Variable valve apparatus of internal combustion engine

A variable valve apparatus is employed a configuration in which, at a high valve lift and high speed operation of an internal combustion engine, an oscillating fulcrum of a transmission arm and a rotation center of a control shaft are arranged between a direction of a component rotating a control shaft of a maximum load which occurs in the oscillating fulcrum of the transmission arm when an oscillating cam oscillates in a valve opening direction and a direction of a component rotating a control shaft of a maximum load opposite thereto which occurs when the oscillating cam oscillates in a valve closing direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable valve apparatus of an internal combustion engine, which varies the phase of an intake valve or an exhaust valve.

2. Description of the Related Art

Many reciprocating engines mounted in automobiles include a variable valve apparatus for changing the phases of an intake valve and an exhaust valve, for reasons of engine gas emission countermeasures, fuel consumption reduction and the like.

Many of such variable valve apparatuses employ a structure in which the phase of a cam formed on a camshaft is replaced with an oscillating cam in which a base circular zone and a lift zone are ranging. Specifically, a structure is employed in which an oscillating range of the oscillating cam is changed, whereby a valve opening period and a valve lift amount of the intake valve and the exhaust valve driven via a rocker arm are varied continuously.

In order to improve a pumping loss, a structure is proposed in Jpn. Pat. Appln. KOKAI Publication No. 2003-239712 in which a transmission arm is interposed between a cam and an oscillating cam, and the transmission arm is oscillatably supported by a control shaft.

Specifically, the transmission arm is moved by the turning displacement of the control shaft. A contact position of transmission arm and the cam is changed by moving the transmission arm. By changing the contact position of the transmission arm and the cam, the valve characteristics, that is, a valve opening period, valve open-close timing and a valve lift volume are continuously varied.

In such a variable valve apparatus, it is known that, when an engine is operated at a high valve lift and at a high speed, a force for driving an intake valve or an exhaust valve becomes large by a positive acceleration zone of a cam lift just after opening the valve and just before closing the valve.

As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-239712, in most variable valve apparatuses using a transmission arm, a valve driving force at opening the valve, and a reaction force working onto a contact point portion of an oscillating cam and a contact point portion of a cam at closing the valve are applied to an oscillating fulcrum of the transmission arm in a same direction at the high valve lift and high speed operation.

In the structure in which the resultant force of these forces works on the oscillating fulcrum, a load amount to be added is large. Therefore, when the force for driving the valve becomes large and the like, an excessive load is likely to work on the oscillating fulcrum of the transmission arm.

In particular, when an excessive load works on the control shaft, there occurs a deformation under torsion in the control shaft. Therefore, there is a fear that preset valve characteristics, that is, valve lift amount and the like may not be reproduced. Further, an actuator having a large capacity and a large size enough to generate a torque to overcome an excessive torque is required.

In particular, in the case of a multicylinder engine in which valve characteristics of each cylinder are varied by a common control shaft, the influence of the deformation under torsion of the control shaft tends to become larger in the cylinders away from the actuator in comparison with the cylinders near the actuator that turns the control shaft.

Therefore, in the multicylinder engines, there occur differences in the valve lift amount and the valve opening period among the cylinders, and there occur differences in the combustion conditions among the cylinders, which causes vibration in the engine, degrades the output, and degrades the fuel consumption.

Under such circumferences, in these variable valve apparatuses, countermeasures must be taken by use of a strong oscillating fulcrum durable to an excessive load and a highly rigid control shaft.

However, these countermeasures make the structure of the variable valve apparatus complicated, and additionally make the structure around the control shaft including the control shaft large.

BRIEF SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a variable valve apparatus of an internal combustion engine of a simple and compact structure, in which a load working on an oscillating fulcrum of transmission shaft is suppressed at a high valve lift and high speed operation.

In order to achieve the above object, according to one aspect of the present invention, there is employed a configuration in which, at a high valve lift and high speed operation of an internal combustion engine, an oscillating fulcrum of a transmission arm and a rotation center of a control shaft are arranged between a direction of a component rotating a control shaft of a maximum load which occurs in the oscillating fulcrum of the transmission arm when an oscillating cam oscillates in a valve opening direction and a direction of a component rotating a control shaft of a maximum load opposite thereto which occurs when the oscillating cam oscillates in a valve closing direction.

