Variable-actuation, electro-hydraulic system and device controlling the valves of internal combustion engines

Abstract

 Control system of an electro-hydraulic device actuating the valves of internal combustion engines, comprising a cylinder-hydraulic piston assembly for each engine valve (3) to be actuated and means to transfer the piston displacement to the engine valve stem, means (10) to supply control fluid to the cylinder chamber (1) of said cylinder-piston assembly for actuating said piston (2) in the opening direction of the engine valve (3), in contrast to spring means (4) which recall said engine valve (3) into the closed position thereof. The cylinder chamber (1) is put in communication, alternately, on one side with a high pressure source (HP) and, on the other side, with a low pressure chamber (LP), through a single, 3-way servovalve (10), which is switched into a "state b" for putting the chamber (1) in communication with the high pressure source (HP), for a time (T1) corresponding to an initial part only of the opening run of the engine valve (3) or, alternatively, into a "state a", for putting said chamber (1) in communication with the low pressure (LP), for an initial part only of the closing run of the engine valve (3). The 3-way valve (10) comprises a shutter valve (12), slidable between a home position ("state a") and an active position ("state b") under the action of a pressure differential acting on the opposite sides thereof, and of a control electromagnet (21).

Description

[0001] It is known that the inlet and outlet valves of internal combustion engines control the flows of gasses coming in and going out of the combustion chamber of each engine cylinder. From their origins to date, these valves are generally driven by cams, integral with a so-called camshaft, which is brought into rotation in sync with the movement of the engine shaft.

[0002] Valve opening and tuning times are established so as to optimise engine efficiency in a certain working range and for certain load conditions; with the known camshaft distribution systems, these times cannot be changed during engine operation. This implies that, departing from such optimal conditions, engine efficiency drops.

[0003] For the purpose of improving the efficiency of internal combustion engines, and at the same time of making the operation thereof more compatible with modern environmental protection requirements, a variety of studies have been carried out and engines have been developed wherein various strategies for the variable actuation of engine valves are implemented; controlled by an electronic data processing unit equipped with a suitable software, these provide to accomplish, cycle by cycle and cylinder by cylinder:

· stroke variation;

· duration and stroke variation;

· lift variation;

· combined variation of the lift and of the duration;

· cyclic exclusion of one or more valves.

[0004] A system of this type, for controlling the valves of internal combustion engines, which provides an electro-hydraulic actuation to obtain a desmodromic-type control of the valves, called in short WA (acronym of Variable Valve Actuators), is disclosed for example in patent application MI2005A.001810 of 28.09.2005, in the name of the same Applicant.

[0005] The general structure of this WA system is diagrammatically shown in enclosed fig. 1. As can be detected from this drawing, the system provides

• an actuator consisting of a cylinder 1 and of a piston 2, the latter acting directly on engine valve 3, with which it is integral, in opposition to a respective contrast spring 4;
• a source of high pressure HP, which can be put in communication with the chamber 1 of the actuator by actuating a semi-automatic electrovalve 6, of the "normally closed" type;
• a source of low pressure LP, not necessarily at room pressure, with which the chamber 1 of the actuator is normally in communication and wherefrom chamber 1 is isolated when electrovalve 7 is actuated, of the "normally open" type;
• a positive-displacement pump 8, which provides to keep the source HP and the circuitry connected thereto under pressure;
• a gas-charged accumulator 9, having the function of dampening any pressure oscillations of the high pressure source HP.

[0006] The two electrovalves 6 and 7 are two one-way, semi-automatic valves. In particular, electrovalve 6 is normally closed and allows the passage of fluid from the circuit HP to the chamber 1 of the cylinder only if it is actuated from outside; however, it automatically allows the passage of fluid in the opposite direction, i.e. when the pressure in working chamber 1 is higher than the one in circuit HP by a set value, established during the design step. Electrovalve 7 is normally open and, when it is actuated, prevents fluid from flowing out of chamber 1 to chamber LP; moreover, it is one-way in the opposite direction, i.e. it automatically allows fluid passage to low pressure tank LP, if the pressure in chamber 1 is smaller than said low pressure LP.

