Patent Publication Number: US-8122861-B2

Title: Two-stroke cycle combustion engine of air scavenging type

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a two-stroke cycle combustion engine of an air scavenging type which may be used as a drive source for a portable work machine such as, for example, a brush cutter. 
     2. Description of the Prior Art 
     The two-stroke cycle combustion engine of a scavenging type has been well known, in which air used for leading scavenging is supplied into the combustion chamber after it has been temporarily introduced into a leading portion of the scavenging passage during the scavenging stroke. In this combustion engine, in order to secure a stabilized rotation during the idling condition, an air valve in the air passage is closed and only the air/fuel mixture is introduced into the crank chamber from an air/fuel mixture passage so that the air/fuel mixture of an optimum concentration can be supplied from the scavenging passage into the combustion chamber during the idling condition. On the other hand, if a rapid acceleration is performed from the idling condition to a full throttle condition, an air valve is brought to a fully opened condition at once. 
     In this respect, because of a low speed rotation taking place with a mixture valve in the air/fuel mixture passage fully opened, the negative pressure at the venturi tube within the carburetor is low and, therefore, the amount of the air/fuel mixture supplied from the carburetor towards the crank chamber is still insufficient. Accordingly, the air/fuel mixture pooled within the crank chamber is substantially supplied to the combustion chamber. But since a substantial amount of a scavenging air is introduced into the combustion chamber immediately after the full opening of the throttle, the air/fuel mixture tends to be leaned within the combustion chamber. For this reason, at the time the engine operating condition begins to change from the idling condition to the rapid accelerating condition, the air/fuel mixture of a concentration required for the rapid acceleration will not be supplied to the combustion chamber and, therefore, an acceleration failure and/or a failure of the combustion engine to rotate is apt to occur. 
     In view of the above, it may be contemplated to use, during the idling condition, the air/fuel mixture which has been leaned beforehand, but this appears to result in an increase of the idling opening, accompanied by opening of the air valve by which air enters into the combustion chamber in a quantity enough to render the rotation to be instable. Also, where a start operating mechanism of a lift-up type is employed in which a needle valve is lifted to increase a fuel supply, the amount of lift decreases in a quantity corresponding to the increase of the idling opening and, therefore, the air/fuel mixture during the start of operation of the combustion engine will not be enriched sufficiently, resulting in reduction in engine startability. 
     In contrast thereto, the two-stroke cycle combustion engine has been suggested of a design, in which an auxiliary passage for supplying a leading air to the scavenging passage during full opening or minimum opening of the air valve is employed in the air passage so that the amount of air in the air/fuel mixture can be reduced in a quantity corresponding to the amount of air flowing through the auxiliary passage during the idling condition to thereby allow the air/fuel mixture of an enriched concentration to be supplied into the crank chamber (See, for example, the Patent Document 1 listed below). In this two-stroke cycle combustion engine, since the amount of the air flow in the air/fuel mixture passage is reduced in a quantity corresponding to the amount of air flowing through the auxiliary passage during the idling condition, the concentration of the air/fuel mixture flowing within the air/fuel mixture passage tends to increase. 
     Accordingly, despite that during the idling condition, the respective amounts of air and fuel to be introduced into the combustion engine are set to values substantially equal to those hitherto employed, the air/fuel mixture, which is more enriched than that hitherto employed, is supplied to the combustion chamber at the time the engine operating condition begins to change from the idling condition to the rapid accelerating condition. In view of this, even if this air/fuel mixture is leaned in admixture of a portion of the leading air, the concentration of the air/fuel mixture supplied into the combustion chamber is maintained at a value required for the rapid acceleration and, therefore, the combustion engine can be smoothly accelerated. 
     [Patent Document 1] JP Laid-open Patent Publication No. 2007-239463 
     The two-stroke cycle combustion engine disclosed in the Patent Document 1 referred to above has, however, been found having such a problem that since the flow within the auxiliary passage and the mixture passage, where the mixture valve is disposed, relies on the negative pressure developed inside the crank chamber and, during the idling condition, the air flows into the air passage through the auxiliary passage and the amount of air flowing through the mixture throttle valve tends to change under the influence of air flowing through the auxiliary passage. Accordingly, the amount of fuel to be supplied, which is determined depending on the amount of air flowing through the mixture valve, tends to fluctuate, resulting in reduction in rotational stability of the combustion engine during the idling condition. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention is intended to provide a two-stroke cycle combustion engine of an air scavenging type, in which not only can transit from the idling condition to the rapid accelerating condition take place smoothly, but the combustion engine can be favorably started. 
     In order to accomplish the foregoing object of the present invention, there is provided a two-stroke cycle combustion engine of an air scavenging type in which an air/fuel mixture and an air are introduced into a combustion chamber through a scavenging passage, which engine includes a valve unit for adjusting the opening of each of an air passage for supplying the air to the scavenging passage and an air/fuel mixture passage for supplying the air/fuel mixture to the scavenging passage; and an auxiliary air introducing passage for introducing an auxiliary air to the air/fuel mixture passage at a location downstream of the valve unit. The air referred to above is preferably introduced into the combustion chamber prior to introduction of the air/fuel mixture into the combustion chamber. 
