Abstract:
In an exhaust emission control system for an internal combustion engine, an exhaust gas recirculation device that operates so as to partially open an exhaust valve to introduce exhaust gas into a cylinder in an intake stroke is provided; a secondary air passage capable of supplying secondary air to an exhaust passage is connected to the exhaust passage; and a one-way valve opened by negative pressure applied from the cylinder to the exhaust passage following an operation of the exhaust gas recirculation device in the intake stroke is provided in the secondary air passage. Accordingly, it is possible to provide an exhaust emission control system for an internal combustion engine that allows secondary air to be supplied into an exhaust passage to reduce HC and CO concentrations and also a NOx concentration in an exhaust gas without using an air pump regardless of an exhaust system structure of the internal combustion engine.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust emission control system for an internal combustion engine which supplies secondary air into an exhaust passage to oxidize and remove HC and CO in an exhaust gas, and which introduces an exhaust gas into a cylinder in an intake stroke to prevent generation of NOx during combustion. 
     2. Description of the Related Art 
     Japanese Utility Model Registration No. 2535339 discloses an exhaust emission control system for an internal combustion engine including: a secondary air passage connected to an exhaust passage; and a one-way valve that is provided in the secondary air passage and opened by negative pressure by exhaust pulsation in an exhaust stroke in order to supply secondary air into the exhaust passage to oxidize and remove HC and CO in an exhaust gas. Japanese Patent Application Laid-Open No. 2005-240793 also discloses an exhaust emission control system for an internal combustion engine including an exhaust gas recirculation device that operates so as to partially open an exhaust valve to introduce an exhaust gas into a cylinder in an intake stroke in order to prevent generation of NOx during combustion by introducing exhaust gas into the cylinder in the intake stroke. 
     For an internal combustion engine having high muffler back pressure or a short exhaust passage length, sufficient negative pressure by exhaust pulsation is not created in an exhaust passage. Thus, even if the exhaust emission control system disclosed in the above-described Japanese Utility Model Registration No. 2535339 is employed, the one-way valve in the secondary air passage is not opened, and secondary air cannot be supplied to the exhaust passage, thereby preventing a desired effect from being obtained. In such a case, an air pump is connected to the secondary air passage to pump the secondary air into the exhaust passage, but the use of the air pump increases the cost of the exhaust emission control system. 
     SUMMARY OF THE INVENTION 
     The present invention is achieved in view of these circumstances, and has an object to provide an exhaust emission control system for an internal combustion engine that allows secondary air to be supplied into an exhaust passage to reduce HC and CO concentrations and also a NOx concentration in an exhaust gas by using an exhaust gas recirculation device without using an air pump regardless of an exhaust system structure of the internal combustion engine. 
     In order to achieve the object, according to a first feature of the present invention, there is provided an exhaust emission control system for an internal combustion engine comprising: an exhaust gas recirculation device that operates so as to partially open an exhaust valve to introduce an exhaust gas into a cylinder in an intake stroke; a secondary air passage that is connected to an exhaust passage and can supply secondary air to the exhaust passage; and a one-way valve that is provided in the secondary air passage and opened by negative pressure applied from the cylinder to the exhaust passage following an operation of the exhaust gas recirculation device in the intake stroke. 
     According to the first feature of the present invention, the exhaust gas recirculation device operates to partially open the exhaust valve in the intake stoke. This causes an exhaust gas remaining in the exhaust passage to be drawn into the cylinder, that is, the exhaust gas is recirculated. This prevents an excessive increase in combustion temperature during combustion of air-fuel mixture to reduce a NOx concentration in the exhaust gas. Further, in the intake stroke, the exhaust valve is partially opened and the negative pressure in the cylinder is applied to the exhaust passage. The negative pressure opens the one-way valve to bring the secondary air passage into communication with the exhaust passage, the secondary air is drawn into the exhaust passage and further into the cylinder, and the secondary air drawn into the exhaust passage remains in the exhaust passage. Thus, the remaining secondary air reacts with HC and CO in the exhaust gas exhausted to the exhaust passage in a later exhaust stroke to reduce HC and CO concentrations in the exhaust gas. Further, the secondary air drawn into a combustion chamber increases charging efficiency in a cylinder bore to increase an output of the engine. This can reduce HC and CO concentrations and also a NOx concentration in the exhaust gas and increase an output of the engine without using an air pump regardless of an exhaust system structure of the internal combustion engine. 
