Abstract:
An engine having an operating orientation includes an engine housing having a crankcase and a single cylinder bore, and a single piston configured to reciprocate within the single cylinder bore to generate pressure pulses in the crankcase. A breather chamber is at least partially defined by the crankcase and is disposed adjacent the single cylinder bore. The breather chamber includes a lowermost wall in the operating orientation that defines a drain aperture through which a lubricant flows into the crankcase. A breather cover is coupled to the crankcase and cooperates with the crankcase to fully define the breather chamber. The breather cover includes a bottom in the operating orientation and a wall extending substantially vertically from the bottom in the operating orientation to at least partially define a cover space. The breather cover defines an air-lubricant mixture aperture configured to allow for the passage of the air-lubricant mixture into the cover space. The wall defines a cover space drain aperture spaced a first non-zero distance above the bottom to facilitate the discharge of lubricant from the cover space to the breather chamber. The cover space drain aperture is spaced a second non-zero distance above the drain aperture when the engine is in the operating orientation.

Description:
RELATED APPLICATION DATA 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 10/919,919, filed Aug. 17, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to engines, and more particularly to low-cost, single cylinder engines. 
       BACKGROUND OF THE INVENTION 
       [0003]    Government regulations pertaining to exhaust emissions of small engines, such as those utilized in lawnmowers, lawn tractors, string trimmers, etc., have become increasingly strict. More particularly, such regulations govern the amount of hydrocarbons and nitrous oxides exhausted by the engine. Currently, several different engine technologies are available for decreasing hydrocarbon emissions, such as, for example, sophisticated fuel injection systems and exhaust catalyst devices. These or other more sophisticated technologies are difficult to incorporate into small engines and are expensive. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides, in one aspect, an engine having an operating orientation. The engine includes an engine housing including a crankcase and a single cylinder bore, and a single piston configured to reciprocate within the single cylinder bore to generate pressure pulses in the crankcase. A breather chamber is at least partially defined by the crankcase and is disposed adjacent the single cylinder bore. The breather chamber includes a lowermost wall in the operating orientation that defines a drain aperture through which a lubricant flows into the crankcase. A breather cover is coupled to the crankcase and cooperates with the crankcase to fully define the breather chamber. The breather cover includes a bottom in the operating orientation and a wall extending substantially vertically from the bottom in the operating orientation to at least partially define a cover space. The breather cover defines an air-lubricant mixture aperture configured to allow for the passage of the air-lubricant mixture into the cover space. The wall defines a cover space drain aperture spaced a first non-zero distance above the bottom to facilitate the discharge of lubricant from the cover space to the breather chamber. The cover space drain aperture is spaced a second non-zero distance above the drain aperture when the engine is in the operating orientation. 
         [0005]    The present invention provides, in another aspect, an engine positioned in an operating orientation and including an engine housing having a crankcase and a single cylinder bore, and a single piston configured to reciprocate within the single cylinder bore to generate pressure pulses in the crankcase. A plurality of breather chamber walls is formed as part of the housing to at least partially define a breather chamber. A lowermost of the breather chamber walls in the operating orientation defines a drain aperture that provides for the passage of fluid between the crankcase and the breather chamber. A breather cover includes a bottom in the operating orientation and a side wall that at least partially enclose a cover space. The breather cover is coupled to the housing and cooperates with the plurality of breather chamber walls to completely define the breather chamber. The side wall includes an air-lubricant mixture aperture that provides for fluid communication between the breather chamber and the cover space, and a cover space drain aperture that allows for the passage of a liquid from the cover space to the breather chamber. The breather chamber contains a portion of the liquid such that the liquid defines a liquid level. The cover space drain aperture is positioned a non-zero distance above the liquid level. 
         [0006]    In another aspect, the invention provides an engine that includes an engine housing including a crankcase and a single cylinder bore. The engine housing at least partially defines a breather chamber having a bottom in an engine operating orientation. The breather chamber is positioned adjacent the cylinder bore and includes a drain aperture that provides for fluid communication with the crankcase. A single piston is configured to reciprocate within the single cylinder bore to generate pressure pulses in the crankcase. A breather cover is positioned adjacent the breather chamber and includes a cover space. The breather cover includes a bottom in the engine operating orientation and a wall including an air-lubricant mixture aperture and a cover space drain aperture. The breather cover is configured to receive a flow of air-oil from the breather chamber via the air-lubricant mixture aperture, separate the oil from the flow, and discharge the oil to the breather chamber via the cover space drain aperture. The cover space drain aperture is positioned a non-zero distance above the bottom of the breather chamber. 
         [0007]    In yet another aspect, the invention provides a method of separating oil and air from an air-oil mixture in an engine. The method includes reciprocating a single piston in a single cylinder to produce pressure pulses within a housing, and forcing the air-oil mixture into a breather chamber in response to the pressure pulses. The breather chamber includes a lowermost surface in an engine operating orientation. The method also includes passing the air-oil mixture into a cover space via an air-lubricant mixture aperture and in response to the pressure pulses, separating the oil from the air-oil mixture within the cover space, collecting the oil within the cover space, and discharging the oil from the cover space via a cover space drain aperture positioned a non-zero distance above the lowermost surface. 
