Abstract:
A piston ( 20 ) for an internal combustion engine includes an upper crown ( 22 ) and a lower crown ( 24 ) that are each fabricated using a same strong material, such as steel and fastened together using a plurality of fasteners ( 50 ). The piston ( 20 ) provides increased strength resulting in improved design flexibility and smaller sized engines that produce the same amount of power. Piston ring grooves ( 100, 102, 104 ) are also provided in either one or both of the upper and lower crowns ( 22, 24 ) which control piston combustion height. A deeper combustion bowl ( 72 ) is also available with the piston ( 20 ).

Description:
This application is a divisional of application Ser. No. 09/198,297, filed Nov. 23, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a piston for large bore internal combustion engines and in particular, to an articulated piston having a crown portion formed by connecting an upper crown to a lower crown using fasteners. 
     BACKGROUND OF THE INVENTION 
     It is known to use pistons for large bore internal combustion engines. Typically, pistons have essentially two functional parts, a crown and a skirt. There are two basic designs for large bore pistons, articulating and non-articulating. Articulating pistons have separate skirts that can pivot about a wrist pin relative to the crown. In comparison, non-articulating pistons have skirts that are integral with the crown. It is also known to divide the crown into upper and lower portions that are subsequently fastened together to form a three-piece piston. The lower crown includes a pair of pin bosses which each have a pin bore to receive the wrist pin. 
     Some non-articulating pistons are called composite pistons because they have a steel upper crown connected to an aluminum, or other light alloy, lower crown to reduce weight. Steel is employed in the upper crown to maintain an adequate degree of strength in the head region, which is exposed to very high combustion temperatures and pressures. The steel upper crown is often connected to the aluminum lower crown using precision bolts. 
     The hybrid metal composition also sacrifices the overall strength of the piston. During operation, pistons experience two separate loads. First, the crown experiences a vertical load from the combustion of the gas gathered in the combustion bowl at the top of the upper crown. It is important to select a material that is strong enough to withstand the combustion load, for example, steel. Next, the skirt receives a side load from the mechanical motion of the crankshaft. In non-articulated pistons, the skirt is integral with the lower crown therefore, both loads are imposed on the entire piston. As a result, the movement and performance of the crown interferes with the movement of the skirt. 
     Also, hybrid steel/aluminum pistons typically have piston ring grooves located in the side walls of the upper crown because aluminum side walls are not strong enough. If the ring grooves are located in the aluminum side walls, they must be reinforced with a stronger metal alloy to accommodate the vertical load experienced during combustion. 
     The combined load on the crown and skirt also causes severe fretting wear in known bolted, non-articulated pistons, and reduces service life. Fretting wear occurs in generally stationary joints when very minor amounts of relative movement create microwelding between components, such as between an upper crown and a lower crown. Microwelds are formed and then broken in successive movements between the crown parts. Further, increased fretting wear occurs at the steel/aluminum interface increasing the amount of contamination particles in precision engines and reducing service life of the piston and engine. 
     The wrist pin is mounted in the pin bores of both pin bosses, constituting a bearing, and must be adequately supported against the piston. Thus the pin boss is the pivot of the piston, the wrist pin and connecting rod assembly. In highly loaded piston applications, a particularly complicated design of the pin boss is needed when aluminum or other light metal alloys are used for the pin boss material. Incipient cracks can occur in the wrist pin if the maximum allowable specific pressure is exceeded. To avoid such cracks in the wrist pin, it is known to profile the pin bore to relieve the stresses caused by deformation of the wrist pin. In a non-articulated piston with an aluminum lower crown, the thickness of the walls around the pin bores and precision profiling of the pin bore itself are necessary to overcome the weakness of the aluminum material. 
