Patent Application
20180179921
The disclosure of Japanese Patent Application No. 2016-250730 filed on Dec. 26, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
The disclosure relates to a variable valve mechanism that is used in a valve actuating system of an engine and, more particularly, to a cam-changing variable valve mechanism configured to select any one of a plurality of cams by sliding a cam unit, fitted around a camshaft, in an axial direction (hereinafter, also referred to as cam axial direction).
Conventionally, there is known a cam-changing variable valve mechanism as a variable valve mechanism that is able to change the lift characteristic of each intake valve of an engine, as described in, for example, Japanese Patent Application Publication No. 2010-520395 (JP 2010-520395 A). In the cam-changing variable valve mechanism, a cam carrier (cam unit) including a plurality of cams is fitted around an intake camshaft. The cam-changing variable valve mechanism is configured to select any one of the cams by sliding the cam carrier in the axial direction. In this example, two intake valves are provided for each cylinder of the engine, and each intake valve is driven by the selected one of the cams via a corresponding rocker arm.
That is, the cam carrier for each cylinder, fitted around the intake camshaft, includes the plurality of cams having mutually different heights in correspondence with each of the two intake valves. When the cam unit is caused to slide in the cam axial direction, any one of the cams presses the corresponding rocker arm. In addition, a spiral guide groove is provided on the outer periphery of the cam carrier. When a shift pin is engaged with the guide groove from the outer side, the cam carrier slides in the cam axial direction while rotating with the rotation of the camshaft.
The structure of such a valve actuating system will be described with reference to
When the rocker arms 15 that rock in that way are viewed from above, the rocker arms 15 are ordinarily arranged substantially parallel to the corresponding cams 41, that is, perpendicular to the cam axial direction (axis X). However, actually, due to manufacturing tolerances, or the like, the cams 41 (indicated by the imaginary lines) can be slightly inclined with respect to the corresponding rocker arms 15 (the inclination angle is denoted by θ in the drawing) as exaggeratedly shown in
That is, when each cam 41 presses the corresponding rocker arm 15, the cam 41 receives reaction force from a valve spring 18 via the rocker arm 15. However, when the rocker arm 15 and the cam 41 are inclined with respect to each other as described above, the valve spring reaction force that acts on the cam 41 and, by extension, the cam unit 4, via the rocker arm 15 includes a component in the axis X direction. Therefore, an unexpected slide of the cam unit 4 can occur.
The disclosure reduces occurrence of an unexpected slide of a cam unit due to reaction force from a valve spring in a variable valve mechanism configured to change the lift characteristic of a valve by sliding the cam unit.
In an aspect of the disclosure, for example, when two intake valves are provided for each cylinder, valve spring reaction forces of the two intake valves act on the corresponding cam unit in opposite directions along the cam axial direction, thus cancelling out sliding forces. Specifically, the aspect of the disclosure provides a variable valve mechanism mounted on an engine. The variable valve mechanism includes a cam unit and rocker arms. The cam unit is fitted around a camshaft. The cam unit includes two sets of a plurality of cams. Any one of the plurality of cams is selected by sliding the cam unit in an axial direction. The engine may be a multi-cylinder engine.
Two intake valves or two exhaust valves or both are provided for each cylinder. Each of the two intake valves or two exhaust valves or both is configured to be driven by the selected one of the cams via a corresponding one of the rocker arms. Each rocker aim includes a support portion rockably supported by a cylinder head of the engine, and a pressing portion configured to press a stem of a corresponding one of the valves. The support portion of any one of the two rocker arms for each cylinder deviates to one side in the axial direction with respect to the corresponding pressing portion. The support portion of the other one of the rocker arms deviates to the other side in the axial direction with respect to the corresponding pressing portion.
With the thus configured variable valve mechanism, when the cylinder is viewed from above the cylinder head, the cams are slightly inclined with respect to the corresponding rocker arms due to manufacturing tolerances, so, as described with reference to
However, with the above configuration, the two rocker arms for each cylinder are intentionally not arranged perpendicularly to the cam axial direction but slightly inclined with respect to the cam axial direction, and the orientations of the inclined two rocker aims are opposite to each other. That is, as described above, the support portion of any one of the rocker arms deviates to one side in the cam axial direction with respect to the pressing portion, and the support portion of the other one of the rocker arms deviates to the other side in the cam axial direction with respect to the pressing portion (see
With such inclined arrangement of the two rocker arms, forces respectively act on the two cams for each cylinder from the rocker arms in opposite directions along the cam axial direction. That is, a force from one of the rocker arms is headed toward one side in the cam axial direction, and a force from the other one of the rocker arms is headed toward the other side in the cam axial direction. Forces in the cam axial direction, which respectively act on the two cams for each cylinder due to valve spring reaction forces, cancel out each other in this way, so it is possible to suppress a slide of each cam unit.
