Abstract:
A two-stroke internal combustion engine having an air control arrangement which controls scavenging air flow during rapid engine acceleration to optimize the acceleration and maximum power output of the engine while maintaining low exhaust emissions from the engine. Desirably, the air control arrangement may comprise a valve that throttles the air passage to one half or less of its total flow area at a medium engine load or less and completely or fully opens at a medium engine load or more of the engine. Normally, the air control valve opens in unison with the throttle valve, but during rapid acceleration of the engine the air control valve opens later or more slowly than the throttle valve to control the supply of scavenging air to the combustion chamber of the engine thereby enhancing rapid acceleration of the engine.

Description:
REFERENCE TO RELATED APPLICATIONS 
     Applicant claims priority of Japanese patent applications, Ser. No. 2000-069271, filed Mar. 13, 2000; Ser. No. 2000-137441, filed May 10, 2000; and 2000-138376, filed May 11, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to internal combustion engines and more particularly to a stratified scavenging two-stroke internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Generally, in a so-called stratified scavenging two stroke internal combustion engine a scavenging air supply is introduced into a combustion chamber of the engine after a combustion event has occurred and before a fuel and air mixture is delivered from a crankcase chamber of the engine to facilitate exhausting the combusted gas from the combustion chamber and to provide some air to facilitate combustion of a subsequently delivered fuel and air mixture. During rapid acceleration of the engine, the scavenging air enters the combustion chamber at a high flow rate which tends to dilute the fuel and air mixture making it overly lean and thereby materially deteriorating the acceleration, performance and stability of the engine. 
     When the scavenging air supply is limited or throttled during rapid acceleration of the engine, the stability of the acceleration of the engine is improved because dilution of the fuel and air mixture is prevented but the maximum power output of the engine is significantly reduced. If the fuel mixture passage is widened or enlarged, the requirements for acceleration and maximum power output can be satisfied, but there is an increased and unacceptably high level of exhaust emissions from the engine. 
     SUMMARY OF THE INVENTION 
     A two-stroke internal combustion engine having an air control which controls scavenging air flow during rapid engine acceleration to optimize the acceleration and maximum power output of the engine while maintaining low exhaust emissions from the engine. Desirably, the air control may comprise a valve that throttles the air passage to one half or less of its total flow area at a medium engine load or less and completely or fully opens at a medium engine load or more of the engine. Normally, the air control valve opens in unison with the throttle valve, but during rapid acceleration of the engine the air control valve opens later or more slowly than the throttle valve to control the supply of scavenging air to the combustion chamber of the engine thereby enhancing rapid acceleration of the engine. The air control valve eventually fully opens to increase the maximum power output of the engine and the fuel mixture passage need not be widened to avoid excessive exhaust emissions. 
     In one form, the air control may be a butterfly or disk type valve driven for rotation by the rotation of the throttle valve through a linkage. In another form, the air control may be a plunger type valve biased by a spring to delay its opening upon rapid acceleration of the engine. In yet another form, the air control may comprise a read type valve. In yet another form, the air control comprises an air passage between the carburetor and engine which is longer than the fuel and air mixture passage so that upon rapid engine acceleration, which tends to draw increased air into the combustion chamber, the scavenging air has a longer path to travel than the fuel and air mixture. Hence, less air is drawn into the engine during rapid acceleration to prevent undue dilution of the fuel mixture in the combustion chamber and enable smooth, stable acceleration. In any form, the flow of air to the combustion chamber during rapid acceleration of the engine is controlled to limit the air flow into the combustion chamber and thereby provide a desired fuel and air mixture suitable to enable rapid acceleration without loss of power output from the engine and without increasing the exhaust emissions of the engine. 
