Abstract:
A low speed cam is disposed on a cam shaft. A low speed sub-rocker arm actuated by the low speed cam is pivotally connected to a main rocker arm. A connecting member is supported by the main rocker arm. The connecting member has both a first condition wherein the sub-rocker arm and the main rocker arm are fixed to each other to constitute a single unit and a second condition wherein the sub-rocker arm and the main rocker arm are disengaged from each other. A hydraulically actuating mechanism has a hydraulic work chamber. The mechanism induces the first condition of the connecting member upon discharge of hydraulic fluid from the work chamber and induces the second condition upon feeding of hydraulic fluid to the work chamber. A control unit causes the hydraulically actuating mechanism to induce the first condition of the connecting member when the engine stops.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to valve operating devices of internal combustion engine, and more particularly to the valve operating devices of a type wherein the valve lifting is controlled in accordance with the operating condition of the engine. 
     2. Description of Related Art 
     In order to clarify the task of the present invention, one related valve operating device of the above-mentioned type will be briefly described in the following, which is shown in Japanese Utility Model First Provisional Publication 6-73301. 
     In the device, low speed cams used for all operation ranges of the engine are mounted on a cam shaft. Each low speed cam slidably contacts a main rocker arm to actuate intake or exhaust valves. The main rocker arm is pivotally mounted on a rocker shaft. Beside the low speed cam, medium speed and high speed cams are also mounted on the cam shaft, which are used for middle and high speed operation ranges of the engine respectively. The medium and high speed cams slidably contact respective sub-locker arms which are pivotally mounted on the rocker shaft beside the above-mentioned locker arm. During operation of the engine, the two sub-rocker arms are selectively fixed to the main locker arm by means of a switching mechanism. 
     The switching mechanism comprises generally two connecting levers which are pivotally connected to the main rocker arm. One of the connecting levers is operated for fixing one of sub-rocker arms to the main rocker arm, and the other connecting lever is operated for fixing the other sub-rocker arm to the main rocker arm. Thus, when one of the sub-rocker arms is fixed to the main rocker arm, these two rocker arms constitute a single unit which is pivotally actuated by selected one of the medium and high speed cams that actually contacts a cam follower of the sub-rocker arm. Thus, in this case, the opening/closing operation of the intake or exhaust valves is timed by the selected one of the medium and high speed cams. 
     In the multi-cylinder internal combustion engines, for reducing a pumping loss at the time of a low load operation, various measures have been hitherto proposed and put into practical use. One of them is shown in Japanese Patent First Provisional Publication 5-248215. In the pumping loss reduction measure of this publication, the intake and/or exhaust valves of given cylinders are made inoperative under a low load operation of the engine. For making the intake and exhaust valves inoperative, the publication discloses an arrangement wherein a sub-rocker arm can be selectively fixed to a rocker shaft to which a main rocker arm for operating the intake or exhaust valves is fixed and wherein a hydraulically actuated connecting plunger for fixing the two rocker arms is slidably received in a receiving hole formed in the rocker shaft. When the connecting plunger is disengaged from the sub-rocker arm, the sub-rocker arm becomes pivotal relative to the main rocker arm, and thus the pivoting movement of the sub-rocker arm induced by rotation of an associated cam does not induce the pivoting movement of the main rocker arm. Thus, in this case, the intake valves and/or the exhaust valves of the given cylinders are forced to take their rest condition even under operation of the engine, which reduces the pumping loss of the engine. In the disclosed measure of the publication, in order to disengage the connecting plunger from the sub-rocker arm, it is needed to stop feeding of hydraulic pressure to a hydraulic work chamber for the connecting plunger. Upon stopping of the pressure feeding, the connecting plunger is retracted into the receiving hole of the rocker shaft due to a biasing force of a coil spring. 
     SUMMARY OF THE INVENTION 
     In the arrangement of the publication 5-248215, for engaging the connecting plunger with the sub-rocker arm, it is needed to feed the hydraulic pressure to the hydraulic work chamber for the connecting plunger. However, due to inherent construction of the arrangement, feeding of sufficient hydraulic pressure to the hydraulic work chamber is not quickly carried out, especially in winter. Thus, upon restarting of the engine, it tends to occur that the intake and/or exhaust valves of the certain cylinders keep the rest condition for a certain time, which induces a non-smoothed engine starting. 
     It is therefore an object of the present invention to provide a valve operating device of an internal combustion engine, which can provide the engine with a smoothed engine starting even when the engine was subjected to the pumping loss reduction operation before engine stopping. 
