Valve drive system for an internal combustion engine

Abstract: A valve drive system for an internal combustion engine in which a plurality of intake or exhaust valves are disposed radially in a cylinder: Said intake or exhaust valves are each driven by a respective rocker arm having a boss supported for rotation on a cylinder head through a rocker pin, and an intake or exhaust camshaft formed with three-dimensional cams for engaging the rocking ends of the rocker arms in sliding relation. The boss of said rocker arm is coupled to said rocker pin for tilting movement in the direction perpendicular to the axis of the rocker pin.

Description: This invention relates to a valve drive system for an internal combustion engine comprising at least one cylinder having a plurality of intake or exhaust valves, said intake or exhaust valves are each driven by a respective rocker arm having a boss supported for rotation on a cylinder head through a rocker pin, and an intake or exhaust camshaft formed with three-dimensional cams for engaging the rocking ends of the rocker arms in sliding relation. A system has been known having, for example, four radially disposed intake and exhaust valves for each cylinder, as an engine formed with a semi-spherical combustion chamber to improve combustion efficiency. In the valve drive system used for this type of engine, since the opening/closing directions of the intake and exhaust valves are different for each valve when viewed from the direction perpendicular to the axes of the cam shafts, a complicated construction is adopted to transmit the rotation of the cam shafts to each valve (see, for example, JP-A-59-29709). The valve drive system disclosed in the patent publication is arranged such that one cam shaft is supported at the center of a cylinder head for rotation, the cam surface of the cam shaft is formed parallel with the axial direction of the cam shaft, and two rocker arms for each intake or exhaust valve are disposed between the cam surface and the intake or exhaust valve. A first rocker arm of one of the two rocker arms is supported for rocking movement on a first support shaft mounted parallel with the cam shaft. with one end engaged with the cam surface and the other end extending toward the intake or exhaust valve. The other second rocker arm is supported for rocking movement on a second support shaft mounted in the direction perpendicular to the axis of the intake or exhaust valve, with the underside of the rocking end in contact with the intake or exhaust valve. The opposite side (top surface) from the intake or exhaust valve at this rocking end is engaged with the other end of the first rocker arm. That is, the valve drive system is arranged such that parallel transfer of the cam surface is converted, by two rocker arms, to the parallel movement in the direction parallel with the axis of the intake or exhaust valve. However, the valve drive system described above has a drawback of higher manufacturing costs and larger size because of the number of the rocker arms being large. In order to realize the construction of one rocker arm for one intake or exhaust valve to achieve cost reduction and smaller size, it is contemplated that the system is arranged such that the cam surface is inclined perpendicular to the axial direction of the intake or exhaust valve to form the so-called three-dimensional cam and engages the second rocker arm in sliding relation. In implementation of this system, however, a problem is raised of lubrication of the contact portion between the three-dimensional cam and the rocker arm. The lubrication of the contact portion is achieved by oil film of lubricating oil formed between the cam surface of the three-dimensional cam and the sliding surface of the rocker arm. It is well known that the oil film is retained when the foregoing two components are in line contact with each other, but broken when they come into point contact. However, when the three-dimensional cam is manufactured as an industrial product, the contact state between the cam surface and the foregoing sliding surface tends to be in point contact due to manufacturing error of the cam surface, and breakage of the oil film causes wear of the sliding portion. Forming of a highly accurate three-dimensional cam surface requires very long grinding work hours, resulting in a significant cost increase. Accordingly, it is an objective of the present invention to provide a valve drive system as indicated above facilitating the use of three dimensional cams without a drop in lubrication ability and cost increase and effecting cost reduction and smaller size by decreasing the number of rocker arms compared with the conventional valve drive system. According to the present invention, this objective is solved for a valve drive system as indicated above in that the boss of said rocker arm is coupled to said rocker pin for tilting movement in the direction perpendicular to the axis of the rocker pin. According to this invention, the rocker arm is tiltable with respect to the rocker pin so as to follow the three-dimensional cam surface, so that the cam surface can be in line contact with the sliding surface of the rocker arm throughout its circumference. The patent publication US 5 445 117 shows a configuration in that a roller is in line contact with a cam surface. An engine valve drive system according to another embodiment of the invention is characterized by the engine valve drive system according to the foregoing invention, wherein the position at which said three-dimensional cam engages said rocker arm, is located closer to said rocker pin than the position at which the rocker arm engages the intake or exhaust valve. According to this invention, compared with the system in which the three-dimensional cam engages the rocker arm at the position corresponding to the intake or exhaust valve, the amount of lift of the three-dimensional cam can be set relatively low for the same opening degree of the intake or exhaust valve. Another embodiment of the invention is characterized by the valve drive system for engines according to the foregoing invention, wherein said rocker pin is inclined with respect to the axis of said cam shaft when viewed from the direction of the cylinder axis. According to this invention, the rocker arm is able to rock along the stroking direction of the intake or exhaust valve. Moreover, in the valve driving mechanism in which the rocker arms are rotatably supported with the rocker shafts, a constitution is generally employed in which the camshafts and the rocker shafts are disposed parallel to each other. On the other hand, in a valve driving mechanism for the internal combustion engine in which the intake and exhaust valves are disposed radially in three dimensions, it is impossible to dispose the camshafts and the rocker shafts parallel to each other. As a result, in order to drive the rocker arms on the intake and exhaust sides with one intake cam shaft on the intake side and one exhaust camshaft on the exhaust side, it is inevitable to use three-dimensional cams formed on the intake and exhaust camshafts. However, in case the three-dimensional cam is used as described above, in order that the three-dimensional cam and the