Abstract:
A piston for a compression ignition internal combustion engine includes a crown portion, torroidal portion, and a reentrant portion. The piston further has a recess portion about a central axis of the piston designed to reduce temperatures near a tip portion of the fuel injector.

Description:
TECHNICAL FIELD 
     This disclosure relates to a compression ignition, internal combustion engine. More specifically this disclosure relates to a piston with improved performance. 
     BACKGROUND 
     Design of internal combustion engines requires delicately balancing competing requirements of low emissions and low fuel consumption. Governments generally limit the production of various emissions including NOx, smoke, soot, and unburned hydrocarbons. Reducing NOx emissions may be accomplished through various techniques. Many of these techniques require lowering combustion temperatures and in turn increasing fuel consumption. 
     Users of the engines in industrial environments such as work machines require that they operate over a wide range of speeds and loads while still meeting the emission requirements and while achieving reasonable fuel consumption. In particular, high speed (up to 2500 rpm), medium-bore (cylinder bores between 100 mm and 175 mm), compression ignition engines used in work machines may repeatedly cycle between a high speed, high load condition and an idle condition. Meeting emissions requirements through these transients requires a flexible combustion system. Additionally, compared with light duty operation seen in automotive engines, these engines operate a larger portion of their life in conditions that may contribute to fouling of fuel injectors. 
     To meet these challenges, designers must work with various tools to achieve a combustion cycle that meets the above needs. These tools include fuel injection equipment, air flow control, and design of the combustion chamber. In small bore (bore diameters of less than 100 mm) engines, air system geometries may be used to introduce air into the combustion chamber in a manner that generates swirling motion within the combustion chamber. The smaller bore engines may operate at higher speeds (in excess of 2500 rpm) and require faster mixing of fuel and air. The air system creates a swirling motion to increase mixing of fuel and air. Combustion chambers with swirl tend to have a narrower throat area compared with the overall piston diameter as shown in U.S. Pat. No. 5,000,144 issued to Schweinzer et al. on 19 Mar. 1991 and European Patent Application No. 0 911 500 published on 28 Apr. 1999. The narrow throat area creates a greater squish area between a top of the piston and cylinder head. Fuel injected into these combustion chambers is intended to enter a torroidal portion without contacting a floor portion. Schweinzer also shows a recess that allows the piston to approach a top dead center position in the cylinder without hitting a fuel injector tip. However, Schweinzer does not discuss interaction of air in the recess with performance of the injector. 
     Large-bore (180 mm diameter or greater), medium speed (between 900 and 1500 rpm), compression ignition engines tend to use quiescent or semi-quiescent open combustion chamber designs. These designs introduce air into the combustion chamber in a manner that generates little or no swirling motion of the gases about a central axis of the combustion chamber. Higher fuel injection pressures in these types of combustion chambers create motion to promote mixing of fuel and air. Also, finer drop sizes increase surface area exposed to air. These combustion chamber designs also have less squish area available to provide air to the torroidal section. U.S. Pat. No. 7,438,039 issued to Poola et al. on 21 Oct. 2008 discloses using an acute angle reentrant on a large bore, medium speed diesel to improve air flow in a quiescent or semiquiescent combustion chamber. Poola also teaches placing a recess near a tip of the fuel injector. The recess in Poola generally is thought of as an aid in removal of the piston from the engine for servicing. Again, Poola does not explain the interaction of air in the recess with fuel injector tip. 
     None of these references discuss the importance of improved air flow around the tip of the fuel injector. Without appropriate air flow, combustion characteristics of the engine may change over the its life or during certain conditions. For instance, high temperatures about the tip of the injector may cause increased fouling of the fuel injector tip over time. These changes may reduce the ability of the engine to meet both the customer requirements of low fuel consumption and the regulatory requirement of low emissions. 
     The current piston disclosed in this application addresses one or more aspects set out above to improve combustion in a medium-bore, high speed, compression ignition engine. 
