EP0759119B1

EP0759119B1 - Valve control mechanism

Info

Publication number: EP0759119B1
Application number: EP95917428A
Authority: EP
Inventors: Jeffrey Allen
Original assignee: Lotus Cars Ltd
Current assignee: Lotus Cars Ltd
Priority date: 1994-05-03
Filing date: 1995-05-03
Publication date: 1999-11-03
Anticipated expiration: 2015-05-03
Also published as: DE69513165T2; JP3865771B2; ES2141350T3; WO1995030081A1; JPH09512603A; DE69513165D1; EP0759119A1

Description

The present invention relates to a valve mechanism for controlling a cylinder head valve of an internal combustion engine.

It is common practice in internal combustion engines to have poppet valves which open and close inlet and exhaust ports in the cylinder head. It is also common practice to control the motion of the poppet valves by the use of a cam shaft. The cam shaft rotates in time with the rotation of the engine and has a plurality of cams. A tappet assembly is provided for each poppet valve, each tappet assembly engaging one cam of the cam shaft. The tappet assemblies relay motion from the cams of the cam shaft to the poppet valves.

In conventional engines the lift of a particular cam is always transmitted to the controlled poppet valve, in all engine conditions. However, this is not always desirable. There are many engines today which have four or more poppet valves for each cylinder head. This improves performance of the engine at high engine speeds and loads. However, at low engine speeds or loads use of all four valves is not necessary and indeed can be detrimental to engine emissions and fuel economy. Therefore, there have been proposed in the past various cam mechanisms which can de-activate a poppet valve in certain engine operating conditions.

In DE-2952037 there is shown a valve control mechanism which allows de-activation of a poppet valve. The valve control mechanism comprises a cylindrical member which has a bore axially therethrough, the bore being closed at one end. The stem of the controlled poppet valve is slidable in the bore in the cylindrical member. A locking member is provided in the cylindrical member which is movable radially of the cylindrical member. The locking member is movable between a first position in which the poppet valve and the cylindrical member are free to slide relative to one another and a second position in which the locking member engages the top of the stem of the poppet valve so that the poppet valve and the cylindrical member move together. The cylindrical member of the tappet assembly engages a cam of a cam shaft to the engine. Thus, when the locking member is in its second position, the lift of the cam engaged by the cylindrical member is transmitted by the cylindrical member to the poppet valve and the poppet valve is activated. However, when the locking member of the tappet assembly is in its first position, the cylindrical member can slide relative to the poppet valve and the lift of the cam therefore causes only relative motion between the cylindrical member and the poppet valve and no lift is relayed to the poppet valve; in other words the poppet valve is de-activated.

The system of DE-2952037 requires that the valve stem of the poppet valve is itself movable within the cylindrical member of the tappet assembly. This has certain disadvantages. First of all, the arrangement requires modification of existing components, such as the valve stem of the poppet valve and the cylinder head construction. Secondly, there is a problem faced if the valve when de-activated remains fully closed at all times. Generally speaking, in fuel injection engines fuel is injected onto the back of the poppet valves, for release into the cylinder as air passes the poppet valves. If a poppet valve remains fully closed for a period of time, then a puddle of fuel builds up behind the poppet valve, leading to undesirable effects when the poppet valve is subsequently opened.

To enable use of the system of DE-2952037 it is necessary to employ precise machining techniques in manufacturing of an engine cylinder head. The system of DE-2952037 requires the stem of a poppet valve to move within a bore of a cylindrical member. The poppet valve will be located in use in a first bore in the cylinder head and the cylindrical member will be located in a second bore in the cylinder head. In practice, both bores would be machined separately. To use the system of DE-2952037 careful machining of the two bores would be necessary to ensure axial alignment.