     According to a second feature of the present invention, in addition to the first feature, a valve box that houses the one-way valve is formed in a head cover coupled to a cylinder head. 
     According to the second feature of the present invention, the need for a special support structure for the valve box for the one-way valve can be eliminated. 
     According to a third feature of the present invention, in addition to the first feature, a valve box that houses the one-way valve is mounted to an exhaust pipe coupled to a cylinder head. 
     According to the third feature of the present invention, the valve box for the one-way valve can be set using dead space around the exhaust pipe. 
     The exhaust passage corresponds to an exhaust port  16   a  and an exhaust pipe  65  of an embodiment of the present invention explained below. The cylinder corresponds to a cylinder bore  3   a.    
     The above description, other objects, characteristics and advantages of the present invention will be clear from detailed descriptions which will be provided for the preferred embodiments referring to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional side view of an internal combustion engine including a valve operating system according to a first embodiment of the present invention; 
         FIG. 2  is a sectional view taken along the line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a sectional view taken along the line  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a sectional view taken along the line  4 - 4  in  FIG. 3 ; 
         FIG. 5  is a sectional view taken along the line  5 - 5  in  FIG. 3 ; 
         FIG. 6  is a view explaining an operation corresponding to  FIG. 5 ; 
         FIG. 7  is a sectional view taken along the line  7 - 7  in  FIG. 3 ; 
         FIG. 8  is a view explaining an operation corresponding to  FIG. 7 ; 
         FIG. 9  is a diagram showing operation regions of a decompression cam member and an exhaust gas recirculation cam member; 
         FIG. 10  is a diagram showing a relationship between a rotation angle of a crankshaft and opening/closing timings of an intake valve and an exhaust valve; and 
         FIG. 11  is a view showing a second embodiment of the present invention and corresponding to  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention will be explained below based on  FIGS. 1 to 10 . 
     In  FIGS. 1 to 3 , an engine body  1  of an internal combustion engine E comprises a crankcase  2 , a cylinder block  3  extending obliquely upward from one side of the crankcase  2 , and a cylinder head  4  joined to an upper end surface of the cylinder block  3 . The crankcase  2  houses a crankshaft  6  supported on left and right sidewalls thereof via bearings  5  and  5 . The crankshaft  6  is connected to a piston  7  fitted in a cylinder bore  3   a  of the cylinder block  3  via a connecting rod  8 . A flywheel  10  is secured to one end of the crankshaft  6  protruding outside the crankcase  2 . An annular cooling fan  11  is secured to an outer side surface of the flywheel  10 . A starting cylinder shaft  12  protruding axially outward from the cooling fan  11  is secured on a center of the flywheel  10 . A known recoil starter  13  that can engage the starting cylinder shaft  12  to crank the crankshaft  6  is mounted to the crankcase  2  via a bracket  14 . The other end of the crankshaft  6  protrudes outside the other sidewall of the crankcase  2  as an output end. 
     In the cylinder head  4 , a combustion chamber  15  connecting to the cylinder bore  3   a , and an intake port  16   i  and an exhaust port  16   e  having inner ends opening into the combustion chamber  15  are formed, and also an intake valve  17   i  and an exhaust valve  17   e  that open and close the intake port  16   i  and the exhaust port  16   e  are provided. A valve operating system  20  that opens and closes these intake valve  17   i  and the exhaust valve  17   e  is provided on a region from the crankcase  2  to the cylinder head  4 . 
     This valve operating system  20  will be described. 