A structure for inclining the two rocker arms for each cylinder in mutually opposite directions may be as follows. When the cylinder head includes, for each cylinder, mounting holes for mounting lash adjusters that respectively support the two rocker arms and insertion holes through which stems of the two valves are inserted, a distance between centers of the two mounting holes may be longer than a distance between centers of the two insertion holes.
That is, generally, the layout of the two valves for each cylinder of the engine is determined on the basis of the configuration of combustion chambers. Thus, the layout of the insertion holes for the stems of the valves is also determined. For this reason, when the distance between the mounting holes for the two lash adjusters is set so as to be longer than the distance between the two insertion holes, determined in this way, as described above, it becomes easy to avoid interference between the mounting holes and intake ports, and the flexibility of the shape and layout of the intake ports increases.
In order to suppress a drag of each cam under the friction resistance between the cam and the corresponding rocker arm as described above, a relatively small-diameter section may be formed in at least part of an angular range corresponding to the exhaust stroke of each cylinder in a base circle section of the cam. With this configuration, the friction resistance between the cam and the corresponding rocker arm reduces in the small-diameter section, so a drag of each cam is suppressed. In the exhaust stroke of a cylinder, even when the degree of sealing of the valve decreases in the small-diameter section, no inconvenience occurs.
According to the aspect of the disclosure, in the variable valve mechanism for an engine, configured to change the lift characteristic of each valve by sliding the cam unit, when two intake valves or two exhaust valves or both are provided for each cylinder, valve spring reaction forces are caused to act on the cam unit in opposite directions along the cam axial direction by arranging the corresponding rocker arms such that the rocker arms are inclined in opposite directions. Thus, it is possible to suppress occurrence of an unexpected slide of the cam unit due to valve spring reaction forces.
Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
Hereinafter, an embodiment in which the disclosure is applied to a valve actuating system for an engine will be described. The engine 1 according to the present embodiment is, for example, an in-line four-cylinder gasoline engine 1. As schematically shown in
As shown from above in
A variable valve timing (VVT) 14 is provided at the front end (left end in
For example, the first cylinder 3 (#1) is shown in
The base circles of these low-lift cam 41 and high-lift cam 42 have the same diameter, and are formed into mutually continuous circular arc faces.
That is, each intake valve 10 is a common poppet valve. The retainer 17 is provided at the upper portion of the stem 10a, and receives upward pressing force from a valve spring 18. Thus, as indicated by the continuous lines in
As shown in
In the present embodiment, the cam that lifts the intake valve 10 via the rocker arm 15 as described above is set to any one of the low-lift cam 41 and the high-lift cam 42. That is, as shown in
As shown only in
In order to cause the cam units 4 to slide in that way, a guide groove 44 is provided at the outer periphery of the sleeve 43. A shift pin 51 is engaged with the guide groove 44 as will be described below. In the present embodiment, as shown in
An actuator 5 is arranged above the intake camshaft 12 in correspondence with each of the cylinders 3 and is supported by the cylinder head 2 via, for example, a stay 52 so that each shift pin 51 can be engaged with a corresponding one of the guide grooves 44. The stay 52 extends in the axis X direction. Each actuator 5 is configured to actuate a corresponding one of the shift pins 51 back and forth with the use of an electromagnetic solenoid. When the actuator 5 is in an on state, the shift pin 51 extends and engages with the guide groove 44.
For example, when the thus extended shift pin 51 is engaged with the guide groove 44, the shift pin 51 relatively moves in the circumferential direction on the outer periphery of the cam unit 4 and also moves in the axis X direction along the guide groove 44 (that is, obliquely) with the rotation of the intake camshaft 12, as will be described below additionally with reference to
More specifically, initially, as shown in
When the actuator 5 operates to cause the shift pin 51 to extend in this state, the shift pin 51 is engaged with the straight groove portion 44a located at one side of the guide groove 44 as shown in the top view of
Thus, actually, the shift pin 51 presses the cam unit 4 toward one side in the axis X direction to cause the cam unit 4 to slide, and switches the cam unit 4 into the position in which the high-lift cam 42 is selected (high-lift position). At this time, as shown in the bottom view of
When the cam unit 4 is switched into the high-lift position as described above, the straight groove portion at the other side of the guide groove in the axis X direction, provided in the cam unit 4 for the second cylinder (#2), faces the shift pin 51 of the corresponding actuator 5 although not shown in the drawing. Then, by turning on the actuator 5 to cause the shift pin 51 to engage with the guide groove, it is possible to cause the cam unit 4 to slide to the other side in the axis X direction with the rotation of the intake camshaft 12 and move the cam unit 4 to the low-lift position similarly.