     According to a first aspect of the present invention, there is provided a valve operating device of an internal combustion engine, which comprises low and high speed cams coaxially disposed on a cam shaft, the low speed cam having a lobe that is lower than that of the high speed cam; a main rocker arm pivotally supported by a rocker shaft and operatively contacting an intake or exhaust valve of the engine to actuate the same; first and second sub-rocker arms pivotally supported by the main rocker arm and pivotally actuated by the low and high speed cams respectively; a connecting member supported by the main rocker arm, the connecting member comprising first and second engaging portions which are respectively engageable with first and second engaged portions defined by the first and second sub-rocker arms, so that upon engagement of the first engaging portion with the first engaged portion, the first sub-rocker arm and the main rocker arm become fixed to each other to pivot about the rocker shaft like a single unit, and upon engagement of the second engaging portion with the second engaged portion, the second sub-rocker arm and the main rocker arm become fixed to each other to pivot about the rocker shaft like a single unit; a hydraulically actuating mechanism comprising first and second hydraulic work chambers, the mechanism inducing the engagement between the first engaging portion and the first engaged portion upon discharge of hydraulic fluid from the first work chamber and inducing a disengagement between the first engaging portion and the first engaged portion upon feeding of the hydraulic fluid into the first work chamber, and the mechanism selectively inducing the engagement or disengagement between the second engaging portion and the second engaged portion in accordance with a pressure of hydraulic fluid fed to the second work chamber; a hydraulic pressure producing unit that feeds the first and second work chambers with hydraulic pressure respectively; and a control unit that, in accordance with operation condition of the engine, controls the hydraulic pressure producing unit, so that the hydraulically actuating mechanism has at least first, second and third operation modes, the first mode being a mode wherein disengagement takes place both between the first engaging portion and the first engaged portion and between the second engaging portion and the second disengaged portion, the second mode being a mode wherein engagement takes place between the first engaging portion and the first engaged portion and disengagement takes place between the second engaging portion and the second engaged portion, the third mode being a mode wherein engagement takes place both between the first engaging portion and the first engaged portion and between the second engaging portion and the second engaged potion. 
     According to a second aspect of the present invention, there is provided a valve operating device of an internal combustion engine, which comprises a plurality of cams coaxially disposed on a cam shaft, one of the cam being a low speed cam; a rocker arm pivotally supported by a rocker shaft and operatively contacting an intake or exhaust valve of the engine to actuate the same; a sub-rocker arm pivotally supported by the rocker shaft and pivotally actuated by the low speed cam; a connecting member supported by the main rocker arm, the connecting member having both a first condition wherein the sub-rocker arm and the main rocker arm are fixed to each other to constitute a single unit and a second condition wherein the sub-rocker arm and the main rocker arm are disengaged from each other; a hydraulically actuating mechanism including a hydraulic work chamber, the mechanism inducing the first condition of the connecting member upon discharge of hydraulic fluid from the work chamber and inducing the second condition upon feeding of hydraulic fluid to the work chamber; and a control unit that causes the hydraulically actuating mechanism to induce the first condition of the connecting member when the engine stops. 
     According to a third aspect of the present invention, there is provided a valve operating device of an internal combustion engine, which comprises at least one cam disposed on a cam shaft; a rocker arm pivotally supported by a rocker shaft and operatively contacting an intake or exhaust valve of a cylinder of the engine to actuate the same; a sub-rocker arm pivotally supported by the main rocker arm and pivotally actuated by the cam; a connecting member supported by the main rocker arm, the connecting member having both a first condition wherein the sub-rocker arm and the main rocker arm are fixed to each other to pivot about the rocker shaft like a single unit and a second condition wherein the sub-rocker arm and the main rocker arm are disengaged from each other to fail to transmit a pivotal movement of the sub-rocker arm induced by rotation of the cam to the main rocker arm thereby to stop operation of the intake or exhaust valve; a hydraulically actuating mechanism including a hydraulic work chamber, the mechanism inducing the first condition of the connecting member upon discharge of hydraulic fluid from the work chamber and inducing the second condition upon feeding of hydraulic fluid to the work chamber; and a control unit that causes the hydraulic actuating mechanism to induce the first condition of the connecting member when the engine stops. 
     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of one unit of a valve operating device according to the present invention, showing parts and portions that are incorporated with a low speed cam; 
     FIG. 2 is a schematic view of an internal combustion engine to which the valve operating device of the invention is practically applied: 
     FIG. 3 is a plan view of the unit of the valve operating device of the present invention, which is incorporated with the two intake valves; 
     FIG. 4 is a front view of the unit of the valve operating device of the present invention; 
     FIG. 5 is a view similar to FIG. 1, but showing parts and portions that are incorporated with a high speed cam; 
     FIG. 6 is a view similar to FIG. 1, but showing a different condition of the valve operating device; and 
     FIG. 7 is a view similar to FIG. 5, but showing a different condition of the valve operating device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, the valve operation device of the present invention will be described in detail with reference to the accompanying drawings. For ease of understanding of the invention, various directional terms, such as, upper, lower, right, left, upward, downward, clockwise, cunterclockwise and the like will be used in the description. However, such terms are to be understood with respect to a drawing or drawings on which the corresponding part and portion are illustrated. 
     Referring to FIG. 2, there is schematically shown an internal combustion engine to which a valve operating device of the present invention is practically applied. 