         [0008]    The lubricant control arrangement further includes a breather chamber defined in the engine housing. The breather chamber includes an inlet for receiving an air-lubricant mixture and a drain for returning separated lubricant to the crankcase. The lubricant control arrangement also includes a breather cover positioned in the breather chamber. The breather cover includes an inlet to receive the air-lubricant mixture and define an inlet flow area. The breather cover also includes a first outlet to discharge air. Further, the breather cover includes a second outlet to discharge separated lubricant into the breather chamber. The second outlet is spaced from a lower-most wall in the breather chamber such that the second outlet remains substantially above the separated lubricant accumulated in the breather chamber during operation of the engine. The second outlet defines an outlet flow area less than the inlet flow area to substantially decrease the amount of air-lubricant mixture discharged from the second outlet into the breather chamber. 
         [0009]    Other features and aspects of the present invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawings, wherein like reference numerals indicate like parts: 
           [0011]      FIG. 1  is an exploded perspective view of a reduced-emission, single cylinder air-cooled engine of the present invention. 
           [0012]      FIG. 2  is a top view of an engine housing of the engine of  FIG. 1 , illustrating an intake opening and a reinforced cylinder bore; 
           [0013]      FIG. 3  is a side view of the engine housing of  FIG. 2 , illustrating the reinforced cylinder bore; 
           [0014]      FIG. 4  is another side view of the engine housing of  FIG. 2 , illustrating an exhaust opening and a breather chamber; 
           [0015]      FIG. 5  is an end view of the engine housing of  FIG. 2 , illustrating a piston positioned within the cylinder bore of the engine housing; 
           [0016]      FIG. 6  is a section view of the engine housing of  FIG. 2  through section line  6 - 6 , illustrating tapered intake and exhaust passageways; 
           [0017]      FIG. 7   a  is an enlarged, cross-sectional view of the engine housing of  FIG. 5  through section line  7   a - 7   a , illustrating the interface between the piston rings and the cylinder bore; 
           [0018]      FIG. 7   b  is an enlarged view of the piston rings and the cylinder bore illustrated in  FIG. 7   a.    
           [0019]      FIG. 8  is an enlarged view of the engine housing of  FIG. 2 , illustrating a breather cover exploded from the breather chamber; and 
           [0020]      FIG. 9  is an enlarged, top perspective view of the engine housing of  FIG. 2  illustrating an intake crossover passageway exploded from the engine housing. 
           [0021]      FIG. 10  is an enlarged, top perspective view of the piston of the engine of  FIG. 1 . 
           [0022]      FIG. 11  is a side view of the piston of the engine of  FIG. 1 . 
           [0023]      FIG. 12  is a bottom view of the piston of the engine of  FIG. 1 . 
           [0024]      FIG. 13  is a partial cross-sectional view of the engine housing of  FIG. 2 , illustrating the breather cover. 
       
    
    
       [0025]    Before any features of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including”, “having”, and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of letters to identify elements of a method or process is simply for identification and is not meant to indicate that the elements should be performed in a particular order. 
       DETAILED DESCRIPTION 
       [0026]      FIGS. 1-12  illustrate various features and aspects of a reduced-emission, four-cycle, single cylinder engine  10  (only a portion of which is shown). Such a “small” engine  10  may be configured with a power output as low as about 1 Hp and as high as about 20 Hp to operate engine-driven outdoor power equipment (e.g., lawn mowers, lawn tractors, snow throwers, etc.). The illustrated engine  10  is configured as an approximate 3.5 Hp single-cylinder, air-cooled engine having a displacement of about 9 cubic inches. The illustrated engine  10  is also configured as a vertical shaft engine, however, the engine  10  may also be configured as a horizontal shaft engine. 
         [0027]    With reference to  FIG. 1 , the engine  10  includes an upper engine housing  14  which may be formed as a single piece by any of a number of different processes (e.g., die casting, forging, etc.). The engine housing  14  generally includes a crankcase  18  containing lubricant and a cylinder bore  22  extending from the crankcase  18 . The engine housing  14  also includes a flange  26  at least partially surrounding the cylinder bore  22 . The flange  26  is a substantially flat surface to receive thereon a cylinder head  28 . The cylinder head  28  is fastened to the flange  26  using a plurality of bolts (not shown) around the outer periphery of the cylinder bore  22 . The cylinder head  28  includes a combustion chamber which, in combination with the cylinder bore  22 , is exposed to the combustion of an air/fuel mixture during operation of the engine  10 . 
         [0028]    A crankshaft  29  is rotatably supported at one end by a journal  30  (see  FIG. 2 ) formed on the crankcase  18 , and at the other end by a similar journal formed on a crankcase cover  32  coupled to the crankcase  18 . A piston  34  is attached to the crankshaft  29  via a connecting rod  36  for reciprocating movement in the cylinder bore  22  as is understood in the art. 
         [0029]    The illustrated engine  10  is also configured as a side-valve or an L-head engine including a valve train incorporating a cam shaft gear  202  driven by a crankshaft gear  206  and a cam shaft  210  coupled to the cam shaft gear  202 . The cam shaft  210  includes intake and exhaust cam lobes  214 ,  218  thereon, and respective intake and exhaust valves  50 ,  54  supported in the engine housing  14  for reciprocating movement engage the respective cam lobes  214 ,  218  on the cam shaft  210 . 
