Patent Publication Number: US-7216612-B2

Title: Internal combustion engine having cylinder formed with water jacket and vehicle provided with the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an internal combustion engine having a cylinder formed with a water jacket and a vehicle provided with the same. 
   2. Description of the Related Art 
   In an engine in which a cylinder and a cylinder head are combined, the cylinder is exposed to high-temperature combustion gas in the same manner as the cylinder head. Conventionally, water-cooled type engines having a cylinder are frequently provided with a water jacket. 
   In recent years, the use of a cylinder made of aluminum alloy instead of a cylinder made of cast iron has been proposed in order to make an engine lightweight. Aluminum alloy is superior in thermal conductivity to iron. Therefore, a water jacket having the same shape and dimension as those in a conventional cylinder made of cast iron is conventionally provided in a cylinder made of aluminum alloy on the basis that sufficient cooling can be obtained with the same water jacket. See, for example, Abstract of Pamphlet of International Publication WO2002/053899. 
   The inventors of the present application have found that when using a material having an excellent thermal conductivity as a material for a cylinder, an overall desired performance for an internal combustion engine cannot necessarily be obtained with the same water jacket as a conventional cast iron cylinder. 
   SUMMARY OF THE INVENTION 
   In order to overcome the problems described above, preferred embodiments of the present invention provide a water jacket suited to a cylinder made of a material having an excellent thermal conductivity in order to achieve a high performance internal combustion engine. 
   An internal combustion engine according to a preferred embodiment of the present invention includes a cylinder head, a cylinder having an opposed surface formed with an opening and opposed to the cylinder head, a cylinder inner surface extending from a peripheral edge of the opening in a predetermined cylinder axial direction to define a combustion chamber, a concave water jacket arranged around the opening on the opposed surface substantially in the cylinder axial direction and having a bottom wall defining a concave bottom, a piston provided in the cylinder and having a piston body and a plurality of rings mounted to a periphery of the piston body to be aligned in the cylinder axial direction, the plurality of rings including a bottom ring positioned on a side that is farthest from the cylinder head and having an upper end that constitutes an end toward the cylinder head and a lower end that constitutes an end in opposition to the cylinder head, at least a portion in the cylinder between the cylinder inner surface and the water jacket made from a material having a higher thermal conductivity than that of iron, and the bottom wall of the water jacket being positioned in the cylinder axial direction between the opposed surface and the lower end of the bottom ring when the piston is disposed in a top dead center position. 
   An internal combustion engine according to a preferred embodiment of the present invention includes a cylinder head, a cylinder having an opposed surface formed with an opening and opposed to the cylinder head, a cylinder inner surface extending from a peripheral edge of the opening in a predetermined cylinder axial direction to define a combustion chamber, a concave water jacket arranged around the opening on the opposed surface substantially in the cylinder axial direction and having a bottom wall defining a concave bottom, a piston arranged in the cylinder so as to be able to reciprocate, a piston body and a plurality of rings mounted to a periphery of the piston body to be aligned in the cylinder axial direction, a crankshaft that rotates as the piston reciprocates, the plurality of rings including a top ring positioned on a side nearest to the cylinder head and having an upper end that constitutes an end toward the cylinder head and a lower end that constitutes an end in opposition to the cylinder head, at least a portion in the cylinder between the cylinder inner surface and the water jacket made from a material having a higher thermal conductivity than that of iron, and the bottom wall of the water jacket being positioned in the cylinder axial direction between the upper end of the top ring when a crank angle is about 0° and the upper end of the top ring when the crank angle is about 60°, where the crank angle is made about 0° when the piston is disposed in a top dead center position. 
   An internal combustion engine according to a preferred embodiment of the present invention includes a cylinder head, a cylinder having an opposed surface formed with an opening and opposed to the cylinder head, a cylinder inner surface extending from a peripheral edge of the opening in a predetermined cylinder axial direction to define a combustion chamber having a predetermined diameter, a concave water jacket arranged around the opening on the opposed surface substantially in the cylinder axial direction, a piston provided in the cylinder, at least a portion in the cylinder between the cylinder inner surface and the water jacket made from a material having a higher thermal conductivity than that of iron, and a depth of the water jacket from the opposed surface is at most about 0.33 times the diameter of the combustion chamber. 
