Patent Publication Number: US-9429067-B2

Title: Two-stroke engine with variable scavenging port

Description:
TECHNICAL FIELD 
     The present invention relates to a two-stroke engine, and in particular to a technology for varying the timing of opening and closing a scavenging port. 
     BACKGROUND OF THE INVENTION 
     A two-stroke engine typically includes a scavenging port that communicates with the crank chamber and opens out at a side wall of the cylinder bore so that a mixture containing fuel is supplied from the crank chamber to the cylinder bore via the scavenging port, and this flow displaces or scavenges the combustion gas remaining in the cylinder out of the combustion chamber at the same time. The scavenging port is opened and closed depending on the position of the piston that reciprocates in the cylinder bore such that the scavenging port communicates with the combustion chamber defined above the piston when the piston is near the bottom dead center, and is shut off from the combustion chamber when the piston is away from the bottom dead center. 
     In such a two-stroke engine, it is known to provide a flow guide such as louver fins adjacent to the scavenging openings at the cylinder bore wall in order to create a circumferential component in the scavenging flow. The circumferential component may also be varied by changing the angle of the flow guide. See JP63-183323U. By thus directing the scavenging flow in the circumferential direction, a swirl flow of the mixture is created in the cylinder bore. 
     However, this prior proposal is not configured to change the timing of opening and closing the scavenging port. If the timing of opening and closing the scavenging port can be varied, the volume of the mixture that is supplied to the combustion chamber and the amount of the internal EGR can be varied depending on the operating condition so that the output and efficiency properties of the engine can be improved over a wide operating range. 
     SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a two stroke engine that can vary the timing of opening and closing the scavenging port by using a highly simple structure. 
     To achieve such an object, the present invention provides a two-stroke engine including a piston slidably received in a cylinder bore defined in a cylinder block, a combustion chamber being defined by the cylinder bore and the piston, comprising: a scavenging port having an open end opening out at a part of a cylinder wall defining a side of the cylinder bore, the open end communicating with the combustion chamber when the piston is near a bottom dead center thereof; and a shutter provided on the cylinder wall so as to selectively project into the open end from an upper edge thereof by moving along an axial line of the cylinder bore. 
     By thus changing the effective position of the upper edge of the opening of the scavenging port by using the shutter, the opening and closing timing of the scavenging port can be changed. 
     Typically, the shutter comprises a tubular portion disposed so as to be axially moveable in a coaxial relationship with the cylinder bore. 
     Thereby, even when the scavenging port is provided with a plurality of individual open ends on the side of the cylinder bore, the single tubular portion can open and close all of the open ends at the same time. Also, guiding the tubular member in parallel orientation with the cylinder axial line can be simplified, as compared with a plate member provided for each individual open end. Therefore, the timing of opening and closing each open end can be precisely controlled in a stable manner. 
     According to a preferred embodiment of the present invention, the cylinder bore is defined by a cylinder sleeve, and the open end of the scavenging port comprises a scavenging orifice passed across a thickness of the cylinder sleeve, the tubular portion being wrapped around the cylinder sleeve in an axially slidable manner. 
     Thereby, the mounting of the tubular portion on the cylinder bore and the guiding of the tubular portion along the cylinder axial line are facilitated. 
     According to a certain aspect of the present invention, a part of the cylinder block surrounding the scavenging port is provided with a recess defining a passage leading to the scavenging orifice. 
     Thereby, the scavenging port can be formed by using a highly simple structure. 
     According to a particularly preferred embodiment of the present invention, an annular recess is formed on a part of an outer circumferential surface of the cylinder sleeve provided with the scavenging orifice, and the tubular portion closely surrounds a bottom surface of the annular recess in an axially slidable manner. 
     Thereby, the upper limit and the lower limit of the movement of the tubular portion along the cylinder axial line can be defined by the upper and lower edges (walls) of the annular recess without requiring any additional stopper members. 
     According to a certain embodiment of the present invention, the tubular portion includes a projection received in the scavenging orifice and defining the cylinder bore jointly with the inner circumferential surface of the cylinder sleeve. 
     Thereby, when the tubular portion is closing a part of each scavenging orifice, substantially no gap is created between the piston (or the compression ring thereof) and the inner wall of the cylinder bore so that the scavenging port can be closed without any significant leakage when the piston is in a position to close the scavenging orifice, and the opening and closing timing of the scavenging port can be determined in a precise manner. 
