Patent Publication Number: US-2022220921-A1

Title: Passive prechamber lean burn combustion system

Description:
BACKGROUND 
     One important avenue for improving gasoline engine efficiency is to operate the engine as lean burn, i.e., more air is burnt for a given amount of fuel. Currently, spark ignited gasoline engines use a spark discharge to initiate combustion inside a main chamber of a cylinder. However, standard spark plugs are unable to provide a strong enough arc to burn a lean mixture, resulting in misfire or no-burn condition when the engine is run very lean. 
     Turbulent jet ignition has been proposed as a strategy to ignite a lean mixture in a main chamber. In turbulent jet ignition, a prechamber is used to combust a small quantity of fuel. The burn moves from the prechamber into the main chamber in the form of rich radicals that are hot and have high velocities to penetrate deep into the main chamber and ignite the lean charge in the main chamber. The volume between the prechamber and the main chamber and the size and geometry of the orifice of the prechamber are critical, which make integration of the prechamber into existing engine designs challenging. 
     SUMMARY 
     In a first summary example, a combustion system includes a cylinder having a main chamber and a cylinder head disposed at a top of the cylinder. The cylinder head forms an upper end of the main chamber. The combustion system includes a prechamber adapter having a prechamber volume defined therein and a nozzle formed at a distal end thereof. The nozzle includes a plurality of orifices fluidly connecting the prechamber volume to an external environment of the nozzle. The prechamber adapter is threaded into a bore in the cylinder head and positioned in the cylinder head to expose the nozzle to the main chamber. The combustion system includes a spark plug that is positioned within the prechamber adapter. A spark emitting end of the spark plug is exposed to the prechamber volume. A piston is disposed within the cylinder and movable between a top dead center position and a bottom dead center position. The piston has a piston head forming a lower end of the main chamber. The piston head has a dome shape and a bowl formed in a top center of the dome shape. The bowl selectively carries a charge to the nozzle. The combustion system includes a fuel injector that is positioned to inject fuel into the main chamber. In certain cases, the plurality of orifices of the nozzle may be positioned radially relative to a main axis of the prechamber adapter and circumferentially spaced apart around the nozzle. In certain cases, the prechamber adapter may be centrally positioned relative to the main chamber. In certain cases, the spark plug may be centrally positioned within the prechamber adapter. In certain cases, the bowl may have a shape and size to receive at least a portion of the nozzle and form a wall around the at least a portion of the nozzle. In certain cases, the at least a portion of the nozzle may include the plurality of orifices. In certain cases, the fuel injector may be positioned to inject fuel directly into the main chamber. In certain cases, the fuel injector may be oriented to spray fuel in a direction towards the bowl when the piston is at a select location between the bottom dead center position and the top dead center position. In certain cases, the combustion system may further include an intake port formed in the cylinder head and an additional fuel injector positioned to inject fuel into the main chamber through the intake port. In certain cases, the prechamber adapter may be positioned in the cylinder head to extend the nozzle below the cylinder head and into the main chamber. 
     In a second summary example, a method of combustion includes capturing a combustible mixture in a bowl formed at a top center of a dome-shaped piston head of a piston inside a cylinder. The method includes moving the piston relative to the cylinder to carry the combustible mixture to a nozzle of a prechamber adapter mounted at a cylinder head. The method includes communicating at least a portion of the combustible mixture to a prechamber volume inside the prechamber adapter through a plurality of orifices of the nozzle. The method includes operating a spark plug centrally positioned within the prechamber adapter to ignite the combustible mixture inside the prechamber volume. In certain cases, the method may include supplying fuel and air into a main chamber formed between the piston head and the cylinder head, and the act of capturing the combustible mixture in the bowl may include capturing a portion of the fuel and air in the bowl. In certain cases, the act of capturing the combustible mixture in the bowl may include spraying fuel into the bowl using a fuel injector positioned to inject fuel directly into the main chamber. In certain cases, the act of moving the piston relative to the cylinder to carry the combustible mixture to the nozzle of the prechamber adapter may include moving the piston in a direction towards the cylinder head until at least a portion of the nozzle enters the bowl. In certain cases, the act of moving the piston relative to the cylinder to carry the combustible mixture to the nozzle of the prechamber adapter may include compressing the fuel and air in the main chamber. 
