Abstract:
A hydraulically operated valve control system includes a hydraulic flow divider including a hydraulic valve adapted to distribute, between two lines feeding respectively to actuators coupled to two inlet or outlet valves of a cylinder, the flow of oil coming either from a source of oil under pressure or from the feeding lines. The oil flow is distributed between the two feeding lines on the basis of the ratio of oil flow-rates in these two lines.

Description:
BACKGROUND AND SUMMARY 
     This invention concerns an hydraulically operated valve control system for an internal combustion engine. It also concerns an internal combustion engine equipped with such a system. 
     Internal combustion engines are more and more equipped with multi-valve injection systems where two inlet valves and/or two exhaust valves are provided for each cylinder in order to optimize the flow of the air-fuel mixture or the exhaust gases to or from a combustion chamber. These sets of two valves must be driven in such a manner that the valves have parallel movements, that is the same lift and speed for both valves. 
     EP-A-O 736671 teaches the use of balancing springs which engage a piston fast with each valve in order to move each valve towards a closing position. Such an approach works if the friction forces for each valve and the rigidity of the two springs are identical and if the hydraulic feeding circuits are symmetrical. Such conditions cannot be guaranteed because of the tolerances in the fabrication of the valves, in the fabrication of the springs and in the distribution of the fluids circuits within a cylinder head. Therefore, it is not sure the two valves of the prior art actually have the same movements. 
     U.S. Pat. No. 5,619,965 discloses an arrangement for balancing valves in a hydraulic camless valve train. Valve position sensors are used in conjunction with an electronic control unit to pilot opening and closing of solenoid valves. Such an arrangement is complex and expensive since it requires sensors and solenoid valves dedicated to each inlet valve/exhaust valve of the engine. 
     It is desirable to provide an hydraulically operated valve control system which efficiently controls the movements of two valves, without requiring electronic sensors or other complex and expensive equipments. 
     An aspect of the invention concerns an hydraulic operated valve control system for an internal combustion engine having at least one cylinder provided with two valves driven with oil coming from a source of oil under pressure, each valve being controlled by an hydraulic actuator fed with oil under pressure through a respective feeding line. This system is characterized in that it includes an hydraulic flow divider comprising an hydraulic valve adapted to distribute the flow of oil coming either, from said source or from said two feeding lines between said two feeding lines, depending on the ratio of oil flow-rates in these two lines. 
     Thanks to an aspect of the invention, the hydraulic valve can evenly distribute oil to the two inlet valves or two exhaust valves when these valves are supposed to be lifted. 
     Similarly, when the valves are supposed to be closed, the flow divider of the system of the invention accommodates evenly the two flows coming from the two inlet or exhaust valves. 
     According to further aspects of the invention, the control system might incorporate one or several of the following features:
         The hydraulic valve comprises a valve member which is movable depending on pressure drops created across two throttles located respectively in a connecting line between said source and one of the feeding lines.   The valve member is automatically moved towards a position of balance of the pressure drops across these throttles.   The valve member is advantageously movable in a valve body which is defines a bore, where the valve member is slidably movable and which forms an internal volumes where oil under pressure acts on the valve member in order to move it in translation along a longitudinal axis, these volumes being fluidically connected to the connecting lines either upstream or downstream of the throttles.   The hydraulic valve body defines four internal volumes, two internal volumes being fluidically connected to a first connecting line in fluid connection with a first valve, respectively upstream and downstream of a first throttle located in this first connecting line, whereas the other two internal volumes are fluidically connected to a second connecting line in fluid connection a second valve, respectively upstream and downstream of a second throttle located in the second connecting line.   The pressure within the internal volume connected to the first connecting line upstream of the first throttle and the pressure within the internal volume connected to the second connecting line downstream of the second volume tend to move the valve member in a first direction along the longitudinal axis of the bore, whereas the pressure within the internal volume connected to the first connecting line downstream of the first throttle and the pressure within the internal volume connected to the second connecting line upstream of the second throttle tend to move the valve member in a second direction opposite the first direction.   According to a first embodiment of the invention, the throttles are each provided on a shuttle movable between two positions, depending on the direction of oil flow in the feeding lines. In such a case, the internal volumes of the hydraulic valve body are advantageously connected to the feeding lines upstream or downstream of the corresponding throttle, irrespective the position of the shuttles.   According to another embodiment of the invention, the throttles are provided on fixed part of the connecting lines, check valves being respectively provided between the internal volumes of the hydraulic valve body and the throttles.   The flow divider also includes two solenoid valves connecting selectively the hydraulic valves respectively to the source of oil under pressure and to a low pressure circuit.       