In this structure, at the high valve lift and high speed operation, the rotation center of the control shaft and the oscillating fulcrum of the transmission arm are arranged between a direction of a component rotating the control shaft of a load which occurs in the oscillating fulcrum of the transmission arm when the oscillating cam oscillates in a valve opening direction and a direction a component rotating the control shaft of a load opposite thereto which occurs when the oscillating cam oscillates in a valve closing direction. Consequently, at the operation, the resultant force of the valve driving force and the reaction force thereof in the prior art does not work onto the oscillating fulcrum of the transmission arm, but any one load of the forces works alternately.

Accordingly, by a simple arrangement and structure of the oscillating fulcrum of the transmission arm and the control shaft, it is possible to prevent an excessive load in the rotation direction of the control shaft from working onto the oscillating fulcrum of the transmission arm at the high left and high speed operation. Thereby, it is possible to prevent an excessive torque from occurring in the control shaft at the high valve lift and high speed operation.

As a result, it is possible to suppress the burden working onto the oscillating fulcrum of the transmission arm and the control shaft, and further, it is possible to make compact the peripheral area of the control shaft including the control shaft. Furthermore, it is possible to make compact the actuator for operating the control shaft. In addition, the deformation under torsion occurring in the control shaft is suppressed, and accordingly, it is possible to reproduce preset valve characteristics. Consequently, the output of an internal combustion engine and the fuel consumption are improved.

DETAILED DESCRIPTION OF THE INVENTION

A variable valve apparatus according to a first embodiment of the present invention will be explained with reference toFIGS. 1 to 11hereinafter.

FIG. 1is a plan view of a cylinder head1of a multi-cylinder internal combustion engine, for example, a 4-cylinder reciprocating gasoline engine100with cylinders1aarranged in series.FIG. 2is a detailed cross sectional view of the cylinder head1taken along line A—A shown inFIG. 1.FIG. 3is a plan view showing a part of the cylinder head1enlarged.FIG. 4is an exploded view of a variable valve apparatus20mounted on the cylinder head1.

The cylinder head1will be explained with reference toFIGS. 1 to 3. On a lower surface of the cylinder head1, combustion chambers2are formed, respectively, in the wake of four cylinders1aformed in a cylinder block1cand arranged in series. Note that combustion chamber2is illustrated only one in the figure.

For example, two pieces each of intake port3and exhaust port4, that is, one pair of intake port3and exhaust port4are formed in the combustion chambers2. An intake valve5that opens and closes the intake port3and an exhaust valve6that opens and closes the exhaust port4are assembled on the top of the cylinder head1. For the intake valve5and the exhaust valve6, a normally closed reciprocating valve which is energized in the closing direction by a valve spring7is used, respectively. Note that a piston1bis reciprocatively housed in the cylinder1a. The piston1bis illustrated by chain two-dot, dashed line inFIG. 2.

InFIGS. 1 and 2, reference numeral8denotes, for example, a Single Overhead Camshaft (SOHC) type valve operating system mounted to the upper part of the cylinder head1. The valve operating system8drives the intake valve5and exhaust valve6.

Reference numeral10denotes a camshaft rotatably arranged in the longitudinal direction of the cylinder head1on the top of the combustion chamber2. Reference numeral11denotes a rocker shaft on the intake side rotatably arranged in intake port side with which the camshaft10is sandwiched. The rocker shaft11is also used as a control shaft of the present application.

Reference numeral12is a rocker shaft on the exhaust side arranged and fixed on the exhaust port side. Reference numeral13denotes a support shaft lying above the rocker shaft11and12and located closer to the rocker shaft12than to the rocker shaft11. Rocker shafts11and12and the support shaft13are all configured by shaft members arranged in parallel to the camshaft10.

The camshaft10is rotatably driven along the arrow-mark direction ofFIG. 2by an output from a crankshaft of the engine. Note that the crankshaft is not shown. To each part of the camshaft10, an intake cam15and two exhaust cams16are formed for each combustion chamber2, that is, for each cylinder. The intake cam15is corresponding to the cam of the present invention. The intake cam15is arranged at the overhead center of the combustion chamber2. The exhaust cams16and16are arranged on both sides of the intake cam15, respectively.