         [0030]    The engine  10  may also include a lubrication system to provide lubricant to the working or moving components of the engine  10 . As is understood in the art, the lubrication system may include a dipper or splasher (not shown) coupled to the connecting rod such that rotation of the crankshaft causes the dipper or splasher to be intermittently submerged into the lubricant held in the crankshaft. Such motion results in a lubricant mist circulated throughout the crankcase to lubricate the working components or the moving components of the engine  10 . Alternatively, a slinger may be drivably coupled to the crankshaft or cam shaft to generate the lubricant mist as is understood in the art. 
         [0031]    With reference to  FIG. 7   a , the piston  34  includes multiple piston rings  38 ,  42 ,  46  axially spaced on the piston  34 . The lowest piston ring (as seen on  FIGS. 7   a  and  7   b ), or the oil control ring  38 , is utilized to wipe lubricant from the cylinder bore  22  so that the lubricant is substantially prevented from mixing with the air/fuel mixture or the spent exhaust gases in contact with the upper portion of the piston  34 . The piston rings  42 ,  46  positioned above the oil control ring  38 , or the compression rings  42 ,  46 , are biased against the cylinder bore  22  to substantially seal the portion of the cylinder bore  22  above the piston  34  from the portion of the cylinder bore  22  below the piston  34 . As such, the compression rings  42 ,  46  allow the piston  34  to generate compression in the combustion chamber. Reference is made to U.S. Pat. No. 5,655,433, the entire contents of which is hereby incorporated by reference, for additional discussion relating to additional features and aspects of pistons and piston rings. 
         [0032]    With reference to  FIG. 6 , the engine housing  14  includes an intake opening  58  and an intake passageway  62  downstream of the intake opening  58 . The intake opening  58  is positioned on a first side  66  of the engine housing  14 . The intake passageway  62  is formed of an intake runner  67  downstream of the intake opening  58 , and an intake port  68  downstream of the intake runner  67 . The intake valve  50  is positioned in the intake port  68 , such that during operation of the engine  10 , reciprocating movement of the intake valve  50  allows an air/fuel mixture air to intermittently be drawn through the intake opening  58 , through the intake passageway  62 , past a head  70  of the intake valve  50 , and into the combustion chamber of the cylinder head  28  and the cylinder bore  22  for compression and combustion. 
         [0033]    An intake valve seat insert  74  is coupled to the engine housing  14  by press-fitting or any other known method. The intake valve seat insert  74  includes a chamfered inner peripheral edge that sealingly engages the head  70  of the intake valve  50  to block the entrance of air/fuel mixture into the combustion chamber and the cylinder bore  22 . A valve spring (not shown) may be coupled to the intake valve  50  to bias the intake valve  50  to a “closed” position, in which the head  70  of the intake valve  50  is engaged with the intake valve seat insert  74  to block the intake passageway  62 . The intake valve seat insert  74  may be made from a material that is harder and/or more heat resistant than the material of the engine housing  14 . 
         [0034]    The intake valve  50  is supported in the engine housing  14  for reciprocating movement by a guide  78  integral with the housing  14 . More particularly, a stem portion  82  of the intake valve  50  is supported by the guide  78 . As shown in  FIG. 6 , a stem seal  86  is coupled to the engine housing  14  to receive the stem portion  82  of the intake valve  50 . The stem seal  86  is operable to wipe the stem portion  82  as the intake valve  50  reciprocates, such that lubricant on the stem portion  82  is substantially prevented from entering the combustion chamber. Reference is made to U.S. Pat. No. 6,202,616, which is incorporated herein by reference, for additional discussion relating to the structure and operation of the stem seal  86 . 
         [0035]    The intake passageway  62  may also be in communication with an induction system to provide the air/fuel mixture. Such an induction system may include, for example, an air cleaner (not shown), a carburetor (not shown), and an intake manifold  90  containing an inlet crossover passageway (see  FIG. 9 ). The air cleaner filters the intake air, the carburetor adds fuel to the intake air, and the inlet crossover passageway directs the air/fuel mixture to the intake opening  58 . 
         [0036]    With reference to  FIG. 6 , the engine housing  14  also includes an exhaust opening  94  and an exhaust passageway  98  upstream from the exhaust opening  94 . The exhaust opening  94  is positioned on a second side  102  of the engine housing  14  adjacent the first side  66  of the engine housing  14  having the intake opening  58 . The exhaust passageway  98  is formed of an exhaust runner  99  upstream of the exhaust opening  58 , and an exhaust port  100  upstream of the exhaust runner  99 . The exhaust valve  54  is positioned in the exhaust port  100 , such that during operation of the engine  14 , reciprocating movement of the exhaust valve  54  allows spent exhaust gases to intermittently pass out of the combustion chamber and the cylinder bore  22 , past a head  106  of the exhaust valve  54 , through the exhaust passageway  98 , and through the exhaust opening  94 . 