[0007] The operation of this device and the advantages thereof are described in detail in the above-cited patent application MI2005A.001810, whereto reference can be made for a better understanding of the present invention.

[0008] The object of the present invention is that of improving the device described in the above-cited application MI2005A.001810, in the sense of achieving improved system construction simplicity and hence lower cost, at the same time streamlining the control of the engine valve movement. This object is achieved through the features mentioned in claims 1 and 7.

[0009] Further features and advantages of the invention are more evident from the following detailed description of a preferred embodiment, given purely by way of a non-limiting example and illustrated in the accompanying drawings, wherein:

Fig. 1 refers to the above-described prior art,;

Fig. 2 shows a system diagram similar to that of fig. 1, but designed according to the invention;

Fig. 3 is a diagram showing the lift law of an engine valve actuated by a system such as the one of fig. 2;

Fig. 4 is a diagram similar to that of fig. 3, but concerning a different management mode of the system according to the invention;

Fig. 5 is a diagrammatic axial section of a three-way valve particularly suited to act as control valve of the control fluid according to the present invention; and

Fig. 6 is a diagrammatic section of a possible embodiment of the entire engine valve actuator, according to the present invention.

[0010] As appears immediately evident from the comparison between the system diagrams of figures 1 and 2 - in this second drawing the same reference numbers of fig. 1 are used to refer to the same component elements - the substantial difference of a system according to the invention, compared to that of the above-cited patent application MI2005A.001810, lies in the number of components regulating:

• on the one hand, the supply of the actuating oil from the high pressure source HP to the control volume consisting of the chamber of cylinder 1;
• on the other hand, the outflow of the actuating oil from control volume 1 to the low pressure tank LP, which occurs under the thrust imparted by spring 4 on piston 2.

[0011] As a matter of fact, according to the invention it is provided to replace the two valves 6 and 7 of the device described in the previous patent application MI2005A.001810 - one for each of the operation steps just described - with a single element fulfilling both functions and which can be defined as a "3-way electrovalve", referred to as 10 in Fig. 2.

[0012] As appears clearly from the diagram of Fig. 2, this single servovalve 10 is intended for oil distribution on the 3 lines indicated by letters "A", "B" and "C", each communicating, respectively, with:

• the high pressure source HP (via A);
• the volume 1 overlying hydraulic piston 2 (via B);
• the low pressure source LP (via C)

[0013] According to the invention, servovalve 10 is switched - for example through electromagnetic actuation (as occurs for valves 6 and 7 of the prior art) - exclusively into either one of two possible positions, namely a first position, wherein it allows the flow of fluid in the direction indicated by arrow F1 of Fig. 2, and a second position, wherein it allows the flow of fluid in the direction indicated by arrow F2. More precisely:

• if the valve is in the first position, arrow F1, lines B and C are put in communication, locking line A;
• if the valve is in the second position, arrow F2, communication occurs between lines A and B, while line C is locked.

[0014] In a first management mode - wherein only an adjustment of the lift stroke of the engine valve is provided - the opening and closing runs of the engine valve occur in the way diagrammatically shown in fig. 3, where the uppermost diagram 3A refers to the motion of engine valve 3, the lowermost diagram 3C refers to the motion of servovalve 10, and the middle diagram 3B refers to the actuation of the electromagnet controlling servovalve 10.

[0015] Moreover, three different conditions of the manoeuvres opening and closing engine valve 10 are shown, at three different heights: in the left-hand area (MAX) of the diagram an actuation at maximum opening is shown, in the right-hand area (MIN) an actuation at minimum opening is shown, while in the middle area (INTERM) an actuation at an intermediate opening value is shown.