     With the two-stroke cycle combustion engine according to the present invention, the auxiliary air flowing through the auxiliary air introducing passage can be introduced at a location downstream portion of the air/fuel mixture passage with respect to the direction of flow of the air/fuel mixture towards the combustion chamber. Therefore, the opening of the air/fuel mixture passage during the idling condition can be reduced to allow the air/fuel mixture to be enriched by, for example, adjusting a needle valve for setting the fuel flow in a quantity corresponding to the amount of the air so introduced during the idling condition. Since the amount of the air entering the combustion chamber is the sum of the air flowing through the air/fuel mixture passage and the air flowing through the auxiliary air introducing passage, the total amount of the air is increased. But if the amount of fuel is increased to a value required to achieve the number of idling revolutions about equal to that afforded by the conventional combustion engine, the air/fuel mixture within the combustion chamber will not be too enriched. Accordingly, at the time transit from the idling condition to the rapid acceleration is initiated, the air/fuel mixture pooled within the crank chamber during the idling condition can be supplied into the combustion chamber and, therefore, the combustion engine of the present invention can be accelerated rapidly. Also, since the throttle opening during the idling condition can be reduced as compared with that in the conventional combustion engine, the air passage will not be open large by the valve unit and, therefore, revolution of the combustion engine of the present invention can be stabilized. 
     Also, since the opening of the air/fuel mixture passage or the throttle opening can be reduced, a start operating mechanism, for example, of a lift-up type may be operated so that the lift-up amount is increased to enrich the air/fuel mixture thereby to improve the startability. 
     In a preferred embodiment of the present invention, the auxiliary air introducing passage may have a downstream portion defined in a spacer which is disposed between a carburetor, having the valve unit, and an engine body. This is particularly advantageous that since the downstream portion of the auxiliary air introducing passage is defined in the spacer having a plenty of available space as compared with the carburetor, positioning of the downstream of the auxiliary air passage can be facilitated. 
     In another preferred embodiment of the present invention, the auxiliary air introducing passage referred to above may have a throughhole defined in the carburetor for introducing a clean air, which has passed through an air cleaner, into the air/fuel mixture passage. This is particularly advantageous in that with no need to employ any extra member, and merely with the carburetor being modified or altered in any way whatsoever, the auxiliary air introducing passage can be readily formed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and: 
         FIG. 1  is a front sectional view showing a two-stroke cycle combustion engine of an air scavenging type according to a first preferred embodiment of the present invention; 
         FIG. 2  is a front sectional view showing, on an enlarged scale, a cylinder block of the combustion engine and a crankcase employed therein; 
         FIG. 3  is a cross sectional view taken along the line III-III in  FIG. 2 ; 
         FIG. 4  is a side sectional view showing a carburetor employed in the combustion engine; 
         FIG. 5  is an enlarged sectional view showing a main nozzle and a needle valve employed in the carburetor; 
         FIG. 6  is a top plan view showing the carburetor employed in the combustion engine; 
         FIG. 7  is a front elevational view showing the carburetor employed in the combustion engine; 
         FIG. 8  is a top plan sectional view showing the combustion engine; 
         FIG. 9  is a top plan sectional view showing the two-stroke cycle combustion engine according to a second preferred embodiment of the present invention; 
         FIG. 10  is a top plan sectional view showing the two-stroke cycle combustion engine according to a third preferred embodiment of the present invention; 
         FIG. 11  is a front elevational view showing an important portion of the two-stroke cycle combustion engine according to a fourth preferred embodiment of the present invention; 
         FIG. 12  is a perspective view showing a cleaner casing of an air cleaner employed in the fourth preferred embodiment; 
         FIG. 13  is a left side view showing the cleaner casing with a choke valve shown as fully opened; 
         FIG. 14  is a left side view showing the cleaner casing with the choke valve shown as completely closed; and 
         FIG. 15  is a left side view showing a cleaner casing according to a fifth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, some preferred embodiments of the present invention will be described in detail with particular reference to the accompanying drawings. A two-stroke cycle combustion engine of an air scavenging type according to a first preferred embodiment of the present invention, shown in  FIG. 1 , includes an engine body comprised of a cylinder block  1 , having a combustion chamber  1   a  defined therein, and a crankcase  2  with the cylinder block  1  fixedly mounted atop the crankcase  2 . The cylinder block  1  has one side portion (a left side portion) connected with a carburetor  3  and an air cleaner  4 , both forming respective part of a fuel intake system, and also has the opposite side portion (the right side portion) connected with a muffler  8 , forming a part of an exhaust system, and the crankcase  2  has a fuel tank  9  fitted to a lower portion thereof. The cylinder block  1  has a cylinder bore  1   b  defined therein, in which a reciprocating piston  10  is accommodated for movement in a direction conforming to the longitudinal axis C thereof, which may be a vertical direction as viewed in  FIG. 1 . The crankcase  2  has a crankshaft  12  drivingly supported therein by means of bearing units  11 . 
     A hollow crank pin  13  is disposed at a location offset from the longitudinal axis of the crankshaft  12 , and the crank pin  13  and a hollow piston pin  14  carried by the reciprocating piston  10  are drivingly connected together by means of a connecting rod  20  through a large diameter end bearing unit  18  and a reduced diameter end bearing unit  19 . The crankshaft  12  is formed with crank webs  21 , and an ignition plug P is mounted on a top portion of the cylinder block  1 . 