To the exhaust-side rocker shaft12, a rocker arm18for exhaust valve is rotatably supported for each exhaust cam16, that is, each exhaust valve6as shown inFIGS. 1 and 2. In addition, to the intake side rocker shaft11, a variable valve apparatus20is assembled for each pair of intake cams15, that is, for each pair of intake valves.

The rocker arm18transmits displacement of the exhaust cam16to the exhaust valve6. The variable valve apparatus20transmits displacement of the intake cam15to the intake valves5and5. Due to the rocker arm18and the variable valve apparatus20being driven by each cam15and16, predetermined combustion cycles, for example, four strokes of intake stroke, compression stroke, explosion stroke and exhaust stroke, are formed in the cylinder1ain linkage with the reciprocating motion of the piston1b. Note that reference numeral87inFIG. 2denotes an ignition plug to ignite fuel-air mixture in the combustion chamber2.

To explain the variable valve apparatus20, as shown inFIGS. 1 to 4, the apparatus20comprise a rocker arm25, center rocker arm35, a swing arm45and a support mechanism70. The rocker arm25is oscillatably supported by the rocker shaft.

The swing cam45is combined with the rocker arm25. The swing cam45is equivalent to the oscillating cam of the present invention.

The center rocker arm35transmits displacement of the intake cam15to the swing cam45. The center rocker arm35is equivalent to the transmission arm of the present invention. The support mechanism70oscillatably supports the center rocker arm35to the rocker arm11.

As shown inFIGS. 3 and 4, the rocker arm25is, for example, bifurcate. Specifically the rocker arm25has a pair of rocker shaft arm pieces29and a roller member30. A cylindrical rocker shaft supporting boss26is formed at the center of the each rocker arm piece29.

To one side of the each rocker arm piece29, adjust screw unit27which drives the intake valve is assembled. The roller member30is sandwiched between other ends of the rocker arm pieces29. The roller member30is a contact unit of the present invention.

Note that reference numeral32denotes a short shaft to rotatably pivot the roller member30to the rocker arm piece29. The rocker shaft11is inserted in the bosses26and can oscillate. The roller member30is arranged on the support shaft13side, namely on the center side of the cylinder head1.

The adjust screw units27are arranged at the upper ends of the intake valves5, that is, valve stem end of the intake valve5, respectively. When the rocker arm25oscillates around the rocker shaft11, the intake valves5are driven.

As shown inFIGS. 2 to 4, the swing cam45has a boss portion46, an arm portion47, and a receiving unit48. The boss portion46is cylindrical. The support shaft13is inserted into the boss portion and rotatably fitted.

The arm portion47extends from the boss portion46to the roller member30, that is, rocker shaft. The receiving unit48is formed at the lower part of the arm portion47.

The front end surface of the arm portion47is a cam surface49which transmits displacement to the rocker arm25. The cam surface49extends in the vertical direction. The cam surface49is brought rotatably in contact with the outer circumferential surface of the roller member30of the rocker arm25. The detail of the cam surface49will be described later.

As shown inFIG. 4, the receiving unit48comprises a recessed portion51and a short shaft52. The recessed portion51is formed at the lower surface portion of the lower part of the arm portion47which is directly above the camshaft10.

The short shaft52is rotatably supported in the recessed portion51in the direction same as that of the camshaft10.

Note that reference numeral53denotes a recessed portion which is formed on the outer circumference of the short shaft52portion and has a flat bottom surface.

As shown inFIGS. 2 and 4, to the center rocker arm35, a substantially L-shape member is used. The center rocker arm35has a rotary contact element, for example, a cam follower36which comes rotatably in contact with the cam surface of the intake cam15, and frame-shape holder unit37which rotatably supports the cam follower36.

Specifically, the center rocker arm35has a relay arm portion38and a fulcrum arm portion39. The relay arm portion38extends from the holder unit37towards between the upper rocker shaft11and the support shaft13.

As shown inFIGS. 5 to 8, the fulcrum arm portion39extends from the holder unit37to the bottom side of a shaft portion11cof the rocker shaft11. The shaft portion11cis exposed from between the pair of rocker arm pieces29. The fulcrum arm portion39is, for example, bifurcated.

