Abstract:
A combustion chamber in an opposed-piston, internal-combustion engine is disclosed in which the pistons tops are designed so that when they approach each other, they induce a tumble flow in one or two hemispherical spaces defined in the piston tops. The combustion chamber further includes injectors side mounted in the cylinder wall. In one embodiment, the tumble flows in the two hemispheres are in the same direction and in another embodiment, in opposite directions. In yet another embodiment, there is only one injector and one hemisphere in which a tumble flow is induced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit from U.S. provisional patent applications 61/511,583 filed 26 Jul. 2011 and 61/523,360 filed 14 Aug. 2011. 
    
    
     FIELD 
     The present disclosure relates to shape of the combustion chamber and injector orientation in internal combustion engines. 
     BACKGROUND 
     Thermal efficiency and engine-out emissions from an internal combustion engine are determined by many factors including the combustion chamber shape, the fuel injection nozzle, fuel injection pressure, to name a few. Much is known and much has been studied in typical diesel engine combustion chambers. However, in unconventional engines, less is known about what combustion chamber shape and fuel injection characteristics can provide the desired performance. 
     Such an unconventional engine, an opposed-piston, opposed-cylinder (OPOC) engine  10 , is shown isometrically in  FIG. 1 . An intake piston  12  and an exhaust piston  14  reciprocate within each of first and second cylinders (cylinders not shown to facilitate viewing pistons). An intake piston  12 ′ and an exhaust piston  14  couple to a journal (not visible) of crankshaft  20  via pushrods  16 . An intake piston  12  and exhaust piston  14 ′ couple to two journals (not visible) of crankshaft  20  via pullrods  18 . The engine in  FIG. 1  has two combustion chambers formed between a piston top  22  of intake piston  12  (or  12 ′) and a piston top  24  of exhaust piston  14  (or  14 ′) and the cylinder wall (not shown). The pistons in both cylinders are shown at an intermediate position in  FIG. 1 . Combustion is initiated when the pistons are proximate each other. The piston tops  22  and  24  in  FIG. 1  may not be optimized to provide the desired performance. The piston top  24  has a raised region at the periphery and a flat bowl in the middle of the chamber. To achieve a desired compression ratio, the volume contained in the piston bowls is prescribed. Piston top  24  has a raised region, known by one skilled in the art as squish. The projected area of the squish region is a small portion of the projected area of piston top  24 , whereas the bowl is the greater portion of the projected area. Because of the large area taken up by the bowl, the depth of the bowl is limited. Such a shallow bowl allows little space to accommodate fuel jets from an injector to enter the combustion chamber without significantly impinging on piston top surfaces. 
     SUMMARY 
     A combustion chamber that induces tumble flow is disclosed. The combustion chamber includes a cylinder wall; an intake piston disposed within the cylinder wall; an exhaust piston disposed within the cylinder wall; and a first fuel injector disposed in an opening that pierces the cylinder wall. The pistons are adapted to reciprocate within the cylinder walls. When tops of the pistons are at their closest approach, the combustion chamber located between the tops of the piston forms first and second regions: the first region being substantially a cone proximate the injector with a tip of the cone closer to the first injector and a base of the cone away from the first injector and the second region being substantially a hemisphere with a flat surface of the hemisphere substantially coincident with a base of the cone. The pistons are configured to reciprocate between an upper and a lower position and the cone provides a line-of-sight opening between a tip of the first injector and the hemisphere. A cross section of the pistons taken through a central axis of the cylinder which is 90 degrees rotated from intersecting the injector toward the hemisphere of the combustion chamber shows the tops of the two pistons on each side of the hemispherical region of the combustion chamber sloped so that a thin ribbon that exists between the two piston tops when the pistons are at their closest approach is substantially tangent to a periphery of the hemisphere. When the pistons approach each other, gases between the two pistons are squeezed into the conical and hemispherical region inducing a vortex. The vortex is a tumble flow with an axis of rotation of tumble flow is substantially perpendicular to a central axis of the cylinder wall. A cross section of the pistons coincident with the base of the cone shows the tops of the two pistons on each side of the hemisphere is sloped so that thin ribbons that exist between the two piston tops when the pistons are at their closest approach are substantially tangent to a periphery of the hemisphere. 
