Abstract:
The present invention provides an engine including a secondary air supply apparatus for supplying a secondary air to an exhaust gas passage in a cylinder head. The engine comprises a secondary air passage of the secondary air supply apparatus which extends from an outer peripheral surface of the cylinder head through an under-portion of the exhaust gas passage into the exhaust gas passage. The secondary air passage has a groove geometry of passage portion on a matching surface between the cylinder head and a cylinder body.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an engine including a secondary air supply apparatus for supplying secondary air to an exhaust gas passage in a cylinder head. 
     2. Description of the Related Art 
     As shown in the first to the third conventional examples of  FIGS. 9 to 11 , in an engine including a secondary air supply apparatus, usually secondary air is taken from an air cleaner, and the secondary air is supplied to an exhaust gas passage through secondary air passages having various configurations. 
     In the first conventional example shown in  FIG. 9 , a linearly drill hole is made as a secondary air passage  103  above an exhaust gas passage  101  in a cylinder head  100 , and an inlet  103   a  of the secondary air passage  103  is opened at an upper portion on an outer peripheral surface  100   a  of the cylinder head  100  on to which an exhaust gas outlet  101   a  of the exhaust gas passage  101  is opened. A secondary air pipe  104  is connected to the inlet  103   a . The secondary air pipe  104  is extended toward an air cleaner  106  provided on the opposite side of the exhaust gas outlet, and the secondary air pipe  104  is connected to the air cleaner  106  through a secondary air lead valve  105 . Assuming that the side of the outer peripheral surface  101   a  of the cylinder head  100  is set to “forward” as shown by an arrow, the secondary air passage  103  is rear downwardly extended from the inlet  103   a  to an outlet opened at an upper part of the exhaust gas passage  101  such that the secondary air passage  103  is in communication with the exhaust gas passage  101 . For example, the first conventional engine has a four-valve structure including a pair of intake valves and a pair of exhaust valves in one cylinder, an exhaust upstream portion of the exhaust gas passage  101  is branched into two by a partition  101   b , and the secondary air passage  103  is in communication from above with an exhaust downstream portion where the two branch exhaust gas passages are merged in the exhaust gas passage  101 . 
     In the second conventional example shown in  FIG. 10 , the secondary air lead valve  105  is attached to the outer peripheral surface  100   a  of the cylinder head  100  on which the exhaust gas outlet  101   a  of the exhaust gas passage  101  is opened. In  FIG. 10 , the same component as that of  FIG. 9  is designated by the same numeral or number. The secondary air lead valve  105  is disposed above the exhaust gas outlet  101   a . The secondary air passage  103  is rear downwardly extended in the cylinder head  100  from the lead valve  105 , the secondary air passage  103  reaches the partition  101   b  of the exhaust gas passage  101 , and is in communication with each branch exhaust gas passage on both sides of the partition  101   b  through a lateral passage portion  108 . 
     In the third conventional example shown in  FIG. 11  is disclosed in Japanese Unexamined Patent Application Publication No. 2003-74319, the lead valve  105  is attached to an upper end of a head cover  110  fastened to an upper end of the cylinder head  100 . In  FIG. 11 , the same components as that of  FIG. 9  are designated by the same numerals. Secondary air passages  111  and  103  are formed in the head cover  110  and the cylinder head  100  respectively, and the secondary air passages  111  and  103  are in communication with the upper part of the exhaust gas passage  101 . 
     In the first conventional example shown in  FIG. 9 , when the secondary air passage  103  is drilled in the cylinder head  100 , it is required to make a long stroke hole from the inlet  103   a  to the exhaust gas passage  101  using a long and thin drill. This hole making work becomes troublesome, because the hole making work should be performed such that the drill is not broken and such that the drill is not bent during the work. Further, given that re-burning of unburned gas is promoted by supplying the secondary air to a high-temperature point near a combustion chamber  109  of the engine, preferably the outlet of the secondary air passage  103  is disposed near the combustion chamber  109 . However, when the outlet of the secondary air passage  103  is brought close to the combustion chamber  109 , the secondary air passage  103  is lengthened, which makes the hole making work of the secondary air passage  103  with the drill more difficult. Further, since the secondary air passage  103  is formed above the exhaust gas passage  101  and the secondary air pipe  104  is passed through an upside of the engine from the inlet  103   a  and connected to the air cleaner  106 , a height of the engine is increased in the structure. 
