WO2003067035A1

WO2003067035A1 - Hydraulic control system for a gas exchange valve of an internal combustion engine

Info

Publication number: WO2003067035A1
Application number: PCT/EP2003/050002
Authority: WO
Inventors: Jean-François Berlemont; Dominique Bodart; Manfred Kolkman; Hermann Breitbach
Original assignee: Delphi Technologies, Inc.
Priority date: 2002-02-04
Filing date: 2003-01-27
Publication date: 2003-08-14
Also published as: LU90889B1

Abstract

A hydraulic control system for a gas exchange valve (18) of an internal combustion engine comprises a hydraulic actuator (40) associated with the gas exchange valve (18). A hydraulic pump (50) provides a hydraulic energy supply for the hydraulic actuator (40). A first control valve (100) controls hydraulic energy supply to the hydraulic actuator (40). A mechanical energy accumulator (80) is connected between cam means (72) and the hydraulic pump (50). The cam means (72) is capable of periodically storing mechanical energy into the mechanical energy accumulator (80) and of periodically causing its discharge. The mechanical energy accumulator (80) is capable of restoring mechanical energy that is stored therein to the hydraulic pump (50), for producing the hydraulic energy required for controlling the hydraulic actuator (40).

Description

HYDRAULIC CONTROL SYSTEM FOR A GAS EXCHANGE VALVE OF AN INTERNAL COMBUSTION ENGINE

Field of the invention

The present invention relates to a hydraulic control system for a gas exchange valve of an internal combustion engine.

Background of the invention

The inlet and exhaust valves of internal combustion engines are conventionally operated by camshafts. Cam lobes on the camshaft act on the respec- tive gas exchange valve either directly or indirectly via a rocker arm. However, it is also known to operate the gas exchange valves of internal combustion engines by means of a hydraulic control circuit.

Most hydraulic control systems for gas exchange valves of internal combustion engines are so-called lost-motion systems. They include a single pressure chamber that is sealed on one side by a valve piston and on the other side by a cam piston. The cam piston is cyclically moved by an associated cam to compress the fluid in the pressure chamber. The increased pressure acts on the valve piston, so that the associated gas exchange valve opens. A hydraulic control valve associated with the pressure chamber allows to avoid a pressure build up in the pressure chamber, thereby decoupling the valve piston from the cam piston, i.e. the gas exchange valve remains in its closed position independently of the position of the associated cam. However, such hydraulic control systems provide only limited control possibilities and their integration into the engine block is rather difficult. In order to achieve better control functions and a greater freedom for arranging the cam shaft on the engine, it has been suggested to provide a hydraulic control system for a gas exchange valve of an internal combustion engine including a hydraulic pump cyclically operated by the cam and a hydraulic actuator associated with the gas exchange valve, wherein the hydraulic pump and the hydraulic actuator are functionally interconnected by means of a hydraulic control valve.

Such a hydraulic control system for a gas exchange valve is e.g. dis- closed in U.S. patent 6,227,154. The hydraulic pump includes a cam piston movably arranged in a pump pressure chamber. A rotating cam is capable of cyclically urging the cam piston into a first end position, wherein the pump pressure chamber has a minimum volume. A restoring spring is capable of urging the cam piston into a second end position, wherein the pump pressure chamber has a maximum volume. The hydraulic actuator includes a valve piston movably arranged in an actuator pressure chamber. The hydraulic control system further includes an accumulator chamber 28, in which an accumulator piston is arranged to be moved by pressure acting therein against the force of an accumulator spring. The actuator chamber and the accumulator chamber communicate with the pump pressure chamber by way of a three-port 2-way solenoid valve. In a first indexing position of the solenoid valve, the pump pressure chamber is in hydraulic connection with the actuator chamber and the accumulator chamber is sealed off. In this first indexing position, the opening and closing of the gas exchange valve is governed by the rotating cam. In a second indexing position of the solenoid valve, the pump pressure chamber is in hydraulic connection with the accumulator chamber and the actuator chamber is sealed off. In this second indexing position, the gas exchange valve is maintained in an open position independently of the position of the rotating cam. It will be appreciated that the hydraulic control system for a gas exchange valve disclosed in U.S. patent 6,227,154 has the disadvantage that the accumulator chamber requires quite a lot of space and causes further sealing and mounting problems. Furthermore, the accumulator chamber increases the volume of hydraulic fluid to be compressed, which has a negative effect on the dynamic behaviour of the hydraulic control system. Another disadvantage is that the gas exchange valve can only be closed if the cam piston is in contact with the closing ramp of the cam. It follows that early closing of the gas exchange valve and, consequently, a short valve-opening window cannot be achieved. A further disadvantage is that in case of failure in the control system of the solenoid valve, the gas exchange valve can still be open if the cam piston is in contact with the closing ramp of the cam. Thus, there is a risk of a collision between the engine piston and the open gas exchange valve.

Object of the invention

A technical problem underlying the present invention is to provide an improved hydraulic control system for a gas exchange valve of an internal combustion engine. This problem is solved by a hydraulic control system as claimed in claim 1.