     Some embodiments include a second fuel injector disposed in a second opening that pierces the cylinder wall. The second fuel injector is in an opposed arrangement with respect to the first injector. When tops of the pistons are at their closest approach, the combustion chamber located between the tops of the piston also forms third and fourth regions: the third region being substantially a cone proximate the second injector with a tip of the cone closer to the second injector and a base of the cone away from the second injector and the fourth region being substantially a hemisphere with a flat surface of the hemisphere of the fourth region coincident with a base of the cone of the third region. The hemisphere of the fourth region and the hemisphere of the second region do not overlap. A cross section of the pistons coincident with the base of the cone of the first region shows the tops of the two pistons on each side of the hemisphere of the second region sloped so that thin ribbons that exist between the two piston tops when the pistons are at their closest approach are substantially tangent to a periphery of the hemisphere of the second region and a cross section of the pistons coincident with the base of the cone of the third region shows the tops of the two pistons on each side of the hemisphere of the fourth region sloped so that thin ribbons that exist between the two piston tops when the pistons are at their closest approach are substantially tangent to a periphery of the hemisphere of the fourth region. When the pistons approach each other, gases between the two pistons that are squeezed out into the hemispherical region of the second region generate a tumble flow in a first direction. When the pistons approach each other, gases between the two pistons that are squeezed out into the hemispherical region of the fourth region also generate a tumble flow substantially in the first direction. In an alternative embodiment, when the pistons approach each other, gases between the two pistons that are squeezed out into the hemispherical region of the fourth region generate a tumble flow in a direction having an opposite sense as the first direction. 
     A combustion chamber is disclosed having a cylinder wall; an intake piston disposed within the cylinder wall; an exhaust piston disposed within the cylinder wall; and first and second fuel injectors disposed in first and second openings that pierce the cylinder wall with the first and second injectors substantially opposed to each other. The pistons are adapted to reciprocate within the cylinder walls. When tops of the pistons are at their closest approach, the combustion chamber located between the tops of the piston defines a first cone with a tip of the cone substantially coincident with a tip of the first injector and a base of the cone located away from the first injector; a second cone with a tip of the second cone coincident with a tip of the second injector and a base of the cone located away from the second injector; a first hemisphere with a base of the first hemisphere coincident with a base of the first cone; and a second hemisphere with a base of the second hemisphere coincident with a base of the second cone. When tops of the pistons are at their closest approach, the first and second cones and the first and second hemispheres are arranged substantially along a diameter defined by tips of the first and second injectors and the first and second hemispheres do not intersect. When the pistons approach each other, gases between the tops of the pistons other than between the first and second cones and the first and second hemispheres are squeezed into the first and second cones and the first and second hemispheres; and the piston tops are arranged so that the gases squeezed into the first and second hemispheres generates tumble flows. The intake piston has a raised portion on one side of the a plane intersecting tips of the first and second injectors and parallel to a central axis of the cylinder; the exhaust piston has a corresponding recessed portion on one side of the plane; the intake piston has a recessed portion on the other side of the plane; and the exhaust piston has a corresponding raised portion on the other side of the plane. The tumble flow in the first hemisphere rotates in substantially the same direction as the tumble flow in the second hemisphere. Considering first, second, third, and fourth quadrants of the piston tops, the intake piston has raised portions in the first and third quadrants, the intake piston has recessed portions in the second and fourth quadrants, the exhaust piston has recessed portions in the first and third quadrants, and the exhaust piston has raised portions in the second and fourth quadrants. The raised and recessed portions are exclusive of the cones and hemispheres defined in the piston tops. The second quadrant is located between the first and third quadrants. The raised portions of the piston tops index with the recessed portions of the piston tops to develop a tumble flow in the first hemisphere in a first direction and a tumble flow in the second hemisphere in a second direction with the second direction in an opposite sense with respect to the first direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric drawing of an OPOC engine; 
         FIGS. 2-4  are cross-sectional views of a single-injector, tumble-inducing combustion chamber according to an embodiment of the present disclosure; 
         FIGS. 5 and 6  are cross-sectional views of a dual-injector, tumble-inducing combustion chamber according to an embodiment of the present disclosure in which two tumble flows rotating in substantially the same direction are induced; 
         FIG. 7  is an isometric view of the top of the intake piston of  FIGS. 5-6 ; 
         FIGS. 8 and 9  are cross-sectional views of a dual-injector, tumble-inducing combustion chamber according to an embodiment of the present disclosure with the tumble flows in the hemispherical counter rotating, i.e., in opposite directions; 
         FIG. 10  is an isometric views of the top of the intake piston; 
         FIG. 11  is an isometric view of the top of the exhaust piston, respectively, of  FIG. 10  with counter-rotating tumble flows; 
         FIG. 12  is an illustration of fuel spray and combustion from a single fuel jet. 
