Variable valve event engine
BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates to a variable valve event engine which can vary the operating characteristics of a valve installed in an internal combustion engine, such as an intake valve or an exhaust valve, in accordance with changes in the operating conditions of the engine. 
     Generally, the operating characteristics of a conventional engine are influenced by the operating characteristics of the cams which actuate the valves of the engine. For example, if the shape of the cams is selected so as to produce a high torque when the engine including the cams rotates at a low speed, the engine cannot produce a sufficiently high torque at a high rotating speed. On the other hand, if the shape of the cams is selected so that an engine having the cams can produce a high torque at a high rotating speed, the output of the engine is decreased when the engine speed is low. As a result, a conventional engine cannot always produce a high torque for various rotating speeds of the engine. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a variable valve event engine which can easily vary the operating characteristics of a valve so that the variable valve engine can produce a desired high torque for various rotating speeds of the engine. 
     The above-mentioned object of the present invention is achieved by a variable valve event engine comprising: a rocker shaft disposed along the engine; a rocker arm for actuating a valve of the engine, which arm is pivoted swingably around and slidably along the rocker shaft; a camshaft arranged parallel to the rocker shaft and synchronized with a crankshaft of the engine; and a plurality of adjacent cams fixed in such an arrangement on the camshaft so as to have profiles different from each other, wherein by sliding the rocker arm along the rocker shaft, the valve is selectively actuated by means of one of the cams. 
     According to the present invention, a cam to be used during a low speed and a cam to be used during a high speed, both having differing cam profiles, are disposed and selectively utilized in accordance with the operating conditions of the engine which has the cams installed thereon. 
     A criterion for determining the alternation of the cams will now be explained. Two diagrams (not shown), one showing the operating characteristics of engines with cams for high speeds and the other showing the operating characteristics of engines with cams for low speeds, were prepared with values corresponding to the engine operating characteristics being plotted on the ordinate of each graph while the values corresponding to the engine rotating speeds are plotted on the abscissa thereof. The abscissa is partitioned into two regions, a high speed region and a low speed region, the boundary between the regions is formed by a vertical line defined by the intersection of the above-mentioned two diagrams. During low speed conditions, the cam for low speed use is adopted, while during high speed conditions, the cam for high speed use is adopted. 
     According to the present invention, the cams for low speed and high speed uses can be selectively utilized in accordance with changes in the engine rotating speed, with regard to whether the engine rotating speed is high or low. As a result, the operating torque characteristic of the engine is highly improved if compared with that of a conventional engine which has a single type of cam installed thereon. In addition, the variable valve event engine, according to the present invention, can generate uniform torque characteristics for various engine rotating speeds. The uniform torque obtained by the engine of the present invention can be almost the same as the maximum torque obtained by the conventional engine with a single type cam. 
     The valve timing (including the valve lift) of the intake and exhaust valves mounted on an engine is adjusted in accordance with changes in the operating conditions of the engine, in other words, with changes in the driving conditions of a vehicle on which the engine is mounted. Such adjustment can be effected by means of the present invention for maintaining a high engine efficiency and for improving the output characteristics and the fuel consumption of the engine during both low and high speed conditions. 
     Some embodiments of the present invention will be explained hereinafter with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional elevational view of a first embodiment according to the present invention; 
     FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1; 
     FIG. 3 is a cross-sectional view taken along the line III--III in FIG. 1; 
     FIG. 4 is a cross-sectional elevational view of a second embodiment according to the present invention; 
     FIG. 5 is a cross-sectional view taken along the line V--V in FIG. 4; 
     FIG. 6 is a cross-sectional elevational view of a third embodiment according to the present invention, and 
     FIG. 7 is a cross-sectional elevational view of a modified stopper guide. 
    
    
     With reference to FIG. 1 which is a cross-sectional elevational view of a first embodiment of the present invention, mounted and secured onto a cylinder block 1 with a cylinder bore 1a formed therein is a cylinder head 3 with a combustion chamber wall 3a formed thereon. A piston 5 is slidably and sealingly disposed within the cylinder bore 1a so that the space surrounded by the cylinder bore 1a, the combustion chamber wall 3a and the upper surface of the piston 5 forms a combustion chamber 7. An intake port 9, which is formed in the cylinder head 3 and communicated with an intake manifold (not shown), and an exhaust port (not shown), which is also formed in the cylinder head 3 and communicated with an exhaust manifold (not shown), are both communicated with the combustion chamber 7 via an intake valve 11, which controls the intake of a gas mixture into the combustion chamber 7, and via an exhaust valve (not shown), which controls the flow of exhaust gas from the combustion chamber 7. 