     An aspect of the invention also concerns an internal combustion engine provided with a control system as mentioned here above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on the basis of the following description, which is given in correspondence with the annexed figures as an illustrative example, without restricting the object of the invention. In the annexed figures: 
         FIG. 1  is a schematic view of an internal combustion engine according to the invention comprising a control system according to the invention; 
         FIG. 2  is a schematic view of the flow divider and electronic control unit of the control system of the engine of  FIG. 1 ; 
         FIGS. 3A to 3E  show variations of some physical values, as a function of time, when the control system is being operated; 
         FIG. 4  is a schematic view of a hydraulic valve belonging to the flow divider of  FIG. 2  in a first configuration of work; 
         FIG. 5  is a view similar to  FIG. 4  when the valve is in a second configuration of work; and 
         FIG. 6  is a view similar to  FIG. 4  for a valve according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The camless internal combustion engine E schematically represented on  FIG. 1  comprises several cylinders. One cylinder  1  is partly represented and a piston  2  is slidably movable within cylinder  1 . A combustion chamber  3  is defined between a front face  2   a  of piston  2  and cylinder head  4 . Two inlet ducts  11  and  21  are mounted on cylinder head  4  to feed combustion chamber  3  with fuel. The flow of fuel within ducts  11  and  21  is controlled by two inlet valves  12  and  22  urged to a closed position by two springs  13  and  23  and piloted each by an hydraulic actuator  14  or  24 . 
     Each actuator  14  or  24  is fed with oil under pressure through a respective feeding line  15  or  25 . A hydraulic flow divider  101  is provided to selectively provide actuators  14  and  24  with oil under pressure, when it is necessary to open valves  12  and  22 . 
     Divider  101  is piloted by an electronic control unit  102  and fed with oil under pressure via a main feeding line  103  which comes from a filtration unit  104  fed by a pump  105  pumping oil in a sump  106 . A main exhaust line  107  conveys oil from divider  101  back to sump  106 . 
     Oil coming from pump  105  has a pressure between about 70 and about 210 bars. 
     Cylinder  1  is provided with some other non represented valves, at least an exhaust valve. When it is desired to open valves  12  and  22 , electronic control unit  102  sends to flow divider  101 , an electric signal S-i, via an electric line  1021 . Flow divider  101  converts this signal into a double pressure hydraulic signal S 12 , S 22  adapted to control actuators  14  and  24  in order to lift valves  12  and  22  with respect to their respective seats  16  and  26 . As shown on  FIG. 2 , flow divider  101  comprises an hydraulic valve  110  connected to line  103  via a first solenoid valve  117  and to line  107  via a second solenoid valve  118 . When they are not activated, valves  117  isolates hydraulic valve  110  from main feeding line  103  and valve  118  connects hydraulic valve  110  to main exhaust line  107 . The outlet port of valve  117  and the inlet port of valve  118  are respectively connected to hydraulic valve  110  via a common line  35 . 
     When solenoid valve  117  is activated to allow communication between line  103  and valve  110 , a main flow of oil under pressure flows from line  103  to hydraulic valve  110  with a flow-rate F 0 . This flow-rate is divided by hydraulic valve  110  into two secondary flow-rates Fi and F 2  which convey respectively hydraulic signal S 12  and S 22 . 
     Referring now to  FIG. 3 , several variations of parameters with respect to time should be considered.  FIG. 3A  shows the part of electrical signal Si sent by unit  102  to solenoid valve  117  as a function of time t. One notes Sn 7  this part of signal. Similarly,  FIG. 3B  shows, as a function of time t, the part of signal Sna sent to solenoid valve  118 . Signals Sn 7  and Sue are sent from an instant t 0 , respectively for a first period of time δtn 7  and for a second period time δt-n 8 . 
       FIG. 3C  shows the flow-rate F 0  in line  35  as a result of the opening and closing of solenoid valves  117  and  118 . F 0  is positive when oil flows from valve  117  to valve  110  and negative when oil flows from valve  110  to valve  118 . 
