Abstract:
In ported engine constructions, cooling of piston crowns and cylinder liners results in reduction or elimination of bore/liner distortions, thus ensuring circularity of the bore/piston interface throughout engine operation. Consequently, the need for heavily-tensioned piston rings is eliminated. Such engine constructions incorporate annular low-tension compression seals on the pistons, which substantially reduce port bridge wear during all phases of engine operation while also limiting blow-by during combustion.

Description:
RELATED APPLICATIONS 
       [0001]    The following patents and patent applications, all commonly assigned to the assignee of this application, contain subject matter related to the subject matter of this application: 
         [0002]    U.S. Pat. No. 7,156,056, issued Jan. 2, 2007 for “Two Cycle, Opposed Piston Internal Combustion Engine”; 
         [0003]    WO/2005/124124, published on Dec. 29, 2005 for “Improved Two Cycle, Opposed Piston Internal Combustion Engine”; 
         [0004]    U.S. Pat. No. 7,270,108, issued Sep. 18, 2007 for “Opposed Piston, Homogeneous Charge Pilot Ignition Engine”; 
         [0005]    WO/2006/105390 published on Oct. 5, 2006 for “Opposed Piston, Homogeneous Charge, Pilot Ignition Engine”; 
         [0006]    U.S. Pat. No. 7,334,570, issued Feb. 26, 2008 for “Common Rail Fuel Injection System With Accumulator Injectors”; 
         [0007]    WO/2006/107892 published on Oct. 12, 2006 for “Common Rail Fuel Injection System With Accumulator Injectors”; 
         [0008]    U.S. Pat. No. 7,360,511, issued Apr. 22, 2008 for “Opposed Piston Engine”; 
         [0009]    WO 2007/109122 published on Sep. 27, 2007 for “Opposed Piston Engine”; 
         [0010]    U.S. Pat. No. 7,546,819 issued Jun. 16, 2009 for “Two Stroke, Opposed-Piston Internal Combustion Engine”; 
         [0011]    U.S. Pat. No. 7,549,401 issued Jun. 23, 3009 for “Two-Cycle, Opposed-Piston Internal Combustion Engine”; 
         [0012]    U.S. patent application Ser. No. 11/642,140, filed Dec. 20, 2006, for “Two Cycle, Opposed Piston Internal Combustion Engine”; 
         [0013]    U.S. patent application Ser. No. 11/725,014, filed Mar. 16, 2007, for “Opposed Piston Internal Combustion Engine With Hypocycloidal Drive and Generator Apparatus”, published as US/2007/0215093 on Sep. 20, 2007; 
         [0014]    U.S. Pat. No. 7,591,235 issued Sep. 22, 2009 for “Opposed Piston Engine With Piston Compliance”; 
         [0015]    U.S. patent application Ser. No. 12/075,557, filed Mar. 12, 2008, for “Internal Combustion Engine With Provision for Lubricating Pistons”; 
         [0016]    U.S. patent application Ser. No. 12/456,735, filed Jun. 22, 2009, for “Two-Cycle Opposed-Piston, Internal Combustion Engine”; and, 
         [0017]    U.S. patent application Ser. No. 12/586,352, filed Sep. 21, 2009, for “Opposed-Piston Engine”. 
     
    
     BACKGROUND 
       [0018]    The technical field relates to a ported internal combustion engine. More specifically the technical field relates to a ported internal combustion engine that incorporates low-tension compression seals to achieve high BMEP (brake mean effective pressure) operation. The technical field also relates to an opposed-piston, compression ignition engine in which low-tension compression seals are mounted to the opposed pistons so as to minimize port bridge wear during all phases of engine operation while also limiting blow-by during combustion. 
