Abstract:
A toroidal combustion chamber shape with a side injector is disclosed for an opposed-piston engine. Fuel is injected into the toroidal volume from a fuel injector in the cylinder wall. In one embodiment, fuel is injected from each injector a plurality of times with the timing between the injections such that fuel clouds from each injection remain substantially isolated from each other.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority benefit from U.S. provisional patent application 61/508,151 filed 15 Jul. 2011. 
     
    
     FIELD 
       [0002]    The present disclosure relates to shape of the combustion chamber and injector orientation in internal combustion engines. 
       BACKGROUND 
       [0003]    Thermal efficiency and engine-out emissions from an internal combustion engine are determined by many factors including the combustion system design and the mechanical design. Combustion system design includes combustion chamber shape, the fuel injection nozzle, and the fuel injection pressure, intake manifold and exhaust manifold, etc. All of these together are optimized to achieve mixing quality that leads to effective combustion. Much is known and much has been studied in typical diesel engine combustion systems to determine what chamber shape and fuel injection characteristics lead to the desired output. However, in unconventional engines, less is known about what combustion chamber shape and fuel injection characteristics can provide the desired mixing and engine performance. 
         [0004]    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 , with each intake piston  12  having two 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 are 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. 
       SUMMARY 
       [0005]    An internal combustion engine is disclosed which includes a cylinder wall with first and second pistons adapted to reciprocate therein. The two pistons are disposed in the cylinder in an opposed fashion. A crankshaft having first and second eccentric journals couples to the first and second pistons via first and second connecting rods. At a particular angle of rotation of the crankshaft, the pistons are at their closest approach. The piston top of the first piston has three regions: a center, an outer ring near the periphery of the piston, and an inner ring, all of which have a center that is substantially coincident with a central axis of the cylinder. The piston top of the second piston has three regions: a center, outer ring near the periphery of the piston, and inner ring, all of which have a center that is substantially coincident with the central axis of the cylinder. Surfaces of the pistons are a predetermined distance apart in the outer ring and center regions when the crankshaft is at the particular angle. A volume between the first and second pistons proximate the inner ring substantially forms a toroidal volume when the crankshaft is at the particular angle. The predetermined distance is in the range of 0.5 to 3 mm. 
         [0006]    The engine also includes a fuel injector disposed in the cylinder wall with an axis of the fuel injector roughly normal to the cylinder wall. A channel is defined in the outer ring to provide an opening for line-of-sight access from a tip of the injector to the toroidal volume formed between the pistons. The fuel injector has at least one orifice and the orifice is arranged so that a spray exiting the orifice is largely directed into the toroidal volume. The fuel injector may contain a plurality of orifices from which fuel sprays exit. In some embodiments, two fuel injectors are provided and orifices of the injector are aligned with channels cut into the outer ring of the pistons for line-of-sight access from the tips of the injectors to the toroidal volume formed between the pistons. The first and second injectors are located about 180 degrees around the cylinder from each other. 
         [0007]    In one embodiment, the surfaces of the first and second pistons in the center regions are substantially flat. Alternatively, a surface of the first piston is concave in the central region and the second piston is convex in the central region. The engine is a two-stroke engine; the first piston is an intake piston; the second piston is an exhaust piston; intake ports are defined in the cylinder wall proximate the intake piston; and exhaust ports are defined in the cylinder wall proximate the exhaust piston. 
