Abstract:
A diesel combustion system with a re-entrant piston bowl utilizes momentum interaction with the bowl at multiple locations to cause recirculation of the partially burned fuel and combustion products in a toroidal flow so as to consume soot formed during an earlier portion of the combustion process, while reducing the formation of NO x  and improving fuel efficiency of the engine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   None. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject matter disclosed herein relates to a diesel combustion system including an injector for furnishing fuel directly to the combustion chamber of an engine, as well as to a piston bowl with a floor which interacts with the fuel spray. 
   2. Discussion of the Prior Art 
   Designers of internal combustion engines have been confronted with ever more stringent requirements relating to fuel economy and exhaust emissions. With compression ignition engines, commonly called ‘diesel’ engines, certain exhaust emission components, including oxides of nitrogen (NO x ) and soot, are more difficult to control. Moreover, alterations in engine operating parameters tending to reduce the fixation of nitrogen also generally tend to increase the amount of soot formed during the combustion process. Presently, NO x  and soot are being controlled ever more tightly, and there is a need for a combustion system which will permit reasonable control of both NO x  and soot. 
   NO x  is controlled in some engines by retarding injection timing. This usually has the unfortunate effect of increasing fuel consumption. Because nitrogen fixation is a product of peak combustion temperature, it is known that NO x  emissions may be reduced by lowering the peak flame temperature in an engine. This, however, can have the unfortunate effect of increasing soot produced by the engine. 
     FIG. 3  shows a prior art combustion chamber in which the peripheral exit angle of a piston bowl is in the range of about 81°. Fuel does not impinge upon the floor of the bowl of the combustion system. Rather, the fuel spray impacts the transition region at the outer periphery of the bowl, shown at “I”. This type of flow results in early nitrogen fixation, as well as momentum loss, which limits the recirculation of partially burned fuel and combustion products into the hotter zone of the cylinder, particularly in view of the peripheral exit angle, which is shown at II as being less than 90°, and which is not optimized to redirect and re-entrain burnt material into the hottest part of the combustion chamber. 
   It would be desirable to have an engine combustion system which not only permits injection timing to be advanced to a reasonable degree so as to provide good fuel economy, but also provides lower NO x  and less soot. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, a combustion system for an internal combustion engine includes a cylinder and a cylinder head having a fuel injector with a nozzle mounted so as to spray at a predetermined angle. A piston is slidingly housed within the cylinder. The piston has a bowl formed in a crown portion of the piston, with the bowl defining a combustion chamber. The piston bowl includes a floor with a depressed periphery and a raised center section, with the bowl floor having a midland portion which is impinged upon by fuel spraying from the nozzle. The bowl also has a peripheral exit angle which is greater than 90°, preferably in the range of 105°-115°. The specified peripheral exit angle of the piston bowl causes burning fuel from the injector to first impinge upon the midland portion of the bowl floor, with the fuel and partial products of combustion flowing past the outer periphery of the bowl before recirculating to a middle region of the combustion chamber defined by the bowl, for further burning. 
   According to yet another aspect of the present invention, a method for introducing and burning fuel within the combustion chamber of a compression ignition engine includes spraying fuel from a centrally mounted injector nozzle, such that the fuel spray impinges tangentially upon a midland portion of the floor of the piston bowl comprising a portion of the combustion chamber, followed by allowing the sprayed fuel to react with air inside the combustion chamber as the fuel and primary products of combustion become entrained in a toroidal flow recirculating from the midland portion of the floor to an outward region of the piston bowl and then inward toward a middle region of the combustion chamber. Without wishing to be bound by this theory, it appears that partial quenching occurs as a result of the tangential impingement of the fuel spray upon the midland portion or region of the piston bowl. 
   It is an advantage of the diesel combustion system according to the present invention that fuel consumption may be reduced without increasing soot or particulate emissions, while at the same time maintaining constant NO x  emissions of an engine. 
   It is another advantage of the present diesel combustion system that particulate matter may be maintained at a constant or even reduced level, as compared with existing engines, but with superior fuel economy, and without increasing NO x  emissions. 
   Other advantages, as well as features, of the present invention, will become apparent to the reader of this specification. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a portion of an engine according to the present invention. 
       FIG. 2  shows geometric considerations embodied in the combustion chamber of the engine shown in  FIG. 1 . 
       FIG. 3  shows a prior art combustion chamber. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As shown in  FIG. 1 , engine  10  has a power cylinder,  14 , which is closed by cylinder head  18 . Piston  30  is slidably mounted within cylinder  14 . Piston  30  has a number of piston rings,  34 . Crown  38  of piston  30  has a bowl,  42 , formed therein. Bowl  42  has a floor portion,  44 , with a midland portion of the floor being identified at  46 . The included angle of the apex at the center of bowl  42  is preferably in the range of 140°-150°. 