rocker arm slipper are in line contact with each other, the contact portions of both components need to be precision-machined using special grinding machines, raising problems of longer processing time and higher processing cost. Therefore, to further enhance the line contact between the intake and exhaust cams and the rocker arms without requiring high precision machining so that friction and heat generation on the sliding surfaces of both components are restricted, the following is advantageous. To accomplish the above aspect, there is provided a valve driving mechanism for an internal combustion engine, wherein intake and exhaust valves are disposed radially, and rotation of intake and exhaust camshafts is converted through rocker arms into sliding movement of the intake and exhaust valves to open and close the intake and exhaust ports, characterised in that a slipper for the rocker arm is made as a separate component and supported for swinging freely. According to another embodiment of the invention, a holder which is separate from a cylinder head is attached to the cylinder head, with the holder being made to support the rocker arm. Further, it is possible that the intake and exhaust cams formed on the intake and exhaust camshafts are made in three-dimensional shapes. According to a further embodiment of the invention the rocker shafts for rotatably supporting the rocker arms are tilted in side view relative to the intake and exhaust camshafts. According to a still further embodiment of the invention, for supporting the intake and exhaust camshafts are disposed between a plural number of adjacent intake and exhaust valves. In addition, in the valve driving mechanism of the rocker arm type, when the rocker shafts are disposed between the intake and exhaust camshafts, conventionally the cylinder head has to be divided into two, upper and lower parts. However, when the cylinder head is divided into two, upper and lower parts, the number of parts increases, the constitution becomes complicated, and the number of assembly steps increases. In sports type, high revolution engines with a small angle between valve axes and a large angle between the intake and the exhaust passage axes, it is difficult to dispose rocker arms around the cylinder center. That is, in a constitution in which a common rocker shaft passes through a rocker shaft hole bored across multiple cylinders, the rocker shaft hole will end up in intersecting the plug hole. Furthermore, it is impossible to make by machining a long, small-diameter rocker shaft hole while maintaining a high precision of parallelism between the rocker shaft hole and the camshaft. On the other hand, when a constitution is employed in which the rocker shaft is disposed outside the camshaft, arrangement of the intake and exhaust passages inevitably becomes disadvantageous. Therefore, another aspect of the present invention is to provide a valve drive system for a multi-cylinder engine which makes it possible to employ an integral type of cylinder head while disposing rocker shafts between intake and exhaust camshafts, and to increase rigidity of supporting the rocker arms by supporting them within a compact arrangement. To accomplish the above-described aspect, there is provided a valve drive system for a multi-cylinder engine wherein for each cylinder, a holder as a separate component for supporting the rocker arms is attached below a surface of an integral type of cylinder head to which a head cover is attached. According to another embodiment of the invention, each holder is formed with a rocker shaft hole and a plug hole. According to a further embodiment of the invention, each holder is provided with four rocker shafts parallel to the intake and exhaust camshafts. According to a still further embodiment of the invention, each holder is made of an iron-based material. Further, it is possible that upper half portions of the intake and exhaust camshafts are supported with a common, integral type of bearing cap, with the bearing cap bridging attachment bosses on opposing intake and exhaust sides of a cylinder head. According to another embodiment of the invention, the axial centers of the intake and exhaust camshafts are disposed on the axial center lines of the intake and exhaust valves. According to a further embodiment of the invention, oil receiving ribs are formed on the inside surface of the head cover placed over the top of the cylinder head. Other preferred embodiments of the present invention are laid down in further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: Fig. 1 is a sectional view of an engine incorporating the valve drive system according to this invention; Fig. 2 is a plan view of the cylinder head; Fig. 3 is a perspective view showing the structure of the valve drive system of this invention; Fig. 4 are views showing the rocker arm; Fig. 5 is a sectional view of the boss of the rocker arm; Fig. 6 is an enlarged view of a portion of another embodiment of the valve drive system for engines of this invention; Fig. 7 shows a lateral cross-section of an upper portion (cylinder head portion) of an internal combustion engine provided with the valve driving mechanism according to the invention; Fig. 8 is a plan view of an internal combustion engine provided with the valve driving mechanism according to the invention, with its head cover removed; Fig. 9 is a side view showing the sliding contact state of the cam and the rocker arm of the valve driving mechanism according to the invention; Fig. 10 is a front view showing the sliding contact state of the cam and the rocker arm of the valve driving mechanism according to the invention (as seen in the direction of the arrow A in Fig. 3); Fig. 11 is an oblique view showing the sliding contact state of the cam and the rocker arm of the valve driving mechanism according to the invention; Fig. 12 shows a lateral cross-section of the upper part (cylinder head area) of a multi-cylinder engine provided with a valve driving mechanism of the invention (as seen along the line B-B in Fig. 2); Fig. 13 is a view as seen along the arrows A-A in Fig. 1; and Fig. 14 shows a cross-section as seen along the line C-C in Fig. 2. Now, an embodiment of the valve drive system for engines according to this invention will be described in detail with reference to Figs. 1-5. Fig. 1 is a sectional view of an engine incorporating the valve drive system according to the invention, and Fig. 2 is a plan view of the cylinder head, in which is shown the broken position of Fig. 1 by the line I-I. Fig. 3 is a perspective view showing the structure of the valve drive system according to the invention, and Fig. 4 are views showing the rocker arm, the figure (a) being a plan view, (b) a side view, and (c) a front view as seen from the slipper side. Fig. 5 is a sectional view of the boss of the rocker arm, taken along line V-V of Fig. 4(b). In these figures, numeral 1 designates a cylinder head of an engine according to this embodiment. The cylinder head 1 is for a water-cooled single cylinder DOHC type engine, and formed with an approximately semi-spherical combustion chamber 2, and two sets of an intake port 3 and exhaust port 4 connected to the combustion chamber 2. Between these ports 3, 