Abstract:
To provide a high performance four-stroke internal combustion engine wherein an increase in a side of a cylinder head is prevented and a valve resting mechanism is provided for a valve lifter. In a four-stroke internal combustion engine with a valve resting mechanism includes a valve lifter having a valve resting mechanism interposed between a valve motion cam and a stem of a poppet valve and a valve lifter spring is provided for biasing the valve lifter in a direction in which the valve lifter is normally held in contact with the valve motion cam. A slide pin holder is pressed against the valve lifter by a spring force of the valve lifter spring that is provided on the valve lifter while a shim portion having a predetermined thickness for controlling a relative positional relationship with the valve lifter is formed integrally with a valve lifter top wall. One of a plurality of predetermined valve lifters most suitable for the relative positional relationship between the slide pin holder and the valve lifter is selectively mounted to effect a tappet gap adjustment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2001-191579 filed on Jun. 25, 2001 the entire contents thereof is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a four-stroke internal combustion engine having a valve resting mechanism for a valve lifter. More particularly, to a four-stroke internal combustion engine which includes a relative positional adjustment structure between an intake or exhaust poppet valve stem and a valve lifter, or in other words, a gap adjustment structure.  
           [0004]    2. Description of Background Art  
           [0005]    A valve motion apparatus with a top end of a stem of an intake or exhaust poppet valve pressed down by a valve motion cam with a valve lifter interposed therebetween to drive the intake or exhaust poppet valve to open or close and the intake or exhaust poppet valve is rendered inoperative when necessary is disclosed in Japanese Patent Laid-Open No. 184327/1998 and is advantageous in that it is compact and involves a number of parts.  
           [0006]    In the valve motion apparatus, in order to adjust the relative positional relationship between the stem of the intake or exhaust poppet valve and the valve lifter, a shim for gap adjustment is interposed between a top end of the stem of the intake or exhaust poppet valve and a top wall of the valve lifter.  
           [0007]    In an OHC (overhead cam) type four-stroke internal combustion engine provided with a valve lifter, where a valve resting mechanism is provided for the valve lifter, the height in the direction of an axial line of the valve increases. This is likely to cause deterioration in incorporating the valve lifter in a vehicle because of an increase in the head height particularly in a high compression ratio four-stroke internal combustion engine having a small valve included angle.  
           [0008]    Further, a valve lifter provided with a valve resting mechanism has a weight increased by the weight of the valve resting mechanism when compared with another valve lifter which is not provided with a valve resting mechanism. In addition, the weight of the shim is added to this construction. Consequently, the equivalent weight of the intake or exhaust poppet valve and so forth increases and the valve spring load increases. As a result, an increase in the friction loss of the valve motion apparatus cannot be avoided.  
           [0009]    Furthermore, since a shim is interposed between a valve lifter of a complicated structure which has a built-in valve resting mechanism and a top portion of an intake or exhaust poppet valve, a mounting operation of the shim is cumbersome and it is difficult to perform the mounting operation readily in a short time.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0010]    The subject to be solved by the present invention is to overcome the difficulties described above.  
           [0011]    According to the present invention, a four-stroke internal combustion engine with a valve resting mechanism is provided wherein a valve lifter having a valve resting mechanism is interposed between a valve motion cam and a stem of a poppet valve. A valve lifter spring is provided for biasing the valve lifter in a direction in which the valve lifter is normally held in contact with the valve motion cam. A slide pin holder presses against the valve lifter by a spring force of the valve lifter spring that is provided on the valve lifter while a shim portion having a different thickness for controlling a relative positional relationship with the valve lifter is formed integrally with a top wall of the valve lifter. One of a plurality of different valve lifters which is most suitable for the relative positional relationship between the slide pin holder and the valve lifter is selectively mounted to effect a tappet gap adjust.  
           [0012]    Since the present invention is configured in such a manner as described above, even if a shim is not prepared separately, a tappet gap adjustment is performed.  
           [0013]    Accordingly, the height of the cylinder head is reduced by an amount corresponding to the thickness of a shim, and a reduction in the overall size of the internal combustion engine can be achieved.  
           [0014]    Further, since no shim is used, the equivalent weight of each of the intake and exhaust poppet valve systems is reduced, and the valve spring load is reduced. As a result, the friction loss of the valve motion apparatus decreases.  
           [0015]    Furthermore, since there is no necessity to provide a shim for the valve lifter, the man-hours for assembly are reduced and the productivity is augmented.  
           [0016]    Further, the present invention is configured in such a manner wherein a valve resting mechanism operates with certainty and is simple in structure is obtained.  
           [0017]    Furthermore, where the present invention is configured in such a manner wherein since no shim is provided for the poppet valve side with a valve resting mechanism wherein the distance from the poppet valve to the cam is liable to become long if a valve resting mechanism is provided. Thus, an increase in the height of the cylinder head can be suppressed thereby to achieve a reduction of the overall size of the internal combustion engine. Meanwhile, a shim is provided for the poppet valve side which normally performs an opening and closing motion and wherein there is no possibility that the distance from the poppet valve for which no valve resting mechanism is provided and which normally performs an opening and closing motion to the cam that may increase so that a single type of valve lifter can be used. Consequently, inventory control of valve lifters can be simplified.  
           [0018]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0020]    [0020]FIG. 1 is a schematic side elevational view of a four-stroke internal combustion engine with a valve resting mechanism according to the present invention;  
         [0021]    [0021]FIG. 2 is a top plan view of a front cylinder head with a front head cover removed;  
         [0022]    [0022]FIG. 3 is a vertical sectional side elevational view taken along line III-III of FIG. 2;  
         [0023]    [0023]FIG. 4 is a vertical sectional side elevational view in a state wherein a valve rest state is cancelled in FIG. 3;  
         [0024]    [0024]FIG. 5 is a vertical sectional side elevational view in a state wherein the valve rest state is cancelled and an exhaust valve is opened by a cam;  
         [0025]    [0025]FIG. 6 is an explanatory view illustrating an arrangement state of cylinder holes and intake and exhaust poppet valves;  
         [0026]    [0026]FIG. 7 is a perspective view of a slide pin holder;  
         [0027]    [0027]FIG. 8 is a perspective view of a slide pin;  
         [0028]    [0028]FIG. 9 is an enlarged vertical sectional side elevational view of essential part illustrating a valve resting state of a valve lifter with a valve resting mechanism;  
         [0029]    [0029]FIG. 10 is an enlarged vertical sectional side elevational view of essential part illustrating a valve resting cancelled state of the valve lifter with a valve resting mechanism; and  
         [0030]    [0030]FIG. 11 is a sectional view taken along line XI-XI of FIG. 9.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    In the following, an embodiment of the invention of the present application shown in FIGS.  1  to  11  is described.  
         [0032]    An OHC type four-stroke internal combustion engine  1  incorporated in a motorcycle not shown is a lengthwise V-type internal combustion engine wherein, as shown in FIG. 1, a crankshaft (not shown) is directed in a vehicle widthwise direction and a cylinder on the vehicle body leading side and another cylinder on the vehicle body trailing side are disposed forwardly and backwardly defining an included angle of the right angle. A body of the OHC type four-stroke internal combustion engine  1  includes a cylinder block  2 , a crankcase  3  mounted integrally on a lower face of the cylinder block  2 , a set of two cylinder heads  4  integrally mounted at top ends of a vehicle body leading side cylinder bank and a vehicle body trailing side cylinder bank of the cylinder block  2 , and a set of two head covers  5  for covering over the top portions of the cylinder heads  4 .  
         [0033]    In each of the cylinder blocks  2  located at a leading portion and a trailing portion of the vehicle body, two cylinder holes  6  are juxtaposed in the vehicle body widthwise direction and are disposed at each of vehicle body leading and trailing positions as shown in FIG. 2 (only the cylinder block at the vehicle body leading portion is shown) thereby to form the four-cylinder OHC type four-stroke internal combustion engine  1 . As shown in FIGS.  3  to  5 , a pent-roof type recess  7  is formed at a location of a lower face of each of the cylinder heads  4  on the leading and trailing sides of the vehicle body corresponding to the cylinder hole  6 . A combustion chamber  8  is defined by a piston (not shown) fitted in the cylinder hole  6 , the cylinder hole  6  and the pent-roof type recess  7 .  
         [0034]    Further, in each of the vehicle body leading and trailing side cylinder banks of the V-type four-cylinder OHC type four-stroke internal combustion engine  1 , a carburetor and an intake apparatus such as an intake chamber not shown are disposed on the cylinder included angle side (the side facing with a V-shaped space A between the leading and trailing side cylinder banks shown in FIG. 1, that is, a space defined by the cylinder bank on the vehicle body leading side and the cylinder bank on the vehicle body trailing side), and an exhaust pipe not shown is connected to the outer side (outer side B of the V-shaped space A between the leading and trailing side cylinder banks) of each of the cylinder banks on the vehicle body leading and trailing sides.  
         [0035]    Further, as shown in FIGS.  3  to  5 , an intake port  9  is formed at a vehicle body trailing side portion of the cylinder head  4  on the vehicle body leading side. The intake port  9  has two openings to the combustion chamber  8  to which two intake paths branched on the intake downstream side from a single upstream side intake path connecting to the intake apparatus described above are connected. Meanwhile, an exhaust port  10  is formed at a vehicle body leading side portion of the cylinder head  4  on the vehicle body leading side. The exhaust port  10  has two openings to the combustion chamber  8  to which two upstream side exhaust paths joining together on the exhaust downstream side into a single exhaust connected to an intake pipe not shown are connected. Further, as shown in FIGS. 2 and 6, an intake poppet valve  13   a , an intake poppet valve  13   b  and a pair of exhaust poppet valves  14   a  and  14   b  for openably closing up the two intake openings  11   a  and  11   b  and the two exhaust openings  12   a  and  12   b , respectively, are provided in the cylinder head  4 .  
         [0036]    Also in the vehicle body trailing side cylinder head  4 , an intake port and an exhaust port whose arrangement is reverse in the forward and backward direction to that of the intake port  9  and the exhaust port  10  of the vehicle body leading side cylinder head  4  are formed. In particular, an intake port (not shown) having a similar shape to that of the intake port  9  at the vehicle body trailing side portion of the vehicle body leading side cylinder head  4  is formed in a vehicle body leading side portion of the vehicle body trailing side cylinder head  4 . Further, an exhaust port (not shown) having a similar shape to that of the exhaust port  10  at the vehicle body leading side portion of the vehicle body leading side cylinder head  4  is formed at a vehicle body trailing side portion of the vehicle body trailing side cylinder head  4 .  
         [0037]    Furthermore, as shown in FIGS. 2 and 6, an intake poppet valve  13   a  for which a valve lifter  17  without a valve resting mechanism shown in FIGS.  3  to  5  is provided and which normally performs an opening and closing motion is provided for the intake opening  11   a  positioned on the vehicle body outer side corresponding to each of the cylinder holes  6 . Meanwhile, an exhaust poppet valve  14   a  for which a valve lifter  18  with a valve resting mechanism shown in FIGS.  3  to  5  and which can stop its opening and closing motion is provided for the exhaust opening  12   a  positioned on the vehicle body outer side corresponding to each of the cylinder holes  6 .  
         [0038]    An intake poppet valve  13   b  is provided with a valve lifter  18  with a valve resting mechanism at the intake opening  11   b  positioned on the vehicle body inner side of each of the cylinder holes  6  contrary to the vehicle body outer side intake opening  11   a . Further, an intake poppet valve (not shown in the vertical sectional views) for which provides a valve lifter  17  without a valve resting mechanism, is provided at the exhaust opening  12   b  positioned on the vehicle body inner side of each of the cylinder holes  6  contrary to the vehicle body outer side exhaust opening  12   a.    
         [0039]    In the following, description is given only of the intake poppet valve  13   a  with a valve lifter  17  without a valve resting mechanism provided at the vehicle body outer side intake opening  11   a  of the vehicle body leading side cylinder head  4  and the exhaust poppet valve  14   a  with a valve lifter  18  with a valve resting mechanism provided at the exhaust opening  12   a.    
