Device for controlling the movement of a valve, in particular of an intake valve, of an internal combustion engine

The device is adapted to control the movement of a valve (1) of an internal combustion engine comprising a stem (1a) which may move in translation through an opening (2a) of a hollow guide body (2) and which ends in a mushroom head (1b) cooperating in operation with a valve seat (12) provided in a wall (13) of the engine head. The valve (1) may move in translation with respect to the hollow guide body (2) between a retracted position and a forward position in which the mushroom head (1b) respectively closes and opens the associated valve seat (12).The control device comprises    The arrangement is such that when the electromagnet (3) is de-energised, the valve (1) moves in acceleration towards the open position under the action of the first spring (7) until the action of the latter stops, and then the valve (1) moves in deceleration into the open position under the action of the second spring (8).

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage of International Application No. PCT/IB2007/050566 filed Feb. 22, 2007, claiming priority based on Italian Patent Application No. TO2006A000131, filed Feb. 24, 2008, the contents of all of which are incorporated herein by reference in their entirety.

The present invention relates to a device for controlling the movement of a valve, in particular of an intake valve, of an internal combustion engine.

The invention relates more particularly to a device for controlling the movement of a valve comprising a stem which is adapted to move in translation through an opening of a hollow guide body and which ends in a mushroom head cooperating in operation with a valve seat provided in a wall of the engine head, wherein the valve is adapted to move in translation with respect to the hollow guide body between a retracted position and a forward position in which the mushroom head respectively closes and opens the associated valve seat.

The object of the present invention is to provide a control device which makes it possible to achieve a substantially constant duration of the period of actual opening of the valve irrespective of the speed of revolution (rpm) of the engine.

In practice, a constant duration of the period of opening of the intake valve makes it possible to achieve different load values by acting on the phasing of this period. When this period straddles the bottom dead centre, where the underpressure in the combustion chamber reaches a maximum, full load conditions are obtained. However, the more the period of opening of the valve, appropriately delayed, is partially in the compression phase, the more partial load conditions are achieved. Opening of the intake valve at the bottom dead centre brings about the so-called “hammer” phenomenon which makes it possible to achieve high volumetric efficiency values (supercharging effect), provided that the duration of opening of the valve is such that its closure is ensured before reflux conditions occur.

This and other objects are achieved by the invention which relates to a control device of the type described above, characterised in that it comprises:a stationary electromagnet adapted, when energised, to retain the valve in the retracted closed position,first resilient means associated with the valve and tending to urge it towards the open position, andsecond resilient means also associated with the valve and tending to oppose its transition to the open position,the arrangement being such that when the electromagnet is de-energised, the valve moves in acceleration towards the open position under the action of the first resilient means until the action of the latter stops, and then the valve moves in deceleration into the open position under the action of the second resilient means.

According to a further characteristic feature, when, in a control device of the invention, the electromagnet is de-energised, the mushroom head, starting from the retracted closed position, may, under the action of the first resilient means, move in translation, substantially in a tight manner or with a limited play, into a recess of the wall of the head facing the combustion chamber until the action exerted on the valve by the first resilient means ceases; the arrangement is then such that when the engine is off the valve is retained by the first and second resilient means in a position of equilibrium in which the mushroom head does not protrude out of this recess into the combustion chamber and is preferably disposed substantially flush with this wall.

As will be explained in further detail below, the control device of the invention may, together with the valve itself, be formed as a self-contained unit which may be inserted in a corresponding seat provided in the engine head.

InFIGS. 1 to 3of the accompanying drawings, a valve of an internal combustion engine, in particular an intake valve, is shown overall by1.

In a known manner, the valve1comprises a stem1awhich ends at one end with a mushroom head1b.

The stem1aof the valve1is mounted to move in translation through an opening2ain an end wall2bof a hollow support and guide housing shown overall by2.

On the side oppose the wall2b, an electromagnet shown overall by3is housed in the housing2. In the embodiment shown, this electromagnet comprises a pot-shaped magnetic circuit4having a central projection or polar expansion4asurrounded by a substantially cylindrical jacket4b. The excitation winding5of this electromagnet is disposed about the projection4awithin the jacket4b. The end of the stem1aopposite the mushroom head1bbears a keeper6of ferromagnetic material facing the central portion4aof the magnetic circuit4of the electromagnet3.

Two helical springs, shown respectively by7and8, are disposed about the stem1aof the valve1in the support and guide body or housing2.

The upper spring7extends between the electromagnet3and a plate9rigid with an intermediate portion of the stem1aof the valve.

The lower spring8extends between the plate9and the lower end wall2bof the support and guide housing2.

The assembly formed by the valve1, the support and guide housing2and the other components described above substantially forms a kind of independent “cartridge”, which can therefore be pre-assembled and readily mounted in a corresponding seat10provided in the head of the combustion engine, this seat intersecting the end portion of the intake duct11with which the valve1is associated.

The mushroom head1bof the valve1cooperates with a valve seat12provided in a wall13of the head of the internal combustion engine.

In operation, in the manner described below, the valve1may move in translation between a retracted position (FIG. 1) in which the mushroom head (1b) closes the valve seat12and a forward position (FIG. 3) in which the mushroom head (1b) protrudes into the combustion chamber14, providing the maximum opening.

A recess15adjacent to the valve seat12is provided on the side of the wall13facing the chamber14. The mushroom head1bof the valve may slide tightly or with a limited play in this recess.

When the winding5is energised, the electromagnet3attracts the ferromagnetic keeper6and retains the valve1in the retracted closed position ofFIG. 1. In this condition, the upper spring7(whose elastic constant will be shown below as K1) is compressed between the plate9and the electromagnet3. The lower spring8(whose elastic constant will be shown below as K2) is, however, unloaded.