4, that is, at the center of the combustion chamber 2 is attached an ignition plug (not shown). Two intake valves 5 for opening/closing the intake ports 3 and two exhaust valves 6 for opening/closing the exhaust ports 4 are disposed such that valve shafts 5a, 6a extend radially from the combustion chamber 2 when viewed from the axial direction of the cylinder, as shown in Fig. 2. These intake and exhaust valves 5, 6 are driven by a valve drive system 7 as described hereinafter. The axis of the cylinder is shown in Fig. 1 by the single dot and dash line C. The components through which valve stems 5a, 6a of the intake and exhaust valves 5, 6 pass, as indicated in Fig. 1 by numeral 8, are spring retainers for retaining valve springs (not shown) for biasing the intake and exhaust valves 5, 6 in the direction of valve closing. The spring retainer 8 is formed in a bottomed-cylindrical shape with a bottom (upper side in Fig. 1) penetrated by the intake or exhaust valve 5 or 6, and fitted for sliding movement in a retaining cylinder 9 fixed to the cylinder head 1. The valve spring is spring-loaded between the inner bottom of the spring retainer 8 and the cylinder head 1. The valve drive system 7 for driving the intake and exhaust valves 5, 6 comprises an intake cam shaft 11 and exhaust cam shaft 12, and rocker arms 14, one for each of the intake and exhaust valves, engaged by three-dimensional cams 13 of these cam shafts 11, 12. The intake cam shaft 11 and exhaust cam shaft 12 are provided with the three-dimensional cams 13 at positions corresponding to the intake and exhaust valves 5, 6, and supported for rotation on the cylinder head 1 by a well-known support structure. Cam caps journaling these cam shafts 11, 12 on the cylinder head are designated by numeral 15 in Figs. 1 and 2. These cam shafts 11, 12 are each arranged such that a timing chain sprocket 16 is fixed at one end (lower end in the figure), and the rotation of the crank shaft (not shown) is transmitted through the timing chain (not shown) stretched between the sprocket 16 and the crank shaft. Lubrication of the bearings for supporting both of the cam shafts 11, 12 for rotation and the sliding portions between the three-dimensional cams 13 and the rocker arms 14, is performed by supplying lubricating oil from lubricating oil passaged (not shown) formed in the cam shafts 11, 12 to the sliding portions. The three-dimensional cam 13, as shown in Figs. 2 and 3, is formed with a cam surface 13a inclined such that its diameter is decreased from one end toward the other end of the cam in the axial direction. The inclined angles of the cam surfaces 13a are set so as to correspond to the inclined angles of the valve stems 5a, 6a of the intake and exhaust valves 5, 6 with respect to the axes of the cam shafts 11, 12, such that the cam surfaces 13a in sliding engagement with the rocker arms 14 are parallel, at the contact portions, with the planes perpendicular to the axes to the intake and exhaust valves 5, 6. The rocker arm 14 is formed, as shown in Fig. 4, such that a cylindrical boss 17, and an arm 18 protruding in one direction from the boss 17, are molded integrally, and a slipper 19 engaged by the three-dimensional cam 13 of the cam shaft 11 or 12 is fixed to the arm 18. The rocker arm 14 is, as shown in Figs 1 and 2, fitted, at the boss 17, on a columnar rocker pin 20 of a constant diameter, and supported, for rotary movement, on the cylinder head 1 through the rocker pin 20. The boss 17 constitutes the base section of the rocker arm 14 of this invention. These rocker arms 14 and rocker pins 20 are inclined so as to correspond to the intake and exhaust valves 5, 6 inclining with respect to the axes of the cam shafts 11, 12. That is, like the cam surface 13a, they are inclined so as to be parallel with the planes perpendicular to the axes of the intake and exhaust valves 5, 6. Specifically, the rocker pins 20, as shown in Fig. 2, are inclined by angle α with respect to the axes of the cam shafts 11, 12 so as to correspond the inclination of the intake and exhaust valves 5. 6 when viewed from the axial direction of the cylinder. The angle α is set at approximately one degree for this embodiment. Similarly, the rocker pins 20 are inclined with respect to the axes of the cam shafts 11, 12 when viewed from the direction of the cam shafts 11, 12 (see Fig. 3). The rocker pin 20 for the intake valve 5 and the rocker pin 20 for the exhaust valve 6 on the left. hand side in Fig. 3, and the two rocker pins 20 on the other side, are inclined so as to assume an inverse straddle shape when viewed from the direction of the cam shafts 11, 12. This inclination of the rocker pins 20 allows the rocker arms 14 to rock along the stroking direction of the intake and exhaust valves 5, 6, so that the intake and exhaust valves 5, 6 can be disposed without unreasonable bending load. The rocker pin 20 is fixed to the cylinder head 1 such that one end of the rocker pin 20 on the side of cylinder axis C is fitted in a center projection 21 (see Fig. 2) formed integral with the cylinder head 1, and the other end, in a rocker pin holder 22. The rocker pin holder 22 is formed separate from the cylinder head 1 and fixed to the cylinder head 1 with fixing bolts 23. The arm 18 of the rocker arm 14 is formed, at the tip, with an integral pushing projection 18a for engaging an end cap 24 attached to the valve stem end of the intake or exhaust valve 5 or 6, as shown in Fig. 1, and on the opposite side (upper side in Fig. 1) of the arm 18 from the pushing projection 18a is mounted fixedly the slipper 19. The slipper 19 is formed in the shape with a quadratic surface such that it is convexed on the cam shaft side and extends in the axial direction of the cam shafts 11, 12. In the embodiment, the length and the mounting position of the rocker arm 14, and the mounting position of the cam shaft 11 or 12 are set such that the distance R1 from the contact point between the pushing projection 18a and the end cap 24 to the rotation center (axial center of the rocker pin 20) is larger than the distance R2 from the rotation center to the contact point between the slipper 19 and the three-dimensional cam 13. A pin hole 25 in which the rocker pin 20 is fitted at the boss 17 of the rocker arm 14 is configured, as shown in Fig. 5, such that the inside diameter is constant in the axially central portion and increased gradually from the central portion toward the open end. The central portion with a constant diameter is shown in Fig. 5 by numeral 21a and the portions of tapered hole with gradually changing diameter are shown by numeral 21b. The wall surface inclination angle  is set, for example, at approximately 0.5-2 degrees. Thus, taper forming of the opening side of the pin hole 25 allows the rocker arm 14 to be tilted in the direction perpendicular to the axis of the rocker pin 20, with the rocker pin 20 fitted in the pin hole 25. Further, the rocker arm 14 is formed with a plurality of projections 26 on the axial end face of the boss 17, as shown in Fig. 4. These projections 26 are formed so as to be in contact with the end face of the center projection 21 of the cylinder head 1 and the end face of the rocker pin holder 22. As a result of the projections 26 