GB2268242A

GB2268242A - Engine valve train

Info

Publication number: GB2268242A
Application number: GB9213372A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1992-06-24
Filing date: 1992-06-24
Publication date: 1994-01-05
Also published as: GB9213372D0

Abstract

An engine valve train includes a cam follower 16 having a surface for following a valve actuating cam 12, a tappet for contacting the stem 22 of a valve to transmit movement of the cam follower to the valve, and a hydraulic lash adjuster 20 for dynamically varying the total effective length of the valve train. To reduce the friction on the camshaft 10, a stationary abutment 24 is provided for limiting the movement of the cam follower 16 and maintaining a predetermined minimum clearance between the cam follower 16 and the base circle 14 of the cam 12. <IMAGE>

Description

Title ENGINE VALVE TRAIN Field of the invention The present invention relates to an engine valve train with hydraulic lash adjusters.

The valve train, that is to say all the mechanical elements which follow the movements of the cams on the camshafts and transmit the movement to the engine valves, need to be designed in a manner to be able to allow for differential expansion between the valves and the cylinder head. For this reason, enough free play or lost motion must be left in the valve train to allow for the maximum possible variation.

If set too widely, the tappet clearance can give rise to unacceptable noise from the tappets, especially when the engine is cold and if insufficient clearance is allowed, the valve cannot shut completely under certain conditions.

Setting of this critical clearance is made more onerous by the fact that the clearance is affected by wear in the valve seat and the tappet clearance must be therefore be reset at regular maintenance intervals.

To counteract this variable play in the valve train, it is known to place a hydraulic lash adjuster in the valve train, usually between the valve and the cam follower. A hydraulic lash adjuster is a variable volume chamber which is filled with oil supplied under pressure through a non-return valve from the engine oil pump and which has a small leakage path to allow the oil to escape slowly. The lash adjuster automatically expands under the applied oil pressure and the action of an internal spring to take up all the free play when the cam follower is on the base circle of the cam but remains stiff when the cam follower rides on the cam lobe because the oil within it cannot escape very rapidly.

The disadvantage of using hydraulic lash adjusters is that the oil pressure and spring within it urge the cam follower against the cam at all times and the friction experienced by the cam is therefore increased. In a four cylinder four stroke engine, while one intake valve is open all the other three are closed but their cam followers are still in frictional contact with the base circles of their respective cams. Though the contribution to valve train friction from each of these cam followers may not be great, their cumulative effect is enough to make a noticeable difference.

It can be seen therefore that in the prior art, an engine could either include hydraulic lash adjusters and therefore suffer a loss in efficiency or it could have preset tappet clearances which needed frequent readjustment to maintain the valve train play within acceptable limits for noise and performance.

Object of the invention The present invention seeks to provide an engine which benefits from having hydraulic lash adjusters but has reduced valve train friction.

Summary of the invention According to the present invention, there is provided an engine valve train which includes a cam follower having a surface for following a valve actuating cam, a tappet for contacting the stem of a valve to transmit movement of the cam follower to the valve, and a hydraulic lash adjuster for dynamically varying the total effective length of the valve train, characterised by a stationary abutment for limiting the movement of the cam follower and maintaining a predetermined minimum clearance between the cam follower and the base circle of the cam.

The effective length of the valve train in a conventional system is determined on the one hand by the lash adjuster which acts to extend the effective length and the cam itself which limits the movement of the cam follower. This inevitably leads to increased valve train friction. In the present invention, the limit on the expansion of the valve train under the action of the hydraulic tappet is set not by the cam but by a stationary abutment surface which just prevents the follower from touching the base circle of the cam. If the abutment surface is substantially level with the base circle of the cam, the relative velocity of the cam follower when it comes to rest against the abutment will be nearly zero. The abutment will not therefore be prone to wear and no noise will be generated by the impact.As the base circle of the cam will itself no longer be contacted by the cam follower, it too will not be subject to wear. For all these reasons, once the position of the abutment has been set, it need not be adjusted for the working life of the engine.