     In the drawing, denoted by numeral  4  is an in-line four cylinder type internal combustion engine. That is, the engine  4  has four cylinders  6 A,  6 B,  6 C and  6 D which are aligned. The cylinders  6 A,  6 B,  6 C and  6 D have ignition plugs  8 A,  8 B,  8 C and  8 D respectively. Air intake sides of the four cylinders  6 A,  6 B,  6 C and  6 D are respectively connected to four branched passages of an intake passage  22  which has a throttle valve  19  installed in an upstream part thereof. That is, an air/fuel mixture created in the intake passage  22  is fed to the four cylinders  6 A,  6 B,  6 C and  6 D through the corresponding four branched passages. The air/fuel mixture fed to each cylinder  6 A,  6 B,  6 C or  6 D is combusted due to operation of an ignition system that includes the ignition plugs  8 A,  8 B,  8 C and  8 D, a distributor  12  and an ignition control unit  14 . The ignition control unit  14  includes an ignition coil. The ignition order of the cylinders  6 A,  6 B,  6 C and  6 D is, for example,  6 A→ 6 C→ 6 B→ 6 D. Due to combustion of the air/fuel mixture, combustion gas is created in each cylinder  6 A,  6 B,  6 C or  6 D. The combustion gas is discharged from each cylinder, as an exhaust gas, to an exhaust passage  2  through a corresponding branched passage of the exhaust passage  2 . 
     At an output side of the engine  4 , there is mounted a transmission  10  which inputs an engine power from a crankshaft of the engine  4 . 
     Referring to FIGS. 3 and 4, there is shown one unit of the valve operating device of the present invention. In the illustrated embodiment, the unit is incorporated with one of the four cylinders  6 A,  6 B,  6 C and  6 D to actuate two intake valves  42 A and  42 B of the cylinder. It is to be noted that exhaust valves of the cylinder is actuated by another unit which is substantially the same in construction as the unit for the intake valves. In the illustrated embodiment, a so-called “over head camshaft” system (viz., OHC) is employed by the engine  4  for driving the two intake valves. 
     The valve operating device comprises a main rocker arm  34  that is pivotally supported by a rocker shaft  30  through a hole  34   h  formed therethrough. The main rocker arm  34  is formed with two arm portions  34 A and  34 B that are contactable with respective ends of valve stems of the two intake valves  42 A and  42 B. Between the two arm portions  34 A and  34 B of the main rocker arm  34 , there are pivotally arranged two sub-rocker arms  36  and  38 . 
     For ease of understanding, in the following description, these two sub-rocker arms  36  and  38  will be referred to as high and low speed sub-rocker arms respectively. 
     Above the main rocker arm  34  and the high and low speed sub-rocker arms  36  and  38 , there is arranged a cam shaft  32  which extends in parallel with the rocker shaft  30 . The cam shaft  32  is rotated about its axis in response to rotation of the crankshaft of the engine  4 . That is, the valve operating device comprises generally the main rocker arm  34 , the high and low speed sub-rocker arms  36  and  38  and the cam shaft  32 . 
     The main rocker arm  34  has at its base portion a through hole  34   h  through which the rocker shaft  30  passes. With this, the main rocker arm  34  is pivotally supported by the rocker shaft  30 . The rocker shaft  30  has both ends tightly held by a cylinder head (not shown) of the engine  4 . 
     As is seen from FIG. 3, the main rocker arm  34  is formed near the through hole  34   h  with three bearing portions  34 I,  34 J and  34 K which are spaced from one another. These bearing portions  34 I,  34 J and  34 K are formed with aligned bores  34 I,  34   j  and  34   k  through which a supporting shaft  40  passes. Between the bearing portions  34 I and  34 J, there is arranged a base portion of the high speed sub-rocker arm  36 , and between the bearing portions  34 J and  34 K, there is arranged a base portion of the low speed sub-rocker arm  38 . The base portions of the high and low speed sub-rocker arms  36  and  38  are respectively formed with bearing holes  36   a  and  36   b  (see FIGS. 5 and 1) through which the supporting shaft  40  passes. Both ends of the supporting shaft  40  are held by the bearing portions  34 I and  34 K through respective retainer rings Sla and Sla fitted to the ends. 
     Leading end portions of the two arm portions  34 A and  34 B of the main rocker arm  34  are formed, at portions thereof facing the intake valves  42 A and  42 B, with respective contacting portions  34 C and  34 D which are contactable with the upper ends of the valve stems of the intake valves  42 A and  42 B. The leading end portions of the two arm portions  34 A and  34 B are integrally connected through a connecting portion  34 E. 
     As is seen from FIG. 4, the intake valves  42 A and  42 B are biased toward the contacting portions  34 C and  34 D of the main rocker arm  34  by respective coil springs  44 A and  44 B. Each coil spring  44 A or  44 B is held by a retainer fixed to an end of the valve stem. 