       FIG. 3D  shows the values of flow-rates Fi and F 2  in lines  15  and  25 , respectively. These values are kept substantially identical, as explained here-under. 
     Finally,  FIG. 3E  shows, the lifts Lu and L 12  of valves  11  and  12  as a result of flow-rates Fi and F 2 . In order that lifts L 11  and Li 2  are identical or superimposed on  FIG. 3E , that is in order to have parallel movements of valves  11  and  12 , flow-rates Fi and F 2  must be substantially identical. 
     In order to obtain such identical flow-rates Fi and F 2 , hydraulic valve  110  is constituted as shown on  FIGS. 4 and 5 . Valve  110  comprises a valve body  1101  which defines a main bore  1102  extending along the direction of an axis X 2 . A valve member  1103  in the form of a spool is slidably mounted within bore  1102  and comprises a main portion  1103 A and two lateral portions  1103 - j  and  1103   2 , axially secured to main portion  1103 A thanks to two locking rings  1103 B and  1103 C. Within bore  1102 , spool  1103  is compressed between two springs  1104   i  and  1104   2  which tend to return spool  1103  to a central position within bore  1102 . It is possible to adjust the central position of spool  1103  within bore  1102  thanks to an adjusting screw  1105  which defines the reference surface of spring  1104   1  on its side opposite to spool  1103 . 
     Main portion  1103 A comprises a central rod  1103 D whose diameter Di is significantly smaller than the diameter D 2  of the central part  1102 A of bore  1102  which communicates with line  35 . On either sides of part  1102 A, bore  1102  is provided with two grooves  1102 -ι and  1102   2  whose diameter D′ 2  is substantially larger than the maximum diameter D 3  of spool  1103 . One notes Vi the volume of groove  1102 -ι and of the part of bore  1102  which surrounds central rod  1103 D at the axial level of this groove. One notes V 2  the volume of groove  1102   2  and the portion of bore  1102  which surrounds rod  1103 D at the axial level this groove. 
     Depending on the position of spool  1103  along axis X 2 , volume Vi is smaller, equal or larger than volume V 2 . More precisely, volumes V 1  and V 2  are substantially equal on  FIG. 4  and, if spool  1103  moves towards the left on this figure, volume V 1  becomes larger than volume V 2 . 
     Volumes Vi and V 2  are fed with oil under pressure by the oil flow, as shown by arrows F, when solenoid valve  111  is activated. Around rod  1103 D, the main flow of oil, having flow-rate F 0 , divides itself into two secondary flows having each a flow-rate Fi or F 2 . These flow-rates follow the following equation
 
 F   0   =F   1   +F   2  
 
     A first conduit  1106 - 1  connects volume V 1  to a bore  1107 - 1  where a shuttle  1108 - 1  is movable along a longitudinal axis X 71  of bore  110 T 1 . Shuttle  1108 - 1  is provided with a central longitudinal bore  1109 - 1  which defines a canal for the flow of oil F coming from line  1106   i . This oil flow exits bore  1107   i  through an exhaust conduit  111 O 1  which is connected to line  15 . 
     A throttle  1111   1  is defined within central bore  1109   1  and this throttle creates a pressure drop in bore  1109 -ι when oil flows from conduit  11061  towards conduit  1110   i    
     Similarly, a conduit  1106   2  leads from volume V 2  to a bore  1107   2  where a shuttle  1108   2  is slidably movable along a longitudinal axis X 72  of this bore. Bore  1107   2  is connected by an exhaust conduit  1110   2  to line  25 . A throttle  1111   2  is defined in a central bore  1109   2  of shuttle  1108   2 . Conduit  1106   1  bores  1107   1  and  1109   1  and conduit  1110   1  form together a connecting line CU between bore  1102  and feeding line  15 . Similarly, conduits  1106   2  and  111  O 2  and bores  1107   2  and  1109   2  form together a connecting line CL 2  between bore  1102  and line  25 . 
     Four hydraulic chambers are defined in bore  1102  around spool  1103 . 
     A first chamber  1102 B is defined between portion  1103   i  and screw  1105 . 
     A second chamber  1102 C is defined around portion  1103   i  and is limited by a first end surface  1103 Ai of portion  1103 A. Pressure within chambers  1102 B and  1102 C acts on the end surface of portion  110 S 1  and on surface  1103 Ai to push spool  1103  against the action of spring  1104   2 , that is towards to right on  FIG. 4 , in the direction of arrow A 1 . 