     In  FIGS. 1 to 3 , the valve operating system  20  includes: a camshaft  26  that is supported on the left and right sidewalls of the crankcase  2  and driven by the crankshaft  6  via a timing transmission device  25 ; an intake tappet  27   i  and an exhaust tappet  27   e  that are axially slidably supported by the cylinder block  3  to cause flanges  27   ia  and  27   ea  at lower ends to abut against an intake cam  26   i and an exhaust cam  26   e  of the camshaft  26  in a way to allow sliding contact; an intake push rod  28   i  and an exhaust push rod  28   e  that have lower ends connecting to upper ends of the intake tappet  27   i  and the exhaust tappet  27   e  and upper ends extending above the cylinder head  4 ; and an intake rocker arm  29   i  and an exhaust rocker arm  29   e  that are oscillably supported by a pair of spherical support members  33  and  33  secured to the cylinder head  4 . The intake rocker arm  29   i  is placed so that one end thereof abuts against a head of the intake valve  17   i  and the other end connects to an upper end of the intake push rod  28   i . The exhaust rocker arm  29   e  is placed so that one end thereof abuts against a head of the exhaust valve  17   e  and the other end connects to an upper end of the exhaust push rod  28   e . 
     Thus, when the intake cam  26   i  and the exhaust cam  26   e  push up the intake tappet  27   i  and the exhaust tappet  27   e , respectively, the intake push rod  28   i  and the exhaust push rod  28   e  that move along with these tappets cause the intake rocker arm  29   i  and the exhaust rocker arm  29   e  to oscillate in opening directions of the intake valve  17   i  and the exhaust valve  17   e . An intake valve spring  30   i  and an exhaust valve spring  30   e  that urge the intake valve  17   i  and the exhaust valve  17   e  in closing directions are mounted to the intake valve  17   i  and the exhaust valve  17   e , respectively. A head cover  18  that covers the valve operating system  20  including the intake rocker arm  29   i  and the exhaust rocker arm  29   e  and the like on the cylinder head  4 , is joined to an upper end surface of the cylinder head  4 . 
     The timing transmission device  25  comprises a drive gear  31  secured to the crankshaft  6 , and a driven gear  32  that is secured to the camshaft  26  and driven by the drive gear  31  at a 1/2 reduction ratio. As shown in  FIGS. 3 and 4 , one end of a centrifugal weight  35  formed into a U-shaped so as to surround a half circumference of the camshaft  26  is supported on the driven gear  32  via a pivot  36 . The centrifugal weight  35  has a center of gravity  37  in a U-shaped bending portion thereof, and oscillates around the pivot  36  between a contracted position A (see  FIGS. 4 and 5 ) where the U-shaped bending portion abuts against a hub  32   a  of the driven gear  32  and an extended position B (see  FIG. 8 ) where an outer side surface of the U-shaped bending portion abuts against a stopper  38  protruding on a side surface of the driven gear  32 . The centrifugal weight  35  is provided with a first movable connection  40  and a second movable connection  41  in a middle portion and an oscillation end thereof. Movable ends  43   a  and  44   a  of a first return spring  43  and a second return spring  44  having secured ends connected to a common secured connection  42  of the driven gear  32  and constituted by tension coil springs are connected to the first movable connection  40  and the second movable connection  41 . The first return spring  43  is adapted to always urge the centrifugal weight  35  radially inward with a predetermined set load Fs 1 . A certain play S is provided between the movable end  44   a  of the second return spring  44  and the second movable connection  41 . The second return spring  44  does not resist the oscillation of the centrifugal weight  35  until the centrifugal weight  35  oscillates radially outward through a predetermined angle from the contracted position A. In other words, the second return spring  44  is adapted to exert a repulsive force that urges the centrifugal weight  35  radially inward in cooperation with the first return spring  43  after the centrifugal weight  35  oscillates radially outward through the predetermined angle from the contracted position A. 
     As shown in  FIG. 3 , a drive ring  45 , a decompression cam member  47 , and an exhaust gas recirculation cam member  48  are fitted to the camshaft  26  in order from the side of the centrifugal weight  35  between the centrifugal weight  35  and the exhaust cam  26   e . The drive ring  45  is rotatably fitted to an outer peripheral surface of the camshaft  26 . The centrifugal weight  35  has a pair of connection pieces  35   a  and  35   a  placed so as to hold a connection protrusion  45   c  protruding from an outer peripheral surface of the drive ring  45  therebetween from opposite sides along a rotational direction of the drive ring  45 . The radially outward oscillation of the centrifugal weight  35  causes the rotation of the drive ring  45 . The drive ring  45  has an axial holding groove  45   a  in an inner peripheral surface thereof, and, by this holding grooves  45   a , holds a roller  45   b  extending axially along the camshaft  26  between the driven gear  32  and the exhaust cam  26   e . This roller  45   b  can roll on the outer peripheral surface of the camshaft  26  with a rotation of the drive ring  45  relative to the camshaft  26 . 