         [0019]    A ported internal combustion engine is an internal combustion engine having a cylinder with one or more ports formed therein for the passage of air into and/or out of the bore. For example, the cylinder of a traditional opposed-piston engine includes exhaust and inlet ports cast or machined into the cylinder near respective exhaust and inlet ends of the cylinder liner. Pistons disposed crown-to-crown in the liner&#39;s bore traverse the ports while moving through respective bottom dead center (BDC) positions. Rings are mounted to the pistons to maintain a seal between the pistons and the liner bore, which reduces the passage of combustion gasses between the pistons and the bore (blow-by). The rings are heavily tensioned against the bore to accommodate bore/liner distortion caused by thermal and mechanical stresses. Each piston and its rings traverse a respective port twice during every complete engine cycle. The heavy tension forces the outer surfaces of the rings into a high frictional engagement with the bore and with the port bridges, especially where the rings contact the edges of the port openings. As a consequence, repeated transits by the rings over the ports result in excessive and uneven port bridge wear, and, ultimately, early ring failures. The exhaust piston rings suffer particularly heavy damage due to the high temperatures encountered at the exhaust port. 
         [0020]    As a result of low durability due to bridge wear, traditional ported engines have had very limited acceptance in the markets for land, air, and marine engines. Measures have been proposed to reduce the complex frictional interface between the piston rings and the port bridges. One such step includes excessive lubrication of the piston/bore interface. However, oil consumption in these cases is typically about 2% of fuel consumption, as compared to portless engines in which oil consumption is typically about 0.1% of fuel consumption. Such high oil consumption is not acceptable under modern emission standards. Other measures include rounding and/or ramping the outer edges of the rings, beveling the edges of the port openings, and customizing the shapes of the port openings. However, these solutions add to manufacturing costs and will continue to have only limited effectiveness so long as the rings are heavily tensioned. 
         [0021]    Ported engine constructions have been proposed which incorporate pistons with axially symmetrical construction and coolant structures for cooling pistons and cylinder liners that reduce or eliminate bore/liner distortions throughout engine operation. Because these cooling designs maintain circularity of the bore/piston interface longitudinally of the cylinder throughout engine operation, they eliminate the need for heavily-tensioned rings. An example of such a design in an opposed piston engine construction is found in commonly-owned U.S. Pat. No. 7,360,511, issued Apr. 22, 2008. Accordingly, we have realized that ported internal combustion engines in which circularity of the bore/piston interface is maintained during all phases of engine operation are well-suited for low-tension piston compression seals which substantially reduce wear on port bridges while also limiting blow-by. 
       SUMMARY 
       [0022]    Low-tension compression seals provide an effective seal between the cylinder bore and piston thereby maintaining compression and preventing blow-by, while reducing or eliminating the problem of frictional interaction with the port bridges. 
         [0023]    These objects are achieved by a compression seal device in a ported internal combustion engine in which circularity is maintained between the bore of a ported cylinder and an axially symmetrical piston having a compression seal mounted in an annular groove. The compression seal has an annular bearing surface to maintain a sealing annular contact with the bore, with no clearance between the bearing surface and the bore, in response to a residual low level of compression seal tension in the direction of the bore when the piston is near a bottom dead center (BDC) position. The bearing surface maintains a sealing annular contact with the bore, with no clearance between the bearing surface and the bore, in response to a high level of compression seal tension in the direction of the bore resulting from pressure of combustion acting against an inner peripheral surface of the compression seal when the piston is near a top dead center (TDC) position. 
         [0024]    In a ported engine, a low-tension, essentially circular compression seal is compressed slightly when mounted to a piston so as to permit the piston to be received in the bore of a ported cylinder where the compression seal is very lightly loaded against the bore at an initial low level of tension such that there is no clearance between the bearing surface of the compression seal and the bore. During combustion, when the piston has moved through a top dead center (TDC) position the high-pressure compression gasses act upon the inner peripheral surface of the compression seal, which presses the bearing surface of the seal more tightly against the cylinder bore at a higher level of tension, thereby firmly sealing the space between the bore and the piston and preventing blow-by. As the piston approaches a port and the compression pressure approaches ambient pressure, the compression seal relaxes to its low tension mode, thereby substantially reducing friction between itself and the port bridges as the port is traversed. 
         [0025]    A manufacturing process yields annular compression seals which exhibit very low tension when mounted to a piston in a ported cylinder. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1A  is a plan view of a low-tension compression seal in an uncompressed condition;  FIG. 1B  is a plan view of the low-tension compression seal in a compressed condition;  FIG. 1C  is a side elevation view of the low-tension compression seal in the uncompressed state; and  FIG. 1D  is a side elevation view of the low-tension compression seal in the compressed state. 