being formed on the end face of the boss 17 in this way, the tilting direction of the rocker arm 14 with respect to the rocker pin 20 can be limited. That is, as shown in Fig. 4, forming the projections on both sides of the boss 17 (both sides in the direction perpendicular to the axes of the cam shafts 11, 12 and the cylinder axis C) allows the rocker arm 14 to be tilted clockwise or counter-clockwise in Fig- 4(c). In the valve drive system 7 as described above, the rotation of the intake cam shaft 11 and exhaust cam shaft 12 is transmitted from the three-dimensional cams 13 to the rocker arms 14, and the rocker arms 14 are rotated about the rocker pins 20 to open/close the intake and exhaust valves 5, 6. Regarding the sliding portion between the three-dimensional cam 13 and the slipper 19 of the rocker arm 14, oil film of the lubricating oil is retained when the inclination angle of the cam surface 13a coincides with that of the contact surface of the slipper 19, thus providing good lubrication. When the angle of the cam surface 13a doesn't coincide with that of the sliding surface, due to manufacturing error of the three-dimensional cam 13, that is, when both surfaces are in the state of point contact and a clearance S is produced between them, as shown in Fig. 4(c) by the double dot and dash lines, the rocker arm 14 is tilted with respect to the rocker pin 20 such that the clearance S is eliminated. In other words, the rocker arm 14 is tiltable with respect to the rocker pin 20 so as to follow the cam surface 13a of the three-dimensional cam 13, so that the cam surface 13c comes into line contact throughout the sliding surface of the slipper 19. Therefore, according to this valve drive system 7, oil film of the lubricating oil can be retained reliably between the cam surface 13a and the sliding surface of the slipper 19 without need of forming a highly accurate three-dimensional cam 13. A second embodiment To couple the rocker arm 14 to the rocker pin 20 for tilting movement, a coupling sleeve may be disposed between the boss 17 and the rocker pin 20. Fig. 6 is an enlarged sectional view of a portion of another embodiment of the valve drive system for engines according to this invention, and like or equivalent parts as illustrated in Figs. 1-5 are designated by like reference numerals, omitting detailed descriptions. The rocker arm 14 shown in Fig. 6 is coupled to the rocker pin 20 through a cylindrical sleeve 31. The sleeve 31 is configured such that the inner circumference 31a has a constant diameter and the outer circumference 31b has diameter gradually decreasing from the axially central portion toward the end portion; the rocker pin 20 is received in the inner circumference 31a; and the outer circumference is fitted in the pin hole 25 of the rocker arm 14. In this embodiment, the axially central portion of the outer circumference 31b of the sleeve 31 has a constant diameter. According to this embodiment, manufacture of the valve drive system 7 is simple compared with the first embodiment. This is because forming the tapered surface on the outer circumference 31b of the sleeve 31 is simpler than forming the tapered surface in the pin hole 25 of the rocker arm 14. In the foregoing embodiments, description has been made on examples in which all the intake valves 5 and exhaust valves 6 are disposed radially, but only two intake valves 5 may be disposed radially while two exhaust valves 6 are disposed parallel with each other, or on the contrary, two exhaust valves 6 may be disposed radially while two intake valves are disposed parallel with each other. Also, the number of the intake valves 5 or exhaust valves 6 may be changed as appropriate, for example, three intake valves 5 and two exhaust valves 6. Other embodiments of the invention will be hereinafter described in reference to Figures 7 to 11. FIG. 7 shows a lateral cross section of an upper portion (cylinder head portion) of an internal combustion engine provided with the valve driving mechanism according to the invention. FIG.8 is a plan view of the same internal combustion engine with its head cover removed. FIG. 9 is a side view showing the sliding state of the cam and the rocker arm. FIG. 10 is a view as seen in the direction of the arrow A in FIG. 9. FIG. 11 is an oblique view of FIG. 10. An internal combustion engine 1 according to the invention is of a four-stroke cycle, four-valve type and comprises as shown FIG. 7 a cylinder head 2 made of aluminium alloy, with two intake valves 3 and two exhaust valves 4 (only one for each is shown in FIG. 7). The above-described cylinder head 2 is placed over a cylinder block (not shown) and a head cover 5 is attached over the cylinder head 2. A piston (not shown) is disposed for vertical sliding in a cylinder formed in the cylinder block, with the piston connected through a connecting rod (not shown) to the crankshaft (not shown). As shown in FIG. 7, the cylinder head 2 is formed with two intake passages 6 and two: exhaust passages 7 (only one for each is shown in FIG. 7). The intake ports 6a of the intake passages 6 and the exhaust ports 7a of the exhaust passages 7 respectively opening to the combustion chamber (S) are opened and closed with the intake valves 3 and the exhaust valves 4 according to appropriate timing to exchange gas as intended. Now the constitution of the valve driving mechanism for opening and closing those ports with the intake valves 3 and the exhaust valves 4 according to the invention will be described. As shown in FIG. 7, the intake valve 3 and the exhaust valve 4 are respectively made to pass through and retained with valve guides 8 and 9 press fitted into the cylinder head 2 so as to slide freely and are urged with air springs in the closing direction. That is, valve lifters 10 and 11 respectively attached to the top ends of each intake valve 3 and each exhaust valve 4 are fitted for free sliding within the recesses in housings 13 and 14 secured by means of a plural number of bolts 12 to the cylinder head 2 to form pressure chambers (not shown) in the recesses. Pressurized air supplied from a compressor (not shown) to respective pressure chambers constitutes air springs to urge the intake valve 3 and the exhaust valve 4 in the closing direction as described above. In the internal combustion engine 1 of the invention as shown in FIG. 7, the intake valve 3 and the exhaust valve 4 are disposed to branch out radially in respective, three-dimensional directions. Accordingly the valve lifters 10, 11 and the housings 13, 14 are also disposed radially. As shown in FIG. 8 bearing bosses 2a and 2b on the intake and exhaust sides opposing each other are formed on both outer sides (with respect to respective valve rows) of the intake and exhaust valves 3 and 4 of each cylinder of the cylinder head 2. On the upper surfaces of the bearing bosses 2a and 2b are respectively formed semi-tubular bearings (not shown). An intake camshaft 15 and an exhaust camshaft 16 are respectively rotatably supported with the bearings parallel to each other. Sprockets 17 and 18 are respectively attached to each one end of the intake camshaft 15 and the exhaust camshaft 16, An endless timing belt 19 is routed over the sprockets 17 and 18 and a sprocket (not