     In  FIGS. 5 to 8 , the decompression cam member  47  and the exhaust gas recirculation cam member  48  are fitted to a pair of guide surfaces  55  and  55  formed on opposite side surfaces of the camshaft  26  and parallel to each other so that these cam members slide along a diameter line of the camshaft  26 . This allows the decompression cam member  47  to slide along the guide surfaces  55  and  55  between an operative position C ( FIG. 5 ) and an inoperative position D ( FIG. 6 ). The operative position C and the inoperative position D are determined by inner end surfaces  56   a  and  56   b  of the decompression cam member  47  orthogonal to the guide surfaces  55  and  55  and abutting against the outer peripheral surface of the camshaft  26 . The exhaust gas recirculation cam member  48  can slide along the guide surfaces  55  and  55  between an inoperative position F ( FIG. 7 ) and an operative position G ( FIG. 8 ). The inoperative position F and the operative position G are determined by inner end surfaces  57   a  and  57   b  of the exhaust gas recirculation cam member  48  orthogonal to the guide surfaces  55  and  55  and abutting against the outer peripheral surface of the camshaft  26 . 
     The decompression cam member  47  and the exhaust gas recirculation cam member  48  integrally have, on outer peripheral surfaces thereof, convex cams  47   a  and  48   a  much lower than a nose of the exhaust cam  26   e . The convex cams  47   a  and  48   a  protrude outward beyond a base circle of the exhaust cam  26   e  in the operative positions C and G, and retract into the base circle in the inoperative positions D and F. The convex cam  47   a  of the decompression cam member  47  is placed so as to push up the exhaust tappet  27   e  in a compression stroke of the engine when the convex cam  47   a  is in the operative position C. The convex cam  48   a  of the exhaust gas recirculation cam member  48  is placed so as to push up the exhaust tappet  27   e  in an intake stroke of the engine when the convex cam  48   a  is in the operative position G. 
     As shown in  FIGS. 5 and 6 , a recessed cam  58  that cooperates with the roller  45   b  is formed in a center part of the inner end surface  56   b  of the decompression cam member  47  on the side of the convex cam  47   a . The recessed cam  58  includes: an inclined surface  58   a  that is pressed by the roller  45   b  to force the decompression cam member  47  into the operative position C, when the centrifugal weight  35  is held in the contracted position A by an urging force of the first return spring  43 ; and an arcuate bottom surface  58   b  that prevents interference with the roller  45   b  and allows the decompression cam member  47  to move to the inoperative position D, after the centrifugal weight  35  rotates through a predetermined angle or more from the contracted position A (a state in  FIG. 6 ). By forming the recessed cam  58 , a center of gravity of the decompression cam member  47  is offset from the center thereof to an opposite side from the recessed cam  58 . When the roller  45   b  comes to a position facing the arcuate bottom surface  58   b , the decompression cam member  47  moves to the inoperative position D by a centrifugal force acting on the center of gravity. 
     On the other hand, a recessed cam  59  that cooperates with the roller  45   b  is formed in a center part of the inner end surface  57   b  of the exhaust gas recirculation cam member  48  on the side of the convex cam  48   a . The recessed cam  59  includes: an arcuate bottom surface  59   a  that prevents interference with the roller  45   b  and allows the exhaust gas recirculation cam member  48  to move to the inoperative position F during rotation of the centrifugal weight  35  from the contracted position A through a predetermined angle; and an inclined surface  59   b  that is pressed by the roller  45   b  to force the exhaust gas recirculation cam member  48  into the operative position G when the centrifugal weight  35  rotates radially outward through a predetermined angle or more by a centrifugal force. By forming the recessed cam  59 , a center of gravity of the exhaust gas recirculation cam member  48  is offset from the center thereof to an opposite side from the recessed cam  59 . When the roller  45   b  is in a position facing the arcuate bottom surface  59   a , the exhaust gas recirculation cam member  48  moves to the operative position G by a centrifugal force acting on the center of gravity. 