     In a third summary example, a prechamber device includes a prechamber adapter having a main axis. The prechamber adapter includes an adapter body having an internal bore extending along the main axis, an internal surface having an internal surface threaded portion, and an external surface having an external surface threaded portion. The prechamber adapter includes a nozzle disposed at an end of the adapter body. The nozzle has an internal chamber fluidly connected to the internal bore and a plurality of orifices fluidly connecting the internal chamber to an external environment of the nozzle. The prechamber adapter includes a spark plug centrally positioned within the internal bore and having a spark emitting end exposed to the internal chamber. The spark plug is threadedly engaged with the internal surface threaded portion. In certain cases, the plurality of orifice are radially oriented relative to the main axis and circumferentially spaced apart around the nozzle. In certain cases, the adapter body and nozzle are integrated to form a single-piece structure. In certain cases, the internal surface threaded portion has a metric thread size of M10, and the external surface threaded portion has a metric thread size of M14. In certain cases, the nozzle has a tapered shape. 
     The foregoing general description and the following detailed description are exemplary of the invention and are intended to provide an overview or framework for understanding the nature of the invention as it is claimed. The accompanying drawings are included to provide further understanding of the invention and are incorporated in and constitute a part of the specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The following is a description of the figures in the accompanying drawings. In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not necessarily drawn to scale, and some of these elements may be arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not necessarily intended to convey any information regarding the actual shape of the particular elements and have been solely selected for ease of recognition in the drawing. 
         FIG. 1  is a cross-sectional view of a prechamber adapter. 
         FIG. 2  is a cross-sectional view of a passive prechamber device employing the prechamber adapter of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of a portion of an engine incorporating the passive prechamber device of  FIG. 2 . 
         FIG. 4A  is a perspective view of piston head with a bowl. 
         FIG. 4B  is a schematic diagram of a swirl motion relative to a piston with a bowl. 
         FIGS. 5A-5D  are schematic diagrams illustrating examples of bowl geometries. 
         FIG. 6  is a schematic diagram illustrating a prechamber adapter nozzle received in a piston head bowl. 
         FIG. 7A  is a schematic diagram illustrating the beginning of an intake stroke of the engine shown in  FIG. 3 . 
         FIG. 7B  is a schematic diagram illustrating the beginning of a compression stroke of the engine shown in  FIG. 3 . 
       FIC.  7 C is a schematic diagram illustrating spraying of fuel into a piston head bowl during a compression stroke of the engine shown in  FIG. 3 . 
         FIG. 7D  is a schematic diagram illustrating a prechamber adapter nozzle entering a piston head bowl during a compression stroke of the engine shown in  FIG. 3 . 
         FIG. 7E  is a schematic diagram illustrating a piston head bowl disposed around orifices of a prechamber adapter nozzle at or near the end of a compression stroke of the engine shown in  FIG. 3 . 
         FIG. 7F  is a schematic diagram illustrating the beginning of an exhaust stroke of the engine shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain specific details are set forth in order to provide a thorough understanding of various disclosed implementations and embodiments. However, one skilled in the relevant art will recognize that implementations and embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, and so forth. In other instances, related well known features or processes have not been shown or described in detail to avoid unnecessarily obscuring the implementations and embodiments. For the sake of continuity, and in the interest of conciseness, same or similar reference characters may be used for same or similar objects in multiple figures. 
     A prechamber adapter that enables installation of a passive prechamber in existing engines with minimal to no modification to the engines is described. The prechamber adapter may be used to centrally position a passive prechamber relative to a main chamber of an engine cylinder. A combustion system employing a centrally positioned passive prechamber is described. Also described is a piston head bowl that together with the centrally positioned passive prechamber enables lean combustion in the main chamber with good efficiency. 
