GB2238348A

GB2238348A - Endless loop i.c. engine valve timing varying mechanism

Info

Publication number: GB2238348A
Application number: GB8925855A
Authority: GB
Inventors: Michael James Smith
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1989-11-15
Filing date: 1989-11-15
Publication date: 1991-05-29
Anticipated expiration: 2009-11-15
Also published as: GB8925855D0; GB2238348B

Abstract

Two parallel shafts, eg camshafts, are rotatably connected by an endless drive loop 5, eg a chain or toothed belt, entrained about pulleys 3, 4 on the shafts and the varying mechanism includes pairs of pulleys 6, 7 and 4, 12, The pulleys 6, 7 are adjacent one another on a support 8 which is mounted for controlled pivotal movement about the axis of one shaft, each pulley being equi-spaced from the shaft axis and rotatable about its own axis and each of the pulleys 4, 12 is rotatable about its own axis and equi-spaced from the shaft axis and positioned adjacent a pulley 6 or 7 such that each pulley 6 or 7 is interposed between a pulley 12 or 7 and the one shaft. The loop (5) is entrained such that the pulleys 6, 7 guide it outside an area enclosed thereby and the pulleys 4, 12 guide it within that area. The two pairs of pulleys (6, 7 and 18, 19, Figs. 2 and 3) may be mounted on the support 8. <IMAGE>

Description

A MECHANISM TO VARY THE VALVE TIMING IN AN INTERNAL COMBUSTION ENGINE The present invention relates to a mechanism to vary valve timing in an internal combustion engine.

In order to obtain improved combustion efficiency for an internal combustion engine throughout its speed range it has been recognised as desirable to be able to vary the valve timing according to the speed at which the engine is running.

It is known from US Patent No. 3,441,009, Rafanelli, in which inlet and exhaust camshafts of an engine are driven by an endless drive loop in the form of a chain entrained about pulleys on the camshafts and about a drive pulley on a driving shaft, to provide a mechanism to vary the valve timing. This mechanism comprises first and second pairs of pulleys, the first pair of pulleys being adjacent one another on a support which is mounted for controlled pivotal movement about the axis of one shaft, each first pulley being equi-spaced from said shaft axis and rotatable about its own axis, each of the second pair of pulleys being rotatably mounted on a fixed axis and positioned adjacent a first pulley such that each first pulley is inte-rposed between a second pulley and the one shaft, a drive chain being trained such that the first pair of pulleys guides the chain outside an area enclosed by the chain and the second pair of pulleys guides the chain within said area, wherein pivotal movement of the support about the said shaft axis acts to alter the phase relationship between the shafts.

In this way the lengths of the "runs" between the shafts can be varied in order to vary the valve timing.

A similar arrangement is disclosed in US Patent No. 4,525,151 Toyota, however the first pulleys1 mounted on the support, are not equally spaced from the said shaft axis.

A difficulty arises with the above prior art disclosures that pivotal movement of the support has the effect of altering the tension in the drive loop which has an unacceptable effect upon certainty of valve timing and upon wear of the drive loop and pulleys. Additional tensioning means can also be required.

It is an object of the present invention to provide an improved mechanism to vary valve timing in an internal combustion engine which will substantially maintain drive loop tension during variations of the valve timing.

According to the present invention there is provided a mechanism to vary the valve timing in an internal combustion engine, the mechanism being for operation between two parallel shafts rotatably connected by an endless drive loop entrained about pulleys on the shafts and including first and second pairs of pulleys, the first pair of pulleys being adjacent one another on a support which is mounted for controlled pivotal movement about the axis of one shaft, each first pulley being equi-spaced from said shaft axis and rotatable about its own axis, each of the second pair of pulleys being rotatably mounted on a fixed axis and being equi-spaced from the said shaft axis and positioned adjacent a first pulley such that each first pulley is interposed between a second pulley and the one shaft, the loop being trained such that the first pair of pulleys guides the loop outside an area enclosed by the loop and the second pair of pulleys guides the loop within said area, wherein pivotal movement of the support about the said shaft axis acts to alter the phase relationship between the shafts.

The equi-spacing of the first and second pairs of pulleys respectively from the said shaft axis has been found to greatly to improve consistency of tension in the drive loop during variations in valve timing caused by pivotal movement of the support and hence result in improved accuracy of valve timing and improved wear characteristics in the drive loop.

Preferably the second pair of pulleys is positioned outside the range of controlled pivotal movement of the support further to stabilize tension in the drive loop during pivotal movement of the support.