     Referring now to  FIGS. 1 and 2 , for the purpose of thermal insulation from the cylinder block  1  tending to exhibit an elevated temperature, an insulator  22 , which serves as a spacer, is interposed between the cylinder block  1  and the carburetor  3 . This insulator  22  has an air passage  23  formed therein at an upper portion thereof and also with a air/fuel mixture passage  24  formed therein at a lower portion thereof so as to extend substantially parallel to the air passage  23 . The carburetor  3  includes a single rotary valve unit  28  for adjusting the sectional area of both of the air passage  23  and the air/fuel mixture passage  24 . Also, the cylinder block  1  has a peripheral wall having an exhaust passage  29  defined therein, which exhaust passage  29  has an exhaust port  29   a  defined in the peripheral wall of the cylinder block  1  so as to open at an inner peripheral surface thereof so that exhaust gases (combustion gases) flowing through the exhaust passage  29  can be exhausted to, for example, the atmosphere through the muffler  8 . 
     As best shown in  FIG. 2 , an air/fuel mixture scavenging passage  31  communicating a portion of the combustion chamber  1   a  above the reciprocating piston  10  directly with a portion of the crank chamber  2   a  below the reciprocating piston  10  is defined in part within the cylinder block  1  and in part within the crankcase  2 . On the other hand, an air scavenging passage  30  for communicating the combustion chamber  1   a  with the crank chamber  2   a  through the bearing units  11  for supporting the crankshaft  12  is defined in part within the cylinder block  1  and in part within the crankcase  2  so as to be positioned adjacent the exhaust port  29   a , but remote from the air/fuel mixture scavenging passage  31 . 
     Also, an air/fuel mixture scavenging port  31   a , defined at an upper end of the air/fuel mixture passage  31 , and an air scavenging port  30   a , defined at an upper end of the air scavenging passage  30 , are so designed and so positioned that the air scavenging port  30   a  may have an upper end edge thereof held at a level higher than a similarly upper end edge of the air/fuel mixture scavenging port  31   a  and lower than an upper end edge of the exhaust port  29   a . Accordingly, during the scavenging stroke, scavenging with the air A can take place prior to that with the air/fuel mixture M. It is, however, to be noted that the air scavenging port  30  may have its upper end edge held in flush with or at a level somewhat lower than the upper end edge of the air/fuel mixture scavenging port  31   a . It is also to be noted that in  FIG. 2 , the connecting rod  20  shown in  FIG. 1  is not shown for the sake of clarity. 
     The air passage  23  and the exhaust passage  29  lie on the substantially same line passing the cylinder longitudinal axis C when viewed in a direction conforming to such axis C, as shown in  FIG. 1 , whereas the air/fuel mixture scavenging passage  31  and the air scavenging passage  30  are employed each in a pair in symmetrical relation about a longitudinal axis C 1  of the air passage  23  or a longitudinal axis C 2  of the exhaust passage  29 . The air/fuel mixture scavenging passage  31  and the air scavenging passage  30  are partitioned from each other by means of a substantially vertically extending partition wall  32 . An air introducing port  42  operable to introduce the air A from the air introducing passage  40  therethrough into the air scavenging passage  30  is defined in a passage wall of the air/fuel mixture scavenging passage  31  remote from the combustion chamber  1   a.    
     The air A from the air passage  23  defined in the insulator  22  is temporarily introduced into the air scavenging passage  30  through the air introducing passage  40  in the cylinder block  1  by the effect of a negative pressure developed within the crank chamber  2   a  shown in  FIG. 2  during the intake stroke with the reciprocating piston  10  then ascending. On the other hand, the air/fuel mixture M from the air/fuel mixture passage  24  is directly introduced into the crank chamber  2   a  by way of an air/fuel mixture port  24   a , defined in an inner peripheral surface of the cylinder block  1 , by the effect of the negative pressure developed within the crank chamber  2   a  during the intake stroke with the reciprocating piston  10  then ascending. 
     Referring to  FIG. 3 , the air passage  23  defined in the insulator  22  has a downstream exit port defined therein and provided with a reed valve  41  fitted thereto. This reed valve  41  is so designed and so operable that when the pressure inside the air introducing passage  40  continued from the downstream exit port of the air passage  23  increases to a value higher than a predetermined pressure, the air passage  23  can be closed. The air A for scavenging purpose flowing from the air passage  23  is introduced into the air scavenging passage  30  through the air introducing passage  40  and the air introducing port  42  when the reed valve  41  is in an open position to open the air passage  23 . 
     The air/fuel mixture scavenging passage  31  best shown in  FIG. 2  includes the air/fuel mixture scavenging port  31   a  open at the inner peripheral surface of the cylinder block  1 , a communicating passage  31   b  defined in part in a lower portion of the cylinder block  1  and in part in an upper portion of the crankcase  2  across the joint between the cylinder block  1  and the crankcase  2  so as to extend downwardly from the air/fuel mixture scavenging port  31   a  in a direction towards the upper portion of the crankcase  2 , and an inflow port  31   c  open at an inner peripheral surface of that upper portion of the crankcase  2 . A cylinder inner diametric side of the communicating passage  31  referred to above is covered by an air/fuel mixture scavenging passage wall  35  best shown in  FIG. 3 , and the air/fuel mixture scavenging port  31   a  is defined in an upper portion of the air/fuel mixture scavenging passage wall  35  while the inflow port  31   c  referred to above is defined in a lower portion of the air/fuel mixture scavenging passage wall  35 . The air/fuel mixture M introduced from the air/fuel mixture passage  24  into the crank chamber  2   a  is, during the scavenging stroke with the reciprocating piston  10  then descending, jetted in a direction diagonally upwardly from the air/fuel mixture scavenging port  31   a  towards the combustion chamber  1   a  through the communicating passage  31   b.    