     A third chamber  1102 D is defined around the free end of lateral portion  1103   2  and a fourth chamber  1102 E is defined around portion  1103   2  and limited by a second end surface  1103 A 2  of portion  1103 . Pressure within chambers  1102 D and  1102 E tends to push spool  1103  against the action of spring  1104 - ι  that is towards the left on  FIG. 4 , in the direction of arrow A 2 . 
     Chambers  1102 B and  1102 D, on the one hand, and chambers  1102 C and  1102 E, on the other hand, are symmetrical with respect to a central axis Xi of body  1101 . Shuttle  1108   i  is provided with a first external groove  1112 A and a second external groove  1112 B offset axially with respect to groove  1112 A. Groove  1112 A is connected to central bore  1109   i  via a first canal  1112 C, whereas groove  1112 B is connected to central bore  1109   i  via a second canal  1112 D. Canals  1112 C and  1112 D are located on either sides of throttle  1111   1 . Similarly, shuttle  1108   2  is provided with two external grooves  1122 A and  1122 B and two canals  1122 C and  1122 D located axially on either sides of throttle  1111   2 . 
     When oil flows from solenoid valve  117  to actuators  14  and  24 , oil coming from volumes Vi and V 2  through lines  1106   i  and  1106   2  tends to push shuttles  1108   i  and  110 S 2  in the position of  FIG. 4  where these shuttles lie against first end walls  1113   i  and  1113   2  of these bores  1107   i  and  1107   2 , next to conduits  1110   i  and  1110   2 . 
     In this configuration, groove  1112 A is aligned with the outlet of a conduit  1125 A which extends between bore  1107   i  and chamber  1102 B. Similarly, groove  1112 B is located in front of one of the two outlets of a conduit  1125 B which connects bore  1107   i  to chamber  1102 E. 
     A third conduit  1125 C has its outlet located in front of groove  1122 A when shuttle HO 8   2  is in the position of  FIG. 4  and connects bore  1107   2  to chamber  1102 D. Finally, a fourth conduit  1125 D has two outlets in bore  1107   2 , one of these outlets being located at the level of groove  1122 B in the configuration of  FIG. 4 . Connecting line  1125 D connects bore  11  QT 2  to chamber  1102 C. 
     One considers that, apart from pressure drops at throttles  1111   1  and  1111   2 , pressure drops within valve  110  and actuators  14  and  24  are negligible with respect to the oil pressure values delivered by pump  105 . 
     The construction of hydraulic valve  110  is such that flow-rates Fi and F 2  are automatically adjusted to be equal, so that actuators  14  and  24  are driven in the same manner. 
     One notes R the ratio of flow-rates Fi and F 2  which follows equation:
 
 R=F 1 /F 2
 
     Because of the construction of valve  110 , flow-rate. F 1  is the same in connecting line CL 1  and in feeding line  15 . Similarly, flow-rate F 2  is the same in connecting line CL 2  and feeding line  25 . 
     Considering the configuration of  FIG. 4  where oil is supposed to flow from line  35  to lines  15  and  25 , if more oil flows in line  1106   1  than in line  1106   2 , that is if R is larger than 1, then pressure drop at the level of throttle  1111   1  is higher than pressure drop at the level of throttle  1111   2 . Under such circumstances, the pressure difference between the pressures in chambers  1102 B and  1102 E is larger than the pressure difference between the pressure in chambers  1102 D and  1102 C. The geometry of spool  1103  is such that the end surface of portion  1103   1 , perpendicular to axis X 1 , which undergoes the pressure in chamber  1102 B, has substantially the same area as surface  1103 A 1  which undergoes the pressure in chamber  1102 C. Similarly, the end surface of portion  1103   2  has the same area as surface  1103 A 2  which undergoes the pressure within chamber  1102 E. Therefore, because of the pressure differences between chambers  1102 B and  1102 E, on the one hand, and  1102 D and  1102 C 1  on the other hand, spool  1103  is pushed to the right of  FIG. 4  in direction of arrow A 1 , that is against the action of spring  1104   2 . This implies that volume V 1  decreases, whereas volume V 2  increases so that the cross section of volume V 1  available for oil flow F 1  becomes smaller than the cross section of volume V 2  available for oil flow F 2 . This implies that flow-rate F 1  in line  1106 - 1  decreases and flow-rate F 2  in line  1106   2  increases. Therefore, ratio R decreases up to when it reaches value “1”. 