       FIG. 1  shows an illustrative implementation of a prechamber adapter  100  that may be mounted in a bore in a cylinder head. Prechamber adapter  100  has a main axis  104 , which may be a longitudinal axis. Prechamber adapter  100  includes an adapter body  112  and a nozzle  116  disposed at an end of adapter body  112 . Adapter body  112  and nozzle  116  are axially aligned along main axis  104 . In one implementation, nozzle  116  is formed integrally with adapter body  112  or permanently attached to adapter body  112 , e.g., by welding, to form a single-piece structure. Integral forming means that there is no seam between adapter body  112  and nozzle  116 . Prechamber adapter  100  may be made of a metal or an alloy that can withstand high temperatures that would be encountered during combustion. In one example, prechamber adapter  100  may be made of steel. The cylinder head will typically be made of aluminum. Typically, both prechamber adapter  100  and the cylinder head will have cooling water around them to control their temperatures. In one example, adapter body  112  includes a larger diameter cylindrical section  112   a  and a smaller diameter cylindrical section  112   b . A shoulder  112   c  is formed in a transition area between sections  112   a ,  112   b . Nozzle  116  may be attached to smaller diameter cylindrical section  112   b.    
     Adapter body  112  has an internal surface  120  forming an internal bore  124 . In one example, internal bore  124  extends along main axis  104  and is accessible from an upper end  126  of adapter body  112 . Internal surface  120  includes an internal surface threaded portion  128 . In one example, the threads in internal surface threaded portion  128  are selected to mate with complementary threads on a spark plug. In one example, internal surface threaded portion  128  has a metric thread size of M10 to mate with 10 mm thread spark plugs. Adapter body  112  has an external surface  132 , which includes an external surface threaded portion  136 . In one example, external surface threaded portion  136  may be located on smaller diameter cylindrical section  112   b . In one example, the threads in external surface threaded portion  136  are selected to mate with complementary threads of a bore in a cylinder head. In one example, external surface threaded portion  136  has a metric thread size of M14 to mate with a 14 mm threaded bore in a cylinder head. In general, the threads on internal surface threaded portion  128  may be different from the threads on external surface threaded portion  136 . Shoulder  112   c  on adapter body  112  may serve to limit travel of the prechamber adapter when the adapter body  112  is being threaded into the bore of the cylinder head, e.g., shoulder  112   c  may engage a seat at the inlet of the bore. 
     Nozzle  116  has a side wall  140  and an end wall  144 . End wall  144  forms a terminus of prechamber adapter  100 . End wall  144  is shown as a planar wall. However, it could be a non-planar wall, e.g., a curved or beveled wall, in other examples. Side wall  140  may be a tapered cylindrical wall, giving nozzle  116  a tapered shape. The tapering of nozzle  116  may be in a direction towards end wall  144 , or the terminus of prechamber adapter  100 . In some cases, side wall  140  may be a straight cylindrical wall. Nozzle  116  has an internal chamber  152  defined by walls  140 ,  144 . Internal chamber  152  is fluidly connected to internal bore  124  in adapter body  112 . Orifices  148  are formed in side wall  140  and form fluid paths between internal chamber  152  and an external environment of the nozzle. In one implementation, orifices  148  are circumferentially spaced apart around nozzle  116  and are radially oriented relative to main axis  104 . Although not shown, in some cases, orifices may be provided in end wall  144  to form fluid paths between internal chamber  152  and the external environment. 
       FIG. 2  shows a passive prechamber device  200  including a spark plug  204  received partially inside internal bore  124  of prechamber adapter  100 . Spark plug  204  can be any conventional spark plug known in the art. In general, spark plug  204  may include a threaded shell  205  and a central electrode  206  disposed inside shell  205 . An electrical insulator  207  extends into shell  205  and isolates shell  205  from central electrode  206 . Central electrode  206  protrudes from electrical insulator  207  at a spark emitting end  208  of the spark plug. A ground electrode  209  is positioned at spark emitting end  208  and separated from central electrode  206  by a gap. In one example, spark plug  204  may be a 10 mm thread spark plug, also known as M10 spark plug, which means that the threads on threaded shell  205  will have a metric thread size of M10. Spark plug  204  is supported inside internal bore  124  of prechamber adapter  100  by engaging the threads of threaded shell  105  with the threads of internal threaded portion  128  of adapter body  112 . A washer  210  may provide a sealing interface between spark plug  204  and prechamber adapter  100 . 