[0016] At the beginning of the cycle - instant C₀ of the diagram of fig. 3 - control electrovalve 10 is in "state a", with the corresponding electromagnet off (level OFF): engine valve 3 is closed, resting in its seat, and cannot leave such position until the respective electromagnet controlling servovalve 10 is energised.

[0017] Upon start - corresponding to instant C₁ - current begins to circulate in the winding of the magnet (level ON); control servovalve 10 then switches to move from "state a" to "state b", going through the infinite intermediate states. This passage does not occur in an equally instantaneous manner as the passage of the magnet from the OFF state to the ON state, but implies a certain delay (highlighted in diagram 3C of the electrovalve movement). As soon as servovalve 10 is in the proximity of "state b", it opens the communication between line A and line B, which remains open for the entire phase I of the cycle - between instant C₁ and instant C₂ - while line C remains locked. Due to the communication of the control volume of cylinder 1 with high pressure HP, this pressure then acts on hydraulic piston 2, which drives engine valve 3 into opening, i.e. departing from the seat thereof, proceeding downwards with respect to the drawing and overcoming the thrust of spring 4, which opposes such movement.

[0018] In this phase I, which takes place in the energisation time T1 of the respective electromagnet, servovalve 10 moves, with a certain delay with respect to instant C₁, from "state a" to "state b" and engine valve 3 opens. The opening occurs with a sharp acceleration, as shown by the development of diagram 3A, until position V₁. This position is determined in the design phase by choosing the length of time T1, to the end whereof corresponds instant C₂ of the end of phase I.

[0019] Phase II of the cycle begins at instant C₂, wherein the electromagnet is de-activated, which hence goes from the ON state to the OFF state virtually instantly. To this action corresponds the return of servovalve 10, equally with a certain delay over instant C₂, from "state b" to "state a". This implies the closure of line A (high pressure) and at the same time the opening of the communication between B and C.

[0020] This change - which, as said, occurs at the end of time T1, i.e. in position V₁ of the engine valve - occurs when the engine valve has all the kinetic energy derived from the acceleration received in previous phase I. The engine valve can then continue by inertia in its stroke, until the maximum lift provided (vertex point V₂ of the lift curve, which corresponds to cycle point C₃). This stroke, line A being closed, translates into a drawing of actuation oil from low pressure LP, through line C-B, to the control volume of cylinder 1; as a matter of fact, hydraulic piston 2 - during the opening stroke together with engine valve 3, "launched" by the initial pressure force - causes an increase of overlying volume 1, which leads precisely to said drawing effect.

[0021] The condition described above is maintained as long as the assembly of piston 2 and engine valve 3 loses all its kinetic energy which, on the one hand, is converted into potential energy compressing spring 4 and, on the other hand, is lost due to the (mechanical and hydraulic) frictions which the movement undergoes.

[0022] At point C₃ of the cycle, i.e. at point V₂ of the maximum lift of engine valve 3, phase III begins, wherein valve 3 performs its return stroke in a closed position. In this phase III the stroke of control piston 2 occurs under the thrust of spring 4, previously loaded as said, and forces the fluid to flow out of the volume of cylinder 1 to the low pressure source LP, through line B-C, until accomplishing the approaching of engine valve 3 to its seat. Therefore, if control servovalve 10 remains in "state a", closure of the engine valve is accomplished.

[0023] Summing up the above, it is possible to distinguish three working steps in the type of manoeuvre described (see Fig. 3):

step I: accelerating, opening movement of the engine valve. It begins when, with the engine valve stationary in its seat, "state b" of servovalve 10 for oil control is set, and lasts as long as such state holds, i.e. until point C₂ of the cycle, i.e. point V₁ of the lift;

step II: further opening movement of the engine valve, in deceleration. It begins when, from step I, control valve 10 switches again into "state a" and ends when the engine valve has lost all its kinetic energy, i.e. point C₃ of the cycle, i.e. point V₂ of the maximum lift;

step III: closing movement of the engine valve. It begins immediately after step II and ends when valve 3, pushed by spring 4, ends up closing on its seat, i.e. at point C₄ of the cycle, i.e. again at point V₀ of the lift.