Summary of the invention

A hydraulic control system in accordance with the present invention comprises a hydraulic actuator that is associated with the gas exchange valve for opening and/or closing the latter. A hydraulic pump provides a hydraulic energy supply for the hydraulic actuator. A cam means is associated with the hydraulic pump for periodically providing a mechanical energy supply for the hydraulic pump. A first control valve means is hydraulically connected between the hydraulic pump and the hydraulic actuator for controlling hydraulic energy supply to the hydraulic actuator. In accordance with an important aspect of the present invention, the hydraulic control system further comprises a mechanical energy accumulator connected between the cam means and the hydraulic pump. The cam means is capable of periodically storing mechanical energy into this mechanical energy accumulator and of periodically causing its discharge. The mechanical energy accumulator is capable of restoring mechanical energy stored therein to the hydraulic pump for producing the hydraulic energy required for controlling the hydraulic actuator. It will be appreciated that a hydraulic control system in accordance with the present invention provides, due to its mechanical energy accumulator, great flexibility for controlling the opening and closing of the gas exchange valve. If the hydraulic control valve means is e.g. maintained in an indexing position, in which it provides a hydraulic connection between the hydraulic pump and the hydraulic actuator, the opening and closing of the gas exchange valve is governed by the cam means, substantially as if the cam means would directly act on the gas exchange valve. A timed switching of the hydraulic control valve in an indexing position in which the hydraulic pump is hydraulically sealed off and the actuator pressure chamber is hydraulically connected to a low pressure tank, makes it possible to reduce the lift of the gas exchange valve, and to trigger an early closing or a deferred opening of the gas exchange valve. A timed switching of the hydraulic control valve in an indexing position in which both the hydraulic pump and the hydraulic actuator are hydraulically sealed off, makes it possible to maintain the gas exchange valve in an open position with a variable lift of the valve. It will further be appreciated that — in contrast to the hydraulic control system disclosed in U.S. patent 6,227,154 — a hydraulic control system in accordance with the present invention does not require a voluminous hydraulic accumulator chamber, which causes sealing and mounting problems. It follows that the volume of hydraulic fluid in the system may be rather small, which has a positive effect on the dynamic behaviour of the hydraulic control system.

In an advantageous embodiment of the hydraulic control system, the first control valve means includes a hydraulic control valve with three ports and two indexing positions, wherein: the first port is connected to the hydraulic pump; the second port is connected to the hydraulic actuator; and the third port is connected to the low pressure tank. In the first indexing position, the first port is in hydraulic communication with the second port; and in the second indexing position, the first port is sealed off and the second port is in hydraulic communication with the low-pressure tank. This hydraulic control system makes it possible to reduce the lift of the gas exchange valve, and to trigger an early closing or a deferred opening of the gas exchange valve.

In a preferred embodiment of the hydraulic control system, the first control valve means includes a hydraulic control valve with three ports and three indexing positions, wherein: the first port is connected to the hydraulic pump; the second port is connected to the hydraulic actuator; and the third port is connected to the low pressure tank. In the first indexing position, the first port is in hydraulic communication with the second port; in the second indexing position, the first port is sealed off and the second port is in hydraulic communication with the low pressure tank; and in the third indexing position the first port and the second port are sealed off. This hydraulic control system makes it possible to reduce the lift, to trigger an early closing or a deferred opening of the gas exchange valve and to maintain the gas exchange valve in an open position with a variable lift.

In a preferred embodiment of the present invention, the hydraulic pump includes a pump piston, a pump pressure chamber and a pump piston return spring. The mechanical energy accumulator is capable of urging the pump piston into a first end position, wherein the pump pressure chamber has a minimum volume, thereby performing a compression stroke. The pump piston return spring biases the pump piston into a second end position, wherein the pump pressure chamber has a maximum volume, thereby performing a suction stroke. In order to compensate a loss of hydraulic fluid to the low pressure tank without a separate low pressure feed pump, the hydraulic control system may comprise a suction line providing a hydraulic connection between the pump pressure chamber and a low-pressure tank means. A second control valve means is associated with the suction line for sealing it off during the compres- sion stroke and opening it during the suction stroke of the hydraulic pump. The second control valve means in the suction line is advantageously a spring biased check valve.

If it is desired to have an operation mode in which the opening and closing of the gas exchange valve is governed by the cam means, substantially as if the latter would directly act on the gas exchange valve, then the mechanical energy accumulator is advantageously designed to rigidly transmit the maximum compression force that is normally required for producing the pressure for actuating said hydraulic actuator, but to elastically deform, if the force to be transmitted exceeds said maximum compression force by a certain amount. The mechanical energy accumulator advantageously includes a compression spring, which is advantageously subjected to a pre-compression force that is greater than the maximum compression force to be rigidly transmitted onto the pump piston. A preferred embodiment of such a mechanical energy accumulator further includes a cup-shaped cam follower, which is axially guided in a bore and defines a spring chamber in which the compression spring is housed; and a spring collar, which is axially guided in the spring chamber. This spring chamber includes a bottom surface and a shoulder surface axially spaced from the bottom surface. The compression spring is pre-compressed between the bottom surface of the cup-shaped cam follower and the spring collar bearing on the shoulder surface. The cam means is capable of axially pushing the cup- shaped cam follower into the bore, to produce the compression stroke. The spring collar is in mechanical contact with a pump piston for transmitting a compression stroke to the hydraulic pump. It will be appreciated that such a "cartridge type" mechanical energy accumulator can be pre-assembled and tested before being mounted in the guiding bore, wherein it bears on the pump piston.

