Abstract:
A valve operating mechanism comprising a valve, a cam, a cam shaft for rotating the cam about an axis, a rocker lever rotatably mounted about a rocker lever shaft. The rocker lever is axially movable along the rocker lever shaft. The cam is tapered in the longitudinal direction of the axis and has a uniform cross sectional shape. The rocker lever has an arm formed with a surface engaging the cam and an opposite arm provided with means for engaging the end of a valve stem of the valve.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an internal combustion engine and more particularly to a variable valve timing system for an internal combustion engine. 
     Exhaust gas recirculation (EGR) is widely employed to reduce nitrogen oxides (NOx) emissions from internal combustion engines. EGR can be accomplished either externally or internally. The external method involves diverting the exhaust gases from some point in the exhaust system back to the intake system. To reduce NOx emissions to a certain sufficiently low level, the amount of EGR must be increased considerably. Increasing the flow rate of the exhaust gases through the intake system to meet the demand above will increase the deposition of carbon on the interior wall of the intake system and EGR system to such an degree as to cause deterioration of the induction efficiency of the engine and the reduction efficiency of NOx removal. To alleviate this problem, it has been proposed to increase the internal EGR to suppress the external EGR. All naturally aspirated engines have some inherent EGR; that is, some portion of the products of combustion remain in the cylinder (residual gas) which mix with the incoming fresh charge before combustion is initiated. The amount of residual gas can be controlled by increasing or decreasing the valve overlap period during which both inlet and exhaust valves are open. 
     As well as having the potential for reducing Nox, the variation of valve overlap has a direct effect on engine combustion and power output. Large valve overlap enhances power output at high-speed wide-open throttle operation, while low-speed part-throttle operation requires low overlap for smooth running. These interrelationships indicate that a variable valve timing system capable of adjusting valve overlap while running may be desirable for optimizing engine operation for both emissions and performance. 
     To accomplish this, a known variable valve timing system comprises a cam shaft axially movable by an actuator. The cam shaft has a cam to operate a valve through a rocker arm or lever. The cam has a plurality of valve control tracks each associated with a different range of engine operation. The changeover of the system from one track to another is effected by axial translation of the cam shaft by the actuator. Because the cam shaft is driven by the engine to rotate at high speeds, there exist some difficulties to axially move the cam shaft by the actuator. Because the cam must have different valve control tracks, it is not easy to manufacture. Therefore this known system is complicated in construction. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a simply constructed, variable valve timing system. 
     It is another object of the present invention to provide a variable valve timing system capable of providing large valve overlap period during which both inlet and exhaust valves are open under operation of the engine at high speeds and low valve overlap under operation at low speeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These objects, features and advantages of the present invention will be clear from the following description in connection with the accompanying drawings, of an overhead cam internal combustion engine according to this invention in which: 
     FIG. 1 is a plan view, partly in section, of a variable valve timing system according to the present invention; 
     FIG. 2 is a section through line II--II of FIG. 1; 
     FIG. 3 is a view as viewed, in FIG. 1, along an arrow III; 
     FIG. 4 is an enlarged fragmentary view of FIG. 2; 
     FIGS. 5 and 6 are perspective views of a portion of a rocker arm and a tappet, respectively; and 
     FIG. 7 is a graph depicting an effective valve lift and crank angle on valve overlap; 
     FIG. 8 is a view similar to that of FIG. 1 but showing an alternative arrangement of the hydraulic chambers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the accompanying drawings and more particularly to FIGS. 1-3 thereof, there is shown a variable valve timing system according to the present invention in which an exhaust valve is indicated by reference numeral 10 (see FIG. 2). The exhaust valve 10 has a stem 12 and equipped with a valve spring 14 which biases the valve 10 toward a closed position, the position being illustrated in FIG. 2. The valve 10 is operated by a cam 16 through a rocker lever 18 rockably mounted on a rocker lever shaft 20. The cam 16 is fixedly mounted on or integrally formed with a cam shaft 22 that is driven by an engine crank shaft, not shown, to rotate about an axis parallel to the rocker lever shaft 20. The rocker lever 18 is mounted on the rocker lever shaft 20 at an intermediate portion thereof. It has at one end thereof an adjustable tappet 24 (see FIGS. 4 and 6 also) positioned to contact with a stem end 12a of the stem 12 and at opposite end thereof a follower 26 cooperating with the cam 16. 
     As best seen in FIG. 3, the follower 26 has a surface 26a which is positioned in active position with the cam 16. The surface 26a of the follower 26 extends along a line that is disposed within a plane in which the axis of the cam shaft 22 lies and that forms an angle with the axis of the cam shaft. 
     As best seen in FIG. 1, the rocker lever 18 is movable along the rocker lever shaft 20 between a first position illustrated by solid line in which a clearance between the surface 26a of the follower 26 and the cam 16 (see FIG. 3) is maximum, the clearance being indicated by C 1 , and a second position illustrated by imaginary line in which a clearance between the surface 26a of the follower 26 and the cam 16 is minimum, the clearance being indicated by C 2  (see FIG. 3). 