     Objects, features and advantages of this invention include providing an engine which provides a scavenging air supply to the engine, controls the flow rate of scavenging air at least during rapid engine acceleration, enables smooth, stable and rapid engine acceleration, permits a high maximum engine power output, has relatively low exhaust emissions, improves the responsiveness of the engine, is of relatively simple design economical manufacture and assembly, and in service has a long and useful life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiments and best mode, appended claims and accompanying drawings in which: 
     FIG. 1 is a front sectional view of a stratified scavenging two-stroke engine according to the present invention; 
     FIG. 2 is a plan view of an insulator plate of a stratified scavenging two-stroke engine according to a second embodiment of the present invention; 
     FIG. 3 is a diagram illustrating the relationship between a throttle valve and air valve of the carburetor and an air control valve in the engine; 
     FIG. 4 is a diagram illustrating the relationship between a throttle valve and air valve of the carburetor and an air control valve in the engine; 
     FIG. 5 is a front sectional view of a stratified scavenging two-stroke internal combustion engine according to a third embodiment of the invention; 
     FIG. 6 is a side sectional view of a portion of the engine of FIG. 5; 
     FIG. 7 is a front sectional view of a stratified scavenging two-stroke internal combustion engine according to a fourth embodiment of the present invention; 
     FIG. 8 is a front sectional view of a stratified scavenging two-stroke internal combustion engine according to a fifth embodiment of the invention; 
     FIG. 9 is a side sectional view of a portion of the engine of FIG. 8; 
     FIG. 10 is a front sectional view of a stratified scavenging two-stroke internal combustion engine according to a fifth embodiment of the present invention; 
     FIG. 11 is a side view of an insulator plate of the engine of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings, FIG. 1 illustrates a two-stroke internal combustion engine A according to the present invention, having a carburetor C connected through a heat insulator plate B to the engine. In the engine A, a piston  9  is inserted into a cylinder body  10  to define a combustion chamber  4  closed by a cylinder head  3  above the piston  9  and a crankcase chamber  18  below the piston  9 . An upper end of a connecting rod  12  is connected to the piston  9  by a pin  8 , and a lower end of the connecting rod  12  is connected to a crank arm integral with a balance weight  16   a  and crank shaft  15  by a pin  19 . In the illustrated embodiment, three scavenging passages  7  are provided in a right side wall and front and rear walls, respectively, of the cylinder body  10 , and an exhaust port  5  is provided in a left side wall of the cylinder  10 . A scavenging port  7   a  of each scavenging passage  7  and the exhaust port  5  are communicated with the combustion chamber  4  at a bottom dead center position of the piston  9 . 
     In the carburetor C, a cylindrical, stepped throttle valve  48  is rotatably and vertically movably inserted into a stepped bore  61  perpendicular to an air passage  46  and a fuel mixture passage  50  extending through a carburetor body  47 . The stepped throttle valve  48  is provided with an air valve having a throttle hole  45  capable of being communicated with the air passage  46  and a mixture valve having a throttle hole  55  capable of being communicated with the mixture passage  50 . A fuel nozzle  52  extends into the bore  61  and projects into the throttle hole  55 . A needle  49  carried by the throttle valve  48  is inserted into the fuel nozzle  52  to adjust to the flow area of a fuel jet or opening of the fuel nozzle  52 . The lower end of the fuel nozzle  52  is communicated with a metering chamber  57  via a check valve (not shown) and the fuel jet. A cover  58  is connected to an intermediate plate  53  connected to the lower end of the carburetor body  47 . A diaphragm  54  and gasket  20  are disposed between the cover  58  and plate  53 . A metering chamber  57  and an atmospheric chamber  56  open to the atmosphere through a vent  22  are defined at least in part above and below, respectively, the diaphragm  54 . 
     A valve shaft  42  of the throttle valve  48  extends through a cover plate  43  which is connected to the upper end of the carburetor body  47  to close the stepped bore  61 . A throttle valve lever  41  is connected to the upper end of the valve shaft  42 . A cam surface (not shown) is formed on the lower surface of the throttle valve lever  41  and is engaged to a follower (not shown) upwardly projecting from the cover plate  43  by the force of a return spring  44 . The return spring  44  is disposed so as to surround the valve shaft  42  between the cover plate  43  and the throttle valve  48 , and the upper and lower ends of the return spring  44  are fastened at the cover plate  43  and the throttle valve  48 , respectively. The throttle valve  48  is yieldably biased to an idle position as shown, by the force of the return spring  44 . 
     The carburetor has a diaphragm type fuel pump, not shown, driven by pressure pulses of the crankcase chamber  18  of the engine to draw fuel from a fuel tank and supply it to the metering chamber  57 . Fuel is stored in the metering chamber  57  under substantially constant pressure maintained by an inlet valve, which is opened and closed in response to movement of the diaphragm  54 . 