A preferred embodiment of the hydraulic control system further includes an auxiliary hydraulic connection between a pump pressure chamber and a low-pressure tank means. This auxiliary hydraulic connection is normally closed, except if the pump piston is in its second end position. It prevents an overfilling of the hydraulic system, by allowing an excess of hydraulic fluid in the hydraulic system to overflow into the low pressure tank means when the pump piston is in its second end position. A preferred embodiment of such an auxiliary hydraulic connection advantageously includes an annular groove in the second end of the pump pressure chamber (i.e. the end where the pump piston is located in its second end position). This annular groove is covered by the pump piston when the latter is out of its second end position and uncovered when the latter is in its second end position.

The gas exchange valve to be controlled advantageously includes: a valve head capable of sealing off a valve seat; a valve stem rigidly connected to the valve head; and a closing spring for biasing the valve head onto its valve seat.

The hydraulic actuator may also be a double-acting actuator including an actuator piston axially sealing a first actuator pressure chamber from a second actuator pressure chamber. In this case the first control valve means is advan- tageously switchable into:

(a) a first indexing position, in which it provides a hydraulic connection between the hydraulic pump and the first actuator pressure chamber and a hydraulic connection between the second actuator pressure chamber and a low pressure tank; (b) a second indexing position, in which it seals of the hydraulic pump, the first actuator pressure chamber and the second actuator pressure chamber; and

(c) a third indexing position, in which it provides a hydraulic connection between the hydraulic pump and the second actuator pressure cham- ber and a hydraulic connection between the first actuator pressure chamber and the low-pressure tank.

The second actuator pressure chamber assists the closing spring of the gas exchange valve in providing the closing force. It follows that a weaker valve closing spring can be used, which allows working with lower hydraulic pres- sures without deteriorating the dynamic behaviour of the gas exchange valve.

Brief description of the drawings

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1: is a schematic diagram of a first embodiment of a hydraulic control system in accordance with the present invention;

FIG. 2: is a schematic diagram of a second embodiment of a hydraulic control system in accordance with the present invention; and FIG. 3: is a schematic diagram of a third embodiment of a hydraulic control system in accordance with the present invention.

Detailed description of a preferred embodiment

In FIG. 1 to 3 are schematic diagrams of hydraulic control systems for an internal combustion engine. An engine cylinder is globally identified by reference number 10 and schematically represented by a part of its cylinder head 11. Reference number 12 generally designates an engine piston 12, which is reciprocally movable in the cylinder 10 as indicated by arrow 14. This engine piston 12 is coupled — in a manner known per se — to a rotating camshaft 16, so that the latter is rotated in synchronism with the reciprocating piston 12.

Reference number 18 generally designates a gas exchange valve, for ex- ample a gas inlet valve for admitting combustion air into the cylinder 10 or a gas exhaust valve for evacuating combustion gases from the cylinder 10. This gas exchange valve 18 comprises a valve seat 20, which surrounds a gas exchange opening 22 in the cylinder head 11, and a valve head 24, which is capable of sitting on the valve seat 20 so as to seal off the gas exchange opening 22 in the cylinder head 11. A valve stem 26, which is rigidly connected with one end to the valve head 24, extends axially through the gas exchange opening 22. A closing spring 30 engages a spring collar 32 of the valve stem

26, so as to bias the valve head 24 onto the valve seat 20. In other words, for opening the gas exchange valve 18, i.e. for lifting the valve head 24 from its valve seat 20, the valve stem 26 has to be axially pushed in the direction of arrow 34 against the biasing force of closing spring 30.