[0024] It is clear from the above that, by varying the actuation time (diagram 3B) of the control electromagnet, a greater or smaller opening of engine valve 3 is achieved, as diagrammatically illustrated in fig. 3 for the three different opening positions corresponding to the three different actuation times T1, T2 and T3. In other words, this first management mode allows only an adjustment of the lift value of the engine valve.

[0025] However, it must not be forgotten that a fully variable system for the actuation of engine valves is required to be able to control simultaneously and at will not only the lift value and/or the timing, i.e. instants C₁ and C₂ for the opening and closing, respectively, of the electromagnet controlling the servovalve, but also the time in which valve 3 is kept in an open position. It is easy to understand, though, that the management mode described in the preceding paragraphs, with reference to fig. 3, does not provide at all a step for the prolonged maintenance of the engine valve in an open position.

[0026] For the purpose of achieving full system flexibility, it must hence be provided - in a second management mode, wherein an adjustment of the time in which the valve is kept in an open position is also achieved - to control servovalve 10, for example in the way diagrammatically shown in Fig. 4, i.e. through:

• a lift stroke according to step I and step II already seen before, with reference to cycle points C₀, C₁, C₂, C₃;
• a stabilisation in a new step III, keeping the engine valve in an open position, as better described in the following; and
• a closing run in a step IV, substantially corresponding to step III of the working diagram of fig. 3.

[0027] Step I and step II - as occurred for the first management mode, seen with reference to fig. 3 - determine the lift value of engine valve 3; this determination is obtained, as already said, by simply varying the duration T of the first control impulse of the electromagnet controlling servovalve 10.

[0028] Subsequently, as said before, if servovalve 10 remained in "state a" at the end of step II of the engine valve deceleration, return in a closed position would inevitably and immediately occur.

[0029] Instead, based on this second management mode, at cycle point C₃ a so-called stabilisation or maintenance step III begins, wherein it is provided to inhibit the closing step of the engine valve by reactivating servovalve 10 for at least a short instant t. More precisely, this maintenance step is achieved by setting up a succession of cycles C₃-C₄, C₅-C₆, .... C_n-1-C_n wherein servovalve 10 is brought back into "state b" each time and, after a very short time t, it is set again into "state a".

[0030] Thereby, engine valve 3, which in the initial instants C₃, C₅, ... C_n-1 of each cycle had begun to close again slightly, recovers the desired height in the subsequent final instants C₄, C₆, ... C_n of each cycle: i.e. in those instants in which magnet de-activation becomes necessary for avoiding an excessive new lift of the valve.

[0031] It derives that, in this step III controlling the electromagnet, aimed at maintaining the lift stroke of the engine valve, the control takes up the appearance of a train of impulses. It is hence easy to understand, also thanks to the help of the diagrams of Fig. 4, that the maintenance time of the valve will be a function of the number of actuations which make up the above-said train of impulses and/or the duration of time t of the individual train impulses.

[0032] Maintenance step III is then followed by a step IV closing engine valve 3, which is substantially identical to step III of the embodiment of figure 3. This is the step wherein the electromagnet is in an OFF condition and servovalve 10 is in "state a", while engine valve 3 moves into the closed position, together with piston 2 and under the thrust of spring 4.

[0033] It is important to appreciate that the system described above, both for the first and second mode of management, appears intrinsically safe against any possible failure situation; this equals to saying that, should a failure occur, the system is such as to always bring the engine valve into a closed position.

[0034] One of the possible failures concerns the power driver which supplies the reel energising the electromagnet controlling servovalve 10: in such case servovalve 10, in the absence of actuation by the magnet thereof, normally remains in "state a", so that - regardless of the instant position in the instant of failure - the action of spring 4 will always bring the engine valve to close.

