Patent Publication Number: US-10323580-B2

Title: Isobaric piston assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. patent application claims priority to U.S. Provisional Patent Application No. 62/253,746, filed Nov. 11, 2015 and entitled “Isobaric Piston Assembly,” the entire disclosure of this application being considered part of the disclosure of this application and hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to pistons for power cylinder assemblies of internal combustion engines. 
     2. Related Art 
     There is a continuing desire to increase power density (power per unit volume of displacement) of internal combustion engines to maximize power while minimizing packaging size and mass. One approach some engine manufacturers have taken to reduce packaging size and mass is to reduce the wall thicknesses of various components in the engine including, for example, the cylinder head and the engine block. However, in order to avoid damaging the engine during use, these components must remain sufficiently strong to resist combustion gas pressures within the combustion chambers of the engine. Therefore, in engines with a fixed compression ratio, the compression ratio must be specifically set so as to avoid damaging the engine, but this may compromise the performance of the engine during certain times, such as during start-up of the engine, when a higher compression ratio would be desirable. 
     There remains a continuing need for a high power density engine which has a variable compression ratio so as to optimize the performance of the engine during a wider range of operating conditions. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     One aspect of the present invention provides for a power cylinder assembly for an internal combustion engine which includes a cylinder wall that surrounds a cylinder bore that extends along an axis. A piston assembly is positioned in the cylinder bore, and the piston assembly has a combustion surface that divides the cylinder bore into a combustion chamber on one axial side of the combustion surface and a crank case on an opposite axial side of the combustion surface. The combustion surface is partially defined by a first upper surface of a first piece and partially defined by a second upper surface of a second piece. The second piece is moveable relative to the piston body during operation of the power cylinder assembly to change a compression ratio of the power cylinder assembly. 
     The power cylinder assembly according to this aspect of the present invention is advantageous because it allows for an engine with a reduced package size and reduced mass in addition to improved performance by allowing the power cylinder assembly to operate at a high compression ratio in certain operating conditions and allowing it to operate at a low compression ratio at other operating conditions. The power cylinder assembly automatically switches between the high and low compression ratios without any external input. For example, the power cylinder can operate with a high compression ratio when the engine is cold or operating in a low power demand condition to maximize engine efficiency and fuel mileage. The power cylinder assembly also can operate at a relatively lower compression ratio when increased power is demanded. The lower compression ratio also may protect the engine from damage that could otherwise occur from high combustion pressures. 
     According to another aspect of the present invention, the first piece is a piston body, and the second piece is a part of a plunger assembly. 
     According to yet another aspect of the present invention, the plunger assembly is biased into a resting position by a spring and is moveable relative to the piston body in response to a pressure force in the combustion chamber overcoming a biasing force by the spring. 
     According to still another aspect of the present invention, the plunger assembly includes a plunger cusp which defines a portion of the combustion surface. 
     According to a further aspect of the present invention, the piston body presents an inner wall which surrounds a through passage and wherein the plunger assembly is slidably disposed in the through passage and is sealed against the inner wall to restrict the passage of combustion gasses past the plunger assembly. 
     According to yet a further aspect of the present invention, the plunger assembly further includes a sealing element which is sealed against the inner wall of the piston body. 
     According to still a further aspect of the present invention, the inner wall partially defines an oil gallery on an opposite radial side of the inner wall from the through passage. 
     According to another aspect of the present invention, the combustion surface includes a combustion bowl which is partially defined by the piston body and partially defined by the plunger assembly. 
     According to yet another aspect of the present invention, the piston body is made as a single, integral piece. 
     Another aspect of the present invention is related to a method of operating a power cylinder assembly in an internal combustion engine. The method includes the step of providing a power cylinder assembly which includes a cylinder bore and a piston assembly disposed in the cylinder bore, the piston assembly presenting a combustion surface which separates the cylinder bore into a combustion chamber on one axial side of the combustion surface and a crank case on an opposite axial side of the crank case, and the combustion surface being partially defined by a first upper surface of a first piece and partially defined by a second upper surface of a second piece. The method continues with the step of, during a combustion stroke, maintaining the second upper surface of the second piece in a fixed position relative to the first upper surface of the first piece to provide the power cylinder assembly with a compression ratio in response to pressures of combustion gasses in the combustion chamber being below a predetermined threshold pressure. The method proceeds with the step of, during another combustion stroke, moving the second upper surface of the second piece relative to the first upper surface of the first piece to expand the combustion chamber and reduce the compression ratio of the power cylinder assembly in response to pressures of the combustion gasses in the combustion chamber reaching the predetermined threshold pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description which considered in connection with the accompanying drawings wherein: 
         FIG. 1  is an isometric view of a piston assembly constructed according to one aspect of the present invention; 
         FIG. 2  is a cross-sectional view of a power cylinder assembly during a combustion stroke and wherein a plunger assembly within a piston assembly is in a first position; 
         FIG. 3  is a cross-sectional view of a power cylinder assembly during a combustion stroke and wherein a plunger assembly within a piston assembly is in a second position; 
         FIG. 4A  is a table comparing a change in compression ratio against a diameter of a plunger for a given amount of travel of the plunger assembly; and 
         FIG. 4B  is a plot showing the data of the table shown in  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF THE ENABLING EMBODIMENT 
     Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, an exemplary embodiment of an improved power cylinder assembly  20  for an in internal combustion engines is generally shown in  FIGS. 2 and 3 . The improved power cylinder assembly  20  has an automatically adjustable a compression ratio by modifying a volume of the combustion chamber when the piston is at or near top dead center between the compression and combustion (power) strokes of a conventional four stroke power cycle to maintain combustion pressures below a predetermined threshold pressure. The power cylinder assembly  20  may be easily tuned to prevent the pressure of the combustion gasses in the combustion chamber from exceeding a predetermined threshold pressure by simply altering or selecting certain components of a piston assembly  22  that is positioned in a cylinder bore  24  of the power cylinder assembly  20 . The piston assembly  22  of the exemplary embodiment is particularly suited for use in diesel fueled engines with a high power density (power per unit volume). However, it should be appreciated that the piston assembly  22  could find uses in a range of different types of internal combustion engines including engines with both four and two stroke power cycles and gasoline and diesel fueled engines. 