     The shutter can be actuated by using any per se means. For instance, the shutter may further include a rack extending axially on an outer surface of the tubular portion, and a pinion rotatably supported by the cylinder block and meshing with the rack. The pinion may be turned by using an electric motor which is controlled by an electronic control unit according to the operating condition of the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Now the present invention is described in the following with reference to the appended drawings, in which: 
         FIG. 1  is a vertical sectional view of an engine embodying the present invention (taken along line I-I of  FIG. 2 ); 
         FIG. 2  is a sectional view taken along line II-II of  FIG. 1 ; 
         FIG. 3  is a sectional view taken along line III-III of  FIG. 2 ; 
         FIG. 4  is a diagram showing the mode of operation of a multiple linkage mechanism used in the engine; 
         FIG. 5  is an enlarged fragmentary sectional view of a part of  FIG. 2 ; and 
         FIG. 6  is a view similar to  FIG. 5  showing a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     The present invention is described in the following with respect to a uni-flow type, single cylinder, two-stroke engine (engine E). 
     Referring to  FIGS. 1 and 2 , an engine main body  1  of the engine E is provided with a crankcase  2  defining a crank chamber  2   a  therein, a cylinder block  3  connected to the upper end of the crankcase  2  and defining a cylinder bore  3   a  therein, a cylinder head  4  connected to the upper end of the cylinder block  3  and a head cover  5  attached to the upper end of the cylinder head  4  to define an upper valve chamber  6  in cooperation with the cylinder head  4 . 
     As best shown in  FIG. 2 , the crankcase  2  consists of two crankcase halves  7  having a parting plane extending perpendicularly to the crankshaft axial line  8 X and joined to each other by seven threaded bolts  9  ( FIGS. 1 and 3 ). Each crankcase half  7  includes a side wall  7 S which is provided with an opening through which the corresponding end of a crankshaft  8  projects, and the corresponding end of the crankshaft  8  is rotatably supported by the side wall  7 S via a first bearing B 1 . Thus, the crankshaft  8  is rotatably supported at two ends thereof by the crankcase  2 , and has a crank throw received in the crank chamber  2   a  defined by the crankcase  2 . 
     The crankshaft  8  includes a pair of journals  11  that are rotatively supported by the first bearings B 1 , respectively, a pair of crank webs  12  extending radially from middle parts of the crankshaft  8 , a crankpin  13  extending between the two webs  12  radially offset from and in parallel with the axial line  8 X of the crankshaft  8 , and a pair of extensions  14  extending coaxially from the outer ends of the journals  11  out of the crankcase  2 . Each crank web  12  is formed as a circular disk defining a larger radius than the outer profile of the crankpin  13  so as to serve as a flywheel that stabilizes the rotation of the crankshaft  8  without substantially splashing the lubricating oil in the crank chamber  2   a.    
     Each extension  14  of the crankshaft  8  extends out of the crankcase  2  via a through hole  15  formed in the side wall  7 S of the corresponding crankcase half  7 . The outer side of each ball bearing B 1  is fitted with a seal S 1  to ensure an air tight seal of the crank chamber  2   a . As shown in  FIGS. 2 and 3 , the side wall  7 S of the right crankcase half  7  is integrally formed with a lower valve case  17  protruding therefrom so as to surround the right extension  14  of the crankshaft  8  as seen in  FIG. 2 . 
     The lower valve case  17  is cylindrical in shape with an open outer axial end, and internally defines a lower valve chamber  18 . The opening of the outer end of the lower valve case  17  is closed by a valve chamber lid  19 . The outer axial end of the lower valve case  17  is provided with an annular seal groove  17   a  so that the valve chamber lid  19  may be joined to the opening of the lower valve case  17  in an air tight manner via a second seal member S 2  received in the seal groove  17   a.    
     The right end of the crankshaft  8  seen in  FIG. 2  is passed through a through hole  19   a  formed in the valve chamber lid  19 , and extends further outward. The inner circumference of the through hole  19   a  is provided with a third seal member S 3  for ensuring the airtight condition of the lower valve case  17 , and hence the airtight condition of the crank chamber  2   a.    
     As shown in  FIG. 1 , the central axial line  8 X of the crankshaft  8  or the axial center of the journals  11  is offset from the cylinder axial line  3 X to a side (left side in  FIG. 1 ). The crankpin  13  rotates around the central axial line  8 X of the crankshaft  8  as the crankshaft  8  rotates, and rotatably supports a middle point of a trigonal link  20  via a tubular portion  20   a  of the trigonal link  20 . A second bearing B 2  is interposed between the crankpin  13  and the tubular portion  20   a.    
     The trigonal link  20  includes a pair of plates  20   d  that are joined by the tubular portion  20   a  in a mutually parallel relationship, and a pair of connecting pins (a first connecting pin  20   b  and a second connecting pin  20   c ) fixedly passed between the two plates  20   d . These connecting pins  20   b  and  20   c  and the crankpin  13  form three pivot points that are arranged in a line at a substantially same interval with the crankpin  13  located in the middle. 
