Abstract:
A pressure reactive piston for an internal combustion engine includes an axially directed central bore formed within a piston ring portion of the piston, which houses a slidably mounted crown which cooperates with the central bore to define a gas chamber which is closed off from the environment by means of a flexible gas seal interposed between the crown and the ring portion of the piston.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   None. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject matter disclosed herein relates to a piston for use in a reciprocating internal combustion engine. The piston has a slidably mounted crown forming part of a gas chamber at the top of the piston. The gas chamber acts as a gas spring to suspend the piston crown. 
   2. Related Art 
   Designers of reciprocating internal combustion engines, in general and, more specifically, diesel engines, are faced with increasingly stringent regulatory requirements relating to exhaust emissions. More specifically, future regulations will require less emission of oxides of nitrogen (NOx), particulate matter (PM), and unburned hydrocarbons (HC). It is known that an effective way to control NOx is to decrease the peak temperatures within the combustion chamber, as well as by decreasing the available oxygen through exhaust gas recirculation (EGR). Both of these remedial actions tend, however, to cause increases PM and HC emissions. Fixation of nitrogen occurs at a very high rate above 2000° K. On the other hand, hydrocarbon formation tends to increase sharply below 1500° K. Accordingly, if peak combustion chamber temperature is lowered, NOx may be reduced, but at the expense of producing more hydrocarbon. Late ignition timing, sometimes termed ignition timing retard, may be used to reduce NOx formation. This will have the effect of causing cylinder temperature to fall below 1500° K, resulting in higher hydrocarbon, and increased fuel consumption. 
   It would be possible to simultaneously produce beneficial results regarding emissions of NOx, PM, and HC while not adversely affecting brake specific fuel consumption if peak temperatures could be limited but, nevertheless, be held above 1500° K long enough to completely consume all the fuel. 
   It is desirable to have a pressure reactive piston allowing engine operation in a regime which simultaneously reduces the formation of NOx, PM, and HC, while not adversely affecting fuel consumption. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present pressure reactive piston allows beneficial engine operation by reducing peak temperatures and pressures within the combustion chamber, while allowing energy storage in the form of a compression of a gas housed within a gas chamber in the working piston, so as to permit later expansion of the gas and to, in effect, permit operation as if at high compression, but without the attendant formation of NOx, PM, and without the drawback of additional HC resulting from combustion temperatures which are too low. 
   According to an aspect of the present invention, a pressure reactive piston for an internal combustion engine includes a generally cylindrical trunk having a wrist pin boss and a generally cylindrical ring portion, located above the trunk, with the ring portion having an axially directed central bore and a number of piston ring grooves circumscribing an outer wall of the ring portion. A crown is slidably mounted in the central bore, with the crown cooperating with the central bore to define a gas chamber under the crown. A volume of compressed gas is contained within the gas chamber and is maintained within the gas chamber by a flexible gas seal interposed between the crown and the ring portion. 
   According to another aspect of the present invention, a flexible gas seal employed in the present piston is preferably configured as a metallic bellows or as an elastomeric member. In either case, the flexible gas seal is housed within an annular space defined by a generally cylindrical outer wall of the piston crown and a generally cylindrical inner wall of the piston&#39;s central bore, as formed in the ring portion of the piston. 
   According to another aspect of the present invention, the piston may be configured with a unitary, generally cylindrical ring portion having a retainer step located in an uppermost part of the bore. The retainer step maintains the slidable crown within the piston during operation of an engine equipped with the present piston. Alternatively, according to another aspect of the present invention, the crown may be slidably retained within the piston by means of an annular top land applied to an upper surface of the piston&#39;s ring portion. 
   According to yet another aspect of the present invention, the static pressure of the compressed gas which is installed within the piston&#39;s gas chamber may be selected to be sufficient to prevent the crown from sliding in the compressive direction with respect to the ring portion of the piston during cranking and light load operation of an engine equipped with the piston. 
   It is an advantage of the present pressure reactive piston that the benefits of both lower and higher compression ratio are available with a single piston. For example, the benefits of low compression ratio such as low NOx production, lower frictional losses, lower heat losses, and lower mechanical stress to engine components may be had along with the higher thermal efficiency available with a high compression piston, because movement of the piston crown in response to cylinder pressure will effectively result in a reduction in maximum cylinder temperature and maximum cylinder pressure, while nevertheless allowing work done on the compressed gas in the piston to be recaptured when the piston crown moves with respect to the ring portion of the piston during the expansion stroke of the engine. 
   It is yet another advantage of a pressure reactive piston according to the present invention that, compared with other variable compression ratio pistons, the present piston is fast acting, but in a repeatable fashion and with more robustness than known pressure active pistons using metallic springs or hydraulic operating systems. 
   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  illustrates a piston according to the present invention mounted within an engine. The movable piston crown is located in its highest compression, or extended, position. 
       FIG. 2A  is an enlarged view of the piston of  FIG. 1 . 
       FIG. 2B  is an enlarged view of a portion of  FIG. 2A . 
       FIG. 3A  is similar to  FIG. 2A , but depicts the piston of  FIG. 2A  with the movable piston crown in its fully retracted position. 
       FIG. 3B  is an enlarged view of a portion of  FIG. 3A . 
       FIG. 4  is an alternate embodiment of a piston including an elastomeric gas seal. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As shown in  FIG. 1 , piston  10  is mounted within a cylinder  22 , which is carried within a cylinder block,  26 . Piston  10  is attached to a connecting rod,  14 , by means of a wrist pin,  16 . In turn, connecting rod  14  is attached to a crankshaft,  18 . The engine also includes poppet valves  17 , and a fuel injector,  19 . Those skilled in the art will appreciate in view of this disclosure that a piston according to the present invention may be employed with various types of reciprocating internal combustion engines, such as the illustrated diesel, or spark-ignition, or homogenous charge compression ignition (HCCI) engines, or yet other types of reciprocating engines. 