In the hydraulic control systems of FIG. 1 and FIG. 2, the opening of the gas exchange valve 18 is achieved by means of a single-acting hydraulic actuator generally designated by reference number 40. This single-acting hydraulic actuator includes an actuator piston 42 axially sealing an actuator pressure chamber 44 within a bore 46. The actuator piston 42 is coupled to the valve stem 26 so as to be capable of exerting onto the latter a pushing force in the direction of arrow 34 if the actuator pressure chamber 44 is pressurised with a pressure fluid. For lifting the valve head 24 from its valve seat 20, the pushing force developed by the actuator 40 must overcome the biasing force of the closing spring 30, as well as the pressure acting on the valve head 24 within the engine cylinder 10. It will be noted that in all three hydraulic control systems of FIG. 1 to 3, the actuator piston 42 is connected to the valve stem 26 so as to be capable of transmitting a pulling and a pushing force onto the valve stem 26. With a single-acting actuator 40, as shown in FIG. 1 and 2, it would of course be sufficient to have a loose contact between the actuator piston 42 and the valve stem 26, i.e. a coupling that is exclusively capable of transmitting a pushing force onto the valve stem 26. It remains to be noted that in case of such a loose contact, a spring is advantageously associated with actuator piston 42 to warrant that the actuator piston 42 is always in contact with the valve stem 26 and to avoid a hammering effect. Reference number 50 generally designates a hydraulic pump associated with the hydraulic actuator 40. This hydraulic pump includes a pump piston 52 fitted in a bore 54 wherein it axially seals off a pump pressure chamber 56. The pump piston 52 can be reciprocated between a first end position, wherein the pump pressure chamber 56 has a minimum volume, and a second end position, wherein the pump pressure chamber 56 has a maximum volume. In FIG. 1 the pump piston 52 is shown in its second end position, wherein the pump pressure chamber 56 has its maximum volume. When the pump piston 52 is moved from its second end position into its first end position, the pump piston 52 performs a compression stroke, pressing a hydraulic fluid from the pump pressure cham- ber 56 into a pump line 58, which forms an opening in a bottom surface 59 of the pressure chamber 56. When moved from its first end position into its second end position, the pump piston 52 performs a suction stroke.

Reference number 60 generally designates a low-pressure tank, i.e. a reservoir of hydraulic fluid under atmospheric pressure or under small excess pressure over atmospheric pressure. (Such a reservoir may e.g. be formed by the cylinder head itself.) A first end of a suction line 62 is submerged in the hydraulic fluid in the low-pressure tank 60. Its second end is connected to the pump line 58, but could also lead directly into the pump pressure chamber 56. This suction line 62 includes a check valve 64 with a check valve closing spring 66. The check valve 64 allows fluid flow only in the direction of the low- pressure tank 60, i.e. it seals off the suction line 62 during the compression stroke of the hydraulic pump 50. The check valve 64 opens if the suction pressure developed by the hydraulic pump 50 during the suction stroke is sufficient to overcome the closing force of the check valve closing spring 66. Hydraulic fluid is then sucked from the low-pressure tank 60 through the suction line 62 into the pump pressure chamber 56.

It will be noted that the pump pressure chamber 56 is provided with an annular groove 68 in its cylindrical wall near its second end, i.e. the end where the pump piston 52 is located in its second end position. A leakage line 70 hydraulically connects this annular groove 68 to the low-pressure tank 60. During the compression stroke of the hydraulic pump 50, the pump piston 52 covers the annular groove 68. It then collects pressurised hydraulic fluid leaking in-between the pump piston 52 and the cylindrical wall of the pump pressure chamber 56, and the leakage line 70 evacuates this small leakage flow into the low-pressure tank 60. However, at the end of the suction stroke, i.e. when the pump piston 52 comes into its second end position, the pump piston 52 uncovers the annular groove 68. The pump pressure chamber 56 is now connected through the leakage line 70 to the low-pressure tank 60, so that an excess of hydraulic fluid in the hydraulic circuit can overflow into the low- pressure tank 60. The hydraulic pump 50 is driven and controlled by a cam lobe 72 of the camshaft 16. A pump piston return spring 74 is associated with the pump piston 52 so as to bias it into its second end position, wherein the pump pressure chamber 56 has a maximum volume. During the compression stroke the pump piston return spring 74 is compressed and stores energy for thereafter generat- ing the suction stroke under control of the cam lobe 72.

Reference number 80 generally designates a mechanical energy accumulator connected between the rotating cam lobe 72 and the reciprocation pump piston 52. The rotating cam lobe 72 is capable of periodically storing mechanical energy into the mechanical energy accumulator 80 and of periodically causing its discharge. The mechanical energy accumulator 80 is capable of restoring mechanical energy that is stored therein to the hydraulic pump 50, for producing the hydraulic energy required for controlling the hydraulic actuator 40.

In the preferred embodiment shown in FIG. 1 to 3, the mechanical energy accumulator 80 includes a compression spring 82, a cup-shaped cam follower 84 and a spring collar 86. The cup-shaped cam follower 84 is axially guided in a bore 88, which is arranged in axial alignment with the bore 54, in which the pump piston 52 is fitted. The cup-shaped cam follower 84 may advantageously include a roller element (not shown) forming the contact surface for the rotating cam lobe 72. The spring collar 86 is axially guided in a spring chamber 90 in the cup-shaped cam follower 84. The compression spring 82 is fitted between a bottom surface 92 of the spring chamber 90 and the spring collar 86. The latter has an end position in which it bears on a shoulder surface 94, which may e.g. be formed by a retainer ring 95 fitted into an annular grove of the spring chamber 90. It will be appreciated that the compression spring 82 is subjected, between the bottom surface 92 and the spring collar 86 bearing on the shoulder surface 94, to a pre-compression force. This pre-compression force is advantageously greater than the maximum compression force to be transmitted onto the pump piston 52 for producing the required hydraulic pressure for opening the hydraulic actuator 40. As long as the force to be transmitted onto the pump piston 86 does not exceed this pre-compression force, the compres- sion spring 82 will then behave as a rigid body. However, if the force to be transmitted onto the pump piston 52 exceeds this pre-compression force, the compression spring 82 will elastically deform and absorb the compression stroke developed by the rotating cam lobe 72. It will be appreciated that the mechanical energy accumulator 80 is advantageously a pre-assembled cartridge that is fitted into the bore 88 between the cam lobe 72 and the pump piston 52, wherein the pump piston return spring 82 warrants a close contact between this three elements in all operating conditions.