     The first connecting pin  20   b  located on the side of the cylinder axial line  3 X is pivotally connected to a big end  21   a  of a connecting rod  21  via a third bearing B 3 . A small end  21   b  of the connecting rod  21  is pivotally connected to a piston  22  slidably received in the cylinder bore  3   a  via a piston pin  22   a  and a fourth bearing B 4 . 
     A pivot shaft  23  is fixedly provided in a lower part of the crankcase  2 , on the side remote from the first connecting pin  20   b . The rotational center lines of the pivot shaft  23  and the three pivot points ( 20   a ,  20   b  and  20   c ) are all in parallel to one another. As shown in  FIG. 2 , the pivot shaft  23  is press fitted into a pair of mutually opposing holes  24  formed in the two halves of the crankcase  2 , respectively. A base end  25   a  of a swing link  25  is pivotally connected to the pivot shaft  23  via a fifth bearing B 5 . The swing link  25  extends substantially upward from the base end  25   a  thereof, and an upper end or a free end  25   b  of the swing link  25  is pivotally supported by the second connecting pin  20   c  (remote from the cylinder axial line  3 X) via a sixth bearing B 6 . 
     The engine E is thus provided with a multiple link mechanism  30  which includes the trigonal link  20  and the swing link  25  in addition to the connecting rod  21 . The multiple link mechanism  30  converts the linear reciprocating movement of the piston  22  into a rotational movement of the crankshaft  8 . The dimensions and positions of the various components of the multiple link mechanism  30  are selected and arranged such that a prescribed compression ratio selected for the properties of the particular fuel may be achieved. The compression ratio is selected such that the pre-mixed mixture may self-ignite in an appropriate manner. The fuels that may be used for this engine include gasoline, diesel fuel, kerosene, gas (utility gas, LP gas and so on), etc. 
     Owing to the use of the multiple link mechanism  30 , for the given size of the engine E, the piston stroke L can be maximized so that a larger part of the thermal energy can be converted into kinetic energy, and the thermal efficiency of the engine E can be improved. More specifically, as shown in part (A) of  FIG. 4 , when the piston  22  is at the top dead center, the big end  21   a  of the connecting rod  21  which is connected to the first connecting pin  20   b  at the right end of the trigonal link  20  is located higher than the crankpin  13  by a first distance D 1 . Furthermore, as shown in part (B) of  FIG. 4 , when the piston  22  is at the bottom dead center, the big end  21   a  of the connecting rod  21  is located lower than the crankpin  13  by a second distance D 2 . Therefore, as compared to the conventional engine where the big end  21   a  of the connecting rod  21  is directly connected to the crankpin  13 , the piston stroke L can be extended by the sum of these two distances or by D 1 +D 2 . Therefore, the piston stroke L of the engine E can be extended without increasing the size of the crankcase  2  or the overall height of the engine E. 
     In this engine E, the trajectory T of the big end  21   a  of the connecting rod  21  is vertically elongated, instead of being truly circular, as shown in (A) and (B) of  FIG. 4 . In other words, as compared to the more conventional reciprocating engine having the constant crank radius R, the swing angle of the connecting rod  21  is reduced. Therefore, the interferences between the lower end of the cylinder (or lower end of the cylinder sleeve  42 ) and the connecting rod  21  can be avoided even when the cylinder bore  3   a  is relatively small. Furthermore, the reduction in the swing angle of the connecting rod  21  contributes to the reduction in the thrust loads which the piston  22  applies to the two sides (thrust side and anti-thrust side) of the cylinder wall. 
     As shown in  FIG. 1 , the crank chamber  2   a  is laterally extended in the region of the swing link  25  and is vertically extended in the region directly under the piston  22  so that the trigonal link  20  that undergoes a composite rotational movement, the swing link  25  that undergoes a swinging movement and the connecting rod  21  that undergoes a vertically elongated circular movement may not interfere with one another. The part of the crankcase  2  adjoining the lower end of the cylinder bore  3   a  is formed with a cylindrical recess  31  having a circular cross section (taken along a horizontal plane) substantially coaxial with the cylinder bore  3   a  and surrounding the lower end of the cylinder sleeve  42  such that an annular space communicating with the crank chamber  2   a  is defined around the lower end of the cylinder sleeve  42 . 
     An intake port  32  is formed by a tubular extension of the crankcase  2  extending obliquely upward adjacent to the cylindrical recess  31  in the upper part of the crankcase  2 . The intake port  32  is fitted with a reed valve  33  that permits the flow of air from the intake port  32  to the crank chamber  2   a , and prohibits the flow of air in the opposite direction. The reed valve  33  includes a base member  33   a  consisting of a wedge shaped member having a pointed end directed inward and a pair of openings defined on either slanted sides thereof, a pair of valve elements  33   b  mounted on the base member  33   a  so as to cooperate with the openings thereof and a pair of stoppers  33   c  placed on the backsides of the valve elements  33   b  so as to limit the opening movement of the valve elements  33   b  within a prescribed limit. The reed valve  33  is normally closed, and opens when the piston  22  moves upward and the internal pressure in the crank chamber  2   a  thereby drops. 