     In the above description, the centrifugal weight  35 , the first return spring  43  and the second return spring  44 , and the drive ring  45  cooperate with one another to constitute a common centrifugal mechanism  46  that operates the decompression cam member  47  and the exhaust gas recirculation cam member  48 . The centrifugal mechanism  46 , the roller  45   b , and the decompression cam member  47  constitute a decompression device  61 , and the centrifugal mechanism  46  and the exhaust gas recirculation cam member  48  constitute an exhaust gas recirculation device  62 . 
     Referring again to  FIG. 2 , an ignition plug  21  whose electrode faces the combustion chamber  15  is screwed into the cylinder head  4 . A carburetor  22  connecting to an outer end of the intake port  16   i  is mounted to the cylinder head  4 . An air cleaner  64  is connected to an air inlet of this carburetor  22 . An exhaust pipe  65  connecting to an outer end of the exhaust port  16   e  is mounted to the cylinder head  4 . The exhaust pipe  65  is relatively short, and an exhaust muffler  23  being equipped therewith a three way catalyst converter  66  is connected to a downstream end of the exhaust pipe  65 . 
     A valve box  67  is formed in the head cover  18  that is coupled to an upper end of the cylinder head  4  and covers the valve operating system  20 . This valve box  67  is constituted by a lower box half body  67   a  integrally formed with a ceiling wall of the head cover  18  and having an open upper surface and an upper box half body  67   b  coupled to the lower box half body  67   a  by a bolt. A joint pipe  69  to which an air tube  68  extending from the air cleaner  64  is connected is integrally formed with one side of the upper box half body  67   b . The lower box half body  67   a  communicates with the intake port  16   i  via an air hole  70  provided in the cylinder head  4  and the head cover  18 . The valve box  67 , the air tube  68 , and the air hole  70  constitute a secondary air passage  71 . 
     In the valve box  67 , a one-way valve  73  is provided that is constituted by a reed valve and opened when negative pressure is created in the exhaust port  16   e  to bring the secondary air passage  71  into communication. The one-way valve  73  is held between the lower and upper box half bodies  67   a  and  67   b , and includes a diaphragm plate  74  that partitions the inside of the valve box  67  into an upstream chamber  75   a  on a side of the air tube  68  and a downstream chamber  75   b  on a side of the air hole  70 . This diaphragm plate  74  has a valve hole  76  that provide communication between the chambers  75   a  and  75   b , and a reed valve plate  77  that opens and closes the valve hole  76  and a stopper plate  78  that restricts an opening limit of the reed valve plate  77  are mounted to the diaphragm plate  74  on a side of the downstream chamber  75   b  by a screw  79  or a rivet. The reed valve plate  77  is generally brought into tight contact with the diaphragm plate  74  by its elasticity to close the valve hole  76 , and when pressure in the downstream chamber  75   b  becomes lower than that in the upstream side, the pressure difference causes the reed valve plate  77  to bend toward the stopper plate  78  side to open the valve hole  76 . 
     Next, an operation of this embodiment will be described. 
     When the crankshaft  6  rotates, the camshaft  26  is driven by the crankshaft  6  via the timing transmission device  25  at a 1/2 reduction ratio. In the intake stroke, the intake cam  26   i  pushes up the intake push rod  28   i  via the intake tappet  27   i , causes the intake rocker arm  29   i  to oscillate, and forces the intake valve  17   i  to open against an urging force of the intake valve spring  30   i . In the exhaust stroke, the exhaust cam  26   e  pushes up the exhaust push rod  28   e  via the exhaust tappet  27   e , causes the exhaust rocker arm  29   e  to oscillate, and forces the exhaust valve  17   e  to open against an urging force of the exhaust valve spring  30   e . Such opening/closing timings of the intake valve  17   i  and the exhaust valve  17   e  are shown in  FIG. 10 . 
