GB2268570A

GB2268570A - I.c. engine camshaft drive mechanism

Info

Publication number: GB2268570A
Application number: GB9319852A
Authority: GB
Inventors: Peter Harry Parker
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1989-10-13
Filing date: 1993-09-24
Publication date: 1994-01-12
Anticipated expiration: 2013-09-24
Also published as: GB9319852D0; GB2268570B

Abstract

An input member 41, which drives a member 36, is rotatable about substantially the same axis as a member 43 on the camshaft 18. Drive is transmitted through a peg 39 on the member 41 and a peg 42 on the member 43, the member 36 having a pair of diametrically opposed radial grooves 37, 38 each engaged with one of the pegs through a drive block 44. The member 36 is journalled in an eccentric sleeve 51 rotatable by a pinion 66 in a bore 49 having its axis offset from the axis of the camshaft 18 by an amount equal to the sleeve eccentricity. Sleeve rotation adjusts the timing of camshaft operated valves. <IMAGE>

Description

AN INTERNAL COMBUSTION ENGINE CAMSHAFT DRIVE MECHANISM The invention relates to internal combusion engines and in particular to the camshaft drive mechanisms of such engines.

Camshaft drive mechanisms which incorporate variable valve timing (VVT) are known. The benefits in terms of engine performance are discussed in SAE Technical Paper Series 880386 entitled A Review of Variable Engine Valve Timing" by C Gray which also discusses various variable valve timing mechanisms.

One type of VVT mechanism which gives a very large scope for improving engine performance varies the timing of both the opening and the closing of the valves by cyclic variation of the rotational speed of the cams during otherwise constant rotational speed of the engine crankshaft. GB-A-1311562 shows a VVT mechanism of this type.

The notionally constant rotational speed of an input member produces a cyclic variation in the rotational speed of the camshaft by moving the axis of rotation of a driving member which transmits drive from the input member to the camshaft. One of the two methods of moving the driving member shown includes an eccentric sleeve rotatable in a bore in the engine block.

An object of the invention is to provide an improved VVT mechanism with an eccentric sleeve and which overcomes certain disadvantages of the known VVT mechanism.

According to the invention there is provided a camshaft drive mechanism for an internal combustion engine having an engine block which defines at least one cylinder, inlet and exhaust valves for the cylinder a 20 ghRt, the Q2mshJft drive mechanism comprising # camshaft for operating one of said valves and a variable valve timing (VVT) mechanism which in use drivingly connects the crankshaft to the camshaft and is operative to provide cyclic variation of the rotational speed of the respective camshaft during otherwise constant rotational speed of the engine crankshaft, the VVT mechanism comprising a rotatable driving member for driving connection to the crankshaft, an output member arranged to drive the camshaft and an eccentric sleeve providing a journal for the driving member and being in use rotatable in a bore in the engine block, the driving member having an axis of rotation which is movable relative to the axis of rotation of the output member to vary the valve timing by rotation of the eccentric sleeve, wherein the axis of rotation of the eccentric sleeve is offset from the axis of rotation of the output member.

Preferably the offset is substantially equal to the eccentricity of the eccentric sleeve, in which case the axis of rotation of the driving member may substantially coincide with the axis of rotation of the output member at one operational position of the eccentric sleeve.

In a preferred arrangement, an input member is rotatable about substantially the same axis as the output member and is arranged to be driven by the crankshaft and to transmit drive to the driving member. For example, a peg on the input member and a peg on the output member may be provided such that each is in driving engagement with a respective one of a pair of diametrically opposed radially extending grooves in the driving member.

Conveniently the output member is on the camshaft.

For use in an engine having an engine block which defines at least a second cylinder arranged in line with said one, first, cylinder, a first group of valves comprising the inlet valves for each cylinder and a second group of valves comprising the exhaust valves for each cylinder, a camshaft drive mechanism according to the invention preferably comprises first and second camshafts which in use extend parallel to the crankshaft for operating one of said groups of valves for the first and second cylinders respectively, the second camshaft having an elongate portion extending coaxially through the first camshaft, a first VVT mechanism arranged to transmit drive from the crankshaft to the first camshaft and a second VVT mechanism arranged to transmit drive from the crankshaft to the second camshaft through the elongate portion.Conveniently the eccentric sleeve is common to the first and second VVT mechanisms.

