ENGINE CAMSHAFT DRIVE MECHANISM

Abstract: The invention relates to a drive mechanism for achiev­ ing variable valve timing in an internal combustion engine. A drive crank 69 is coupled to a driven crank 72 by a slotted disc 63 rotatably mounted in a yoke 66 and movable be­ tween on-axis and offset positions relative to a common axis of rotation of the two cranks. To control the position of the yoke 66 a force is applied to the yoke to urge the disc away from the on-axis position, the direction of the force be­ ing such that the reaction force on the yoke, resulting from the rotation of the slotted disc in an offset position, acts to move the yoke 66 and the disc 63 into the on-axis position as the rotational speed of the disc increases.

Description: ENGINE CAMSHAFT DRIVE MECHANISM The present invention relates to an engine camshaft drive mechanism. It is known that the optimum crankshaft angles of a combustion cycle of an internal combustion engine during which the inlet and the exhaust valves should remain open varies as a function of the engine speed. Consequently in deciding on a fixed cam profile one has to select an engine speed at which the performance is optimised and accept reduced efficiency at other speeds. It is not practical to vary the camshaft profile in dependence upon engine speed and in order to achieve a similar effect to a variable cam profile the Applicant has previously proposed in British Patent Application No, 2096695 a system in which the camshaft is driven at variable speed in order to achieve varying opening and closing angles. The camshaft is accelerated and retarded during its rotation in order to alter the effective valve timing. In Patent Specification No. 2096695, Figure 15 shows a system in which the camshaft is driven by way of a circular disc which is rotatably mounted within a yoke. The disc has a slot in which two cranks engage. The first crank is formed on a drive pulley and the second on a driven gear of the camshaft driving train, the pulley and the driven gear being coaxial. Depending'upon the position of the centre of the slotted disc in relation to the axis of rotation of the two cranks, the amplitude of the modulation of the angular speed of the camshaft may be varied. In the above arrangement, when the disc is concentric with the axis of the two cranks then provided the drive crank rotates at constant speed the driven crank will also rotate at the same constant speed. On the other hand, if the disc rotates about an axis offset from the rotational axis of the two cranks, then the mechanical advantage of the coupling between the two cranks will depend upon the angle of rotation and consequently the driven crank will rotate with variable speed, a rotational vibration being added to the speed of the drive crank. This modulation of the angular velocity, which depends on the offset of the axis of the disc, serves to speed up and retard the camshaft so as to advance or retard respectively, the angle of opening of the inlet or the outlet valve. It has been found in the case of the embodiment described in Figure 15 of the above patent specification that the force required to move the disc in order to vary the modulation amplitude is quite substantial and therefore the implementation of a control system to vary the, valve timing in dependence upon engine speed may prove both complicated and expensive. Furthermore, though it is safe to rotate the slotted disc at high speed when it is concentric with the two cranks, the mechanisn is likely to break if the disc is made to rotate at high speed when it is in an offset position. Consequently, any slight delay in the response of the control mechanism moving the yoke carrying the disc is likely to subject the yoke and the disc to unbearable forces and cause a breakdown of the angular velocity modulating mechanism. The present invention seeks to provide a system for controlling the yoke supporting the slotted disc which mitigates the above problems. In accordance with the present invention, there is provided a drive mechanism for achieving variable valve timing in which a drive crank is coupled to a driven crank by a slotted disc rotatably mounted in a yoke and movable betweenon-axis and offset positions relative to a common axis of rotation of the two cranks, wherein means are provided to apply a force to the yoke to urge the disc away from the on-axis position, the direction of the force being such that the reaction force on the yoke, resulting from the rotation of the slotted disc in an offset position, acts to move the yoke and the disc into the on-axis position as the rotational speed of the disc increases. It has been found that if the force acting upon the yoke during rotation of the disc in an eccentric position is averaged over a cycle then it produces a net resultant force in one direction. Though it may be possible to overcome this force and to constrain the disc to adopt any desired position, such a method of control, if adopted, would result in breakage of the entire variable valve timing mechanism, as mentioned above, in the event of malfunctioning of the control system. In the approach adopted by the invention, however, the reaction force acting on the disc and the yoke is arranged to oppose a steady force which acts to urge the disc and the yoke into an eccentric position. Thus, in the arrangement of the invention, in the steady state position when the engine is rotating at low speed the force acting on the yoke as a result of the offset disposition of the disc is not sufficient to displace the yoke and therefore the disc continues to rotate eccentrically. Because the speed is low this presents only an acceptable. load. During such time, the modulation of the speed of rotation on the camshaft is at a maximum and the extent to which the valve timing is varied from its high speed timing angle is at a maximum. As the engine speed increases, the force resulting from the offset rotation also increases and without the need for a complicated control system opposes the constant displacement force to move the disc automatically into a more central position thereby simultaneously reducing the load on the yoke. When the engine reaches the speed at which the camshaft profile optimises the engine performance, the disc will rotate substantially concentrically with the driving and driven crank so that the camshaft speed is then substantially constant during its rotation. It is possible for the force urging the yoke into an offset position of the disc to be derived from a hydraulic piston, this being a means of providing substantially constant force over the stroke of displacement of the yoke. Alternatively, a relatively long spring may be employed to displace the yoke into an offset position of the disc. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 corresponds substantially to Figure 15 of Patent Specification No. 2096695, Figure 2 shows a graph of the force acting on the yoke as a function of the angular position of the driving crank of the camshaft mechanism, and Figure 3 is a schematic section showing a control mechanism for achieving self modulation of the variable valve timing control. In Figure 1, a drive pulley 60 transmits drive to a camshaft 3 through an angular velocity modulating mechanism 62 and a gear train 68. The angular velocity modulating mechanism 62 comprises a yoke 66 which has an annular portion 65 rotatably journalling a disc 63. The disc 63 is formed with a slot 64 which is engaged from one side by a crank 69 which rotates with the input drive pulley 60 and drives a second crank 72 which is connected through a shaft 70 to a gear wheel 73. The latter gear wheel 73 meshes with a gear wheel 74 which is integral with the camshaft 3. In this mechanism, if the centre of the disc 63 is in line with the axes of the drive pulley 60 and the gear wheel 73 then these two will rotate at the same speed as each other. If, however, the yoke 66 is moved so that the disc 63 is no longer concentric with the drive pulley 60 and the gear wheel 73 then the mechanical advantage of the coupling will vary with the angular position and as a consequence of this a modulation is superimposed on the speed of rotation of the drive pulley 60. The extent of speed modulation varies with the amount of eccentricity. In Figure 2, there is shown a graph of the reaction force on the yoke 66 as a function of the angular position of the disc 63. It will be seen that the force is not symmetrical about the angle zero and instead there is a mean force which is dependent upon the level of eccentricity. Figure 3 shows a schematic diagram of an embodiment of the invention which makes use of the reaction force on the yoke to effect control of the level of modulation superimposed on the angular velocity of the camshaft to achieve variable valve timing. In Figure 3, the yoke 66 has an annular portion 65 in which the disc 63 is journalled and two radial extensions designated 10 and 12. An adjustable spring 14 acts to urge the yoke 66 downwards as viewed and a hydraulic piston 18 serves to urge the yoke 66 upwards as viewed. The working chamber of the cylinder 20 of the piston 18 is connected to a source of constant pressure by way of a damping restrictor 22. An over-pressure relief valve 24 serves to limit the pressure in the cylinder 20. In use, in the absence of a reaction force, the constant pressure in cylinder 20 urges the piston 18 upwards and moves the yoke 66 against the action of the spring 14. The purpose of the spring 14 is to achieve fine adjustment of the variable valve timing control. The piston urges the yoke 66 in a direction to increase the eccentricity of the disc 62 (not shown in Figure 3). As the engine speed rotation increases, the reaction force increases and because of the damping in the hydraulic circuit the reation forces are averaged over the cycle and the net force acts to oppose the pressure in the cylinder 20. The piston 18 therefore moves downwards automatically with increased speed to move the disc in a direction to reduce the eccentricity and thus the yoke reaches a position equilibrium in which the reaction force balances the constant pressure from the hydraulic cylinder 20. By appropriately dimensioning the hydraulic forces acting on the yoke and the return spring 14, it is possible to ensure that the yoke adopts at each engine speed a position which at least approximates to the optimum setting of the valve timing at that speed. The advantage offered by this system, apart from its inherent simplicity is the safety resulting from the fact that the reaction force which is likely to cause damage to the variable valve timing mechanism is itself responsible for altering the eccentricity of the disc and consequently there is no risk of the variable valve timing mechanism being in an eccentric position when the engine speed is high.