     FIG. 1  also shows a fuel injector,  22 , having a nozzle,  26 . Fuel injector  22  sprays fuel into combustion chamber  50 , which is defined by piston  30  and cylinder head  18 . As shown in  FIGS. 1 and 2 , fuel leaving nozzle  26  in spray S impinges upon the midland portion,  46 , of piston bowl  42  and then flows to the outer periphery of bowl  42  before recirculating to middle region R of combustion chamber  50  ( FIG. 2 ). Region R is defined by bowl  42  and cylinder head  18 . Cylinder head  18  also includes at least two poppet valves,  52  (one is shown), for admitting air into cylinder  14  and for allowing the removal of exhaust products from cylinder  14 . In a preferred embodiment, injector  22  is located so as to spray in a pattern situated about the geometric center of combustion chamber  50 . This is generally located about the centerline of cylinder  14 . 
   With further reference to  FIG. 2 , it is seen that the peripheral exit angle for the piston bowl of the present combustion system is in the range of 105°-115°, and preferably about 110°. As used herein, the term “peripheral exit angle” means the departure angle at which gases circulate from the piston bowl portion of the combustion chamber to the remainder of the combustion chamber. It is also noted that the spray leaving nozzle  26  has an included conic angle, α, which approximates the conic angle (140°-150°) of bowl  42 . It is important to the function of the present combustion system that the fuel spray impinge tangentially upon the floor of bowl  42  at or near midland portion  46 . Tangential impingement permits some quenching of the combustion, lowering the peak reaction temperature, and consequently reducing the formation of NO X , while producing soot which will be consumed as described below. Tangential impingement at or near midland portion  46  also promotes entrainment of the mixture of air and partially burned fuel smoothly through the peripheral exit portion,  56 , of bowl  42 . The spray pattern from nozzle  26  may be either symmetrical or asymmetrical. The air motion within the combustion chamber may dictate that the fuel spray should either be evenly distributed or biased to a certain portion of the combustion chamber. With the present quiescent chamber, an equally distributed spray is generally preferred. As used herein, the term “quiescent” means a combustion chamber in which the charge air motion prior to the onset of combustion has little swirl or tumble, and mixing of air and fuel is substantially promoted by the fuel injection process. 
   It has been determined that the present combustion system produces excellent results when incorporated in a medium speed diesel engine. Exemplary embodiments include engines having a bore of about 250 mm and a stroke of about 320 mm, when fuel is injected by fuel injector  22  over a period of approximately 30 crank angle degrees, beginning on the compression stroke at a point between 5° BTDC and 5° ATDC. It has further been determined that injector  22  should have a number of exit holes (not shown), with not more than 16-20% of the fuel injected during a single injection event passing through each of the exit holes. 
   The combustion system of the present invention is substantially quiescent and gives rise to a toroidal flow path shown in  FIGS. 1 and 2  in which substantially all the fuel injected during a single combustion event burns during a single toroidal pass through combustion chamber  50 , beginning with impingement of sprayed fuel upon midland portion  46  of bowl floor  44  and continuing during and after transport of partially burned fuel and fuel droplets through peripheral exit portion  56  and into middle region R of combustion chamber  50 . The impingement of the spray upon bowl floor  44 , resulting in partial quenching, produces higher levels of soot, at least initially, with the soot being re-entrained into the hotter zones of the combustion chamber through the momentum interactions with piston bowl  42  at midland region A and peripheral region B of  FIG. 2 . The momentum in the fuel spray is essentially conserved and causes the toroidal recirculation which assures that the partially burned products of combustion, including soot, are recirculated into a hot region, causing consumption of soot and other intermediate combustion products. In this manner, NO x  formation is suppressed by avoiding higher peak combustion temperatures, while at the same time allowing both injection timing advance sufficient to yield good fuel economy, and consumption of soot, so as to avoid particulate matter emissions. 
   An internal combustion engine equipped with the present piston bowl and associated combustion chamber geometry may be configured as a diesel engine or other type of compression ignition engine. 
   The flow of air about the combustion system is substantially quiescent. Moreover, substantially all of the fuel injected during a single combustion event burns during a single toroidal pass through the combustion chamber, beginning with the impingement of sprayed fuel upon a midland portion of the bowl floor and continuing during and after transport of partially burned fuel into a middle region of the combustion chamber. Preferably, fuel is injected by the fuel injector directly into the engine&#39;s cylinder over a period of approximately 30 crank angle degrees using a nozzle having a cone angle of about 140°-150°, with the cone angle of the spray and the piston bowl being approximately equal. 
   Injection preferably begins at approximately 30 crank angle degrees on the compression stroke at a point between 5° BTDC and 5° ATDC. As used herein, the term “ATDC” means  a fter  t op  d ead  c enter on a compression stroke of a four-stroke cycle engine, and “BTDC” means  b efore  t op  d ead  c enter on a compression stroke. 
   The fuel injector has a number of exit holes, with not more than approximately 16%-20% of the fuel being injected during a single injection event passing through each of the exit holes. 
   According to another aspect of the present invention, combustion of the fuel is partially quenched when the fuel spray impinges upon the floor of the piston bowl. This partial quenching decreases the amount of NO x  formed during the combustion process while providing soot which is consumed when the fuel and products of combustion circulate to the middle region of the combustion chamber as part of the previously described toroidal flow. 
   The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims.