GB2194587A

GB2194587A - Variable actuator for a valve

Info

Publication number: GB2194587A
Application number: GB08713764A
Authority: GB
Inventors: Stephen John Charlton; David John Bell; Peter Charles Howard; Andrew John Haines; Stuart Lawrence Bird
Original assignee: South Western Industrial Research Ltd
Current assignee: South Western Industrial Research Ltd
Priority date: 1986-06-12
Filing date: 1987-06-12
Publication date: 1988-03-09
Also published as: WO1987007677A1; GB8713764D0; EP0309468B1; GB8614310D0; EP0309468A1

Abstract

A valve actuator such as an hydraulic tappet. In one embodiment the tappet comprises a cylinder (16) which defines by means of a central wall (17), upper and lower chambers (18, 19). These have respective pistons (20 and 21). Piston (20) is connected to an actuator rod (13) of a valve (11), whilst piston (21) rides on a cam (23). The central wall is formed with two flow paths (24 and 25). The first allows oil to flow from the chamber (19) to the chamber (18) during upward movement of the piston (21) and hence the cam can cause lifting movement of the piston (20) and opening of the valve (11). As the cam (23) falls away the valve (11) closes under the action of spring (14) which results in downward movement of the upper piston (20). The rate of this return is dictated by the rate at which oil can flow through the passage (25) which can be variably restricted by control element (27). Thus, by adjustment of the control element (27) the rate of closing of the valve (11) can be varied. Other embodiments are described but in each the return movement of the upper piston (20) is variably damped.

Description

SPECIFICATION Variable actuator for a valve This invention relates to variable actuators for valves and in particular, but not exclusively, to such actuators for use with internal combustion engine valves.

In at least the majority of automotive production engines the valves are opened and closed by means of a cam whose profile is fixed for all operational speeds, although the rate of opening and closing of the valves increases with engine speed as the rate of rotation of the cam increases. It has been known for some time that the combustion efficiency and torque of an engine can be improved by altering the open period of a valve and that the optimal open period differs with engine speed. Thus, current cam profiles represent a compromise and are therefore unsatisfactory.

Attempts have been made to provide systems whereby the rate of opening can be varied, but for the most part they are overcomplex, provide insufficient adjustment and/or do not fit with current engine configurations.

It is an object of the invention to produce a valve actuator which overcomes at least some of the above problems.

From one aspect, the invention consists in an actuator for a valve comprising bias means for urging the valve into a closed position and means for opening the valve against the bias means including means for variably damping the closing of the valve by the bias means.

The opening means may include a cam and cam follower means for causing opening of the valve in response to movement of it by the cam in a first direction. In this case, the damping means may act on at least part of the cam follower means to disconnect it, and hence the valve, oparatively from the cam for a controllable period Thus, the cam follower means may comprise a hollow elongate body divided into first and second chambers by a generally central wall,an actuator piston for operating the valve disposed in the first chamber, a second camoperated piston in the second chamber and means for interconnecting the first and second pistons so that the valve is opened by the first piston in response to cam induced movement of the second piston, the first chamber containing fluid which can be vented from the chamber through an outlet port in response to closing movement of the valve and the damping means may comprise a variable restriction in that outlet port.

The second chamber may also contain fluid, in which case the central wall may include a fluid flow path from the second chamber to the first chamber, such that cam induced movement of the second piston forces fluid into the first chamber. The outlet port may debouch into the second chamber.

In some embodiments the body may be provided with a fluid input and a fluid output such that the fluid in the cam follower means is changed in each cycle in response to one or both of the pistons. For example, the fluid inlet may be connected to the first chamber and the fluid outlet may be connected to the second chamber. A further fluid input may be connected to the second chamber for supplying fluid under pressure to maintain the second piston in contact with the cam during the valve closing part of the cycle.

In one arrangement the transfer of fluid via the fluid flow path imparts valve opening movement to the first piston in response to cam induced movement of the second piston.

In another, support means may be mounted on the second piston for carrying the first piston during the valve opening part of the cycle, whilst allowing an independent movement of the pistons during the valve closing part of the cycle. In a further arrangement, the first and second pistons may be effectively integral one with the other.