     Referring now to  FIG. 2 , the exemplary embodiment of the piston assembly  22  is shown in a cylinder bore  24 , which extends along an axis, at approximately a top-dead-center position. The piston assembly  22  presents a combustion surface  26  which separates the cylinder bore  24  into a combustion chamber  28  on one axial side of the combustion surface  26  and a crank case  30  on an opposite axial side of the combustion surface  26 . The combustion surface  26  is partially defined by an upper surface of a piston body  32  and partially defined by an upper surface of a plunger assembly  34 . 
     In the exemplary embodiment, the piston body  32  is a single piece of metal (such as steel or a steel alloy). The piston body  32  includes a crown  36  with an upper surface and an annular ring belt with a plurality of circumferentially extending grooves  38  that are spaced axially from one another. A plurality of piston rings  42  are disposed in the grooves  38  and are biased into contact with a wall of the cylinder bore  24  to seal the piston body  32  with the wall as the piston assembly  22  reciprocates up and down within the cylinder bore  24  during operation of the engine. The exemplary embodiment of the piston body  22  additionally includes a pair of diametrically opposed skirt portions  40  for guiding the reciprocating movement of the piston assembly  22  in the cylinder bore  24  and a pair of co-axially aligned pin bosses with pin bores  44  for receiving a wrist pin  45  which connects the piston assembly  22  with a connecting rod  46 . 
     The upper surface of the crown portion  36  has an annularly shaped and generally planar portion  48  and a combustion bowl  50  which is recessed relative to the generally planar portion  48 . The crown  36  further includes a semi-enclosed and annularly shaped cooling gallery  52  that is spaced radially inwardly of the ring grooves  38  for receiving a cooling oil to cool the crown  36  during operation of the engine. Specifically, the exemplary cooling gallery  52  includes a plurality of circumferentially spaced ports  54  that receive the cooling oil into the cooling gallery  52  and allow the cooling oil to escape the cooling gallery  52  and fall down into the crank case  30 . The cooling gallery  52  is formed into the piston body  32  by joining two separately made pieces of the piston body  32  together on opposite radial sides of the cooling gallery  52 . Each of the two pieces includes a circumferentially extending outer wall  56  and a circumferentially extending inner wall  58  that is spaced radially inwardly from the outer wall  56 . The outer walls  56  are joined together to provide the piston body  32  with a single outer wall  56 , and the inner walls  58  are joined together to provide the piston body  32  with a single inner wall  58 . The outer and inner walls  56 ,  58  could be joined together through, for example, friction welding or induction welding. 
     The inner wall  58  of the piston body  32  has an inner surface that surrounds a cylindrically shaped inner chamber  60  (or through passage) which opens to the combustion chamber  28  in one axial direction and towards the crank case  30  in an opposite axial direction. The inner chamber  60  is generally centrally located in the piston body  32  in a radial direction. The piston body  22  also presents a shelf  62  which is located axially below the inner chamber  60  and above the pin bosses. In the exemplary embodiment, the shelf  62  is made integrally with the piece of the piston body  32  that includes the pin bosses and the skirt portions  40 . 
     The plunger assembly  34  is disposed in the inner chamber  60  of the piston body  32  and is movable relative to the piston body  32  in the axial direction during operation of the engine to expand and contract a volume of the combustion chamber  28 . In the exemplary embodiment, the plunger assembly  34  includes a plunger cusp  64 , a plunger body  66 , a sealing ring  68 , and a spring  70 . The plunger cusp  64  has an upper surface which completes the combustion bowl  50  of the combustion surface  26 . The plunger body  66  is bonded with a lower surface of the plunger cusp  64  and has a skirt that extends axially towards the crank case  30 . The sealing ring  68  is a dykes-type ring  68  that has an L-shaped cross-section and is disposed in a ring groove in the plunger body  66  for sealing the plunger assembly  34  against the inner wall  58  of the piston body  32 . The dykes-type ring  68  is a so-called zero tangential tension spring in that it is configured to exert a very low tension force against the inner wall  58  of the piston body  32  during the intake, compression and exhaust strokes of the power cycle, but during the combustion stroke, pressurized combustion gasses can flow into a convoluted space between the sealing ring  68  and the piston body  32  to force the sealing ring  68  out against the inner wall  58  and seal the combustion gasses in the combustion chamber  28 . As such, the friction is minimized during three of the strokes without compromising the sealing ring&#39;s  68  ability to seal combustion gasses in the combustion chamber  28 . This increases the durability, efficiency and operating life of the piston assembly  22 . Additionally, the sealing ring  68  functions to impede the flow of oil upwardly from the crank case  30  below the piston assembly  22  to the combustion chamber  28  above the piston assembly  22 . In the exemplary embodiment, only one sealing ring  68  is employed to seal the plunger assembly  34  with the piston body  32 . However, it should be appreciated that any suitable number of sealing rings may be employed. 
     The plunger assembly  34  is biased by the spring  70  against a stopper  72  into a first position, which is shown in  FIG. 2 , within the inner chamber  60  wherein the upper surface of the plunger cusp  64  is generally flush with the upper surface of the piston body  32  to complete the combustion bowl  50 . In the exemplary embodiment, the stopper  72  is a counterbore in the inner wall  58  of the piston body, and the plunger body  66  presents a radially outwardly extending flange for contacting the stopper  72  to hold the plunger assembly  34  in the first position. The spring  70  is a compression spring which contacts a first spring seat on the plunger body  66  and a second spring seat on the shelf  62 . It should be appreciated that the stopper  72  may take a range of different configurations for establishing the first position of the plunger assembly  34 . Rather than a compression spring  70 , the biasing means for biasing the plunger assembly  34  towards the first position could be, for example, with hydraulic pressure that is sourced from the engine. 
     During operation, when the fuel and air mixture is ignited within the combustion chamber  28  of the piston body  22  during a compression stroke of the piston assembly  20 , if the pressure in the combustion chamber  28  exceeds a predetermined threshold pressure established, at least partially, by the stiffness of the spring  70 , then the pressure exerts a downward force on the plunger assembly  34  which overcomes the biasing force from the spring  70  to urge the plunger assembly  34  downwardly. This downward movement expands the volume of the combustion chamber  28  to maintain the pressure in the combustion chamber  28  at the predetermined threshold pressure and protect the engine from damage that could result from excessive pressures in the combustion chamber  28 . Increasing the stiffness of the spring  70  increases the predetermined threshold pressure, and reducing the stiffness of the spring  70  reduces the predetermined threshold pressure. The compression ratio (ε) of the engine and the change in combustion chamber volume due to plunger travel (ΔV i ) may be calculated as functions of the plunger diameter (ø i ) and the plunger travel distance (γ i ) according to the following equations with γ i  having a maximum value of 50 mm.  FIG. 3  shows the plunger assembly  34  in a second position with a maximum combustion chamber  28  volume. 
     