Where at least two cylinders are provided, each VVT mechanism may comprise a respective one of a pair of pegs on the input member and a peg on the respective output member, each driving member defining a pair of diametrically opposed radial grooves each for driving engagement with a respective one of the pegs.

The output member of the second VVT mechanism may extend through the driving member of the first VVT mechanism. Similarly, the input member of the first VVT mechanism may extend through an aperture in the driving member of the second VVT mechanism.

Conveniently the output member of each WT dhaism iecha! on the respective camshaft in which case the driving member of the second VVT mechanism may comprise the elongate portion of the second camshaft.

The eccentric sleeve may be drivingly connected to a servomotor, conveniently through a non-reversible worm and worm wheel gear drive. The or each eccentric sleeve may have gear teeth to mesh with a control shaft which has a worm wheel meshing with a worm drivingly connected to the servomotor.

The invention will now be described by way of example and with reference to the accompanying drawings, of which : Fig 1 is a diagrammatic perspective view of one embodiment of an internal combustion engine according to the invention and incorporating a camshaft drive mechanism; Fig 2A and 2B are respectively right and left hand portions of a plan view of the cylinder head of the engine shown in Fig 1 showing parts of the camshaft drive mechanism in more detail; Fig 3 is an enlarged view of part of the camshaft drive mechanism shown in Fig 2; Fig 4 is a view similar to Fig 3 showing the camshaft drive mechanism rotated through 90 degrees; Fig 5 is a perspective view of one of the components shown in Figs 3 and 4; Fig 6 is an elevation of another of the components shown in Figs 3 and 4;; Fig 7 is an elevation of part of the camshaft drive mechanism shown in Fig 2A; Fig 8 is a diagram showing on an enlarged scale the geometry of a VVT mechanism incorporated in the camshaft drive mechanism shown in Figs 1 to 7; Fig 9 is a diagram showing part of Fig 8 on a further enlarged scale and with further detail; and Fig 10 is a graph illustrating the variation in valve timing obtained using the VVT mechanism incorporated in Figs 1 to 7.

Referring to Figs 1 to 4 and in particular to Fig 1, an internal combustion engine 11 includes an engine block 12 comprising a crankcase 13 integral with a cylinder block having a bank of four cylinders in line and a cylinder head 14. A crankshaft 15 is journalled in the crankcase and has a drive pulley 16 for a toothed belt 17.

The crankshaft 15 carries a flywheel (not shown) at its end remote from the drive pulley 16 for transmitting the engine output, eg through a clutch to a gearbox. In the conventional manner the cylinders will be referred to as Nos 1 to 4, starting at the drive pully end.

The cylinder head 14 carries inlet and exhaust valves (not shown) for the engine. The inlet valves are operated by four inlet camshafts 18, 19, 21 and 22 for cylinder Nos 1, 2, 3 and 4 respectively. The inlet camshafts 18 and 19 of cylinders Nos 1 and 2 are nested, that is inlet camshaft 19 has an elongate portion 23 which extends coaxially through a bore 24 in inlet camshaft 18.

Inlet camshafts 18 and 19 are both driven by a toothed pulley 25.

An exhaust camshaft 26 extending parallel to the inlet camshafts 18, 19, 20 and 21 is common to all four cylinders and is driven by another toothed pulley 27, pulleys 25 and 27 being driven by the toothed belt 17 and having twice the number of teeth as the drive pulley 16 so as to rotate at half crankshaft speed. The exhaust camshaft 26 acts as a layshaft to transmit drive through another toothed belt 28 and through another pair of pulleys 29 and 31 to the inlet camshafts 21 and 23 of cylinders Nos 3 and 4, pulley 31 having the same number of teeth as pulley 29.

The inlet camshafts 21 and 23 of cylinders Nos 3 and 4 are also nested in the same manner as inlet camshafts 18 and 19.

Each of the inlet camshafts 18, 19, 21 and 22 is driven through a respective variable valve timing (VVT) mechanism, indicated generally at 32, 33, 34 and 35 respectively (Figs 2A & 2B). The VVTmechanisms for cylinders Nos 1 and 2 are grouped together outboard of the camshafts 18 and 23 at one end of the engine block 12 where they are driven by first drive means comprising the drive pulley 16 and toothed belt 17 at the same end and the VVT mechanisms for cylinders Nos 3 and 4 are grouped together outboard of the camshafts at the other end of the engine block where they are driven by second drive means comprising pulleys 29 and 31 and toothed belt 28.