The cam lobe 72, which engages the cup-shaped cam follower 84, can e.g. be divided in four angular annular sectors I, II, III and IV, wherein arrow 96 indicates the sense of rotation of the cam lobe 72. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector I (which can be identified as an opening ramp), it pushes the cam follower 84 into its bore 88. Within this angular sector I, the rotating cam lobe 72 puts mechanical energy into the system. As long as the pre-compressed compression spring 82 behaves as a rigid body, this mechanical energy input (minus friction losses) will be transmitted by the spring collar 86 onto the pump piston 52. However, as soon as the pre-compressed compression spring 82 begins to elastically deform, the mechanical energy will be stored in the mechanical energy accumulator 80. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector II, it maintains the cam follower 84 in its inserted position. Within this angular sector II, the rotating cam lobe 72 can no longer put mechanical energy into the system. However, mechanical energy that has been previously stored into the mechanical energy accumulator 80 can still be restored to the hydraulic pump 50, for producing the hydraulic energy required by the hydraulic actuator 40. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector III (which can be identified as a closing ramp), it allows the cam follower 84 to be raised in its bore 88. Within this angular sector III, mechanical energy stored in the mechanical energy accumulator 80 is gradually restored to the rotating cam lobe 72; i.e. the mechanical energy accumulator 80 is gradually discharged. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector IV, it maintains the cam follower 84 in its raised position. Within this angular sector IV, there is no energy exchange between the rotating cam lobe 72 and the mechanical energy accumulator 80, and the discharged mechanical energy accumulator 80 is no longer capable of providing the mechanical energy required by the hydraulic pump 50 for producing the hydraulic energy for opening the hydraulic actuator 40.

In FIG. 1 , reference number 100 generally designates a hydraulic control valve with three ports 102, 104, 106 and two indexing positions. The pump line 58 is connected to the first port 102 of the control valve 100. The actuator pressure chamber 44 is connected to the second port 104 of the control valve 100. The low-pressure tank 60 is connected to a third port 106 of the control valve 100. In the first indexing position, which is shown on FIG. 1 , the third port 106 is sealed off, and the first port 102 is in hydraulic communication with the second port of the control valve 100, whereby the pump line 58 is in hydraulic communication with the actuator pressure chamber 44. In the second indexing position, the first port 102 is sealed off, and the second port 104 is in hydraulic communication with the third port 106 of the control valve 100, whereby the actuator pressure chamber 44 is in hydraulic communication with the low pressure tank 60. An actuator 108, which is triggered by an engine controller (not shown), allows switching the hydraulic control valve 100 between its first and second indexing position. The non-energised hydraulic control valve 100 is advantageously spring biased in its first indexing position.

The most important operation modes of the hydraulic control system of FIG. 1 can be summarised as follows:

- Let us first assume that the control valve 100 is in its first indexing position, which is shown on FIG. 1. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector I, hydraulic fluid is pumped from the pump pressure chamber 56 into the actuator pressure chamber 44, thereby push- ing the actuator piston 42 in the direction of arrow 34 to open the gas ex- change valve 18. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector II, the gas exchange valve 18 is maintained in the open position, because the pump piston 52 is maintained in its second end position, wherein the pressure is maximum in the hydraulic control system. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector III, the pressure in the spring pressure chamber 56 and the actuator pressure chamber 44 decreases, whereby the valve closing spring 30 becomes capable of closing the gas exchange valve 18. The hydraulic fluid contained in the actuator pressure chamber 44 is now pumped back into the pump pressure chamber 56. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector IV, the gas exchange valve 18 is maintained in the closed position, because the pump piston 52 is maintained in its second end position, wherein the pressure is minimum in the hydraulic control system. It will be appreciated that the mechanical energy accumulator 80 behaves as a rigid body, because the compression force to be transmitted onto the pump piston 52 during angular sector I and II is lower than the pre-compression force of the compression spring 82. In conclusion, in the first indexing position of the control valve 100, the opening and closing of the gas exchange valve 18 is essentially determined by the profile of the cam lobe 72. Let us now assume that the control valve 100 is switched in its second indexing position, during engagement of the angular sector I of the cam lobe 72 with the cam follower 84. This switching results in that the pump line 58 is sealed off and in that the pressure in the actuator pressure chamber 44 is relieved into the low pressure tank 60. It follows that the gas exchange valve 18 closes while the rotating cam lobe 72 still engages the cam follower 84 with its angular sector I and tends to push the cam follower 84 further into its bore 88. As the pump pressure chamber 56 is now entirely sealed off, the pressure force acting onto the pump piston 52 rapidly in- creases. When this force exceeds the pre-compression force of the compression spring 82, the latter elastically deforms and absorbs the downward movement of the cam follower 84. The cam lobe 72 can continue to rotate, but the gas exchange valve 18 is entirely uncoupled from the rotating cam lobe 72. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector III, the compression spring 82 first regains its original length, i.e. the mechanical energy accumulator 80 is discharged. Thereafter the pump piston return spring 74 produces a suction stroke, which opens the check valve 64, so that the pump pressure chamber 56 can be refilled with hydraulic fluid from the low pressure tank 60. When the rotating cam lobe 72 engages the cam follower 84 with its angular sector IV, the pump piston 52 is in its second end position, wherein the pump pressure chamber