In the present embodiment, a lock mechanism 6 is provided between each cam unit 4 and the intake camshaft 12. The lock mechanism 6 is used to hold the position of the cam unit 4 (the low-lift position or the high-lift position) at the time when the cams 41, 42 have been changed as described above. That is, as shown in
Two lock balls 61 are retractably arranged at the outer periphery of the intake camshaft 12 so as to be fitted to the annular groove 43a or the annular groove 43b when the cam unit 4 is in the low-lift position or the high-lift position. That is, in the present embodiment, a through-hole 12a extends through the intake camshaft 12 and opens at two locations on the outer periphery of the intake camshaft 12. The through-hole 12a has a circular cross section. The through-hole 12a accommodates the two lock balls 61 and a coil spring 62 inside.
Those two lock balls 61 are respectively arranged on both ends of the coil spring 62, and are urged by the spring force of the coil spring 62 so as to be pushed outward from openings at both ends of the through-hole 12a. Thus, when the cam unit 4 is in the low-lift position (the right-side position in
On the other hand, when the cam unit 4 is in the high-lift position (the left-side position in
At this time, as the cam unit 4 slides, the lock balls 61 are initially pushed by the annular protrusion 43c, move against the spring force of the coil spring 62, and leave the annular groove 43a. After climbing over the annular protrusion 43c, the lock balls 61 are fitted into the annular groove 43b under the spring force of the coil spring 62. This also applies to the case where the cam unit 4 slides from the high-lift position to the low-lift position.
Incidentally, with the structure that each cam unit 4 is slidably fitted around the intake camshaft 12 as in the case of the above-described cam changing mechanism, each cam unit 4 can slide due to reaction force from the valve springs 18 of the intake valves 10. That is, initially, as described with reference to
The rocker arm 15 that rocks in that way is arranged so as to be parallel to the cam indicated by the imaginary line (the low-lift cam 41 in
If there is a misalignment with the cam 41 in this way, an unexpected slide of the cam unit 4 can occur under reaction force from the valve spring 18, which acts on the cam 41 via the rocker arm 15 and, by extension, the cam unit 4. That is, when the cam 41 rotates to rock the rocker arm 15 as described above, the cam 41 receives reaction force from the valve spring 18 via the rocker arm 15.
At this time, when the rocker arm 15 and the cam 41 are inclined with respect to each other as described above, the cam 41 is dragged in the direction of the axis X by the friction resistance between the rocker arm 15 and the cam 41 (in the present embodiment, the rolling resistance between the cam 41 and the roller 15a). In other words, the valve spring reaction force that acts on the cam 41 and, by extension, the cam unit 4, via the rocker arm 15 includes a component in the axis X direction. Thus, sliding force is added to the cam unit 4.
The magnitude of sliding force that is added to the cam unit 4 may be regarded as being proportional to the magnitude of friction resistance, so the sliding force increases as the reaction force from the valve spring 18 increases. The sliding amount may be expressed by (Perimeter of Cam 41)×tan θ by using the inclination angle θ between the rocker arm 15 and the cam 41. The sliding amount increases as the inclination angle θ increases.
In the present embodiment, since the cams 41 corresponding to the two intake valves 10 in the cam unit 4 for each cylinder 3 are ground as one at the same time (this also applies to the cams 42), inclination with respect to the rocker arm 15 similarly occurs, and the direction of the drag at each of the two rocker arms 15 is the same. For this reason, sliding force that acts on the cam 41 and, by extension, the cam unit 4, tends to increase. If the sliding force overcomes the holding force of the lock mechanism 6, an unexpected slide of the cam unit 4 occurs.
In contrast, for example, it is also conceivable that the spring constant of the coil spring 62 of the lock mechanism 6 is increased or the annular grooves 43a, 43b into which the lock balls 61 are fitted are deepened. However, this increases resistance at the time of causing the cam unit 4 to slide in order to change the cams 41, 42, with the result that an engine rotation speed that is an upper limit for changing the cams 41, 42 decreases. In addition, the coil spring 62 is used in a high-stress state, so there is a concern that the durability of the coil spring 62 decreases.