     If flow-rate F 2  tends to be larger than flow-rate F 1 , that is if R is smaller than 1, the pressure differences work in the other way, so that spool  1103  is moved to the left on  FIG. 4  in the direction of arrow A 2  and the cross section of volume V 2  available for flow-rate F 2  decreases whereas the cross section of volume V 1  available for flow-rate F 1  increases, so that R increases up to when it reaches the values “1”. 
     Therefore, hydraulic valve  110  evenly distributes flow-rate F 0  into two substantially equal flow-rates F 1  and F 2  whose ratio R equals “1” or is automatically adjusted to “1”, so that actuators  14  and  24  are driven in the same way. 
     In the configuration where oil flows from actuators  14  and  24  towards main exhaust line  107  and sump  106 , that is when inlet valves  12  and  22  are being closed, the flow of oil within bores  1107 - 1  and  1107   2  is such that shuttles  1108   1  and  1108   2  are moved away from lines  15  and  25 , as shown in  FIG. 5 . In this configuration, shuttles  1108   i  and  1108   2  lie respectively against second end walls  1114   1  and  1114   2  of bores  1107   1  and  1107   2  on the sides of lines  1106   1  and  1106   2 , that is opposite lines  15  and  25 . 
     Because of this movement of the shuttles, groove  1112 B is connected by conduit  1125 A to chamber  1102 B. On the other hand, groove  1112 A is connected via conduit  1125 B to chamber  1102 E. Thanks to canals  1112 C and  1112 D, chamber  1112 B is at the pressure within central bore  1109   1  upstream of throttle  1111   1 , whereas chamber  1102 E is at the pressure within central bore  1109   1  downstream of throttle  1111   1 . In other words, even if the oil flow direction within lines  15  and C 1   —   i  is reverse with respect to the situation of  FIG. 4 , the pressure difference between chambers  1102 B and  1102 E measures the pressure drop at the level of throttle  1111   1 , as in the configuration of  FIG. 4 . Similarly, the pressure difference between chambers  1102  D and  1102 C measures the pressure drop across throttle  1111   2 . 
     As explained for the configuration of  FIG. 4 , in case more oil flows in line  15  than in line  25 , that is when R is larger than 1, the pressure drop across throttle  1111 - 1  becomes bigger than the pressure drop across throttle  1111   2 . Therefore, that the pressure differences between chambers  1102 B and  1102 E, on the one hand,  1102 D and  1102 C, on the other hand, act on spool  1103 , so that it is moved to the right on  FIG. 4  in the direction of arrow A-i, which partially closes volume \λ and decreases flow F-i. Therefore, R decreases to value “1” and flow-rates F 1  and F 2  are substantially equal. 
     In case the pressure drop across throttle  1111   2  is greater than the pressure drop across throttle  1111   1 , spool  1103  is moved to the left of  FIG. 5 , in the direction of arrow A 2  and R increases to value “1” 
     In the second embodiment of  FIG. 6 , the same elements as in the first embodiment have the same references. The upper part of hydraulic valve  110  is the same as in the first embodiment. A valve spool  1103  is slidably mounted within a bore  1102  provided in a valve body  1101  and defining four chambers  1102 B,  1102 C,  1102 D and  1102 E. No shuttle is used in this embodiment and two throttles  1111   1  and  1111   2  are provided on fixed portions of two conduits  1106   1  and  1106   2  between volumes V 1  and V 2  and feeding lines  15  and  25 . 
     Conduits  1106   1  and  1106   2  constitute each a connecting line CL 1 , respectively CL 2 , between bore  1102  and feeding line  15 , respectively  25 . A first check valve  1116  is provided on connection line CL 1  between bore  1102  and throttle  1111   1 . It allows oil flow only from bore  1102  to throttle  1111   1 . A first conduit  1125 A connects conduit  1106   1 , between check valve  1116  and throttle  1111   1 , to chamber  1102 B. A second conduit  1125 B connects conduit  1106   i , between line  15  and throttle  1111   1 , to chamber  1102 E. Similarly, a third conduit  1125 C connects chamber  1102 D to conduit  1106   2 , between volume V 2  and throttle IIH 2 , and a fourth conduit  1125 D connects chamber  1102 C to conduit  1106   2  between line  25  and throttle  1111   2 . 