         FIGS. 13 and 14  shown an alternative embodiment in which a single combustion bowl is offset from the center; 
         FIGS. 15-18  are illustrations to describe how to form piston tops according to an embodiment of the disclosure; 
         FIGS. 19-21  and  23  are isometric drawings of pistons according to several embodiments of the disclosure; 
         FIG. 22  is a cross-sectional view of the embodiment of  FIG. 21 ; and 
         FIG. 24  is a method to make a piston according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations whether or not explicitly described or illustrated. 
     A cross section of a portion on an OPOC engine illustrating a combustion chamber according to an embodiment of the disclosure is shown in  FIG. 2 . A portion of intake piston  40  and a portion of exhaust piston  42  are shown at their closest position. Piston  40  has grooves  44  and  45  and piston  42  has grooves  46  and  47  to accommodate piston rings. For convenience in illustration, piston rings are not shown in the grooves in  FIG. 2  nor in following figures illustrating pistons. Pistons  40  and  42  reciprocate within cylinder wall  50 . The combustion chamber is the volume enclosed between the tops of pistons  40  and  42  and the cylinder wall  50 . The tops of the pistons in their closest position are separated by at least 0.5 mm. Those skilled in the art appreciate that the minimum distance of separation varies depending on the particulars of the engine including size, tolerances, etc. Such range is provided as an example and not intended to be limiting. 
     In  FIG. 2 , a single-injector embodiment with an injector  60  is shown. The opening between pistons  40  and  42  in region  52  is substantially conical with a tip of the cone located proximate injector  60 . The cross section of the opening increases to accommodate expanding fuel jets emanating from injector  60 . Distal from injector  60  the opening between pistons  40  and  42 , in region  54 , is substantially a hemisphere. Fuel from injector  60  has momentum to travel through region  52  and potentially into region  54 . However, much of the fuel has vaporized and the momentum of the liquid drops is reduced by shear with the compressed gases in the combustion chamber. Thus, if the injector hole size and fuel injection pressure characteristics are chosen carefully, few droplets impact the far wall of the combustion chamber from injector  60 . 
     An alternative cross section, which is rotated 90 degrees from  FIG. 2  is shown in  FIG. 3 , a view from the injector tip. The hemispherical shape of region  54  of  FIG. 2  is more easily viewed in  FIG. 3 . The shape of the tops of pistons  40  and  42  promote tumble flow, i.e., a vortex with an axis of rotation substantially perpendicular with respect to the central axis of the central axis  66  of cylinder walls  50 . A portion  64  of the top of piston  42  angles upward toward axis  66  and a portion  62  of piston  40  angles downward toward axis  66 . As pistons  40  and  42  move toward each other, they force the gases between them to exit tangentially as illustrated by arrow  70 . Similarly, portion  56  of the top of piston  40  and portion  58  of the top of piston  42 , during a compression stroke, cause gases to exit tangentially as illustrated by arrow  72 . The flows shown by arrows  70  and  72  interacting with the hemispherical region of the combustion chamber generate a tumble flow, as illustrated by arrow  74 . Such tumble flow aids in mixing the fuel with the air to improve the combustion efficiency and reduce generation of diesel particulates. 