     In the structure of the second conventional example shown in  FIG. 10 , similarly to the first conventional example, it is necessary that the hole making work of the secondary air passage  103  be performed for a long stroke using a long and thin drill, so that a large amount of time is taken in forming the secondary air passage  103 . Furthermore, the size in the forward and rearward direction of the engine and the height of the engine are increased because the secondary air lead valve  105  is disposed above the exhaust gas outlet  101   a  while attached to the outer peripheral surface (front wall surface)  100   a  of the cylinder head  100 . 
     In the third conventional example shown in  FIG. 11 , the hole making work becomes troublesome because it is necessary to form the secondary air passages  103  and  111  in both the cylinder head  100  and the head cover  110 . Additionally, the total height of the engine is increased because the secondary air lead valve  105  is attached to the upper end of the head cover  110 . 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above described condition, and an object of the present invention is to provide an engine including a secondary air supply apparatus, in which a secondary air passage located in a cylinder head can be easily formed and can be in communication with a exhaust gas passage at a neighborhood of a combustion chamber of the engine while compact engine is maintained. 
     In order to accomplish the foregoing object of the present invention, there is in accordance with the present invention provided an engine including a secondary air supply apparatus for supplying secondary air into an exhaust gas passage in a cylinder head, the engine including a secondary air passage of the secondary air supply apparatus which extends from an outer peripheral surface of the cylinder head through an under-portion of the exhaust gas passage into the exhaust gas passage, the secondary air passage having a groove geometry of a passage portion on a matching surface between the cylinder head and a cylinder body. 
     With this configuration, since the secondary air passage has the groove geometry of the passage portion formed in a groove shape on a matching surface between the cylinder head and the cylinder body, the secondary air passage can be easily formed. That is, the groove geometry of the passage portion can be molded easily during the casting of the cylinder head. 
     Additionally, the groove geometry of the passage portion shortens a portion of the secondary air passage formed by the drill (drill hole). Therefore, breaking or bending of the drill can be effectively prevented. 
     Since the geometry of the passage portion of the secondary air passage is formed on the matching surface between the cylinder head and the cylinder body, the secondary air passage can be formed in a region located below and close to the exhaust gas passage. Therefore, the secondary air passage can be communicated easily with the exhaust gas passage at a point near the combustion chamber of the engine. This enables the secondary air to be supplied to the highest-temperature point of the exhaust gas passage to improve the re-burning efficiency of the unburned gas in the exhaust passage. 
     Preferably, the outer peripheral surface of the cylinder head may include a first side surface on which an exhaust gas outlet of the exhaust gas passage is opened and a second side surface in a direction substantially perpendicular to the first surface, and a secondary air lead valve may be mounted on the second surface, and be fluidly connected with an upstream end of the secondary air passage. 
     With this configuration, in the case where the engine is mounted on a vehicle such that the exhaust gas outlet and the intake air inlet are formed on front and rear surfaces in the vehicle traveling direction, the secondary air lead valves are attached to right and left sidewall surfaces of the cylinder head, which allows the compact size of the engine to be realized in a forward and rearward direction. Additionally, the secondary air lead valve can be easily detached from the right and left sides of the vehicle. 
     Preferably, the exhaust gas passage may be divided into two exhaust gas branch passages by means of a partition, the secondary air passage may include an air passage portion which extends from the groove geometry of the passage portion located under the exhaust gas passage through inside the partition, and is divided into two air branch passage portions, and the two air branch passage portions may be connected with the two exhaust gas branch passages respectively. 