         [0008]    Also disclosed is a method to provide fuel to an opposed-piston, internal-combustion engine including: injecting fuel multiple times into a combustion chamber, which includes: a cylinder wall, an intake piston disposed within the cylinder wall, an exhaust piston disposed within the cylinder wall with a top of the intake piston opposite a top of the exhaust piston. The tops of the intake and exhaust pistons each have three regions: a center, an outer ring near the periphery of the piston, and an inner ring, all of which are have a center substantially coincident with a central axis of the cylinder. Two channels are defined in the inner ring region of the intake piston with the two channels diametrically opposed. A volume between the first and second pistons proximate the inner ring substantially forms a toroidal volume. The combustion chamber further includes first and second fuel injectors disposed in the cylinder wall with the first injector proximate the first channel and the second injector proximate the second channel. The injection can be single event or multiple events based on different operating conditions. The multiple injections are separated in time such that a fuel cloud from a second injection is substantially separate from a fuel cloud from the first injection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an isometric drawing of an OPOC engine; 
           [0010]      FIGS. 2 and 3  are cross sections of a combustion chamber according to an embodiment of the disclosure; 
           [0011]      FIG. 4  is a sketch of the regions on the piston top; 
           [0012]      FIGS. 5 and 6  show the combustion chamber of  FIGS. 2 and 3  at crank angle positions displaced from that of the closest approach of the pistons; 
           [0013]      FIG. 7  is a cross section of a combustion chamber according to an embodiment of the disclosure; 
           [0014]      FIGS. 8 and 9  illustrate intake and exhaust piston tops, respectively, for the combustion chamber of  FIG. 7 ; 
           [0015]      FIG. 10  is an illustration of the piston top of the intake piston associated with the combustion chamber of  FIGS. 2 and 3   
           [0016]      FIGS. 11-16  illustrate the location of fuel clouds from multiple injections into a combustion chamber according to an embodiment of the disclosure; and 
           [0017]      FIG. 17  shows a simulation of fuel injection and combustion in a chamber according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]    In  FIG. 2 , a cross section of a portion of an OPOC engine is shown illustrating a combustion chamber according to an embodiment of the disclosure. 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 (not shown). 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 . 
         [0020]    The cross section illustrated in  FIG. 3  is a rotated 90 degrees with respect to the cross section illustrated in  FIG. 2 . The cross section in  FIG. 3  cuts through injectors  60 . It can be seen that proximate injectors  60 , a pocket  62  is provided to accommodate injectors  60 . Sprays emanating from injectors  60  are discussed below. 
         [0021]    A top of intake piston  140  is shown in  FIG. 4 . The piston is shown having three regions: outer ring  152 , inner ring  154 , and center  156 . Exhaust piston  142  has three corresponding regions: an outer ring, an inner ring, and a center. The majority of the volume of the combustion chamber, when the pistons are in close proximity, is contained in the volume between the inner ring surface of the intake piston and the inner ring surface of the exhaust piston. Outer ring  152  includes passages  158  defined therein to allow for line-of-sight access between fuel injectors (not shown) and the toroidal volume associated with inner ring  154 . As shown in  FIG. 2 , the surfaces of the outer ring of the intake and exhaust pistons are displaced from each other a small distance: at most 2 mm, at least 0.5 mm. Very little of the combustion chamber volume is contained between the pistons in the outer ring region. Similarly, the exhaust piston top and the intake piston top are displaced from each other a very small distance in the center region and thus, very little of the combustion chamber volume is contained between the pistons in the center region. 
         [0022]    The cross section of the combustion chamber volume, as shown in  FIGS. 2 and 3 , is two oval areas  64 . The shape of the combustion chamber in the inner ring region is a surface of revolution generated by revolving oval area  64  in space about a central axis  66  of cylinder  50 . Strictly speaking, a torus is the result of rotating a circle around an axis. However, in the present disclosure, the term torus is used to apply to any  2 -dimensional shape rotated about the central axis. In the embodiment in  FIGS. 2 and 3 , the shape rotated about the central axis is generally curved, but not a circle. Nevertheless, the term torus is applied to the resulting combustion chamber. Furthermore, the term torus is being used to describe a shape in which the cross-sectional area is not constant as taken along points in the revolution. For example in  FIG. 4 , as outer ring  152  is shown as an annulus and is defined by a circle at the interior edge and center  156  is an oval. Dotted lines  160  and  162  are axes of symmetry of center  156 . If the depth of inner ring  154  is substantially constant throughout inner ring  154 , the lesser width near axis  160  indicates that the cross sectional area (taken through the central axis of the cylinder) is less than the cross sectional area near axis  162 . The term toroidal volume is applied to such a situation in which the cross-sectional area varies around the circumference. Also, the smaller cross-sectional area is smaller close to passages  158  because the fuel coming out of the injectors is compact. The cross sectional area is greater in the region of axis  162 , which is farther from the injector tips. At this location, the fuel spray has expanded. With such a configuration, it is easier to avoid fuel droplets from impacting the piston top when the larger cross-sectional area is provided somewhat away from the injector tip. 
