Patent Publication Number: US-6216647-B1

Title: Free piston internal combustion engine with piston head having non-metallic bearing surface

Description:
TECHNICAL FIELD 
     The present invention relates to free piston internal combustion engines, and, more particularly, to piston and cylinder configurations within such engines. 
     BACKGROUND ART 
     Free piston internal combustion engines include one or more pistons which are reciprocally disposed within corresponding combustion cylinders. Each piston is typically rigidly attached to a plunger rod which provides a desired work output. For example, the plunger rod may be used to provide electrical power output by inducing an electrical current, or fluid power output such as pneumatic or hydraulic power output. 
     Pistons used in free piston internal combustion engines typically include a piston head which is entirely constructed from a metallic material such as aluminum or steel. Metals such as aluminum and steel have a relatively high coefficient of thermal expansion. Thus, during operation of the free piston engine, the metallic piston head expands considerably in the radial direction toward the inside surface of the combustion cylinder. Each piston head used in the free piston engine is thus formed with an outside diameter which provides a considerable radial clearance with the inside surface of the combustion cylinder to accommodate the relatively large radial expansion during operation. To prevent blow-by of combustion products past the piston head during operation, the outside peripheral surface of the piston head is formed with one or more piston ring grooves which receive corresponding piston rings therein. The piston rings allow for radial thermal expansion and contraction of the piston head, while at the same time effectively preventing blow-by of combustion products past the piston head. 
     Although piston rings provide valuable functionality as indicated above, it would be desirable to eliminate the use of piston rings to reduce manufacturing and assembly costs. 
     Moreover, to prevent excessive wear between the piston rings and the inside surface of the combustion cylinder, it is necessary to lubricate the piston rings with a suitable lubricant. The lubrication system for lubricating the piston rings may require additional porting and/or other structure to effect proper lubrication, which in turn increases the size and complexity of the engine. Additionally, the lubricating oil may increase undesirable emissions from the engine. 
     Another problem with using conventional piston and cylinder arrangements including a metallic combustion cylinder and metallic piston head with piston rings is that suitable fluid cooling channels must be provided within the combustion cylinder to effect the proper cooling of the combustion cylinder and piston head. These cooling fluid channels again increase the size and complexity of the engine. 
     The present invention is directed to overcoming one or more of the problems as set forth above. 
     SUMMARY OF THE INVENTION 
     The present invention provides a free piston internal combustion engine with a piston head having a non-metallic radially outside bearing surface with low friction, low thermal expansion and high temperature resistance properties. 
     In one aspect of the invention, a free piston internal combustion engine includes a combustion cylinder having an inside surface. A piston is reciprocally disposed within the combustion cylinder. The piston includes a piston head and a plunger rod attached to the piston head. The piston head has a cylindrical outside surface lying closely adjacent to and defining a bearing surface with the inside surface of the combustion cylinder. The bearing surface consists essentially of a non-metallic material. 
     An advantage of the present invention is that the need for lubricating oil and cooling fluid in the free piston engine is eliminated, thereby eliminating the increased physical size and decreased efficiency losses associated with such structure. 
     Another advantage is that the radial clearance between the piston head and cylinder inside surface is substantially reduced or eliminated, thereby eliminating the need for piston ring grooves and piston rings in the piston head. 
     Yet another advantage is that the portion of the piston head defining the bearing surface is constructed from a material having low friction, low thermal expansion and high temperature resistance properties. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a simplified side, sectional view of a portion of a free piston internal combustion engine with an embodiment of a piston of the present invention disposed therein; 
     FIG. 2 is a side, sectional view of the piston shown in FIG. 1; 
     FIG. 3 is a side, sectional view of another embodiment of a piston of the present invention; 
     FIG. 4 is a side, sectional view of yet another embodiment of a piston head of the present invention; 
     FIG. 5 is a rear view of the piston head of FIG. 4; 
     FIG. 6 is a perspective view of an embodiment of a plunger which may be used with the piston head of FIGS. 4 and 5; and 
     FIG. 7 is a side, sectional view of still another embodiment of a piston of the present invention. 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a simplified side, sectional view of an embodiment of a portion of a free piston internal combustion engine  10  including an embodiment of a piston  12  of the present invention, shown in more detail in FIG.  2 . 
     Free piston engine  10  includes a combustion cylinder  14  with a combustion air inlet  16  and an exhaust outlet  18 . In this embodiment, combustion cylinder  14  has a substantially circular cross section. Combustion air is transported through combustion air inlet  16  and an air scavenging channel into combustion chamber  22  when piston  12  is at or near a bottom dead center (BDC) position. An appropriate fuel, such as a selected grade of diesel fuel, is injected into combustion chamber  22  as piston  12  moves toward a top dead center (TDC) position using a controllable fuel injector system, schematically shown and referenced as  24 . The stroke length (S) of piston  12  between the BDC position and TDC position may be fixed or variable. 