     With this configuration, in the engine which includes the pair of exhaust valves in one cylinder, the secondary air passage can be communicated easily with each branch exhaust gas passage close to the combustion chamber. 
     Preferably, the groove geometry of passage portion may be formed with a groove formed on the matching surface of the cylinder head. 
     With this configuration, the secondary air passage can be formed only by casting and machining (hole making) the cylinder head, and it is not necessary to machine the cylinder. 
     According to the present invention, preferably, the groove geometry of the passage portion may be formed with both a groove formed on the matching surface of the cylinder head and a groove formed on the matching surface of the cylinder. 
     With this configuration, a sectional shape of the secondary air passage can widely be formed in the desired shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be become more apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a right side view showing a V-2 cylinder engine including a secondary air supply apparatus according to an embodiment of the present invention; 
         FIG. 2  is a plan view showing the V-2 cylinder engine of  FIG. 1 ; 
         FIG. 3  shows a sectional view taken along a line III-III of  FIG. 2  of a front cylinder in the V-2 cylinder engine of  FIG. 1 ; 
         FIG. 4  shows a partially cut out sectional view (bottom view of a cylinder head of the forward cylinder) taken along a line IV-IV of  FIG. 3 ; 
         FIG. 5  shows a sectional view (top view of a cylinder body of the forward cylinder) taken along a line V-V of  FIG. 3 ; 
         FIG. 6  shows a sectional view taken along a line VI-VI of  FIG. 4 ; 
         FIG. 7  shows an enlarged longitudinal sectional view of a secondary air on-off valve of the engine of  FIG. 4 ; 
         FIG. 8  is a sectional view of the same portion as that of  FIG. 6  showing a modification of the secondary air passage; 
         FIG. 9  is a longitudinal sectional view showing a cylinder head of a first conventional example; 
         FIG. 10  is a longitudinal sectional view showing a cylinder head of a second conventional example; and 
         FIG. 11  is a left side view showing an engine of a third conventional example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 to 7  show a V-2 cylinder engine for vehicle according to an embodiment of the present invention, and the V-2 cylinder engine is mainly mounted on a four-wheeled all terrain vehicle (A.T.V.).  FIG. 1  is a right side view showing the V-2 cylinder engine,  FIG. 2  is a plan view showing the V-2 cylinder engine of  FIG. 1 ,  FIG. 3  is an enlarged sectional view taken along a line III-III of a front cylinder  1  of  FIG. 2  when viewed from the left side,  FIG. 4  is a partially cut out sectional view (bottom view of a cylinder head  5 ) taken along a line IV-IV of  FIG. 3 ,  FIG. 5  is a sectional view (top view of a cylinder body  4 ) taken along  FIG. 6  is a sectioned view taken along a line VI-VI of  FIG. 4 , and  FIG. 7  is an enlarged longitudinal sectional view of a secondary air on-off valve  44 . An axial direction of a crankshaft of the engine is set to a right and left direction of the engine, a direction perpendicular to the axial direction is set to a forward and rearward direction. 
     [Entire Structure of Engine] 
     Referring to  FIG. 1 , a front cylinder  1  bent forward and a rear cylinder  2  bent rearward are fastened to an upper end of a forward portion of a crank case  3 . Both cylinders  1 ,  2  are arranged in V-shape when viewed from a lateral direction. Each of the cylinders  1  and  2  includes cylinder bodies  4  and  14  coupled to the upper end portion of the crank case  3  with bolts, cylinder heads  5  and  15  fastened to the upper ends of the cylinder bodies  4  and  14  with cylinder fastening bolts, and head covers  6  and  16  and the like coupled to the upper end of the cylinder heads  5  and  15  with bolts, respectively. Intake air inlets  8  and  18  are formed in a rear wall surface  5   b  of the cylinder head  5  of the front cylinder  1  and a front wall surface  15   a  of the cylinder head  15  of the rear cylinder  2  respectively. Intake air outlets of front and rear throttle devices  21  and  22  are connected to the intake air inlets  8  and  18  of the cylinder heads  5  and  15  respectively. Intake air inlets of the throttle devices  21  and  22  are connected to front and rear cleaner outlets  25  and  26  in a bottom wall of an air cleaner  24  disposed above the throttle devices  21  and  22 , respectively. 