Where fluid inlets and outlets are not provided there may be a top-up port which can debouch into either chamber but preferably opens on the first chamber.

In some embodiments, a bypass flow path may also be provided allowing flow from the first to the second chamber under certain operating conditions.

From another aspect, the invention consists in an actuator for a valve comprising bias means for urging the valve into a closed position, and means for opening the valve against the action of the bias means, the valve opening means including a cam, cam follower means for causing opening of the valve in response to movement of the cam in a first direction and control means for controlling the rate of return of the cam follower means in the reverse direction under action of the bias means, the arrangement being such that at least part of the cam follower means is operatively disconnectable from the cam for a controllable period.

In a preferred embodiment the actuator is used to control the operation of one or more valves of an internal combustion engine, and is particularly suitabie for use with the inlet valves of such an engine.

In that case, the control means may be controlled in response to an operating parameter of the engine and preferably in accordance with a parameter which represents or reflects engine speed. For example, the control means may vary the rate of flow in accordance with the inlet manifold pressure, the oil pressure, or an engine speed detection device. The valves may be controlled individually, or all the inlet valves may be controlled simultaneously by a linking mechanism.

The control may be based on a mechanical or electrical system. If it is electrical the control may be continuous and designed to maintain the valve operation in accordance with a pre-determined "optimum" performance.

If it- is mechanical, then it may be desirable to inhibit the operation of the control under certain operating conditions, so that the movement of the valve is dictated solely by the profile of the cam follower. This can be achieved by the bypass means mentioned above and would typically be used at low engine oil temperatures during start-up. The closure of the bypass may be temperature controlled.

It is preferred that the profile of the cam is selected in accordance with the desired low speed (e.g. 1,000 r.p.m. and under) performance of the- engine and that the outlet port is opened increasingly as engine speed increases.

The cylinder and other portions of the opening means may be formed in the engine casing.

Although the invention has been defined above, it is to be understood that it includes any inventive combination of the features set out above or in the following description.

The invention may be performed in various ways, and a specific embodiment will now be described, with reference to the following drawings, in which: Figure 1 is a partly schematic view of a valve train including the actuator of the invention; Figure 2 is a graph of the valve opening with respect to time; Figure 3 is a graph of the controlled opening area against engine speed and its variation with temperature; Figure 4 is a schematic view of a bypass; Figure 5 illustrates a dampening arrangement for use in the invention; Figure 6 is a schematic view of a centralised control system; Figure 7 is a schematic view of an individual control system, and Figure 8 is a cross sectional view of an alternative embodiment of an actuator.

Figure 1 illustrates schematically a valve train generally indicated at 10. As is conventional,the valve 11 is controlled by a rocker arm 12 which is itself actuated by a rod 13.

A return spring 14 is provided to re-seat the valve.

In the Applicant's invention, the valve train, or opening means, includes an hydraulic section generally indicated at 15. This comprises an open-ended cylinder 16 which is divided by a central wall 17, into upper and lower chambers 18,19 for receiving respective pistons 20,21. The upper piston 20 is coupled to the rod 13, whilst the bottom half 22 of the lower piston 21 is configured as a plate for engaging a cam 23.

First and second flow paths 24,25 are defined through the central wall. The first is designed to allow flow from the lower chamber 19 to the upper chamber 18 and includes a non-return valve 26. The second, 25, is provided to allow flow in the reverse direction and has a control element 27, which can be inserted and withdrawn at right-angles to the direction of flow therethrough to vary the effective cross-sectional area of the flow path 25 and hence the rate of flow therethrough.

For the reasons set out above, the cam is profiled to operate the valve in accordance with the optimal low speed conditions. In this arrangement, the cam 23 lifts the lower piston 21 compressing oil in the lower chamber 19 and hence forcing it into the upper chamber 18, with the result that the upper piston 20, and hence the rod 13, is lifted causing opening of the valve 11. As the cam passes its peak projection, the movement is reversed due to the action of spring 14. The rate at which the upper piston 20 falls back and hence the rate at which the lower piston 21 moves back to the position in which the bottom half 22 engages the circle of the cam 23, is determined by the rate at which fluid flows back into the lower chamber 19, but it can never be faster than that allowed by the cam 23.

