WO1992007173A1

WO1992007173A1 - An actuator

Info

Publication number: WO1992007173A1
Application number: PCT/GB1991/001803
Authority: WO
Inventors: David Burke; Giles Muddell
Original assignee: Lotus Cars Limited
Priority date: 1990-10-16
Filing date: 1991-10-16
Publication date: 1992-04-30
Also published as: GB9022448D0

Abstract

The present invention relates to actuator (11) comprising a piston (16) with a cylinder (17). The piston (16) divides the cylinder (17) with at least two chambers. An orifice (53) is provided in an end surface of the cylinder (17). The said end surface and the nearest piston surface are shaped so that as the piston (16) approaches the said end surface the flow of fluid from between the said end surface and the said piston surface to the orifice is restricted. Preferably the end surface and the piston (16) define between them two subchambers (54, 57) and an interconnecting passage (58) for the last portion of motion of the piston (16) towards the end surface. The passage (58) decreases in size as the piston (16) approaches the end surface and increasingly restricts flow of fluid between the two subchambers (57, 54).

Description

AN ACTUATOR

The invention relates to an improvement in the design of actuators which use pressurised hydraulic fluid or compressed gas.

The improvement in actuator design shall be discussed with reference to a particular use of a hydraulic actuator, but the improved hydraulic actuator can be used to replace a normal hydraulic actuator in any use. The actuator shall be discussed with reference to a field in which its advantages are clear.

The motion of valve gear at the cylinder head is normally controlled by the rotation of one or more cam shafts. The cam shafts act to open and close the inlet and exhaust valves of the internal combustion engine. The timing of the opening and closing of the exhaust valves is determined by the profile of the cam shaft. Different cam shaft profiles are chosen for different engines.

The ideal valve for use as inlet or exhaust valve would be a valve that was either open or closed, with no intermediate position. Having a valve partially open leads to losses, since the flow of gases into or out of the combustion chamber is restricted. However, the acceleration and deceleration of valves has been limited in the past by the need to maintain the integrity of mechanical linkages operating the valve. In usual systems, only a certain limit of acceleration and deceleration of the valve is possible before the valve stem becomes separated from the mechanical member acting upon it.

Systems have been proposed in the past which provide an alternative to the activation of valve gear by cam shafts. Such an alternative can be found in U.K. Patent GB 2061560B in the name of Daimler-Benz. In the system described in GB 2061560B a valve element is connected to working piston which can move within a cylinder. The position of the valve element is varied by the movement of the working piston. The working piston is subjected to a pre-tensioning by a spring which biases the piston to move in one direction. The movement of the piston is controlled by the supply of pressurised medium into a chamber defined by one surface of the piston and the inner surface of the cylinder. The force acting on the piston due to presence of pressurised medium in the chamber acts against the resilient means. An electronic controller is used to control the supply of the pressurised medium to the chamber and thereby controls the movement of the valve element.

In U.S. Patent 4188925 the valve gear at the cylinder head is controlled by the supply of pressurised medium to an actuator which pushes against the top of the valve stem, against the action of a spring. A control system controls the supply of pressurised fluid to control the motion of the valve.

In U.S. 4612883 once again a system is described in which a valve is controlled by supplying pressurised fluid to one side of the piston, the force applied by the pressurised fluid acting against a biasing force supplied by a spring.

Difficulties are encountered in controlling the motion of valves which open and shut the inlet and exhaust ports of internal combustion engines since the valves operate at a very high speed. It is difficult to control the motion of the valves at such high frequencies, for two reasons. Firstly, most electronic control systems do not have a frequency response range which enables them to exactly control the motion of a valve moving at high speed. Secondly, it has been found that the compliance of the hydraulic medium used in an actuator leads to inaccuracies in control. Generally the actuators used to power the valve gear in systems of the prior art have used either oil as a hydraulic fluid or have used pressurised air. Both of these mediums are not perfectly incompressible mediums and the compressibility of these mediums leads to problems of control.

The critical region of motion of a valve which opens and closes an inlet or exhaust port for internal combustion engine is the region of motion of the valve nearest the valve seat. The valve during this region of motion undergoes considerable acceleration and decceleration. It has been found that the limited frequency response of an electronic control system causes difficulties in controlling the motion of the valve in this region. The applicant has found during research that the valve cannot be sufficiently deccelerated by the electronic control system and therefore the valve impacts with the valve seat. This impact leads to two serious problems in the operation of internal combustion engines. The first problem is one of noise. The noise generated by the impact of a valve with its valve seat is significant and can be heard in the operation of an internal combustion engine. The second problem is more serious. The life of an engine is cut short by wear of valve seats. Obviously, the impact of a valve with its associated valve seat causes wear of the valve seat.