     In one implementation, spark plug  204  is centrally positioned within prechamber adapter  100 , which means that spark plug  204  is coaxial with prechamber adapter  100 . Spark emitting end  208  is in opposing relation to end wall  144  of nozzle  116  and forms an upper part of a prechamber volume  216 . End wall  144  of nozzle  116  forms a lower part of prechamber volume  216 . As shown, at least a part of prechamber volume  216  occupies internal chamber  152  within nozzle  116 . Electrodes  206 ,  209  at spark emitting end  208  are exposed to prechamber volume  216 . 
       FIG. 3  shows a combustion system  300  incorporating passive prechamber device  200 . Combustion system  300  includes a cylinder  304  formed within an engine body or engine block  308  (only a portion of the engine block is shown, and only one cylinder in the engine block is shown—an engine block may have several cylinders). A piston  312  is arranged to move back and forth (or up and down) within cylinder  304 , typically between a top dead center (TDC) and a bottom dead center (BDC). TDC is the position of the piston when the piston is at the top of its stroke, and BDC is the position of the piston when the piston is at the bottom of its stroke. Piston  312  may be connected to a crankshaft (not shown) by a connecting rod (not shown). The crankshaft will convert the reciprocating motion of piston  312  into rotary motion, as is well known in the art. A cylinder head  316  is mounted at the top of cylinder  304  (only a portion of the cylinder head is shown). Piston  312  has a cylindrical piston body  318  and a piston head  320  formed on top of cylindrical piston body  318 . Piston head  320  is in opposing relation to cylinder head  316 . Cylinder  308  includes a main chamber  324  for combustion of a fuel-air mixture. Piston head  320  forms a lower end of main chamber  324 , and cylinder head  316  forms an upper end of main chamber  324 . 
     Passive prechamber device  200  is mounted to cylinder head  316 . In one example, cylinder head  316  includes a threaded bore  328 , and passive prechamber device  200  is mounted to cylinder head  316  by making up a threaded connection between threaded bore  328  and external surface threaded portion  136  on prechamber adapter  100 . Threaded bore  328  opens to main chamber  324  such that when the threaded connection is made up, nozzle  116  is exposed to main chamber  324 . In one example, nozzle  116  extends below cylinder head  316  into main chamber  324  such that orifices  148  of nozzle  116  are positioned within main chamber  324 . In one implementation, the arrangement of passive prechamber device  200  in cylinder head  316  is such that prechamber volume  216  and spark plug  204  are centrally positioned relative to main chamber  324 . This may be achieved, for example, by aligning the main axis of prechamber adapter  100  with the axial axis (or piston axis) of piston  312  (or the axial axis of cylinder  304 ), as shown. In this case, nozzle  116  is also centrally positioned relative to main chamber  324 . 
     In one implementation, piston head  320  has a dome shape. A bowl  332 , i.e., a concave depression, is formed at the top center of the dome shape of piston head  320 . As more clearly shown in  FIG. 4A , bowl  332  is centrally located on piston head  320 . Bowl  332  is used to carry a charge to the prechamber nozzle during an engine cycle. Examples of bowl geometries are shown in  FIGS. 5A-5D . The bowl geometry shown in  FIG. 5A  has a straight cylindrical side wall  333   a  and a flat bottom wall  333   b . The bowl geometry shown in  FIG. 5B  has a tapered cylindrical side wall  334   a  and a flat bottom wall  334   b . In  FIG. 5C , the bowl geometry has a straight cylindrical side wall  335   a  and a curved bottom wall  335   b . In  FIG. 5D , the bowl geometry has a continuous curved wall  336 . Any of the geometries shown in  FIGS. 5A-5D  may form the basis for optimizing the bowl geometry for the purposes of providing the prechamber nozzle with a charge. In some cases, the bowl geometry may be influenced by the shape of the prechamber nozzle. Additional recessed areas, indicated as  331  in  FIG. 4A , for example, may be formed in piston head  320 . These recessed areas may play a role in the flow dynamics inside the main chamber as well as reduce the volume of the cylinder taken up by the dome shape of the piston head. 