   With the internal combustion engine, at least a portion of the cylinder between the cylinder inner surface and the water jacket is made from a material having a higher thermal conductivity than that of iron, and the water jacket is formed to be relatively shallow. Therefore, while the cylinder is effectively cooled by the cooling water in the water jacket it is also possible to prevent excessive cooling. Accordingly, it is possible to achieve improved engine performance while maintaining the cooling capacity. 
   Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view showing a motorcycle; 
       FIG. 2  is a view showing the construction of an engine; 
       FIG. 3  is a cross sectional view showing an essential part of the engine; 
       FIG. 4  is a bottom view showing a cylinder head; 
       FIG. 5  is a cross sectional view taken along the line V—V in  FIG. 4 ; 
       FIG. 6  is a cross sectional view showing, in enlarged scale, the area of a water jacket of a cylinder; 
       FIG. 7  is a bottom view showing a cylinder head; 
       FIG. 8  is a bottom view showing a gasket; 
       FIG. 9  is a graph illustrating the relationship between a crank angle and a rate of combustion mass; and 
       FIG. 10  is a partial cross-sectional view showing a cylinder according to a modification of preferred embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   As shown in  FIG. 1 , a vehicle according to a preferred embodiment includes a motorcycle  1 . However, a vehicle according to preferred embodiments of the present invention is not limited to the motorcycle  1 . A vehicle according to the present invention may include other saddle-ride type vehicles or vehicles other than saddle-ride type vehicles. In addition, “motorcycle” includes scooters, etc. in addition to so-called motorbikes. 
   The motorcycle  1  includes a vehicle body  2 , a front wheel  3  and a rear wheel  4 , which are mounted to the vehicle body  2 , and an engine  5  that drives the rear wheel  4  through a drive chain or the like (not shown). In the present preferred embodiment, the engine  5  preferably includes a single cylinder 4-cycle internal combustion engine, for example. However, the engine  5  is in no way limited in the number of cylinders, etc. 
   As shown in  FIG. 2 , the engine  5  includes an engine body  10 , an intake passage  11  , and an exhaust passage  12 . The engine body  10  includes a crankcase  21 , in which a crankshaft  42  or the like (see  FIG. 3 ) is accommodated, a cylinder  22  unified with the crankcase  21 , and a cylinder head  23  mounted to the cylinder  22 . According to the present preferred embodiment, the crankcase  21  and the cylinder  22  are unified together to form a cylinder block. However, the crankcase  21  and the cylinder  22  may be formed separately and assembled to each other. 
   The intake passage  11  includes an intake pipe  15  connected to an air cleaner (not shown), a throttle body  16 , and an intake port  54  formed in the cylinder head  23 . A downstream end of the intake pipe  15  is connected to an upstream end of the throttle body  16 , and a downstream end of the throttle body  16  is connected to the cylinder head  23 . A throttle valve  13  is provided within the throttle body  16 . An injector  14  is mounted to the cylinder head  23 . That is, the injector  14  is arranged in the intake passage  11  downstream of the throttle valve  13 . Accordingly, the injector  14  jets a fuel between the throttle valve  13  and an intake opening  52  (see  FIG. 3 ), as described later. 
   The exhaust passage  12  includes an exhaust port  55  arranged on the cylinder head  23 , an exhaust pipe  17  connected to the cylinder head  23 , a catalyst casing  18  provided on the exhaust pipe  17 , and a muffler  19  provided at a tip end of the exhaust pipe  17 . A ternary catalyst  7  is accommodated within the catalyst casing  18 . 
   As shown in  FIG. 3 , a cylinder inner surface  31  defines a column shaped cylinder chamber  32  inside the cylinder  22 , and a piston  40  is accommodated in the cylinder chamber  32 . An upper side, as shown in  FIG. 3 , is provided on the cylinder head  23  and a lower side, as shown in  FIG. 3 , is provided on the cylinder  22 . However, the vertical relationship between the cylinder head  23  and the cylinder  22  is actually dependent upon a position and orientation of the engine  5  as mounted. Here, “upper side” and “lower side” as referred to do not necessarily mean an upper side and a lower side in a state in which the engine  5  is mounted. 
   The piston  40  includes a substantially cylindrical shaped piston body  43 , a top ring  45  mounted to a side surface, that is, an outer peripheral surface of the piston body  43 , a second ring  46 , and an oil ring  47 . The plurality of piston rings, that is, the top ring  45 , the second ring  46 , and the oil ring  47  are arranged in this order from above to below. 