         [0023]    The center region of the top of intake piston  40  has a concave shape and the center region of the top of exhaust piston  42  has a convex shape; these nest together. The top of exhaust piston  42  is at a higher temperature than the top of intake piston  40  because the exhaust gases exit through exhaust ports proximate exhaust piston  42 . Thus, it is an advantage for exhaust piston  42  to have a convex shape with no corners that might generate hot spots. Corners  68  on center region  56  of the top of intake piston  40  could be problematic on an exhaust piston, but are less likely to present issues on an intake piston. 
         [0024]    Pistons  40  and  42  are at their closest approach in  FIGS. 2 and 3 . The combustion chamber at a position  10  crank angle degrees rotated away from the position of closest approach is shown in  FIGS. 5 and 30  crank degrees rotated in  FIG. 6 . 
         [0025]    In an alternative embodiment in  FIG. 7 , center regions  96  of the intake and exhaust pistons are flat. The bulk of the combustion chamber in this alternative is yielded by revolving region  98  about central axis  66  of cylinder  50 . 
         [0026]    In  FIG. 8 , a view of the top of intake piston  80  is shown with orifices on injectors  60  situated so that fuel jets  106  travel through channels in the piston top. The channels are not separately visible in the view in  FIG. 8 . Fuel jets  106  are directed into the inner ring region  102 , which is depressed with respect to center region  104  and outer ring region  100 . In the embodiment shown in  FIG. 8 , four fuel jets emanate from injector  60 , with one of the fuel jets not visible. There is a very small angle between the individual jets. Alternatively, an injector with a different number of jets may be used. The jets are directed along a tangent of the surface of the toroidal volume to limit the amount of fuel droplets coming in direct contact with the piston top. The rounded surfaces of the torus help to direct the flow toward the center of the toroidal volume. 
         [0027]    In  FIG. 9 , the top of exhaust piston  82  has a raised outer ring  120  with the inner ring  122  and  124  being at the same level of depression. Injectors  60  are shown directing fuel through channels (not separately shown) into inner ring region  122  at a direction substantially tangent to an interior edge of outer ring  120 . There is just one pair of injectors  60 , but illustrated in both  FIGS. 8 and 9  to show how the fuel jets interact with the piston tops of the pistons. 
         [0028]    In  FIG. 10 , a detail of the top of intake piston  40  of  FIGS. 2 and 3  is shown. A channel  130  is provided through outer ring  126  of the piston top to allow fuel jets to exit into the inner channel. Fuel jets are not shown in channel  130 . Opposite channel  130  is another channel which is not visible due to the jets  132   a,    132   b,    132   c , and  132   d  being illustrated, thereby not allowing a view of the channel. Fuel jets  132   a - d  are directed into the depression associated with inner ring  127 . Center region  128  is oval. This allows a wider space in inner ring  127  to accommodate the fuel jets. Edge  136  is the visible edge of outer ring  126  from this view. The dashed line  138  shows that inner ring  127  is slightly re-entrant. There is no such undercut along the 2-2 section. 
         [0029]    A method of distributing fuel into the cylinder is illustrated in  FIGS. 11-16 . Piston top  200  has channels  202  through which fuel jets can be sprayed. Piston top  200  has three regions: center  208 , inner ring  206  and outer ring  204 . A swirl flow  210  is developed, as shown in  FIG. 11 . In  FIG. 12 , fuel jets  212  are first injected. In  FIG. 13 , illustrating a snapshot later in time, fuel jets  212  rotate in inner ring  206  due to the momentum of the jets themselves as well as the swirl  200 . Fuel jets  212  become fuel clouds in  FIG. 13 . In  FIG. 14 , an even later snapshot, a second injection causes fuel jets  214  to enter inner ring  206 . The timing of the second injection is such that the tips of fuel jets  214  substantially do not overlap with fuel jets (now clouds)  212 . In  FIG. 15 , fuel jets  212  and  214  are now both fuel clouds and have moved around inner ring  214  further. At a later time, in  FIG. 16 , a third injection produces fuel jets  216  with the timing of the third injection so that none of the clouds substantially overlap. Furthermore, the third cloud from the first injector does not overlap with the first cloud from the second injector. 