Since the VVT mechanisms 32 and 33 for cylinders Nos 1 and 2 are essentially similar to the VVT mechanisms for cylinders Nos 3 and 4 it will be convenient to describe only the VVT mechanisms 32 and 33 of cylinders Nos 1 and 2 in detail and with particular reference to Figs 3 and 4. Fig 3 shows the details of the VVT mechanism 32 of cylinder No 1 to better effect whereas Fig 4 shows the details of the VVT mechanism 33 of cylinder No 2 to better effect.

VVT mechanism 32 includes a driving member 36 which defines a pair of diametrically opposed radially extending grooves 37 and 38 of rectangular section.

Groove 37 is in driving engagement with a peg 39 on an input member 41 and groove 38 is in driving engagement with a peg 42 on an output member in the form of a bearing portion 43 of the inlet camshaft 18 of No 1 cylinder. The pegs 39 and 42 drive through rectangular drive blocks 44 each of which is rotatable on its respective peg 39 or 42 and is a close sliding pit in its respective groove 37 or 38.

The input member 41 is rotatable about substantially the same axis as the camshafts 18 and 19, being carried by ball bearings 45 in a housing 46 attached to the cylinder head 14. Pulley 25 is spigotted onto the input member 41 and is retained by a cap screw 47. A peg 48 engages a slot in the pulley 25 to transmit drive and provide an angular location.

The housing 46 forms part of the engine block 12 and defines a bore 49 whose axis is offset from the axis of rotation of the inlet camshafts 18, 19, 21 and 23. An eccentric sleeve 51 is rotatable in the bore 49 and provides the outer races of a pair of needle roller bearings 52 and 53. The inner race of bearing 52 is a press fit on the outer diameter of driving member 36 so that the driving member is journalled in the eccentric sleeve 51.

VVT mechanism 33 includes a driving member 54 which defines a pair of diametrically opposed radially extending grooves'55 and 56 of rectangular section. Groove 55 is in driving engagement with another peg 57 on the input member 41 and groove 56 is in driving engagement with a peg 58 on an output member in the form of a radially extending lobe 59 which is part of the elongate portion 23 of the inlet camshaft 19 of cylinder No 2. The pegs 57 and 58 drive through rectangular drive blocks 61 in a similar manner to pegs 39 and 42.

The inner race of bearing 53 is a press fit on the outer diameter of driving member 54 so that the driving member 54 is journalled in the eccentric sleeve 51 which is thus common to the adjacent VVT mechanisms 32 and 33.

The driving member 54 of VVT mechanism 33 has an aperture 62 angularly spaced from the grooves 37 and 38 to allow a boss 63 on the input member 41 to extend through with clearance, the boss 63 being drilled to receive the peg 39. To minimise the overall length of the adjacent VVT mechanisms 32 and 33 and to keep the length of the boss 63 to a minimum, a recess 64 in the driving member 36 is provided to partially accommodate the lobe 59 with clearance. Fig 5 shows the driving member 36 in detail perspective and also shows an axial bore 65 which provides a clearance aperture through which the elongate portion 23 of the inlet camshaft 19 extends.

Each VVT mechanism 32, 33, 34 and 35 produces a cyclic variation in the speed of the respective camshaft by moving the axis of rotation of the respective driving member relative to the axis of rotation of the output member on the camshaft. This is achieved by rotating the eccentric sleeve 51 which is shown in more detail and on an enlarged scale in Fig 6. The outer diameter of the sleeve 51 which rotates in the bore 49 is represented by D1 and the inner diameter which provides the outer race of the needle roller bearings 52 and 53 is represented by D2. The eccentricity of the sleeve is represented by E and this dimension is made substantially equal to the offset between the axis of rotation of the inlet camshafts 18, 19, 21 and 22 and the axis of the bore 49 in the housing 46.

Rotary control of the eccentric sleeve 51 is achieved by rotation of a gear pinion 66 which meshes with gear teeth 67 on the outer periphery of the eccentric sleeve. The pinion 66 is part of a control shaft 68 which extends from one end of the cylinder head 14 to the other and also includes a worm wheel 69 on a hollow shaft 71.