To the front end, i.e. top end surface, of the relay arm portion38, a gradient surface40is formed as a drive surface. The gradient surface40tilts in such a manner that the rocker shaft11side is lower and the support shaft13side is higher. The front end of the relay arm portion38is inserted into the recessed portion53of the swing cam45. With this, the center rocker arm35is interposed between the intake cam15and the swing cam45. The gradient surface40of the arm unit38is slidably abutted on a receiving surface53aformed at the bottom surface of the recessed portion53. By this, displacement of the intake cam15is transmitted to the swing cam45from the relay arm portion38while being accompanied by slides.

As shown inFIGS. 2 and 4, the support mechanism70has a support unit77and an adjusting unit80. The support unit77has a control arm72. The control arm72oscillatably supports the center rocker arm35. The adjusting unit80adjust the position of the center rocker arm35.

Now, the support unit77will be explained. A through hole73is formed on a lower peripheral wall of the shaft portion11c. The through hole portion11extends in a direction orthogonal to the center of axle of the shaft portion11c. The control arm72is formed to have a rod portion74having a circular cross section, a disk-shaped pin joining piece75formed on one end of the rod portion74, and a support hole75aformed on the pin joining piece75.

The support hole75ais shown inFIG. 4. The end of the shaft74is inserted into the through hole73from the bottom of the shaft portion11c. Note that the inserted rod portion74can move in the axial direction and rotate in the circumferential direction. The end of the rod portion74impinges against a component of the adjusting unit80described later.

The pin joining piece75is inserted in the fulcrum arm portion39. A pin42is inserted in the arm unit39and the support hole75a, thereby allowing the front end of the fulcrum arm portion39and the end of the control arm72protruding from the shaft portion11cto rotatably join each other in the protruding direction, that is, direction orthogonal to the center of axle of the camshaft10of the intake cam15.

Since the fulcrum arm portion39and the control arm72are joined together, the center rocker arm35oscillates up and down, using the pin42as fulcrum, when the in the intake cam15. In linkage with the motion of the center rocker arm35, the swing cam45is periodically oscillated with the support shaft13used as the fulcrum, the short shaft52used as the point of action, that is, point at which a load from the center rocker arm35works on, and the cam surface49used as the point of force, that is, as point at which the rocker arm25is driven.

Note that the rocker arm25, the center rocker arm35, and the swing cam45are mutually energized by energizing means, for example, a pusher86, in a direction to come in close contact to each other to secure smooth movement.

As shown inFIGS. 1 and 4, for example, a control motor43as an actuator is connected to the end of the rocker shaft11. The rocker shaft11is driven, or rotated around the center of axle by the control motor43. By this rotation of the rocker shaft11, the control arm72can be varied from a substantially perpendicular posture shown in, for example,FIGS. 5 and 6to a posture greatly tilted to the camshaft rotating direction shown inFIGS. 7 and 8.

The center rocker arm35is moved, that is, displaced in the direction intersecting with the axial direction of the shaft portion11cfrom this change of posture of the control arm72. That is, as shown inFIGS. 5 to 8, the position at which the follower rolling intake contact cam follower36and the intake cam15can be varied in the early injection directions or the late injection direction.

Because the rotary contact position is variable, the posture of the cam surface49of the swing cam45is varied too. That can simultaneously and continuously vary an opening and closing timing, a valve opening period, and a valve lift volume of the intake valve5.

Specifically, a curvature which varies the distance from the center of, for example, the support shaft13is used for the cam surface49. As shown inFIG. 2, the cam surface49has a base circular zone α and a lift zone β. The circular zone α is the upper side of the cam surface49. The base circular zone α is a circular arc surface centering around the center of axle of the support shaft13.

The lift zone β is the lower side part of the cam surface49. The lift zone β has a first portion γ1and a second portion γ2. The first portion γ1extends from the base circular zone α and curves the opposite direction opposite to the direction in which base circular zone α curves. The second portion γ2extends from the first portion γ1. The second portion γ2curves in the opposite direction opposite to the direction in which the first portion γ1curves. Specifically, the base lift zone β is a circular arc surface similar to a cam shape of a lift area of, for example, the intake cam15.