     To the outer end of the intake port  32  is connected a throttle body  34  so as to define an intake passage  34   a  extending vertically as a smooth continuation of the intake port  32 . A throttle valve  34   b  is pivotally mounted on a horizontal shaft for selectively closing and opening the intake passage  34   a . A fuel injector  35  is also mounted on the throttle body  34  with an injection nozzle  35   a  thereof directed into a part of the intake passage  34   a  somewhat downstream of the throttle valve  34   b . The axial line of the fuel injector  35  is disposed obliquely so as to be directed to the reed valve  33 , and fuel is injected into the intake passage  34   a  in synchronism with the opening of the reed valve  33 . The upstream end of the throttle body  34  is connected to an L shaped intake pipe  36  including a vertical section connected to the throttle body  34  and a horizontal section extending away from the cylinder block  3 . 
     Four stud bolts  38  are secured to the upper side of the crankcase  2  and extend upward around the cylinder bore  3   a  at a regular interval as can be seen from  FIG. 1 . The cylinder block  3  and the cylinder head  4  are secured to the crankcase  2  by passing the stud bolts  38  therethrough and threading acorn nuts  39  onto the upper ends of the stud bolts  38 . 
     As shown in  FIGS. 1 and 2 , the cylinder block  3  is provided with a bore  41  having a circular cross section passed therethrough, and the cylinder sleeve  42  is fitted into this bore  41  with the lower end thereof extending into the cylindrical recess  31  mentioned above. The bore  41  is provided with a large diameter section in an upper end thereof defining an annular shoulder  41   a  facing upward, and the cylinder sleeve  42  is provided with a radial flange  42   b  configured to rest on this annular shoulder  41   a . The upper end part of the cylinder sleeve  42  (or the part thereof located above the radial flange  42   b ) defines an annular space  41   b  in cooperation with the large diameter section of the bore  41  of the cylinder block  3 . 
     The cylinder sleeve  42  is provided with a constant inner diameter over the entire length thereof except for the lower end thereof which is chamfered, and the cylinder bore  3   a  is defined by an inner circumferential surface  42   a  of the cylinder sleeve  42 . The outer diameter of the cylinder sleeve  42  is also constant over the entire length thereof except for the lower end thereof which is reduced in diameter over a certain length and a part adjacent to the upper end thereof which is provided with the radial flange  42   b  defining an annular shoulder surface abutting the annular shoulder  41   a  to determine the axial position of the cylinder sleeve  42  relative to the cylinder block  3 . The upper end of the cylinder sleeve  42  is flush with the upper end surface of the cylinder block  3 , and the cylinder sleeve  42  is provided with a somewhat greater vertical dimension than the cylinder block  3  so that the lower end of the cylinder sleeve  42  projects out of the lower end of the cylinder block  3  into the cylindrical recess  31  of the crankcase  2 . 
     A pair of scavenging orifices  42   c  which are identically shaped and dimensioned are formed on either side of the cylinder sleeve  42 , and arranged 180 degrees apart about the cylinder axial line  3 X at a same elevation. As shown in  FIG. 5 , each scavenging orifice  42   c  is provided with an upper edge  42   d  located higher than the parting plane between the cylinder block  3  and the crankcase  2 , and a lower edge  42   e  located lower than this parting plane. The upper end (compression ring) of the piston  22  is located higher than the upper edge  42   d  of the scavenging orifices  42   c  at least when the piston  22  is at the top dead center, and is located lower than the upper edge  42   d  of the scavenging orifices  42   c  when the piston  22  is at the bottom dead center. Preferably, the upper end of the piston  22  is located higher than the upper edge  42   d  of the scavenging orifices  42   c  when the piston  22  is at the top dead center, and is located lower than the lower edge  42   d  of the scavenging orifices  42   c  when the piston  22  is at the bottom dead center. 
     As shown in  FIGS. 1, 2 and 5 , an axially intermediate part of the cylinder sleeve  42  is formed with an annular recess  70  having a certain vertical width on the outer circumference thereof. The annular recess  70  is provided with a bottom surface  70   a  extending circumferentially concentric to the cylinder axial line  3 X. The upper wall  70   b  and the lower wall  70   c  defining the upper and lower edge of the annular recess  70  extend substantially vertically with respect to the bottom surface  70   a  so that the vertical cross section of the annular recess  70  is substantially rectangular. In other words, the annular recess  70  continues with the remaining part of the outer circumferential surface of the cylinder sleeve  42  via the steps defined by the upper wall  70   b  and the lower wall  70   c.    