56 is in communication with the low pressure tank 60 through the leakage line 70. The control valve 100 may now be switched back in its first indexing position to generate the next opening stroke of the gas exchange valve 18 when the rotating cam lobe 72 engages the cam follower 84 with its angular sector I.

It will be appreciated in the hydraulic control system of FIG. 1 provides multiple possibilities for controlling the opening and closing of the gas exchange valve 18. If the hydraulic control valve 100 is maintained in its first indexing position, the opening and closing of the gas exchange valve 18 is governed by the cam lobe 72, substantially as if the cam lobe 72 would directly act on the valve stem 26. An early closing of the gas exchange valve 18 can be obtained by switching the hydraulic control valve 100 from its first indexing position into its second indexing position, when the rotating cam lobe 72 engages the cam follower 84 either with its angular sector I or with its angular sector II. A reduced lift of the gas exchange valve 18 can be obtained by switching the hydraulic control valve 100 from its first indexing position into its second indexing position, when the rotating cam lobe 72 engages the cam follower 84 with its angular sector I. A deferred opening of the gas exchange valve 18 can be obtained if the hydraulic control valve 100 is in its second indexing position when the rotating cam lobe 72 engages the cam follower 84 with its angular sector I or II and is then switched into its second indexing position while the rotating cam lobe 72 still engages the cam follower 84 with its angular sector I or II. It will be noted that when the rotating cam lobe 72 engages the cam follower 84 with its angular sector II, mechanical energy stored in the mechanical energy accumulator 80 will be used for producing the hydraulic energy required for opening the hydraulic actuator 40. Furthermore, if the rotating cam lobe 72 engages the cam follower 84 with its angular sector IV, the gas exchange valve 18 is always entirely closed, independently of the indexing position of the hydraulic control valve 100, which eliminates a collision risk between the valve head 24 and the engine piston 12 in case of a failure of the hydraulic control valve 100 or in its control circuit.

The hydraulic control system of FIG. 2 distinguishes itself from the hydraulic control system of FIG. 1 , exclusively in that the hydraulic control valve 200 has three indexing positions. The first and second indexing positions are the same as those described above for the hydraulic control valve 100. In the third indexing position all three ports 202, 204 and 206 of the hydraulic control valve 200 are sealed off. An actuator 208, which is triggered by an engine controller (not shown), allows switching the hydraulic control valve 200 between its first, second and third indexing positions. In the absence of current, the hydraulic control valve 200 is advantageously spring biased in its first indexing position.

In addition to the possibilities for controlling the opening and closing of the gas exchange valve 18 described above for the hydraulic control system of FIG. 1 , the hydraulic control system of FIG. 2 allows to maintain the gas exchange valve 18 in an open position independently of the position of the cam lobe 72. Furthermore, by switching the hydraulic control valve 200 into the third indexing position when the rotating cam lobe 72 engages the cam follower 84 with its angular sector I or II, it is possible to limit the lift of the gas exchange valve 18 to a desired value and to maintain this lift for a desired period of time.

The hydraulic control system of FIG. 3 distinguishes itself from the hy- draulic control system of FIG. 2, exclusively in that the opening and closing of the gas exchange valve 18 is achieved by means of a double-acting hydraulic actuator generally designated by reference number 340. The latter includes an actuator piston 342 axially sealing a first actuator pressure chamber 344 from a second actuator pressure chamber 344'. The first actuator pressure chamber 344 is exactly an equivalent of the actuator pressure chamber 44 in the single- acting actuator of FIG. 1 and 2. The object of the second actuator pressure chamber 344' is to assist the valve closing spring 30 in providing the closing force. It follows that a weaker valve closing spring 30 can be used, which allows working with lower pressures without deteriorating the dynamic behav- iour of the gas exchange valve 18.