The difficulties of controlling the decceleration of a valve as it approaches its valve seat are heightened by the compliance of the hydraulic medium present in the actuator controlling motion of the valve. Since the valve undergoes a high level of decceleration as it approaches the valve seat, significant forces are exerted upon the hydraulic medium within the actuator. The hydraulic medium tends to compress and the compression leads to undesired motion of the valve. Further, when the compressed fluid recovers an undesired force is placed upon the valve, which tend to cause motion of the valve away from the valve seat. The valve tends to "bounce" on the hydraulic fluid in its associated actuator. Obviously, this "bouncing" is undesirable.

The present invention provides an actuator comprising a piston within a cylinder, said piston dividing the said cylinder into at least two chambers, and an orifice in an end surface of the cylinder, wherein the said end surface and the nearest piston surface are shaped so that as the piston approaches the said end surface the flow of fluid from between the said end surface and the said piston surface to the orifice is restricted.

Preferably the end surface of the cylinder and and the nearest piston surface are shaped such that as the piston approaches the end surface they define between them at least two separate subchambers, one subchamber adjacent the orifice, and a passage between the chambers, the passage becoming increasingly constricted as the piston approaches the end surface, thereby increasingly restricting flow of fluid between the subchambers.

Preferably both of the end surfaces of the cylinder and both of the piston surfaces are configured such that the flow of fluid out of both of the chambers defined between the end surfaces and the piston surfaces is restricted as the piston moves towards both of the ends of the cylinder.

Preferably at least one end surface of the cylinder comprises a frusto-conical protrusion and the nearest piston surface comprises a corresponding frusto-conical indentation and the orifice of the connecting means is preferably positioned in the flat surface of the frusto-conical protrusion of the end surface of the piston.

Preferably the actuator of the invention further comprises adjustment means connected to at least one end surface, which adjustment means can move one end surface of the cylinder relative to the other end surface in an axial direction. Preferably the movable end surface is connected to a second piston within a second cylinder and when the adjustment means cause the said movable end surface to move by supplying fluid to at least one of chambers defined between the surfaces of the second piston and the inner surface of the second cylinder.

In a preferred embodiment of the invention the fluid supplied to the second cylinder is provided from the same source as the pressurised fluid supplied to the first cylinder of the actuator and wherein the adjustment means comprises a pressure regulator such that the pressure of the fluid supplied to the second cylinder is lower than the pressure of the fluid supplied to the first cylinder of the actuator.

The invention also provides an internal combustion engine which uses the hydraulic actuator of the invention to open and close an inlet and/or exhaust valve in a working cylinder thereof.

A preferred embodiment of the present invention will be described with reference to the accompanying drawings in which;

Figure 1 is a schematic representation of a portion of a first embodiment of the apparatus including a pump, a reservoir, a control valve, a hydraulic actuator and an engine valve.

Figure 2 is a cross-section of part of the hydraulic actuator of the first embodiment of the apparatus. Figure 3 is a schematic representation of the control system of the apparatus.

Figure 4 is a schematic representation of a portion of a second embodiment of the invention.

Figure 5 is a cross-section of a part of the second embodiment.

Referring to Figure 1, a first embodiment of the apparatus can be seen to comprise a control valve 10 and a hydraulic actuator 11 connected to the control valve 10 by two passages 12 and 14.

The control valve 10 is a standard control servo-valve. In the preferred embodiment in an engine with a maximum revolutionary speed of 7000 rpm the servo-valve has a frequency response of around 350 H_. The actuator 11 comprises a piston 16 movable within a cylinder defined by the walls 17. The piston

16 is directly connected by a rod 18 to a valve 19 which opens and closes an aperture opening onto a cylinder of an internal combustion engine.

The piston 16 is caused to move with respect to the cylinder 17 by the supply of fluid under pressure through the two pipes 14 and 12 to both sides of the piston 16.

The control valve 10 is shown schematically in Figure 2 with outlets 14 and 12 and with two inlets 20 and 21. Inlet 20 is connected to a pump 24 which supplies hydraulic fluid under pressure and to a reservoir 22 of pressurised hydraulic fluid.