         [0030]      FIGS. 3 ,  4  and  6  show the combustion chamber shape from when the pistons are at their position of closest approach ( FIG. 3 ) as they move away from each other ( FIGS. 5 and 6 ).  FIGS. 3 ,  4  and  6  can be considered in reverse order to show the combustion chamber shape as the pistons are moving toward each other. By considering the change of the combustion chamber from  FIG. 6  to  FIG. 5 , the air that is between pistons  40  and  42  in the outer ring portion  52  is squished into the inner ring portion  54 . Similarly, air between pistons  40  and  42  in the center  56  is squished into the inner ring portion  54 . The movement caused by these squish flows is shown by arrows  58   a - d . Because the opening connecting the volume associated with the outer ring region  52  with the volume associated with the inner ring region  54  is tangent to the inner ring region  54 , a tumble flow is induced. Similarly, the opening connecting the volume associated with the center region  56  is tangent to the inner ring region, also promoting a tumble flow. The flow exiting from the squish regions induces flows in the direction of arrows  58   a  and  58   b  which causes a clockwise tumble, as view in the cross section illustrated in  FIG. 5 . The flow exiting from the squish regions induces flows in the direction of arrows  58   c  and  58   d  which causes a counter clockwise tumble. 
         [0031]    In  FIG. 17 , a representation of modeling results is shown. Two injectors  250  inject fuel primarily into inner ring region  254 , which is between center  252  and outer ring  256 . The intake ports (not shown) are angled such that a swirl flow is induced by incoming gases into the cylinder, as shown by clockwise arrow  248 . Injectors  250  inject fuel tangentially into inner ring region  254  in the direction of the swirl, as shown by arrow  248 . Thus, fuel droplets are carried by the swirl flow. 
         [0032]    A limitation in obtaining satisfactory combustion at the highest load condition is utilizing the air in the cylinder. This is accomplished by the fuel droplets being relatively uniformly mixed in the air at the highest torque operating condition in which the most fuel is injected. The representation in  FIG. 17  is for a 100 mm bore cylinder with a swirl ratio of 5 at the highest torque condition, i.e., longest fuel pulse width anticipated. The crank angle illustrated in  FIG. 17  is about 20 degrees into the expansion stroke, which is also the end of the fuel injection interval. Liquid droplets  260  and  262  are contained mostly in inner ring  254 . Droplets  260  and  262  are shown much larger than in reality so that they can be viewed in  FIG. 17 . Much of the fuel has vaporized and combustion is occurring. Surfaces  270  and  272  are isothermal surfaces which are indicative of the surface of the flame. Some of the combustion is occurring in outer ring  256  having spilled out of inner ring  254 . In  FIG. 17 , it can be seen that tips  274  and  276  of combustion surfaces  270  and  272 , respectively, do not overlap. Based on hole sizes on the injector tip, injection pressure, the number of orifices on the injector, and the swirl ratio, the air utilization, as illustrated in  FIG. 17  in which the combustion surfaces do not overlap but encompass most of inner ring  254 , can be obtained. 
         [0033]    Small orifices in the injector create small droplets that vaporize more readily. Such small droplets are helpful in avoiding soot formation. However, small droplets have low inertia and do not travel far into the chamber, which is harmful for air utilization. By injecting the fuel in the same direction as the swirl flow, small droplets are carried by the flow to access unused air away from the injector, thereby facilitating the injection of smaller droplets than could otherwise be used. 
         [0034]    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.