, thereby closing the valve.

Duct911of rocker shaft91is connected, via duct912, to a first chamber915realized in rocker9. First chamber915is connected to check valve chamber976through seat974. First chamber915is opposite the check valve chamber976with respect to the seat, so that fluid pressure in the first chamber915tends to push the ball away from the seat, thereby opening the check valve. A check valve actuation member978is housed in chamber915, also for forcing the opening of the valve. Actuation member978is movable with respect to chamber915, which has a cylindrical form, along axis X97. Actuation member978comprises an outer sleeve9780. Actuation member978further comprises a pushing pin extending along axis X97and adapted to make a contact with ball970. A further spring is provided to act on the actuation member978so as to push it in the direction in which it forces the ball970off the seat974, thereby forcing the opening of the check valve. When thud pressure is delivered to chamber915though duct912, 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 ball970, which can therefore open and close as a normal check valve, essentially based on the pressure differential on both sides of the valve. Actuation member978also comprises a central pin9784extending along axis X97opposite to pushing pin9782. Central pin9784extends in the vicinity of an end of chamber915which opens by a hole917, on the outside of rocker9.

According to the invention, a stopper13is provided which is fast with a housing of the engine F and adapted to exert, on a member of the rocker9, a variable force for opening the fluid releasing valve.

Preferably, the force exerted by the stopper13on said member is adapted to overcome a force keeping said valve in a closed position only when the piston95has to be moved from its second position to its first position.

Preferably, the variable force exerted by the stopper13increases when the rocker rotates from a valve closing position to its valve opening position.

In this embodiment, the stopper3is an elastic, stopper and the element of the stopper with which thestopvalve97, the check valve being the valve which performs the function of releasing fluid from the chamber101. Therefore, an elastic stopper13is adapted to cooperate, via actuation member978, with check valve97. Stopper13comprises a contact element, here in the form of a pushrod131extending along a longitudinal axis X13and having a pushing end132. Pushing end132is adapted to cooperate with central pin9784, through hole917. Stopper13is hidden onFIG. 1for the simplicity of the drawing.

Stopper13comprises a cylindrical housing jacket134which has an open upper end1340and a lower end1342which is fast with a housing E1of the engine E. Pushrod131is mounted in jacket134and is adapted to move translationally with respect to jacket134along axis X13.

In the vicinity of open end1340, jacket134comprises a stopper element1344which limits the translation of pushrod131along axis X13towards rocker9. Pushrod131also comprises a peripheral collar1311. A main compression spring136is mounted between peripheral collar1311and end1342so as to urge pushrod131against, stopper element1344.

In this embodiment, valve actuation mechanism S operates in the following way during an engine brake operation: prior to the rotation of rocker9from a valve closing position towards a valve opening position in the direction of arrow R1, a clearance C1separates central pin9784from pushing end132of pushrod131, as shown onFIG. 2or may be instead provided between actuation member978and bail974. In other words, in this embodiment, the clearance C1entails 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 chamber915so that actuation member978does not interfere with ball970. When rotation of rocker9begins, due to the cam follower93cooperating with a main valve lift sector of the cam22, a contact is made between central pin9784and pushrod131as shown onFIG. 3. At this time, piston95has been moved to its second position and check valve97is closed due to the action of its bias spring972and of the pressure inside chamber976, both acting, on the hall970. Piston95is thereby prevented from going back into its first position under the action of a force F7exerted by valve bridge7and induced by springs41and51.

According to a non-shown variant, the clearance between central pin9784and pushing end132prior to the rotation of rocker9may be inexistent. Spring136may be designed to keep a permanent contact between central pin9784and pushing end132.

In the configuration ofFIGS. 2 and 3, fluid pressure in chamber976exerts a force Fp on ball970, which urges ball970against seat element974. The contact between central pin9784and pushrod131induces a translation of pushrod131towards end1342and a subsequent deformation of main spring136. In this configuration, as the deformation of main spring136is relatively low, the compression force F136exerted by main spring136on pushrod131remains inferior to fluid pressure force Fp. The fluid pressure force Fp depends essentially on the force which is acting on activation piston95, i.e. the force of the return spring41of valve4. The fluid pressure in chamber101and in chamber976can be in the order of 20 bars.