     On the other hand, the air scavenging passage  30  includes the air scavenging port  30   a  open at the inner peripheral surface of the cylinder block  1  and a communicating passage  30   b  defined in part in the lower portion of the cylinder block  1  and in part in the upper portion of the crankcase  2  across the joint between the cylinder block  1  and the crankcase  2  so as to extend downwardly from the air scavenging port  30   a  in a direction towards outer side faces of the crank bearing units  11 , which is located in a portion of the crankcase  2  intermediate of the height of the latter. A cylinder inner diametric side of the communicating passage  30   b  referred to above is covered by an air scavenging passage wall  36 , and the air scavenging port  30   a  referred to previously is defined in an upper portion of the air scavenging passage wall  36 . This communicating passage  30   b  best shown in  FIG. 2  has a lower end communicated with the crank chamber  2   a  through a gap between inner and outer rings of each of the bearing units  11  and then through a gap between the bearing units  11  and the adjacent crank webs  21 . 
     The air A introduced into the air scavenging passage  30  from the air passage  23 , shown in  FIG. 3 , through the air introducing passage  40  is, during the scavenging stroke with the reciprocating piston  10  then descending, jetted in a direction diagonally upwardly from the air scavenging port  30   a  towards the combustion chamber  1   a  through the communicating passage  30   b . Accordingly, the air A shown in  FIG. 3  serves to block the air/fuel mixture M, thereby effectively suppressing an undesirable blow-off of the air/fuel mixture M from the exhaust passage  29  to the atmosphere. 
     The two-stroke cycle combustion engine of the structure described hereinabove operates in the following manner. During the intake stroke, as the reciprocating piston  10  then ascending within the cylinder bore  1   b  in the cylinder block  1  approaches a position adjacent the top dead center, accompanied by a negative pressure developed inside the crank chamber  2   a  and the cylinder block  1  below the piston  10 , the air/fuel mixture M is directly introduced into the crank chamber  2   a  from the air/fuel mixture port  24   a  open at the inner peripheral surface of the cylinder block  1 . Since at this time, a negative pressure is developed also in the air scavenging passage  30  communicated with the crank chamber  2   a  through the bearing units  11 , the pressure inside the air introducing passage  40  shown in  FIG. 3  and communicated with the air scavenging passage  30  is reduced to a negative value and the reed valve  41  fitted to the exit of the air passage  23  in the insulator  22  is accordingly opened to allow the air A from the air passage  23  to be introduced temporarily into the air scavenging passage  30  through the air introducing passage  40 . 
     In this way, during the intake stroke, the air A is introduced into the air scavenging passage  30  at all times when the reed valve  41  is opened by the effect of the negative pressure inside the crank chamber  2   a  shown in  FIG. 2 . For this reason, a sufficient amount of air necessary to avoid the undesirable blow-off can be secured within the air scavenging passage  30 . 
     Thereafter, and during the scavenging stroke, the air A is introduced into the combustion chamber  2   a  from the air scavenging port  30   a  open at the inner peripheral surface of the cylinder block  1  at a somewhat high level as shown in  FIG. 3  and, a short moment later, the air/fuel mixture M is introduced into the combustion chamber  2   a  from the air/fuel mixture scavenging port  31   a . Because of such introducing timing and, also, because the air A is introduced into the combustion chamber  1   a  from a location nearer the exhaust port  29   a  than a location from which the air/fuel mixture M is introduced, exhaust gases can be pushed out and discharged from the exhaust port  29   a  by the air A that has been introduced earlier than the air/fuel mixture M. Accordingly, the undesirable blow-off of the air/fuel mixture M from the exhaust gases  29   a  can be avoided. 
     In the description that follows, the structure according to the gist of the first preferred embodiment of the present invention will be discussed. As shown in  FIG. 1 , the carburetor  3  has a carburetor body  43  having a carburetor air/fuel mixture passage  48  defined therein for supplying the air/fuel mixture M towards the engine body E and a carburetor air passage  49  defined therein for supplying the scavenging air A towards the engine body E so as to extend parallel to the carburetor air/fuel mixture passage  48 . The carburetor body  43  includes the single rotary valve unit  28  supported thereby so as to extend across the passages  48  and  49  in a direction substantially perpendicular thereto for pivotal movement. As best shown in  FIG. 4 , the rotary valve unit  28  includes an air/fuel mixture valve  50  for adjusting the opening of the carburetor air/fuel mixture passage  48  and an air valve  51  formed integrally and coaxially with the air/fuel mixture valve  50  for adjusting the opening of the carburetor air passage  49 . 
     More specifically, the air/fuel mixture valve  50  and the air valve  51  have an air/fuel mixture flow aperture  50   a  and an air flow aperture  51   a , which are defined therein, respectively, for adjusting the respective openings of the carburetor air/fuel mixture passage  48  and the carburetor air passage  49 . Those valves  50  and  51  are pivotable about the longitudinal axis C 3  extending in a direction substantially perpendicular to the passages  48  and  49  for adjusting the respective openings of the passages  48  and  49 . 