   The piston body  43  is connected to an upper end of a connecting rod  41  and a lower end of the connecting rod  41  is connected to a crankshaft  42 . The piston  40  reciprocates between a predetermined top dead center position (a position indicated by solid lines in  FIG. 3 ) and a bottom dead center position (a position indicated by two-dot chain lines in  FIG. 3 ) within the cylinder  22 . As the piston  40  reciprocates, the crankshaft  42  rotates. 
   The cylinder  22  is preferably made from a material having a higher thermal conductivity than that of iron. Preferably, the material of the cylinder  22  has a thermal conductivity of at least about 90 W/(m·K) at, for example, about 0° C. According to the present preferred embodiment, the cylinder  22  is preferably formed from an aluminum alloy. However, the material of the cylinder  22  may be another aluminum based material or made from other materials. 
   As shown in  FIG. 3 , a water jacket  34  is formed on an upper surface of the cylinder  22 , that is, a surface  33  opposed to the cylinder head  23 . The water jacket  34  is formed by a concave groove that is concave in an axial direction (a vertical direction in  FIG. 3 ), referred simply to below as “cylinder axial direction” of the cylinder  22 . Also, the water jacket  34  is arranged so as to surround an entire periphery of the cylinder chamber  32  as viewed in the cylinder axial direction (see  FIG. 8 ). The cylinder  22  includes the opposed surface  33  formed with an opening  35  (see  FIG. 5 ) with the cylinder inner surface  31  extending from a peripheral edge of the opening  35  in the cylinder axial direction, and the water jacket  34  arranged around the opening  35  on the opposed surface  33 . 
   In the specification of the present application, a deepest portion, that is, a lowermost wall surface of the water jacket  34 , is referred to as “bottom wall”  36 . The water jacket  34  will be described later in detail. 
   A pent roof type recess  51  is preferably formed on a lower surface  60  of the cylinder head  23  to cover an upper region of the cylinder chamber  32 . However, the recess  51  is not limited in shape but may be, for example, semi-spherical or multi-spherical in shape. A combustion chamber  44  is defined by the recess  51 , the cylinder inner surface  31 , and an upper surface of the piston  40 . 
   As shown in  FIG. 4 , the recess  51  is preferably formed with two intake openings  52  and two exhaust openings  53 , for example. The intake openings  52  are provided on a rear side (a left side in  FIG. 4 ) of the vehicle body  2  to be arranged in a right and left direction (a vertical direction in  FIG. 4 ) of the vehicle body  2 . The exhaust openings  53  are provided on a front side (a right side in  FIG. 4 ) of the vehicle body  2  to be arranged in the right and left direction of the vehicle body  2 . 
   As shown in  FIG. 3 , the cylinder head  23  is formed with intake ports  54  that are in communication with the combustion chamber  44  through the respective intake openings  52 , and exhaust ports  55  that are in communication with the combustion chamber  44  through the respective exhaust openings  53 . As shown in  FIG. 4 , the intake ports  54  join together to communicate with the throttle body  16 . Also, the exhaust ports  55  join together to communicate with the exhaust pipe  17  (see  FIG. 2 ). 
   As shown in  FIG. 3 , the cylinder head  23  is provided with intake valves  56  that open and close the intake openings  52 , and exhaust valves  57  that open and close the exhaust openings  53 . The intake valves  56  and the exhaust valves  57 , respectively, are biased in directions in which the intake openings  52  and the exhaust openings  53  are closed. Also, the cylinder head  23  is provided with rocker arms  58 ,  59 , respectively, that periodically open and close the intake valves  56  and the exhaust valves  57 . However, avalve operating mechanism that opens and closes the intake valves  56  and the exhaust valves  57  is in no way limiting. 
   As shown in  FIG. 5 , the cylinder head  23  is provided with an ignition plug  63 . The ignition plug  63  includes a plug body  66 , a central electrode  64  provided at a tip end of the plug body  66 , and a lateral electrode  65 . The central electrode  64  and the lateral electrode  65  project toward the combustion chamber  44  from the recess  51  of the cylinder head  23 . 