In WO 91/12413 there is illustrated a valve control mechanism which has first and second tappet members mounted co-axially in a bore in an engine, the outer tappet member engaging a first cam mounted on a camshaft of the engine and the inner tappet member engaging a second cam on the camshaft of lower lift than the first cam. The inner tappet is slidable in a bore which extends axially along the whole length of the outer tappet. The inner tappet abuts the top of a stem of a cylinder head valve of the engine. Locking means are provided to lock the inner and outer tappets to move together. When the tappets are unlocked then the valve is controlled by the inner tappet which follows the profile of the lower lift cam. When the tappets are locked then the valve is controlled by the lift of the higher lift cam.

JP-A-61-118515 describes a valve control mechanism suitable for use in a valve train which transmits lift from a cam located on a camshaft of an internal combustion engine to a cylinder head valve of the internal combustion engine. The valve control mechanism has a first abutment member in abutment with the cam and a second abutment member in abutment with the top of the stem of the cylinder head valve. The valve control mechanism comprises a first tappet member slidable in a bore in the engine and a second tappet member movable relative to the first tappet member. A locking pin is slidably located in the second tappet member and can be slid to interengage the first and second members and thereby lock the first and second tappet members to move together. When the first and second tappet members are locked to move together then the valve control mechanism transmits all of the lift of the cam to the cylinder head valve. When the first and second tappet members are allowed to move relative to each other at least a part of the lift of the cam causes relative motion between the first and second tappet members rather than lift of the cylinder head valve whereby the valve control mechanism reduces the amount of lift transmitted from the cam to the cylinder head valve. One of the tappet members is connectable with the camshaft only via the other tappet member. The first tappet member is an outer tappet member which has a bore therein and the second tappet member is an inner tappet member which is slidable in the bore of the outer tappet member. The bore in the outer tappet member has a closed end.

Biasing means acts between the two tappet members to bias them into a position relative to each other in which the locking pin can slide to interengage the tappet members.

In JP-A-61-118515 there is no provision for a stop to limit the downward motion of the inner tappet member relative to the outer tappet member.

The present invention provides a valve control mechanism suitable for use in a valve train which transmits lift from a cam located on a camshaft of an internal combustion engine to a cylinder head valve of the internal combustion engine and which has a first abutment member in abutment with the cam and a second abutment member in abutment with the top of the stem of the cylinder head valve, the valve control mechanism comprising:

a first tappet member slidable in a bore in the engine,

a second tappet member movable relative to the first tappet member, and

locking means capable of locking the first and second tappet members to move together, wherein:

when the locking means locks the first and second tappet members to move together the valve control mechanism transmits all of the lift of the cam to the cylinder head valve;

when the locking means allows the first and second tappet members to move relative to each other at least a part of the lift of the cam causes relative motion between the first and second tappet members rather than lift of the cylinder head valve, whereby the valve control mechanism reduces the amount of lift transmitted from the cam to the cylinder head valve;

one of the tappet members is connectable with the camshaft only via the other tappet member;

the first tappet member is an outer tappet member which has a bore therein and the second tappet member is an inner tappet member which is slidable in the bore of the outer tappet member;

the locking means comprises a locking pin slidable between a first position in which the locking pin interengages the inner and outer tappet members and a second position in which the locking pin disengages the inner and outer tappet members; and

biasing means is provided to act between the inner and outer tappet members to bias the inner and outer tappet members into a position relative to each other in which the locking pin can slide from the second position to the first position to interengage the inner and outer tappet members;

characterised in that:
on the outer tappet member means is provided to limit motion of the inner tappet member relative to the outer tappet member, the biasing means biasing the inner tappet member into engagement with the means limiting motion of the inner tappet member.

The invention provides a valve control mechanism which can be used in an engine without extensive modification of existing components. The mechanism can be easily converted to allow small valve lift rather than total deactivation. The mechanism removes the need for close alignment of two different bores in the engine cylinder head. The mechanism is equally applicable in overhead camshaft and push rod engines.