         [0037]    An exhaust valve seat insert  110  is coupled to the engine housing  14  by press-fitting or other known methods. The exhaust valve seat insert  110  includes a chamfered inner peripheral edge that sealingly engages the head  106  of the exhaust valve  54  to block spent exhaust gases from exiting the combustion chamber and the cylinder bore  22 . A valve spring (not shown) may be coupled to the exhaust valve  54  to bias the exhaust valve  54  to a “closed” position, in which the head  106  of the exhaust valve  54  is engaged with the exhaust valve seat insert  110  to block the exhaust passageway  98 . The exhaust valve seat insert  110  may be made from a material that is harder and/or more heat resistant than the material of the engine housing  14 . 
         [0038]    The exhaust valve  54  is supported in the engine housing  14  for reciprocating movement by a valve guide  114  positioned in the housing  14 . More particularly, a stem portion  118  of the exhaust valve  54  is supported by the valve guide  114 . Like the exhaust valve seat insert  110 , the valve guide  114  may be made from material that is harder and/or more heat resistant than the material of the engine housing  14 . As such, the valve guide  114  supporting the stem portion  118  of the exhaust valve  54  may lead to improved sealing of the exhaust valve  54  and the exhaust valve seat  110 . 
         [0039]    The exhaust passageway  98  may also be in communication with an exhaust system (not shown) to discharge the spent exhaust gases. Such an exhaust system may include, for example, an exhaust manifold receiving the spent exhaust gases from the exhaust opening  94  and a muffler. 
         [0040]    With reference to  FIG. 8  and  FIG. 13 , the engine  10  may also include a breather cover  122  engageable with a breather chamber  126  formed in the engine housing  14 . The breather cover  122  includes two pieces that attach to one another to define a cover space (shown in  FIG. 13 ) and generally removes lubricant entrained in an air/lubricant mixture (i.e., the lubricant mist) present in the crankcase  18 . During operation of the engine  10 , a quantity of air/lubricant mixture is displaced from the crankcase  18  into the breather chamber  126  via an inlet passageway  130  (shown in  FIGS. 8 and 9 ) when crankcase pressure increases during the power stroke or the intake stroke of the piston  34  (i.e., during a downward stroke of the piston  34 , as shown in  FIG. 7   a ). 
         [0041]    As shown in  FIG. 8  and  FIG. 13 , the breather cover  122  includes an air/lubricant inlet  134  to receive the air/lubricant mixture or breather gases in from the breather chamber  126 . The breather cover  122  includes internal baffling structure to separate the entrained lubricant from the oil-laden breather gases. The baffling structure causes the entrained lubricant to precipitate out of the mixture and accumulate in the bottom of the breather cover  122 , while the breather gases are discharged from the breather cover  122  via a first outlet  138  (shown in  FIGS. 8 and 13 ). The engine housing  14  includes a passageway  142  (shown in  FIG. 13 ) for recirculating the breather gases from the breather cover  122  to the induction system downstream of the air cleaner so the breather gases may be burned by the engine  10 . 
         [0042]    The breather cover  122  also includes a second outlet  146  positioned toward the bottom of the breather cover  122  (as shown in  FIG. 8  and  FIG. 13 ). The separated lubricant is discharged from the breather cover  122  via the second outlet  146  and returned to the breather chamber  126 . The breather chamber  126  includes a drain  150  communicating the breather chamber  126  with the crankcase  18 , such that the separated lubricant may drain from the breather chamber  126  back to the crankcase  18  for reuse by the engine  10 . 
         [0043]    It is expected that various combinations of features and aspects of the engine  10  will enable the engine  10 , without using a sophisticated fuel injection system or expensive exhaust catalysts, to operate at decreased levels of hydrocarbon emissions compared to other four-cycle single cylinder small engines. It is expected that various combinations of features and aspects of the engine  10  as described herein will reduce the amount of hydrocarbon emissions output by about 50 percent without using a sophisticated fuel injection system or expensive exhaust catalysts. 
         [0044]    With reference to  FIG. 6 , the engine  10  utilizes a valve sealing arrangement that is expected to decrease hydrocarbon emissions output of the engine. In the illustrated construction, the intake valve seat insert  74  has a radial thickness T 1  between about 1.8 mm and about 2.2 mm, while the exhaust valve seat insert  110  has a radial thickness T 2  between about 1.8 mm and about 2.2 mm. In some embodiments of the engine  10 , the axial thickness of the intake valve seat insert  74  is equal to about twice the radial thickness T 1 . In other embodiments of the engine  10 , the axial thickness of the exhaust valve seat insert  110  is equal to about twice the radial thickness T 2 . 