   An annular-shaped water jacket  61  is arranged in the cylinder head  23 . The water jacket  61  is arranged in a position corresponding to the water jacket  34  in the cylinder  22  to substantially surround an entire periphery of the combustion chamber  44  as viewed in the cylinder axial direction. 
   As shown in  FIG. 7 , the lower surface  60  of the cylinder head  23  is formed with a plurality of openings  67  that are in communication with the water jacket  61 . The openings  67  are also provided in positions opposed to the water jacket  34  in the cylinder  22  and arranged circumferentially at intervals to surround the periphery of the combustion chamber  44  as viewed in the cylinder axial direction. 
   The cylinder head  23  is also provided with an inlet port  61   b  for introduction of cooling water and an outlet port  61   a  for discharge of the cooling water. An introduction passage  69   b  is formed in the cylinder head  23  through which the cooling water introduced from the inlet port  61   b  is conducted to the water jacket  61  and a return passage  69 a through which the cooling water in the water jacket  61  is conducted to the outlet port  61   a . In addition, while an illustration is omitted, a water pump is mounted to the cylinder head  23  and the cooling water fed from the water pump is introduced into the water jacket  61  through the inlet port  61   b  and the introduction passage  69   b.    
   As shown in  FIG. 5 , a gasket  62  is interposed between the cylinder head  23  and the cylinder  22 . As shown in  FIG. 8 , the gasket  62  is preferably formed with two introduction communication holes  68   b  and preferably a single return communication hole  68   a . The introduction communication holes  68   b  are provided in the vicinity of the introduction passage  69   b  (see  FIG. 7 ) of the cylinder head  23  to overlap the openings  67  positioned in the vicinity of the introduction passage  69   b  in the cylinder axial direction (a direction perpendicular to the plane of  FIG. 7 ). The return communication hole  68   a  is provided in the vicinity of the return passage  69   a  (see  FIG. 7 ) of the cylinder head  23  to overlap the openings  67  positioned in the vicinity of the return passage  69   a  in the cylinder axial direction. However, the introduction communication holes  68   b  and the return communication hole  68   a  are in no way limited in position, shape, and number. 
   Subsequently, flow of the cooling water will be described. The cooling water having been introduced from the inlet port  61   b  of the cylinder head  23  flows into the water jacket  61  through the introduction passage  69   b . Then some of the water in the water jacket  61  flows into the water jacket  34  in the cylinder  22  through the introduction communication holes  68   b  and the openings  67  corresponding thereto. The cooling water having flowed into the water jacket  34  flows through the water jacket  34  and then returns to the water jacket  61  of the cylinder head  23  through the return communication hole  68   a  and the opening  67  corresponding thereto. The water returned to the water jacket  61  from the water jacket  34  joins the water having flowed through the water jacket  61  in order to be discharged from the outlet port  61   a  through the return passage  69   a . In this manner, the cooling water flows through the water jacket  61  and the water jacket  34  whereby the cylinder head  23  and the cylinder  22  are cooled. 
   Subsequently, the water jacket  34  of the cylinder  22  will be described in detail. 
   A curve in  FIG. 9  represents the relationship between a crank angle and a degree in which burning in the combustion chamber  44  proceeds. In  FIG. 9 , an axis of abscissas represents a crank angle with a top dead center position of compression as a reference (crank angle=0) and an axis of ordinates represents a rate of combustion mass. In addition, the curve remains substantially the same even when an engine speed is varied. Here, the engine speed is in the order of about 3,000 rpm to about 5,000 rpm. 
   As seen from  FIG. 9 , burning rapidly proceeds when the crank angle is about −10° to about 30° and the rate of combustion mass becomes about 90% when the crank angle is about 30°. When the crank angle is over about 30° burning gently proceeds, and when the crank angle is about 60° burning is substantially wholly terminated. In this manner, burning in the combustion chamber  44  is mostly generated in a stage before the crank angle becomes about 30°. Therefore, an increase in heating value is caused before the crank angle becomes about 30°, and a decrease in heating value is caused after the crank angle becomes about 30°. Also, when the crank angle is over about 60°, the heating value is substantially decreased as compared with the stage before the crank angle becomes about 30°. 