     As is seen from FIGS. 1 and 4, the low speed sub-rocker arm  38  is formed, at an upper surface thereof facing the cam shaft  32 , with a cam follower  38 A which slidably contacts a low speed cam  32 D tightly disposed on the cam shaft  32 . The low speed sub-rocker arm  38  is further formed, at a lower surface thereof, with a recess  38 B to which an engaging portion  48 A of an after-mentioned connecting lever  48  is engageable. 
     As is seen from FIG. 4, the recess  38 B is formed with a wall  38 g that extends perpendicular to the axis of the supporting shaft  40  and faces leftward in the drawing, that is, toward the high speed sub-rocker arm  36 . 
     As is seen from FIG. 1, the low speed sub-rocker arm  38  is formed at its lower surface with a projection  38 C which holds an upper end of a coil spring  56  which is operatively interposed between the main rocker arm  34  and the low speed sub-rocker arm  38 . A lower end of the coil spring  56  is held by a projection  34   pa  formed on the main rocker arm  34 . With the force of the coil spring  56 , the low speed sub-rocker arm  38  is biased toward the cam shaft  32 , that is, biased to pivot in a counterclockwise direction in FIG.  1 . 
     It is to be noted that the coil spring  56  is arranged between the main rocker arm  34  and the low speed sub-rocker arm  38  without using a conventionally used spring holder that is to be received in the main rocker arm  34 . This means that in the illustrated embodiment, there is no need of worrying about a friction inevitably produced between the spring holder and the internal wall of the main rocker arm  34 . Furthermore, such simple arrangement of the coil spring  56  between the two rocker arms  34  and  38  brings about reduction in number of parts and simplification in machining the rocker arms  34  and  38 . Furthermore, when the two projections  38 C and  34 pa are arranged to contact each other, the coil spring  56  is protected from being applied with an undesirable shearing force. 
     As is seen from FIG. 1, the low speed sub-rocker arm  38  is formed at the base portion thereof with a projection  38   d  that extends downward. The main rocker arm  34  is formed near the through hole  34   h  with a stepped portion (no numeral). The projection  38   d  of the low speed sub-rocker arm  38  is able to abut against the stepped portion of the main rocker arm  34 , and thus excessive upward pivoting of the low speed sub-rocker arm  38  relative to the main rocker arm  34  is suppressed. 
     As is seen from FIGS. 4 and 5, the high speed sub-rocker arm  36  is formed, at an upper surface thereof facing the cam shaft  32 , with a cam follower  36 A which slidably contacts a high speed cam  32 C tightly disposed on the cam shaft  32 . The high speed sub-rocker arm  36  is further formed, at a lower surface thereof, with a recess  36 B to which an engaging portion  48 B of the next-mentioned connecting lever  48  is engageable. 
     Thus, it is to be noted that the connecting lever  48  has two engaging portions, which are, the engaging portion  48 A which is engageable with the recess  38 B of the low speed sub-rocker arm  38  and the engaging portion  48 B which is engageable with the recess  36 B of the high speed sub-rocker arm  36 . This arrangement will be well seen from FIG.  4 . As will be understood when comparing FIGS. 1 and 5, the length of the recess  36 B measured with respect to the traveling path of the connecting lever  48  is shorter than that of the above-mentioned recess  38 B of the low speed sub-rocker arm  38 . 
     As is seen from FIG. 4, the recess  36 B of the high speed sub-rocker arm  36  is formed with a wall  36   g  that extends perpendicular to the axis of the supporting shaft  40  and faces rightward in the drawing, that is, toward the wall  38   g  of the recess  38 B of the low speed sub-rocker arm  38 . 
     That is, in a rest condition of the low and high speed sub-rocker arms  38  and  36 , the respective recesses  38 B and  36 B face each other. 
     As is seen from FIG. 5, the high speed sub-rocker arm  36  is formed at its lower surface with a projection  36 C which holes an upper end-of a coil spring  58  which is operatively interposed between the main rocker arm  34  and the high speed sub-rocker arm  36 . A lower end of the coil spring  58  is held by a projection  34   pb  formed on the main rocker arm  34 . With the force of the coil spring  58 , the high speed sub-rocker arm  36  is biased toward the cam shaft  32 , that is, biased to pivot in a counterclockwise direction in FIG.  5 . 
     It is to be noted that the coil spring  58  is arranged between the main rocker arm  34  and the high speed sub-rocker arm  36  without using a conventionally used spring holder that is to be received in the main rocker arm  34 . Thus, there is no need of worrying about a friction inevitably produced between the spring holder and the internal wall of the main rocker arm  34 . Furthermore, such simple arrangement of the coil spring  58  between the two rocker arms  34  and  36  brings about reduction in number of parts and simplification in machining the main rocker arms  34  and  36 . Furthermore, when the two projections  36 C and  34   pb  are arranged to contact each other, the coil spring  58  is protected from being applied with an undesirable shearing force. 