         [0045]    By sizing the radial thickness of the intake and exhaust valve seat inserts  74 ,  110  according to the above-referenced values, the inserts  74 ,  110  present less of a barrier to the dissipation of heat from the valves  50 ,  54  since the heat conducts through a shorter distance before reaching the engine housing  14 . As such, less heat may be “trapped” by the inserts  74 ,  110  and a more uniform dissipation of heat from the valves  50 ,  54  may occur, resulting in reduced temperature and decreased warpage or distortion of the inserts  74 ,  110  and the valves  50 ,  54 . Further, it is expected that sizing the radial thickness of the intake and exhaust valve seat inserts  74 ,  110  according to the above-referenced values may allow more effective sealing of the intake and exhaust valves  50 ,  54  and the respective inserts  74 ,  110  during engine operation, potentially prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0046]    The valve sealing arrangement may also include spacing the intake and exhaust valve seat inserts  74 ,  110  by a wall thickness W between about 2.5 mm and about 5 mm. By sizing the wall thickness W according to the above-referenced values, heat transfer between the inserts  74 ,  110  may be reduced, allowing more uniform temperatures of the inserts  74 ,  110 . As a result, more uniform temperatures of the inserts  74 ,  110  may reduce warpage or distortion of the inserts  74 ,  110  during operation of the engine  10 . Further, sizing the wall thickness W according to the above-referenced values may lead to improved sealing of the intake and exhaust valves  50 ,  54  and the respective inserts  74 ,  110  during operation of the engine  10 . It is therefore expected that such improved valve sealing may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0047]    The valve sealing arrangement may also include positioning the valve guide  114  in a reinforced portion of the engine housing  14  to stabilize the valve guide  114 , and therefore, support the stem portion  118  of the exhaust valve  54  to stabilize the reciprocating movement of the exhaust valve  54 . In addition, the valve sealing arrangement may include reinforcing a portion of the engine housing  14  to provide additional support to the stem portion  82  of the intake valve  50  to stabilize reciprocating movement of the intake valve  50 . More particularly, with reference to  FIG. 2 , a rib  154  is formed on a portion of the engine housing  14  supporting the stem portion  82  of the intake valve  50 . The rib  154  may substantially prevent undesirable lateral movement of the intake valve  50  during operation of the engine  10 . By stabilizing the intake and exhaust valves  50 ,  54  during reciprocating movement, more effective sealing is promoted between the valve head  106  and the intake and exhaust valve seat inserts  74 ,  110  during engine operation. As such, the useful life of the engine  10  may be prolonged, performance of the engine  10  may be increased, and the hydrocarbon emissions output of the engine  10  may be decreased. 
         [0048]    With reference to  FIG. 6 , the valve sealing arrangement may further include positioning the stem seal  86  in sliding contact with the stem portion  82  of the intake valve  50  during reciprocating movement of the intake valve  50 . As discussed above, the stem seal  86  wipes the stem portion  82  of the intake valve  50  to substantially prevent lubricant from entering the intake passageway  62  and being drawn into the combustion chamber for combustion with the air/fuel mixture. Such combustion of lubricant may result in an increased hydrocarbon emissions output. By substantially sealing the lubricant from the intake passageway  62  and thus the combustion chamber, the useful life of the engine  10  may be prolonged, performance of the engine  10  may be increased, and the hydrocarbon emissions output of the engine  10  may be decreased. 
         [0049]    The valve sealing arrangement may also include spacing the exhaust opening  94  and the exhaust runner  99  a dimension D 1 . High temperature exhaust gases are discharged from the exhaust opening  94 . As such, spacing the exhaust opening  94  and the exhaust valve seat insert  110  by dimension D 1  may facilitate more uniform cooling and/or a lower temperature of the exhaust valve seat insert  110 . With reference to  FIG. 6 , the exhaust runner  99  is spaced from the exhaust valve seat insert  110  by a dimension D 1  between about 6 mm and about 12 mm. By spacing the exhaust runner  99  and the exhaust valve seat insert  110  according to the above-referenced values, more uniform cooling or lower temperatures of the exhaust valve seat insert  110  may result which, in turn, may promote more effective sealing of the exhaust valve  54  and the exhaust valve seat insert  110  during engine operation. As such, the life of the engine  10  may be prolonged, performance of the engine  10  may be increased, and the hydrocarbon emissions output of the engine  10  may be decreased. 
         [0050]    With reference to  FIGS. 5 ,  6 , and  9 , the engine  10  utilizes an air flow arrangement that is expected to decrease hydrocarbon emissions output of the engine  10 . The air flow arrangement includes forming the inlet crossover passageway in the intake manifold  90  (see  FIG. 9 ) such that the inlet crossover passageway has a substantially constant cross-sectional area along the its length to increase the flow efficiency of the intake air therethrough. Reference is made to U.S. patent application Ser. No. 10/779,363 filed Feb. 13, 2004, the entire contents of which is incorporated herein by reference, for additional discussion relating to the inlet crossover passageway. The inlet crossover passageway may define a constant cross-sectional shape, and thus a constant cross-sectional area, or the inlet crossover passageway may define a varying cross-sectional shape while maintaining a constant cross-sectional area. By increasing the flow efficiency of the intake air and/or the air/fuel mixture through the inlet crossover passageway, more efficient combustion may result during operation of the engine  10 . It is therefore expected that such improved air flow may result in increased performance of the engine  10  and decreased hydrocarbon emissions output of the engine  10 . 
         [0051]    Also, the inlet crossover passageway draws intake air from a location spaced from the exhaust opening  94 . More particularly, the inlet crossover passageway draws intake air from a location adjacent a third side  160  of the engine housing  14  opposite the second side  102 . This enables the engine  10  to draw a cooler intake charge (i.e., the air/fuel mixture) into the combustion chamber. 