shown) attached to one end of the above-mentioned crankshaft. It may also be constituted to transmit the rotation of the crankshaft through a multiple-gear train to the intake and exhaust camshafts 15 and 16. Both upper halves of the intake camshaft 15 and the exhaust camshaft 16 are respectively supported with bearing caps 20 and 21 attached to the top surfaces of the bearing bosses 2a and 2b of the cylinder head 2 using bolts 22. Two intake cams 15a are formed side by side integrally with parts of the intake camshaft 15 opposite the two intake valves 3. Likewise, two exhaust cams 16a are formed side by side integrally with parts of the exhaust camshaft 16 opposite the two exhaust valves 4. Those intake and exhaust cams 15a and 16a are made in three-dimensional shape with their sliding surfaces (peripheral surfaces) in tapered shape. The valve driving mechanism of the invention is of the rocker arm type as shown in FIGs. 7 and 8 in which the four rocker arms 25 and 26 swinging about the rocker shafts 23 and 24 are disposed between the intake camshaft 15 and the exhaust camshaft 16, the rotation of the intake camshaft 15 and the exhaust camshaft 16 is converted through the rocker arms 25 and 26 into sliding movement of the intake valve 3 and the exhaust valve 4 so as to open and close the intake and exhaust ports by driving the intake valve 3 and the exhaust valve 4 according to appropriate timing and exchange gas as intended. In this embodiment as shown in FIG. 7, a holder 27 as a separate component is secured using a bolt 28 in a space, formed below the top surface (the surface to which the head cover 5 is attached) and between the intake and exhaust camshafts 15 and 16, to support four rocker shafts 23, 24 which in turn support four rocker arms 25, 26 for swinging. The holder 27 is made of an iron-based material having higher strength and rigidity than aluminium alloy and, as shown in FIG. 8. its central portion has a plug hole 27a. The top surfaces of the fore-ends of the rocker arms 25, 26 extending sideways from the holder 27 are in contact with the intake and exhaust cams 15a, 16a through slippers 29, 30. The underside surfaces of the fore-ends of the rocker arms 25, 26 are in contact with the top ends of the intake and exhaust valves 3, 4. In the valve driving mechanism of this embodiment as shown in FIG. 7, the centers of the intake and exhaust camshafts 15 and 16 are located on the axial center lines of the intake and exhaust valves 3, 4. In the internal combustion engine 1 of this embodiment, the intake and exhaust cams 15a, 16a are made in three-dimensional shape because the intake and exhaust valves 3, 4 are disposed radially in three-dimensions as described above. The taper angles of the sliding surfaces of the intake and exhaust cams 15a, 16a are designed so that the axes of the intake and exhaust valves 3, 4 intersect the sliding surfaces at right angles. The rocker shafts 23, 24 for bearing-supporting the rocker arms 25, 26 on the holder 27 are disposed generally parallel (within a deviation angle of 1 degree) to the intake and exhaust camshafts 15 and 16 in plan view. In side view, however, they are disposed with tilt angles of the intake and exhaust valves 3, 4 relative to the intake and exhaust camshafts 15 and 16 (relative to the crankshaft direction). In the valve driving mechanism of this embodiment, the slippers 29, 30 of the rocker arms 25, 26 are separate components from the rocker arms 25, 26 and supported for free swinging in vertical planes parallel to the axes of the rocker shafts 23, 24 (in planes parallel to the paper surface of FIG. 10). Now the constitution of the slipper 29 and the rocker arm 25 on the intake side is described in reference to FIGs. 9 to 11, since the constitution of the slipper 30 and the rocker arm 26 on the exhaust aide is similar to that on the intake side, drawings and explanations therefor are omitted. As shown in FIG. 9. the top surface (sliding contact surface) 29a of the slipper 29 is made in convex or arcuate curved shape as seen in the axial direction of the rocker shaft 23. The underside (supported surface) 29b of the slipper 29 is made in convex or arcuate curved shape as seen in the direction normal to the rocker shaft 23. On the other hand as shown in FIG. 10, the supporting surface 25a of the rocker arm 25 for supporting the slipper 29 is made complementarily concave corresponding to the convex surface shape of the underside 29b of the slipper 29. The slipper 29 is supported for swinging, in a vertical plane which is parallel to the axis of the rocker shaft 23, as its convex underside 29b is fitted and received in the concave supporting surface 25a, and is in line contact with the point "a" shown in FIG. 9 on the sliding surface of the intake cam 15a. Likewise, the slipper 30 of the rocker arm 26 is also supported for swinging, in a vertical plane which is parallel to the axis of the rocker shaft 24, and in line contact with the exhaust cam 16a. Now the function of the valve driving mechanism of the invention is explained. When the internal combustion engine 1 is started and its crankshaft (not shown) is rotated, the crankshaft rotation with its speed reduced to a half through the sprocket (not shown), the timing belt 19, and the sprockets 17, 18 (shown in FIG. 8) is transmitted to the intake and exhaust camshafts 15 and 16, so that the intake and exhaust camshafts 15, 16 and the intake and exhaust cams 15a, 16a are driven for rotation at a specified speed (half the rotation speed of the crankshaft). when the intake and exhaust cams 15a, 16a are driven for rotation as described above, the rocker arms 25, 26 are pushed down with the intake and exhaust cams 15a, 16a in indirect contact with the rocker arms 25, 26 through the slippers 29, 30 according to appropriate timing, so that the rocker arms 25, 26 depress the intake and exhaust valves 3, 4 against the urging force of the air spring and that the ports are respectively opened for specified periods of time to perform intended gas exchange. As described above in this embodiment, since the slippers 29, 30 of the rocker arms 25, 26 are made as separate components and supported for free swinging, the slippers 29, 30 swing on the rocker arms 25, 26 due to dimensional errors in machining and in assembly work, so that the contact between the slippers 29, 30 and the intake and exhaust cams 15a, 16a is maintained in a stabilized line contact state. Therefore, it is possible to realize the line contact and restrict friction and heat generation on the sliding surfaces of the components without requiring high precision machining of the slippers 29, 30 and the intake and exhaust cams 15a, 16a. In this embodiment, since the bearings for supporting the intake and exhaust camshafts 15, 16 are provided also between the two the intake valves 3 and the two exhaust valves 4, the rigidity for supporting the intake and exhaust camshafts 15, 16 is increased and the deflective deformation of the intake and exhaust camshafts 15, 16 is restricted to a small amount. In this embodiment, since the centers ot the intake and exhaust camshafts 15 and 16 are located on the axial center lines of the intake and exhaust valves 3, 4, loads acting onto the rocker shafts 23, 24 