     The combustion chamber, per the view in  FIG. 3 , shows that the piston tops have an upward slope, as considered from left to right to facilitate generating tumble flow in the combustion chamber. 
     In  FIG. 4 , jets  68  exit from injector  60  into the combustion chamber. Tips of jets  68  have not reached region  54  at the time illustrated in  FIG. 4 . In  FIG. 4 , three jets are visible with additional jets possibly being occluded by the visible jets. However, any number of jets may exit injector  60 . 
     It is desirable to have one injector supply fuel to the combustion chamber. However, if jets  68  from the one injector are unable to access the air in the cylinder to effectively utilize inducted air, a second injector may be provided in the cylinder. Such an embodiment with two injectors  160  in cylinder  150  is shown in  FIG. 5 . Two combustion chamber portions that are smaller versions of the combustion chamber of  FIGS. 2 and 3  are provided in  FIG. 5 . Regions  152  of the combustion chamber that are proximate injectors  160  are substantially conical; regions  154  of the combustion chamber that are distal from injector  160  substantially form a hemisphere. 
     An alternative view of the pistons in  FIG. 5  is shown in  FIG. 6 . The alternative view is rotated 90 degrees with respect to  FIG. 5 , i.e., a view as seen by a tip of one of injectors  160 . A portion  162  of the surface of piston  142  and a portion  164  of piston  140  are angled upward to the right so that during a compression stroke, gases between portions  162  and  164  are squeezed as shown by arrow  170 . Analogously, a portion  158  of piston  142  and a portion  156  of piston  140  slope upwards as taken from left to right so that gases between portions  156  and  158  are directed as shown by arrow  172 . These flows, as illustrated by arrows  170  and  172 , form a tumble flow as illustrated by circular arrow  174 . 
     The top of piston  140  is shown isometrically in  FIG. 7  illustrating portions  158  and  164  in which the tumble in the two bowls rotates in the same general direction. 
     An alternative with counter-rotating tumble flows is shown in  FIG. 8 . Two injectors  260  are disposed in cylinder  250  and the volume between pistons  240  and  242  form two combustion chambers. Regions  252  of the combustion chamber that are proximate injectors  260  are substantially conical; regions  254  of the combustion chamber that are distal from injector  260  substantially form a hemisphere. Referring back to the embodiment in  FIG. 5 , the view of the combustion chamber shows that the primary portions of the combustion chamber surface is formed in intake piston  140 .  FIGS. 5-7  are different views of the same embodiment in which the tumble flows rotate in substantially the same direction.  FIGS. 8-11  are views of an embodiment in which the tumble flows substantially counter-rotate. In the view of the combustion chamber illustrated in  FIG. 8 , in which the tumbles are counter rotating. The portion of the combustion chamber visible in  FIG. 9  causes a tumble flow  274  from the jets of gases  270  and  272  that are squeezed out when pistons  240  and  242  move toward each other during a compression stroke. 
     An isometric view of the top of piston  240  is shown in  FIG. 10 . Rather than the raised portion of the piston being on one side of the piston, as is the case in  FIG. 7 , raised portions  280  of piston  240  are opposite each other (across from each other with respect to axis  266 ), i.e., in quadrants across from each other with respect to central axis  266 . Recessed portions  282  of the top of piston  240  are also arranged opposite each other. In  FIG. 11 , an isometric view of exhaust piston  242  is show with jets  268  spraying into the combustion chamber portions. Three jets  268  from each injector  260  are visible in  FIG. 11 . Additional jets may exit injector  260 , but are not visible in  FIG. 11 . Alternatively, an injector with fewer or more jets may be used. Exhaust piston  242  has raised portions  290  diametrically opposed to each other and depressed portions  292  diametrically opposed to each other. Depressed portions  290  of exhaust piston  242  move toward raised portions  280  of intake piston  240  during reciprocation during operation. Depressed portions  282  of intake piston  240  move toward raised portions  292  of exhaust piston  242 . Due to the depressed portions of each piston being adjacent a recessed portion, the direction of the tumble flow in the two combustion chamber portions are of opposite sense or counter-rotating. 