     Exhaust gas outlets  9  and  19  are formed in a front wall surface  5   a  (a first side surface) of the cylinder head  5  of the front cylinder  1  and a rear wall surface  15   b  (a first side surface) of the cylinder head  15  of the rear cylinder  2  respectively. Exhaust gas pipes  31  and  32  are connected to the exhaust gas outlets  9  and  19  respectively, and the exhaust gas pipes  31  and  32  are extended rearward and connected to an exhaust muffler (not shown). 
     Referring to  FIG. 3 , a matching surface  5   f  located at the lower end of the cylinder head  5  is overlapped with a matching surface  4   f  located at the upper end of the cylinder  4 , and the cylinder head  5  is fastened to the cylinder  4  with the bolts as described above. Referring to  FIG. 4 , an exhaust gas passage  20  formed in the cylinder head  5  of the front cylinder  1  is extended toward a cylinder center O 1  from the exhaust gas outlet  9  formed in the front lower wall surface  5   a  of the cylinder head  5 , and the exhaust gas passage  20  is branched at a midpoint into right and left branch exhaust gas passages  20   a  and  20   a  by a partition  23 . The branch exhaust gas passages  20   a  and  20   a  are extended toward a pair of exhaust ports  11  and  11  opened to a combustion chamber  28 , and the branch exhaust gas passages  20   a  and  20   a  are in communication with the exhaust ports  11  and  11  respectively. Plural cooling water jackets  35  are formed around the combustion chamber  28 . Although not shown, the exhaust gas passage of the rear cylinder  2  of  FIG. 2  has the structure similar to the exhaust gas passage of the front cylinder  1  except that the fore-and-aft direction is reversely formed. 
     (Configuration of Secondary Air Supply Apparatus) 
     Referring to  FIG. 1 , similarly to the cleaner outlets  25  and  26  for the intake air, a secondary air outlet  40  is in communication with a clean side space of the air cleaner  24  is formed in the bottom wall of the air cleaner  24 , and a secondary air source pipe  41  extended substantially downward is connected to the secondary air outlet  40 . A secondary air on-off valve  44  is disposed near a bottom of a V-bank formed between the front and rear cylinders  1  and  2 , and a lower end portion of the secondary air source pipe  41  is connected to a secondary air inlet  45  formed at an upper end of the secondary air on-off valve  44 . 
     Referring to  FIG. 2 , the secondary air on-off valve  44  is disposed near a right end portion of the bottom of the V-bank, a secondary air outlet  48  for supplying the second air into the front cylinder  1  is formed at the end on the right side of the secondary air on-off valve  44 , a secondary air outlet  49  for supplying the second air into the rear cylinder  2  is formed at the end on the left side of the secondary air on-off valve  44 , and a secondary air pipe  51  for supplying the second air into the front cylinder  1  and a secondary air pipe  52  for supplying the second air into the rear cylinder  2  are connected to the secondary air outlets  48  and  49  respectively. The secondary air pipe  51  for supplying the second air into the front cylinder  1  is extended along a right side face of the front cylinder  1 , and the secondary air pipe  51  is connected to an inlet  54   a  of the lead valve  54  for the front cylinder  1  attached to a right sidewall surface  5   c  of the cylinder head  5  of the front cylinder  1 . On the other hand, the secondary air pipe  52  transverses leftward the bottom of the V-bank, and is extended rearward along a left side face of the rear cylinder  2 , and the secondary air pipe  52  is connected to an inlet  55   a  of a lead valve  55  for the rear cylinder  2  attached to a left sidewall surface  15   d  of the cylinder head  15  of the rear cylinder  2 . A negative-pressure supply pipe  56  is connected to a lower end of the secondary air on-off valve  44 , the negative-pressure supply pipe  56  is extended toward the intake air inlet  18  of the rear cylinder  2 , and is connected to an intake air negative-pressure generation portion in the intake air inlet  18 . 