are reduced, so that the durability of the rocker shafts 23, 24 is improved. while the above description is related to the application of the invention to a four-valve type engine having two intake valves and two exhaust valves, it is a matter of course that this invention is also applicable to any other internal combustion engines as long as they employ the rocker arms in the valve driving mechanism. Further embodiments of the invention will be hereinafter described in reference to the Figures 12 to 14. FIG. 12 shows a lateral cross section of the upper part (cylinder head area) of a multi-cylinder engine provided with a valve driving mechanism of the invention (as seen along the line B-B in FIG. 13). FIG. 13 is a view as seen along the line A-A in FIG. 12 FIG. 14 shows a cross section as seen along the line C-C in FIG. 13. An internal combustion engine 1 of the invention is of a four-stroke cycle, four-valve type and comprises as shown FIG. 12 for each cylinder, a cylinder head 2 made of aluminium alloy, with two intake valves 3 and two exhaust valves 4 (only one for each is shown in FIG. 12). The above-described cylinder head 2 is placed over a cylinder block (not shown). A piston (not shown) is disposed for vertical sliding in each cylinder formed in the cylinder block, with the piston connected through a connecting rod (not shown) to the crankshaft (not shown). As shown in FIG. 12 the cylinder head 2 is provided with two intake passages 5 and two exhaust passages 6 for each cylinder (only one for each is shown in FIG. 12). The intake port 5a of the intake passage 5 and the exhaust port 6a of the exhaust passage 6 respectively opening to the combustion chamber (S) are opened and closed with the intake valves 3 and the exhaust valves 4 according to appropriate timing to exchange gas as intended. Now the constitution of the valve driving mechanism for opening and closing those ports with the intake valves 3 and the exhaust valves 4 according to the invention will be described. As shown in F IG. 12, the intake valve 3 and the exhaust valve 4 are respectively made to pass through and retained with valve guides 7 and 8 press-fitted into the cylinder head 2 so as to slide freely and are urged with an air spring in the closing direction. That is, valve lifters 9 and 10 respectively attached to the top ends of each intake valve 3 and each exhaust valve 4 are fitted for free sliding within the recesses 11a and 12a formed in housings 11 and 12 attached to the cylinder head 2 to form pressure chambers S1 and S2 in the recesses 11a and 12a. Pressurized air supplied from a compressor (not shown) through passages 11b and 12b to respective pressure chambers constitutes the air spring to urge the intake valve 3 and the exhaust valve 4 in the closing direction as described above. In the multi-cylinder engine 1 of the embodiment as shown in FIG. 12, the intake and exhaust valves 8 and 4 in lateral cross section (in the direction of engine width) are tilted or disposed radially to diverge upward, and in longitudinal cross section (longitudinally) they are parallel to each other and vertical. As shown in FIG. 13 head attachment bosses 2a and 2b on the intake and exhaust sides opposing each other are formed on both outer sides (with respect to respective valve rows) of the intake and exhaust valves 3 and 4 of each cylinder of the cylinder head 2. On the upper surfaces of the head attachment bosses 2a and 2b are respectively formed semi-tubular, double bearings 2a-1 and 2b-1. The double bearings 2a-1 and 2b-1 each has in its central area a round bolt insertion hole 2c. The cylinder head 2 is attached onto the top of the cylinder block (not shown) using head bolts 13 passed through the bolt insertion holes 2c. Ahead cover 14 made of aluminium alloy is placed over the top surface of the cylinder head 2 (as shown in FIGs. 12 and 14). As shown in FIG. 12 an intake camshaft 15 and an exhaust camshaft 16 are rotatably supported with the double bearings 2a-1 and 2b-1 parallel to each other in the longitudinal direction (in the direction perpendicular to the surface of FIG. 12 on the top surfaces on the intake and exhaust sides of the cylinder head 2. The upper halves of the intake camshaft 15 and the exhaust camshaft 16 are supported with an integral. type of bearing cap 17 made of aluminium alloy. The bearing cap 17 is attached on the top surfaces of the attachment bosses 2a and 2b on the opposing intake and exhaust sides of the cylinder head 2 by means of four attachment bolts 18, so that an integral type of bearing cap 17 bridges the attachment bosses 2a and 2b on the opposing intake and exhaust sides. For each cylinder, two (only one is shown) intake cams 15a are formed side by side integrally with parts of the intake camshaft 15 opposite the two intake valves 8. Likewise, two (only one is shown) exhaust cams 16a are formed side by side integrally with parts of the exhaust camshaft 16 opposite the two exhaust valves 4. As shown in FIG. 12, insides of the intake and exhaust camshafts 15 and 16 are provided with oil passages 21 and 22 of a ring shape in cross section, as formed between the camshaft 15 and a pipe member 19, and between the camshaft 16 and a pipe member 20. The intake and exhaust cams 15a and 16a are respectively bored with oil holes 15b and 16b in fluid communication with the oil passages 21 and 22. The journal portions of the intake and exhaust camshafts 15 and 16 are respectively bored with oil holes 15c and 16c. As shown in FIGs. 12 and 14 ribs 23 and 24 for receiving oil thrown up from the oil holes 15b and 16b respectively bored in the intake and exhaust cams 15a and 16a are provided at positions on the intake and exhaust sides of the inside surface of the head cover 14. The valve driving mechanism of the invention is of the rocker arm type as shown in FIGs. 12 and 13 in which the four rocker arms 27 and 28 swinging about the rocker shafts 25 and 26 are disposed between the intake camshaft 15 and the exhaust camshaft 16, the rotation of the intake camshaft 15 and the exhaust camshaft 16 is converted through the rocker arms 27 and 28 into sliding movement of the intake valve 3 and the exhaust valve 4 so as to open and close the intake and exhaust ports with the intake valve 3 and the exhaust valve 4 according to appropriate timing and to exchange gas as intended. In this embodiment, a constitution is employed in which a space is formed below the top surface (on which the head cover 14 is attached) of the cylinder head 2 and between the intake and exhaust camshafts 15 and 16. A holder 29 as a separate component for each cylinder is accommodated and secured in this space using two-large bolts 30 and a small bolt 31, to support four rocker shafts 25, 26 which in turn support four rocker arms 27, 28 for swinging. The holder 29 is made of an iron-based material which is higher in both strength and rigidity than aluminium alloy and, as shown in FIG. 14, its central portion has a plug hole 29a and its both side portions have cut grooves 29b for the rocker arms 27 and 28 to fit in. The holder 29 is also provided with four rocker shaft holes 29c (shown in FIG. 12) for the rocker shafts 25 and 26 to fit in, parallel to the intake and exhaust camshafts 16 and 16 (in the