When the rotation of rocker9goes further, pushrod131reaches a position, along axis X13, which induces an increased deformation of main spring136and an increased compression force F136. At this time, corresponding to a third configuration represented onFIG. 4, force F136exerted by main spring136becomes superior to fluid pressure Fp. Force F136, transmitted to ball970via actuation member978, then lifts ball970away from seat element974. Check valve97is opened and pressure in chamber976is therefore reduced because some fluid is released from the chamber through the check valve97. The pressure in chamber101can eventually fall to the value of the engine brake control pressure delivered by ducts911and912, which can for example be in the order of 3 bars. This allows piston95to be pushed back to its first position. This position of the pushrod can be associated to a corresponding position of the rocker9between its valve closing and opening position and to a corresponding timing within the opening/closing cycle of valves4and5. 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 chamber101.

Preferably, check valve97is opened before contact, is made between piston95and slider block71so that the elastic force exerted by spring41on valve4, and transmitted to slider block71, overcomes the fluid pressure force Fp in piston chamber101. This allows to push back piston95towards its first position and to ensure valves4and5are substantially synchronized at closure.

The stiffness of main compression spring136is determined to obtain a pushing back of piston95in its first position at the time when valves4and5reach a lift value superior to the engine brake lift value, preferably close to maximal lift value of the valves4and5. Therefore the stiffness of main compression spring136is determined so that the deformation of main spring136, for such lift value of the valves, i.e. for the corresponding position of the rocker, and hence for the corresponding position of the rocker9, induces a compression force F136superior to the fluid pressure force Fp in chamber976.

During the rotation of rocker9in the opposite direction relative to rotation R1, the elastic means of stopper9induce an hysteresis effect on the opening/closing of the fluid releasing valve, which is here check valve97. Indeed after the rocker9has 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 valve97, and in this embodiment through pushrod131and actuation member978. Thereby, the fluid releasing valve, here check valve97, remains opened during most of the rotation of rocker9back 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 ofFIGS. 2 to 4, such force is essentially the force of spring972which pushes back the ball970towards the seat. In any case, it can be noted that the pressure in chamber101is then only the pressure delivered by ducts911and912, 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 valve5, would have a certain main lift value (the maximum displacement of the valve5when 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, piston95cannot 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 chamber976fluidly connected to the piston chamber101. In other words, when the check valve is closed and when a pressure is present in chamber101, said pressure tends to maintain the reset valve in its closed position. Therefore, the variable force exerted by the stopper13needs 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 F136needs 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 onFIG. 5. In this embodiment, a jacket134, of an elastic stopper13fast with a housing E1of the engine E, comprises a central stopper sleeve1346which extends around axis X13in the interior of main spring136. Stopper sleeve1346comprises an abutment surface1347facing pushrod131.

In this embodiment, pushrod131comprises, opposite to pushing end132, an inner portion which defines an annular edge1315, which faces surface1347.

This embodiment operates in the following way: in a first phase, main spring136is deformed as in the first embodiment. Force F136therefore increases at a progressive rate. At the time check valve97must be opened, annular edge1315of pushrod131comes into abutment with abutment surface1347of jacket134. This induces the exertion of a large force on pushrod131and therefore on actuation member978, inducing the opening of check valve97. Piston95housed in a non-shown bore similar to bore94, can then be moved back in its first position. The position of abutment surface1347along axis X13, with respect to jacket134is determined to correspond to the rotation angle reached by rocker9at the moment when check valve97must be opened; i.e. at the fluid release triggering position.

A third embodiment of the invention is represented onFIG. 6. In this embodiment, an elastic stopper13fast with a housing E1of engine E comprises an auxiliary spring138, which extends along axis X13radially in the interior of main compression spring136. Auxiliary spring138extends from a base surface1350of jacket134and exerts a force F138on pushrod131.

This embodiment works in the following way: in the initial configuration of valve actuation mechanism S corresponding toFIG. 2. When the rocker is in a valve closing position, only auxiliary spring.138cooperates with pushrod131, which is not in contact with actuation member978. Main spring136is offset, along axis X13, by a clearance C2. When contact is made between pushing end132and actuation member978, deformation of auxiliary spring138begins and lasts until peripheral edge1311of pushrod131makes a contact with main spring136. The stiffness of auxiliary spring38is set to a value inferior to the stiffness value of main spring136. This implies that when cooperation between main spring,36and pushrod131begins, a force similar to force. F136is exerted on pushrod131. The stiffness of main spring136is set to a value implying that said force is directly superior to force Fp, allowing check valve97, which is housed in a non-shown bore similar to bore94, to be driven back to its first position. Clearance C2between main spring136and peripheral edge1311is set to a value allowing auxiliary spring138to be deformed until check valve97must be opened.