     The rotary valve unit  28  has an upper surface formed with a valve shaft  52  so as to project coaxially therefrom, which shaft  52  extends through a lid member  53  closing an upper open end of the carburetor body  43  and is then rotatably supported by such lid member  53 . The carburetor body  43  is provided with a main nozzle  54  for a fuel (gasoline) extending coaxially through a bottom portion of the air/fuel mixture valve  50  so as to project into the air/fuel mixture aperture  50   a , and a fuel injecting port  54   a  shown in  FIG. 5  is defined in a portion of a peripheral wall of the main nozzle  54 . 
     On the other hand, a needle valve  58  extends coaxially with the rotary valve unit  28  to the air/fuel mixture aperture  50   a  in the air/fuel mixture valve  50  through the hollow of the valve shaft  52 , and this needle valve  58  has an upper end  58   a  formed with a male thread that is threaded into an internally threaded hole  52   a  defined in the valve shaft  52 . The needle valve  58  has a minus (−) shaped groove  58   b  defined in a top face thereof so that when the needle valve  58  is turned with a screw driver engaged in the minus shaped groove  58   b , the needle valve  58  can be moved up or down relative to the rotary valve unit  28  to thereby adjust the opening of the fuel injecting port  54   a  of the main nozzle  54 , that is, adjust the amount of fuel injected. This needle valve  58  has a lower end inserted into the main nozzle  54 . 
     As best shown in  FIG. 5 , the fuel injecting port  54   a  of the main nozzle  54  is formed in a shape similar to the shape of the elongated, but inverted triangle so that the opening of the fuel injecting port  54   a , that is, the opening of the main nozzle  54  can be adjusted by the needle valve  58  then moving upward or downward. Also, a coil spring  62  is interposed between an upper face of the air valve  51  and a lower surface of the lid member  53  so that the rotary valve unit  28  can be returned from a throttle full open position to an idling opening by the effect of a spring force thereof. 
     A fuel reserving body  59  having a diaphragm pump (not shown) built therein is connected with a bottom region of the carburetor body  43  and the fuel can be supplied from this fuel reserving body  59  to the main nozzle  54 . The valve shaft  52  has an upper end portion extending upwardly through the lid member  53  and a valve operating lever  61  is connected with that upper end portion of the valve shaft  52 . As shown in  FIG. 6 , the valve operating lever  61  has a cam portion  61   a  of a sector shape as viewed from top and also has an intermediate portion, which is a root portion thereof, fixed to the valve shaft  52 . A cylindrical connecting member  63  is provided in an end portion of the valve operating lever  61  remote from the cam portion  61  so as to protrude therefrom. A remote operating cable  65  having one end connected with a manually operated throttle lever (not shown) has the opposite end provided with an engagement member  66  that is connected with the connecting member  63 . The valve operating lever  61  is applied a rotary restoring force from the coil spring  62  shown in  FIG. 4  and is accordingly normally biased in a direction counter to the arrow headed line P so that the valve operating lever  61  can be maintained at a position where it is held in engagement with a free end  64   a  of an adjustment bolt  64  during the idling condition. 
     As best shown in  FIG. 7 , the cam portion of the valve operating lever  61  has a cam surface  61  defined in a lower surface thereof, with which an upper end  68   a  of a guide pin  68  projecting from the lid member  53  is held in contact. In other words, the cam surface  61   b  is applied an elastic force by the effect of a restoring force of the coil spring  62 , shown in  FIG. 4 , acting on the valve shaft  52 , so as to be oriented downwardly and is therefore held in contact with the upper end of the cam guide pin  68 . Accordingly, when the connecting member  63  is pulled in a direction shown by the arrow headed line Q, shown in  FIG. 6 , through the remote operating cable  65 , the cam portion  61   a  of the valve operating lever  61  pivots in the direction shown by the arrow headed line P in the manner hereinabove described, with the cam surface  61   b  held in sliding contact with the cam guide pin  68  as shown in  FIG. 7 , and, therefore, the valve operating lever  61  is pulled upwards against the resilient force of the coil spring  62  shown in  FIG. 4 . At this time, the rotary valve unit  28  shown in  FIG. 4  is pivoted together with the valve operating lever  61  to thereby adjust the respective openings of the carburetor air/fuel mixture passage  48  and the carburetor air passage  49  to a large value by means of the associated apertures  50   a  and  51   a , shown in  FIG. 4 , of the air/fuel mixture valve  50  and the air valve  51 . Simultaneously therewith, the needle valve  58  ascends together with the valve operating lever  61  to increase the opening of the fuel injecting port  54   a  with a substantial amount of fuel consequently supplied to the carburetor air/fuel mixture passage  48 . In this way, the combustion engine is controlled as to its revolution in dependence on manipulation of the throttle lever. 
     The carburetor  3  is provided with a lift-up type start operating mechanism S shown in  FIG. 6 . This start operating mechanism S includes a lift up lever  69  having its left end (as viewed in  FIG. 6 ) provided with an operating portion  69   a , and a shank portion  69   b  on the right side of the lift up lever  69 . The shank portion  69   b  is inserted into a bearing unit  53   a  disposed at a position of a left end, as viewed in  FIG. 6 , of the lid member  53  so that the lift-up lever  69  is supported for pivotal movement about the longitudinal axis C 4  of the lever  69 . An actuating piece  69   c  is provided at a free end of the shank portion  69   b  of the lift up lever  69  so as to project therefrom and is held in contact with the lower surface of the cam portion  61   a  of the valve operating lever  61 . 