     SUMMARY OF INVENTION 
     In a first aspect a piston for an engine is disclosed having a crown portion with an outer diameter and an inner diameter wherein a ratio the inner diameter to outer diameter is greater than 0.65. The piston is a reentrant design bowl including a reentrant portion, torroidal portion, and floor portion leading to a recess portion about a central axis of the piston. The floor portion has a floor angle of about 65 to 70 degrees. The recess portion has a recess depth that is less than a maximum bowl depth. 
     In a further aspect, a piston for an internal combustion engine is disclosed having a crown portion having an outer diameter and an inner diameter wherein a ratio of the inner diameter to the outer diameter is greater than 0.65. The reentrant bowl design includes a reentrant portion, torroidal portion, and a floor portion. A recess portion is connected to the floor portion by a recess transition portion. The recess portion has a recess depth that is less than the bowl depth. 
     In yet another aspect, a piston for an internal combustion engine is disclosed having a crown positioned about a central axis. An outer diameter of the crown is about 105 mm and the ratio of the inner diameter to the outer diameter is between about 0.65 and 0.75. The reentrant bowl design includes a reentrant portion with a reentrant angle of about 63 to 68 degrees. A torroidal portion defines a maximum bowl depth. A recess portion has a ratio of a recess diameter to the inner diameter being about 0.09. 
     These and additional features will become clearer from the following specification of a preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a partial cross-sectional view of a compression ignition engine; 
         FIG. 2  shows a cross-sectional view of the piston of  FIG. 1 ; 
         FIG. 3  shows a top view of the piston of  FIG. 2 . 
         FIG. 4  shows an enlarged cross-sectional detail of “A” in  FIG. 2 ; and 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Generally, the same or corresponding reference numbers will be used throughout the drawings to refer to the same or corresponding parts. It should be appreciated that the terms “upper,” “lower,” “top,” “bottom,” “up,” “down,” and other terms related to orientation are being used solely to facilitate the description of the objects as they are depicted in the figures and should not be viewed as limiting the scope of this description to the orientations associated with each of these terms. All dimensions provided should be understood to include conventional tolerances associated with manufacturing processes such as machining, casting, or the like. 
     As best shown in  FIG. 1 , an engine  10  is made up of a block  20  defining a cylinder  30 . A piston  40  is positioned in the cylinder in a slideable manner. The cylinder  30  may also be formed by a cylinder liner (not shown) positioned in the block  20  wherein the cylinder liner defines the cylinder  30 . A cylinder head  50  connects to the block  20 . The cylinder head  50  has a cylinder facing portion  60 . The cylinder facing portion  60 , the piston  40 , and the cylinder  30  define a combustion chamber  70 . A fuel injector  80  is positioned in the cylinder head  50  and has a tip portion  90  with a plurality of nozzles (not shown) adapted to direct fuel into the combustion chamber  70 . The cylinder head  50  also defines at least one inlet port  100  and at least one exhaust port  110 . An inlet valve  120  moves within the cylinder head  50  to at least partially block the inlet port  100 . Similarly, an exhaust valve  130  is positioned in the cylinder head  50  to at least partially block the exhaust port  130 . The tip portion  90  of the fuel injector  80  has an injection angle IA where the injection angle IA is defined as angle between a piston central axis  140  and a nozzle central axis  150 . 
     The piston as shown in  FIG. 2  has a land  160 , a skirt  170 , a crown  180 , and a bowl  190 . The land  160  has a first end portion  200  and a second end portion  210 . The second end portion contains a ring groove  220 . The crown portion  180  is proximate the second end portion  210  of the land portion  160 . The skirt  170  is adjacent the first end portion  200  of the land  160 . The bowl  190  has a bowl volume Vb defined by a crown transition portion  250 , a reentrant portion  260 , a torroidal portion  270 , a floor portion  280 , and a recess portion  290 . In the present embodiment, the bowl volume Vb is about 57 cc. The crown transition portion  250  is preferably a radius of 1.5 mm that transitions from the crown  180  to the reentrant portion  260 . However, a smaller radius or edge may also be used for the crown transition portion  250 . The reentrant portion  260  is a partial conical surface connecting the crown transition portion  250  with the torroidal portion  270  and has a reentrant angle RA of between 63 and 68 degrees with the crown  180 . The recess portion  290  is a partial spherical surface formed by a radius of about 9 mm with a recess depth  295  of about 9.4 mm from the crown  180 . The recess portion  290  in the present embodiment has a volume Vr of about 0.1 cc. The volume of the recess may also be described by the equation Vr≧KVb where K is a constant of about 0.002. 