     According to the present invention, a heat insulator plate B is disposed between the cylinder body  10  and the carburetor C. The insulator plate B has an air passage  36  communicating the air passage  46  of the carburetor C with a scavenging passage  7  and a fuel mixture passage  40  communicating the fuel mixture passage  50 , of the carburetor C with an intake part  17  in the engine A through a check valve  39 . An air control valve  35  is connected on a valve shaft  32  and is disposed in the air passage  36  for controlling air flow from the air passage  46  to the scavenging passage  7  through a port  14   a  of the engine having a check valve  14 . A lever  31  is connected to the upper end of the valve shaft  32 , and a return spring  34  is disposed between the lever  31  and the upper wall of the heat insulator plate B. The lever  31  and the throttle valve  41  are connected by a linkage  33 . A second valve  37  is connected to the lower end of the valve shaft  32 . The second valve  37  is designed to open and close a passage  38  connecting a portion of the air passage  36  downstream of the air control valve  35  and the mixture passage  40  in the insulator plate B. 
     The air control valve  35  carried by the heat insulator plate B has a minimum opening area which is about ½ of the normal or fully open area of the air passage  36  and need not be fully closed at any time. The lever  31  on the valve shaft  32  of the air control valve  35  and the throttle valve lever  41  are connected by a linkage  33  such that the air control valve  35  opens more slowly or later by a phase difference than the throttle valve  48 . Since the air control valve  35  may have a smaller working angle of rotation than the throttle valve  48 , the air control valve  35  is connected so that it is rarely operated or moved below a medium engine load or a medium opening position of the throttle valve  48  (i.e. when the throttle valve  48  is mid-way between its idle and wide open positions). The air control valve  35  is preferably fully opened when the throttle valve  48  is in the vicinity of its wide-open position. 
     At idle and low speed and low load engine operation the second valve  37  is open to communicate the mixture passage  40  and the air passage  36  through passage  38 . At high speed and or high load engine operation the second valve  37  is closed. When the passage  38  is opened during low speed engine operation, a part of the fuel and air mixture is supplied to the air passage  36  and fuel which stays in the air passage  36  assists acceleration in case the engine is rapidly accelerated. However, during high-speed or high load engine operation, corresponding to wide open throttle valve  48  position, the passage  38  is closed to avoid increasing the emission of harmful components of exhaust gases. 
     During the operation of the engine, when the piston  9  moves toward top dead center the pressure in the crankcase chamber  18  decreases, thereby drawing a fuel and air mixture from the carburetor C through the mixture passage  40 , the check valve  39  and the intake port  17  and into the crankcase chamber  18 . Air flows into the crankcase chamber  18  through the air passages  46  and  36 , the check valve  14  and the scavenging passage  7 . That is, a condition exists wherein the scavenging passage  7  is filled with air and the fuel and air mixture in the crankcase chamber  18  is made lean by air. 
     When a fuel and air mixture in the combustion chamber  4  is ignited by a spark plug  2 , the pressure in the combustion chamber  4  rapidly increases, and as the piston  9  moves down toward bottom dead center, the pressure in the crankcase chamber  18  increases. When the piston  9  moves down to a given position the exhaust port  5  opens so that the combustion gas in the combustion chamber  4  may flow out of the exhaust port  5  and the pressure in the combustion chamber  4  rapidly decreases. At the same time, the scavenging ports  7   a  of the scavenging passages  7  open so that first air in the scavenging passage  7  flows into the combustion chamber  4  and then the fuel and air mixture in the crankcase chamber  18  flows into the combustion chamber  4  via the scavenging passage  7 . 
     FIG. 3 shows how the quantity of air changes relative to the operating position of the fuel mixture valve (represented by throttle hole  55 ) in response to the throttle valve  48 , the air valve (represented by throttle hole  45 ) and the air control valve  35 . The air control valve  35  relatively rapidly opens when the fuel mixture valve moves toward its fully or wide-open position. There is a time or phase difference in operation or movement of the air control valve  35  from the idle or low speed position and the full open or high speed position when the engine is rapidly accelerated. During rapid acceleration, the engine speed or R.P.M. rapidly increases and the fuel mixture valve (throttle hole  55 ) is fully opened before the air control valve  35  is fully opened so that a rich fuel and air mixture (a mixture not made lean by air from the scavenging passage  7 ) is supplied from the crankcase chamber  18  to the combustion chamber  4  to assist the acceleration and the increase in the engine R.P.M. In other words, the delay in fully opening the air control valve  35  after the fuel mixture valve is fully opened throttles or limits the flow of air through the air passage  36 . This reduces the amount of air mixed with the fuel and air mixture transferred from the crankcase chamber  18  to the combustion chamber  4  providing a rich fuel and air mixture in the combustion chamber to support the rapid engine acceleration. The delay in fully opening the air control valve  35  is sufficient to support rapid acceleration even if the valve  35  relatively quickly opens after initial rapid acceleration because the delay permits the engine speed to rapidly increase to a desired and sufficiently high speed. 
     In the embodiment shown in FIG. 2, to improve the acceleration of the engine by delaying the full opening of the air control valve  35  relative to the throttle valve  48  when the engine is rapidly accelerated, an air damper  70  is provided on the upper wall of the heat insulator B. In the air damper  70 , a piston  68  is fitted in a cylinder  63  having a hole  72 , and a rod  68   a  projecting from the piston  68  extends out of the hole  72  and into contact with the lever  31  connected to the valve shaft  32  by the force of a spring  66  disposed between a wall plate  64  and the piston  68 . The linkage  33  extends through and is supported on the lever  31  and a spring  62  is disposed between the lever  31  and a spring seat  61   a  connected to the end of the link  33 . Wall plate  64  has an opening or throttle hole  65  and defines in part a chamber  67 . In the present embodiment, the passage  38  and the second valve  37  are not provided. 
     Even if the throttle valve lever  41  is rapidly turned to rapidly move the throttle valve  48  toward its fully open position when the engine is rapidly accelerated, the spring  62  is compressed so that the lever  31  turns later or more slowly than the throttle valve  48  due to the action of the air damper  70 . As the lever  31  rotates, the piston  68  is displaced by the lever  31  against the force of the spring  66  and gradually moves leftward (as viewed in FIG.  2 ), and the air control valve  35  moves to its fully open position after the throttle valve  48  of the carburetor C, by a delay period, has been fully opened as shown in FIG.  4 . 
     Second Embodiment 
     As shown in FIGS. 5 and 6, in accordance with a second embodiment of an engine of the present invention, for improving the responsiveness of the engine during rapid acceleration without sacrificing the maximum power output of the engine, an air control valve  35   a  is disposed in the air passage  36  of the insulator plate B′ between the air valve  45  (not shown) and the check valve (shown as a reed valve)  14  provided at an inlet of the scavenging passage  7 . To facilitate the description of this embodiment, essentially only the insulator plate B′ and associated components are shown. The engine A, carburetor C and their associated components may be the same as shown in FIG.  1  and described with reference thereto. A valve chamber  100  having a rectangular shape in section is provided in the air passage  36  of the heat insulator plate B′. In this embodiment the air control valve  35   a  is a reed valve disposed in part in communication with the air passage  36 . A partially curved or bent guide plate  102  and valve plate  104  are superimposed on each other and fixed to the side wall of the valve chamber  30  by means of a rivet  105  or other fastener. The reed valve  35   a  partially closes the air passage  36  when in its unflexed state shown in FIG.  5  and is flexed to a position permitting an essentially free flow of air through the air passage  36  by a sufficiently high vacuum signal acting thereon. In other words, when flexed, the valve plate  104  bears on the bent portion of the guide plate  102  and permits an increased airflow through the air passage  36  compared to its unflexed state. 
     The check valve  14  is provided in a valve chamber in the sidewall of the cylinder body  10 . A partially curved or bent guide plate  106  and a check valve plate  108  are superimposed on each other and fixed to the side wall of the valve chamber by means of a rivet or other fastener. The check valve  14  is flexible, has low rigidity, and rapidly opens the air passage  36  when the scavenging passage  7  assumes vacuum pressure. 
     As shown in FIG. 6, the heat insulator plate B′ is secured to the side wall of the cylinder body  10  by means of bolts extending through bolt insert holes  110  provided in the front and rear edges. A check valve (reed valve)  39  disposed at an inlet of intake port  17  is secured to a valve chamber provided on the heat insulator plate B′, and a partially curved or bent guide plate  112  (FIG. 5) and a check valve plate  114  are superimposed on each other and secured to a side wall of the valve chamber by means of a rivet  116  or other fastener. The check valve  39  is flexible, has low rigidity and quickly opens the mixture passage  40  when the intake port  17  assumes vacuum pressure. 