Another hollow shaft 72 has splined end pieces 73 and 74, one end piece 73 connecting with hollow shaft 71 and the other end piece 74 connecting with another hollow shaft 75 which carries the pinion 66 and has skew gear teeth 76 for meshing with a skew gear on a feedback potentiometer (not shown).

A rotary servomotor (not shown) has a worm which meshes with the worm wheel 69 to rotate the control shaft 68 and hence the eccentric sleeve 51, the position of the eccentric sleeve being determined from the feedback potentiometer. A further means of feedback of the position of the eccentric sleeve is provided by three teeth 77, 78 and 79 on the inlet camshaft 18. With the camshaft 18 rotating in the direction of arrow B (Fig 7), the leading edge of tooth 77 represents the start of the opening of the inlet valves and the leading edge of tooth 78 represents the end of the closing of the inlet valves. The leading edge of tooth 79 represents the angle of maximum lift of the cam.

An inductive transducer (not shown) senses the movement of the teeth 77, 78 and 79 and provides signals for a control system used to operate the servomotor. The signals from teeth 77 and 78 indicate the inlet valve opening period and the signal from tooth 79 is used as a control check to ensure correct operation during rapid engine acceleration.

The effect of rotating the eccentric sleeve is shown in Figs 8 and 9. In Fig 8 the pegs 39 and 42 of the VVT mechanism of cylinder No 1 are shown diagrammatically and the line of motion of the inlet valves is indicated by a line V-V. Point 0 corresponds to the axis of rotation of the input member 41 and the inlet camshaft 18 and point P corresponds to the axis of rotation of the driving member 36.

The dimension O-P is conveniently referred to as the eccentricity of the driving member 36 for a particular setting of the eccentric sleeve 51 but is to be distinguished from the eccentricity E of the eccentric sleeve which is represented as dimension P-A where A is the axis of the bore 49 in which the eccentric sleeve rotates.

Points P and A are also shown in Fig 6.

The effect of varying the angular position of the eccentric sleeve 51 is to be seen in Fig 9. When the sleeve is rotated to bring point P coincident with point 0 then the axes of rotation of the input member 41, the camshaft 18 and the driving member 36 all coincide and the drive from the input member to the camshaft is without any cyclic variation.

When the eccentric sleeve 51 is moved such that P is at point P2 the eccentricity O-P is at a maximum.

Taking the inlet valve motion line V-V as a datum, the angle which O-P makes from the datum can be referred to as the eccentricity angle G. Fig 10 shows how the inlet cam opening and closing point is advanced or retarded with variations in the eccentricity angle G.

The curves show the variation of valve opening angle VO and curves of valve closing angle VC plotted against the eccentricity angle G for various values of the eccentricity OPexpressed in a non-dimensional form as the eccentricity ratio R. The eccentricity ratio R 'is the ratio of the eccentricity O-P to the radial distance between the axis of rotation of the camshaft 18 (point 0) and the centre of each of the drive pegs 39 and 42.

Also shown in Fig 10 are control law curves C1, C2, C3 and C4 whist intDrsuct thp curuQt of valve opening and value closinq angle; These control law curves are a characteristic of the invention and result from the axis of rotation of the eccentric sleeve 51 being offset from the axis of rotation of the output member comprising the bearing portion 43 on the inlet camshaft 18. The control law curves C1 and C2 are in effect continuous as are curves C3 and C4. The 180 degree jump (from 2100 to 300) represents the change from advancing the valve opening to retarding it.

Points P1, 0 and P2 are shown on the control law curves. Thus at the eccentric sleeve position P1 the inlet valve opening is advanced and the valve closing is retarded. At position 0 there is no advance or retard of opening or closing and the inlet valve operates according to the basic characteristic of the inlet cam. At point P3 the valve opening is retarded whereas the closing point remains close to the basic cam characteristic.

Position P1 represents the requirement for high power at high engine speeds and P3 represents the requirement for efficient engine running at idle or tickover.

Position 0 can be chosen to correspond to a medium power output and engine speed, for example main road cruising and, because there is no movement of the drive blocks 44 in the grooves 37 and 38 at this position, wear of the VVT mechanism is minimised.