     Since the constructions of the intake valve 11 and the exhaust valve are similar to each other, only the construction of the intake valve 11 will now be described. The intake valve 11 comprises a valve body 11a, which cooperates with a valve seat 9a formed at the opening portion of the intake port 9 of the combustion chamber 7 for controlling the intake of the gas mixture, and a valve rod 11b which is fixed to the valve body 11a. The valve rod 11b is slidably and sealingly inserted into the cylinder head 3 and has a spring retainer 13 fixed on its rear portion which projects from the cylinder head 3. A compression spring 15 is mounted between the spring retainer 13 and the upper surface of the cylinder head 3 for urging the intake valve 11 upwardly so that the valve body 11a can abut against the valve seat 9a. Mounted on the valve rod 11b of the intake valve 11 is a cylindrical-shaped valve lifter 17 which is slidable within a cylindrical recess 3b formed in the cylinder head 3. When the upper surface of the valve lifter 17 is pushed downwardly, the intake valve 11 is opened. A rocker arm 19 adapted to be swingable around a rocker shaft 21 has a knocker 25 which is threadedly secured with a lock nut 22 to one end thereof so that the length of the knocker 25 is adjustable. A cam member 27 is in abutment with a rocker arm pad 19b formed at the other end of the rocker arm 19 so that, when the cam 27 is rotated in the direction designated by the arrow a in FIG. 1, the knocker 25 pushes the valve lifter 17 downwardly for opening the intake valve 11. 
     As illustrated in FIG. 2, the cam member 27 includes two adjacent cams 27a and 27b which are fixed on a camshaft 29 disposed parallel to the rocker shaft 21. The cam profiles of the cams 27a and 27b are different from each other with respect to their valve lifts and/or valve timings, for example, the cam 27a has a cam profile preferable for use during a low speed condition and the cam 27b has a cam profile preferable for use during a high speed condition. It should be noted that the number of cams is not limited to two but may be three or more according to preference, and that the base circular portions of the cams 27a and 27b have substantially the same radii. The camshaft 29 is synchronized with the crankshaft of the engine (not shown) and rotated in a direction designated by the arrow a (FIG. 1). 
     As mentioned above, the rocker arm 19 is not only swingably pivoted to the rocker shaft 21 but also adapted to be slidable along the rocker shaft 21 so that the rocker arm 19 can selectively transmit the movement of each of the cams 27a or 27b to the valve lifter 17. The slide mechanism of the rocker arm 19 will now be explained with reference to FIGS. 1 through 3. The rocker shaft 21 is made of a hollow cylinder and the rocker arm 19 is swingably and slidably inserted onto the outside wall of the hollow cylinder. A rocker arm slide shaft 23 is disposed inside the hollow cylinder with a certain clearance therebetween and extends along the rocker shaft 21. One end 23a of the rocker arm slide shaft 23 is connected to a linear movement mechanism, such as a hydraulic cylinder 24 (illustrated in FIG. 2), a pneumatic cylinder or a mechanism comprising a cam and a driving motor, for moving the rocker arm slide shaft 23 along the rocker shaft 21. The rocker arm slide shaft 23 has two annular-shaped stops 31 and 33 fixed thereon with a certain distance therebetween along the lengthwise direction of the slide shaft 23. Two movable stops 35 and 37 are slidably mounted between the stops 31 and 33 on the rocker arm slide shaft 23 with a small distance therebetween. Connecting compression springs 39 and 41 are installed between the stops 31 and 35, and 33 and 37, respectively. According to the above-mentioned construction, stops 35 and 37 are located at predetermined positions which are determined by the biasing forces generated by the compression springs 39 and 41. As illustrated in FIG. 3, the rocker shaft 21 has a slot 21a formed thereon through which a bolt 43 threaded to the rocker arm 19 extends to a space located between the stop 35 and 37, so that the rocker arm 19 is adapted to be movable with the stops 35 and 37. In the first embodiment which is illustrated in the accompanying FIG. 2, the rocker shaft 21 has two annular grooves 21b and 21c formed therearound at positions which correspond to the cams 27a and 27b. The rocker arm 19 has a small hole 19e formed therein. A ball 45 which is capable of being selectively engaged with the annular groove 21b or 21c is retained in the hole 19e and then urged by a spring 47 so that the rocker arm 19 is in position. A screw bolt 49 is used for retaining the spring 47. 