     The upper wall  70   b  of the annular recess  70  is located higher than the upper edge  42   d  of the scavenging orifices  42   c , and the lower wall  70   c  of the annular recess  70  is located intermediate between the upper edge  42   d  and the lower edge  42   e  of the scavenging orifices  42   c . In other words, the scavenging orifices  42   c  straddle across the lower wall  70   c  of the annular recess  70  so that an upper half of each scavenging orifice  42   c  opens out at the bottom surface  70   a  of the annular recess  70  and the lower half of each scavenging orifice  42   c  opens out at the part of the outer circumferential surface of the cylinder sleeve  42  located below the annular recess  70 . 
     As shown in  FIGS. 1 and 2 , the lower end part of the cylinder block  3  surrounding the cylinder sleeve  42  is formed with an annular recess  3   b  concentrically surrounding the cylinder axial line  3 X. This annular recess  3   b  extends vertically (widthwise) such that the lower end thereof opens out at the lower surface of the cylinder block  3  and communicates with the cylindrical recess  31 . The upper end of the annular recess  3   b  extends up to the upper edge  42   d  of the scavenging orifices  42   c . Alternatively, the upper end of the annular recess  3   b  may extend slightly beyond the upper edge  42   d  of the scavenging orifices  42   c.    
     The scavenging orifices  42   c , the cylindrical recess  31  and the corresponding annular recess  3   b  jointly form a scavenging port  43  that communicates the crank chamber  2   a  and the cylinder bore  3   a  with each other. In particular, the scavenging orifices  42   c  defining the open end of the scavenging port  43  on the side of the cylinder bore  3   a . The upper end of the annular recess  3   b  is defined by a curved wall surface curving toward the cylinder bore  42  as one moves upward so that the mixture flowing upward through the annular recess  3   b  may be smoothly guided to the scavenging orifices  42   c  on the side part of the cylinder bore  3   a.    
     As shown in  FIG. 5 , a shutter  73  is fitted on the outer circumferential surface of the cylinder sleeve  42  so as to be moveable in the axial direction (along the cylinder axial line  3 X). The shutter  73  includes a thin-walled tubular portion  74  and a rack  75  fixedly attached to the outer surface of the tubular portion  74  and extending in the axial direction. The rack  75  is provided with rack teeth  75   a  facing radially outward. 
     The tubular portion  74  of the shutter  73  is received in the annular recess  70  of the cylinder sleeve  42  in a coaxial relationship to the cylinder sleeve  42  (cylinder bore  3   a ). The inner circumferential surface of the tubular portion  74  is in close contact with the bottom surface  70   a  of the annular recess  70  in a mutually slidable manner. The axial length (width) of the tubular portion  74  is somewhat smaller than the distance between the upper wall  70   b  and the lower wall  70   c  (the width) of the annular recess  70  so that the tubular portion  74  is moveable along the cylinder axial line  3 X within the annular recess  70 . The upper and lower limits of this movement are defined by the upper wall  70   b  and the lower wall  70   c  of the annular recess  70 , respectively. Therefore, the tubular portion  74  is moveable along the cylinder axial line  3 X within the annular recess  70  within a prescribed range. 
     As shown in  FIG. 5 , the rack  75  is provided on one side of the tubular portion  74 . The cylinder block  3  is formed with a recess  77  extending vertically from the annular recess  3   b  of the cylinder block  3  to accommodate the rack  75  therein over the entire vertical stroke of the shutter  73  and permit the vertical movement of the shutter  73  without interfering with the cylinder block  3 . The side surfaces of the rack  75  may be in slidable engagement with the side walls of the recess  77  to restrict the rotation of the shutter  73  around the cylinder axial line  3 X. 
     A pinion  78  is rotatably supported by the cylinder block  3  via a pinion shaft  81 , and meshes with the rack teeth  75   a  of the rack  75 . The cylinder block  3  is formed with a cavity  79  to receive the pinion  78  therein, and the cylinder block  3  is formed with a corresponding external bulge  82  to create the space for the pinion  78  without increasing the overall size of the cylinder block  3 . As shown in  FIG. 3 , the pinion shaft  81  extends perpendicularly to both the crank axial line  8 X and the cylinder axial line  3 X, and has an outer end extending out of the cylinder block  3  and connected to the output shaft of an electric motor  84 . The electric motor  84  is fixedly attached to the outer surface of the cylinder block  3  via a bracket  85 . Therefore, when the electric motor  84  is activated, the pinion  78  rotates, and the tubular portion  74  of the shutter  73  moves along the cylinder axial line  3 X owing to the meshing between the pinion  78  and the rack teeth  75   a  of the rack  75 . The electric motor  84  is controlled by an electronic control unit (ECU) not shown in the drawings according to the operating condition of the engine. 