The abutment may be used in conjunction with any form of cam follower, be it of the rocker arm type, finger follower type or a direct acting bucket type.

The abutment may be set by means of a screw adjustment or shims may be fitted to position the abutment surface in the precise location in relation to the valve train to stop the cam follower at the desired distance from the base circle of the cam.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing which is a section through a direct acting bucket type cam follower positioned between a cam and a valve.

Detailed description of the preferred embodiment In the single drawing, a camshaft 10 has a cam 12, the base circle portion of which is designated 14, being the portion opposite the cam lobe. A cam follower 16 urged upwards as viewed by a valve stem 22 (the latter being in turn urged upwards by a conventional valve spring which is not shown) and is guided in a bore formed in the cylinder head 18 of the engine. A conventional hydraulic lash adjuster 20, which is built into the bucket 16, takes up whatever gap there is between the bucket 16 and the valve stem 22. The hydraulic lash adjuster 20 is typically a hydraulic jack supplied with oil from the engine oil pump through a non-return valve and having a slow leakage path for the oil.The details of the lash adjuster are not shown in the drawing as they are well known in themselves and the construction of the lash adjuster is not itself material to an understanding of the invention.

It suffices for the present purposes to appreciate that the lash adjuster tries at all times to expand under the action of the oil pressure supplied to it and also an internal spring. The force is not very large and is certainly not sufficient to compress the valve spring but it does urge the ends of the jack apart until such time as they meet enough resistance to overcome the force of the jack.

In a conventional engine, when the valve is closed against its seat in the cylinder head, the cam follower 16 continues to move until such time as it rests on the base circle 14 of the cam 12. This causes the drag on the cam 12 which the present invention sets out to reduce.

To achieve this objective, the illustrated preferred embodiment of the invention provides an abutment 24. This is formed as a grub screw mounted in a suitable bracket 26 which is secured by a bolt 28 to the cylinder head 18. The abutment surface is adjusted by turning the grub screw 24 and locking it in a position in which the cam follower 16 is stopped short of the base circle 14 of the cam 12 and just fails to make contact with it. The cam 12 can therefore continue to rotate without extra drag from the cam followers but the lash adjuster 20 can also continue to operate in its normal manner against the abutment 24 so that there is no necessity to readjust a tappet clearance at regular service intervals.

The position of the grub screw 24 does not require frequent adjustment as it only defines a fixed position between components which are not prone to wear or differential expansion.

It will be appreciated that other forms of cam follower can be used and in the case, for example, of a rocker arm or finger follower, the lash adjuster can be provided anywhere along the valve train at which it can take up free play and the abutment can be located at any convenient location at which it will limit the movement of the cam follower before it starts riding on the base circle of the cam.

It will also be appreciated that the grub screw 24 is only one way of establishing a presettable abutment and other means, for example using shims, may alternatively be employed. In all cases, the invention offers the advantage that once the position of the abutment has been set, it should not require resetting during the normal lifetime of the engine.

Claims

1. An engine valve train which includes a cam follower having a surface for following a valve actuating cam, a tappet for contacting the stem of a valve to transmit movement of the cam follower to the valve, and a hydraulic lash adjuster for dynamically varying the total effective length of the valve train, characterised by a stationary abutment for limiting the movement of the cam follower and maintaining a predetermined minimum clearance between the cam follower and the base circle of the cam.

2. An engine valve train as claimed in claim 1, the position of the abutment is set by means of a screw adjustment.

3. An engine valve train as claimed in claim 2, wherein the adjustment screw is mounted in a bracket which in use is fixed in relation to the engine cylinder head.

4. An engine valve train as claimed in any preceding claim, wherein the cam follower is of the direct acting bucket type.

5. An engine valve train constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

6. An internal combustion engine having a valve train as claimed in any preceding claim.