The tappet member which is connectable with the cam could be in direct engagement with the cam or indirect engagement with the cam (e.g. through a rockable cam follower member). The other tappet member would normally abut the stem of a cylinder head valve in an overhead camshaft engine or a push-rod in a push-rod engine.

In one embodiment the means provided to limit motion of the inner tappet member comprises a snap-ring.

The valve control mechanism can be configured to transmit none of the lift of the cam to the cylinder head valve when the locking means allows the inner and outer tappet members to move relative to each other.

Alternatively the valve control mechanism can be configured to transmit a part of the lift of the cam to the cylinder head valve when the locking means allows the inner and outer tappet members to move relative to each other. For instance, the bore in the outer tappet member could have a closed end and in each rotation of the cam, as the lift of the cam initially increases the inner tappet member can slide along the bore of the outer tappet member and then the inner tappet member can abut the closed end of the bore, whereafter further increasing lift of the cam causes lift of the cylinder head valve.

Preferably the inner tappet member has a surface which in use faces the closed end of the bore and has oil retaining means provided on the surface, the oil retaining means in use retaining a film of oil on the surface which acts to damp relative motion between the inner and outer tappet member as the surface nears abutment with the closed end of the bore.

In a first preferred embodiment of the invention the locking means comprises a locking pin slidable in a bore provided in the inner tappet member and
a slot provided in the outer tappet member, wherein:
the locking pin is slidable between a first position in which the locking pin extends outwardly from the inner tappet member to engage the slot in the outer tappet member and a second position in which the locking pin is out of engagement with the slot in the outer tappet member.

Preferably in the first preferred embodiment the locking means comprises spring means for biasing the locking pin into the second position, and hydraulic fluid supply means operable to supply hydraulic fluid to the valve control mechanism to apply pressure on the locking pin to slide the locking pin from the second position to the first position against the biasing force of the spring means.

Preferably, a hydraulic lash adjuster is provided in a closed bore in the inner tappet member, the hydraulic lash adjuster in use extending to compensate for wear of components of the engine.

Preferably, the valve control mechanism comprises hydraulic control means for controlling the locking means, the hydraulic control means switching the locking means between the first and second operating conditions by controlling the pressure of hydraulic fluid supplied to the locking means.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross-sectional view through a part of the cylinder head of an engine, showing in cross-section a first embodiment of valve control mechanism according to the invention; and

Figure 2 is a cross-sectional view of a part of a cylinder head of an engine, showing in cross-section a valve control mechanism according to a second embodiment of the invention.

In Figure 1 there can be seen a cam shaft 10 on which there is provided a cam 11. The cam shaft 10 will be rotated by a drive system (eg. a cam belt, chain or gear train) at a speed related to the speed of rotation of the crank shaft of the engine in which the cam shaft is present.

The lift of the cam 11 is relayed to a controlled poppet valve via a valve control mechanism 1.

The cam 11 engages a top surface of a cylindrical outer tappet member 12 of the valve control mechanism 1. The outer tappet member 12 is slidable in a bore provided in the cylinder head of an engine. The cylindrical tappet member 12 contains therein an inner tappet member 13 which is slidable relative to the outer tappet member 12 in a closed bore provided in the outer tappet member 12. The inner tappet member 13 is cylindrical in nature and has an outer diameter which corresponds to the diameter of the bore in the outer tappet member 12.

Acting between the outer tappet member 12 and the inner tappet member 13 is a spring 14.

A cylindrical bore is provided in the centre of the cylindrical inner tappet 13. In the cylindrical bore there is provided a hydraulic lash adjuster 15, of conventional construction. The hydraulic lash adjuster 15 acts between the inner tappet member 13 and the top of the valve 17 which is a poppet valve of an internal combustion engine. The poppet valve 17 would typically be an inlet valve, but could also be an exhaust valve.

Attached to the valve 17 is a spring retainer 16. Acting on the spring retainer 16 is a valve spring 18 which biases the valve 17 into abutment with its valve seat (not shown).