         [0052]    With reference to  FIG. 6 , the intake passageway  62  has first and second cross-sectional areas defined by respective first and second planes  161 ,  162  passing substantially transversely through the intake passageway  62 . The first cross-sectional area is larger than the second cross-sectional area and disposed further from the intake opening  58  than the second cross-sectional area to increase flow efficiency of the intake air and/or the air/fuel mixture through the intake passageway  62 . In the illustrated construction, the intake port  68  has a conical shape defining an included angle A 1  between about 8 degrees and about 15 degrees. By increasing the flow efficiency of the intake air and/or the air/fuel mixture through the intake passageway  62 , more efficient combustion may result during operation of the engine  10 . It is therefore expected that such improved air flow may result in increased performance of the engine  10  and decreased hydrocarbon emissions output of the engine  10 . 
         [0053]    Likewise, the exhaust passageway  98  has third and fourth cross-sectional areas defined by respective third and fourth planes  163 ,  164  passing substantially transversely through the exhaust passageway  98 . The third cross-sectional area is larger than the fourth cross-sectional area and disposed closer to the exhaust opening  94  than the fourth cross-sectional area to increase flow efficiency of exhaust gases through the exhaust passageway  98 . In the illustrated construction, the exhaust runner  99  has a conical shape defining an included angle A 2  between about 4 degrees and about 10 degrees. By increasing the flow of exhaust gases through the exhaust passageway  98 , more efficient combustion may result during operation of the engine  10 . It is therefore expected that such improved air flow may result in increased performance of the engine  10  and decreased hydrocarbon emissions output of the engine  10 . 
         [0054]    With reference to  FIG. 9 , the engine  10  utilizes a lubricant control arrangement that is expected to decrease hydrocarbon emissions output of the engine  10 . With reference to  FIG. 9 , the lubricant control arrangement includes reinforcing a portion  170  of the engine housing  14  adjacent the flange  26  to decrease deflection of the flange  26  and/or deflection of the cylinder bore  22  during operation of the engine  10 . The reinforced portion  170  of the engine housing  14  is on the first side  66  of the engine housing  14  in a location that is covered by the intake manifold  90  when the intake manifold  90  is coupled to the engine housing  14 . 
         [0055]    By not sufficiently reinforcing the portion of the engine housing  10  adjacent the flange  26 , deflection of the flange  26  and/or the cylinder bore  22  may occur due to the forces exerted on the cylinder head  28  during engine operation. More particularly, the forces exerted on the cylinder head  28  during engine operation want to separate the cylinder head  28  from the engine housing  14 . However, the cylinder head  28  is secured to the engine housing  14  by multiple bolts. As a result, the forces are absorbed by the engine housing  14 . Insufficient reinforcement around the cylinder bore  22  may allow the cylinder bore  22  to deflect, which may prevent the piston rings  38 ,  42 ,  46  from effectively sealing against the cylinder bore  22  during engine operation. If the piston rings  38 ,  42 ,  46  do not effectively seal against the cylinder bore  22 , lubricant may be allowed to enter the combustion chamber where it is burnt. The burned lubricant, therefore, may create deposits on the piston  34  or in the combustion chamber that may likely result in decreased performance of the engine  10  and increased hydrocarbon emissions output of the engine  10 . 
         [0056]    However, by providing the reinforced portion  170  in the engine housing  14 , the cylinder bore  22  is less likely to deflect during operation of the engine  10 . Further, the reinforced portion  170  of the engine housing  14  may lead to improved sealing of the piston rings  38 ,  42 ,  46  to the cylinder bore  22  during engine operation, thereby reducing the amount of lubricant that enter the cylinder bore  22  and combustion chamber. Such improved sealing of the piston rings  38 ,  42 ,  46  to the cylinder bore  22  during combustion may also reduce blow-by of combustion gases into the crankcase  18 . It is therefore expected that such improved lubricant control may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0057]    With reference to  FIG. 7   a , the lubricant control arrangement also includes sizing the radial thickness of the compression rings  42 ,  46  to facilitate radially outward deflection of the compression rings  42 ,  46  to more effectively seal against the cylinder bore  22 . In the illustrated construction, the radial thickness T 3  of the compression rings  42 ,  46  may be between about 2.3 mm and about 2.7 mm. 
         [0058]    The lubricant control arrangement further includes sizing the axial thickness of the compression rings  42 ,  46  to facilitate sealing against the cylinder bore  22 . In the illustrated construction, the axial thickness T 4  of the compression rings  42 ,  46  may be between about 1 mm and about 1.5 mm. By providing compression rings  42 ,  46  of decreased radial and axial thickness, lubricant is less likely to enter the combustion chamber during engine operation. It is therefore expected that such improved lubricant control may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0059]    The lubricant control arrangement also includes utilizing the oil control ring  38  to wipe lubricant from the cylinder bore  22  preferentially during the power stroke and the intake stroke of the engine  10 . In other words, the oil control ring  38  is configured to wipe oil from the cylinder bore  22  preferentially in one direction. In the illustrated construction, the oil control ring  38  includes two wipers  174  biased against the cylinder bore  22  and downwardly angled to wipe oil from the cylinder bore  22  to return the oil to the crankcase  18 . Some oil control rings utilize wipers configured to wipe oil from the cylinder as the piston reciprocates both upward and downward. Such a configuration may be less efficient in wiping lubricant from the cylinder, and some lubricant may be allowed to enter the combustion chamber. 