           [0027]      FIG. 2  is a side sectional view of an inlet side of a ported cylinder showing a bore with a piston disposed therein;  FIG. 2A  is an enlarged view of a portion of the piston showing details of a compression seal mounted thereto. 
           [0028]      FIG. 3  is a partial schematic illustration of opposed pistons near top dead center when combustion occurs. 
           [0029]      FIG. 4A  is a perspective view of a piston crown having a circumferential groove for seating a low-tension compression seal.  FIGS. 4B ,  4 C, and  4 D are side sectional views of the piston crown of  FIG. 4A  showing three arrangements for mounting low-tension compression seals.  FIG. 4E  is s perspective view of a low-tension compression seal with a “Z-shaped” gap. 
           [0030]      FIG. 5  is a schematic view of an optional piston squish zone to enhance operation of the low-tension compression seal. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    A low tension compression seal for use on a piston in a ported internal combustion engine is illustrated in one or more of the above-described drawings, and is disclosed in detail in the following description. Preferably, a “low tension compression seal” is an annular device which, when mounted to a piston received in the bore of a ported engine, is loaded against the bore by a tension of 3 Newtons, or less, such that there is no clearance between the bearing surface of the compression seal and the bore when the engine is not operating; more desirably still, the low tension compression seal is loaded against the bore by a tension of nominally 0 Newtons such that there is no clearance between the bearing surface of the compression seal and the bore when the engine is not operating. 
         [0032]    A low-tension compression seal (hereinafter, “compression seal”) is shown in  FIG. 1A , where the view is toward a side surface of the compression seal and  FIG. 1C , where the view is toward an annular outer peripheral surface (hereinafter, a “bearing surface”) of the compression seal. In these figures, the compression seal  100 , which has a body  110  with a generally circular or annular shape, is shown in a “free” state where it is has not been compressed diametrically and is not subject to combustion pressures. A gap  113  opens between a bearing surface  114  and an annular inner peripheral surface  115 . An alignment notch  116  is cut into the inner peripheral surface  115 . Preferably, the alignment notch  116  is positioned at the inside end of the gap  113  in order to align the gap with a bridge of an inlet or exhaust port of a ported internal combustion engine in which the compression seal is to be mounted. 
         [0033]    As per  FIGS. 1B and 1D , when the compression seal  100  is diametrically compressed as it would be when mounted to a piston received in the bore of a cylinder, the bearing and inner peripheral surfaces define concentric circular shapes. An outside diameter  117  (D o ) is measured with respect to an edge of the bearing surface  114 . An inside diameter  118  (D i ) is measured with respect to an edge of the inner peripheral surface  115 . As seen in the elevation views of  FIGS. 1C and 1D , the compression seal body  110  has opposing side surfaces  119  and  120 . Preferably, the side surfaces  119  and  120  are annular, flat and parallel to a horizontal surface, and essentially perpendicular to the bearing and inner peripheral surfaces  114  and  115 . As per  FIGS. 1B and 1D , the gap  113  in the compression seal  100  remains slightly open when the bearing surface  114  is seated against the bore; for example, the gap  113  may be open by about 0.33 mm. This slight opening enables the compression seal to undergo thermal expansion during engine operation. The annular compression seal  100  is preferably constituted of conventional materials, such as steel or cast iron with an admixture of carbon and one or more alloys, and may be conventionally coated or plated, as required by any particular application. 