A bore is provided diametrically across the inner tappet member 13. In the bore there are located two locking

pins

20 and 21. The locking pins 20 and 21 are respectively inwardly biased by

springs

22 and 23. The spring 22 acts between the locking pin 20 and a shoulder 24 provided in the diametrically extending bore. The spring 23 acts between the locking pin 21 and a shoulder 25 provided in the diametrically extending bore.

A snap-ring 26 is provided extending round the interior surface of the outer tappet 12, the snap-ring 26 limiting the downward motion of the inner tappet member 13 relative to the outer tappet member 12. The spring 14 biases the inner tappet member 13 into engagement with the snap-ring 26.

In use, the valve control mechanism 1 has two operating conditions. In the first operating condition, which is shown in Figure 1, the locking pins 20 and 21 are held inward by the biasing springs 23 and 22. Thus, the locking pins 20 and 21 are held out of engagement with the outer cylindrical member 12 and the outer tappet member 12 can move relative to the inner tappet member 13. Hence, the lift of the cam 11 in the first operating condition is not transmitted to the valve 17, because the lift of the cam 11 is taken up by relative motion between the outer tappet member and the inner tappet member 13. It should be appreciated that the valve spring 18 is stiffer than the spring 14 and therefore the spring 14 compresses to allow relative motion between

tappet members

12 and 13, before the valve spring 18 allows any movement of the valve 17.

In the second operating condition of the tappet assembly 1 the locking pins 20 and 21 are pushed radially outwardly from the inner tappet member 13 to engage

slots

27 and 28 provided in the outer tappet member 12. The locking pins 20 and 21 are pushed radially outwardly by hydraulic pressure acting on their radially innermost surfaces. The hydraulic lash adjuster 15 and the radially innermost surfaces of the locking pins 20 and 21 are both connected to a supply of hydraulic fluid through a passage in the inner member 13 which is not shown. The passage in the inner member 13 will extend out from the plane of the cross-section to an aperture in the exterior surface of the inner tappet member 13. The aperture in the exterior surface of the inner tappet member will align with a slot provided in the outer tappet member 12, which will in turn align with a passage for hydraulic fluid provided in the cylinder head of the engine.

Once the locking pins 20 and 21 have been extended under the influence of hydraulic pressure, they will engage the

slots

27 and 28 in the outer tappet member 12. Thus, the outer tappet member 12 is locked to the inner tappet member 13 and both move together. Consequently, the lift of the cam 11 is transmitted through the outer tappet member 12 and the two locking

pins

20 and 21 to the inner tappet member 13 and thence to the valve 17, so that the valve is given the full lift of the cam 11 and is activated.

The supply of hydraulic fluid to the innermost surfaces of the locking pins 20 and 21 will be controlled by a control system which is not shown in the drawings. The control system will be able to switch the hydraulic pressure from low pressure, which is sufficient only to supply fluid to the hydraulic lash adjuster 15, to a high pressure which is sufficient to overcome the biasing force of the

springs

22 and 23, in order that the locking pins 20 and 21 can be extended radially outwardly. When the hydraulic pressure is switched back from high pressure to low pressure, the biasing springs 22 and 23 will return the locking pins 20 and 21 to a retracted position, so that the outer tappet member 12 can move relative to the inner tappet member 13 (the valve thus being de-activated).

The snap-ring 26 limits the downward motion of the inner tappet member 13 relative to the outer tapper member 12. This then gives the hydraulic lash adjuster 15 a fixed reference to work from. Hydraulic lash adjusters such as the adjuster 15 are well known in the prior art and therefore the hydraulic lash adjuster 15 will not be described in detail in the specification. It is suffice to say that the hydraulic lash adjuster 15 will extend to take up any wear that arises through use.

