GB2241552A - Cam mechanism for operating the valve of an i.c. engine - Google Patents

Abstract

Cams A and B operate on a poppet valve 1 via rockers 6a and 6b and an hydraulic tappet 5. The timing of the two cams A and B is adjustable, e.g. by jockeys 21a and 21b, so that the cams can be advanced or retarded with respect to the engine so as to vary the dwell angle. In order to increase the dwell angle still further a third cam C acts directly on the hydraulic tappet 5 to hold the valve open during the period when cam A would otherwise allow it to close and cam B is not yet in position to hold it open. <IMAGE>

Description

CAM MECHANISM TECHNICAL FIELD OF THE INVENTION This invention relates to a cam mechanism which is particularly suited to controlling the valve gear of an internal combustion engine.

BACKGROUND In conventional engines the inlet and exhaust valves are each operated by a single cam, the profile of which is normally chosen for optimum performance of the engine around its average running speed. Because the cam profile is fixed, the performance of the engine declines significantly as the engine speed moves away from that of optimum performance.

It has previously been proposed to use two cams to effectively vary the overall profile of the cam system.

(See for example GB 511 853, GB 812 303, US 3 687 010, GB 2 088 471 A, US 4 530 318, and US 4 583 501.) In general however, the range of adjustment which can be achieved with the earlier proposals is insufficient to optimise the performance of the engine over more than a limited range of running speeds.

An object of the present invention may be viewed as being to provide a cam mechanism which permits the cam profile to be varied over a considerable range.

SUMMARY OF THE INVENTION The present invention proposes a cam mechanism in which first and second rotary cams act on a body in mutual co-operation so as to move the body to an operating position for a proportion of each rotational cycle referred to herein as the lift period, and means are provided for adjusting the relative angular positions of both cams so as to vary the lift period, the mechanism being distinguished by the provision of a third rotary cam which is arranged such that it can hold the body in the operating position during an intermediate phase of the lift period between phases in which the other two cams respectively act on the body.

The third cam may thus act to effectively extend the lift period and hence the dwell angle of the mechanism.

(The term "dwell angle" refers to the part of the lift period during which the body is in the operating position and is not in motion.) In order to provide a compact operating arrangement the third cam preferably acts directly on the body and the other two cams act via rocker levers.

The body is preferably an hydraulic tappet which is disposed at the upper end of a valve stem, which may form part of the valve gear of an internal combustion engine. The use of an hydraulic tappet obviates the need to provide manually set clearances for the three cams in order to ensure that the valve closes fully.

Such manual setting would be a very lengthy procedure and would be a considerable drawback to the commercial application of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is exemplified in the accompanying drawings, in which: Figure 1 is a schematic view of a cam mechanism of the invention arranged to operate the valve gear of an internal combustion engine, Figure 2 is a diagrammatic representation of effective cam profiles achieved with the mechanism, and Figure 3 is a graph to show the improved engine performance achieved with the mechanism of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 shows an axially slidable poppet valve l of a four stroke internal combustion engine. The valve is urged upwardly to a closed position by a compression spring 2 which is located about the valve stem 3 to act against a flange 4 on the valve stem. The upper end of the valve stem 3 carries an hydraulic tappet 5 of the bucket type, which is acted upon by two diametrically opposed rocker levers 6a and 6b. The two rocker levers are each pivoted for rocking movement about their centres 7, their inner fingers 8a, 8b being arranged to act downwardly upon the body 5 and their outer fingers 9a, 9b being arranged to co-operate with respective rotary cams A and B.Respective springs 10a and lOb (which in practice will usually be of the hair spring type) act on the outer fingers 9a and 9b to urge them into contact with the respective cams.

The two cams A and B are fixed to rotate with respective camshafts 15a and 15b. The crankshaft 16 of the engine drives a sprocket 17 (usually known as the half engine speed sprocket) via a chain or drive belt 18. The sprocket 17 is fixed for rotation with a drive shaft 19 which drives the respective camshafts 15a and 15b via respective belts or chains 20a and 20b. The lengths of the two extents of each belt or chain are determined by an adjustable jockey wheel 21a, 21b which acts on one extent whilst a spring biassed idler 22a, 22b acts on the other extent to take up the slack and maintain the tension in the belt.

A third cam C fixed with a respective rotary camshaft 15c also acts downwardly on the body 5. The camshaft 15c is driven from the drive shaft 19 via a respective belt or chain 20c.

It will thus be appreciated that by varying the positions of the jockeys 21a, 21b the rotational angle of the two cams A and B can be adjusted relative to the sprocket 17. Since it will not normally be necessary to adjust the angular position of cam C no jockey wheel or idler is provided for the respective belt or chain 15c, but clearly these could be provided if so required.

In this example the profile of the three cams A, B and C i s s is substantially identical, although this is not necessarily so.