   Since the combustion chamber  44  is varied in volume according to a position of the piston  40 , it is varied in volume according to the crank angle. Specifically, when the crank angle is about 0°, the combustion chamber  44  becomes a minimum in volume, thereafter increases in volume as the crank angle is increased, and becomes a maximum when the crank angle is about 180°. Here, combustion gas is varied in temperature as the combustion chamber  44  is varied in volume and is decreased as the combustion chamber  44  is increased in volume. Therefore, combustion gas lowers in temperature as the crank angle is increased. Accordingly, after burning is mostly terminated and combustion gas is rapidly lowered in temperature as the crank angle is increased. 
   As a result, the heating value of the cylinder inner surface  31  is greatly dispersed. More specifically, the upper portion of the cylinder inner surface  31  where the burning is mostly generated receives a large amount of heat, and the lower portion of the cylinder inner surface  31  where burning has been mostly terminated receives a small amount of heat. 
   Since conventional cylinders made of cast iron are low in thermal conductivity, the velocity at which heat is conducted in an interior of a cylinder (that is, the heat flux) is small. Therefore, a water jacket in a cylinder is formed relatively deep so as to cool the whole cylinder evenly. However, in the case where a cylinder is made of a material having a higher thermal conductivity than that of iron, the velocity at which heat is conducted in an interior of the cylinder is increased. Accordingly, the cylinder is rapidly cooled by cooling water in a water jacket, so that when the water jacket is formed too deep, there is a fear that the cylinder is cooled locally more than is needed. 
   When the cylinder is low in temperature, however, a lubricating oil present between a piston and the cylinder becomes low in temperature in some cases such that an increase in viscosity inhibits the lubricating property. Therefore, the friction of the piston becomes large so as to cause a fear of a decrease in output of the engine. 
   According to the present preferred embodiment, those portions of the cylinder  22  which need a high degree of cooling are specifically cooled while excessive cooling is avoided in those portions which need a low degree of cooling. The water jacket  34  is formed to be relatively shallow on the basis that the cylinder  22  should neither be cooled locally too much nor too little. Specifically, as shown in  FIG. 6 , the bottom wall  36  of the water jacket  34  is positioned above a lower end  47   b  of the oil ring  47  when the piston  40  is disposed in the top dead center position. According to the present preferred embodiment, a depth L 1  of the water jacket  34  is smaller than a distance L 2  between the opposed surface  33  of the cylinder  22  and the lower end  47   b  of the oil ring  47  when the piston  40  is disposed in the top dead center position. 
   In addition, the depth of the water jacket  34  is preferably larger than a distance between the opposed surface  33  of the cylinder  22  and an upper end  45   a  of the top ring  45  when the piston  40  is disposed in the top dead center position. That is, the bottom wall  36  of the water jacket  34  is preferably positioned between the upper end  45   a  of the top ring  45  when the piston  40  is disposed in the top dead center position and the lower end  47   b  of the oil ring  47 . 
   According to the present preferred embodiment, the bottom wall  36  of the water jacket  34  is positioned in the cylinder axial direction, between the upper end  45   a  of the top ring  45  when the crank angle is about 0° and the upper end  45   a  of the top ring  45  when the crank angle is about 60°. The reason why the upper end  45   a  of the top ring  45  is made a reference point is that the piston body  43  and the cylinder  22  contact each other through the piston rings  45  to  47 , and the upper end  45   a  of the top ring  45  is an uppermost portion among those portions (that is, the piston rings  45  to  47 ) through which heat is conducted to the cylinder  22  from the piston body  43 . 
   Also according to the present preferred embodiment, a distance between the opposed surface  33  of the cylinder  22  and the upper end  45   a  of the top ring  45  when the crank angle is about 60° is about 0.33 times a cylinder bore diameter D (see  FIG. 5 ) of the combustion chamber  44 . The depth L 1  of the water jacket  34  is at most about 0.33 times the diameter D of the combustion chamber  44 . While the diameter D is not especially limited in value, the diameter D according to the present preferred embodiment is about 50 mm to about 60 mm. 
   As described above, burning in the combustion chamber  44  is mostly generated before the crank angle is about 30°. The water jacket  34  may be made more shallow in order to position the bottom wall  36  between the upper end  45   a  of the top ring  45  when the crank angle is about 0° and the upper end  45   a  of the top ring  45  when the crank angle is about 30°. Here, a distance between the opposed surface  33  of the cylinder  22  and the upper end  45   a  of the top ring  45  when the crank angle is about 30° is about 0.09 times the diameter of the combustion chamber  44 . Hereupon, the depth LI of the water jacket  34  from the opposed surface  33  may preferably be made about 0.09 to about 0.33 times the diameter of the combustion chamber  44 . 