     The shape of the piston dome can be adjusted to achieve a desired compression ratio, which is important when tuning the combustion system to achieve efficient and clean combustion. Compression ratio is the maximum volume of the combustion chamber (main chamber volume and prechamber volume), i.e., when the piston is at BDC, divided by the volume when the piston is in the full-compression position, i.e., when the piston is at TDC. For example, given an initial piston dome design such as shown in  FIG. 4A , surface  337   a  where bowl  332  is formed can be moved up (i.e., the height of surface  337   a  on the piston head increased) to increase the compression ratio or moved down (i.e., the height of surface  337   a  on the piston head decreased) to decrease the compression ratio. When moving up surface  337   a  to increase compression ratio, surfaces  337   b ,  337   c  adjacent to surface  337   a  are not moved up so that they do not interfere with the engine cylinder valves during operation. In addition, surfaces  337   d ,  337   e  (surface  337   e  is in opposing relation to  337   d ) may be moved outboard to adjust compression ratio. In general, surfaces  337   a - 337   e  can be suitably adjusted to achieve a desired compression ratio while preventing interference of the piston head surfaces with the engine cylinder valves during operation. 
     Returning to  FIG. 3 , intake air entering into main chamber  324  creates a swirl in main chamber  324 . The swirl is useful in mixing fuel and air inside the main chamber. The higher the engine load, the stronger the swirl may be as more air is admitted into the main chamber. Arrows  330  in  FIG. 4B  illustrate a swirl motion that may occur in the main chamber relative to piston head  320 . As shown, the swirl moves in a clockwise fashion around the piston axis and therefore around bowl  332 . The swirl motion can have the effect of sweeping a charge (i.e., a mixture of fuel and air) toward bowl  332 . The dome shape of piston head  320  can enable the swirl motion to keep the charge concentrated at and above bowl  332  until the upward motion of piston  312  moves bowl  332  to the prechamber nozzle. 
     In one example, as illustrated in  FIG. 6 , bowl  332  may be shaped and sized such that at least a portion of nozzle  116  can be received inside bowl  332  during upward motion of the piston. In one example, the positioning of nozzle  116  may be such that at least a portion of nozzle  116  is received inside bowl  332  when piston  312  is at TDC. In one case, the portion of nozzle  116  received inside bowl  332  may include orifices  148  such that bowl  332  circumscribes orifices  148 . When nozzle  116  is received inside bowl  332 , the side wall of bowl  332  will guide the charge to orifices  148 . For example, a flow column will form between the side wall of bowl  332  and the side wall of nozzle  116  from which flow can enter into orifices  148 . The angle of the side wall of bowl  332  (or the shape of bowl  332 ) and the annular gap between the side wall of bowl  332  and the side wall of nozzle  116  may be suitably selected to achieve a desired flow pattern of the charge around orifices  148 . In practice, the piston will “rock” at TDC position. Therefore, the gap between the side wall of bowl  332  and the side wall of nozzle  116  should not be too tight that there would be physical contact between bowl  332  and nozzle  116  due to rocking motion of the piston. Typically, a gap of about 2 mm all around nozzle  116  will suffice, but this is not intended to be limiting. 
     Returning to  FIG. 3 , cylinder head  316  includes an intake passage  340  terminating in an intake port  344 . Cylinder head  316  includes an exhaust passage  348  terminating in an exhaust port  352 . Intake port  344  and exhaust port  352  are in the part of cylinder head  316  forming an upper end of main chamber  324 . An intake valve  356  is arranged to control opening and closing of intake port  344 . When intake port  344  is open, air can be drawn into main chamber  324  from intake passage  340 . Although not shown, intake passage  340  is connected to a source of air in a conventional manner. The air in intake passage  340  may be ambient air or a mixture of ambient air and recirculated exhaust gases. A valve not shown may be positioned to control flow of air into intake passage  340 . An exhaust valve  360  is arranged to control opening and closing of exhaust port  352 . When exhaust port  352  is open, exhaust gases can be pushed out of main chamber  324  into exhaust passage  348 . Opening and closing of valves may be controlled by an engine control unit (not shown separately) according to an engine operation plan. 