A fourth embodiment of the invention is represented onFIGS. 7 and 8. In this embodiment, each rocker9comprises a reset valve99housed in a chamber999of rocker9, fluidly connected to chamber101and adapted to reduce fluid pressure in chamber101by purging fluid via a non-shown discharge duct or to the outside of rocker9. Reset valve99is biased towards its closed position, with a ball991of reset valve being biased against a seat995, by a force F993exerted by a compression spring993along a longitudinal axis X99of reset valve99. More predominantly, reset valve99is also kept in its closed position by a fluid pressure force Fp exerted by fluid in chamber999. Said pressure reflects the pressure in chamber10, and in most cases is equal to the pressure in chamber101. In other words, when the reset valve is closed and when a pressure is present in chamber101said pressure tends to maintain the reset valve in its closed position. Reset valve is distinct from the check valve97as described in relation to the preceding embodiment, in that it is not provided between the chamber101and the control fluid source which can be formed by the ducts911and912of previous embodiments. Such check valve97may be present in this embodiment, although not described here.

A contact element, such as a pushrod131, of an elastic stopper13fast with a housing E1of the engine E, may exert, from outside of the rocker, a force F136on the ball911to open the valve, by lifting the ball991from the seat995, against, the action of the compression spring993. When force F136becomes superior to forces F993and Fp, ball991is lifted away from seat995, allowing fluid to flow outside rocker9through a hole997directly following seat995along the fluid stream direction. Piston95housed in bore94can 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 F136needs 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 onFIG. 9. In this embodiment, each rocker9comprises a discharge valve103, 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 portion950of piston95housed in bore94. Discharge valve is a normally closed valve which is opened by the fluid pressure in the chamber101when such pressure exceeds a predetermined threshold to allow fluid flow out of the chamber101. The discharge valve103forms the valve for releasing fluid from the chamber101. As an example, discharge valve103shown onFIG. 9is kept in sealing contact with a seat952of piston95by a compression spring1035exerting a force F1035. Seat952extends around a hole954which fluidly connects chamber101with a hollow portion950of piston95. Piston95comprises two bleed passages956which fluidly connect hollow portion950with the outside of piston95and rocker9.

In this embodiment, an elastic stopper13fast with a housing E1of engine E cooperates, for example via a contact element similar to pushrod131, with a surface958of piston95. Discharge valve103is movable with respect to seat952along axis X95.

This embodiment works in the same manner as in the previous embodiments. When contact is made between pushrod131and surface958, main compression spring136is first deformed until compression force F136becomes superior to fluid pressure force Fp exerted by fluid in chamber101on piston95. At this time, as pushrod131stops movement of piston95along axis X95, fluid pressure force Fp is then exerted on discharge valve103through hole954. When fluid pressure force Fp becomes superior to compression force F1035exerted by spring1035on discharge valve103, discharge valve103opens. As discharge valve103is not anymore in sealing contact with seat952, fluid is purged from chamber101to hollow portion950and then outside of piston95. Thus, piston95can be pushed back in its first position. In this case, the exertion of force F136permits to overcome force F1035to open discharge valve103, without the stopper acting, directly on the discharge valve, only due to the increase of pressure in chamber101created by force F136exerted 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 chamber101when pressure in chamber101exceeds a certain threshold due to the force exerted by the stopper on the activation piston.

A sixth embodiment of the invention is represented onFIGS. 10 and 11in which the exhaust valves and the valve opening actuator are not shown.

Valve actuation mechanism S also comprises a stopper13, which comprises elastic means136which are stressed when the rocker travels from its valve closing position to its valve opening position. The stopper13may have a fork-shaped contact element135, for example with a half-circular shape extending between two parallel fingers. The contact element135is connected to the engine housing E1by elastic means which are here embodied as a compression spring136. The part of the engine E housing E1to which the stopper13is 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 piston95comprises a first element9501which has a hollow portion9502and comprises a tubular peripheral wall9503parallel to axis X95. A plane circular wall9507extends perpendicularly to axis X95from an end of peripheral wall9503on the side of piston chamber101. Plane wall9507comprises a central hole9509aligned with axis X95. Central hole9509forms a fluid passageway between chamber101and hollow portion9502of first element9501.