The oscillating range of the swing cam45is varied when rotary contact position where the cam follower36rotary contacts the intake cam15is displaced in the early or late injection direction of the intake cam15. When the oscillating range of the swing cam45is varied, the region of the cam surface49with which the roller member30comes in contact is varied. More specifically, it is intended that the ratio of the base circular zone α and the lift zone β where the roller member30comes and goes is varied while the phase of the intake cam15is shifted to the early injection direction or late injection direction.

To the adjusting unit80, a structure to support the end of the inserted control arm72by a screw member82is adopted as shown in, for example,FIGS. 2to4. Specifically, the screw member82is screwed from a point that is opposite to through hole73in the shaft portion11cin such a manner as to freely advance and retreat. That is, the screw member82is screwed from upper peripheral wall of the shaft portion11c. The insertion end of the screw member82impinges against the end of the control arm72halfway in the passage73and supports the control arm72.

As a consequence, operating to rotate the screw member81varies the protrusion rate of the rod portion74protruding from the shaft member11c. The volume of the protruding portion of the rod portion74is varied. When the protrusion rate of the rod portion74is varied, the rotary contact position of the cam follower36with which the intake cam15comes in contact is varied. On the basis of the changes of the rotary contact position of the cam follower36with which the intake cam15comes in contact with, valve opening time and the valve closing time of the intake valve5are adjusted.

Reference numeral83denotes, for example, a cruciform groove formed on the top end surface of the screw member82to operate to rotate the screw member82. Reference numeral84denotes a lock nut driven into the end of the screw member82. Reference numeral84adenotes a notch which forms a bearing surface of the lock nut84.

With reference toFIGS. 5 to 8, discussion will be made on the operation of the variable valve apparatus20obtained by the configuration described above. Now, assume that the camshaft10is rotated by the operation of an engine as shown in the arrow mark direction ofFIG. 2.

In this case, the cam follower36of the center rocker arm35contacts the intake cam15and is tracer-driven by the cam profile of the cam15. By this, the center rocker arm35oscillates in the vertical direction with the pin42set as the oscillating fulcrum.

The receiving surface53aof the swing cam45is transmitted the oscillation displacement of the center rocker arm35through the gradient surface40. Now, since the receiving surface53aand the gradient surface40are slidable, the swing cam45repeats oscillating movement of being pressed up or lowered by the gradient surface40while sliding on the gradient surface40. Oscillation of the swing cam45allows the cam surface49to reciprocate in the vertical direction.

Because, in this case, the cam surface49is rotatably in contact with the roller member30of the rocker arm25, the roller member30is periodically pressed by the cam surface49. The rocker arm25oscillates when pressure is applied thereto, and opens or closes the pair of intake valves5, with the rocker shaft11as a support point.

Now, assume that the engine is operated at a high speed by operation of an accelerator pedal. After the motor43as a actuator receives acceleration signal, the motor43rotates the rocker shaft11and rotates the control arm72to the spot where, for example, the maximum valve lift volume is secured, for example, where the control arm72achieves the vertical posture as shown inFIGS. 5 and 6.

Then, the center rocker arm35displaces along the rotating direction on the intake cam15in response to the rotation of the control arm72. As a consequence, the position where the center rocker arm35comes in rotary contact with the intake cam15is deviated in the early or late injection direction on the intake cam15. Therefore the cam face49of the swing cam45fixed to the position where the cam surface49of the swing cam45achieves an angle close to perpendicularity as shown inFIGS. 5 and 6.

By the posture of the cam surface49, a region where the roller member30of the cam surface49comes and goes as shown inFIGS. 5 and 6is set to a region which brings the maximum valve lift volume, that is, to the shortest base circular zone α and the longest lift zone β. That is, the rocker arm25is driven by the cam surface portion made by the narrow base circular zone α and the longest lift zone β. Consequently, the intake valve5is opened and closed at the maximum valve lift volume as shown in the graph of A1of, for example,FIG. 9, and further, at an opening and closing timing that follows the intake stroke.