     In  FIG. 12 , a representation of combustion of a diesel jet is shown. The fuel emanates from an orifice  300  of a fuel injector (not shown). The liquid drops travel through a region  302  with vaporization occurring. The fuel jets spreads in region  304  and due to vaporization of the fuel, a fuel rich zone develops in region  304 . The jet continues forward and autoignition of premixed fuel and air ensues when fuel and air in a combustible mixture reach a temperature for a sufficient duration to autoignite. After the premixed fuel burns, a diffusion flame forms on the periphery of the jet in region  306 . Soot forms within region  308 , much of which is burned when the soot mixes with air. The fuel from the jet is contained substantially within a conical region  320  connected with a hemispherical region  322 . The combustion chambers described herein are substantially conical with a hemisphere at the end, i.e., similar to the envelope which contains the fuel jet shown in  FIG. 12 . 
     An embodiment in which the combustion chamber is defined preferentially in a piston  350  in  FIGS. 13 and 14 . In  FIG. 13 , it can be seen that piston  350  has a deep bowl while piston  352  has a shallower bowl. Also shown in  FIG. 13  is an end view of fuel jets  354  from an injector (not shown). The example in  FIG. 13  is a four jet injector at a location in which the jets have overlapped.  FIG. 14  is a cross section taken at 90 degrees rotated from  FIG. 13  in which the cross section is taken through injector  356 . 
     To aid in the description of the combustion chamber, a series of piston shapes leading up to the embodiment in  FIGS. 13 and 14  are used. The intake and exhaust pistons, other than in the area of the combustion chamber, are substantially conical. Blanks of the pistons are shown in cross section in  FIG. 15 : piston  370  is conical (in a positive fashion) and piston  372  is negatively conical. 
     If the combustion chamber were to be taken out of the center from the exhaust piston as illustrated in  FIG. 16 , a tumble flow would not be generated. The squish flow on both sides is directed upwards as illustrated by the arrows. By displacing the combustion chamber toward one side, a feature can be added to cause the flow to tumble. 
     In the cross section shown in  FIG. 17 , the combustion bowl  360  is offset to the left of central axis  358  toward the left. On the left hand side of combustion bowl  360 , the piston tops of both pistons slope upwards to the right. On the right hand side of combustion chamber, the interface between the two pistons also slope upwards to the right. However, this deviates from the purely conical shape, which is indicated by dashed line  361 . A portion of the cone that would be in exhaust piston  350  is removed, i.e., the portion indicated by region  362 . Region  362  is part of intake piston  352  (but would be part of exhaust piston if the conical shapes of  FIG. 15  had remained). The benefit of this feature shown by region  362  is illustrated in  FIG. 18 . The squish flow generated from the interface between intake piston  352  and exhaust piston  350  when they approach each other on the left hand side of combustion bowl  360  causes an upward flow, similar to that shown in  FIG. 16 . An arrow is illustrating this upward flow in  FIG. 18 . On the right hand side of combustion bowl  360 , a downward flow is generated when the pistons approach each other thereby causing a tumble flow in combustion bowl  17 , as illustrated by the circular arrow. 
     In  FIG. 19 , an isometric view of piston  350  is shown. As discussed above in regards to the cross-sectional view of piston  350  in  FIG. 17 , the shape of the piston on one side of combustion bowl  360  is different than on the other side. A transition region  364  is provided across from injector  356 . In such a location, the transition region has little impact generating tumble flow as the desired geometry is provided along the majority of the fuel jet trajectory. 