         [0034]      FIG. 2  illustrates a piston and cylinder assembly of a ported internal combustion engine in which a low tension compression seal  100  is mounted to a piston. Preferably, but not necessarily, the engine has an opposed-piston construction and operates with a two-stroke cycle. Further, in order to simplify the description, only one compression seal is shown, although this should not limit the application of the principles illustrated, as two or more compression seals can be mounted to the piston.  FIG. 2  shows the inlet side of an opposed-piton cylinder assembly, with the understanding that the explanation applies to the exhaust side of the assembly in the same manner. In this regard, see, for example, FIG. 4A of US publication 2006/0157003 A1. In  FIG. 2 , the opposed piston engine includes a cylinder  210  with a piston  219  having a crown  220  and a skirt  230 . The skirt  230  may be mounted to the crown  220  in a compound piston structure, or formed as one piece with the crown, in a unitary piston structure. The cylinder  210  has an inlet port  222  cast or machined into the cylinder near one end thereof. The inlet port  222  is constituted of an annular sequence of openings  223  alternating with bridges  224  between the openings  223 . The openings  223  permit the passage of pressurized air from an inlet manifold  225  into the bore  226  of the cylinder  210 . The bore  226  is defined by the inside cylindrical surface  227  of the cylinder  210 . The bore  226  has a diameter  228 . In  FIG. 2 , the crown  220  is near a bottom dead center (BDC) position. During engine operation, as the piston  219  reciprocates in the bore  226 , the crown  220  moves between top dead center (TDC) and BDC positions, traveling back and forth across bridges  224  of the inlet port  222  during each cycle of operation. The front surfaces of the piston crowns may be contoured as seen in  FIG. 2 , or may be planar. 
         [0035]    As per  FIG. 2 , each of the bore and piston has an axially symmetrical construction, and it is desirable that the axial symmetries of the cylinder bore  226  and the piston crown  220  be maintained substantially throughout all phases of the operational cycle of the engine. With this condition met, circularity of the piston/bore interface is ensured, thereby permitting the use of low-tension compression seals in a ported internal combustion engine. Preferably, but not necessarily, respective cooling structures are tailored to direct liquid coolant on the cylinder liner and the back surface of the piston crown in such a manner as to achieve this condition. For example, liquid coolant directed through cylinder coolant channels  229  on the outer surface of the cylinder liner and a radially symmetrical piston cooling structure  231  on the back surface of the piston crown, both taught in cross-referenced U.S. Pat. No. 7,360,511 and U.S. Pat. No. 7,549,401 (incorporated herein by this reference), maintain the axial symmetries of the bore  226  and the crown  220 . Together, these cooling structures direct liquid coolant so as to maintain circularity of the cylinder bore/piston interface, longitudinally of the cylinder, during all phases of engine operation, thereby eliminating the need for high-tension piston rings. 
         [0036]    Refer now to  FIG. 2A , an enlarged view of the circled portion seen in  FIG. 2 , in which the spacing between the outer side surface of the piston  219  and the bore of the cylinder  210  is exaggerated for a clearer understanding of the following description. As seen in  FIG. 2A , a circumferential groove  235  is provided in the outer side surface of the piston  219 , preferably in or near the crown  220 . For example, although not necessarily, the groove may be constituted as a gap defined in a compound piston structure when the crown  220  is joined to the upper end of the skirt  230  at  236  (by threading, for example). Alternatively, the groove may be machined or cast into a unitary piston structure. In either case, the compression seal  100  is mounted in the groove  235  in the piston  219 . The floor  237  of the groove  235  defines a groove diameter. The groove  235  has opposing walls  239  and  240 , with the wall  239  being relatively nearer to the crown  220  than the wall  240 . 
         [0037]    With reference to  FIGS. 1A and 1C , when the compression seal  100  is in its free state, the outside diameter of the compression seal is very slightly larger than the diameter of the bore  226 . Thus, as per  FIGS. 1B and 1D , when the compression seal  100  is mounted in the groove  235  and compressed slightly in order for the piston to be inserted into the bore  226 , the outside diameter of the compression seal  100  is equal to the diameter of the bore  226 , in which case the annular bearing surface of the compression seal  100  is very lightly loaded against the bore so that there is unbreached low-tension contact between the bearing surface and the bore. In other words, in the compressed state, there is a residual low level of compression seal tension in the direction of the bore and there is no clearance between the annular bearing surface and the bore. There is a slight opening in the bearing surface where the gap is located; however, this opening becomes negligible when the compression seal expands during engine operation in response to the heat of combustion. The inside diameter of the compression seal  100  is slightly greater than the diameter of the groove  235  shown in  FIG. 2A , and the thickness of the compression seal is slightly less than the width of the groove. 