     Referring now to FIG. 2, piston  12  is shown in greater detail. Piston  12  is reciprocally disposed within combustion cylinder  14  and generally includes a piston head  26  which is attached to a plunger rod  28  using a mounting flange  30 . 
     Piston head  26  includes a non-metallic cap  32  which is connected to a metallic skirt  34 . Cap  32  has a cylindrical outside surface  36  with a diameter which is larger than the outside diameter of skirt  34 . Outside surface  36  lies closely adjacent to and defines a bearing surface with an inside surface  38  (FIG. 1) of combustion cylinder  14 . In the embodiment shown, outside surface  36  of cap  32  and inside surface  38  of combustion cylinder  14  have a radial operating clearance therebetween of between approximately 0.000 and 0.002 inch, preferably between approximately 0.000 and 0.001 inch, and more preferably approximately 0.000 inch. The term “radial operating clearance”, as used herein, means the radial clearance between outside surface  36  of cap  32  and inside surface  38  of combustion cylinder  14  when free piston engine  10  is under operating conditions. That is, the radial operating clearance is designed to be within the range as set forth herein when piston  12  and combustion cylinder  14  are at an operating temperature. 
     As shown in FIG. 2, outside surface  36  of cap  32  does not include any piston ring grooves therein, and accordingly does not carry any piston rings. To prevent excessive blow-by of exhaust products during the return stroke of piston  12 , and to prevent excessive wear between outside surface  36  of cap  32  and inside surface  38  of combustion cylinder  14 , cap  32  is formed from a material having selected physical properties. More particularly, cap  32  is formed from a non-metallic material having a relatively low coefficient of thermal expansion, low coefficient of friction and high temperature resistance. Examples of such non-metallic materials which have been found to be suitable include composite materials and ceramic materials. In the embodiment shown, cap  32  is formed from a carbon-carbon composite material having carbon reinforcing fibers within a carbon matrix. The carbon matrix may include carbon powder within a suitable resin. The carbon reinforcing fibers may be randomly oriented chopped fibers or may be longer filaments which are either randomly oriented or oriented in one or more directions. 
     The non-metallic material from which cap  32  is constructed preferably has a coefficient of thermal expansion of between approximately 0.5 and 10 ppm/° C. In the embodiment shown, the carbon-carbon composite material from which cap  32  is constructed has a coefficient of thermal expansion of between approximately 1 and 2 ppm/° C. Moreover, the non-metallic material from which cap  32  is constructed preferably has a coefficient of friction of between 0.01 and 0.15. In the embodiment shown, the carbon—carbon composite material from which cap  32  is constructed has a coefficient of friction of approximately 0.10. Additionally, the non-metallic material from which cap  32  is constructed preferably has a temperature resistance up to an average temperature of between approximately 400° C. and 2500° C. In the embodiment shown, the carbon-carbon composite material from which cap  32  is constructed has a temperature resistance up to an average temperature of approximately 500° C. The temperature resistance may be increased by applying a coating on the face of cap  32  adjacent combustion chamber  22 . 
     Skirt  34  is formed from a suitable metallic material, such as aluminum or steel. In the embodiment shown, skirt  34  is formed from aluminum. Since the coefficient of thermal expansion of metallic skirt  34  is larger than the coefficient of thermal expansion of cap  32 , the outside diameter of skirt  34  when at a non-operating temperature is small enough so that the outside diameter of skirt  34  does not exceed the outside diameter of outside surface  36  when at an operating temperature. That is, skirt  34  is not intended to be a primary bearing surface with inside surface  38  of combustion cylinder  14 . of course, some intermittent contact may occur between the outside diameter of skirt  34  and inside surface of  38 ; however, skirt  34  is not intended to be a primary bearing surface. 
     Cap  32  and skirt  34  are connected together such that cap  32  may move a limited extent in a radial direction relative to skirt  34 . More particularly, cap  32  includes a stepped inner surface  40  with a diameter which is larger than an outside diameter of a shoulder  42  of skirt  34 . In the embodiment shown, a radial clearance of between approximately 0.001 and 0.003 inch, and more preferably approximately 0.002 inch is formed between inner surface  40  and shoulder  42 . A plurality of radially extending holes  44  (four holes in the embodiment shown) receive respective set screws  46  therein which are threadingly engaged with shoulder  42 . The inside diameter of each hole  44  is larger than the outside diameter of a corresponding set screws  46  so that set screws  46  retain cap  32  to skirt  34  while at the same allowing relative movement therebetween. Plunger rod  32  may be carried by a pair is of bearings along the axial length thereof which do not perfectly align with the longitudinal axis of combustion cylinder  14  because of manufacturing tolerances, etc. By allowing cap  32  to move in a radial direction relative to skirt  34 , lateral loads on plunger rod  28  during reciprocation within free piston engine  10  are reduced or eliminated. 