     In  FIG. 5 , the upper most position of the shutter  73  is indicated by solid lines, and the lower most position of the shutter  73  is indicated by double-dot chain-dot lines. At the upper most position, the lower edge  74   b  of the tubular portion  74  coincides with the upper edge  42   d  of the scavenging orifices  42   c . At the lower most position, the lower edge  74   b  of the tubular portion  74  opposes a lower part of the scavenging orifices  42   c.    
     Therefore, when the shutter  73  moves downward along the cylinder axial line  3 X from the upper most position, the lower edge  74   b  of the tubular portion  74  passes the upper edge  42   d  of the scavenging orifices  42   c , and projects into the scavenging orifices  42   c  so as to define a new upper edge of the opening of the scavenging port  43  at the cylinder bore, this new opening being narrower than that defined by the scavenging orifices  42   c . In other words, when the position of the shutter  73  is changed along the cylinder axial line  3 X, the timing of starting the communication between the scavenging port  43  and the combustion chamber  44  (the part of the cylinder bore  3   a  defined above the piston  22 ) or the opening timing of the scavenging port  43  during the downward stroke of the piston  22 , and the timing of ending the communication between the scavenging port  43  and the combustion chamber  44  (the part of the cylinder bore  3   a  defined above the piston  22 ) or the closing timing of the scavenging port  43  during the upward stroke of the piston  22  can be changed. By changing the opening timing and the closing timing of the scavenging port  43 , the duration of the open state of the scavenging port  43  or the open period can be changed. 
     As shown in  FIGS. 1 and 2 , the part of the lower surface of the cylinder head  4  corresponding to the cylinder bore  3   a  is recessed in a dome-shape (dome-shaped recess  4   a ) so as to define a combustion chamber  44  jointly with the top surface of the piston  22 . An annular groove  4   b  is formed in the lower surface of the cylinder head  4  concentrically around the dome-shaped recess  4   a  which aligns with the annular recess  41   b  defined between the upper part of the cylinder sleeve  42  and the surrounding wall of the cylinder block  3  such that a water jacket  45  surrounding the dome-shaped space  4   a  of the cylinder head  4  and the upper part of the cylinder bore  3   a  is defined jointly by the annular space  41   b  and the annular groove  4   b.    
     The cylinder head  4  is further provided with an exhaust port  46  opening out at the top end of the combustion chamber  44  and a plug hole for receiving a spark plug  47  therein. In the illustrated embodiment, the spark plug  47  is normally activated only at the time of starting the engine to ignite the mixture in the combustion chamber  44 . The exhaust port  46  is provided with an exhaust valve  48  consisting of a poppet valve to selectively close and open the exhaust port  46 . The exhaust valve  48  includes a valve stem which is slidably guided by the cylinder head  4  at an angle to the cylinder axial line  3 X, and the stem end of the exhaust valve  48  extends into the upper valve chamber  6  containing a part of the valve actuating mechanism  50  for actuating the exhaust valve  48  via the stem end thereof. 
     The valve actuating mechanism  50  includes a valve spring  51  that resiliently urges the exhaust valve  48  in the closing direction (upward), an upper rocker shaft  53  supported by a block  52  provided on the cylinder head  4  and an upper rocker arm  54  rotatably supported by the upper rocker shaft  53 . The upper rocker shaft  53  extends substantially perpendicularly to the crankshaft  8 , and the upper rocker arm  54  extends substantially in parallel to the crankshaft  8 . One end of the upper rocker arm  54  is provided with a socket  54   a  engaging the upper end  55   a  of the pushrod  55 , and the other end of the upper rocker arm  54  is provided with a tappet adjuster  54   b  consisting of the screw which engages the stem end of the exhaust valve  48 . The upper end  55   a  of the pushrod  55  is given with a semi-spherical shape, and the socket  54   a  of the rocker arm  54  receives the upper end  55   a  of the pushrod  55  in a complementary manner, allowing a certain sliding movement between them. 
     As shown in  FIGS. 2 and 3 , the pushrod  55  extends substantially vertically along a side of the cylinder block  3 , and is received in a tubular rod case  56  having an upper end connected to the cylinder head  4  and a lower end connected to the lower valve case  17 . In the illustrated embodiment, the rod case  56  extends along the exterior of the cylinder block  3 . 
     Because the crankshaft  8  is offset from the cylinder axial line  3 X ( FIG. 1 ), as best shown in  FIG. 3 , the lower end of the rod case  56  is connected to a part of the upper wall of the lower valve case  17  laterally offset from the crankshaft  8 . The lower valve chamber  18  receives the remaining part of the valve actuating mechanism  50 . The lower wall of the lower valve case  17  is provided with a drain hole  57  for expelling the lubricating oil in the lower valve chamber  18  which is usually closed by a drain plug  58 . 