     One alternative for attaching an upper crown to a lower crown is friction welding. However, pistons of 250 mm diameters and more are very heavy and are relatively low volume items. In addition, many friction welding machines do not have enough radial clearance to assemble large diameter pistons. Thus, it is not usually economically feasible to invest in larger friction welding equipment to assemble low volume, large diameter pistons. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an articulated type of piston having a crown portion formed by connecting an upper crown to a lower crown using fasteners. By connecting the upper and lower crown with fasteners, the present invention has greater flexibility for manufacturing, piston design and locations of piston ring grooves. 
     In the present invention, the upper and lower crowns are made from similar metals, preferably steel. Using steel for both portions of the crown allows for increased flexibility in the location of the piston ring grooves. For example, the piston ring grooves can be located in both the upper and lower crowns because steel does not require reinforcement. By locating at least one compression ring groove in the lower crown, the compression height of the piston can be reduced. The compression height is defined by the distance from the top of the piston to the center line of the pin bore. Reducing the piston compression height results in smaller engine designs that provide the same power as corresponding larger engines. Smaller engines require smaller envelopes in vehicles and are lighter in weight. 
     It was generally accepted that similar materials have a tendency to attract and weld to each other on a microstructure level. In the present invention, however, fretting wear surprisingly decreased compared to known hybrid non-articulating pistons. 
     In addition, the articulated piston configuration of the present invention allocates the vertical and side loads between the crown and the skirt, individually. The present invention increases strength and loading providing ability to sustain higher peak combustion pressures. Generally, the weight of the present invention is comparable to the hybrid steel/aluminum type, but with potential to increase engine ratings and lower overall engine weight by providing stronger pistons. 
     In the hybrid steel/aluminum configuration, the strength of the aluminum lower crown determined the depth of the combustion bowl. The steel upper and lower crown configuration of the present invention, however, increases the ability to form deeper combustion bowls. 
     In the present invention, using a steel lower crown reduces the need to profile the pin bore because the steel material is strong enough to endure the stress caused by the piston pin. 
     The steel upper and lower crown configuration of the present invention increases the strength of the crown and the pin bores, the ease of manufacturing, and flexibility of the location of piston rings. Thus, the present invention provides a smaller overall engine with the same power and less overall weight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description: 
     FIG. 1 is an exploded view of a piston assembly according to the present invention. 
     FIG. 2 is a sectioned perspective view of a piston assembly according to the present invention. 
     FIG. 3 is a cross-sectional elevational view of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an exploded view of a piston assembly  20  having an upper crown or crown part  22  that is designed to be attached to a lower crown or crown part  24 . A separate skirt  26  is also provided for articulating connection to lower crown  24  by a wrist pin (not shown). A pair of bushings  28  are optionally installed into pin bores  30  in downwardly depending pin bosses  31  to provide a bearing surface for engaging the wrist pin (not shown), which connects piston assembly  20  to a connecting rod (not shown). Upper crown  22  includes a plurality of circumferentially spaced apart bosses  32  having fastener bores  34  that align with corresponding bosses  36  in fastener bores  38  located on lower crown  24 . In addition, upper and lower crowns  22 ,  24  each have a pair of locating bores  40  for corresponding alignment to receive locating pins  42 . Locating pins  42  aid during assembly to prevent rotation of upper crown  22  relative to lower crown  24 . In addition, locating pins  42  and locating bores  40  serve as orientation indicators to ensure that if a piston assembly  20  is ever taken apart for service. Thus, upper crown  22  will be oriented on lower crown  24  the same as before servicing. 
     A plurality of fasteners  50  are used to securely attach upper crown  22  to lower crown  24 . Fasteners  50  are shown in the form of studs  52 , sleeves  54  and nuts  56 . However, any suitable type of fastener can be used, including but not limited to, bolts, pins, studs, and screws. Studs  52  include upper and lower threaded ends  58 ,  60  with a necked down body portion  62  in between. Sleeves  54  are designed to pass over lower threaded ends  60  and seat in lower crown bores  38 . Nuts  56  are threaded onto lower threaded ends  60  and are tightened to engage and axially pre-load sleeves  54 . 