direction perpendicular to the surface of FIG. 12). As shown in FIG. 14 an ignition plug 23 is made to pass through the plug holes 29a and 2d bored respectively through the holder 29 and the cylinder head 2. The ignition plug 32 is screwed into the cylinder head 2 with its electrode portion 32a facing the central area of the combustion chamber S. The top surfaces of the fore-ends of the rocker arms 27, 28 extending sideways from the holder 29 are in contact with the intake and exhaust cams 15a, 16a through slippers 33, 34. The underside surfaces of the fore-ends of the rocker arms 27, 28 are in contact with the top ends of the intake and exhaust valves 3, 4. In the valve driving mechanism of this embodiment as shown in FIG. 12, the centers of the intake and exhaust camshafts 15 and 16 are located on the axial center lines of the intake and exhaust valves 3, 4. Now the function of the valve driving mechanism of the invention is explained. When the internal combustion engine 1 is started and its crankshaft (not shown) is rotated, the crankshaft rotation is transmitted to the intake and exhaust camshafts 15 and 16, so that the intake and exhaust camshafts 15, 16 and the intake and exhaust cams 15a, 16a are driven for rotation at a specified speed (half the rotation speed of the crankshaft). When the intake and exhaust cams 15a, 16a are driven for rotation as described above, the rocker arms 27, 28 are pushed down with the intake and exhaust cams 15a, 16a in contact with the fore-ends of the rocker arms 27, 28 according to appropriate timing, so that the rocker arms 27, 28 depress the intake and exhaust valves 3, 4 against the urging force of the air spring and that the intake and exhaust ports are opened for specified periods of time to perform intended gas exchange. In this embodiment as described above, for each cylinder, since the holder 29 for supporting the rocker arms 27, 28 is attached below the head cover attachment surface of the cylinder head 2, it is possible to make the cylinder head 2 as a single, integral component in spite of employing the constitution in which the rocker shafts 25, 26 for swingably supporting the rocker arms 27, 28 are disposed between the intake and exhaust camshafts 15, 16. Therefore, it is unnecessary to divide the cylinder head into two, upper and lower parts as in the conventional design. As a result, the constitution is simplified, and the number of components, the number of assembly steps, and cost are reduced. In this embodiment, each holder 29 is made of an iron-based material having high strength and rigidity as a separate component from the cylinder head. The upper halves of the intake and the exhaust camshafts 15, 16 are supported with the integral type of bearing cap 17 which bridges the attachment bosses 2a, 2b located on both opposing intake and exhaust sides of the cylinder head 2. As a result, rigidity of supporting the rocker arms 27, 28 is enhanced and the rocker arms 27, 28 are supported with a compact arrangement and a high rigidity. Since the centers of the intake and exhaust camshafts 15 and 16 are located on the axial center lines of the intake and exhaust valves 3, 4, loads acting on the rocker shafts 25, 26 are reduced and the durability of the rocker shafts 25, 26 is improved. In this embodiment, since the oil receiving ribs 23, 24 are provided on the inside surface of the head cover 4 attached over the cylinder head 2, oil sprayed out of the oil holes 15b, 16b bored in the intake and exhaust cams 15a, 16a is received with the ribs 23, 24 and drops by its own weight onto the sliding parts between the intake and exhaust cams 15a, 16a and the rocker arms 27, 28 and serves to lubricate and cool those parts, wear between the intake and exhaust cams 15a, 16a and the rocker arms 27, 28 is prevented, and heat generation due to friction is restricted. While the above description is related to the application of the invention to a four-valve type engine having intake and exhaust valves, two for each, it is a matter of course that this invention is also applicable to any other multi-cylinder engines as long as they employ the rocker arms in the valve driving mechanism. According to this invention as described above, the rocker arm is tiltable with respect to the rocker pin so as to follow the three-dimensional cam surface, so that the cam surface can be in line contact with the sliding surface of the rocker arm throughout its circumference. Therefore, oil film of the lubricating oil can be retained reliably between the cam surface and the sliding surface of the rocker arm without need of forming a highly accurate three-dimensional cam. Thus, the valve drive system according to this invention is able to utilize the three-dimensional cam to reduce the number of the rocker arms while improving productivity of the three-dimensional cam and durability of the sliding portion, thereby effecting cost reduction and smaller size. According to another invention in which the contact position between the rocker arm and the three-dimensional cam is located closer to the rocker pin, compared with a system in which the three-dimensional cam engages the rocker arm at a position corresponding to the intake or exhaust valve or a position further away from the rocker pin than the position corresponding to the intake or exhaust valve, the amount of lift of the three-dimensional cam can be set relatively low and the length of the rocker arm can be shorter, for the same opening degree of the intake or exhaust valve. Therefore, the inertial mass of the three-dimensional cam and the rocker arm can be smaller, thereby providing a valve drive system suited for high speed type engines. According to another invention in which the rocker pin is tiltable with respect to the cam shaft, the rocker arm is able to rock along the stroking direction of the intake or exhaust valve, therefore the intake or exhaust valve can be disposed without unreasonable bending load. As is clear from the above description, with this invention, an effect is obtained that, since the slippers of the rocker arms are made as separate components and supported for free swinging, it is possible to maintain the contact between the slippers and the intake and exhaust cams in the state of line contact in a stabilized manner as the slippers swing on the rocker arms due to dimensional errors in machining and in assembly work, without requiring high precision machining of the slippers and the intake and exhaust cams. As is clear from the above description, according to the invention, in the valve driving mechanism for a multi-cylinder engine wherein rocker shafts for swingably shaft-supporting rocker arms are disposed between intake and exhaust camshafts, rotation of the intake and exhaust camshafts is converted into sliding movement of intake and exhaust valves to open and close intake and exhaust ports, since the holder for supporting the rocket arms is secured in a position below the head cover attachment surface of the integral type of cylinder head, effects are provided that the integral type of cylinder head can he employed in spite of disposing the rocker shafts between the intake and exhaust camshafts, and that the rocker arms are supported compactly with a high rigidity.