     The valve actuating mechanism  50  further comprises a cam  61  carried by the part of the crankshaft  8  extending into the lower valve chamber  18 , a lower rocker shaft  63  supported by the side wall  7 S of the crankcase  2  and the valve chamber lid  19  in parallel with the crankshaft  8  and a lower rocker arm  64  pivotally supported by the lower rocker shaft  63  for cooperation with the cam  61 . In other words, one of the extensions  14  of the crankshaft  8  (the right end thereof in  FIG. 2 ) serves as the camshaft  66  for the cam  61 . 
     As shown in  FIG. 3 , the lower rocker arm  64  includes a tubular portion  64   a  rotatably supported by the lower rocker shaft  63 , a first arm  64   b  extending from the tubular portion  64   a  toward the crankshaft  8 , a roller  64   c  pivotally supported by the free end of the first arm  64   b  to make a rolling contact with the cam  61 , a second arm  64   d  extending from the tubular portion  64   a  away from the first arm  64   b , and a receiving portion  64   e  formed in the free end of the second arm  64   d  to support the lower end  55   b  of the pushrod  55 . The lower end of the pushrod  55  is given with a semi-spherical shape, and the receiving portion  64   e  is formed as a recess complementary to the semi-spherical lower end of the pushrod  55  so as to receive the lower end of the pushrod  55  in a mutually slidable manner. 
     The engine E described above operates as described in the following at the time of start-up. Referring to  FIG. 1 , in the upward stroke of the piston  22 , owing to the depressurization of the crank chamber  2   a , the reed valve  33  opens. As a result, a mixture of the fresh air metered by the throttle valve  34   b  and the fuel injected into this fresh air by the fuel injector  35  is drawn into the crank chamber  2   a  via the reed valve  33  and the intake port  32 . Meanwhile, the mixture in the cylinder bore  3   a  is compressed by the piston  22 , and is ignited by the spark from the spark plug  47  when the piston  22  is near the top dead center. 
     The piston  22  then undergoes a downward stroke, and because the reed valve  33  is closed at this time, the mixture in the crank chamber  2   a  is prevented from flowing back to the throttle valve  34   b , and compressed. During the downward stroke of the piston  22 , before the piston  22  opens the scavenging port  43 , the exhaust valve  48  actuated by the valve actuating mechanism  50  according to the cam profile of the cam  61  opens the exhaust port  46 . Once the piston  22  opens the scavenging port  43 , the compressed mixture is introduced into the cylinder bore  3   a  (combustion chamber  44 ) via the scavenging port  43 . The combustion gas in the combustion chamber  44  is displaced by this mixture, and is expelled from the exhaust port  46  while part of the combustion gas remains in the combustion chamber  44  as EGR gas. The valve opening timing of the exhaust valve  48  is determined such that the amount of the EGR gas remaining in the combustion chamber  44  is great enough for the self-ignition of the mixture to take place owing to the rise in the temperature of the mixture in the combustion chamber  44  under compression with the increase in the amount of the EGR gas. 
     When the piston  22  undergoes an upward stroke once again, the piston  22  closes the scavenging port  43 , and, thereafter, the exhaust valve  48  actuated by the first cam  61  closes the exhaust port  46 . As a result, the mixture in the cylinder bore  3   a  (combustion chamber  44 ) is compressed while the crank chamber  2   a  is depressurized, causing the mixture to be drawn thereinto via the reed valve  33 . Once the engine E is brought into a stable operation, the mixture is self-ignited as the piston  22  comes near the top dead center, and the combustion gas created by the resulting combustion pushes down the piston  22 . 
     The engine E thus performs a two-stroke operation. In particular, spark ignition using the spark plug  47  is required at the time of start up, but once the engine starts operating in a stable manner, a two-stroke operation based on a homogeneous charge compression ignition is performed. The scavenging flow from the scavenging port  43  to the exhaust port  46  via the cylinder bore  3   a  is guided along a relatively straight path, or the so-called “uni-flow scavenging” can be achieved. 
     The engine E described above allows the opening timing, the closing timing and the open period of the scavenging port  43  to be adjusted owing to the provision of the shutter  73 . As the lower edge  74   b  of the tubular portion  74  of the shutter  73  defines the upper edge of the open end of the scavenging port  43  on the side of the cylinder bore  3   a , the displacement of the shutter  73  along the cylinder axial line  3 X changes the timing of the upper end of the piston  22  passing the lower edge  74   b  of the tubular portion  74 , and hence the open timing and the closing timing of the scavenging port  43 . A certain gap corresponding to the thickness of the cylinder sleeve  42  is created between the outer circumferential surface of the piston  22  and the inner circumferential surface of the tubular portion  74 , but the cross sectional area of the this gap is so small as compared to the cross sectional area of the scavenging port  43  that the effect of this gap on the communication state between the combustion chamber  44  and the scavenging port  43  is not significant. 