         [0040]    An intake camshaft  19  is disposed on an extension line of a stem  15   a  and above the stem  15   a  of the intake poppet valve  13   a , and an exhaust camshaft  20  is disposed on an extension line of a stem  16   a  and above the stem  16   a  of the exhaust poppet valve  14   a . The intake camshaft  19  and the exhaust camshaft  20  are each mounted for rotation on the cylinder head  4  by a camshaft holder  23  positioned at a mid-portion in the vehicle body widthwise direction and another camshaft holder  24  is positioned on the right side in the vehicle body widthwise direction as shown in FIG. 2. An intake cam  21   a  of the intake camshaft  19  and an exhaust cam  22   a  of the exhaust camshaft  20  for each of the cylinder holes  6  are held in contact with top faces of a valve lifter  17   a  without a valve resting mechanism of the intake poppet valve  13   a  and a valve lifter  18   a  with a valve resting mechanism of the exhaust poppet valve  14   a , respectively. A driven sprocket wheel  25  is mounted integrally at the vehicle body right end of each of the intake camshaft  19  and the exhaust camshaft  20 . An endless chain not shown extends between a driving sprocket wheel (not shown) integral with a crankshaft not shown and the driven sprocket wheel  25 . When the OHC type four-stroke internal combustion engine  1  is placed into an operative state, an intake cam  21   a  and an exhaust cam  22   a  are driven to rotate in the same direction at a speed equal to one half the speed of rotation of the crankshaft.  
         [0041]    In the intake poppet valve  13   a  for which the valve lifter  17   a  without a valve resting mechanism is provided, a valve guide tube  26   a  for guiding and supporting the stem  15   a  of the intake poppet valve  13   a  for sliding movement is formed to be longer by a length equal to that of a valve resting mechanism. A retainer  27  is fitted at a top portion of the stem  15   a  of the intake poppet valve  13   a  and is coupled integrally to the top end of the stem  15   a  by means of a cotter pin  28 . Two inner and outer valve springs  30  and  31  are interposed in parallel to each other between a valve spring receiving piece  29  in the proximity of an upper portion of a valve guide tube  26  and the retainer  27  such that the intake poppet valve  13   a  is normally biased in a direction in which it closes the intake opening  11  a of the intake port  9  by the spring force of the valve springs  30  and  31 . A shim  33  is fitted in a central hole of the retainer  27  between the top end of the stem  15   a  of the intake poppet valve  13   a  and a top wall  32   a  of the valve lifter  17   a  without a valve resting mechanism so that the top wall  32   a  of the valve lifter  17   a  without a valve resting mechanism is biased in a direction in which it closely contacts with the intake cam  21   a  by the spring force of the valve springs  30  and  31 .  
         [0042]    Meanwhile, in the exhaust poppet valve  14   a  for which the valve lifter  18   a  with a valve resting mechanism is provided, a valve guide tube  34   a  for guiding and supporting the stem  16   a  of the exhaust poppet valve  14   a  for sliding movement is formed longer by a length equal to that of a valve resting mechanism. A retainer  35  is fitted not at a top end but at an upper portion of the stem  16   a  of the exhaust poppet valve  14   a  and is coupled integrally to the upper portion of the stem  16   a  by means of a cotter pin  36 . A valve spring  38  is interposed between a valve spring receiving piece  37  in the proximity of the upper portion of the valve guide tube  34   a  and the retainer  35  while another valve spring  39  having a greater coil diameter than the valve spring  38  is interposed between the valve spring receiving piece  37  and the valve lifter  18   a  with a valve resting mechanism. Consequently, the exhaust poppet valve  14   a  is normally biased in a direction in which it normally closes up the exhaust opening  12   a  of the exhaust port  10  by the spring force of the valve spring  38  while a top wall  40   a  of the valve lifter  18   a  with a valve resting mechanism is biased in a direction in which it closely contacts with the exhaust cam  22  by the spring force of the valve spring  39 .  
         [0043]    At a central portion of the top wall  40   a  of the valve lifter  18   a  with a valve resting mechanism, an increased thickness portion  57  which serves as a shim is formed with a thickness a little greater than that of an outer peripheral portion of the top wall  40   a . Several valve lifters  18   a  with a valve resting mechanism are prepared wherein the increased thickness shim portion  57  is different in thickness.  
         [0044]    Now, a valve resting mechanism of the valve lifter  18   a  with a valve resting mechanism is described.  