     Referring to  FIG. 4 , a secondary air entrance chamber  66  is formed in a front end portion of a right sidewall surface (a second side surface)  5   c  of the cylinder head  5  of the front cylinder  1 , and the lead valve  54  for the front cylinder is attached to the position corresponding to the secondary air entrance chamber  66  with the bolts. The lead valve  54  for the front cylinder includes a valve case  60 , a valve support  62  having a valve hole  61 , a flexible thin-plate valve body (lead)  63  made of stainless steel or a resin, and a guide  64  which regulates the maximum opening of the valve body  63 . The valve body  63  is disposed such that the valve hole  61  is covered therewith from the left side. A rear end portion of the valve body  63  is fixed to the valve support  62  with a screw  67 . The valve body  63  is pressed and opened leftward by a difference in pressure between the inside of the valve case  60  and the secondary air entrance chamber  66  (pressure inside the valve case&gt;pressure in the secondary air chamber), which causes the secondary air to flow only from the inside of the valve case  60  into the secondary air entrance chamber  66 . The rear end portion of the guide  64  is fixed to the valve support  62  along with the valve body  63 , the guide  64  is inclined toward a left forward direction with respect to the valve support  62  to restrict the opening of the valve body  63  within a predetermined opening. 
     In the cylinder head  5  of the front cylinder  1 , a secondary air passage  70  is formed from the secondary air entrance chamber  66  to the right and left branch exhaust gas passages  20   a  and  20   a . The secondary air passage  70  includes a first passage portion  71 , a second passage portion  72 , a third passage portion  73 , and a fourth passage portion  74 . As shown in  FIG. 6 , the first passage portion  71  is linearly extended toward the left downward direction from the secondary air entrance chamber  66 , and reaches a matching surface  5   f  at the lower end of the cylinder head  5 . The second passage portion  72  is extended leftward from the left end portion of the first passage portion  71  to the region in the neighborhood below the exhaust gas passage  20 . As shown in  FIG. 3 , the third passage portion  73  is extended toward the rear upward direction from the left end portion of the second passage portion  72  to the inside of the partition  23 . The fourth passage portion  74  is branched into right and left from the rear upper end portion of the third passage portion  73 , and is opened to the side face of each branch exhaust gas passage  20   a.    
     Referring to  FIG. 6 , the second passage portion  72  of the secondary air passage  70  includes a groove formed in the matching surface  5   f  located at the lower end of the cylinder head  5 , and the second passage portion  72  is molded during the casting or die-casting of the cylinder head  5 . On the other hand, the first passage portion  71  is formed in a so-called drill hole which is made using the drill after the casting or die-casting of the cylinder head  5 . Referring to  FIG. 3 , the third passage portion  73  is a so-called drill hole which is made using the drill after the casting of the cylinder head  5  like the first passage portion  71 . The fourth passage portion  74  is molded during the casting of the cylinder head  5 . 
     Referring to  FIG. 5 , in this embodiment, the cooling water jacket  36  is opened on the matching surface  4   f  located at the upper end of the cylinder  4 , whereas a passage portion for secondary air is not formed. 
     Although not shown, the secondary air passage having the structure similar to that of the front cylinder  1  is formed in the cylinder head  15  of the rear cylinder  2  of  FIG. 1 . However, in the secondary air passage of the rear cylinder  2 , the right and left and the front and rear are inversely formed compared with the secondary air passage of the front cylinder  1 . 