[0035] However, the failure can also concern the control system: in such case the provisions of the above-mentioned patent application MI2005A.001810 are applicable, and specifically the locking system between ECU and VVA shown in fig. 7 of the aforecited application, explicit reference whereto is hence made here. As a matter of fact, this locking system may be applied regardless of the provision of two distinct control electrovalves 6 and 7 (as in the above-mentioned application MI2005A.001810), or of a single, 3-way servovalve 10 as provided by the present invention.

[0036] Finally, the failure may concern the high pressure circuit: once again, engine valve 3, even though open, returns into a closed position in its seat, being driven only by the action of spring 4, regardless of the fact that control servovalve 10 is in "state a" or in "state b" (in this connection, reference can also be made to what is disclosed in the above-cited patent application MI2005A.001810 on failures of the high-pressure circuit).

[0037] In the following description reference is made to fig. 5, wherein a 3-way servovalve 10 is shown, for the distribution of actuation oil, which is capable of fulfilling particularly well the above-described functions of the system according to the invention.

[0038] The servovalve 10 shown here consists of a base body 11, in the lower part whereof the three pipes are firstly highlighted forming the three lines A, B, C, according to what is shown in fig. 2.

[0039] In the middle part of body 11 there is formed a guide for a sliding member 12, referred to as shutter valve in the following; guide and shutter valve 12 have cylindrical symmetry with respect to valve axis X-X.

[0040] Shutter valve 12 has a conical-shape, lower end 12a, which has the function of intercepting the communication between line A and line B when resting on the seat thereof. Such shutter valve 12 separates, with a perfect seal, two chambers 13 and 14, formed in the lower and upper part, respectively, of the guide of shutter valve 12. Moreover, these chambers 13, 14 are constantly in mutual communication through channel 15, which has a small, sized section.

[0041] If As indicates the area of the upper surface 12b of said shutter valve 12, subject to the pressure existing in volume 14, and if Ai indicates the area of the lower surface 12a thereof, subject to the pressure existing in volume 13 (the area Ai being equal to As less the area of the hole of line B shut by conical shutter valve 12a), one immediately understands that the shutter valve undergoes the following forces:

1. F_s = p_s* A_s : directed downwards;

2. F_e : directed downwards;

3. F_i = p_i* A_i : directed upwards;

where p_s, p_i and F_e refer to the pressure values in volume 14 and in volume 13 and the value of the force imparted by spring 17, respectively. The force F_e of spring 17 is normally low and, in the home condition shown in fig. 5 ("state a"), it adds to the differential F_s - F_i of the forces of hydraulic origin.

[0042] As appears clearly from Fig. 5, volume 14 as well as communicating, as already mentioned, with the high pressure source HP through channel 15 and line A, is also capable of being put in communication with a volume 18 formed in a body 11a integral with body 11. For this purpose, volume 18 - which, in the home condition shown in fig. 5, is at a low pressure - the communication between volumes 14 and 18 is intercepted by a ball valve 19, which acts on conical seat 20 formed in body 11a and which can be opened in the way described in the following.

[0043] When ball 19 closes on its own seat 20, the underlying volume 14 - being capable of communicating only with volume 13 and hence with high pressure HP through line A - will be at the same pressure of volume 13 and of the HP, which is here referred to as P. In this condition the resulting force on shutter valve 12 will be equal to:

[0044] Such force is directed downwards (since the area As exceeds Ai by an amount equal to, as said, the area of the hole of line B), and pushes the conical part 12a of the shutter valve onto its seat, interrupting the connection between A and B.

[0045] The above-described arrangement, also in the light of the following, corresponds to the position of "state a" of servovalve 10 (see fig. 3), also referred to as home position, and is accomplished when the electromagnet is in an OFF state.

[0046] Equally in a home position, with electromagnet 21 OFF, a spring 22 pushes magnetic anchor 23 away from the same electromagnet 21, i.e. downwards with respect to the drawing, and said anchor 23 pushes ball 19 against its own seat 20, in opposition to the action of the hydraulic pressure from below: as a result the separation of the two volumes 18 and 14 is obtained.