         [0045]    As shown in FIGS. 9 and 10, the valve resting mechanism includes a slide pin holder  43  shown in FIG. 7 which is fitted within a cylindrical circumferential wall  42  of the valve lifter  18   a  with a valve resting mechanism such that it can move along the sliding direction (upward or downward direction) of the valve lifter  18   a  with a valve resting mechanism, a slide pin  45  shown in FIG. 8 which is fitted for sliding movement in a pin hole  44  of the slide pin holder  43  such that it can be removably engaged with the stem  16   a  of the exhaust poppet valve  14   a , a guide pin  47  extending through the slide pin holder  43  and capable of loosely fitting in a guide groove  46  formed at one end portion of the slide pin  45 , a pin spring  49  interposed between the other end of the slide pin  45  and a bottom portion of the pin hole  44  of the slide pin holder  43  for biasing the bottom portion of the guide groove  46  of the slide pin  45  in a direction in which it contacts with the guide pin  47 , and a hydraulic driving apparatus  50  for pressing the slide pin  45  towards the pin spring  49  against a spring of a stem through-hole  48 .  
         [0046]    In a state shown in FIG. 9 wherein oil pressure is not supplied to an oil pressure path  51  on one end side of the pin hole  44  by the hydraulic driving apparatus  50  and therefore the bottom portion of the guide groove  46  of the slide pin  45  is held in contact with the guide pin  47  by the pin spring  49 , the stem through-hole  48  is formed in the slide pin  45  and positioned in the direction of an extension line of the stem  16   a  of the exhaust poppet valve  14   a  such that the stem  16   a  of the exhaust poppet valve  14   a  can be slidably moved in the stem through-hole  48  of the slide pin  45 .  
         [0047]    Further, in the hydraulic driving apparatus  50 , as shown in FIGS. 9 and 10, the oil pressure path  51  which is connected through a control valve (not shown) to an exhaust port of a hydraulic pump not shown provided in the OHC type four-stroke internal combustion engine  1  is formed in the cylinder head  4 . A circumferential recessed groove  53  directed in a circumferential direction is formed in a lifter guide hole  52  of the valve lifter  18  with a valve resting mechanism provided on the cylinder head  4 . The oil pressure path  51  and the circumferential recessed groove  53  are in communication with each other by a communication hole  54 .  
         [0048]    Further, a side hole  55  is formed in the cylindrical circumferential wall  42  of the valve lifter  18  with a valve resting mechanism for communicating with the circumferential recessed groove  53  of the lifter guide hole  52  at whichever position the valve lifter  18   a  with a valve resting mechanism is positioned when the valve lifter  18   a  with a valve resting mechanism is moved upwardly or downwardly by the exhaust cam  22   a . As shown in FIG. 7, a circumferential recessed groove  56  which is in communication with the side hole  55  is formed on an outer peripheral face of the slide pin holder  43 . When oil pressure is supplied into the oil pressure path  51 , the oil pressure is introduced into an opening of the pin hole  44  from the oil pressure path  51  through the communication hole  54 , circumferential recessed groove  53 , side hole  55  and circumferential recessed groove  56 . The pressure of the oil pressure overcomes the spring force of the pin spring  49  to move the slide pin  45  toward the pin spring  49  (refer to FIGS. 4, 5 and  10 ) until the stem  16   a  of the exhaust poppet valve  14   a  is arrested by the slide pin  45 . A valve lifter  18   b  with a valve resting mechanism is provided in the intake opening  11   b  positioned on the vehicle body inner side, conversely to the intake opening  11   a  on the vehicle body outer side. Meanwhile, a valve lifter  17   b  without a valve resting mechanism is provided at the exhaust opening  12   b  positioned on the vehicle body inner side.  
         [0049]    Since the embodiment shown in FIGS.  1  to  11  is configured in such a manner as described above, in a state wherein the OHC type four-stroke internal combustion engine  1  operates at a low speed or with a low load and no oil pressure is supplied into the oil pressure path  51 , the slide pin  45  is biased and moved in a direction in which it moves away from the pin spring  49  by the spring force of the pin spring  49  until the bottom portion of the guide groove  46  is arrested by the guide pin  47  in a state wherein the stem through-hole  48  is positioned immediately above the stem  15   b  or  16   a  as shown in FIGS. 3 and 9.  