     Conduit  1106   2  is provided with a check valve  1117  located between volume V 2  and throttle  1111   2 . Check valve  1117  allows oil flow only from bore  1102  to throttle 
     A fifth conduit  1125 E connects conduit  1106   1 , between check valve  1116  and throttle  1111   1 , to conduit  1106   2 , between check valve  1117  and volume V 2 . Another check valve  1118  is mounted on conduit  1125 E and allows oil to flow only from line  1106 - 1  to line  1106   2 . 
     A sixth conduit  1125 F connects conduit  1106   21  between check valve  1117  and throttle  1111   2 , to conduit  11 Oe 1 , between volume V 1  and check valve  1116 . Another check valve  1119  is mounted on conduit  1125 F and allows oil flow only from conduit  1106   2  to conduit  1106   1 . 
     In case oil flows from line  35  to lines  15  and  25 , volumes V 1  and V 2  are connected to throttles  1111   1  and  1111   2  respectively through check valves  1116  and  1117 . If, for instance, ratio R defined as above is higher than 1, that is if flow-rate F 1  in line  15  is larger than flow-rate F 2  in line  25 , the pressure drop across throttle  111 I 1  is higher than the pressure drop across throttle  1111   2 . Then the pressure differences sensed through conduits  1125 A,  1125 B on the one side,  1125 C and  1125 D, on the other side, are such that spool  1103  is moved to the right on  FIG. 6 , in the direction of arrow A-t, against the action of a return spring  1104   21  which decreases volume V 1 , its corresponding cross section and the flow in line  1106 -t, so that the differences between flow-rates Fi and F 2  decreases. Therefore, ratio R decreases up to value 
     Similarly, spool is moved to the left on  FIG. 6  in the direction of arrow A 2 , against the action of a return spring  1104 - 1 , if flow F 2  is larger than flow F 1 , that is if ratio R is smaller than 1. So, flow-rate F 2  decreases and flow-rate Fi increases and ratio R increases up to value “1”. 
     In the case of oil flow from lines  15  and  25  to line  35 , that is in a configuration corresponding to  FIG. 5  for the first embodiment, oil flows from throttle  111 I 1  to volume V 2  through conduit  1125 E. Similarly, oil flows from throttle  1111   2  to volume V 1  through conduit  1125 F. In case the pressure drop across throttle  1111   1  is higher than the pressure drop across throttle  1111   2 , this difference is sensed through conduits  1125 A,  1125 B 1    1125 C and  1125 D, which induces that spool  1103  moves to the left of  FIG. 6  in the direction of arrow A 2 , which decreases volume V 2  and increases volume V 1 , so that the differences between the flow-rates F 1  and F 2  is reduced. 
     Throttles  1111   1  and  1111   2  have been represented in connecting lines CL 1  and CL 2  which are different from feeding lines  15  and  25 . However, connecting lines CL 1  and CL 2  could be parts of lines  15  and  25 . 
     The invention has been described when used to control two inlet valves  11  and  12  of a cylinder. It may also be used to control exhaust valves. 
     In both embodiments described, the valve member  1103  is subject to a first force proportional to the flow in one feeding line, this first force acting along a first direction. The valve member is also subject to a second force proportional to the flow in the other feeding line, this second force acting along an opposite direction. These forces are due to the pressure acting on the relevant surfaces of the valve member. The valve member has a flow directing portion which directs the incoming flow to the two feeding lines which is proportional to an offset compared to a centre position where it delivers the same flow to both feeding lines. The balance of the two forces move the valve member in a direction where its flow directing portion will correct an unbalance in the two flows, by a negative feedback relationship. An overpressure (or overflow) in one feeding line will tend to force the valve member in a direction where it will restrict the flow in that feeding line. 
     Each first and second force is directly derived from the pressure difference on both sides of a throttle in the corresponding feeding line. Such force is created by directing a pressure collected upstream of the throttle on one side of a piston, and directing a pressure collected downstream of the throttle to the other side of the piston, said piston being in fact formed by two opposite surfaces of the valve member. The first and the second force are therefore each function of the difference between the actions of the upstream pressure and the downstream pressure for their respective throttle. 