     As is seen from FIG. 5, the high speed sub-rocker arm  36  is formed at the base portion thereof with a projection  36 d that extends downward. The main rocker arm  34  is formed near the through hole  34   h  with a stepped portion (no numeral). The projection  36   d  of the high speed sub-rocker arm  36  is able to abut against the stepped portion of the main rocker arm  34 , and thus excessive upward pivoting of the high speed sub-rocker arm  36  relative to the main rocker arm  34  is suppressed. 
     As is seen from FIGS. 1 and 4, the low speed cam  32 D is tightly disposed about the cam shaft  32 , which slidably contacts the cam follower  38 A to determine the lift degree of the intake valves  42 A and  42 B when the engine  4  is in a lower speed operation mode. 
     As is seen from FIGS. 4 and 5, the high speed cam  32 C is tightly disposed about the cam shaft  32  beside the low speed cam  32 D, which slidably contacts the cam follower  36 A to determine the lift degree of the intake valves  42 A and  42 B when the engine  4  is in a high speed operation mode. 
     As will become apparent when comparing FIGS. 1 and 5, the maximum eccentricity (viz., lobe) of the low speed cam  32 D relative to the axis of the cam shaft  32  is smaller than that of the high speed cam  32 C. Although not shown in the drawings, a so-called variable valve open/close timing unit is installed at one end of the cam shaft  32  to adjust the cam face angle of the cam shaft  32 . 
     As is understood from FIGS. 1 and 4, at a lower portion of the main rocker arm  34 , that is, below high and low speed sub-rocker arms  36  and  38 , there is arranged a supporting shaft  46  which extends in parallel with the cam shaft  32 . For supporting the supporting shaft  46 , two spaced bearing portions  34 F and  34 F are formed on the main rocker arm  34 . Both ends of the supporting shaft  46  are held by the bearing portions  34 F and  34 F through respective retainer rings SLa and SLb fitted to the ends. 
     The connecting lever  48  is pivotally supported by the supporting shaft  46 . The connecting lever  48  is integrally formed with two engaging portions, which are the engaging portion  48 B which is selectively engageable with the recess  36 B of the high speed sub-rocker arm  36  and the engaging portion  48 A which is selectively engageable with the recess  38 B of the low speed sub-rocker arm  38 . These two engaging portions  48 A and  48 B are spaced from each other in a direction parallel with the axis of the supporting shaft  46 . 
     As will be understood when comparing FIGS. 1 and 5, the engaging portion  48 A is arranged nearer to the rocker shaft  30  than the other engaging portion  48 B by a predetermined angle which the connecting lever  48  can pivot. Accordingly, when a top end  48   b  of the engaging portion  48 A is shifted from a position shown by a solid line in FIG. 1 to an engaging position shown by a phantom line, the other engaging portion  48 B is shifted from a position shown by a phantom line in FIG. 5 to a position shown by a solid line. That is, upon counterclockwise pivoting from OFF position in FIGS. 1 and 5, the engaging portion  48 A can arrive at ON position faster than the other engaging portion  48 B. 
     As is seen from FIG. 1, the top end  48   b  of the engaging portion  48 A is shaped roundly to achieve a smoothed engagement with the recess  38 B of the low speed sub-rocker arm  38 . Furthermore, as is seen from FIG. 5, a top end  48   d  of the other engaging portion  48 B is shaped roundly to achieve a smoothed engagement with the recess  36 B of the high speed sub-rocker arm  36 . 
     As is seen from FIG. 4, a return spring  50  is arranged, which has a middle portion engaged with a lower portion of the connecting lever  48  and both ends held by both ends of the supporting shaft  46 . With this return spring  50 , the connecting lever  48  is biased to pivot in a direction to move the two engaging portions  48 A and  48 B away from the respective low and high speed sub-rocker arms  38  and  36 , that is, in a clockwise direction in FIGS. 1 and 5. 
     As is seen from FIG. 1, the rocker shaft  30  is formed with two axially extending hydraulic passages  30   ar  and  30   br . These passages  30   ar  and  30   br  are connected to an after-mentioned hydraulic circuit. 
     The main rocker arm  34  is formed, at a portion facing the engaging portion  48 A of the connecting lever  48 , with a hydraulic work chamber  34   r  which is communicated with the hydraulic passage  30   ar  through hydraulic passages  30   cr ,  34   oa ,  34   ob  and  34   oc . As shown, the work chamber  34   r  is formed near its open end  34   ra  with an annular groove to which the hydraulic passage  34   oc  is exposed. Within the hydraulic work chamber  34   r , there is slidably received a piston  52 . The piston  52  has a shoulder portion to which the hydraulic pressure in the work chamber  34   r  is practically applied. As shown, the exposed end of the piston  52  is rounded. The hydraulic passages  34   oa  and  34   ob  each have an end sealed with a plug member  60 A or  60 B. One end of the hydraulic passage  34   oc  is connected to a space that is defined between a leading portion of the piston  52  and an inner wall of the hydraulic work chamber  34   r.    