First element9501is mounted within a corresponding cylinder bore94created in the rocker9in the continuation of the chamber101and having the same axis X95and first element is adapted to move in translation with respect to rocker9along axis X95.

Piston95further comprises a central member9551housed in hollow portion9502of first element9501and movable in translation with respect to first element9501, and subsequently with respect to rocker9, along axis X95. Hollow portion9502is defined as the inside of the tubular peripheral wall9503. Central member9551comprises two bleed passages959adapted to let fluid flow from hollow portion9502of first element9501to the outside of rocker9. Central member9551may comprise only one bleed passage959.

Central member9551comprises a pin9559having a form corresponding to the form of central hole9509. Pin9559extends from a planar annular surface9561adapted to come in abutment against a portion of plane wall9507, which acts as a stop, under action of a traction force F9563exerted by a spring9563arranged between first element9501and central member9551. The cooperation between pin9559and surface991forms a fluid releasing valve105.

Piston95has a pushing surface963realized on a pin964which extends from a surface961of central member9551for cooperation with a valve opening actuator such as valve bridge7or more particularly, in the case of single valve brake technology as described above, with a slider block of a valve bridge.

Contact element135of stopper13is adapted to cooperate with an annular outer edge9513of first element9501, located on the outside of rocker9, without interfering with the central member95551.

Valve actuationmeclranismSorks'i holloiway: when rocker9is in a position corresponding to the closed state of valves4and5, a clearance C1separates edge9513from contact element135of stopper13. Prior to the engine brake valve openings, piston95is moved to its second position thanks to a fluid pressure raise in chamber101.

Once the two engine brake valve openings have been realized, thanks to a rotation R1of rocker9, a main exhaust opening of valves4and5is to be performed. Therefore, during the opening of valves4and5, piston95must be pushed back to its first position. When rotation R1of rocker9approaches its maximal angular value, contact is made between edge9513and fingers136of fork stopper13. At this moment, the exertion of a force F136by stopper13on first element9501begins.

The exertion of force F136on edge9513, which increases as the rocker travels towards its valve opening position induces a movement of first element9501along axis X95with respect to central member955under action of fluid pressure force Fp exerted on pin9559.

Planar annular surface9561therefore becomes remote from plane wall9507, as shown onFIG. 4, causing fluid releasing valve105to open and provoking fluid flow inside hollow portion9502of first element9501. Fluid is purged outside rocker9via bleed passages959which are realized in base portion9557of central member9551. Central member9551is moved towards chamber101under action of spring9563, until a contact is made again between surface9561and wall9507. Piston95as a whole is then pushed in its first position under action of valve opening actuator, which exerts a force F7on central member9551induced by the springs which return the exhaust valves to their closed positions.

In other words, during a movement of the rocker9towards the opening of the valves4and5corresponding to a main exhaust event, the stopper will progressively block the movement of first element9501with respect to the engine casing. Due to the fact that the rocker continues its movement towards the valve bridge7, the pressure in the main chamber, acting on the pin9559causes the central member9551to continue the movement in the direction of the valve bridge. Therefore, there is a tendency for the central member9551and the first element9501to separate, and when the pin9559escapes of hole9509, the control fluid contained in chamber101can be discharged though the central hole9509and then through bleed passages959.

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 spring136. The pitch between the coils of compression spring136is determined so that the force increase needed to overcome the force which keeps check valve97, reset valve99or discharge valve103in 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 ofFIGS. 1 to 4, compression spring136can have a relatively low pitch between its coils in the vicinity of pushrod131, and an increasing pitch towards end1342, so that the deformation of compression spring136induces an increase of compression force F136according 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 piston95.

According, to a non-shown embodiment, piston95is adapted to exert valve opening effort F9on the whole of valve bridge7. Both valves4and5are connected to valve bridge7so 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 ofFIGS. 5 and 6may apply to the embodiments ofFIGS. 7 to 11.