Advantageously the range of pivotal movement of the support is symmetrical about a plane passing through the said shaft axis and bisecting the angle formed between the second pair of pulleys about the shaft axis.

Owing to the symmetry of the arrangement of the first and second pairs of pulleys, a symetrical range of pivotal movement will guarantee that if the mechanism is used with the support rotated to the two extremeties of movement substantially identical drive loop tensions will result in both positions.

Further to assist in stabilising drive loop tension during operation of the mechanism the inner radius of the second pair of pulleys about the said shaft axis can be arranged substantially to equal the outer radius of the first pair of pulleys thereabout. Detailed investigation (as set out in table 1) has revealed that tension variation for pivotal movement of the support to different positions is least for this arrangement of pulleys.

The support may be controlled for said pivotal movement by means rotatable only in one direction. Errors of backlash in the pivotal control means are therefore eliminated.

Advantageously the pivotal control means includes a circular cam rotatable in conjunction with a double-acting follower thereby to provide reciprocal pivotal movement.

The one shaft may be a first camshaft and one of the second pair of pulleys may be mounted on a second camshaft. A variable phase difference between the two camshafts is thus provided by the mechanism. In a preferred arrangement the one shaft is an inlet camshaft and the other shaft is an exhaust camshaft. Variation of inlet valve timing in relation to the exhaust valve timing which remains constant in relation to the crankshaft is thus provided.

The invention also provides an internal combustion engine including a mechanism in which the one shaft is an inlet camshaft and the other shaft is an exhaust camshaft.

Drive from a crankshaft of the engine to the camshafts may be via an end of the exhaust camshaft remote from the mechanism and the camshaft axes may be across a motor vehicle in which the engine is installed with the inlet camshaft nearer the front of the vehicle. With this arrangement the use of a relatively large drive pulley on the camshaft nearer the front of the vehicle can be avoided thus allowing a lowered bonnet line.

In an alternative arrangement to enable the camshaft pulleys to be kept as small as possible, drive to the camshafts may be taken from a double diameter idler pulley around a smaller pulley of which the drive loop for the camshafts is entrained and a larger pulley of which a further drive loop from the engine crankshaft is entrained.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawing of which Figure 1 is an end elevational view of an internal combustion engine incorporating a mechanism according to the invention, Figure 2 is a perspective schematic view of an alternative camshaft drive arrangement embodying the mechanism of the present invention, and Figure 3 shows schematically a preferred pulley arrangement.

Referring to Figure 1, an internal combustion engine 1 has a crankshaft pulley 2 to drive twin overhead camshafts, the inlet camshaft of which is driven by an inlet camshaft pulley 3 and the exhaust camshaft of which is driven by an exhaust camshaft pulley 4 by means of a drive loop in the form of a toothed belt 5 entrained around the pulleys. A first pair of idler pulleys 6,7 are mounted equi-spaced from the inlet camshaft axis for rotation about their own axes on a support 8. The support 8 is mounted for controlled pivotal movement about the inlet camshaft axis. The pivotal control of the support is provided by an electric motor 9 having a circular eccentrically mounted cam 10 rotatable thereon within a double acting cam follower in the form of a slot 11.A second pair of pulleys take the form of the exhaust camshaft pulley 4 and a large idler pulley 12, each of the latter being mounted on a fixed axis equi-spaced from the inlet camshaft axis.

It will be seen that the second pair of pulleys 12,4 are positioned outside the range of controlled pivotal movement of the support 8.

The electric motor 9 and cam 10 control the pivotal movement of the support 8 by rotation of the cam 10 in only one direction.

It will be appreciated that such unidirectional rotation of the cam 10 will provide reciprocal pivotal movement of the support 8 and backlash errors are thereby avoided.

When the support 8 is rotated anticlockwise about the inlet camshaft axis the inlet camshaft pulley 3 will be similarly rotated anticlockwise relative to the exhaust camshaft pulley 4 and to the crankshaft pulley 2. Assuming clockwise rotation of the crankshaft pulley 2 as viewed, such anticlockwise pivotal movement of the support 8 will have the effect of retarding the inlet valve timing. Conversely, clockwise rotation of the support 8 will result in advancement of the inlet valve timing.

With two to one gearing between the camshaft pulleys and the crankshaft pulley respectively a rotation of X0 of the support about the inlet camshaft axis will result in a rotation of 2X of the crankshaft pulley, assuming the inlet camshaft pulley 3 remains stationary. Thus it will be seen that a very small pivotal movement of the support will result in a significant valve timing phase change between inlet and exhaust valves.