In addition, when low and medium rotating operations are carried out, the drive of the control motor43rotates the rocker shaft11in the direction in which the pin42close to the intake cam15as shown inFIGS. 7 and 8. Then, in response to the rotation of the rocker shaft11, the center rocker arm35moves on the intake cam15to the front side of the rotating direction. As a result, the rotary contact position between the center rocker arm35and the intake cam15is deviated in the early injection direction on the intake cam15as shown inFIGS. 7 and 8. By the change of this rotary contact position, the valve opening time of the cam phase is quickened. In addition, the gradient surface40slides from the initial position to the early injection direction on the receiving surface53ain response to the shift of the center rocker arm35.

By the shift of the center rocker arm35in this case, the swing cam45changes the posture to have the cam surface49tilted to the down side as shown inFIGS. 7 and 8. As the gradient increases, the region of the cam surface49in which the roller member30comes and goes is changed to a region in which the base circular zone α gradually increases and the lift zone β gradually decreases.

As the cam profile of the varied cam surface49is being transmitted to the roller member30, the rocker arm25is oscillatably driven while the valve opening time is quickened.

Accordingly, the intake valve5is controlled from the maximum valve lift volume A1shown in, for example,FIG. 9to the minimum valve lift volume A6at the spot where the control arm72is tilted to the maximum. That is, the intake valve5holds the timing to open the valve substantially same at the maximum valve lift period from the high rotating operation to low rotating operation of the engine. The valve lift volume is continuously varied with varying the valve-close timing greatly while being the low valve lift volume. Needless to say, the engine100is 4 cylinders engine, and the rocker shaft11, that is, control shaft is used in common among cylinders. Thus, this kind of variation of characteristics of the intake valve5takes place in all the cylinders1a.

For the rocker shaft11and the center rocker arm35that vary valve phases as described above, contrivance is made to reduce burdens of the load working onto these components.

To this contrivance, a technique is employed in which, as shown inFIG. 10, at the high valve lift and high speed operation of the engine, the maximum load at the time of opening the valve, and the maximum load at the time of closing the valve are made to work alternately on the rocker shaft11and the oscillating fulcrum S1, that is, oscillation center of the center rocker arm35. Note that A1and around it inFIG. 9show the characteristics of the engine which is in the high valve lift and high speed operation.

For this technique, a structure is employed in which, at the high valve lift and high speed operation as shown inFIG. 6, a direction of the load, that is, direction of α3working onto the oscillating fulcrum S1of the center rocker arm35near the maximum lift is arranged substantially in parallel with a line L3which connects the oscillating fulcrum S1and a center S2of the rocker shaft11, that is, control shaft.

Note that α3inFIG. 6is a load that works onto the oscillating fulcrum S1of the center rocker arm35at the moment of the maximum lift. α3is the resultant force of a load α1occurring in a normal direction L1of the surface where the intake cam15and the center rocker arm35contact and a load β1occurring in a normal direction of the surface where contacts the center rocker arm35and the swing cam45.

The direction and the size of the load α3change continuously while the swing cam45oscillates. At the high valve lift and high speed operation as shown inFIG. 10, the load occurring in the oscillating fulcrum, that is, trace of the load α3inFIG. 6changes from Q1to Q2when the swing cam45oscillates in association with the lift of the intake cam15.

When the swing cam45rotates in the valve opening direction, the maximum load works on the oscillating fulcrum S1from the rotation center S2of the rocker shaft11to one side, that is, right side as shown by the trace Q1.

When the swing cam45rotates in the valve closing direction a load works on the oscillating fulcrum S1from a rotation center S2of the rocker shaft11to the other side, that is, left side as shown by the trace Q2.

The oscillating fulcrum S1of the center rocker arm35and the rotation center S2of the rocker shaft11are arranged at the area between the direction T1of a component rotating the rocker shaft11of a maximum load P1brought by the trace Q1, that is, maximum load occurring in the oscillating fulcrum S1when the swing cam45rotates in the valve opening direction, and the direction T2of a component rotating the rocker shaft11of a maximum load P2brought by the trace Q2, that is, a maximum load occurring in the oscillating fulcrum S1, namely, an alternate area R shown inFIG. 10where the load directions become reverse alternately to work alternately onto the oscillating fulcrum S1at the high valve lift and high speed operation.