         [0038]    Operation of a ported internal combustion engine with one or more low-tension compression seals will now be described using an opposed piston engine as an illustrative example. With reference to  FIG. 3 , before operation of the ported internal combustion engine commences, each compression seal  100  is loaded by a residual low level of tension in the direction of the bore  226 , which urges the bearing surface of the compression seal into engagement against the bore, with no clearance between the bearing surface and the bore. When engine operation commences, as the pistons  219   a  and  219   b  move toward their TDC positions, air is compressed between the crowns  220 . Combustion occurs in the space  310  when fuel injected into the compressed air ignites as the pistons move through their TDC positions. The pressure (P) produced by combustion acts against the crowns  220 , propelling the pistons toward their BDC positions. Some of the combustion gas pressure leaks behind each compression seal  100  and acts against the inner peripheral surface of each compression seal to increase the level of tension in the direction of the bore  226 , which urges the bearing surface of the compression seal into a highly-tensioned engagement against the bore, thereby effecting a tight seal that prevents blow-by. After combustion, as the pistons move away from each other and toward their BDC positions, the combustion pressure P continually drops until it reaches an ambient level. As the pressure P drops, the force acting against each compression seal falls until the bearing surface is once again loaded against the bore only by the residual low level of compression seal tension in the direction of the bore with no clearance between the annular bearing surface and the bore as the piston transits the port bridges. 
         [0039]      FIGS. 4A-4D  illustrate a crown  400  such as can be incorporated into the construction of each of a pair of opposed pistons. The piston construction may include assembly of the crown to a skirt as taught in cross-referenced U.S. Pat. No. 7,549,401 and U.S. Pat. No. 7,360,511, for example. Alternately, the crown may be an inseparable element of a piston with a unitary construction. The crown preferably is constituted of a durable material such as steel, cast iron, aluminum, or aluminum hybrid, as required by any particular application. For example, we have used crowns made of 4130 alloy steel. 
         [0040]    As per  FIGS. 4B-4D , the crown  400  includes a front surface  402  which faces the combustion space and a back surface  404 . As per  FIG. 4A , the construction of the crown includes a coolant structure for radially symmetrical impingement cooling of the back surface  404 . Such cooling maintains axial symmetry of the crown  400  during all phases of engine operation. In this regard, liquid coolant is conducted in a tubular piston rod (as seen in  FIG. 2 , for example) to a central point  405  from which streams of liquid coolant flow through a coolant structure constituted of an radially-symmetric array of channels  406  centered on the longitudinal axis of the crown to the interior surface of the skirt (not shown). The crown includes at least one circumferential groove  410  for mounting a low-tension compression seal. 
         [0041]    As per  FIGS. 4B ,  4 C, and  4 D, low-tension compression seals  100  are mounted to the crown  400  as a pair in a single circumferential groove  410  in the side surface of the crown ( FIG. 4B ), as a pair mounted in separate grooves  410  ( FIG. 4C ), or singly in a single groove  410  ( FIG. 4D ). If mounted pair-wise, the low tension compression seals  100  of a pair are mutually rotated so as to maintain their gaps  113  out of alignment. Further, if necessitated by design or performance requirements, one or more alignment protuberances (not seen) are formed in the floor of a groove  410  wherein a low tension compression seal  100  is mounted. Each protuberance is shaped to fit into an alignment notch  116  on the inside surface  115  (seen in  FIG. 1A ) of a compression seal. A groove  410  formed to receive a pair of compression seals may have one protuberance, in which case, the gap and alignment notch of at least one compression seal would be mutually offset. Any protuberance formed in a groove is located on the side of the crown so as to prevent a compression seal it retains from rotating in the groove and maintain the gap of the compression seal in alignment with a bridge of the port which the compression seal traverses. Such alignment prevents the gap from being damaged and/or worn by interacting with edges of port openings. 
         [0042]      FIG. 4E  illustrates a low-tension compression seal  450  with a “Z-shaped” gap  453  in which a top surface portion  455  of the gap overlays a bottom surface portion  456  of the gap, thereby eliminating a path for combustion gasses to escape through gap  453 . 