     The actuating piece  69   c  referred to above is in the form of a flat plate as best shown in  FIGS. 6 and 7 . Accordingly, when at the time the combustion engine is to be started, the lift up lever  69  is manually pivoted 90° from a stop position by the effect of a pivoting operation of the operating portion  69   a , the actuating piece  69   c  is lifted upwardly to push the valve operating lever  61  upwardly as shown by the double dotted line in  FIG. 4 . Accordingly, the rotary valve unit  28  and the needle valve  58  are elevated together with the valve operating lever  61  to increase the opening of the fuel injecting port  54   a  of the main nozzle  54  to increase the amount of fuel to be supplied and, consequently, a proper amount of fuel can be supplied to facilitate starting of the combustion engine. 
     When after the combustion engine has been started, the operating portion  69   a  best shown in  FIG. 6  is manually operated to return the lift up lever  69  back to the initial position, the valve operating lever  61  then pushed upwardly by the actuating piece  69   c  in the manner described above is lowered, with the combustion engine establishing an idling condition. Also, even though the lift up lever  69  is not manually returned to the initial position, operating the throttle lever in a direction of acceleration can result in pivotal movement of the valve operating lever  61  and the rotary valve unit  28  ( FIG. 4 ) in the direction shown by the arrow headed line P through the remote operating cable  65 , with the valve operating lever  61  consequently pushed upwards by means of the guide pin  68 . Therefore, when the lower surface of the valve operating lever  61  separates from the actuating piece  69   c  of the lift up lever  69 , the pushing force applied from the valve operating lever  61  to the actuating piece  69  is released. As a result, the lift up lever  69  is automatically returned to the initial position, best shown in  FIGS. 6 and 7 , by the effect of the restoring force of a coil spring  73 , which has been engaged at its opposite ends with the operating portion  69   a  and a cylindrical portion  53   a  of the bearing unit of the lid member  53 , respectively, and the combustion engine is consequently brought in the idling condition. 
     Referring now to  FIG. 8 , an auxiliary air passage  70  is formed so as to bypass the air/fuel mixture valve  50  of the rotary valve unit  28 . This auxiliary air introducing passage  70  has a throughhole  70   a  defined therein as a part thereof so as to extend through the carburetor body  43  of the carburetor  3 . 
     The air cleaner  4  referred to previously is formed with an upstream communicating hole  70   b  positioned in a clean air chamber  4   a , which is on a clean side in the air cleaner  4 , and communicated with an upstream opening of the throughhole  70   a  for the air A. Also, the insulator  22  referred to previously is formed with a communicating groove  70   c  defined therein for communicating a downstream opening of the throughhole  70   a  and the air/fuel mixture passage  24 . The auxiliary air introducing passage  70  referred to above and communicating between the clean air chamber  4   a  of the air cleaner  4  and a downstream side of the rotary valve unit  28  is formed by the throughhole  70   a , an upstream communicating hole  70   b  and a communicating hole  70   c.    
     The air cleaner  4  is of a type made up of a cleaner casing  85  and a cleaner covering  86  both cooperating with each other to define a cleaner space, including the clean air chamber  4   a  referred to previously, therebetween, and a cleaner element  72  accommodated within such cleaner space. This air cleaner  4  draws an air A from the outside through an air intake tube  71 , and then introduce the air A into the carburetor air/fuel mixture passage  48  and the carburetor air passage  49 , both shown in  FIG. 1 , after the air A so drawn from the outside has been substantially purified by the cleaner element  72  and subsequently flowed into the clean air chamber  4   a . In addition, a part of the substantially purified air within the clean air chamber  4   a  is introduced through the auxiliary air introducing passage  70  as an auxiliary air into the air/fuel mixture passage  24  on the downstream side of the rotary valve unit  28 . 
     As hereinabove described, in the two-stroke cycle combustion engine according to the foregoing first preferred embodiment of the present invention, the use is made of the auxiliary air introducing passage  70  so that an auxiliary air A can be introduced into the air/fuel mixture passage  24  on the downstream side of the rotary valve unit  28 . Accordingly, the needle valve  58  of the carburetor  3  can be shifted upwards during the idling condition to increase the amount of fuel to be supplied in a quantity corresponding to the amount of the auxiliary air so introduced, allowing the number of idling revolutions of the combustion engine to be adjusted to a value comparable to that afforded by the conventional standard combustion engine. In other words, it is sufficient to set the initial position of the air/fuel mixture valve  50  of the rotary valve unit  28  so that the opening of the air/fuel mixture valve  50  in the carburetor air/fuel mixture passage  48  is reduced so as to render the amount of the air flowing through the carburetor air/fuel mixture passage  48  small, while adjusting the needle valve  58  in the manner described above. 
     The amount of the air flowing into the combustion chamber  1   a  during the idling condition corresponds to the sum of the air flowing through the air/fuel mixture passage  24  and that flowing through the auxiliary air introducing passage  70 . Accordingly, since the total amount of the air is increased, the amount of fuel can be increased correspondingly by determining the position of the needle valve  56  so that the number of idling revolutions of the combustion engine which may be equal to that afforded by the conventional standard combustion engine can be attained. As a result, the air/fuel mixture within the combustion chamber  1   a  will not be enriched excessively and, therefore, a smooth idling operation can be performed. 