       
         
           
             
               
                 
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     The piston assembly  22  advantageously allows for a variable compression ratio during operation of the engine. For example, the piston assembly  22  allows for a relatively high compression ratio (with the plunger assembly  34  in the first position shown in  FIG. 4 ) during start-up (cold) operating conditions of the engine and during times when less power is demanded. Then, when greater power is demanded, the plunger assembly  34  is automatically urged downwardly to expand the combustion chamber  28  and reduce the compression ratio during the combustion strokes to allow the engine to produce increased power without exceeding the predetermined threshold pressure in the combustion chamber  28 . The predetermined threshold pressure can be easily established by selecting a compression spring  70  with an appropriate stiffness. 
     Another aspect of the present invention is related to a method of operating a power cylinder assembly  20 , such as the power cylinder assembly shown in  FIGS. 2 and 3  and discussed above. The method includes the step of, during a combustion stroke, maintaining an upper surface of the plunger cusp  64  in a fixed position relative to an upper surface of the piston body  32  to provide the power cylinder assembly  20  with a compression ratio in response to pressures of combustion gasses in the combustion chamber  28  being below a predetermined threshold pressure. The method proceeds with the step of, during another combustion stroke, moving the upper surface of the plunger cusp  64  relative to the upper surface of the piston body  32  to expand the combustion chamber  28  and reduce the compression ratio of the power cylinder assembly  20  in response to pressures in of the combustion gasses in the combustion chamber  28  reaching the predetermined threshold pressure. 
     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.