     By changing the opening timing, the closing timing and the open period of the scavenging port  43 , the amount of the mixture that is delivered to the combustion chamber  44  from the scavenging port  43  and the amount of the internal EGR that remains in the combustion chamber  44  can be adjusted. For instance, when the opening timing is delayed while the closing timing is advanced, thereby decreasing the opening period, the supply of the mixture into the combustion chamber  44  can be reduced, and the amount of the internal EGR can be increased. The position control for the shutter  73  in changing the opening timing, the closing timing and the open period of the scavenging port  43  can be performed in a continuous manner depending on the load condition and other operating conditions of the engine which may be determined by the engine rotational speed and the depression of the accelerator pedal. 
     The tubular portion  74  of the shutter  73  is coaxial with the cylinder bore  3   a  and the cylinder sleeve  42 , and is moveable with respect to the cylinder bore  3   a  in the direction of the cylinder axial line  3 X. Therefore, even when there are a plurality of scavenging orifices  42   c  around the cylinder bore  3   a , the single shutter  73  can open and close all of the scavenging orifices  42   c . The shutter  73  is subjected to radial loadings from the mixture that passes through the scavenging orifices  42   c  and the mixture compressed in the cylinder bore  3   a , but because the tubular portion  74  is annular and surrounds the outer circumferential surface of the cylinder sleeve  42 , the shutter  73  is prevented from deflecting in any direction, and can open and close the scavenging orifices  42   c  in a favorable and stable manner. 
     Because the tubular portion  74  of the shutter  73  is received in the annular recess  70  formed in the outer circumferential surface of the cylinder sleeve  42 , the upper limit and the lower limit of the movement of the shutter  73  along the cylinder axial line  3 X can be defined without requiring any special stopper members or stopper features. 
     A second embodiment of the present invention is described in the following with reference to  FIG. 6 . In the following description, the parts corresponding to those of the previous embodiment are denoted with like numerals without necessarily repeating the description of such parts. In this embodiment, when the shutter  73  is at the upper most position thereof, the lower edge  74   b  of the tubular portion  74  is located below the upper edge  42   d  of the scavenging orifices  42   c . In other words, the lower edge  74   b  of the tubular portion  74  is always located below the upper edge  42   d  of the scavenging orifices  42   c , and defines the upper edge of the scavenging port  43  at the open end thereof on the side of the cylinder bore  3   a.    
     The part of the tubular portion  74  corresponding to each scavenging orifice  42   c  is provided with a thick-walled portion defining a projection  90  projecting into the scavenging orifice  42   c , and the inner surface  90   a  of the projection  90  defines an inner circumferential surface continuous with the inner circumferential surface of the cylinder sleeve  42 . In other words, the projection  90  defines a part of the cylinder wall surface defining the cylinder bore  3   a  such that the oil ring and the compression ring fitted in the outer circumference of the piston  22  slides along the inner surface  90   a  of the projection  90 . 
     According to the second embodiment of the present invention, even when the wall thickness of the cylinder sleeve  42  is significant, no radial gap is created between the shutter  73  (tubular portion  74 ) and the piston  22 . Therefore, when the upper end of the piston  22  is located below the upper edge  42   d  of the scavenging orifices  42   c , and above the lower edge  74   b  of the tubular portion  74 , communication between the combustion chamber  44  and the scavenging port  43  can be shut substantially completely. 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention. For instance, the shutter  73  included the tubular portion  74  for closing the scavenging orifices  42   c  in the foregoing embodiments, but may also consist of a plurality of plate members that are provided so as to correspond to the individual scavenging orifices  42   c.    
     A rack and pinion mechanism consisting of the rack  75  and the pinion  78  was used for moving the shutter  73  along the cylinder axial line  3 X in the foregoing embodiment, but any other per se known arrangement such as those using electromagnetic force can also be used. 
     The tubular portion  74  of the shutter  73  was received in the annular recess  70  so that the upper and lower limits of the axial movement of the shutter  73  may be limited by the upper and lower walls of the annular recess  70  in the foregoing embodiments, but it is also possible to eliminate the lower wall  70   c  (or have the bottom surface  70   a  to extend to the lower edge of the cylinder sleeve  42 ). This simplifies the assembling of the tubular portion  74  into the annular recess  70 . 
     The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application as well as the contents of the prior art references mentioned in this application are incorporated in this application by reference.