As shown, the cam mechanism is operable either to fully activate or fully de-activate the valve 17. This arises because the lift of the cam 11 is less than the relative motion that is allowed between the outer tappet 12 and the inner tappet 13 before the top surface of the inner tappet 13 engages the underside of the topmost surface of the outer tappet member 12. However, the valve control mechanism can be modified so that the lift of the cam 11 is greater than the greatest permissible relative motion between the

tappet members

12 and 13. In such a case, the valve will never be completely de-activated. Instead, when the locking pins 20 and 21 are retracted and

tappet members

12 and 13 can move freely relative to each other, the cam 11 will cause motion of the tappet members relative to each other until a point when the top surface of the inner member 13 contacts the underside of the top surface of the tappet member 12. Thereafter lift is transmitted from the cam to the valve 17. Therefore, in the first operating condition of the valve control mechanism the valve 17 receives a small amount of lift of short duration and is not completely de-activated.

Providing a small lift rather than full de-activation can in fact be preferable in a fuel injected engine. In a fuel injected engine, fuel is generally sprayed onto the rear surfaces of the inlet valves. If an inlet valve is fully de-activated then a puddle of fuel can develop whilst the valve is de-activated, leading to undesirable effects when the valve is next opened. With a minimal amount and duration of lift, the build-up of fuel behind a de-activated inlet valve can be eliminated, whilst the advantages of de-activation the valve are maintained.

The valve control mechanism 1 will operate in its first operating condition (with the valve de-activated or with minimal lift) for low speed and low load operation of the engine. The tappet assembly will operate in a second operating condition (with the controlled valve receiving full lift) at high engine speeds and loads. This is achieved by the control system (not shown) which switches the pressure of the hydraulic fluid supplied to the locking pins 20 and 21 from a low to a high pressure (and vice-versa) at certain sensed engine speeds and loads.

The locking pins 20 and 21 are arranged to extend outwardly from the inner member 13 to engage the outer tappet member 12 so as to minimise the reciprocating mass of the valve control mechanism 1 in the first operating condition. This decreases engine losses when the valve control mechanism 1 is in its first operating condition.

A second embodiment of valve control mechanism according to the invention is shown in Figure 2. In Figure 2, there can still be seen a cam shaft 10 with a cam 11 arranged thereon for rotation therewith. The second embodiment of valve control mechanism is denoted by the reference numeral 2 and is in many respects similar to the valve control mechanism 1 already described. The valve control mechanism 2 comprises an outer tappet member 112 which has therein an inner tappet member 113 which is slidable in a closed bore in the outer tappet member 112 relative to the outer tappet member 112. A spring 114 acts between the inner tappet member 113 and the outer tappet member 112 and biases the inner tappet member 113 into engagement with a snap ring 126 provided on the interior of the outer tappet member 112.

The second valve control mechanism 2 does not have a hydraulic lash adjuster and the inner tappet member 113 directly abuts a poppet valve 117 controlled by the tappet assembly. The valve 117 is biased into engagement with its valve seat by the valve spring 118 which acts on a spring retainer 116 attached to the valve 117.

Two locking

pins

120 and 121 are provided in a diametrically extending bore in the inner tappet member 113. The locking pins 120 and 121 are respectively biassed by

springs

122 and 123 into abutment with a spring seat 130 provided at the centre of the inner tappet member 113 for retaining the spring 114. The spring 122 acts between the locking pin 120 and a shoulder 124 provided in the diametrically extending bore. Similarly, the spring 123 acts between the locking pin 121 and the shoulder 125 provided in the diametrically extending bore.

Instead of having a hydraulic lash adjuster, the tappet assembly 2 makes use of a mechanical shim 131 interposed between the outer tappet member 112 and the cam 11. The shim 131 can be replaced by shims of different thicknesses, in order to achieve the correct working clearance and to compensate for wear of components in the engine (e.g. wear of the cams).