     In  FIG. 9 , the centrifugal weight  35  is held in the contracted position A by the set load Fs 1  of the first return spring  43  as shown in  FIGS. 4 and 5  in a starting rotation region a of the engine from the engine rotational speed Ne of zero to a predetermined rotational speed Ne 1  lower than an idling rotational speed. At this time, the centrifugal weight  35  causes the roller  45   b  of the drive ring  45  positioned via the connection protrusion  45   c  and the connection pieces  35   a  and  35   a  that engage each other, to press the inclined surface  58   a  of the recessed cam  58  of the decompression cam member  47 . Thus, the decompression cam member  47  is held in the operative position C where the convex cam  47   a  protrudes outward beyond the base circle of the exhaust cam  26   e . Therefore, if the recoil starter  13  is operated to start the internal combustion engine E, the crankshaft  6  is cranked from the starter  13  via the starting cylinder shaft  12 , and at the same time, the camshaft  26  is driven via the timing transmission device  25 . Consequently, as described above, in the compression stroke of the piston  7 , the convex cam  47   a  of the decompression cam member  47  slightly pushes up the exhaust tappet  27   e  to slightly open the exhaust valve  17   e  via the exhaust push rod  28   e  and the exhaust rocker arm  29   e . The timing at this time is shown in  FIG. 10 . As a result, a part of compression gas in the cylinder bore  3   a  is discharged through the exhaust port  16   e  to prevent an increase in a compression pressure of the gas, thus reducing an operation load of the starter  13 . Therefore, the crankshaft  6  can be cranked relatively lightly and swiftly, thereby allowing easy starting of the engine. 
     When the engine starts and the engine rotational speed Ne exits the starting rotation region a, as shown in  FIG. 6 , moment of the centrifugal weight  35  around the pivot  36  by a centrifugal force Fw overcomes moment of the centrifugal weight  35  around the pivot  36  by the set load Fs 1  of the first return spring  43 , to cause the centrifugal weight  35  to oscillate radially outward from the contracted position A. Such oscillation is transmitted from the connection piece  35   a  to the connection protrusion  45   c , to rotate the drive ring  45  counterclockwise and move the roller  45   b  to a position facing the bottom surface  58   b  of the recessed cam  58  of the exhaust gas recirculation cam member  48 . Thus, the decompression cam member  47  moves to the inoperative position D by the centrifugal force without interference by the roller  45   b , thereby retracting the convex cam  47   a  into the base circle of the exhaust cam  26   e.    
     During this process, in the exhaust gas recirculation cam member  48 , the bottom surface  59   a  of the recessed cam  59  faces the roller  45   b  of the drive ring  45  as shown in  FIG. 7 , and thus the exhaust gas recirculation cam member  48  is held in the inoperative position F by the centrifugal force without interference by the roller  45   b , thereby retracting the convex cam  48   a  into the base circle of the exhaust cam  26   e.    
     Thus, the exhaust valve  17   e  is controlled to be opened and closed only depending on the operation of the exhaust cam  26   e  as usual. Such a state continues in a low speed operation region b of the engine where the engine rotational speed Ne is Ne 1  to Ne 2 . With the movement of the centrifugal weight  35  at about the time when the engine rotational speed Ne reaches Ne 2 , as shown in  FIG. 7 , the play S between the second movable connection  41  of the centrifugal weight  35  and the movable end  44   a  of the second return spring  44  disappears, so that action of the second return spring  44  on the centrifugal weight  35  starts. Thus, after that, the position of the centrifugal weight  35  is determined by a balance between moment of the centrifugal weight  35  around the pivot  36  due to a return force Fs 1  of the first return spring  43  and a return force Fs 2  of the second return spring  44 , and moment of the centrifugal weight  35  around the pivot  36  due to the centrifugal force Fw, so that an oscillation speed of the centrifugal weight  35  according to an increase in the engine rotational speed Ne slows. 