     On the other hand, the air control valve  35   a , is less flexible and has a greater rigidity than valves  14  and  39 . Accordingly, even if the air valve (throttle hole  45 ) and the fuel mixture valve (throttle hole  55 ) are rapidly fully opened during rapid acceleration of the engine, the check valves  14  and  39  are already opened by vacuum pressure in the crankcase chamber  18  so that a fuel and air mixture from the mixture passage  50  rapidly flows to the intake port  17  via the mixture passage  40 . However, the air control valve  35   a  temporarily remains in its unflexed or closed position and throttles the airflow through the air passage  36 . When the rotational speed of the engine increases thereby increasing the vacuum pressure in the scavenging passage  7  beyond a threshold pressure, the air control valve  35   a  is flexed to its open position moving valve plate  104  against the guide plate  102  to permit increased fluid flow through the air passage  36 . 
     As will be apparent from the foregoing, for low speed and low load engine operation, air passage  36  need not be fully closed, and preferably has a restricted flow area controlled by the air control valve  35   a  which permits low speed operation and sufficient engine acceleration. When the internal combustion engine is rapidly accelerated, even if the throttle valve lever  41  is rapidly turned to fully open throttle valve  48 , the air control valve  35   a  opens later than the check valve  39  due to the greater rigidity of its valve plate  104 . Accordingly, the air control valve  35   a  throttles airflow through the air passage until a relatively high engine speed is obtained to ensure that a rich enough fuel and air mixture is provided to the engine to support its rapid acceleration. When the engine attains a high enough speed or load, the vacuum generated in the crankcase chamber  18  will move the valve  35   a  to its flexed position permitting greater air flow through the air passage  36  and to the engine to ensure sufficient air is provided for maximum engine power output and to avoid an overly rich fuel mixture and accompanying high exhaust emissions. 
     In the embodiment shown in FIG. 7, a moveable plate  120 , in place of the reed valve  35   a  acts as the air control valve in the air passage  36 . The movable plate  120  is supported in the air passage  36  by means of a shaft  122 . A lever  124  is connected to the outer end of the shaft  122  for connecting the movable plate  120 , and a spring  126  yieldably biases the plate  120  towards its closed position reducing the flow area of the air passage  36 . The movable plate  120  provided in the air passage reduces the flow area of the air passage  36  by about ½ of its unrestricted or normal flow area, and need not ever fully close the air passage  36 . The movable plate  120  is movable to a second or fully open position permitting a substantial free air flow through air passage  36  in response to a sufficiently high vacuum pressure in the air passage  36 . When the internal combustion engine is rapidly accelerated, even if the throttle valve lever  41  is rapidly turned to fully open the throttle valve  48 , opening or movement of the movable plate  120  to its second position is resisted by the force of the spring  126  so that it opens later than the check valve  39 . 
     Accordingly, the movable plate  120  as an air control opens later by a phase difference than the stepped throttle valve  48 . When the stepped throttle valve  48  is in a position less than or closer to idle than a medium opening between idle and wide open, the movable plate  120  is rarely operated, and the movable plate  120  opens against the force of the spring  126  in the vicinity of the wide open position of the throttle valve  48 . 
     While in the above-described embodiment, a rotary throttle valve comprising an integral configuration of an air valve (throttle hole  45 ) and a fuel mixture valve (throttle hole  55 ) is provided on the carburetor body, it is noted that the present invention is not limited to the carburetor of this type, and can be applied to other types of carburetors. 
     Third Embodiment 
     In a third embodiment of the invention, as shown in FIGS. 8-9, the air control is an air passage  36 ′ of the insulator plate B″ which is made longer than the mixture passage  40  to control air flow through passage  36 ′ and enhance engine responsiveness at the time of rapid acceleration without sacrificing the maximum output power of the internal combustion engine. To accomplish this, a baffle plate  150  is provided midway in the air passage  36 ′ and an air passage  36   a  passing upward and over the baffle plate  150  is connected to the air passage  36 ′. The insulator plate B″ comprises a pair of left and right plates  152 ,  154 , between which the baffle plate  150  is disposed. As shown in FIG. 9, the insulator plate B″ is secured to the right side wall of the cylinder body  10  by bolts received through bolt holes  156  of the insulator plate B″. 