     Within a blind bore formed in the piston  52 , there is disposed a coil spring  62  which has one end seated on the bottom of the hydraulic work chamber  34   r  and the other end seated on the bottom of the blind bore. With this coil spring  62 , the piston  52  is biased rightward in FIG. 1, that is, in a direction in which the leading portion of the piston  52  projects outward through an open end  34   ra . The biasing force produced by the coil spring  62  is greater than that of the return spring  50  that biases the connecting lever  48 . As shown, the leading top of the piston  52  is in contact with a downward projection  48   a  of the engaging portion  48 A of the connecting lever  48 . 
     When the hydraulic work chamber  34   r  is fed with a certain hydraulic pressure through the hydraulic passages  30   ar ,  30   cr ,  34   oa ,  34   ob  and  34   oc , the piston  52  is retracted into the work chamber  34   r  against the force of the coil spring  62  and the leading top of the piston  52  becomes flush with an outer surface of the main rocker arm  34  as is shown in FIG.  1 . With this, the connecting lever  48  is permitted to pivot in a clockwise direction in FIG. 1 due to the force of the return spring  50 . Upon this, as is shown by a solid line, the engaging portion  48 A of the connecting lever  48  is disengaged from the low speed sub-rocker arm  38 . 
     While, when the hydraulic pressure is discharged from the hydraulic work chamber  34   r , the piston  52  is forced to take its projected position due to the force of the coil spring  62  causing the leading top thereof to largely project from the outer surface of the main rocker arm  34  as is shown by a phantom line in FIG.  1 . Thus, in this case, the connecting lever  48  is pivoted in a counterclockwise direction. 
     As is seen from FIG. 5, the main rocker arm  34  is formed, at a portion facing the engaging portion  48 B of the connecting lever  48 , with a hydraulic work chamber  34   or  which is communicated with the hydraulic passage  30   br  through hydraulic passages  30   dr  and  34   od . Within the hydraulic work chamber  34   or , there is slidably received a piston  54 . As shown, the exposed end of the piston  54  is rounded. 
     As is seen from FIG. 7, when the hydraulic work chamber  34   or  is fed with a certain hydraulic pressure through the hydraulic passages  30   dr  and  34   od , the piston  54  is projected outward through an open end of the work chamber  34   or . With this, a downward projection  48   c  of the engaging portion  48 B of the connecting lever  48  is pushed rightward in the drawing pivoting the connecting lever  48  in a counterclockwise direction against the force of the return spring  50 , that is, in a direction to cause the top end  48   d  of the engaging portion  48 B to near the high speed sub-rocker arm  36 . 
     While, when the hydraulic pressure is discharged from the hydraulic work chamber  34   or , the piston  54  is retracted into the work chamber  34   or  due to the force of the return spring  50 . That is, in this case, the connecting lever  48  is pivoted in a counterclockwise direction in FIG. 5, that is, in a direction to move the top end  48   d  of the engaging portion  48 B away from the high speed sub-rocker arm  36 . 
     As is shown in FIG. 2, for feeding the above-mentioned hydraulic work chambers  34   r  and  34   or  with a given hydraulic pressure, there is provided a hydraulic pressure producing unit  72 . The hydraulic pressure producing unit  72  is controlled by an engine control unit  70  in accordance with the operation condition of the engine  4 . In fact, the valve lifting control, valve stopping control and ignition timing control are all carried out by the engine control unit  70 . 
     The hydraulic pressure producing unit  72  comprises generally a plurality of hydraulic passages whose one ends are connected to an outlet side of an oil pump and a plurality of electromagnetic valves respectively installed in the hydraulic passages. The other ends of the hydraulic passages are respectively connected to hydraulic passages defined in the engine  4 , and the oil pump is operated to pump up the hydraulic fluid in an oil pan of the engine  4 . 
     The hydraulic passages are grouped into two which are independent from each other. That is, for example, one group is applied to the hydraulic passages  30   ar  and  30   br  which are provided for only the cylinders  6 B and  6 C, and the other group is applied to the hydraulic passages  30   ar  and  30   br  which are provided for only the other cylinders  6 A and  6 D. 
     Upon receiving an instruction signal from the engine control unit  70 , each electromagnetic valve functions to feed the hydraulic work chamber  34   r  or  34   or  with an adjusted hydraulic pressure. 
     Inputted into the engine control unit  70  are an engine speed signal S n  produced by an engine speed sensor  16  mounted to the distributor  12 , a crank angle signal S c  produced by a crank angle sensor  18  mounted to the distributor  12 , a cooling water temperature signal S w  produced by a temperature sensor  17  installed in a cooling water jacket of the engine  4 , a throttle angle signal S t  produced by a throttle angle sensor  20  which senses the opening angle of the throttle valve  19 , an intake air rate signal S a  produced by an air flow meter and an intake negative pressure signal S b  produced by an intake pressure sensor. 