         [0043]    An optional opposed-piston configuration including low-tension compression seals is illustrated in  FIG. 5 . The crowns of a pair of opposed pistons disposed in the bore of a cylinder liner are shown at or near TDC, when fuel is injected between the crowns in order to initiate combustion. Two low-tension compression seals  100  are mounted to each crown. As illustrated, the front surface of each crown has a concave contour  510 , preferably, although not necessarily, in the shape of a symmetrical hemispherical bowl. At the end of a compression stroke, the opposing concave contours  510  define a section of a spherical space  512  into which a fresh charge of air has been compressed, and into which fuel is injected to initiate combustion. The flat circular peripheries  515  of the crowns are in close proximity at TDC and form an annular squish zone around the quasi-spherical space. An opening  517  through the squish zone is formed by opposing notches in the crown peripheries; the opening  517  is aligned with an injector nozzle  520  so that fuel can be injected into the quasi-spherical space  512 . When the front surfaces of the pistons are this closely spaced, little or no injected fuel is able to pass through the squish zone onto the cylinder bore. One potential benefit realized thereby is reduction or elimination of unburned fuel on the bore surface near TDC that could otherwise reduce the effectiveness of the low-tension compression seals at and immediately following combustion. 
         [0044]    Manufacturing application: We have manufactured low-tension compression seals exhibiting an estimated residual low level of tension in the direction of the bore as low as three (3) Newtons when compressed by a diametrically-applied force sufficient to reduce the gap in a seal as would occur in a cylinder bore of 80 millimeters (mm); desirably, the gap is reduced to about 0.3 mm. The starting material was a tube of 440A stainless steel 83 mm in diameter. The tube was heated to 1800° F. and maintained at that temperature for four (4) hours, oil cleansed, and then tempered at 600° F. for four (4) hours, and again oil cleansed. The interior and exterior surfaces of the tube were then turn finished to an outer diameter of 80.4 mm and an inner diameter of 73.9 mm. A low-tension compression seal was manufactured by cutting an annular piece with a thickness of 1 mm from the finished tube with a numerically-controlled mill. The opposing side surfaces of the annular piece were lapped flat and the inside and bearing surfaces were deburred using a hand tool. Per ISO 6621-4, 8.1, a half moon-shaped alignment notch was formed in the inner surface using a 2.38 mm end mill at 73.85 mm diameter. The alignment notch was then located in the mill and the annular piece was split to form the gap. The split was made with a saw and the resulting gap was de-burred. (We formed Z-shaped gaps by use of an Electrical Discharge Machine (EDM) with a “Z” shaped wire with which the overlapping notch was cut). The annular piece was then mounted on a mandrel and the bearing surface was lapped with an 80 mm diameter round lapping tool until it was “light tight” per ISO 6621-4, 7.2. We noted that oscillating the mandrel during lapping would impose a slight barrel shape on the bearing surface. Once light tightness was achieved, a nitride layer was applied to the annular piece per ISO 6621-4, 10.3.2, NT070. Finally, a chromium nitride layer was deposited on the bearing surface. 
         [0045]    Referring now to  FIGS. 1A and 2 , when manufactured as disclosed, the outside diameter of low-tension compression seals (D o ) for an 80 mm cylinder bore was typically 80.4 mm+0.05, −0.00 mm, and the inside diameter (D i ) of the cylinder bore was measured at 80.0 mm+/−0.0075 mm. The typical thickness of the low-tension compression seals, measured between the opposing side surfaces, was 1.0 mm. When such a low-tension compression seal was mounted to a piston disposed in the 80 mm diameter bore, the annular interface between the annular bearing surface and the bore was light tight, indicating that no clearance occurred between the annular bearing surface and the bore. 
         [0046]    The scope of patent protection afforded the novel articles and methods described and illustrated herein may suitably comprise, consist of, or consist essentially of the low-tension compression seal, piston, and ported cylinder. Further, the novel articles and methods disclosed and illustrated herein may suitably be practiced in the absence of any element or step which is not specifically disclosed in the specification, illustrated in the drawings, and/or exemplified in the embodiments of this specification. Moreover, although one or more inventions are described with reference to preferred embodiments, it should be understood that various modifications can be made without departing from the spirit of the invention. Any invention described herein is limited only by the following claims.