     (Secondary Air On-Off Valve) 
     Referring to  FIG. 7 , usually the secondary air on-off valve  44  is referred to as air cut valve, a secondary air inlet  45  formed at the upper end of the secondary air on-off valve  44  is formed in a cylindrical shape projected upward, a negative-pressure inlet port  59  formed at the lower end of the secondary air on-off valve  44  is formed in a cylindrical shape projected leftward, the secondary air outlet  48  for the front cylinder  1  formed at the right end of the secondary air on-off valve  44  is formed in a cylindrical shape projected rightward, and the secondary air outlet  49  for the rear cylinder  2  formed at the left end of the secondary air on-off valve  44  is formed in a cylindrical shape projected leftward. A valve chamber  82  is formed in an upper portion of a valve case  80 , and a negative-pressure actuating chamber  83  is formed in a lower portion of the valve case  80 . The valve chamber  82  is communicated with the secondary air inlet  45  while accommodating a valve body  81 . The negative-pressure actuating chamber  83  is communicated with the negative-pressure inlet port  59 . The valve chamber  82  is communicated with the right and left secondary air outlets  49  and  48  through the valve hole  84 , and the valve chamber  82  includes a valve seat  85 . The valve body  81  is seated on the valve seat  85  to cut off between the valve chamber  82  and both the secondary air outlets  48  and  49 , thereby closing the secondary air on-off valve  44 . An upper side of the negative-pressure actuating chamber  83  is covered with a flexible rubber diaphragm  86 , and the diaphragm  86  is biased upward by a valve spring  89  while fixed to a lower end portion of a connecting rod  87  by a pair of clamping plates  88 . The connecting rod  87  is extended upward, and the valve body  81  is fixed to the upper end portion of the connecting rod  87 . That is, when the negative pressure is not supplied to the negative-pressure actuating chamber  83  or when the negative pressure is lowered, the diaphragm  86  and the valve body  81  is moved upward by the valve spring  89  to keep the valve hole  84  opened. On the contrary, when at least a predetermined amount or more of negative pressure is supplied to the negative-pressure actuating chamber  83 , the negative pressure bends downward the diaphragm  86  against the valve spring  89 , which lowers the valve body  81  against the connecting rod  87  to close the valve hole  84 . 
     (Secondary Air Flow) 
     (1) Referring to  FIGS. 1 and 2 , when the negative pressure of the negative-pressure generating portion of the intake air inlet  18  of the cylinder head  15  is zero or lower than a predetermined value like the case in which the engine is stopped or idled, the secondary air on-off valve  44  is opened as shown in  FIG. 7 . Accordingly, part of the air in the clean side space of the air cleaner  24  passes through the secondary air on-off valve  44  from the secondary air source pipe  41 , and the part of the air is distributed as the secondary air to the right and left secondary air supply pipes  52  and  51 . The secondary air distributed to the secondary air supply pipe  51  for the front cylinder is supplied to the lead valve  54  for the front cylinder, and the secondary air distributed to the secondary air supply pipe  52  for the rear cylinder is supplied to the lead valve  55  of the rear cylinder  2 . 
     (2) Referring to  FIG. 4 , the secondary air supplied to the valve case  60  of the lead valve  54  for the front cylinder presses and opens the valve body  63  to flow into the secondary air entrance chamber  66 , the secondary air flows leftward through the first and second passage portions  71  and  72  of the secondary air passage  70  to reach an under-position of the exhaust gas passage  20 . Then, the secondary air flows in the third passage portion  73  from the left end portion of the second passage portion  72  toward the rear upward direction, enters the partition  23  of the branch exhaust gas passages  20   a  and  20   a  to reach the neighborhood of the downstream end portion of the exhaust gas, i.e., the neighborhood of the exhaust port  11 . Then, the secondary air is branched into right and left in the fourth passage portion  74 , and is supplied to the right and left branch exhaust gas passages  20   a  and  20   a  and used to re-burn the unburned gas. 
     In the rear cylinder  2  of  FIG. 1 , basically similarly to the front cylinder  1 , the secondary air is supplied to the neighborhood in the downstream end portion of the exhaust gas of the right and left branch exhaust gas passages through the secondary air passage. 