     Since when transit from the idling condition to the rapid acceleration is initiated, the enriched air/fuel mixture pooled within the crank chamber  2   a  is supplied into the combustion chamber  1   a , the rapid acceleration can be performed smoothly. Also, since the opening of the air/fuel mixture valve  50  during the idling condition, that is, the throttle opening during the idling condition can be reduced as compared with that in the conventional standard combustion engine, the opening of the air valve  51  provided in the same rotary valve unit  28  having the air mixture valve  50  can also be reduced. Accordingly, there is no possibility of the air passage to be opened large, allowing the revolution of the combustion engine to be stabilized. 
     Since as hereinabove described, the throttle opening during the idling condition can be reduced to a value smaller than that afforded with the conventional standard combustion engine, the release position (return position) of the lift up lever  69  for starting can be lowered. Accordingly, the lift-up amount or distance of the needle valve  58  when the lift up lever  69  is pivoted to an operating position at the time of engine start, that is, the amount of the fuel injection port  54   a  of the main nozzle  54  to be increased at the time of engine start can be increased. Considering that the fuel injecting port  54   a  of the main nozzle  54  is so shaped as to represent the inverted triangular shape as shown and described with particular reference to  FIG. 5 , increase of the amount of the needle valve  56  lifted up results in increase of the amount of fuel supplied and hence, the startability can be improved. 
     Also, in the two-stroke cycle combustion engine of the structure hereinbefore described, the communicating groove  70   c  forming a downstream portion of the auxiliary air introducing passage  70  is formed in the insulator  22  disposed between the carburetor  3  and the engine body E. Accordingly, a portion of the auxiliary air introducing passage  70  can be easily provided in the insulator  22  which has an ample space as compared with that in the carburetor  3 . In addition, with no need to add any extra member, the auxiliary air introducing passage  70  can be provided if the carburetor  3 , the insulator  22  and the air cleaner  4  are merely modified. 
     It is to be noted that as shown by the double dotted chain line in  FIG. 7 , the communicating groove  70   c  may be formed in a mating surface  3   a  of the insulator  22  of the carburetor  3 . 
       FIG. 9  illustrates in a top sectional representation the combustion engine according to a second preferred embodiment of the present invention. It is to be noted that component parts shown in  FIG. 9 , but similar to those shown particularly in  FIG. 8  are designated by like reference numerals and the details are not therefore reiterated for the sake of brevity. 
     In the second preferred embodiment of the present invention, the auxiliary air introducing passage  70 A is made up of a delivery tube  78  fitted to a wall surface of the air cleaner  4  so as to extend therethrough with one end thereof positioned inside the clean air chamber  4   a , an inflow tube  79  fitted to the insulator  22  so as to extend thereinto from outside, a connecting tube  80  connecting the delivery tube  78  with the delivery tube  79 , and a communicating groove  81  defined in the insulator  22  for communicating the inflow tube  79  with the air/fuel mixture passage  24 . Accordingly, in this second embodiment of the present invention, the auxiliary air introducing passage  70 A can easily provided with no need to modify the carburetor  3  for this purpose. 
       FIG. 10  illustrates in a top sectional representation the combustion engine according to a third preferred embodiment of the present invention. It is to be noted that component parts shown in  FIG. 9 , but similar to those shown particularly in  FIG. 8  are designated by like reference numerals and the details are not therefore reiterated for the sake of brevity. 
     In the third preferred embodiment of the present invention, an auxiliary air cleaner  4 A is additionally employed, which includes a compact cleaner casing  82  having an air inflow port  82   a  and an air outflow port  82   b  both defined therein and a cleaner element  83  accommodated within the interior of the cleaner casing  82  and is operable to substantially purify the external air with the cleaner element  83 . An air introducing tube  87  has an upstream portion thereof fitted into the air outflow port  82   b  open into a clean air chamber  82   c  of the cleaner casing  82  and also has a downstream portion fitted into an air introducing hole  75  defined in the insulator  22  in communication with the air/fuel mixture passage  24 . This air introducing tube  87  and the air introducing hole  75  altogether form an auxiliary air introducing passage  79 B that connects the auxiliary air cleaner  4 A with the air/fuel mixture passage  24 . 
     Accordingly, even in this third embodiment, the auxiliary air introducing passage  70 B can be provided with no need to modify the carburetor  3  for this purpose. 
     As discussed above, since the carburetor  3  employed in any of the embodiments shown in and described with particular reference to  FIGS. 9 and 10 , respectively, need not be modified or altered in anyway whatsoever as is the case with that in the second embodiment described previously, the versatility of the carburetor  3  can be expanded. 
     The two-stroke cycle combustion engine according to a fourth preferred embodiment of the present invention is shown fragmentarily in  FIG. 11  in a front elevational representation. In the fourth embodiment, as the start operating mechanism, a standard choke mechanism is employed in place of the start lift up lever  69  shown in and described with particular reference to  FIG. 1 . 
     Referring now to  FIG. 11 , a choke valve CH is provided in the air cleaner  4 .  FIG. 12  illustrates a perspective representation showing the interior of the cleaner casing  85  of the air cleaner  4  and  FIG. 13  illustrates a left side representation showing the cleaner casing  85 . As best shown in  FIG. 12 , an air inflow hole  88  of a shape nearly similar to the shape of an hen egg is defined at a generally center portion of the cleaner casing  85 , and an upper air delivery hole  89  and a lower air delivery hole  90  are defined in a portion of the cleaner casing  85  downstream of the air inflow hole  88  and positioned one above the other. Accordingly, air having passed through the air inflow hole  88  flows in part into the upper air delivery hole  89  and in part into the lower air delivery hole  90 . 