In consideration of such a situation, in the present embodiment, arrangement of the two rocker arms 15 for each cylinder 3 is devised such that reaction force that acts on the cam unit 4 from the valve spring 18 and reaction force that acts on the cam unit 4 from the other valve spring 18 are set in the opposite directions along the axis X direction. With this configuration, since sliding forces that act on the cam unit 4 via the corresponding two rocker arms 15 are cancelled, an unexpected slide of the cam unit 4 is suppressed.
Specifically, as shown in
With this configuration, the valve spring reaction force that is input to the distal end portion 15c of the one of the rocker arms 15 and that acts on the cam 41 or the cam 42 (not shown in
In order to lay out the two rocker arms 15 in that way, in the present embodiment, when the cylinder head 2 is viewed from above as shown in
In
Generally, in the engine 1 as described in the present embodiment, the layout of the two intake valves 10 for each cylinder 3 is determined on the basis of the configuration of a corresponding combustion chamber, with the result that the distance D2 between the two valve insertion holes 2b is determined. If the distance D1 between the two adjuster mounting holes 2a is increased with respect to the distance D2, interference between the adjuster mounting holes 2a and the intake ports (not shown in
In the above-described engine 1 according to the present embodiment, in the case where the cam changing mechanism that changes the two cams 41, 42 by sliding the cam unit 4 mounted on the intake camshaft 12 is provided, when the rocker arms 15 corresponding to the two intake valves 10 for each cylinder 3 are arranged so as to be inclined in opposite directions, reaction force that acts on the cam unit 4 from the valve spring 18 and reaction force that acts on the cam unit 4 from the other valve spring 18 act in opposite directions along the axis X direction and cancel out each other. Thus, it is possible to suppress an unexpected slide of the cam unit 4 due to valve spring reaction force.
The configuration of the disclosure is not limited to those described in the above embodiment. The embodiment is only illustrative, and the application, and the like, of the configuration of the disclosure are, of course, not limited. For example, in the embodiment, the low-lift cam 41 and the high-lift cam 42 are provided in the cam unit 4 for each intake valve 10, and the lift characteristic is switched in high and low two steps; however, the disclosure is not limited to this configuration. For example, the lift characteristic may be switched in three steps.
In the embodiment, the cam units 4 for the first and second cylinders 3 (#1, #2) are integrally coupled to each other by the sleeve 43, and, similarly, the cam units 4 for the third and fourth cylinders 3 (#3, #4) are also integrally coupled to each other; however, the disclosure is not limited to this configuration. The cam units 4 for the first to fourth cylinders 3 (#1 to #4) may be configured to slide independently of one another. In this case, each guide groove 44 may have various known shapes, such as a Y-shaped guide groove described in JP 2010-520395 A.
In the embodiment, in order to cancel out valve spring reaction forces that act on the cam unit 4 via the two rocker arms 15 for each cylinder 3 in the axis X direction, those two rocker arms 15 are inclined in opposite directions and are arranged so as to form a divergent shape in
In order to suppress a drag of the cam 41 or cam 42 under the friction resistance between the cam 41 or cam 42 and the rocker arm 15, it is effective to devise the cam profile. That is, as shown in
With this configuration, the friction resistance with the rocker arm 15 reduces in the small-diameter section, and a drag of the cam 41 or cam 42 is suppressed, so an unexpected slide of the cam unit 4 is difficult to occur. In an exhaust stroke, even when the degree of sealing of the intake valve 10 decreases in the small-diameter section, no inconvenience occurs. In
Furthermore, in the embodiment, the example in which the cam changing mechanism is provided at the intake side in the valve actuating system of the engine 1 is described. Instead, the cam changing mechanism may be provided at the exhaust side or may be provided at both sides. The engine 1 is not limited to an in-line four-cylinder engine. The engine 1 may be an in-line two-cylinder, three-cylinder, five-cylinder or more. The disclosure is applicable to not only an in-line engine but also various cylinder arrangement engine, such as a V-engine.
The disclosure is able to suppress an unexpected slide of a cam unit due to reaction force from a valve spring in a cam-changing variable valve mechanism provided in a valve actuating system of an engine, and is highly effective when applied to, for example, an engine mounted on an automobile.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-250730 | Dec 2016 | JP | national |