     Piston  352  is shown isometrically in  FIG. 20  and shows the offset nature of the combustion chamber and separately shows the combustion chamber. It is difficult to discern that piston  352  is concave from the two-dimensional drawing in  FIG. 20 . Nevertheless, as piston  352  is concave, it is known to one skilled in the art, that combustion bowl  356  is less deep than in embodiments in  FIGS. 2-11 . This may present an advantage in scavenging the combustion bowl region. However, the embodiments in  FIGS. 2-11  are lighter and have fewer regions at which hot spots could form and thus may have some other advantages. The selection of the combustion chamber shape may depend on the ultimate application. 
     As discussed above, the  352  can be consider as starting out as a cone defined in the piston top, i.e, a negative cone. However, due to the desire to promote tumble flow, the region  361 , as shown in  FIG. 17 , is built up. Thus, in some embodiments, the piston blank for piston  352  is not a negative cone, but has additional material formed in region  361 . Region  361  has a fairly pointed tip extending downwardly toward exhaust piston  350 . This forms a ridge in piston  352 . It is advantageous that combustion bowl  360  is offset so that the ridge in region  361  is more nearly centrally located than it would be if combustion bowl  360  were centrally located. Thus, interference of the intake flow by the ridge of region  361  is minimized. 
     In the above discussion, an injector with one or more orifices is discussed and shown in various figures. Alternatively, an injector with an outwardly opening pintle can be used. Such an injector provides a spray which is a hollow cone. The angle of the cone can be varied by varying the geometry of the injector tip. In  FIG. 21 , an isometric view of exhaust piston  350  is shown with a conical spray  382  is directed into combustion bowl  362 . A cross section of the pistons and the conical spray is also illustrated in  FIG. 22 . Such a spray may benefit vaporization by allowing air to access the inner and outer surfaces of the conical spray. A pintle-type injector can be used in place of the multi-hole injector in any of the embodiments. 
       FIGS. 2-4  show a single-injector embodiment while  FIGS. 5-7  show a dual-injector embodiment that is analogous to the embodiment of  FIGS. 2-4 . That is, the combustion bowls in  FIGS. 5-7  are scaled down proportionally to accommodate two of the bowls shown in  FIGS. 2-4 . The embodiment of the single-injector embodiment shown in  FIGS. 13-14  can be similarly extended to a dual-injector embodiment. 
     In  FIG. 23  piston  350  is shown in an isometric view. The combustion bowl is comprised of a reentrant portion of a sphere  380  and a conical region  382  that provides a passage from an injector tip region  390  (injector not shown) to the portion of sphere  380 . Material is removed from the blank piston in the region of  361 . Referring back to  FIG. 18 , this provides the ability, in cooperation with piston  352 , to direct the gases downward into combustion bowl  360 . 
     One method of making a piston is shown in  FIG. 24 . A piston is formed that has a top that is convexly conical  400 . This is referred to a vertical cone for the purposes of discussion when viewing the piston with its central axis oriented vertically. The piston may be a unitary piston or be made of a plurality of elements. The portion including the piston top includes the cone. A spherical combustion bowl is formed in the cone and is offset from a central location  402 . The portion of the combustion bowl formed in the exhaust piston is reentrant in the embodiment shown in  FIG. 23 . The sphere that is defined in the exhaust piston is a truncated sphere as a portion of the combustion bowl is also formed in the intake piston (not shown). A horizontally-arranged conical passage is defined in the piston top  404 . The tip of the cone is arranged near the tip of the injector with the base of the cone coinciding with the sphere. The cone opens up to the combustion bowl to allow fuel jets, which are expanding after exiting the injector, to access the combustion bowl. A portion of the remaining cone is removed on one side of the combustion bowl to provide a recess. The recess in the exhaust piston with the corresponding built up area on the intake piston (as shown in  FIG. 23 ) direct flow downwardly into the combustion bowl to promote tumble flow. Processes  402 - 204  in  FIG. 24  can be performed in any order. 
     While the best mode has been described in detail with respect to particular embodiments, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are characterized as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.