Claims: CLAIMS 1. A drive mechanism for achieving variable valve timing in an internal combustion engine, in which a drive crank is coupled to a driven crank by a slotted disc rotatably mounted in a yoke and movable between onaxis and offset positions relative to a common axis of rotation of the two cranks, characterised in that means (14,20) are provided to apply a force to the yoke (66) to urge the disc (63) way from the on-axis position, the direction of the force being such that the reaction force on the yoke (66), resulting from the rotation of the slotted disc (63) in an offset position, acts to move the yoke and the disc into the on-axis position as the rotational speed of the disc increases. 2. A drive mechanism according to claim 1, wherein the means for applying a force to the yoke (6'6) comprises a hydraulic piston (18) serving to apply a substantially constant force in all positions of the yoke. 3. A drive mechanism according to claim 2, wherein the piston (18) is connected to a source of constant pressure and a flow restrictor (22) is provided in the hydraulic circuit in order to damp the reaction force on the yoke. 4. A drive mechanism according to claim 2 or 3, in which an adjustment spring (14) acts on the yoke (66) in a direction opposing the force of the hydraulic piston (18). 5. A drive mechanism according to claim 1, in which the means for applying a force to the yoke comprises, a spring urging the yoke into an eccentric position relative to the axes of rotation of the two cranks.

Inventors: MA, THOMAS TSOI-HEI
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1983
Main classification: F01L1
IPC codes: F01L1, F01L13