[0047] If instead servovalve 10 is to be switched into "state b", electromagnet 21 is energised so as to attract anchor 23, overcoming the action of spring 22 and releasing ball 19. Thereby a connection between volumes 18 and 14 is suddenly accomplished, causing a sharp pressure drop in the latter; in actual fact, the actuating oil which blows by into small-section channel 15 cannot compensate the pressure drop in volume 14. This results in the change of sign of force F_tot, since this time ps (and hence Fs) is very low compared to pi (and hence to Fi). Concerning instead force Fe, as said, it is very small if compared to pressure forces Fs and Fi: as a matter of fact, everything is sized so that the sum, in formula (1), changes sign only depending on the hydraulic pressure forces. Changing the sign of F_tot hence means "launching" shutter valve 12 upwards (with respect to the drawing) until a mechanical stroke stop (not shown in detail, but which can be considered coinciding with the lower surface of body 11a in the diagrammatic representation of fig. 5), once reached which, servovalve 10 is in the "state b" position.

[0048] It is obvious that, by bringing the magnet in the OFF position, and hence causing spring 22 to act closing ball valve 19, the sign of F_tot changes again and the servovalve returns in the "state a" position.

[0049] It must further be added that in the body of shutter valve 12 an oblique conduit 24 is formed, which puts in communication the lower end of its cone 12a - in turn constantly in communication with line B - with an own circumferential slit 25. This slit is practised on the wall of the same shutter valve 12, in an intermediate position of the height thereof (middle position in the embodiment shown), in any case such as to be - when servovalve 10 is in the "state a" position shown in fig. 5 - perfectly aligned with a further radial slit 26, obtained in body 11 and communicating with line C.

[0050] Therefore, with servovalve 10 in "state a", the connection of line B with line C is accomplished. Following energisation of the control magnet, shutter valve 12 moves from the low position, shown in fig. 5 ("state a"), to the raised position (not shown) corresponding to resting against the upper stroke stop ("state b"); cone 12 then, departing from its seat, opens the communication between line A and line B. At the same time, slits 25 and 26 immediately move into misalignment, closing the connection between line B and line C. It is important to notice that, given the very narrow shape of slits 25, 26, as soon as shutter valve 12 moves, such slits go into misalignment and close line B-C, and this even before a substantial amount of high pressure oil can flow back from line A to C through conduit 24.

[0051] It is then understandable that, by the arrangement described here:

• in the "state a" position of servovalve 10 - which is the normally enabled one (magnet OFF) - the connection between A and B is intercepted, while the one between B and C is enabled;
• in the "state b" position of servovalve 10, the connection between B and C is intercepted, while the one between A and B is enabled.

[0052] Fig. 6 shows, as said, a diagrammatic section of a possible embodiment of the entire engine valve actuator, according to the present invention.

[0053] Valve 3 can be noticed sliding within a valve guide 30, which is housed in a body 31 integral with the cylinder head (not shown). A spring 32, which rests on one side in a recess formed in body 31 and, on the other side, against a disc 33 integral at the upper end 3a of rod 3 of the engine valve, imparts its action in the sense of always bringing the engine valve into its closed position (as shown in fig. 6).

[0054] In mounting 34, arranged above body 31, there is formed a cylindrical recess 1, wherein the piston 2 actuating valve 3 is housed. Cylinder 1 extends into a chamber 1a, connected to the line B feeding the actuating oil supplied by servovalve 10, said servovalve being in turn mounted above mounting 34.

[0055] In mounting 34 there is also formed a supply pipe 35 of high pressure HP, which is directly connected with line A of servovalve 10. Fig. 6 also shows the connection of line C of servovalve 10 to the low pressure.

[0056] It is understood that the invention is not to be considered limited to the specific arrangement illustrated above, which is only an example embodiment thereof, in particular concerning the valve structure illustrated in fig. 5; different variants are hence possible, all within the reach of a person skilled in the field, without departing from the scope of protection of the invention, as defined by the following claims.