In FIG. 3, reference number 300 generally designates a hydraulic control valve with four ports and three indexing positions that is used to control the double-acting hydraulic actuator 340. The pump line 58 is connected to the first port 302 of the control valve 300. The first actuator pressure chamber 344 is connected to the second port 304 of the control valve 300. The second actuator pressure chamber 344' is connected to the third port 304' of the control valve 300. The low-pressure tank 60 is connected to the fourth port 306 of the control valve 300. In the first indexing position, which is shown on FIG. 3, the first port 302 is in hydraulic communication with the second port 304 of the control valve 300, whereby the pump line 58 is in hydraulic communication with the first actuator pressure chamber 344, and the third port 304' is in hydraulic communication with the fourth port 306 of the control valve 300, whereby the second actuator pressure chamber 344' is in hydraulic communication with the low pressure tank 60. In the second indexing position, the first port 302 is in hydraulic communication with the third port 304' of the control valve 300, whereby the pump line 58 is in hydraulic communication with the second actuator pressure chamber 344', and the second port 304 is in hydraulic communication with the fourth port 306 of the control valve 300, whereby the first actuator pressure chamber 344 is in hydraulic communication with the low pressure tank 60. In the third indexing position, all the four ports 302, 304, 304' and 306 are sealed off. An actuator 308, which is triggered by an engine controller (not shown), allows switching the hydraulic control valve 300 between its first, second and third indexing positions. In the absence of current, the hydraulic control valve 300 is advantageously spring biased in its first indexing position. The hydraulic control system of FIG. 3 basically provides the same possibilities for controlling the opening and closing of the gas exchange valve 18 as the hydraulic control system of FIG. 2. It will however be appreciated that if the hydraulic control valve 300 is switched into its second position as long as the rotating cam lobe 72 engages the cam follower 84 with its angular sector I or II, a pressurisation of the second actuator pressure chamber 344' will assist the valve closing spring 30 in accelerating the actuator piston 42 in the closing direction.

It will be noted that hydraulic control systems in accordance with the invention are advantageously provided for the inlet valves of the engine cylin- ders, whereas an exhaust valve camshaft can still mechanically actuate the exhaust valves of the engine cylinders. The cam lobes 72 associated with the hydraulic pumps 50 can then be provided on the exhaust camshaft, and no separate camshaft is required for actuating the hydraulic pumps 50.

Claims

1. A hydraulic control system for a gas exchange valve (18) of an internal combustion engine, comprising: a hydraulic actuator (40) associated with said gas exchange valve (18) for opening and/or closing the latter; a hydraulic pump (50) associated with said hydraulic actuator (40) for providing a hydraulic energy supply for said hydraulic actuator (40); a cam means (72) associated with said hydraulic pump (50) for periodically providing a mechanical energy supply for said hydraulic pump (50); first control valve means (100, 200, 300) hydraulically connected between said hydraulic pump (50) and said hydraulic actuator (40) for controlling hydraulic energy supply to said hydraulic actuator (40); characterised by a mechanical energy accumulator (80) connected between said cam means (72) and said hydraulic pump (50), said cam means (72) being capable of periodically storing mechanical energy into said mechanical energy accumulator (80) and of periodically causing its discharge, and said mechanical energy accumulator (80) being capable of restoring mechanical energy stored therein to said hydraulic pump (50) for producing the hydraulic en- ergy required for controlling said hydraulic actuator (40).

2. The hydraulic control system as claimed in claim 1 , wherein said first control valve means (100, 200, 300) is switchable into a first indexing position, in which it provides a hydraulic connection between said hydraulic pump (50) and said hydraulic actuator (40), and in at least one other operating posi- tion, in which said hydraulic pump (50) is hydraulically sealed off, and said hydraulic actuator (40) is either hydraulically connected to a low pressure tank means (60) or hydraulically sealed off.

3. The hydraulic control system as claimed in claim 2, wherein said first control valve means (100) includes a hydraulic control valve with three ports (102, 104, 106) and two indexing positions, wherein: said first port (102) is connected to said pump (50); said second port (104) is connected to said hydraulic actuator (40); said third port (106) is connected to said low-pressure tank (60); in said first indexing position, said first port (102) is in hydraulic communication with said second port (104); and in said second indexing position, said first port (102) is hydraulically sealed off and said second port (104) is in hydraulic communication with said low pressure tank (60).

4. The hydraulic control system as claimed in claim 2, wherein said first control valve means (200) includes a hydraulic control valve with three ports (202, 204, 206) and three indexing positions, wherein: said first port (202) is connected to said pump (50); said second port (204) is connected said hydraulic actuator (40); said third port (206) is connected to said low-pressure tank (60); in said first indexing position, said first port (202) is in hydraulic communication with said second port (204); in said second indexing position, said first port (202) is hydraulically sealed off and said second port (204) is in hydraulic communication with said low pressure tank (60); and in said third indexing position, said first port (202) and said second port (204) are hydraulically sealed off. 5. The hydraulic control system as claimed in any one of claims 1 to 4, wherein said hydraulic pump (50) includes: a pump piston (52) and a pump pressure chamber (56), said mechanical energy accumulator (80) being capable of urging said pump piston (52) into a first end position wherein said pump pressure chamber (56) has a minimum volume, thereby performing a compression stroke; and a pump piston return spring (74) associated with said pump piston (52) so as to bias it into a second end position wherein said pump pressure chamber (56) has a maximum volume, thereby performing a suction stroke.