     Preferably, both upper and lower crowns  22 ,  24  are fabricated from the same material to decrease the amount of fretting wear at the interface joint. Preferably, upper and lower crowns  22 ,  24  are both made from steel to provide increased strength and permit greater flexibility in piston design. 
     FIGS. 2 and 3 show piston assembly  20  including upper crown  22  having an annular outer ring member  66  with a circumferentially extending outer sidewall  68  and an uppermost surface  70 . A combustion bowl  72  is located interior of outer ring member  66  and forms a generally depressed recess  74  having a very slightly convex top surface  76 . Fabricating lower crown  24  from steel instead of a weaker material, such as aluminum, increases overall piston strength and rigidity and permits combustion bowl  72  to have a deeper design. If a lighter weight material is used for lower crown  24 , then the strength of the piston is reduced and would not permit increasing the depth of combustion bowl  72  since the cross-sectional thicknesses would be reduced resulting in increased flexing of pin bosses  31 . Thus, combustion bowl top surface  72  can have a substantially increased depth D 1  from uppermost surface  70 . For example, depth D 1  can be up to approximately 15% of the piston diameter, so a piston having an outer diameter of 250 mm can have combustion bowl top surface  76  located approximately 37 mm from uppermost surface  70 . However, any suitable combustion bowl shape or configuration can be used with the present invention. 
     Upper crown  22  also includes an annular ridge or inner wall  80  that depends downwardly from a bottom surface  81  of combustion bowl  72  and is circumferentially continuous. Bosses  32  are formed radially outwardly and adjacent to ridge  80  to provide increased strength where fastener bores  34  are located. In addition, an annular recess  82  is formed between outer sidewall  68  and combustion bowl  72  to define an upper part of a generally continuous cooling gallery  84 . 
     Lower crown  24  has corresponding features that matingly engage with features on upper crown  22 . Specifically, lower crown  24  includes an annular recess  83 , and an annular outer ring member  86  having an outer sidewall  88 . An uppermost surface  90  of ring member  86  is designed to matingly engage ring member  66  of upper crown  22 . Also, an annular ridge or inner wall  92  projects upwardly for mating engagement with upper crown annular ridge  80  and includes lower locating bores  40 . Additionally, lower bosses  36  and lower fastener bores  38  are located radially outward of locating bores  40  and adjacent to annular ridge  92 . A counter bore  94  is optionally provided in fastener bore  38  to receive sleeves  54 . Preferably, counter bore  94  has a slightly larger diameter than nut  56  to allow full compression loading of sleeve  54  to develop a fully tensioned fastening system. 
     Using steel to fabricate both upper and lower crowns  22 ,  24  provides increased flexibility in the location of piston ring grooves. For example, a first combustion ring groove  100  can be located on outer sidewall  68  of upper crown  22 . Second and third combustion ring grooves  102 ,  104  can be located on sidewall  88  of lower crown  24  because the lower crown  24  is fabricated from high strength steel. An oilscraper ring groove  106  is located below third combustion ring groove  104  and prevents oil from entering a combustion chamber (not shown). Although four total ring grooves are shown, any suitable number of ring grooves are contemplated for the present invention. Moreover, the ring grooves can be located on either upper crown  22  or lower crown  24  or both. As a result, compression height H can be reduced. The compression height is defined by the distance from uppermost surface  70  to the center line of pin bores  30 . For example, a 250 mm diameter piston can be produced that has a compression height of approximately 175 mm, or approximately 70% of the piston diameter. Reducing the piston compression height allows for smaller engine designs that provide the same power as corresponding larger engines. Smaller engines also require smaller envelopes in vehicles and are lighter in weight. 
     Skirt  26  surrounds pin bosses  31  and is preferably made of aluminum to reduce weight. However, any suitable material can be used. 
     Preferred embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.