     In the normal running of the internal combustion engine, when the piston  9  moves upward toward its top dead center position, the pressure in the crankcase chamber  18  decreases, and a fuel and air mixture flows into the crankcase chamber  18  via the mixture passages  50  and  40 , the check valve  39  and the intake port  17 . Air flows into the crankcase chamber  18  via the air passages  46 ,  36 ′ and  36   a,  the check valve  14  and the scavenging passage  7 . That is, a condition occurs wherein the scavenging passage  7  is filled with air, and a fuel and air mixture in made lean by air in the crankcase chamber  18 . Then, the fuel and air mixture in the combustion chamber  4  is ignited by a spark plug, the pressure in the combustion chamber  4  rapidly increases, the piston  9  is driven towards a bottom dead center position, and the pressure in the crankcase chamber  18  increases. 
     When the piston  9  moves down to a certain position the exhaust port  5  opens so that the combustion gas of the combustion chamber  4  flows out of the exhaust port  5  and the pressure of the combustion chamber  4  rapidly decreases. At the same time, the scavenging ports  7   a  are opened to the combustion chamber  4  so that first, air in the scavenging passage  7  flows into the combustion chamber  4 , and then the fuel and air mixture in the crankcase chamber  18  flows into the combustion chamber  4  via the scavenging passage  7 . 
     During acceleration of the internal combustion engine, even if the air control valve  45  and the mixture valve  55  are rapidly and fully opened, a fuel and air mixture from the carburetor C rapidly flows into the crankcase chamber  18  via the mixture passages  50 ,  40 , the check valve  39  and the intake port  17 . On the other hand, air from the air passage  46  flows into the crankcase chamber  18  later than the fuel and air mixture via the air passage  36 ′ which is longer than the mixture passage  40  and includes air passage  36   a.  Therefore, the quantity or flow rate of air reaching the crankcase chamber  18  is temporarily reduced compared to an engine having an air passage and mixture passage of the same effective length. Because the flow of air to the engine is delayed or reduced, the fuel and air mixture in the crankcase chamber  18  and that delivered to the combustion chamber is somewhat rich and the accelerating characteristics of the internal combustion engine are enhanced. Desirably, only the initial airflow upon rapid acceleration of the engine is delayed. At high engine speed other than rapid acceleration, a desired flow rate of air reaches the engine to provide maximum engine power output. 
     In the embodiment shown in FIGS. 10 and 11, an inlet and an outlet of the air passage  36 ″ are arranged to be concentric, and a spiral air passage  36   b  formed in insulator plate B′″ outside of the mixture passage  40  is connected between the inlet and the outlet. Accordingly, the air passage  36 ″ including passage  36   b  has an effective length greater than the fuel and air mixture passage  40 . In use, an effect similar to that of the embodiments shown in FIGS. 8 and 9 is obtained with a relative delay in the flow of air to the engine upon rapid acceleration of the engine due to the greater distance the air must flow through the passages  36 ″ and  36   b.  In both examples, there are no moving parts increasing reliability and simplifying the design, manufacture and assembly of the engine. 
     While in the above-described embodiment, a rotary throttle valve having an integral configuration of an air valve and a mixture valve is provide in the carburetor, it is noted that the present invention is not limited to throttle valves and carburetors of this type, and can be applied to other types of throttle valves and carburetors. 
     As described above, the present invention provides a stratified scavenging two-stroke engine in which air is introduced into a scavenging passage of an internal combustion engine, a mixture of air and fuel is introduced into a crankcase chamber of the internal combustion engine, air in the scavenging passage is guided to discharge exhaust gases of a combustion chamber when the engine is scavenged, and the fuel and air mixture is then supplied to the combustion chamber. An air control temporarily restricts or delays scavenging air flow to the combustion chamber of the engine. Therefore, when the engine is rapidly accelerated, a reduced flow rate or volume of air flows into the scavenging passage and the quantity of air reaching the crankcase chamber  18  and combustion chamber  4  is temporarily reduced compared to an engine without the air control. Accordingly, the fuel and air mixture delivered to the engine when it is rapidly accelerating is somewhat rich and the accelerating characteristics of the engine are enhanced. Thereafter, the flow rate of air is essentially not affected by the air control to improve the maximum engine power output.