     Combustion cylinder  14 , in the embodiment shown, includes a liner  52  which defines inside surface  38 . Liner  52  is formed from a non-metallic material having physical properties which are similar to the non-metallic material from which cap  32  is formed, as described above. In the embodiment shown, liner  52  is also formed from a carbon-carbon composite material with physical properties which are substantially the same as the carbon-carbon composite material from which cap  32  is formed. Since the carbon—carbon composite material from which each of outside surface  36  and inside surface  38  are formed has a relatively low coefficient of friction, wear between outside surface  36  and inside surface  38  is minimized. Moreover, since the carbon-carbon composite material from which each of outside surface  36  and inside surface  38  are formed has a relatively low coefficient of thermal expansion, the radial operating clearance therebetween can be maintained at a minimum distance (e.g., 0.000 inch), thereby preventing blow-by of combustion products during operation. 
     To assemble piston  12 , bolt  48  is passed through mounting flange  30  and screwed into an end of plunger rod  28 . Mounting flange  30  is then placed within metal skirt  34  and a plurality of bolts  50  are used to attach skirt  34  with mounting flange  30 . Cap  32  is then placed over the end of skirt  34  and the plurality of set screws  46  are passed through the corresponding holes  44  in cap  32  and screwed into shoulder  42  of skirt  34 . Piston  12  may then be installed within free piston engine  10 . 
     Referring now to FIG. 3, there is shown a side, sectional view of another embodiment of a piston  56  of the present invention, including a piston head  58  and plunger rod  60 . Piston head  58  is formed entirely from a carbon—carbon composite material having carbon reinforcing fibers  62  which are oriented within piston head  58  generally as shown to provide strength to piston head  58  upon axial loading in either direction by plunger rod  60 . Piston head  58  includes a hub  64  with an opening  66  having an inside diameter which is larger than an outside diameter of plunger rod  60  to thereby provide a desired radial operating clearance therebetween. A snap ring  68  attaches hub  64  to plunger rod  60 , while at the same time allowing relative radial movement therebetween. 
     Referring now to FIGS. 4-6, there is shown yet another embodiment of a piston including a piston head  72  (FIGS. 4 and 5) which is attached with a plunger rod  74  (FIG.  6 ). Piston head  72 , in the embodiment shown, is constructed entirely from a carbon—carbon composite material. A pair of locking flanges  76  project radially inwardly from opposite sides of skirt  78 . A hub  80  attached to plunger rod  74  is placed against a rear face  82  of piston head  72 . Plunger rod  74  is rotated so that ears  84  projecting radially outwardly from hub  82  are disposed between locking flanges  76  and rear face  82 . A set screw or bolt  86  passes through a hole  88  in hub  80  and is threadingly engaged with a hole  90  in rear face  82  of piston head  72 . The radial clearance between the inside diameter of hole  88  and the shaft of bolt  86 , as well as the radial operating clearance between ears  84  and the inside diameter of skirt  78 , allow relative radial movement between plunger rod  74  and piston head  72  during operation. 
     Referring now to FIG. 7, there is shown a side, sectional view of still another embodiment of a piston  94  of the present invention, including a piston head  96  and a plunger rod  98 . Piston head  96  is formed from a carbon—carbon composite material with physical properties as described above. Metallic plunger rod  98  is attached to a metallic mounting hub  100  using a bolt  102 . A plurality of bolts or pins  104  which extend radially through skirt  106  of piston head  98  interconnect piston head  96  with plunger rod  98  while at the same time allowing relative movement therebetween within a desired range, dependent upon the specific application. 
     In the embodiments shown in the drawings and described above, piston heads  26 ,  58 ,  72  and  96  each include a generally flat face on the side facing combustion chamber  22 . However, it is to be appreciated that the shape of the face disposed adjacent to combustion chamber  22  may vary, dependent upon the specific application. 
     Moreover, in the embodiments shown in the drawings, piston heads  26 ,  58 ,  72  and  96  have a cylindrical cross-sectional shape. However, piston heads  26 ,  58 ,  72  and  96  and combustion cylinder  14  may have any desired cross-sectional shape such as oval, rectangular, square, star, etc. 
     INDUSTRIAL APPLICABILITY 
     During use, the selected piston  12 ,  56 ,  72  and  74 , or  94  is reciprocally disposed within combustion cylinder  14 . The selected piston travels between a BDC position and a TDC position during a compression stroke, and between a TDC position and BDC position during a return stroke. Combustion air is introduced into combustion chamber  22  through combustion inlet  16  and air scavenging channel  20 . Fuel is controllably injected into combustion chamber  22  using a fuel injector  24 . The non-metallic, carbon—carbon bearing surfaces defined by the outside bearing surface of the piston head and inside surface  38  of combustion cylinder  14  allow the piston to be used within combustion cylinder  22  without the use of piston ring grooves or piston rings. 
     The portion of the piston head defining the bearing surface is constructed from a material having low friction, low thermal expansion and high temperature resistance properties. The need for lubricating oil and cooling fluid in the free piston engine is thus eliminated, thereby eliminating the increased physical size and decreased efficiency losses associated with such structure. Additionally, the radial clearance between the piston head and cylinder inside surface is substantially reduced or eliminated, thereby eliminating the need for piston ring grooves and piston rings in the piston head. 
     Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.