Claims: A valve drive system for an internal combustion engine comprising at least one cylinder having a plurality of intake or exhaust valves (5,6), said intake or exhaust valves (5,6) are each driven by a respective rocker arm (14) having a boss (17) supported for rotation on a cylinder head (18) through a rocker pin (20), and an intake or exhaust camshaft (11,12) formed with three-dimensional cams (13) for engaging the rocking ends of the rocker arms (14) in sliding relation, characterized in that the boss (17) of said rocker arm (14) is coupled to said rocker pin (20) for tilting movement in the direction perpendicular to the axis of the rocker pin (20). Valve drive system according to claim 1, wherein the position at which said three-dimensional cam (13) engages said rocker arm (14), is located closer to said rocker pin (20) than the position at which the rocker arm (14) engages the intake or exhaust valve (5,6). Valve drive system according to claim 1 or 2, wherein said rocker pin (20) is inclined with respect to the axis of said cam shaft (11,12) when viewed from the direction of the cylinder axis. Valve drive system according to at least one of the preceding claims 1 to 3, characterized in that the rocker arm (14) is coupled to the rocker pin (20) through a cylindrical sleeve (31 ). Valve drive system according to claim 4, characterized in that the sleeve (31) is configured such that an inner circumference (31 a) has a constant diameter and an outer circumference (31 b) has a diameter gradually decreasing from the axial central portions toward the end portion, and that the rocker pin (20) is received in the inner circumference (31 a) and the outer circumference (31 b) is fitted in a pin hole (25) of the rocker arm (14). Valve drive system according to at least one of the preceding claims 1 to 5, characterized in that a slipper (29,30) for the rocker arm (25,26) is made as a separate part and supported for swinging freely. Valve drive system according to claim 6, characterized in that a holder (27) which is separate from a cylinder head is attached to the cylinder head, with the holder (27) being made to support the rocker arm. Valve drive system according to claim 6 or 7, characterized in that the intake and exhaust cams (15a,16a) formed on the intake and exhaust camshafts (15,16) are made in three-dimensional shape. Valve drive system according to claim 8, characterized in that the rocker shafts (23,24) for rotatably supporting the rocker arms (25,26) are tilted in side view relative to the intake and exhaust camshafts (15,16). Valve drive system according to one of the preceding claims 6 to 8 or 9, characterized in that the bearings for supporting the intake and exhaust camshafts (15,16) are disposed between a plural number of adjacent intake and exhaust valves (3,4). Valve drive system according to at least one of the preceding claims 1 to 10, characterized in that the valve drive system is for a multi-cylinder internal combustion engine. Valve drive system according to claim 11, characterized in that rocker shafts (25,26) for swingably shaft-supporting rocker arms (27,28) are disposed between intake and exhaust camshafts (15,16) for each cylinder, a holder (29) as a separate component for supporting the rocker arms (27,28) is attached below a surface of an integral type of cylinder head (2) to which a head cover (14) is attached. Valve drive system according to claim 12, characterized in that each holder (29) is formed with a rocker shaft hole (29c) and a plug hole (29a). Valve drive system according to claim 12 or 13, characterized in that each holder (29) supports four rocker shafts (25,26) parallel to the intake and exhaust camshafts (15,16). Valve drive system according to one of claims 12, 13 or 14, characterized in that each holder (29) is made of an iron-based material. Valve drive system according to claim 12, characterized in that upper halves of the intake and exhaust camshafts (15,16) are supported with a common, integral type of bearing cap (17), with the bearing cap (17) bridging attachment bosses (2a,2b) on opposing intake and exhaust sides of the cylinder head (2). Valve drive system according to claim 12, characterized in that the axial centers of the intake and exhaust camshafts (15,16) are disposed on the axial center lines of the intake and exhaust valves (3,4). Valve drive system according to claim 12, characterized in that oil receiving ribs (23,24) are formed on the inside surface of the head cover (14) placed over the top of the cylinder head (2).

Inventors: NARITA MATUNORI, NISHI KENGO, SAIJO YUJI, TAKAHASHI TOSHIYUKI, NARITA, MATUNORI, NISHI, KENGO, SAIJO, YUJI, TAKAHASHI, TOSHIYUKI, Narita, Matunori, c/o Yamaha Hatsudoki K. K., Nishi, Kengo, c/o Yamaha Hatsudoki K. K., Saijo, Yuji, c/o Yamaha Hatsudoki K. K., Takahashi, Toshiyuki, c/o Yamaha Hatsudoki K. K.
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1999
Main classification: F01L1
IPC codes: F01L1