Claims

1. Control system of an electro-hydraulic device for the actuation of the valves of internal combustion engines, of the type comprising a cylinder-hydraulic piston assembly for each engine valve (3) to be actuated, and means for transferring the piston displacement to the engine valve stem, means (10) for supplying control fluid to the chamber (1) of the cylinder of said cylinder-piston assembly, for actuating said piston (2) in the opening direction of the engine valve (3), in opposition to spring means (4) which recall said engine valve (3) towards its closed position, said cylinder chamber (1) alternately being put in communication on one side with a high pressure source (HP) and, on the other side, with a low-pressure chamber (LP),
characterised in that the communication of the cylinder chamber (1) with the high pressure source (HP) and with the low pressure chamber (LP) is accomplished through a single, 3-way servovalve (10), and in that
said servovalve (10) is switched into an active position ("state b"), to put in communication cylinder chamber (1) with a line (A) to the high pressure source (HP), for an initial portion only of the opening run of the engine valve (3),
or, alternatively, said servovalve (10) is switched into a home position ("state a"), to put in communication cylinder chamber (1) with a line (C) to the low pressure chamber (LP) for an initial portion only of the closing run of the engine valve (3).

2. Control system as claimed in claim 1), characterised in that the actuation of the engine valve (3) when opening is accomplished in two steps:

- a first step, when the engine valve (3) accelerates, wherein said servovalve (10) is switched into an active position ("state b") putting in communication line (B) to chamber (1) with the line (A) of high pressure (HP), for a time (T1) corresponding to said initial portion of the opening run of said engine valve (3), and

- a second step, when the engine valve (3) decelerates, wherein said servovalve (10) is switched into a home position ("state a") putting in communication line (B) to chamber (1) with the line (C) of low pressure (LP), for a time corresponding to at least a portion of the remaining opening run of the engine valve (3),

in this second step, due to the vacuum which is produced in the chamber (1) by the further inertial advancement of the piston (2), an amount of fluid coming from the low pressure chamber (LP) being drawn into said cylinder chamber (1).

3. Control system as claimed in claim 2), characterised in that the lift variation of the engine valve (3) depends on the length of time (T1).

4. Control system as claimed in claim 1) or 2), characterised in that the actuation of the engine valve (3) when closing is performed in two steps:

- a third step, when the engine valve (3) accelerates, wherein said servovalve (10) is switched into a home position putting line (B) of chamber (1) in communication with the line (C) of low pressure (LP), so that said spring means (4) can push said engine valve (3) into its closed position, discharging the pressurised fluid to line (C), for a time corresponding to said initial portion of the closing run of said engine valve (3), and

- a fourth step, when the engine valve (13) decelerates, wherein said servovalve (10) is switched into an active position, putting line (B) of chamber (1) in communication with the line (A) of high pressure (HP), for a time corresponding to at least part of the remaining closing run of said engine valve (3), the advancement of the piston (2) under the thrust of said spring means (4) being braked by the outflow of pressurised fluid in said chamber (1) in contrast to the pressure in the high pressure source (HP).

5. Control system as claimed in claim 2) and 4), characterised in that

- between said opening steps of the engine valve (3) and the closing steps of the same there is provided a step which keeps the engine valve (3) in an open position,

- said maintenance step being actuated by the repeated switching of said servovalve (10) between said home position ("state a") and said active position ("state b"), putting line (B) of chamber (1) in communication with the line (A) of high pressure (HP) for a preset sequence of short times (t).

6. Control system as claimed in claim 5), characterised in that the maintenance time of the engine valve (3) is determined by the sum of said short times (t).