     A fuel injector  364  is mounted in cylinder head  316  and has a nozzle  365  exposed to intake passage  340 . Fuel injector  364  can be operated to inject fuel into the air flowing along intake passage  340  to intake port  344 . The intake air will entrain the injected fuel and enter main chamber  324  if intake valve  356  is in the open position. Fuel injector  364  may be described as a port injector. A fuel injector  368  is mounted in cylinder head  316  and has a nozzle  369  exposed directly to main chamber  324  through an opening  372  in the part of cylinder  316  forming an upper end of main chamber  324 . In some cases, nozzle  369  may be exposed directly to main chamber  324  through an opening at a side of cylinder  304 . Fuel injector  368  may be described as a direct injector. In one example, fuel injector  368  may be oriented such that there is at least one piston position between BDC and TDC where there is a line of sight  370  (dashed line) between nozzle  369  and bowl  332  that allows fuel injector  368  to spray fuel directly into bowl  332  (or in the direction of bowl  332 ). Operation of fuel injectors  364 ,  368  may be controlled by the engine control unit according to an engine operation plan. 
     In one example, the engine operates the cylinder on a four-stroke cycle including an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. At the beginning of the intake stroke, intake valve  356  is open, exhaust valve  360  is closed, and piston  312  is at TDC, as shown in  FIG. 7A . As piston  312  moves away from TDC, air in intake passage  340  is drawn into main chamber  324  through intake port  344 . Fuel injector  364  may be operated to inject fuel into the air that is drawn into main chamber  324  through intake port  344 . Optionally, injector  368  may be operated to inject fuel into main chamber  324 . The motion of piston  312  may cause mixing of the air and fuel supplied into main chamber  324 . The swirl action created by the air inside main chamber  324  helps with mixing the air and fuel. The dome shape of piston head  320  may help with improving homogeneity of the fuel-air mixture. Some of the fuel-air mixture inside main chamber  324  will enter bowl  332  in piston head  320 . When piston  312  reaches BDC, intake valve  356  closes. 
     At the beginning of the compression stroke, as shown in  FIG. 7B , piston  312  is at BDC and intake valve  356  and exhaust valve  360  are closed. Piston  312  starts to move towards TDC, compressing the fuel-air mixture inside main chamber  324 . As piston  312  moves towards TDC, bowl  332  carries a charge of fuel and air towards nozzle  116  of prechamber device  200 . In one example, when piston  312  is at a predetermined position between TDC and BDC where fuel can be sprayed directly into bowl  332 , fuel injector  368  is operated to spray fuel into bowl  332 , as illustrated in  FIG. 7C  along lines of sight  370  (dashed lines), thereby increasing the richness of the fuel-air mixture inside bowl  332 . As piston  312  approaches TDC, nozzle  116  enters bowl  332 , as shown in  FIG. 7D . At TDC, a portion of nozzle  116  including orifices  148  may be fully inside bowl  332  so that the side wall of bowl  332  wraps around orifices  148 , as shown in  FIG. 7E . The side wall of bowl  332  will guide the charge carried by bowl  332  to orifices  148 , allowing filling of prechamber volume  216  with the charge. 
     Prior to or close to the end of the compression stroke, spark plug  204  is fired, igniting the charge in prechamber volume  216 . The spark timing is such that turbulent jets emanate from nozzle  116  as piston  312  moves outward, i.e., in a direction away from cylinder head  316  or towards the bottom of cylinder  304 , on the power stroke. The turbulent jets ignite the lean charge in main chamber  324 . High pressure gases produced from combustion of the charge in main chamber  324  will expand and push piston  312  down and towards BDC, generating force on the crank and shaft and useful work. When piston  312  is at BDC, exhaust valve  360  opens to begin the exhaust stroke, as shown in  FIG. 7F . During the exhaust stroke, piston  312  pushes exhaust gases out of main chamber  324  into exhaust passage  348  through exhaust port  352 . 
     The detailed description along with the summary and abstract are not intended to be exhaustive or to limit the embodiments to the precise forms described. Although specific embodiments, implementations, and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art.