         [0060]    By providing the oil control ring  38  having directional wipers  174 , lubricant is less likely to enter the combustion chamber during engine operation. It is therefore expected that such improved lubricant control may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0061]    With reference to  FIG. 8  and  FIG. 13 , the lubricant control arrangement further includes positioning the second outlet  146  in the breather cover  122  above the level of accumulated lubricant (represented by line  178 ) in the breather chamber  126 . In the illustrated construction, the second outlet  146  is positioned a dimension D 2  of at least 6 mm from a lower-most wall  182  in the breather chamber  126  such that the second outlet  146  remains substantially above the separated lubricant accumulated in the breather chamber  126  during operation of the engine  10 . Positioning the second outlet  146  as shown in  FIG. 8  and  FIG. 13  also allows the engine  10  to be tipped during normal operation without substantially submerging the second outlet  146  in the accumulated lubricant in the breather chamber  126 . 
         [0062]    If the second outlet  146  is positioned substantially below the level illustrated in  FIG. 8  and  FIG. 13 , pressure pulses in the breather chamber  126  due to the reciprocating motion of the piston  34  may cause the accumulated lubricant to re-enter the breather cover  122  via the second outlet  146 . If the accumulated lubricant is allowed to re-enter the breather cover  122 , the lubricant may become re-mixed with the air in the breather cover  122  and discharged from the air outlet  138  for re-introduction into the engine  10 . If this is allowed to occur, lubricant may be allowed to enter the combustion chamber where it may be burnt. The burned lubricant, therefore, may create deposits on the piston  34  and/or in the combustion chamber that may likely result in decreased performance of the engine  10  and increased hydrocarbon emissions output of the engine  10 . 
         [0063]    However, by providing the improved breather cover  122  having the second outlet  146  spaced sufficiently far from the lower-most wall  182  in the breather chamber  126 , accumulated lubricant is less likely to re-enter the breather cover  122  via the second outlet  146 , thereby more effectively preventing lubricant from entering the combustion chamber and being burned. It is therefore expected that such improved lubricant control may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0064]    In addition, the second outlet  146  is sized to control air leakage back into the crankcase  18 . More particularly, the second outlet  146  is formed as a circular aperture having a diameter between about 0.5 mm and about 2 mm, which yields a flow area of between about 0.2 mm 2  and about 3.1 mm 2 , and the inlet  134  is formed as a circular aperture yielding a flow area substantially larger than the flow area of the second outlet  146 . Sizing the second outlet  146  as described above increases the efficiency of the breather cover  122  by decreasing the amount of oil-laden breather gases that leak through the second outlet  146 , while facilitating the precipitated oil in the breather cover  122  to drain into the breather chamber  126  through the second outlet  146 . 
         [0065]    With reference to  FIGS. 7   a - 8 , the engine  10  utilizes a crankcase breather arrangement that is expected to decrease hydrocarbon emissions output of the engine  10 . More particularly, with reference to  FIG. 7   a , the crankcase breather arrangement includes sizing the radial thickness of the compression rings  42 ,  46  to facilitate radially outward deflection of the compression rings  42 ,  46  to more effectively seal against the cylinder, as discussed above. The crankcase breather arrangement also includes sizing the axial thickness of the compression rings  42 ,  46  to facilitate sealing against the cylinder, as discussed above. 
         [0066]    By sizing the compression rings  42 ,  46  according to the above values, the piston  34  may be more effectively sealed against the cylinder bore  22 . As a result, it is less likely that blow-by of the combusting air/fuel mixture will occur, and that the breather cover  122  may function more efficiently. It is therefore expected that such improved crankcase breathing may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0067]    With reference to  FIG. 8  and  FIG. 13 , the crankcase breather arrangement also includes positioning the second outlet  146  in the breather cover  122  above the level of accumulated oil in the breather chamber  126 , as previously discussed. By providing the improved breather cover  122  having the second outlet  146  spaced sufficiently far from the lower-most wall  182  in the breather chamber  126 , accumulated lubricant is less likely to re-enter the breather cover  122  via the second outlet  146 , thereby more effectively preventing lubricant from entering the combustion chamber and being burned. It is therefore expected that such improved crankcase breathing may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0068]    With reference to  FIGS. 10-12 , the piston  34  includes a substantially circular head portion  212  and a skirt  216  extending from the head portion  212 . The substantially circular head portion  212  generally defines at its outer periphery a cylindrical plane  220  (see  FIG. 10 ). The head portion  212  includes a plurality of grooves therein to receive the rings  38 ,  42 ,  46 , as discussed above. 