     When the throttle devices  21  and  22  are closed while the engine runs in a high revolving speed range, because the intake air negative pressure is increased, the secondary air on-off valve  44  is closed (air cut off state). When the revolving speed of the engine is decreased, the intake air negative pressure is lowered to open the secondary air on-off valve (air flowing state). 
     Effects of Embodiment 
     According to the present embodiment, referring to  FIG. 4 , in the first to fourth passage portions  71  to  74  constituting the secondary air passage  70 , because the second passage portion  72  is formed in the groove shape on the matching surface  5   f  of the cylinder head  5  with the cylinder body  4 , the second passage member  72  can simultaneously be molded during the casting of the cylinder head  5 . 
     The second passage portion  72  is formed in the groove shape in the matching surface  5   f  of the cylinder head  5  with the cylinder body  4 . Therefore, compared with the conventional example in which the whole of the secondary air passage is substantially formed by the drill hole, the degrees of freedom of the shape and layout design are increased in the secondary air passage  70 , the external piping is decreased, the secondary air passage  70  can be easily formed in the layout in which the cooling water jacket  35  is avoided without enlarging the cylinder head  5 . 
     The second passage portion  72  is formed in the groove shape in the matching surface  5   f  of the cylinder head  5  with the cylinder body  4 . Therefore, compared with the conventional example in which the whole of the secondary air passage is substantially formed by the drill hole, the first and fourth passage portions  71  and  74  made by the drilling can be shortened, the drill is prevented to break or bent during the hole making work because the short drill can be used, and the hole making work becomes easy in the first and fourth passage portions  71  and  74 . 
     The groove-shaped second passage portion  72  is formed on the matching surface  5   f  of the cylinder head  5  with the cylinder body  4 . Therefore, the secondary air passage  70  can easily be guided to the region in the neighborhood below the exhaust gas passage  20 , and can be guided from the region in the neighborhood below the exhaust gas passage  20  to the point near the combustion chamber of the branch exhaust gas passages  20   a  and  20   a  by the short third passage portion  73 . Accordingly, while the compact cylinder head  5  is maintained, the secondary air can be supplied to a high-temperature region of the exhaust gas passage  20  to improve the re-burning efficiency of the unburned gas. 
     Referring to  FIG. 2 , the lead valves  54  and  55  for the front and rear cylinders are attached to the right and left sidewall surfaces  5   c  and  15   d  of the cylinder heads  5  and  15  respectively. Therefore, the size in the fore-and-aft direction and the height of the engine can be decreased in the fore-and-aft direction of the engine, and the lead valves  54  and  55  can easily detachably be attached from the right and left sides of the vehicle. 
     Referring to  FIG. 4 , in the engine which includes the pair of exhaust valves (exhaust port  11 ) in one cylinder, the third passage portion  73  is formed in the partition  23  between the branch exhaust gas passages  20   a  and  20   a , so that the secondary air passage  70  can easily be opened in the branch exhaust gas passages  20   a  and  20   a  at the point near the combustion chamber  28 . 
     Other Embodiments 
     (1)  FIG. 8  shows a modification of the secondary air passage  70  in the cylinder head  5 . The second passage portion  72  includes a groove portion  72   a  and a groove portion  72   b . The groove portion  72   a  is formed on the matching surface  5   f  located at the lower end of the cylinder head  5 , and the groove portion  72   b  is formed on the matching surface  4   f  located at the upper end of the cylinder body  4 . 
     (2) In  FIG. 8 , the second passage portion  72  of the secondary air passage  70  may be formed only by the groove portion  72   b  which is formed in the matching surface  4   f  located at the upper end of the cylinder body  4 . 
     (3) The engine of the embodiment mentioned above is a four-valve engine including the pair of intake valves and the pair of exhaust valves in the one cylinder. Alternatively, the invention can also be applied to the engine which includes a single exhaust valve in the one cylinder. 
     (4) It is to be understood that the invention should not be limited to the structure in the above-described preferred embodiments, and therefore, that various modifications can be applied within the scope of claims.