     Next, when the engine rotational speed Ne exceeds Ne 2  and the internal combustion engine E enters a high speed operation region C, the centrifugal weight  35  finally reaches the extended position B where the centrifugal weight  35  abuts against the stopper  38  on the driven gear  32 , and the further counterclockwise rotation of the drive ring  45  accompanying with it causes the roller  45   b  to press the inclined surface  59   b  of the recessed cam  59  of the exhaust gas recirculation cam member  48  and move the exhaust gas recirculation cam member  48  to the operative position G against the centrifugal force. Therefore, the convex cam  48   a  of the exhaust gas recirculation cam member  48  protrudes beyond the base circle of the exhaust cam  26   e . Thus, as described above, in the intake stroke, the convex cam  48   a  of the exhaust gas recirculation cam member  48  slightly pushes up the exhaust tappet  27   e  to slightly open the exhaust valve  17   e  via the exhaust push rod  28   e  and the exhaust rocker arm  29   e  (see  FIGS. 1 and 2 ). As a result, an exhaust gas remaining in the exhaust port  16   e  is drawn into the cylinder bore  3   a , that is, the exhaust gas is recirculated. This exhaust gas can suppress an excessive increase in combustion temperature during combustion of air-fuel mixture in a later expansion stroke, to reduce a NOx concentration in the exhaust gas. 
     In the above-described intake stroke, as shown in  FIGS. 1 and 2 , the exhaust valve  17   e  is slightly opened to apply the negative pressure in the cylinder bore  3   a  to the exhaust port  16   e . This negative pressure is transferred from the air hole  70  to the downstream chamber  75   b  of the valve box  67  to create the pressure difference between the downstream chamber  75   b  and the upstream chamber  75   a . Thus, the reed valve plate  77  bends toward the downstream chamber  75   b  to open the valve hole  76  in the diaphragm plate  74 . Specifically, the one-way valve  73  is opened to bring the secondary air passage  71  into communication with the exhaust port  16   e , and thus secondary air filtered by the air cleaner  64  is drawn through the secondary air passage  71  into the exhaust port  16   e  and further into the cylinder bore  3   a . In a short time thereafter, the intake stroke finishes and the intake and exhaust valves  17   i  and  17   e  are closed. Thus, the secondary air drawn into the exhaust port  16   e  remains in the exhaust port  16   e . Thus, the remaining secondary air reacts with HC and CO in the exhaust gas exhausted to the exhaust port  16   e  in a later exhaust stroke to reduce HC and CO in the exhaust gas. Further, the secondary air drawn into the cylinder bore  3   a  increases charging efficiency of air/fuel mixture to increase an output of the engine E. 
     This can reduce HC and CO concentrations and also a NOx concentration in the exhaust gas and increase an output of the engine without using an air pump regardless of an exhaust system structure of the internal combustion engine. The exhaust gas passes through the exhaust muffler  23  including the three way catalyst converter  66 , thereby further facilitating reduction and removal of NOx and oxidation and removal of HC and CO remaining in the exhaust gas. 
     Now, in the first embodiment, the valve box  67  that houses the one-way valve  73  is integrally formed with the head cover  18 , thereby eliminating the need for a special support structure for the valve box  67  for the one-way valve  73 . 
     Next, a second embodiment of the present invention shown in  FIG. 11  will be explained. 
     In this second embodiment, a valve box  67  that houses a one-way valve  73  is secured by a screw  80  to an exhaust pipe  65  connected to a cylinder head  4 , and a short air hole  70  connecting to a downstream chamber  75   b  of the valve box  67  opens into the exhaust pipe  65 . Other configurations are the same as in the first embodiment. Thus, components corresponding to those in the first embodiment are denoted by the same reference numerals in  FIG. 11 , and overlapping descriptions will be omitted. 
     According to this second embodiment, the valve box  67  for the one-way valve  73  is mounted to the exhaust pipe  65 , and thus the valve box  67  for the one-way valve  73  can be set using dead space around the exhaust pipe  65 . 
     The present invention is not limited to the above-mentioned embodiments and may be modified in a variety of ways as long as the modifications do not depart from its gist. For example, the exhaust muffler  23  does not need to always be equipped therewith the three way catalyst converter  66 .