6. The hydraulic control system as claimed in 5, further comprising: a suction line (62) providing a hydraulic connection between said pump pressure chamber (56) and a low pressure tank means (60); and a second control valve means (64) associated with said suction line (62) for sealing off said suction line (62) during said compression stroke and opening it during said suction stroke.

7. The hydraulic control system as claimed in 6, wherein said second control valve means (64) is a spring biased check valve. 8. The hydraulic control system as claimed in any one of claims 1 to 8, wherein: said hydraulic pump (50) includes a pump piston (52) and a pump pressure chamber (56); and said mechanical energy accumulator (80) is designed to rigidly transmit onto said pump piston (52) a maximum compression force required for actuating said hydraulic actuator (40), but to elastically deform, if the force to be transmitted on said pump piston (52) exceeds said maximum compression force by a certain amount.

9. The hydraulic control system as claimed in any one of claims 1 to 8, wherein: said mechanical energy accumulator (80) includes a compression spring (82).

10. The hydraulic control system as claimed in claim 9, wherein: said mechanical energy accumulator (80) includes a compression spring (82) that is subjected to a pre-compression force that is greater than a compression force to be rigidly transmitted onto said pump piston (52).

11. The hydraulic control system as claimed in claim 10, wherein: said hydraulic pump includes a pump piston (52) and a pump pressure chamber (56), said pump piston (52) being movable between a first end position wherein said pump pressure chamber (56) has a minimum volume and a second end position wherein said pump pressure chamber (56) has a maximum volume; and said mechanical energy accumulator (80) further includes: a cup-shaped cam follower (84) that is axially guided in a guiding bore (88), said cup-shaped cam follower (84) defining a spring chamber (90) in which said compression spring (82) is housed, said spring chamber (90) including a bottom surface (92) and a shoulder surface (94), said shoulder surface (94) being axially spaced from said bottom surface

(92); a spring collar (86) that is axially guided in said spring chamber (90), said compression spring (82) being pre-compressed between said bottom surface (92) of said cup-shaped cam follower (84) and said spring collar (86) bearing on said shoulder surface (94); said cam means (72) being capable of axially pushing said cup-shaped cam follower (84) into said guiding bore (88), so as to produce said compression stroke; and said spring collar (86) being in mechanical contact with said pump pis- ton (52) for transmitting a compression stroke to said pump piston (52).

12. The hydraulic control system as claimed in any one of claims 1 to 11, wherein:

said hydraulic pump includes a pump piston (52) and a pump pressure chamber (56), said pump piston (52) being movable between a first end position wherein said pump pressure chamber (56) has a minimum volume and a second end position wherein said pump pressure chamber (56) has a maximum volume; and said hydraulic control system further includes: low pressure tank means (60); and an auxiliary hydraulic connection (68, 70) between said pump pressure chamber (56) and said low-pressure tank means (60), said auxiliary hydraulic connection (68, 70) being normally closed, except if the pump piston (52) is in its second end position. 13. The hydraulic control system as claimed in claim 12, wherein: said pump pressure chamber (56) has a first end where said pump piston (52) is located in its first end position and to which said pump line (58) is hydraulically connected; said pump pressure chamber (56) has a second end where said pump pis- ton (52) is located in its second end position; said auxiliary hydraulic connection includes: an annular groove (68) in said second end of said pump pressure chamber (56), said annular groove being covered by said pump piston (52) when the latter is out of its second end position and uncovered when the latter is in its second end position; and a leakage line (70) providing a hydraulic connection between said annular groove (68) and said low-pressure tank means (60).

14. The hydraulic control system as claimed in any one of claims 1 to 13, wherein said gas exchange valve (18) includes: a valve head (24) capable of sealing off a valve seat (20); a valve stem (26) rigidly connected to said valve head (24); and a closing spring (30) for biasing said valve head (24) onto said valve seat (20).

5. The hydraulic control system as claimed in any one of claims 1 to 14, wherein: said hydraulic actuator (340) is a double acting actuator including an actuator piston (342) axially sealing a first actuator pressure chamber (344) from a second actuator pressure chamber (344'); and said first control valve means (300) is switchable into:

(a) a first indexing position, in which it provides a hydraulic connection between said hydraulic pump (50) and said first actuator pressure chamber (344) and a hydraulic connection between said second ac- tuator pressure chamber (344') and a low pressure tank (60);

(b) a second indexing position, in which it seals off said hydraulic pump (50), said first actuator pressure chamber (344) and said second actuator pressure chamber (344'); and

(c) a third indexing position, in which it provides a hydraulic connection between said pump line (58) and said second actuator pressure chamber (344') and a hydraulic connection between said first actuator pressure chamber (344) and said low-pressure tank (60).