When the winding5of the electromagnet3is de-energised, the valve1is no longer electromagnetically retained in the retracted closed position, and under the action exerted on it by the upper spring7, it moves downwards with an accelerating movement until it reaches a position of equilibrium shown inFIG. 2; in this condition, both springs7and8are substantially unloaded and in equilibrium with one another and the valve1is held in a position in which the mushroom head1bagain extends within the recess15without projecting into the combustion chamber14, preferably with its lower end surface substantially flush with the lower surface or face of the wall13. InFIG. 1, the stroke of the valve1between the retracted closed position and the position of equilibrium shown inFIG. 2has an extension L2. This extension L2is substantially equal to the extent of pre-compression of the spring7with the valve1closed.

As a result of the presence of the recess15of extension L2, the pressure differential between the intake duct11and the combustion chamber14at the moment at which it is desired to open the valve helps to impart a quantity of movement to this valve1. Similarly, during the phase of further closing of the valve, when the associated piston of the combustion engine is rising again, the pressure differential (this time of opposite sign) tends to urge the mushroom head1bof the valve1into the recess15, promoting the closure of the valve.

In practice, the mechanical work performed as a result of the above pressure differentials is useful in compensating the energy dissipated as a result of friction forces.

An equation of the movement of the valve1during the opening phase will now be described.

When the electromagnet3is de-energised, the valve1is subject to the concurrent thrusts exerted by the upper spring7and by the air pressure in the intake duct11.

Using an x-coordinate axis oriented downwards (FIG. 1, where x=0 corresponding to the closed position of the valve1), the force Fairwhich the pressure differential Δp between the intake duct11and the combustion chamber14exerts on the mushroom1bof the valve may be expressed as a function of the stroke x by the relationship:
Fair(x)=Δp(x).S
where S is the exposed surface of the valve.

For simplicity, and as a rough approximation, the following assumptions may be made as regards the pressure differential Δp(x):during the phase of opening of the valve, as long as the valve is still closed as it is moving in the recess15, it can be assumed that the pressure differential (positive, i.e. with an underpressure in the chamber14) is:

Δ⁢⁢p⁡(x)=p0,where⁢⁢x≤L2⁢⁢and⁢⁢ⅆxⅆt>0during the phase of actual opening (x>L2), it is assumed that Δp(x) is zero:
Δp(x)=0, wherex>L2;during the phase of closure of the valve, when the latter returns to the recess15, it is assumed that the pressure differential is inverted:

The forces of the springs7and8then act on the valve1:

Upper spring: this spring exerts a force
F1(x)=K1·(L2−x) andF2(x)=0, wherex≦L2

Lower spring: this spring exerts a force
F2(x)=0 andF2(x)=K2·(L2−x), wherex>L2.

Lastly, the friction forces between the stem1aof the valve and the guide opening2a, and between the plate and the surrounding wall of the support and guide housing2, also play a part. In relation to these friction forces, it can be assumed that they can be described by a constant term of sign opposite that of the velocity of the valve, as a result of the pressure of the sealing members (present, but not shown in the drawings) and by a term proportional to the modulus of the velocity of opening of the valve1:

This provides the following equation of movement:

On the basis of the equation of movement given above, it is possible to calculate the values of some parameters of the device for controlling the movement of the valve.

Let us assume, for instance, that it is wished to achieve an overall time of actual opening of the valve (i.e. the time during which the mushroom1bis disposed externally to the recess15) of between 5 and 6 ms.

The following values are assumed for the area and the mass of the valve1,
S=7 cm2and m=30 g.

Reasonable values for the damping coefficients are, for instance, as follows:
a=5Nandb=1,5N·s/m

On the basis of the above numerical data, a time of actual opening of the order of 5÷6 ms is obtained with the following values of the parameters L2, K1and K2:L2=4.0 mmK1=105000 N/mK2=11800 N/m

In order to prevent knock in the closing phase of the valve, which occurs when the work due to the pressure differential exceeds the energy lost by friction, it may be advantageous to provide a pneumatic brake in the last part of the closure stroke of the valve1, between the magnetic keeper6and the electromagnet3. To this end, provision may be made for the movement of the keeper6to take place substantially in a tight manner relative to the polar expansions of the magnetic circuit4.

On the basis of the numerical data given above, the lift profile of the valve1as a function of the value assumed for p0(pressure differential in the phase of movement of the mushroom1bwithin the recess15) is shown inFIG. 4.

As can be seen in this Figure, the maximum lift of the valve is equal to 12 mm with an actual opening time of 5 ms.

The values of the retaining force which the electromagnet3has to exert for the three values of p0to which the graphs ofFIG. 4refer are as follows:p0=0.9 bar: F=483 Np0=0.7 bar: F=469 Np0=0.5 bar: F=455 N

If the magnetic force exerted on the keeper6is described by the expression F=s·B2/(2μ0) and if it is assumed that s=3 cm2(area of the section of the portion4aof the magnetic circuit4of the electromagnet), the following value is obtained for the magnetic induction B:

In operation, the control device of the valve is initialised when the first compression occurs in the associated cylinder of the engine: the pressure in the combustion chamber14causes the full closure of the valve1until the keeper6is hooked by the energised electromagnet3.

Associating respective devices for controlling movement of the present invention with the intake valves of an internal combustion engine makes it possible to achieve the opening of these valves in a fixed time, irrespective of the number of revolutions per unit of time of the engine.

The exhaust valves may be traditionally controlled, for instance by a camshaft, or by associated devices for controlling movement of the type described above, obviously driven with a different phasing.

Without prejudice to the principle of the invention, its embodiments and details may obviously be widely varied with respect to what has been described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the invention as set out in the accompanying claims.