Accordingly, by varying the restriction caused by control element 27, the pistons 20,21 can be caused to return more slowly than would be dictated by the cam 23, with the result that the valve opening time can be adjustably increased.

Referring to Figure 2, it will be seen that lines A to C represent increasing cross-sections for the second flow path 25, and hence reduced valve opening times.

It will also be noted from both Figure 2 and Figure 3 that as the speed increases, the cross-section of the second flow path 25 is also increased, i.e. the actual valve open time is reduced. At first sight, this is somewhat surprising, because conventional theory says that one should increase valve opening as speed increases. What the Applicants have realised is that by adjusting the actual valve opening time, they achieve a different effect in terms of the engine crank angle at different speeds. Thus, 1 millisecond of delay in valve closure at 1,000 r.p.m. corresponds to a crank rotation of far less degrees than it does at 4,000 r.p.m.

The insertion and withdrawal of control element 27 can be achieved in many ways and it is preferred that it should be microprocessor controlled and electrically driven in accordance with engine speed and optimum operated engine conditions pre-programmed into the processor. However, such facilities are currently only available on the most up-market ranges of cars, and so mechanical control may be needed in the mass market. Figures 6 and 7 illustrate two mechanical control systems; the first being based on the pressure in the inlet manifold 28, which is converted to mechanical linear movement by a diaphragm spring unit 29 to move a linear cam 30 which causes orthogonal linear movement of a control rod 31 which, in turn, rotates a rocking lever 32 to produce axial movement of the element 27.

The Figure 7 embodiment again used a linear cam and is based on the premise that the mean pressure in the lower chamber 19 is a reflection of speed. The linear cam 30 will be moved in accordance with that pressure.

It will be noted that in Figure 1 a top-up port 33 is provided in the lower chamber and, as is mentioned above this is fed by the engine oil pump. It has been found that in some circumstances it is desirable for this port to enter the upper chamber through the non-return valve, because when the engine is not in use, the valve 11 will re-seat if the engine is stopped with the valve in its open position, due to hydraulic oil leakage. The valve train therefore gets out of calibration with the cam.

With the-top-up port in the upper chamber, the oil pressure provided by the engine pump will lift the upper piston 20 and re-position the valve 11.

It will be seen from Figure 3 that the second flow path cross-section varies significantly with temperature below about 40"C. For normal running, this does not matter as most modern engines operate at around 100 C, but problems can arise on start-up if the element 27 is under mechanical control. (A microprocessor can of course allow for such temperature changes). Accordingly, it is envisaged that the cylinder 16 may be provided with a further bypass between the upper and lower chambers 18,19 for allowing flow from the upper chamber to the lower chamber when the oil temperature is below 40"C. When this temperature is reached, the bypass will be closed off and the second flow path crosssection will control the action of the valve 11.

Such a bypass is illustrated in Figure 4.

It has been found desirable to damp the last part of the downward movement of the upper piston to provide controlled seating of the valve and an appropriate cushioning arrangement is illustrated in Figure 5.

In certain uses, particularly in high speed operation, cavitation or frothing of the oil may occur in the second !chamber 19. It is considered that this may be reduced or overcome by replacing the oil in the chambers 18,19 in each cycle. One arrangement for achieving this is illustrated in Figure 8.

Here, the upper chamber is provided with an oil inlet 40 having a non-return valve 41 whilst the lower chamber 19 has a fluid outlet 42 having a non-return valve 43. In this construction, the first flow path 24 has been dispensed with and instead the upper piston 20 is lifted directly by the lower piston 21 by means of legs (one of which is shown at 44) which support the upper piston 20.

Thus, in operation, as the cam 23 lifts the lower piston 20, oil is pushed out of the chamber 19 through the outlet 42. At the same time the upper piston is lifted by legs 44 and draws fresh oil into the chamber 18 via inlet 40. During the closing cycle the rate of the return of the upper piston is determined, as before, by the degree to which the passage 25 is restricted by the control element 27.