In Figure 1 it can also be seen that the control valve 10 is connected via inlet 21 to an exhaust 25 for hydraulic fluid.

The control valve 10 is controlled by signals provided by the processing means 40, which signals are provided to the line 43.

The first preferred embodiment of the invention can also be seen to comprise an end 26 of the cylinder

17 which is movable axially with respect to the other end 27. The end 26 is connected to a second piston 28 which is movable with a second cylinder defined by walls 29.

Adjustment means is provided to adjust the position of the end 26 of the cylinder 11 with respect to the end 27. These adjustment means move the end 26 by supplying hydraulic fluid to a chamber 30 defined between the piston 28 and the walls 29 of the second cylinder. The hydraulic fluid is supplied by a line 31. The line 31 is connected to the source of pressurised fluid 24 and the reservoir 22 via a pressure regulating valve 32. An orifice 33 is positioned in the line 31 to restrict the flow of fluid into the chamber 30.

The actuator of the invention is specially designed to increase the resistance offered to the flow of fluid out of the cylinder 11 as the piston 16 approaches the end 26 of the cylinder. This feature is shown schematically in Figure 1 as resistance means 50. This feature of the invention will be hereinafter described at length.

An expanded cross-sectional view of part of the first embodiment of hydraulic actuator 11 is shown in Figure 2. In Figure 2 there can be seen the tube 14, the piston 16 and the cylinder defined by the walls 17.

The control system of the aparatus can be seen in Figure 3. The processing means of the apparatus is contained within the dotted line 40. From Figure 3 it can be seen that there are three inputs to the processing means. The first of these inputs is an electrical signal corresponding to the crank angle. The crank angle is measured at 41. The crank angle signal corresponds to an instantaneous measurement of the angle that the crankshaft makes with an arbitrary fixed reference. The instantaneous measurement is differentiated at 42 to give the revolutionary speed of the engine in revolutions per minute. This parameter forms another input into the processing means 40. The crank angle measurement and the RPM measurement are used not only by the processing means 40 but can also used by processing means for controlling ignition timing and fuel injection.

The third input to the processing means 40 is a position measurement. Position measuring means 49 measures the position of the piston 16 with respect to the cylinder formed by the walls 17. The position measurement is input to the processing means. The signal is used in the closed loop control system of the processing means 40, the signal providing the necessary feedback loop.

The processing means 40 has one output 43. The output is an electrical signal which controls motion of the control valve 10. The control valve 10 then controls the motion of the actuator 11.

The physical operation of the first embodiment of the actuator 11 shall now be described.

Referring firstly to Figure 1 of the drawings, the piston 16 can move within the cylinder 17 and is connected via a rod 18 to a valve 19. In the arrangement described herein the valve 19 opens and closes an aperture which opens on to a cylinder of the internal combustion engine. The valve can open and close either an inlet aperture opening on to the cylinder or an exhaust aperture.

The piston 16 is caused to move within the cylinder 17 by applying a pressure difference thereacross. The pressure difference is applied by supplying hydraulic fluid under pressure to one side of the piston 16, whilst connecting the hydraulic fluid on the other side of the piston 16 to a sink. The control valve 10 is used to control the motion of the piston 16. If the control valve 10 is used to cause the valve 19 to move towards the valve seat then the control valve acts to connect the line 12 to line 20. Line 20 receives a supply of hydraulic fluid under pressure from a pump 24 and a reservoir of pressurised hydraulic fluid 22.

Whilst the control valve 10 connects the line 12 to the line 20 to supply hydraulic fluid under pressure to the lower side of the piston 16, the valve also acts to connect line 15 to a sink 21. Sink 21 is an exhaust for fluid. By connecting the line 15 to the sink 21 the control valve enables fluid to flow from the upper portion of the cylinder 17 through the line 15 and out to the sink 21.

The reservoir of pressurised hydraulic fluid 22 is used to maintain the supply pressure at 20 at an approximately constant value. The pump 21 is powered by motion of the engine which varies during use of the vehicle and therefore the supply of hydraulic fluid from pump 21 can vary. The reservoir 22 contains a supply of pressurised fluid and acts to stabilise the fluctuations in supply of pressurised hydraulic fluid by the pump 21.