         [0069]    With continued reference to  FIG. 10 , the skirt  216  includes a curved first portion  224 , at least a portion of which is substantially co-planar with the cylindrical plane  220 . The skirt  216  also includes a substantially flat second portion  228  having an aperture  232  therethrough for receiving a connecting pin (not shown). The connecting pin rotatably couples the piston  34  to the connecting rod  36  as is understood in the art. The skirt  216  further includes a substantially elliptical third portion  236  connecting the curved first portion  224  and the substantially flat second portion  228 . As shown in  FIG. 12 , the substantially flat second portion  228  and the substantially elliptical third portion  236  are located radially inward of the cylindrical plane  220 . 
         [0070]    With reference to  FIG. 12 , at least a portion of the curved first portion  224  is located radially inward of the cylindrical plane  220 . Specifically, point P 1  on the outer periphery of the curved first portion  224  is located on a portion of the curved first portion  224  that is coplanar with the cylindrical plane  220 , while points P 2 , P 3  on the outer periphery of the curved first portion  224  are located on respective portions of the curved first portion  224  that are spaced radially inward of the cylindrical plane  220 . In other words, the spacing between the first curved portion  224  and a cylinder wall  240  of the cylinder bore  22  is the smallest at point P 1 , while the spacing between the curved first portion  224  and the cylinder wall  240  increases moving from point P 1  to point P 2 , and from point P 1  to point P 3 . In the illustrated construction, all of the points P 1 , P 2 , P 3  are located in a common horizontal plane (not shown) passing through the middle of the skirt  216  (see  FIG. 11 ). 
         [0071]    This shape of the curved first portion  224  allows the piston  34  to be tightly fit into the cylinder bore  22  at point P 1 . In some constructions of the engine  10 , a clearance of 0.013 mm can be used between the curved first portion  224  and the cylinder wall  240  at point P 1 . Points P 2 , P 3  are located at portions of the curved first portion  224  that experience a greater amount of thermal expansion during operation of the engine  10 . By spacing these portions of the curved first portion  224  inwardly from the cylinder bore  22 , these portions are allowed to grow without substantially affecting operation of the engine  10 . The piston  34  can be fitted tightly to the cylinder bore  22  at point P 1  to provide improved stability of the piston  34  as it moves in the cylinder bore  22 , while allowing adequate clearance at points P 2 , P 3  for thermal expansion during operation of the engine  10 . As a result of increasing the stability of the piston  34  in the cylinder bore  22 , the movement of the piston rings  38 ,  42 ,  46  in the cylinder bore  22  can also be stabilized. It is therefore expected that such improved piston and ring stability may yield reduced oil consumption and reduced amounts of burned oil deposits on the piston  34  and/or in the combustion chamber, thereby reducing hydrocarbon emissions from the engine  10 . It is also expected that such improved piston and ring stability may yield reduced blow-by of combustion gases into the crankcase  18 , thereby reducing the amount of combustion gases passing through the breather cover  122  and into the combustion chamber. Further, it is expected that such improved piston and ring stability may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0072]    With reference to  FIG. 11 , the first portion  224  of the skirt  216  is spaced from the cylinder wall  240  a variable clearance from an end of the skirt  216  adjacent the head portion  212  to an opposite end of the skirt  216 . More particularly, the smallest clearance (indicated by CL 1 ) between the first portion  224  of the skirt  216  and the cylinder wall  240  occurs about midway between the opposite ends of the skirt  216 . Further, larger clearances (indicated by CL 2  and CL 3 ) between the first portion  224  of the skirt  216  and the cylinder wall  240  occur toward the opposite ends of the skirt  216 . In the illustrated construction, clearance CL 1  may be about 0.013 mm, clearance CL 2  may be about 0.150 mm, and clearance CL 3  may be about 0.025 mm. 
         [0073]    As a result, the curved first portion  224 , as viewed in  FIG. 11 , is substantially arcuate with a tight fit against the cylinder wall  240  at a location on the skirt  216  corresponding with clearance CL 1 . The increased clearance CL 2  allows for thermal expansion of the skirt  216  toward the cylinder wall  240 . The increased clearance CL 3  provides additional clearance for improved lubrication between the skirt  216  and the cylinder wall  240 . In operation, therefore, the resultant fit of the piston  34  provides improved stability of the piston  34  as it moves in the cylinder bore  22 . As a result of increasing the stability of the piston  34  in the cylinder bore  22 , the movement of the piston rings  38 ,  42 ,  46  in the cylinder bore  22  can also be stabilized. It is therefore expected that such improved piston and ring stability may yield reduced oil consumption and reduced amounts of burned oil deposits on the piston  34  and/or in the combustion chamber, thereby reducing hydrocarbon emissions from the engine  10 . It is also expected that such improved piston and ring stability may yield reduced blow-by of combustion gases into the crankcase  18 , thereby reducing the amount of combustion gases passing through the breather cover  122  and into the combustion chamber. Further, it is expected that such improved piston and ring stability may lead to prolonging the useful life of the engine  10 , increasing the performance of the engine  10 , and decreasing the hydrocarbon emissions output of the engine  10 . 
         [0074]    It should be understood that the reduced emission, single cylinder engine  10  of the present invention may incorporate one or more of the valve sealing arrangement, the lubricant control arrangement, the air flow arrangement, and the crankcase breather arrangement. 
         [0075]    Various aspects of the invention are set forth in the following claims.