     The crown  180  as best shown in  FIG. 3  is ring shaped and has an inner diameter  230  measured from the intersection of the crown transition portion with the crown  180 . An outer diameter of the crown  240  is measured from the land  160 . The recess portion  290  has a recess diameter  297  measured at a location where a line tangent to the recess portion is perpendicular with the piston central axis. In the present embodiment, the outer diameter is about 105 mm. The ratio of the inner diameter  230  to outer diameter  240  is between 0.65 and 0.75. The ratio of the recess diameter  297  to the inner diameter  240  is about 0.09. 
     Greater detail of the floor portion in  FIG. 4  shows a floor angle FA of between 65 and 70 degrees defined in reference to the piston central axis  140 . The recess transition portion  300  connects the floor portion  280  with the recess portion  290 . A floor transition portion  310  connects the floor portion  280  with the torroidal portion  270 . Both the recess transition portion  300  and the floor transition portion  310  may be formed by radiuses of 3 mm or less. The torroidal portion  270  is formed by a radius  320  and connects the floor transition portion  310  with the reentrant portion  260 . In this embodiment, the radius  320  is about 9 mm with maximum bowl depth  330  of about 16.8 mm. 
     Industrial Applicability 
     During operation, the piston  40  moves downward drawing an oxidant like air through the inlet port  100  past the intake valve  120  (intake stroke). The inlet port  100  closes at some time prior to operation of the fuel injector  80  introducing fuel into the combustion chamber  70 . As the piston  40  moves toward the cylinder head  50  and the both the inlet port  100  and exhaust port  110  are blocked by the respective inlet valve  120  and exhaust valve  130 , the piston  40  compresses the oxidant within the combustion chamber  70  (compression stroke) including the recess portion  290 . The piston  40  eventually begins to slow and changes direction such that the piston  40  travels away from the cylinder head  50  (working stroke). The fuel injector  80  will supply at least some fuel as the piston  40  nears the transition from the compression stroke to the working stroke (also known as the top dead center position). 
     In the present embodiment, the fuel injector  80  directs a fuel jet portion  340  toward the torroidal portion without contacting the floor portion  280 . However, a fuel plume portion  350  will vaporize and come in contact with the floor portion  280 . The fuel plume portion  350  contacting the floor portion  280  slows the combustion process and reduces the rate of temperature rise thus reducing NOx formation. The plume portion  350  moves along the floor portion  280  into the torroidal portion  270  where the floor transition portion  310  allows additional air from the torroidal portion  270  to further mix with un-combusted fuel (not shown). This further mixing increases combustion of the un-combusted fuel and reduces formation of soot. Similarly, the reentrant angle RA of the reentrant portion  260  promotes additional mixing of air into un-combusted fuel and oxidation of soot 
     Air retained in the recess portion  290  during the compression stroke provides additional air for mixing with fuel exiting the fuel injector  80 . In particular, the recess portion  290  reduces surface temperatures of the fuel injector  80  by increasing both motion and volume of air near the tip portion  90  at a start of fuel injection. The current embodiment reduces combustion temperatures by about 100 K (180 R) and allows timing of fuel injection to be advanced in order to improve fuel consumption while still meeting emissions requirements. Reducing combustion temperatures near the fuel the tip portion  90  limits fouling of the tip portion  90  and may improve the injector  80  operational life. 
     Although the preferred embodiments of this disclosure have been described herein, improvements and modifications may be incorporated without departing from the scope from the following claims.