     As best seen in FIGS. 4-6, the adjustable tappet 24 or clearance adjustor has a portion 24a which contacts with the stem end 12a. The portion 24a extends in a direction parallel to the rocker lever shaft 20 so that the portion 24a is in contact with the stem end 12a irrespective of positions of the rocker 18 during its movement along the rocker lever shaft 20. The adjustable tappet 24 has a plunger portion 24b formed with a screw thread. The plunger portion 24b extends through an elongated hole 18a of the rocker arm 18 and the tappet 24 can be appropriately positioned relative to the stem end 12a by means of nuts 28a and 28b. 
     The rocker lever 18 is biased toward the illustrated position by a return spring 30 compressedly disposed between a spring retainer 32 securely attached to the rocker lever shaft 20 or cylinder head and the rocker lever 18, as shown in FIG. 1. The rocker lever 18 is moved against the action of the return spring 30 toward the position illustrated by the imaginary line by means of a piston 34 of a hydraulic actuator. A stopper 36 is provided to define the position illustrated by the imaginary line. The hydraulic actuator comprises a cylinder 38 in which the piston 34 is sealingly slidably disposed to define a chamber 40. The chamber 40 communicates with a lubricating oil supply pump, not shown, driven by the engine through a conduit 41. The pump provides lubricating oil having a pressure which increases with increase of revolution speed of the engine (engine rpm). To supply the lubricating oil to portions of the rocker lever shaft 20 which require lubrication, the rocker lever shaft 20 is hollowed to provide a passage 20a for the lubricating oil. 
     Although in the preceding the conduit 41 is provided to establish communication between the chambers 40 and 40&#39; and the pump, the communication may be established by a passage 42 drilled through a body defining the cylinders 38 and 38&#39;. The passage 42 communicates at one end with the cylinders 38 and 38&#39; and opens at the other end to the passage 20a formed in the rocker lever shaft 20. 
     During operation at idle or at low speed, the rocker lever 18 is in the position illustrated by the solid line because pressure of the lubricating oil supplied by the oil pump is not high enough to cause the piston 34 to move the rocker lever 18 rightwardly against the return spring 30. When the pressure increases above a predetermined level, the piston 34 commences to move the rocker lever 18 toward the position illustrated by the imaginary line against the return spring 30. During operation at high speed in which the pressure of the lubricating oil is higher than the predetermined level, the rocker lever 18 moves toward the position illustrated by the imaginary line. It will be recognized that during operation at idle or low speed, the rocker lever 18 is in the position illustrated by the solid line and the clearance between the cam 16 and the follower 26 is maximum, and during operation at high speed, the clearance decreases as engine speed increases. 
     Referring to the cam 16, it is tapered axially so the edge of the cam 16 makes line-to-line contact with the surface 26a of the follower 26 and has the same profile in any section taken transversely through the cam 16. 
     Referring to FIG. 1, an inlet valve, not shown, is operated by a similar cam 16&#39; to the cam 16 through a similar rocker lever 18&#39; to the rocker lever 18. The rocker lever 18&#39; is in the position illustrated by solid line during operation at idle and low speed because force of a return spring 30&#39; overcomes force of a piston by pressure of oil within a chamber 40&#39;. The rocker lever 18&#39; is moved toward a position illustrated by imaginary line during operation at high speed, decreasing a clearance between the cam 16 and its follower 26&#39; formed on the rocker lever 18&#39; from C 1  (the maximum) to C 2  (the minimum). 
     The operation of the variable valve timing system described in the preceding will be explained. 
     When the engine operates at idle or at low speed, the rocker levers 18 and 18&#39; for the exhaust and inlet valves, respectively are in the positions illustrated by the solid lines, the clearance between each of the cams and its associated follower is maximum (C 1 ). Under this condition, valve overlap period during which the inlet and exhaust valves are open is the lowest, as indicated at L 1  as shown in FIG. 7. 
     When the engine operates at high speed, the rocker levers 18 and 18&#39; take the positions illustrated by the imaginary lines thereby the clearance reducing to the minimum (C 2 ). Under this condition, the valve overlap becomes the largest, as indicated at L 2  as shown in FIG. 7. 
     It will now be recognized that the internal EGR during operation at idle or low speed is much for smooth running and for fuel economy, while the internal EGR during operation at high speed is great. 
     Although in the preceding embodiment the valve overlap is varied by varying the valve timing of both of the inlet and exhaust valves, the variation of the valve overlap may be effected by varying the valve timing of the exhaust valve alone by selectively supplying oil or fluid under pressure into chamber 40 only. 
     The valve overlap may be varied by varying the valve timing of the inlet valve alone by selectively supplying oil or fluid under pressure into chamber 40&#39; only. This causes throttling of fresh intake charge because during operation at high speed opening timing of the inlet valve is delayed. Thus the intake charge will be mixed well to provide high output during operation at high speed.