The reservoir 22 also helps to overcome problems provided by the inertia of the fluid. The system requires the quick motion of fluid. If the supply to the valve were limited to the fluid supplied by the pump 24 then the valve may not be able to receive enough fluid due to inertia of the fluid in the supply line. In practice the reservoir 22 should be placed as near the control valve 10 as possible. In practice the apparatus of the invention has been found to be limited in performance by the frequency response of the control valve 10. Further, the control system is not sensitive enough to control the decceleration of the engine valve onto the seat of the cylinder head. The problem is enhanced since the fluid contained within the hydraulic actuator is not a perfectly incompressible fluid. The fluid tends to compress under rapid decceleration of the valve. This compression causes inaccuracies in control. Further inaccuracies are caused when the fluid tries to expand, applying pressure to the piston. In effect the compressibility of the fluid within the hydraulic actuator causes "bouncing" of the controlled valve. In order to overcome these difficulties the actuator has been specially designed as will now be described.

Referring to Figure 2 of the drawings the actuator 11 can be seen to comprise the piston 16 within the cylinder defined by the wall 17. The supply line 14 can be seen in Figure 2 connected to the interior of the cylinder 17 by an orifice 51. Fluid can flow from the line 40 through passages 52 and 53 defined in the end 26 to the upper chamber of the actuator 11.

The end 26 has a frusto-conical protrusion 55. The passage 53 opens onto the upper chamber via an orifice situated in the flat portion of the frusto-conical protrusion 55.

The piston 16 has a frusto-conical indentation 56 which corresponds to the frusto-conical protrusion 55. In normal operation of the actuator 11 motion of the piston 16 towards the end 26 of the actuator 11 causes fluid to be expelled from the chamber 54 from the passages 53 and 52 to the passage 14. No constriction is placed upon the flow of fluid out of the upper chamber, other than the constrictions always present in the actuator. However, the flow of fluid from the upper chamber is constricted for the last part of the motion of the piston 16 towards the end 26.

In the last portion of the motion of the piston 16 toward the end 26, fluid becomes trapped in an annular subchamber 57 surrounding the protrusion 55. The piston 16 and upper surface 26 of the cylinder define between them during the last portion of motion of the piston toward the upper surface two subchambers 54 and 57 and an interconnecting passage 58. Fluid can only flow from the annular subchamber 57 out through passage 53 by passing through a restricted annular passage 58 defined between the uppermost surface of the piston 16 and the end 26 of the cylinder. Since the flow is constricted by the narrow aperture greater resistance is offered to the flow of fluid out of the annular subchamber 57. The increased resistance causes damping of the motion of the piston 16 with respect to the cylinder 17. The increased resistance slows the motion of the piston 16.

As the piston 16 approaches nearer and nearer the end 26, the flow of fluid from the annular subchamber 57 becomes more and more constricted. The flow is constricted by the narrow passage 58 formed between the sides of the frusto-conical protrusion 55 and the sides of the frusto-conical indentation 56. The annular aperture continually narrows as the piston 16 moves towards the end 26. The resistance to motion of the piston 56 is therefore continually progressively increased as the piston 16 reaches its limits of motion towards the end 26. The progressive increase in resistance enables deceleration of the valve 19, preventing impact between the valve 19 and its respective valve seat.

The device minimises the difficulties caused by compressibility of the hydraulic fluid within the actuator, since very little or no fluid is actually contained in the annular subchamber 57 as the piston 16 reaches the limit of its motion. Thus the problem of "bouncing", previously mentioned is largely eliminated.

Since the operation of the device relies upon there being little or no volume of hydraulic fluid lying between the top surface of the piston 16 and the end 26 when the valve 19 is against its respective valve seat, it is necessary to include adjustment means which allow the end 26 to meet with the top of the piston 56. The adjustment means must also adjust for thermal expansion and production tolerances. The adjustment is effected by connecting the end 26 to a piston 28, which moves with a second cylinder defined by walls 29. In a preferred embodiment, the end portion 26 is formed in one component with the piston 28. However, the piston 26 could alternatively be connected by connecting means to the piston 28.

A chamber 30 is defined between the piston 28 and the walls of the second cylinder. Hydraulic fluid is introduced into the chamber 30 through line 31 from a pressure regulating valve (shown in Figure 1) which is connected to the supply of hydraulic fluid under pressure.