   The water jacket  34  is recessed from the opposed surface  33  substantially in the cylinder axial direction and is not especially limited in specific shape. According to the present preferred embodiment, as shown in  FIG. 6 , the bottom of the water jacket  34  preferably has an arcuate-shaped cross section but the bottom may be formed to be otherwise curved or polygonal-curved, for example. The cross section of the water jacket  34  may be substantially rectangular or substantially inverse-triangular or some other suitable shape. According to the present preferred embodiment, the water jacket  34  has a substantially constant transverse width in portions other than the bottom thereof, but the transverse width of the portions may be varied in the cylinder axial direction. According to the present preferred embodiment, the water jacket  34  has a substantially constant depth in a circumferential direction but the water jacket  34  may have different depths in different places. 
   With the engine  5  according to the present preferred embodiment, cooling can be carried out according to the material property of the cylinder  22  and the burning property in the combustion chamber  44 . That is, an upper portion of the cylinder  22  can be positively cooled by the cooling water in the water jacket  34 . On the other hand, other cooling can be avoided in a lower portion of the cylinder  22 . Therefore, it is possible to prevent a temperature increase in the cylinder  22  and to prevent a temperature drop in the lubricating oil in order to maintain a favorable lubricating property of the oil. Accordingly, it is possible to prevent increased friction of the piston  40  to achieve improved engine performance. 
   As described above, according to the present preferred embodiment, it is possible not only to simply cool the cylinder  22  but also to obtain a water jacket  34  that contributes to an improvement in the overall performance of the engine  5 . Accordingly, it is possible to increase the engine performance, and hence, the motorcycle performance. 
   In addition, the material or construction of the cylinder  22  is not limited to those in the preferred embodiment described above but is susceptible to various modifications. The cylinder  22  may be formed from an aluminum alloy with a plating layer provided on the cylinder inner surface  31 , or formed from an aluminum alloy, etc. with silicone deposited on the cylinder inner surface  31 , for example. 
   The cylinder  22  may include a cylinder body and a sleeve fitted into the cylinder body. The water jacket  34  is not limited to a so-called dry-type construction, in which the cooling water indirectly cools the sleeve through the cylinder body, but may have a wet-type construction, in which the cooling water directly cools the sleeve. For example, as shown in  FIG. 10 , a cylinder  22  may include a cylinder body  22   b  and a sleeve  22   a , and a portion of a water jacket  34  may be defined by an outer peripheral surface of the sleeve  22   a.    
   In this case, it is sufficient that at least the sleeve  22   a  be formed from a material having a higher thermal conductivity than that of iron and the cylinder body  22   b  may be formed from iron, or a material having a lower thermal conductivity than that of iron. Of course, the cylinder body  22   b  may be also formed from a material having a higher thermal conductivity than that of iron. In this manner, with the engine  5  according to preferred embodiments of the present invention, it is sufficient that at least a portion in the cylinder  22  between the cylinder inner surface  31  and the water jacket  34  be formed from a material having a higher thermal conductivity than that of iron. Even in such a preferred embodiment, the same effect as that of the previous preferred embodiments can be obtained. 
   Also, it is sufficient that a portion in the cylinder  22  between the cylinder inner surface  31  and the water jacket  34  be higher (for example, at least about 90 W/(m·K) at about 0° C.) in thermal conductivity than iron, or the cylinder may be formed from a material which is locally different in thermal conductivity. 
   In addition, the manufacturing method of the cylinder  22  is in no way limited. The cylinder  22  can be manufactured by a known manufacturing method or by a method or a material disclosed in, for example, JP-A-2002-180104, Pamphlet of International Publication WO2002/053899, or Pamphlet of International Publication WO2004/002658. Contents of these documents are incorporated by reference herein. 
   In the present preferred embodiments, the piston rings are preferably three in number, for example, but the piston rings are in no way limited in number. 
   The present invention is not limited to the preferred embodiments described above, but various variations and modifications may be made without departing from the scope of the present invention. The presently disclosed preferred embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalence thereof are intended to be embraced therein.