It will be noted that, at least to some extent, the rate of return of the upper piston 20, and hence the alve 11, can be independent of the rate of return of he lower piston 21. This provides the possibility of 'topping-up' the second chamber 19 through an additional inlet 45. This option could provide two advantages; first, the lower piston 21 can be allowed to stay in contact with the cam 23 at all times, because the upper piston 20 is temporarily disconnected from the lower piston 21 during descent; and second, the pressure in the chamber 19 can be kept up to further reduce frothing of the oil.

In a further modification the passage 25 can be provided in a side wall of the chamber 18 allowing the oil to be passed directly into the engine's oil supply. In this case, the oil in the chamber 19 could be replaced by a spring or other bias means arranged to return the lower piston 21 during the return cycle.

In a further variation, the upper piston 20 may be integrally formed with or connected to the legs 44 so that the two pistons travel together. In this case, the rate of movement on the upward stroke is determined by the action of the cam on the lower piston 20, whilst the rate of movement on the downward or closing stroke is determined by the degree of damping induced by the control element 27.

It will be appreciated that the systems described is equally applicable to an overhead cam shaft arrangement and indeed may be used in almost any orientation.

Claims

1. An actuator for a valve comprising bias means for urging the valve into a closed position and means for opening the valve against the bias means including means for variably damping the closing of the valve by the bias means.

2. An actuator as claimed in claim 1 wherein the opening means includes a cam and cam follower means for causing opening of the valve in response to movement of it by the cam in a first direction and wherein the damping means acts on at least part of the cam follower means to disconnect it, and hence the valve, operatively from the cam for a controllable period.

3. An actuator as claimed in claim 1 wherein the cam follower means comprises a hollow elongate body divided into first and second chambers by a generally central wall, an actuator piston for operating the valve disposed in the first chamber, a second camoperated piston in the second chamber, and means for interconnecting the first and second pistons so that the valve is open by the first piston in response to cam induced movement of the second piston, the first chamber containing a fluid which can be vented from the chamber through an outlet port in response to closing movement of the valve and wherein the damping means comprises a variable restriction in the outlet port.

4. An actuator as claimed in claim 3 wherein the second chamber also contains fluid and wherein the central wall includes a fluid flow path from the second chamber to the first chamber such that cam induced movement -of the second piston forces fluid into the first chamber.

5. An actuator as claimed in claim 4 wherein the outlet port debouches into the second chamber.

6. An actuator as claimed in claim 4 or claim 5 wherein the body is provided with a fluid input and a fluid output such that the fluid in the cam follower means is changed each cycle in response to movement of one or both of the pistons.

7. An actuator as claimed in claim 6 wherein the fluid inlet is connected to the first chamber and the fluid outlet is connected to the second chamber.

8. An actuator as claimed in claim 6 or claim 7 wherein there is a further fluid inlet connected to the second chamber for supplying fluid under pressure to maintain the second piston in contact with the cam during the valve closing part of the cycle.

9. An actuator as claimed in any one of claims 4 to 8 wherein the transfer of fluid via the fluid flow path imparts valve opening movement to the first piston in response to cam induced movement of the second piston.

10. An actuator as claimed in any one of claims 4 to 8 further comprising support means mounted on the second piston for carrying the first piston during the valve opening part of the cycle whilst allowing independent movement of the pistons during the valve closing part of the cycle.

11. An actuator as claimed in any one of claims 4 to 7 wherein the first and second pistons are integral.

12. An actuator substantially as hereinbefore described with reference to the accompanying drawings.

13. An actuator for a valve comprising bias means-for urging the valve into a closed position, and means for opening the valve against the action of the bias means, the valve opening means including a cam, cam follower means for causing opening of the valve in response to movement of the cam in a first direction and control means for controlling the rate of return of the cam follower means in the reverse direction under action of the bias means, the arrangement being such that at least part of the cam follower means is operatively disconnectable from the cam for a controllable period.

14. An internal combustion engine including a valve actuator as claimed in any one of the preceding claims.

15. An engine as claimed in claim 14 wherein the damping means are controlled in response to an operating parameter which represents or reflects engine speed.