The pressure regulating valve 32 regulates the pressure of the fluid supplied to the chamber 30 to roughly one tenth of the system pressure. When the valve 19 is against its respective valve seat, the orifice 33 allows a small flow of fluid into the cavity 30, causing the end 26 to move until it mates with the top surface of the piston 16.

The end 26 is always prevented from moving any great distance since the orifice 33 restricts the flow of fluid into the chamber 30. Also, the compressibility of the fluid in the chamber 30 is negligible since the area of the piston 28 is comparatively large and the volume of fluid contained in the chamber 30 comparatively small.

The second embodiment of the invention is shown in figures 4 and 5. Essentially it is the same as the first embodiment and differs in only one aspect. The only difference between the two embodiments is a difference in the top portion of the actuator 11. There is provided in the second embodiment a second channel 60 in communication with the chamber 30. The second embodiment 60 allows fluid to flow out of the chamber 30 via a narrow orifice provided by member 61 to a reservoir of fluid or fluid return, 62.

In the second embodiment the pressure of the fluid in chamber 30 is regulated by controlling the flow of fluid through the chamber. Again the pressure of the fluid in chamber will be kept substantially less than system pressure.

Whilst the system described above uses pressurised hydraulic fluid, the applicant envisages that compressed gas could also be used to cause motion of the piston 16. However, the use of hydraulic fluid is suited to the use of the apparatus in an internal combustion engine since pressurised hydraulic fluid is readily available in the form of oil pressurised by an oil pump powered by the output of the engine.

Whilst the fluid used in the system hereinbefore described is supplied to chambers on both sides of the piston 16, the applicant envisages a system wherein fluid need only be supplied to one side. If resilient means are provided to bias the piston in one direction then fluid or gas need only be supplied to one chamber, the force exerted by the pressurised fluid or gas acting to move the piston relative to the cylinder against the force provided by the resilient means.

Whilst the actuator above is configured such that the motion of the piston is damped in only one direction, the applicant envisages that the damping of the motion of the piston in both directions may be desirable in certain circumstances.

It will be clear from the foregoing that the invention provides an actuator which can overcome the difficulties presented by limited frequency response of an electronic control system and by the compressible nature of the fluid used within the actuator controlled by the control system.

Claims

1. An actuator comprising a piston within a cylinder, said piston dividing the said cylinder into at least two chambers, and an orifice in an end surface of the cylinder, wherein the said end surface and the nearest piston surface are shaped so that as the piston approaches the said end surface the flow of fluid from between the said end surface and the said piston surface to the orifice is restricted.

2. An actuator as claimed in Claim 1 wherein the end surface of the cylinder and the nearest piston surface are shaped such that as the piston approaches the end surface they define between them at least two separate subchambers, one chamber adjacent the orifice, and a passage between the subchambers, the passage becoming increasingly constricted as the piston approaches the end surface, thereby increasingly restricting the flow of fluid between the subchambers.

3. An actuator as claimed in Claim 1 or Claim 2 wherein both of the end surfaces of the cylinder and both of the piston surfaces are configured such that the flow of fluid out of both of the chambers defined between the end surfaces and the piston surfaces is restricted as the piston moves towards both of the ends of the cylinder.

4. An actuator as claimed in any one of the preceding claims wherein at least one end surface of the cylinder comprises a frusto-conical protrusion and the nearest piston surface comprises a corresponding frusto-conical indentation.

5. An actuator as claimed in Claim 4 wherein the orifice of the connecting means is positioned in the flat surface of the frusto-conical protrusion of the end surface of the piston.

6. An actuator as claimed in any one of the the preceding claims further comprising adjustment means connected to at least one end surface, which adjustment means can move one end surface of the cylinder relative to the other end surface in an axial direction.

7. An actuator as claimed in Claim 6 wherein the movable end surface is connected to a second piston within a second cylinder and when the adjustment means cause the said movable end surface to move by supplying fluid to at least one of chambers defined between the surfaces of the second piston and the inner surface of the second cylinder.

8. An actuator as claimed in Claim 7 wherein the fluid supplied to the second cylinder is provided from the same source as the pressurised fluid supplied to the first cylinder of the actuator and wherein the adjustment means comprises a pressure regulator such that the pressure of the fluid supplied to the second cylinder is lower than the pressure of the fluid supplied to the first cylinder of the actuator.

9. An internal combustion engine which uses an hydraulic actuator as claimed in any one of the preceding claims to open and close an inlet and/or exhaust valve in a working cylinder thereof.

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

11. An internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.