The top surface of the inner tappet member 113 is provided with an oil retainer 140 which takes the form of a ridge on the top surface of the inner tappet member 113. The oil retainer 140 retains in use a film of oil on the top of the inner tappet member 113. This is useful when valve control mechanism 2 is configured such that the lift of the cam 11 exceeds the maximum relative motion permitted between the inner tappet member 113 and the outer tappet member 112. As explained previously, when this occurs, the controlled valve 117 is provided with a small amount of lift which is transmitted from the cam 11 to the poppet valve 117 once the upper surface of the inner tappet 113 abuts the underside of the top surface of the outer tappet 112. By providing a film of oil the impact between the upper surface of the inner tappet 113 and the underside of the top surface of tappet 112 is dampened, avoiding excessive noise and wear. Thus it can be seen in Figure 2 that the underside of the outer tappet member 112 is in fact provided with a ridge 132 which runs around the perimeter of the underside surface. As the top surface inner tappet member 113 approaches the underside of the outer tappet member 112, the ridge 132 and the oil retainer ridge 140 cooperate to define an ever-reducing annular gap through which oil is forced. This is very effective in damping the final motion of the inner tappet member 123 into abutment with the outer tappet member 112.

Whilst the second embodiment shown in Figure 2 is configured such that the valve control mechanism in its first operating condition still imparts to the controlled valve 117 a small lift, the tappet assembly could equally well be configured to provide full valve de-activation if desired.

As with the first embodiment, the embodiment shown in Figure 2 has locking pins which extend radially outwardly of the inner tappet so as to reduce the reciprocating mass of the tappet assembly in the valve de-activated condition.

As with the first embodiment, the second embodiment will have oil passages which enable supply of hydraulic pressure to the radially innermost surfaces of the locking pins 120, 121, so that the locking pins 120 and 121 can be extended under the application of hydraulic pressure. In fact, in the embodiment shown the chamber 133 located below the spring seat 130 will be connected to an oil passage which extends through the inner tappet 113 to open onto the exterior surface of the inner tappet 113 at an aperture which will align with a longitudinally extending slot provided in the outer tappet 112. The slot in the outer tappet 112 will in turn align with an opening 4 of an oil passage provided in the cylinder head. A control mechanism, not shown in the drawings, will be provided for switching the hydraulic pressure supplied between a low pressure at low engine speeds and loads and the high pressure at high engine speeds and loads.

When the hydraulic pressure is switched from low to high, the

pins

120, 121 are forced radially outwardly to engage

slots

127 and 128 provided in the outer tappet 112 so that both the inner tappet 113 and the outer tappet 112 move together and the lift of cam 11 is transmitted to the valve 117. When the oil pressure is switched back from high to low, the

springs

122 and 123 return the locking pins 120 and 121 to a retracted position in which the outer tappet 112 is free to move relative to the inner tappet 113 and the valve 117 is de-activated (or subject only to a small lift).

In the embodiments of Figures 1 and 2 the

inner tappets

13 and 113 remain disconnected from the

outer tappets

12 and 112 when oil pressure is low, due to the biasing force of

springs

22, 24 and 122, 124.

The valve control mechanisms of the invention are simple and compact in nature and do not require substantial modification of the cylinder head of an engine. In fact, it is envisaged that the hydraulic lash adjusters present in conventional engines could simply be replaced by valve control mechanisms according to the present invention, to give existing engines the possibility of valve de-activation. A large number of engines already have hydraulic lash adjusters and oil passages in the cylinder head supplying the hydraulic lash adjusters. It would be a simple matter to replace the hydraulic lash adjusters with tappet assemblies according to the invention and to then provide the engine with means for switching the pressure in the existing oil passages between a high and a low pressure. The valve control mechanism of the invention does not require any machining of the stems of the valves used in the engine and does not require special machining of passages in the cylinder head.

Whilst the illustrated embodiments of valve control mechanism are shown in use in an overhead cam engine, the embodiments could be used in push-rod engines. In such a case the inner tappet would engage a push-rod rather than a valve stem.