     When the rocker arm slide shaft 23 is moved to the left from the position illustrated in FIG. 2 by means of the hydraulic cylinder 24, the contacting compression spring 41 is compressed by the stop 33 so that the stop 37 is urged to the left. However, since the ball 45 is engaged with the annular groove 21b formed around the rocker shaft 21, the stop 37 cannot be moved for a while. On the other hand, as the contacting compression spring 41 is compressed, potential energy is retained in the compression spring 41. When the retained potential energy becomes more than a predetermined value, due to the urging force generated by the compression spring 41, the ball 45 is disengaged from the annular groove 21b. The rocker arm 19 is next moved within a short period to a position corresponding to the other cam 27b by means of the potential energy retained in the compression spring 41 and then positioned there by engaging the ball with the other annular groove 21c. The strength of the connecting compression spring 41 is so adjusted that the rocker arm 19 can be moved a certain distance between the cams 27a and 27b, i.e., the distance between the annular grooves 21b and 21c. Similarly the rocker arm 19 is moved to the right as seen in FIG. 2, by means of the connecting compression spring 39 when the rocker arm slide shaft 23 is moved to the right. The strength of the compression spring 39 is likewise adjusted so that the rocker arm 19 can be moved a certain distance between the cams 27a and 27b. The amount of movement of the rocker arm 19 can be limited by covers 51 and 53 which are inserted on the rocker shaft 21 as illustrated in FIG. 2, in addition to the ball 45 and the annular grooves 21b and 21c which are also used for limiting the movement of the rocker arm 19. 
     It is preferable that the above-mentioned movement of the rocker arm 19 be effected while the rocker arm pad 19b (FIG. 1) is in abutment with a base circular portion of one of the adjacent cams 27a and 27b. If movement of the rocker arm 19 is not effected under such condition, the rocker arm 19 and/or cams 27a and 27b may be abraded or damaged when the rocker arm 19 is being moved. This is because the cams 27a and 27b have different cam profiles with respect to the valve lifts and/or valve timings. As a result, the smooth operation of the engine is disturbed. 
     In this embodiment, a stopper guide 55 is disposed between the two adjacent cams 27a and 27b (FIG. 2) to permit the rocker arm 19, to move only when the rocker arm is in abutment with one of the base circular portions of the cams 27a and 27b. Referring to FIG. 1 again, one end 55a of the stopper guide 55 is fixed, at an intermediate portion in the base circles of the cams 27a and 27b, to the camshaft 29 and the other end 55b of the stopper guide 55 is partially wrapped around the camshaft 29 at a trailing portion with respect to the rotational direction of the camshaft 29. The intermediate portion of the stopper guide 55 for connecting both the ends 55a and 55b is formed in a circular arch shape, and the outside periphery of the intermediate portion of the stopper guide bulges out from the base circles of the cams 27a and 27b by a certain amount &#34;t&#34;, for example, 1 mm. The stopper guide 55 made of a spring steel strip is so constructed and arranged that the stopper guide 55 is rigid against a force acting on the side thereof (i.e., in a direction perpendicular to the sheet on which FIG. 1 is illustrated) but flexible against a force acting on the surface thereof (i.e., in a direction parallel to the sheet on which FIG. 1 is illustrated). As a result, when the rocker arm pad 19b formed on the rocker arm 19 is in abutment with a base circular portion but free from the stopper guide 55 of one of the cams 27a and 27b, and when the rocker arm 19 is commenced to move, the rocker arm 19 cannot be prevented from moving by the stopper guide 55 and is thus caused to move by the energy retaining and rapidly moving mechanism. In addition, even if the rocker arm pad 19b should run onto the stopper guide 55 while the rocker arm 19 is moving, the movement of the rocker arm 19 still cannot be prevented because the stopper guide may be easily deflected elastically. On the other hand, while the rocker arm pad 19b formed on the rocker arm 19 is in abutment with a portion free from the base circular portion, such as a lift portion, of one of the cams 27a and 27b, the rocker arm 19 is prevented from moving because the urging forces generated by the connecting compression springs 27a and 27b are so adjusted that the urging forces are smaller than the frictional forces occurring between the cam 27a or 27b and the rocker arm pad 19b while the intake valve 11 is being opened by the cam 27a or 27b. In addition, while the rocker arm pad 19b is in abutment with a rear half portion of the base circle of the cam 27a or 27b, in other words, a portion where the stopper guide 55 is disposed, the movement of the rocker arm 19 is prevented by the projecting portion of the stopper guide 55. As a result, the movement of the rocker arm 19 is prevented while it is in abutment with a portion free from the base circles of the cam 27a or 27b so that defects, such as vibrations of the cam, are fully prevented from occurring. In this case, the rocker arm 19 which has been prevented from moving is moved when the rocker arm 19 comes into abutment with the base circular portion of the cam 27a or 27b. 