     It is to be noted that reference numeral  95  represents insertion holes through which corresponding bolts (not shown) required to connect the cleaner casing  85  and the cleaner covering  86  together are passed. 
     As shown in  FIG. 13  in the side view, the upper air delivery hole  89  represents a round shape whereas the lower air delivery hole  90  represents a modified elongated shape. The upper air delivery hole  89  serves to deliver the air to the carburetor air passage  49  best shown in  FIG. 1  and the lower air delivery hole  90  deliver the air to the carburetor air/fuel mixture passage  48  also best shown in  FIG. 1 . Also, an auxiliary air hole  91  is defined at a location laterally of the lower air delivery hole  90  for supplying the auxiliary air towards the throughhole  70   a , defined in the carburetor  3  and forming a major passage portion of the auxiliary air introducing passage  70 , as is the case with the previously described first embodiment, or for supplying the auxiliary air towards the auxiliary air introducing passage  70 A including the connecting tube  90  shown in  FIG. 9 , as is the case with the previously described second embodiment. 
     The cleaner casing  85  shown in  FIG. 12  has a support pin  93  pivotally disposed therein and extending parallel to a direction X in which the air inflow hole  88  opens, and the choke valve CH is fixedly mounted on this support pin  93  for pivotal movement together with the support pin  93 . This choke valve CH has a valve body  92  positioned inwardly of the cleaner casing  85 , and choke lever  94  for adjustably driving the valve body  92  is fixedly connected with the support pin  93  and is positioned outside the cleaner casing  86 . As best shown in  FIG. 11 , the choke lever  94  protrudes forwardly from the cleaner casing  85  and can be moved up and down manually to pivot the choke valve CH, best shown in  FIG. 13 , to thereby adjust the amount of the air to be sucked. 
     Although the valve body  92  of the choke valve CH so far described above is in the form of a flat plate, it may be of a rotary valve type as is the case with the rotary valve unit  28  and, in any event, it may be of any suitable construction provided that the air inflow port  88  can be selectively opened or closed. 
     When the choke lever  94  is pushed upwards to assume a fully closed position as shown in  FIG. 14  at the time the combustion engine is desired to be started, the valve body  92  closes the air inflow port  88 . The valve body  92  is provided with a passage shutter  98  for closing the auxiliary air hole  91 , that is, the auxiliary air introducing passage  70 , when the valve body  92  is in position to close the air inflow hole  88 , through a stay  99  so as to protrude laterally of the valve body  92 . Accordingly, at the time of engine start, air flows into the air inflow hole  88  only from a small hole  92   a  defined in the choke valve body  92 . The passage shutter  98  referred to above is, when the choke valve CH is in a fully opened position as shown in  FIG. 13 , positioned upstream of the air inflow hole  88 . 
     Thus, even in the two-stroke cycle combustion engine utilizing the standard choke valve CH, a favorable startability of the engine can be maintained. In other words, if the air passage  23  and the air/fuel mixture passage  24  are throttled down by the choke valve CH at the time of engine start, a substantial amount of air would flow from the auxiliary air hole  91  into the auxiliary air introducing passage  70  to lean the air/fuel mixture, accompanied by lowering the startability. However, this problem can be solved in the fourth embodiment, since the auxiliary air introducing passage  70  is closed by the passage shutter  98  in the manner hereinabove described. 
     On the other hand, when at any time other than the starting of the combustion engine, the choke lever  94  is pushed downwards to assume a fully opened position as shown in  FIG. 13 , the choke valve CH is held in position to fully open the air inflow hole  88  and also to open the auxiliary air introducing passage  70  and, therefore, transit from the idling condition towards the rapid acceleration can be accomplished smoothly in a manner similar to that described in connection with the first embodiment of the present invention hereinbefore fully described. 
     In a fifth preferred embodiment of the present invention shown in  FIG. 15 , the stay  99  for supporting the passage shutter  98  is formed with a slot  100 . While when the valve body  92  of the choke valve CH is held in position to fully open the air introducing passage  98 , the stay  99  for supporting the passage shutter  98  is positioned upstream of the air inflow hole  88  enough to somewhat disturb the amount of air flowing into the air introducing hole  88  as in the case of the fourth embodiment, the fifth embodiment shown in  FIG. 15  is effective to allow a substantially large amount of air to be smoothly introduced into the air inflow hole  88  through the slot  100  defined in the stay  99 . 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings which are used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention. Accordingly, such changes and modifications are, unless they depart from the scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
               1 : Combustion chamber 
               3 : Carburetor 
               4 : Air cleaner 
               4 A: Auxiliary air cleaner 
               22 : Insulator (Spacer) 
               23 : Air passage 
               24 : Air/fuel mixture passage 
               28 : Rotary valve unit 
               30 : Air scavenging passage (Scavenging passage) 
               31 : Air/fuel mixture scavenging passage (Scavenging passage) 
               50 : Air/fuel mixture valve (Valve) 
               51 : Air valve (Valve) 
               54 : Main nozzle 
               58 : Needle valve 
               69 : Lift up lever 
               70 ,  70 A,  70 B: Auxiliary air introducing passage 
               70   a : Throughhole 
               80 : Connecting tube (Tube) 
               98 : Passage shutter 
             A: Air 
             CH: Choke valve 
             E: Engine body 
             M: Air/fuel mixture 
             S: Start operating mechanism