By this arrangement, not the resultant force of the load in the valve opening direction and the load in the valve closing direction, but one of the loads works onto the rocker shaft11alternately at the high valve lift and high speed operation. With the structure, a counterclockwise torque is made to occur in the rocker shaft11when the swing cam45rotates in the valve opening direction, a clockwise torque is made to occur in the rocker shaft11when the swing cam45rotates in the valve closing direction. Note that, the counter clockwise is assumed positive. Clockwise is assumed negative.

The oscillating fulcrum S1and the rotation center S2are arranged at a position where the load in the T1direction where the maximum torque in the counterclockwise direction is generated to the rocker shaft11, and the load in the T2direction where the maximum torque in the clockwise direction is generated to the rocker shaft11are substantially same such that the torques work to the rocker shaft11become substantially equal in the clockwise direction and the counterclockwise direction at the high valve lift and high speed operation.

Further, in order that positive and negative torques per cylinder occurring in the rocker shaft11should be offset on the common rocker shaft11, the oscillating fulcrum S1of the center rocker arm35at the high valve lift and high speed operation is arranged in a position where the direction of the torque of the rocker shaft11occurring when the swing cam45of the next cylinder oscillates in the valve opening direction becomes reverse with respect to the torque of the rocker shaft11occurring when the swing cam45of the current cylinder oscillates in the valve closing direction.

Further, as shown inFIG. 11, at the high valve lift and high speed operation, the oscillating fulcrum S1of the center rocker arm35is arranged such that the valve of the next cylinder opens, that is, next cylinder starts the valve opening operation earlier than the timing to generate the maximum load which occurs in the oscillating fulcrum S1of the current cylinder when the swing cam45oscillates in the valve closing direction, or the valve of the current cylinder closes, that is, current cylinder ends the valve closing operation later than the timing to generate the maximum load which occurs when the swing cam45of the next cylinder oscillates in the valve opening direction.

At the high valve lift and high speed operation in which valve characteristics near A1and A2inFIG. 9, the position of the rotation center S2of the rocker shaft11to the oscillating fulcrum S1of the center rocker arm35is arranged so that the component rotating the rocker shaft of the maximum lad P1occurring at the time of opening the valve and the component rotating the rocker shaft11of the maximum load P2occurring at the time of closing the valve are substantially offset.

Consequently, the maximum load in the direction T1occurring on the rocker shaft11at the time of opening the valve and the maximum load in the direction T2occurring on the rocker shaft11at the time of closing the valve can be set small, and as a result, the torque working onto the rocker shaft11can be set small.

Therefore, only by a simple arrangement and structure of the oscillating fulcrum S1of the center rocker arm35and the rotation center S2of the rocker shaft11, it is possible to suppress the deformation under torsion of the rocker arm11caused by an excessive load. As a consequence, it is possible to reproduce the set valve characteristics, and to improve the engine output and improve the fuel consumption.

Further, because the burdens given to the oscillating fulcrum S1of the center rocker arm35and the rocker shaft11, that is, control shaft are suppressed, it is needless to use highly rigid members or components to the oscillating fulcrum S1and the rocker shaft11, and it is possible to make compact the peripheral area of the shaft11including the rocker shaft11.

Furthermore, the actuator for rotating the rocker shaft11, herein, the control motor43have only to be a motor capable of generating a torque enough to overcome the larger torque component of the loads P1, P2, and it is possible to attain the purpose with a small motor.

Moreover, the maximum loads P1, P2work as loads of bending, etc. to the rocker shaft11and the supporting mechanism70in particular, the control arm72. However, the position of the rotation center S2of the rocker shaft11is arranged so that the component rotating the rocker shaft11of the maximum load P1occurring at the time of opening the valve and the component rotating the rocker shaft11of the maximum load P2occurring at the time of closing the valve are substantially offset. Thereby, it is possible to make the cross sectional shape of the rocker shaft11substantially symmetrical to L3connecting S1and S2.

Consequently, cross sectional shape of the rocker shaft11can be compact by making the most suitable shape adapted for both of the maximum loads P1, P2. Moreover, also with regard to the control arm72in the same manner, the bending load can be set minimum, so that it is possible to prevent the lift changes owing to deflection and fretting wear of the holding portions, and perform a compact design.