Whilst in the illustrated embodiments the outer tappet engages a cam and the inner tappet engages a valve, the mechanism could be used inverted. Also it is not necessary for the tappet members to directly abut a cam and a valve, but instead the tappet members could be part of a larger mechanism for relaying lift from a cam to a valve.

In the illustrated embodiments the hydraulic supply to the inner tappet member is achieved through aligned bores in the inner and outer tappet members and a bore in the cylinder head which aligns with the bore in the outer tappet member. However, to keep the alignment throughout operation it may be necessary to provide means to prevent the tappet members rotating relative to one another and relative to the cylinder head. The inner and outer tappets can be held in a fixed rotational alignment by insertion of a first pin in matched axially extending grooves on the inner surface of the outer tappet member and the outer surface of the inner tappet member. The outer tappet can be held itself on a fixed rotational position by insertion of a second pin in matched axially extending grooves on the outer surface of the outer tappet member and the inward surface of the cylinder head bore. Alternatively an oil gallery could be provided around the circumference of the outward surface of the outer tappet member or an inward surface of the bore in the cylinder head, to remove the need for fixing the rotational position of the outer tappet member. The gallery would allow hydraulic fluid to be supplied no matter what the relative rotational position of the outer tappet member, as well as having the advantage of providing a lubricating film between abutting surfaces. Whatever configuration is used it must be ensured that the bores and/or galleries are always covered throughout the maximum possible range of relative axial displacements (e.g. the bore in the inner tappet member is always covered by the outer tappet member); otherwise an air lock might develop.

Claims

A valve control mechanism (1;2) suitable for use in a valve train which transmits lift from a cam (11) located on a camshaft (10) of an internal combustion engine to a cylinder head valve (17; 117) of the internal combustion engine and which has a first abutment member (12; 131) in abutment with the cam (11) and a second abutment member (13; 113) in abutment with the top of the stem of the cylinder head valve (17; 117) the valve control mechanism comprising:

a first tappet member (12; 112) slidable in a bore in the engine,

a second tappet member (13; 113) movable relative to the first tappet member (12; 112) and

locking means (20,21,22,23,24,25,27,28; 120,121,122,123,124,125,127,128) capable of locking the first (12; 112) and second (13; 113) tappet members to move together, wherein:

when the locking means (20,21,22,23,24,25,27,28; 120,121,122,123,124,125,127,128) locks the first (12; 112) and second (13; 113) tappet members to move together the valve control mechanism (1; 2) transmits all of the lift of the cam (11) to the cylinder head valve (17; 117);

when the locking means (20,21,22,23,24,25,27,28; 120,121,122,123,124,125,127,128) allows the first (12; 112) and second (13; 113) tappet members to move relative to each other at least a part of the lift of the cam (11) causes relative motion between the first (12; 112) and second (13; 113) tappet members rather than lift of the cylinder head valve (17;117) whereby the valve control mechanism (1;2) reduces the amount of lift transmitted from the cam (11) to the cylinder head valve (17; 117);

one of the tappet members (13; 113) is connectable with the camshaft (10) only via the other tappet member (12; 112);

the first tappet member (12; 112) is an outer tappet member (12; 112) which has a bore therein and the second tappet member (13; 113) is an inner tappet member (13; 113) which is slidable in the bore of the outer tappet member (12; 112);

the locking means (20,21,22,23,24,25,27,28; 120,121,122,123,124,125,127,128) comprises a locking pin (20, 21; 120, 121) slidable between a first position in which the locking pin (20, 21; 120,121) interengages the inner (13; 113) and outer (12; 112) tappet members and a second position in which the locking pin (20, 21: 120,121) disengages the inner (13; 113) and outer (12; 112) tappet members; and

biasing means (14; 114) is provided to act between the inner (13; 113) and outer (12; 112) tappet members to bias the inner (13; 113) and outer (12; 112) tappet members into a position relative to each other in which the locking pin (20, 21; 120, 121) can slide from the second position to the first position to interengage the inner (13; 113) and outer (12;112) tappet members; characterised in that