         [0050]    In this low speed-low load operation state, the top portions of the stems  15   b  and  16   a  of the intake poppet valve  13   b  and the exhaust poppet valve  14   a  can extend through and be freely slidably moved relative to the stem through-hole  48  of the slide pin  45 . Consequently, even if the valve lifters  18   a  and  18   b  with a valve resting mechanism are driven to move upwardly and downwardly by the intake cam  21  a and the exhaust cam  22   a , the intake poppet valve  13   b  and the exhaust poppet valve  14   a  are held in a closed state and are set in a valve resting state, respectively.  
         [0051]    However, if the OHC type four-stroke internal combustion engine  1  is operated at a high speed or with a high load and oil pressure is supplied into the oil pressure path  51 , then the oil pressure is introduced from the oil pressure path  51  into the pin hole  44  through the communication hole  54 , circumferential recessed groove  53 , side hole  55  and circumferential recessed groove  56 . Consequently, the pressure of the oil pressure at an entrance portion of the pin hole  44  overcomes the spring force of the pin spring  49  to drive the slide pin  45  to move in a direction in which it approaches the pin spring  49  until the stems  15   b  and  16   a  of the intake poppet valve  13   b  and the exhaust poppet valve  14   a  are each arrested by a bottom cutaway portion  45   a  of the slide pin  45  as shown in FIGS. 4, 5 and  10 . Consequently, as shown in FIG. 5, the intake poppet valve  13   b  and the exhaust poppet valve  14   a  are opened and closed.  
         [0052]    Further, since the valve resting mechanism is built in each of the valve lifters  18   a  and  18   b , the valve lifters  18   a  and  18   b  with a valve resting mechanism are likely to be formed with an increased upward and downward dimension. However, since the shim  33  is not provided for each of the valve lifters  18   a  and  18   b  with a valve resting mechanism, the height of the valve lifters  18   a  and  18   b  with a valve resting mechanism is reduced as much. Thus, even if it is tried to decrease the valve included angle of the intake poppet valves  13  and the exhaust poppet valves  14  in order to reduce the combustion chamber  8  to raise the compression ratio, an increase in the height of the cylinder heads  4  is prevented, and an increase in the size of the OHC type four-stroke internal combustion engine  1  is prevented.  
         [0053]    Further, since the shim  33  is not provided on any of the valve lifters  18   a  and  18   b  with a valve resting mechanism, the equivalent weight of each of the intake poppet valve  13   b  and the exhaust poppet valve  14   a  is reduced, and the spring load of the valve spring  31  is reduced. Consequently, the power loss in opening and closing of the intake poppet valve  13   b  and the exhaust poppet valve  14   a  is reduced.  
         [0054]    Furthermore, since the amount of work for assembling the shim  33  to each of the valve lifters  18   a  and  18   b  with a valve resting mechanism is unnecessary, the man-hours for assembly are reduced and the productivity is augmented.  
         [0055]    In addition, since the valve resting mechanism is not provided on each of the valve lifters  17   a  and  17   b  without a valve resting mechanism, even if the shim  33  is provided to the valve lifters, the height of the valve lifters  17   a  and  17   b  without a valve resting mechanism in the upward and downward direction can be made substantially equal to that of the valve lifters  18   a  and  18   b  with a valve resting mechanism. Consequently, one kind of valve guide tube can be used for the valve guide tubes  34  of the valve lifters  17   a  and  17   b  without a valve resting mechanism, and therefore, the inventory control of the valve lifters  17   a  and  17   b  without a valve resting mechanism can be simplified with an anticipated reduction in the cost.  
         [0056]    Further, in a low speed or low load operation state wherein the exhaust poppet valve  14   a  and the intake poppet valve  13   b  are controlled to a rest state by the valve lifters  18   a  and  18   b  with a valve resting mechanism, the intake poppet valve  13   a  and the exhaust poppet valve  14   b  which normally perform an opening and closing motion are positioned in a diagonal direction as shown in FIGS. 2 and 6. Therefore, a swirl is generated in the air fuel mixture in the combustion chamber  8 , and firing is performed with certainty. Consequently, the generation of unburned gas is suppressed and fuel cost is improved.  
         [0057]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.