Although the invention has been particularly described with reference to a four cylinder in line engine it is readily, adapted to other engine configurations as previously discussed. For a bank of three cylinders the VVT mechanism 34 of cylinder No 3 would be deleted and cylinder No 4 becomes cylinder No 3. For a bank of two cylinders the VVT mechanisms 34 and 35 of cylinders Nos 3 and 4 would be deleted. Alternatively, the VVT mechanisms 33 and 34 of cylinders Nos 2 and 3 could be deleted and cylinder No 4 becomes cylinder No 3.

Claims

1. A camshaft drive mechanism for an internal combustion engine having an engine block which defines at least one cylinder, inlet and exhaust valves for the cylinder and a crankshaft, the camshaft drive mechanism comprising a camshaft for operating one of said valves and a variable valve timing (VVT) mechanism which in use drivingly connects the crankshaft to the camshaft and is operative te provide cyclic variation of the rotational speed of the respective camshaft during otherwise constant rotational speed of the engine crankshaft, the VVT mechanism comprising a rotatable driving member for driving connection to the crankshaft, an output member arranged to drive the camshaft and an eccentric sleeve providing a journal for the driving member and being in use rotatable in a bore in the engine block, the driving member having an axis of rotation which is movable relative to the axis of rotation of the output member to vary the valve timing by rotation of the eccentric sleeve, wherein the axis of rotation of the eccentric sleeve is offset from the axis of rotation of the output member.

2. A camshaft drive mechanism according to Claim 1, wherein the offset is substantially equal to the eccentricity of the eccentric sleeve.

3. A camshaft drive mechanism according to Claim 2, wherein the axis of rotation of the driving member substantially coincides with the axis of rotation of the output member at one operational position of the eccentric sleeve.

4. A camshaft drive mechanism according to any preceding claim1 wherein an input member is rotatable about substantially the same axis as the output member and is in use arranged to be driven by the crankshaft and to transmit drive to the driving member.

5. A camshaft drive mechanism according to Claim 4, wherein a peg on the input member and a peg on the output member are each in driving engagement with a respective one of a pair of diametrically opposed radially extending grooves in the driving member.

6. A camshaft drive mechanism according to any preceding claim, wherein the output member is on the camshaft.

7. A camshaft drive mechanism according to any preceding claim for an internal combustion engine having an engine block which defines at least a second cylinder arranged in line with said one, first1 cylinder, a first group of valves comprising the inlet valves for each cylinder and a second group of valves comprising the exhaust valves for each cylinder, wherein the camshaft drive mechanism comprises first and second camshafts which in use extend parallel to the crankshaft for operating one of said groups of valves for the first and second cylinders respectively, the second camshaft has an elongate portion extending coaxially through the first camshaft, a first VVT mechanism is arranged so that in use drive is transmitted from the crankshaft to the first camshaft and a second VVT mechanism is arranged so that in use drive is transmitted from the crankshaft to the second camshaft through the elongate portion.

8. A camshaft drive mechanism according to Claim 7, wherein the eccentric sleeve is common to the first and second VVT mechanisms.

9. A camshaft drive mechanism according to either Claim 7 or Claim 8 when the respective claim is dependent upon Claim 5, wherein each VVT mechanism comprises a respective one of a pair of pegs on the input member and a peg on the respective output member, each driving member defining a pair of diametrically opposed radial grooves each for driving engagement with a respective one of the pegs.

10. A camshaft drive mechanism according to any of Claims 7 to 9, wherein the output member of the second VVT mechanism extends through the driving member of the first VVT mechanism.

11. A camshaft drive mechanism according to any of Claims 7 to 10, wherein the input member of the first VVT mechanism extends through an aperture in the driving member of the second VVT mechanism.

12. A camshaft drive mechanism according to any of Claims 7 to 11, wherein the output member of each VVT mechanism is on the respective camshaft.

13. A camshaft drive mechanism according to Claim 12, wherein the driving member of the second VVT mechanism comprises the elongate portion of the second camshaft.

14. A camshaft drive mechanism according to any preceding claim, wherein the eccentric sleeve is drivingly connected to a servomotor.

15. A camshaft drive mechanism according to Claims 14, wherein the servomotor is arranged to drive through a non-reversible worm and worm wheel gear drive.

16. A camshaft drive mechanism according to Claims 15, wherein the eccentric sleeve has gear teeth meshing with a control shaft which has a worm wheel meshing with a worm drivingly connected to the servomotor.

17. A camshaft drive mechanism substantially as described herein with reference to the accompanying drawings.