on the outer tappet member (12; 112) means (26; 126) is provided to limit motion of the inner tappet member (13; 113) relative to the outer tappet member (12; 112), the biasing means (14;114) biasing the inner tappet member (13; 113) into engagement with the means (26; 126) limiting motion of the inner tappet member (13;113).
A valve control mechanism as claimed in claim 1 wherein the means (26; 126) provided to limit motion of the inner tappet member (13; 113) comprises a snap-ring (26;126).
A valve control mechanism as claimed in claim 1 or claim 2 wherein the valve control mechanism (1; 2) transmits none of the lift of the cam (11) to the cylinder head valve (17; 117) when the locking means (20,21,22,23,24,25,27,28;120,121,122,123,124,125,127, 128) allows the inner (13; 113) and outer (12; 112) tappet members to move relative to each other.
A valve control mechanism as claimed in claim 1 or claim 2 wherein the valve control mechanism (1; 2) transmits a part of the lift of the cam (11) to the cylinder head valve (17'; 117) when the locking means (20,21,22,23,24,25,27,28;120,121,122,123,124,125,127, 128) allows the inner (13; 113) and outer (12; 112) tappet members to move relative to each other.
A valve control mechanism as claimed in claim 4 wherein the bore in the outer tapet member (12; 112) has a closed end and, in each rotation of the cam (11), as the lift of the cam (11) initially increases the inner tappet member (13; 113) slides along the bore of the outer tappet member (12; 112) until the inner tappet member (13; 113) abuts the closed end of the bore, whereafter further increasing lift of the cam (11) causes lift of the cylinder head valve (17; 117).
A valve control mechanism as claimed in claim 5 wherein the inner tappet member (13; 113) has a surface which in use faces the closed end of the bore and has oil retaining means (140) provided on the surface, the oil retaining means (140) in use retaining a film of oil on the surface which acts to damp relative motion between the inner (13; 113) and outer (12; 112) tappet members as the surface nears abutment with the closed end of the bore.
A valve control mechanism as claimed in any one of the preceding claims wherein:

the locking pin (20, 21; 120,121) is slidable in a bore provided in the inner tappet member (13; 113);

a slot (27,28; 127,128) is provided in the outer tappet member (12; 112); and

the locking pin (20,21; 120,121) in the first position extends outwardly from the inner tappet member (13; 113) to engage the slot (27,28; 127,128) in the outer tappet member (12; 112) and the locking pin (20,21; 120,121) in the second position is out of engagement with the slot (27,28; 127,128) in the outer tappet member (12; 112).
A valve control mechanism as claimed in claim 7 wherein the locking means (20,21,22,23,24,25,27,28; 120,121,122,123,124,125,127,128) comprises spring means (22,23;122,123) for biasing the locking pin (20,21; 120,121) into the second position, and hydraulic fluid supply means operable to supply hydraulic fluid to the valve control mechanism (1; 2) to apply pressure on the locking pin (20, 21; 120,121) to slide the locking pin (20,21; 120,121) from the second position to the first position against the biasing force of the spring means (22,23;122,123).
A valve control mechanism as claimed in any one of the preceding claims wherein a hydraulic lash adjuster (15) is provided in a closed bore in the inner tappet member (13), the hydraulic lash adjuster (15) in use extending to compensate for wear of components of the engine.
A valve control mechanism as claimed in any one of the preceding claims comprising hydraulic control means for controlling the locking means (20,21,22,23,24,25,27,28;120,121,122,123,124,125,127, 128),the hydraulic control means switching the locking means (20,21,22,23,24,25,27,28;120,121,122,123, 124,125,127,128) between the first and second operating conditions by controlling the pressure of hydraulic fluid supplied to the locking means (20,21,22,23,24,25,27,28;120,121,122,123,124,125,127, 128).