     In the engine control unit  70 , based on the engine speed signal S n  and the intake negative pressure signal S b , a reference spark-advance value is determined, based on the cooling water temperature signal S w , a correction value for the spark-advance value is determined, and based on the reference spark-advance value and the correction value, an effective spark-advance value is determined. Furthermore, in the engine control unit  70 , in accordance with the crank angle signal S c  and the determined effective spark-advance value, an ignition timing control signal C i  is produced and led into the ignition control unit  14 . With this, as has been mentioned hereinabove, at first, ignition is carried out in the cylinder  6 A, then in the cylinder  6 C, then in the cylinder  6 B and then in the cylinder  6 D. 
     In the valve lift degree switching control, based on the engine speed signal S n  and the throttle angle signal S t , or the intake air rate signal S a  and the cooling water temperature signal S w , the engine control unit  70  stops feeding of hydraulic pressure to the hydraulic passages  30   ar  and  30   br  of all of the cylinders  6 A,  6 B,  6 C and  6 D at the time of engine starting. Thus, as is seen from FIG. 6, at the engine starting, the piston  52  takes its projected position causing the engaging portion  48 A of the connecting lever  48  to operatively engage with the recess  38 B of the low speed sub-rocker arm  38 . While, as is seen from FIG. 5, at this engine starting, the piston  54  assumes its retracted position causing the other engaging portion  48 B of the connecting lever  48  to be released from the corresponding recess  36 B. 
     That is, in this case, the engaging portion  48 A becomes operative and thus, the main rocker arm  34  is actuated by the low speed cam  32 D, as is shown in FIG.  6 . Thus, the opening/closing operation of the intake valves  42 A and  42 B is timed by the low speed cam  32 D. Accordingly, the engine starting is smoothly and assuredly carried out. 
     Furthermore, based on the engine speed signal S n  and the throttle angle signal S t  or the intake air rate signal S a  and the cooling water temperature signal S w , the engine control unit  70  stops feeding of hydraulic pressure to the hydraulic passages  30   ar  and  30   br  of all of the cylinders  6 A,  6 B,  6 C and  6 D when the engine  4  runs at a lower speed (viz., lower than 5,000 rpm) in a medium to high load. Under this low speed operation condition of the engine  4 , only the engaging portion  48 A of the connecting lever  48  becomes operative for the reason as has been mentioned in the section of engine starting. Thus, the opening/closing operation of the intake valves  42 A and  42 B is timed by the low speed cam  32 D. 
     Furthermore, based on the engine speed signal S n  and the throttle angle signal S t  or the intake air rate signal S a  and the cooling water temperature signal S w , the engine control unit  70  carries out feeding of hydraulic pressure to only the hydraulic work chambers  34   or  of all of the cylinders  6 A,  6 B,  6 C and  6 D through the hydraulic passages  30   br  when the engine  4  runs at a higher speed (viz., 5,000 rpm to 8,000 rpm) in a medium to high load. In fact, for feeding the hydraulic pressure to the hydraulic work chambers  34   or , the hydraulic pressure producing unit  72  receives a corresponding instruction signal C db  from the engine control unit  70 . 
     As is seen from FIG. 7, upon supply of hydraulic pressure to the hydraulic work chamber  34   or  through the hydraulic passage  30   br , the piston  54  is shifted to take its projected position, and thus, the engaging portion  48 B of the connecting lever  48  is brought into engagement with the recess  36 B of the high speed sub-rocker arm  36 . While, as is seen from FIG. 6, because the hydraulic work chamber  34   r  is not fed with hydraulic pressure, the piston  52  keeps its projected position, and thus the engagement between the engaging portion  48 A of the connecting lever  48  and the recess  38 B of the low speed sub-rocker arm  38  is kept. That is, in this condition, both the engaging portions  48 B and  48 A of the connecting lever  48  are engaged with the corresponding recesses  36 B and  38 B of the high and low speed sub-rocker arms  36  and  38 , respectively. That is, both the sub-rocker arms  36  and  38  are fixed to the main rocker arm  34  to act as a single unit. 
     Accordingly, as is understood from FIG. 7, the main rocker arm  34  is actuated by the high speed cam  32 C. That is, the opening/closing operation of the intake valves  42 A and  42 B is timed by the high speed cam  32 C. As is seen from this drawing, the construction of the high speed cam  32 C is the same as that of the low speed cam  32 D except the radially projected cam portion, and the radially projected cam portion of the high speed cam  32 C is higher than that of the low speed cam  32 D. Thus, the pivoting movement of the rocker cam  34  is effected by only the high speed cam  32 C that slidably contacts the cam follower  36 A of the high speed sub-rocker arm  36 . In other words, rotation of the low speed cam  32 D has substantially no effect on the pivoting movement of the rocker cam  34 . 