   Piston  10  includes a trunk,  30 , which incorporates a wrist pin boss  34 . The upper part of the piston includes a ring portion,  38 , having an outer wall  40 , and a number of piston ring grooves,  42 . In the embodiment of  FIGS. 1-3B , ring portion  38  is surmounted by an annular top land,  74 , having an inner diameter  78 , whose function will be explained below. 
   Piston  10  also includes an axially directed bore,  46 , formed in ring portion  38 . Axially directed bore  46  has an inner wall,  48 , upon which a slidable piston crown,  50 , is mounted. 
   Slidable crown  50  has two outer walls,  51  and  52 . Outer wall  51  is at the lower part of slidable crown  50  and is slidably engaged with generally cylindrical inner wall  48  of axially directed central bore  46 . The upper portion of outer wall  52  of piston crown  50  slidably rides upon the interior diametral surface  78  of annual top land  74 . 
   Floor  47  of axially directed bore  46  and the underside of piston crown  50  form a gas chamber,  60 , having a pre-charged volume of gas,  62 , contained therein. The gas pressure is selected so that piston crown  50  will not move in a compressive direction in response to cylinder pressures encountered during at least cranking of an engine. More preferably, piston crown  50  will remain immovable with respect to the remainder of piston  10  during not only cranking but also during light load operation of an engine. This allows piston  10  to function as a higher compression ratio piston, giving excellent thermal efficiency, while not decreasing peak combustion temperature during operating regimes in which nitrogen fixation does not typically occur to a prohibitive extent. Accordingly, in  FIGS. 1-2B , piston crown  50  is shown at its highest compression ratio position, whereas in  FIGS. 3A-3B , piston crown  50  is shown in its lowest compression ratio position. 
   Compressed gas  62  is contained within gas chamber  60  by means of a flexible gas seal, which is illustrated at  64  in  FIGS. 1-3B  and  70  in  FIG. 4 . As shown in  FIGS. 1-3B , a flexible gas seal may be rendered as a folded metallic bellows,  64 . In  FIG. 4 , a flexible gas seal is illustrated as an elastomeric member,  70 . What is important is that the flexible gas seal be bonded to the relatively moving parts of piston  10  so that gas  62  is maintained within gas chamber  60 . In its metallic configuration,  64 , the bellows may be bonded to crown  50  and either top land  74  or one-piece ring portion and retainer  44  ( FIG. 4 ), by methods such as brazing, welding, and other methods known to those skilled in the art and suggested by this disclosure. It should be understood that another advantage of the present piston resides in the fact that the gas pressures acting across the flexible gas seal are essentially equal when piston crown  50  is moving with respect to the remainder of piston  10 . In effect, the gas seal must support a large pressure difference only when it is collapsed (when crown  50  is fully extended). Moreover, the gas seal is well-supported between crown  50  and bore  46 . 
   In the embodiment illustrated in  FIGS. 1-3B , piston crown  50  is confined within axially directed bore  46  by annular top land  74 , which is connected with ring portion  38  either by welding, such as electron beam welding or fusion welding shown at  90 , or by threaded fasteners, threaded engagement, or by other types of bonding known to those skilled in the art and suggested by this disclosure. When crown  50  is fully extended, bellows  64  is fully stacked and prevents any further upward travel of crown  50  with respect to the remainder of piston  10 . 
   In the embodiment of  FIG. 4 , a single one-piece ring portion and retainer,  44 , maintains piston crown  50  in slidable engagement with piston  10 . In this embodiment ( FIG. 4 ), piston crown  50  and either elastomeric seal  70  or flexible gas seal  64  are first bonded to crown  50  and to ring portion and retainer  44  before ring portion and retainer  44  are welded or bonded to trunk  30 , as shown at  92  in  FIG. 4 . As before, such bonding may alternatively be accomplished by means of threaded fasteners or by complimentary threaded sections on ring portion  44  and trunk  30  or other types of joining known to those skilled in the art and suggested by this disclosure. A stepped portion,  83 , of bore  46 , prevents crown  50  from extending outwardly from the remainder of piston  10  to an extent greater than that shown in  FIG. 4 . 
   In addition to gas  62  contained within gas chamber  60 , the gas chamber may also include a cooling, or heat transfer, medium,  63  ( FIGS. 2A and 4 ), such as an aqueous based fluid containing ethylene glycol or organic acid technology coolant or some other type of antifreeze and heat transfer medium, with the heat transfer medium being stored as a liquid at room temperature, but available to move up and down within gas chamber  60  in response to the movement of piston  10 . Preferably, cooling medium  63  is selected so as to change phase during operation of an engine equipped with piston  10 . As is known to those skilled in the art, phase change may be employed to transfer heat very efficiently, with the cooling medium condensing on floor  47  of ring portion  38 . The presence of gas chamber  60  would be expected to increase the temperature on top of crown  50 , but for the fact that the movement of crown  50  so as to achieve a lower effective compression ratio during maximum load operation of the engine means that higher temperature regimes will be avoided; the use of a heat transfer medium within gas chamber  60  is a further aid to avoidance of excessive peak chamber temperatures. 
   Gas chamber  60  presents another advantage inasmuch as the size of the gas chamber may be adjusted so as to change the gas spring rate acting upon piston crown  50 . Moreover, selection of cooling medium  63  from a class of materials which are solid at lower temperatures, but which eventually liquefy and ultimately vaporize at higher temperatures, would promote more stable operation of an engine by increasing the gas spring rate of piston  10 . 
   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.