DESCRIPTION OF THE PRIOR ART: In internal combustion engines for vehicles it is sometimes desirable to have the facility for switching between different operating modes. For example, it is possible to switch between a conventional symmetrical cycle and an asymmetrical cycle, such as a so-called Miller cycle, by varying the timing of the inlet valve closure during the engine's induction stroke. The advantage in being able to switch between these different operating modes lies, for example, in the ability to vary the effective compression ratio of the engine in order to optimize efficiency and reduce exhaust emissions to a minimum. For this purpose, therefore, variable valve actuation is necessary.

The experimental use of electronically controlled hydraulic actuators for variable valve actuation as an

alternative to mechanical valve systems is already known. These known systems are at present still very expensive and still not sufficiently reliable and robust and require a very sophisticated system of timing in order, among other things, to avoid collision between valves and piston and to cope with a viscosity that varies with temperature.

Other mechanical systems are disclosed, for example by US patent 4829949, which permit mechanical change-over from one operating mode to another. These known systems are mechanically complex and call for extremely high precision both in manufacturing and servicing.

One known method is to achieve variable valve movements by means of mechanical-hydraulic systems of the"lost motion"type. These are characterized by a hydraulic link which forms part of the mechanical valve mechanism and which can be alternately opened or closed by means of a solenoid valve, for example. When closed, the movement of the valve is controlled by the cam curve in the same way as in a conventional mechanical system.

When valve closure is to be advanced, for example, the hydraulic link is opened by means of the solenoid valve, so that the valve ceases to follow the cam curve and can thereby be made to close.

The disadvantage with the system described above is that the energy stored in the valve spring, which in a conventional mechanical system is largely recovered as the valve falls, due to the fact that the valve spring force imparts a driving torque to the camshaft, is for the most part lost in a system of the"lost motion" type when the engine is operating in Miller cycle mode.

For engine designs in which the Miller cycle represents the most commonly occurring mode of operation, this energy loss may have a significant impact on the fuel

consumption of the engine during a collective drive test cycle.

SUMMARY OF THE INVENTION: The present invention relates to a mechanical/ hydraulic valve system in which the aforementioned disadvantages of systems of the"lost motion"type have been eliminated by functioning as a conventional mechanical system in Miller cycle mode. In operating conditions where for various reasons it is desirable to reduce or eliminate the Miller stage, for example in transient conditions or in certain load/engine speed ranges, the mechanical contact between cam curve and valve is broken in that closure of the valve is delayed for an adjustable period of time by means of a hydraulic apparatus counteracting the valve spring closing force.

It may be advantageous in certain operating conditions to set the closing point of the inlet valve to a position situated between early closure and normal closure. The energy loss occurring when the mechanical contact is broken can be reduced by allowing the valve to follow the cam curve during the first part of the valve closing movement, the energy stored in the valve spring being recovered in the form of a drive torque on the camshaft for the part of the closing movement that occurs before the mechanical contact is broken. This method can also be used for the normal closing point and also to particular advantage at lower engine load when the air quantity which must pass through the inlet valve is small and the flow losses are also small despite the reduced valve area in the final phase of the valve closing movement. In this context it is worth pointing out that the engine pistons are always near their bottom dead center in the final phase of inlet valve closure at the normal closing time. This means that the rate of flow through the valve is always relatively low, even at high engine load, which keeps the flow losses to a

moderate level. The present invention permits optimization of the inlet valve closure sequence by minimizing the sum of the mechanical losses in the valve system and the flow losses.

In its simplest embodiment the hydraulic apparatus comprises a hydraulic medium, a cylinder and a piston running in the cylinder, the cylinder being alternately connected to a pump or an outlet, or alternatively sealed, by means of a solenoid valve.

To this end the apparatus according to the invention is characterized in that the control members are arranged so that closure of the valve can be delayed for an adjustable period of time by means of a hydraulically adjustable force counteracting the valve spring closing force.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail below with reference to an examplary embodiment which is shown in the drawings attached, in which Fig. 1 is a graph curve showing the movement of the inlet valve in the case of a valve mechanism according to the invention, Fig. 2 shows a diagram of a valve mechanism according to a first examplary embodiment of the invention, and Fig. 3-5 show smaller scale diagrams of three different variants of the valve mechanism according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS : The graph curve shown in Fig. 1 illustrates how an inlet valve works in an internal combustion engine according to the present invention. The engine is designed so that it can switch between a first operating mode corresponding to the solid lines 10 in Fig. 1, and other operating modes corresponding to the lines lla-llc in

Fig. 1. Here the inlet valve according to the line 10 follows the rising ramp and falling ramp on a camshaft cam, which is designed to operate the internal combustion engine on a Miller cycle with maximum advancement of the inlet valve closure.

This mode of operation means that the inlet valve closes early enough in the inlet phase to allow the gas volume enclosed in the cylinder to expand before the piston reaches its bottom dead center following the inlet phase. The temperature of the gas volume is thereby reduced, so that subsequent compression and ignition can occur at reduced temperature level, which allows the NOx content of the engine exhaust gases to be reduced, whilst at the same time permitting an increase in engine efficiency.

In a second operating mode the inlet valve according to line lla follows the rising ramp of the cam curve but then leaves the falling ramp, so that the inlet valve closes at a later point in time (crank angle). Line lla here represents a conventional symmetrical cycle. This common symmetrical cycle may be advantageous when the engine is operating under transient conditions and in certain parts of the engine working range.

In a third operating mode the inlet valve according to line llb follows the rising ramp of the cam curve but leaves the falling ramp, so that the inlet valve closes at a point in time situated between early and normal closure.

In a fourth operating mode the inlet valve according to line lie follows the rising ramp and the first part of the falling ramp of the cam curve, but then leaves the falling ramp, so that the inlet valve closes at a point in time situated between early and normal closure.

The valve mechanism shown in schematic form in Fig. 2 is located in a cylinder head and comprises double inlet valves 12 with valve springs 13 and a common yoke 14.

The yoke is acted upon in a known manner by a rocker 15 which is pivotally supported on a rocker shaft 16. The rocker 15 has a valve pressure arm 17 on one side of the shaft and a cam follower 18 on the other side, the follower being provided with a rocker arm roller 19 which interacts with an overhead camshaft 20.

Alternatively a camshaft located at a lower level in the engine can interact with the rocker by way of a valve tappet and a push rod.

The yoke 14 is mounted on a piston rod 21 which is supported so that it is vertically displaceable by a piston 22 in a cylinder 23. The end 24 of the cylinder is provided with sealing against the piston rod 21, so that a fluid-tight space 25 is formed between the piston and the end. The space 25 is connected via a pipe and a control valve 26 to a pressure pump 27. Fig. 2 shows the control valve 26 in an active position in which the pump 27 can deliver hydraulic fluid from a reservoir 28 to the space 25 via a non-return valve 29. In the other position of the control valve the hydraulic fluid can be dumped from the space 25 into the reservoir 28.

The valve mechanism therefore normally follows the lifting curve 10, the control valve 26 being situated in its inactive control position. In this position unpressurized hydraulic fluid can flow freely between the space 25 and the reservoir 28, whilst the yoke 14 moves down and up under the action of the rocker in one direction and the valve springs 13 in the other direction.

When the engine control unit registers that it is time to switch to another operating mode, the control valve is made to assume its active position (as shown in Fig.

2), the next depression of the yoke 14, under the action

of the valve mechanism, enabling the pump 27 to fill the space 25 with hydraulic fluid from the reservoir 28.

Once the downward movement is completed and the yoke commences its upward movement, the yoke is prevented for a suitable period of time from moving upwards in Fig. 2, under the action of the non-return valve 29. The upward movement of the yoke is initiated at a suitable point in time by the control valve being returned to its inactive position again. The inlet valves can thereby be closed at a suitable crank angle.

Information from the angular position of the camshaft or the crankshaft may be used in order to give the precise timing for switching the control valve 26 from the active to the inactive position in order to bring about the desired valve closure. This process is repeated until the engine control unit registers that it is time to switch to another operating mode.

The piston cylinder 21-25 or any other part of the valve mechanism may suitably be provided with damping elements which brake the movement of the valves before they land on their valve seats.

Fig. 3 and 4 show variants of the invention in which the piston cylinder 21-25 is located so that it acts on the rocker.

Fig. 5 shows a further variant of the invention in which the piston cylinder is connected to the valve yoke 14 by way of an angle arm 30.

The apparatus according to the invention has been demonstrated in its application to an inlet valve. It is also possible to apply the apparatus to an exhaust valve. This can be used, for example, for internal return of the exhaust gas, so-called internal exhaust gas recirculation (EGR), a variation of the exhaust valve closing sequence being capable of influencing the

quantity of internal EGR by adjusting the overlap between the inlet and exhaust valves on completion of the exhaust stroke.

The invention must not be regarded as being limited to the examplary embodiments described above, a number of further variants and modifications being feasible within the scope of the following patent claims. For example, the piston cylinder 21-25 may be designed differently as may the control valve 26. The piston cylinder 21-25 may, for example, act directly on a valve. In the case of a camshaft located low down, the piston cylinder 21-25 may interact with the push rod or the valve tappet.