7. Electro-hydraulic device for the actuation of the valves of internal combustion engines, in particular for the actuation of the system as in any one of the preceding claims, of the type comprising a cylinder-hydraulic piston assembly for each engine valve (3) to be actuated and means to transfer the piston displacement to the engine valve stem, means (10) for supplying control fluid to the cylinder chamber (1) of said cylinder-piston assembly, for the purpose of actuating said piston (2) in the opening direction of the engine valve (3), in contrast to spring means (4) recalling said engine valve (3) into its closed position, said cylinder chamber (1) being in communication, on one side, with a high pressure source (HP), and with a low pressure chamber (LP) on the other side,
characterised in that said means (10) for putting in communication said cylinder chamber (1) with the high pressure source (HP) or with the low pressure chamber (LP), respectively, consist of a single, 3-way, actuatable servovalve (10), which puts in communication said chamber (1) with a line (A) to the high pressure source (HP) or, alternatively, with a line (C) to the low pressure chamber (LP).

8. Electro-hydraulic device as claimed in claim 7), characterised in that said 3-way electrovalve comprises a shutter valve slidable between a home position ("state a"), wherein it puts in communication said cylinder chamber (1) with the low pressure (LP), and an active position ("state b"), wherein it puts in communication said cylinder chamber (1) with the high pressure source (HP).

9. Electro-hydraulic device as claimed in claim 7) or 8), characterised in that said home position ("state a") corresponds to the de-energised condition of the electromagnet (21) and said active position ("state b") corresponds to the energised condition of the electromagnet (21).

10. Electro-hydraulic device as claimed in any one of claims 7) to 9), characterised in that it comprises means (19, 23) for creating a pressure differential on the opposite sides of said shutter valve (12), said electromagnet (21) acting directly on said means (19, 23).

11. Electro-hydraulic device as claimed in claim 8) or 10), characterised in that said shutter valve (12) consists of a cylindrical-symmetry body slidable within an axial housing of the valve body (11), said shutter valve having a first surface (12a) forming a closing cone of a supply conduit (B) to said cylinder chamber (1), said cone being housed in a first working volume (13), and a second surface (12b), opposite to the first one with respect to the sliding direction of the shutter valve and housed within a second working volume (14), the area of said first surface (12a) subject to the pressure existing in the first working volume (13) being smaller than the area of said second surface (12b) subject to the pressure existing in the second working volume (14).

12. Electro-hydraulic device as claimed in claim 8), 10) or 11), characterised in that in the body of said shutter valve there is formed a conduit (24), which begins from the vertex of said cone surface (12a) and extends as far as a circumferential slit (25) halfway through the height of said shutter valve.

13. Electro-hydraulic device as claimed in claim 12), characterised in that said conduit (24) opens, at the vertex of said cone surface (12a), in correspondence of the supply line (B) of the actuating oil to said cylinder chamber (1).

14. Electro-hydraulic device as claimed in claim 12), characterised in that said intermediate circumferential slit (25) of said shutter valve lies, when the shutter valve is in the home position ("state a"), coinciding with a slit (26) which communicates with the line (C) to the low pressure chamber (LP).

15. Electro-hydraulic device as claimed in claim 8), 10) or 11), characterised in that said first working volume (13) is in direct communication with the line (A) to the high pressure source (HP).

16. Electro-hydraulic device as claimed in claim 8), 10) or 11), characterised in that said first working volume (13) is in direct communication with said second working volume (14) through a small-section, calibrated conduit (15).

17. Electro-hydraulic device as claimed in claim 8), 10) or 11), characterised in that with said valve body (11) there is associated an electromagnet body (11a), between the two bodies (11, 11a) a passageway being formed, which is closed by a valve (19) driven directly by the electromagnet (21).

18. Electro-hydraulic device as claimed in claim 17), characterised in that said passageway closed by the valve (19) puts in communication said second working volume (14) with a low-pressure volume, housing the electromagnet (21).

19. Electro-hydraulic device as claimed in claim 17) or 18), characterised in that said electromagnet (21) comprises an anchor (23) which, in its home position ("state a") is pushed by spring means (22) against said ball valve (19), and pushes said valve into closing on its seat, locking said passageway between said two bodies (11, 11a).

Drawing