     In the first embodiment, the shuttles act as circuit inverters to switch the connections between the pressure collecting points on both sides of the throttle, so that the upstream pressure and the downstream pressure always act on the same side of the piston, irrespective of the direction of flow across the throttle. This means that whatever the sign of the pressure difference across one throttle (which is positive for one flow direction and negative for the other flow direction), the valve member will tend to be displaced in the same direction when considering the action of one the first or second force. In the second embodiment, contrary to the first embodiment, the valve member will tend to be displaced in opposite directions when considering the action of one of the first or second force, depending on the direction of low through the corresponding throttle. Therefore, in the second embodiment, the check valves switch the connections between the flow directing portion of the valve member and the two feeding lines, so that they are inverted. This allows that, although the displacement of the valve member will depend on the sign of an over-pressure (or over-flow) in one feeding line, the resulting displacement will nevertheless be a flow restriction in the feeding line which has the strongest flow in absolute value. 
     LIST OF REFERENCES 
     
         
           1  cylinder 
           2  piston 
           2   a  front face 
           3  combustion chamber 
           4  cylinder head 
           11 ,  21  inlets ducts 
           12 ,  22  inlet valves 
           13 ,  23  springs 
           14 ,  24  hydraulic actuators 
           15 ,  25  feeding line 
           16 ,  26  seats 
           35  common line 
           101  hydraulic flow divider 
           102  electronic control unit 
           1021  electric line 
           103  main feeding line 
           104  filtration unit 
           105  pump 
           106  sump 
           107  main exhaust line 
           110  hydraulic valve 
           1101  valve body 
           1102  bore 
           1102 A central part 
           1102   1  groove 
           1102   2  groove 
           1102 B chamber 
           1102 C chamber 
           1102 D chamber 
           1102 E chamber 
           1103  valve member or spool 
           1103 A main portion 
           1103 A 1  end surface 
           1103 A 2  end surface 
           1103   1  lateral portion 
           1103   2  lateral portion 
           1103 B locking ring 
           1103 C locking ring 
           1103 D central rod 
           1104   1  spring 
           1104   2  spring 
           1105  adjusting screw 
           1106   1 , conduit 
           1106   2  conduit 
           1107   1  bore 
           1107   2  bore 
           1108   1  shuttle 
           1108   2  shuttle 
           1109   1  central bore 
           1109   2  central bore 
           1110   1  exhaust conduit 
           1110   2  exhaust conduit 
           1111   1  throttle 
           1111   2  throttle 
           1112 A external groove 
           1112 B external groove 
           1112 C canal 
           1112 D canal 
           1113   1  first end wall of bore  1107   1    
           1113   2  first end wall—of bore  1107   2    
           1114   1  second end wall of bore  1107   1    
           1114   2  second end wall of bore  1107   2    
           1122 A external groove 
           1122 B external groove 
           1122 C canal 
           1122 D canal 
           1125 A conduit 
           1125 B conduit 
           1125 C conduit 
           1125 D conduit 
           1125 E conduit 
           1125 F conduit 
           1116  check valve 
           1117  check valve 
           1118  check valve 
           1119  check valve 
           117  solenoid valve 
           118  solenoid valve 
         A 1  arrow 
         A 2  arrow 
         CL 1  connecting line 
         CL 2  connecting line 
         D 1  diameter of  1103 D 
         D 2  diameter of central part of  1102   
         D′ 2  diameter of  1102   1  and  1102   2    
         D 3  diameter of  1103   
         E engine 
         F arrows (oil flow) 
         F 0  flow-rate in line  35   
         F 1  flow-rate in line  15   
         F 2  flow-rate in line  25   
         L 11  lift of valve  11   
         L 12  lift of valve  12   
         R ratio F 1 /F 2    
         S 1  electrical signal 
         S 12  hydraulic signal 
         S 22  hydraulic signal 
         S 117  part of signal S 1    
         S- 118  part of signal S 1  t time to instant 
         δt 117  period of time 
         δt 118  period of time 
         V 1  volume of  1102   1    
         V 2  volume of  1102   2    
         X 1  axis of body  1101   
         X 2  axis of body  1102   
         X 71  axis of  1107   1    
         X 72  axis of  1107   2