     To facilitate the smooth movement of the rocker arm 19, the width of the rocker arm pad 19b measured along the cam shaft 19 should preferably be greater than the width of the clearance between the cams 27a and 27b. In other words, the clearance between the cams 27a and 27b should preferably be as small as possible in order to maintain a minimum amount of movement of the rocker arm 19. The amount of movement of the rocker arm 19 is so limited that the knocker 25 threaded at the front portion of the rocker arm 19 cannot be disengaged from the valve lifter 17 when the rocker arm 19 is moved. 
     In an embodiment according to the present invention, the abutment between the rocker arm 19 and the cam 27a or 27b is selectively changed by means of the drive mechanism, such as the hydraulic cylinder 24 (FIG. 2) connected to the rocker arm slide shaft 23, when the engine rotating speed exceeds a predetermined value. 
     Various alterations of the above-described embodiment are possible. For example, the movable stoppers 35 and 37 illustrated in FIGS. 2 and 3 may be formed in one body (not shown) which has a radial hole for inserting the screw bolt 43. 
     In the above-mentioned altered embodiment, the rocker shaft 21 is formed in a hollow cylinder which has the rocker arm slide shaft 23 positioned thereon. However, in a second embodiment which will now be explained with reference to FIGS. 4 and 5, a rocker shaft 121 is formed in a solid cylinder and a rocker arm slide shaft 123 is disposed parallel to the rocker shaft 121. Because the reference numerals used for indicating the parts in the second embodiment are the same as those in the first embodiment, an explanation therefor is omitted from herein. The rocker arm 19 is carried on the rocker shaft 121 so that the rocker arm 19 can be swingable about and slidable along the rocker shaft 121. Referring to FIG. 5, the rocker arm slide shaft 123, which is disposed parallel to the rocker shaft 121, has two stationary stops 31 and 33 fixed on the rocker arm slide shaft 123, a movable stop 135 slidably inserted onto the rocker arm slide shaft 123, and compression springs 39 and 41 disposed between stops 31, 33 and 135. A groove 135a formed on the movable stop 135 engages with an annular projection 19c formed on the rocker arm 19. Between the rocker arm 19 and the rocker shaft 121 is disposed a spring urging mechanism (not shown) which is similar to that of the first embodiment and which comprises annular grooves, a ball and a spring for urging the ball. 
     Referring to FIG. 6, a third embodiment of the present invention will now be explained. Instead of the connecting compression springs 39 and 41, which are used in the first and second embodiments for transmitting the movement of the rocker arm slide shaft 23 or 123 to the rocker arm 19, a plate spring 139 is used for a connecting spring in the third embodiment, one end of which spring is fixed to the rocker arm slide shaft 123 and the other end of which engages with a groove 19d formed on the rocker arm 19. Since the other construction of the third embodiment is similar to that of the second embodiment, further explanation therefor is accordingly not provided herein. 
     An alteration of the stopper guide of the present invention, which is disposed at a space between the cams, will now be explained with reference to FIG. 7. The stopper guide 55 illustrated in FIGS. 1 and 4 has an end fixed onto the camshaft 29, another end formed into a free end, and a circular-shaped arch which connects both ends. The stopper guide 155 illustrated in FIG. 7 is, however, made of a corrugated plate extending between its fixed end portion and its free end portion, the upper peripheries of which plate slightly projecting beyond the base circles of the cams 27a and 27b and the lower peripheries of which plate being in contact with the outer surface of the camshaft 29. The wave length P, i.e., a distance between the two adjacent upper peripheries, is so selected that the rocker arm pad 19b (FIGS. 1 and 4) can smoothly come into contact with the upper peripheries. Accordingly, this construction can improve the rigidity of the stopper guide 155 against a force acting on a side thereof.