The hydraulic tappet adjuster 5 is a device which automatically takes up any clearance between the cams and the valve stem during the non-lift phase, but which forms a substantially rigid link during the lift period. In general, the tappet adjuster includes an inverted bucket-like outer body which contains a fixed plunger working within a cup shaped piston which acts on the valve stem 3. There is a very accurate clearance fit defining a leakage gap between the plunger and the piston, and both of these parts enclose a high pressure chamber containing an oil cushion which rigidly transmits the actuating force from the cams to the valve stem. The high pressure chamber also contains a compression spring which acts to urge the plunger and piston apart and ensure that there is no clearance between the cams and the valve stem during the non-lift phase of the cam. With each cam lift a small but defined quantity of oil is urged out of the high pressure chamber through the leakage gap, but when the cams are not producing lift, oil can return to the high pressure chamber via a valve. The leakage during each lift ensures that any positive length changes in the valve gear are automatically compensated.

In operation of the engine, rotation of the sprocket 17 causes the three cams to rotate and act on the body 5 to move the valve downwards, cams A and B acting via the rocker levers 6a and 6b. In Fig. 1 the mechanism is shown in the configuration for low engine speeds, the three cams A, B and C being so arranged that they all operate the valve 1 simultaneously. The performance of the engine is thus determined by the common profile of the cams.

If jockey wheel 21a is now moved in the direction of arrow X cam A is advanced with respect to the sprocket 17 thereby causing the valve to open earlier in the cycle. Initially cam A will hold the valve open, but during the lift period when the cam is acting to lift the valve off its seat, control of the valve is transferred from cam A to cam B so that the dwell angle is increased. The timing of the closure of the valve is unaffected since the timing of cam B (and of cam C, of course) is unchanged. On the other hand, the valve can be made to close later in the rotational cycle by moving jockey wheel 21b in direction Y so that cam B is retarded relative to sprocket 17.

It will be appreciated that whilst the use of jockey wheels and idlers is a simple and inexpensive way of adjusting the relative timing of cams A and B other means of achieveing the same result could equally well be used.

If the dwell angle is increased to more than about twice the minimum dwell angle, cam C comes into operation. Cam C extends the maximum dwell angle by holding the valve open between the action of cam A and cam B, so that the maximum dwell angle is effectively three times that of a single cam. Under these conditions the motion of the valve has three phases: (i) an opening phase during which the valve is controlled by cam A, (ii) a middle phase during which the valve is held open by cam C, and (iii) a closing phase in which the valve is controlled by cam B. Fig.

2 shows the effective cam profiles during minimum, intermediate and maximum dwell conditions.

The effective characteristics of the cam can thus be controlled in accordance with increasing engine speed, thereby allowing the engine to be run at optimum settings for a wide range of engine speeds. In a conventional four stroke engine the cam profile has to be a compromise between that required for easy starting and good low speed torque and that required for high engine speeds. It is therefore usual to arrange the cam profile for optimum performance at mid engine speeds. A measure of the performance of the cam profile is given by a parameter known as Brake Mean Effective Pressure (BMEP). Fig. 3 shows the BMEP curve for a normal engine, which has a single peak at mid engine speed. Fig. 3 also shows the BMEP curve for a similar engine fitted with the mechanism of the invention.The cam profile is now optimised throughout most of the speed range of the engine so that the curve is almost flat, apart from high engine speeds. At such speeds the effective cam profile is deliberately tailored to tail off the performance of the engine by setting the adjustment back to the low engine speed configuration in order to reduce the power available and prevent possible engine damage. The engine would thus out-perform a normal engine at all normal operating speeds other than the speed chosen for optimum performance of the normal engine. The shaded area between the two curves represents the improvement in engine performance which is attainable by the mechanism of the invention.

It will be appreciated that in engines having all the inlet and exhaust valves for the cylinders arranged in a line these valves may be operated from a single set of three camshafts operating respective cams A, B and C, but for engines having the inlet and exhaust valves arranged in two separate rows two sets of three camshafts would be required.

Claims (5)

1. A cam mechanism in which first and second rotary cams act on a body in mutual co-operation so as to move the body to an operating position for a proportion of each rotational cycle referred to herein as the lift period, and means are provided for adjusting the relative angular positions of both cams so as to vary the lift period, the mechanism being distinguished by the provision of a third rotary cam which is arranged such that it can hold the body in the operating position during an intermediate phase of the lift period between phases in which the other two cams respectively act on the body.

2. A cam mechanism according to Claim 1, in which the third cam acts directly on the body and the other two cams act via rocker levers.

3. A cam mechanism according to Claim 1 or 2, in which the body is an hydraulic tappet disposed at the upper end of a valve stem.

4. A cam mechanism according to Claim 3, in which the valve stem is part of the valve gear of an internal combustion engine.

5. A cam mechanism substantially as described with reference to the drawings.