     In the valve stopping control, based on the engine speed signal S n  and the throttle angle signal S t , or the intake air rate signal S a  and the cooling water temperature signal S w , the engine control unit  70  carries out feeding of hydraulic pressure to only the hydraulic passages  30   ar  of the cylinders  6 B and  6 C when the engine  4  runs at a lower speed (viz., 750 rpm to 3,000 rpm) in idling or low load. In fact, for feeding the hydraulic pressure to only the hydraulic passages  30   ar  of the cylinders  6 B and  6 C, the hydraulic pressure producing unit  72  receives a corresponding instruction signal C da  from the engine control unit  70 . It is now to be noted that in this condition, the engine control unit  70  does not feed the hydraulic pressure to the hydraulic passages  30   ar  of the other cylinders  6 A and  6 D. 
     Accordingly, as is understood from FIG. 1, the hydraulic work chambers  34   r  for the cylinders  6 B and  6 C are fed with hydraulic pressure through the hydraulic passages  30   ar , while, as is understood from FIG. 5, the hydraulic work chambers  34   or  for the cylinders  6 B and  6 C are not fed with hydraulic pressure. Thus, both the piston  52  (see FIG. 1) and piston  54  (see FIG. 5) take their retracted positions. Accordingly, as is seen from these drawings, both the engaging portions  48 A and  48 B of the connecting lever  48  are released from the corresponding recesses  38 B and  36 B of the low and high speed sub-rocker arms  38  and  36 . Thus, under this valve stopping control, these low and high speed sub-rocker arms  38  and  36  are freely pivotal about the supporting shaft  40  relative to the main rocker arm  34 . Thus, the intake valves  42 A and  42 B of the cylinders  6 B and  6 C assume their rest condition, which can reduce a pumping loss of the engine  4 . 
     As is described hereinabove, under this condition, the low and high speed sub-rocker arms  38  and  36  freely pivot relative to the main rocker arm  34 . Thus, pivoting movement of the low speed sub-rocker arm  38  induced by rotation of the low speed cam  32 D (see FIG. 1) is absorbed by the coil spring  56 , and pivoting movement of the high speed sub-rocker arm  36  induced by rotation of the high speed cam  32 C (see FIG. 5) is absorbed by the coil spring  58 . 
     During this operation, the hydraulic passages  30   ar  and  30   br  for the cylinders  6 A and  6 D are not fed with hydraulic pressure. Thus, the piston  52  for each of these cylinders  6 A and  6 D assumes the projected position (see FIG. 6) causing the engaging portion  48 A of the connecting lever  48  to engage with the recess  38 B of the low speed sub-rocker arm  38 , while the piston  54  for each of the cylinders  6 A and  6 D assumes its retracted position (see FIG. 5) causing the engaging portion  48 B of the connecting lever  48  to release from the recess  36 B of the high speed sub-rocker arm  36 . That is, under this condition, only the low speed sub-rocker arm  38  for each of the cylinders  6 A and  6 D is fixed to the main rocker arm  34  to act as a single unit. Thus, the opening/closing operation of the intake valves  42 A and  42 B for the cylinders  6 A and  6 D is timed by the low speed cam  32 D. 
     As will be understood from the foregoing description, first, second and third operation modes are provided by the valve operating device. That is, in the first operation mode, both of the low speed sub-rocker arm  38  and the high speed sub-rocker arm  36  are disengaged from the main rocker arm  34 . Thus, in this case, the main rocker arm  34  does not operate and thus the intake valves  42 A and  42 B assume their rest condition, which can reduce a pumping loss of the engine  4 . In the second operation mode, only the low speed sub-rocker arm  38  is fixed to the main rocker arm  34 . Thus, in this case, the intake valves  42 A and  42 B are controlled by the low speed cam  32 D through the main rocker arm  34 . In the third operation mode, both the low and high speed sub-rocker arms  38  and  36  are fixed to the main rocker arm  34 . Thus, in this case, the intake valves  42 A and  42 B are controlled by the high speed cam  42 C through the rocker cam  34 . 
     Furthermore, in the present invention, the second operation mode (which is achieved when only the low speed sub-rocker arm  38  is fixed to the main rocker arm  34 ) is carried out when the hydraulic pressure is discharged from the hydraulic work chamber  34   r . This brings about the following advantage. That is, when the engine  4  is stopped, the feeding of hydraulic pressure to the work chamber  34   r  is also stopped. Thus, upon stopping the engine  4 , the second operation mode, that is, the fixing between the low speed sub-rocker arm  38  and the main rocker arm  34 , is instantly assumed by the valve operating device. Thus, subsequent engine starting is smoothly carried out. 
     Although, in the above-mentioned embodiment, for coupling each of the sub-rocker arms  36  and  38  with the main rocker arm  34 , the arrangement using the connecting lever  48  pivotally supported on the main rocker arm  34  is employed, other arrangements such as those disclosed in U.S. Pat. Nos. 6,125,805 and 5,445,115 may be employed which uses a non-pivotal connecting member. 
     The entire contents of Japanese Patent Applications 11338017 (filed Nov. 29, 1999) are incorporated herein by reference. 
     Although the invention has been described above with reference to the embodiment of the invention, the invention is not limited to the embodiment described above. Various modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teachings.