In particular, a structure is employed in which the variable valve apparatus20is driven per cylinder by use of the common rocker shaft11, that is, control shaft. In this case, as shown inFIG. 10, the oscillating fulcrum S1of the center rocker arm35is arranged in the position where the direction of the torque of the rocker shaft11occurring when the swing cam45of the next cylinder oscillates in the valve opening direction becomes reverse with respect to the torque of the rocker shaft11occurring when the swing cam45of the current cylinder oscillates in the valve closing direction.

For this reason, as shown inFIG. 11, with regard to the positive and negative torques, that is, clockwise and counterclockwise torques occurring in the rocker shaft11, the torque for the previous cylinder shown in a thin line as shown inFIG. 11, the torque for the current cylinder shown in a broken line, and the torque of the next cylinder shown in a thin broken line are offset mutually.

Therefore, with regard to the torque on the rocker shaft11, there occurs only a torque whose torque peak is low as shown in the torque shown in a thick line inFIG. 11, and whose average torque value is small. Accordingly, the burdens to the rocker shaft11, that is, burdens to the control shaft and the control motor43are small even in a multicylinder engine.

Moreover, as shown inFIG. 11, the valve of the next cylinder is set in order to close earlier than the timing to generate the maximum load of the rocker shaft11occurring when the valve of the current cylinder closes, or the valve of the current cylinder is set in order to close the valve later than the timing to generate the maximum load occurring when the valve of the next cylinder opens. Consequently, it is possible to effectively reduce the maximum values of respective loads such as the maximum load for the previous cylinder, the maximum load for the current cylinder, and the maximum load for the next cylinder, and thus, the burdens to the rocker shaft11, that is, control shaft and the control motor43can be made further smaller.

A load direction which works onto the oscillating fulcrum S1of the center rocker arm35when the intake valve5is near its maximum lift, and a line which connects the oscillating fulcrum S1of the center rocker arm35and the rotation center S2of the rocker shaft11are substantially in parallel with each other. In this structure, at the high valve lift and high speed operation, the rotation center of the rocker shaft11and the oscillating fulcrum S1of the center rocker arm35are easily arranged between a direction of a component rotating the rocker shaft lit of a load which occurs in the oscillating fulcrum S1of the center rocker arm35when the oscillating cam45oscillates in a valve opening direction and a direction of a component rotating the rocker shaft11of a load opposite thereto which occurs when the oscillating cam45oscillates in a valve closing direction.

Now, with reference toFIGS. 12 and 13, a variable valve apparatus according to a second embodiment of the present invention will be described. Note that the configurations having the same functions as those in the first embodiment are denoted by the same reference numerals and the description thereof is not repeated.

In the present embodiment, it is difference that the variable valve apparatus20is provided at the exhaust side. Other structures may be the same as those in the first embodiment. The difference will be described in detail.

FIG. 12is a plan view of a cylinder head1mounted the variable valve apparatus20according to this embodiment.FIG. 13is a cross sectional view taken along line B—B shown inFIG. 12the cylinder head1.

As shown inFIGS. 12 and 13, rocker shaft12of the exhaust side is provided the variable valve apparatus20per the pair of the exhaust cam16, that is, the pair of the exhaust valve6. The a rocker arm18afor the intake is rotatably supported by the rocker shaft11of the intake valve15per intake cam15, that is intake valve15. The present embodiment can also provides the same advantageous effects as those provided by the first embodiment.

Note that the present invention is not limited to the first and second embodiments described above, and the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, in the above embodiment, the structure is employed in which the rocker shaft at the intake side is used also as the control shaft. However, a structure may be made in which a control shaft is employed separately.

Furthermore, in the first and second embodiments, the present invention is applied to an engine of an SOHC type valve operating system. A structure where the intake valve and the exhaust valve are driven by one camshaft is used for the SOHC type valve operating system. However, the present invention is not limited thereto, and the present invention may be applied to an engine of a Double Overhead Camshaft (DOHC) type valve operating system. A structure having a camshaft exclusive for the intake side and another camshaft exclusive for the exhaust side is used for the DOHC type valve operating system.