Referring to Figure 2, items with functions corresponding to those illustrated in Figure 1 are shown with the same reference numerals.

An overhead inlet camshaft 13 and exhaust camshaft 14 operate in a transversely mounted engine (not shown), the exhaust camshaft 14 being driven via an exhaust camshaft drive pulley 15 which is connected via a drive belt 16 to a crankshaft pulley 2. The exhaust camshaft drive pulley is twice the size of the crankshaft pulley to provide the appropriate reduction gearing to drive the camshafts at half engine speed.

Interposed between the exhaust camshaft and the inlet camshaft is a mechanism to vary the valve timing according to the present invention. The exhaust camshaft 14 transmits drive to the inlet camshaft 13 by means of an exhaust camshaft pulley 4 transmitting drive through a transfer drive belt 17 to an inlet camshaft pulley 3. It will be noted that the inlet and exhaust camshaft pulleys 3,4 are of considerably lesser diameter than the exhaust camshaft drive pulley, the latter acting to provide the necessary reduction gearing for camshaft drive and the inlet and exhaust camshaft pulleys 3,4 merely acting to transfer drive from one camshaft to the other. Accordingly, the camshaft pulley 3 at the front of the engine can be made much smaller than is normal for known twin overhead camshaft arrangements and the vehicle bonnet line can consequently be lowered.

The mechanism to vary the valve timing between the inlet and exhaust camshafts 3,4 comprises a support 8 pivoted about the axis of the inlet camshaft 3 which supports a first pair of idler pulleys 6,7 thereon and a second pair of idler pulleys 18,19, each rotating on its own fixed axis. Valve timing variation between the inlet and the exhaust camshafts is thus achieved neatly and simply with the added advantage that the height of the front part of the engine can be reduced owing to the arrangement allowing the use of a smaller than usual forward camshaft pulley in the form of the inlet pulley 3.

Figure 3 shows an optimum geometry for the arrangement of the first and second pairs of pulleys 6,7 and 18,19 in relation to a pulley 20 on one shaft (not shown).

In this arrangement the inner radius of the second pair of pulleys 18,19 about the axis of the one shaft is substantially equal to the outer radius of the first pair of pulleys 6,7 about this axis.

Claims

1. A mechanism to vary the valve timing in an internal combustion engine, the mechanism being for operation between two parallel shafts rotatably connected by an endless drive loop entrained about pulleys on the shafts and including first and second pairs of pulleys, the first pair of pulleys being adjacent one another on a support which is mounted for controlled pivotal movement about the axis of one shaft, each first pulley being equi-spaced from said shaft axis and rotatable about its own axis, each of the second pair of pulleys being rotatably mounted on a fixed axis and being equi-spaced from the said shaft axis and positioned adjacent a first pulley such that each first pulley is interposed between a second pulley and the one shaft, the loop being trained such that the first pair of pulleys guides the loop outside an area enclosed by the loop and the second pair of pulleys guides the loop within said area, wherein pivotal movement of the support about the said shaft axis acts to alter the phase relationship between the shafts.

2. A mechanism as in claim 1, wherein the second pair of pulleys is positioned outside the range of controlled pivotal movement of the support.

3. A mechanism as in claim 1 or 2, wherein the said range of pivotal movement of the support is symmetrical about a plane passing through the said shaft axis and bisecting the angle formed between the second pair of pulleys about the shaft axis.

4. A mechanism as in claim 1, 2 or 3, wherein the inner radius of the second pair of pulleys about the said shaft axis substantially equals the outer radius of the first pair of pulleys thereabout.

5. A mechanism as in any of the preceding claims, wherein the support is controlled for said pivotal movement by means rotatable only in one direction.

6. A mechanism as in claim 5, wherein said pivotal control means includes a circular cam rotatable in conjunction with a double-acting follower thereby to provide reciprocal pivotal movement.

7. A mechanism as in any of the preceding claims, wherein the one shaft is a first camshaft and one of the second pair of pulleys is mounted on a second camshaft.

8. A mechanism as in any of claims 1 to 6, wherein the one shaft is an inlet camshaft and the other shaft is an exhaust camshaft.

9. An internal combustion engine including a mechanism as in claim 8.

10. An internal combustion engine as in claim 9, wherein drive from a crankshaft of the engine to the camshafts is via an end of the exhaust camshaft remote from the mechanism.

11. A motor vehicle having an internal combustion engine as in claim 10 having its camshaft axes across the vehicle with the inlet camshaft nearer the front of the vehicle.

12. A mechanism to vary the valve timing in an internal combustion engine substantially as described herein with reference to the accompanying drawing.