Abstract:
A variable compression ratio device may include a piston assembly having double pistons of which the volume may be varied, a lifter relatively moving one piston of the double pistons with respect to the other piston of the double pistons, a guide unit engaged with the piston assembly and guiding a movement of the one piston with respect to the other piston, and a locking unit selectively coupling the one piston to the other piston.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to Korean Patent Application No. 10-2013-0022420 filed on Feb. 28, 2013, the entire contents of which is incorporated herein for all purposes by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a variable compression ratio device, and more particularly, to a variable compression ratio device capable of varying a compression ratio by varying a protrusion height of an inner piston which is coupled to an outer piston so as to move in an axial direction, and an internal combustion engine including the same. 
         [0004]    2. Description of Related Art 
         [0005]    In general, a compression ratio of an internal combustion engine is represented by a ratio of a maximum volume of a combustion chamber before compression to a minimum volume of the combustion chamber after compression during a compression stroke of the internal combustion engine. 
         [0006]    When the compression ratio of the internal combustion engine is increased, the output of the internal combustion engine increases. However, when the compression ratio of the internal combustion engine is excessively high, a so-called knocking may occur to reduce the output of the internal combustion engine. Furthermore, the internal combustion engine may be overheated or a valve or piston trouble may occur in the internal combustion engine. 
         [0007]    Thus, the compression ratio of the internal combustion engine is set to a specific value within a proper range before a knocking occurs. When the compression ratio is properly varied according to a load of the internal combustion engine, the fuel efficiency and output of the internal combustion engine may be improved. Therefore, various methods for varying the compression ratio of the internal combustion engine have been proposed. 
         [0008]    The methods for varying the compression ratio of the internal combustion engine may include methods for varying the volume of a combustion chamber during a compression stroke. 
         [0009]    For example, the level of top dead center of the piston may be varied during the compression stroke, or the volume of a secondary combustion chamber provided in a cylinder head may be increased or decreased. 
         [0010]    U.S. Pat. No. 7,284,512 has disclosed a double piston structure having an outer piston and an inner piston, and the outer piston varies a compression ratio while moved vertically by a hydraulic actuator. 
         [0011]    The variable compression ratio device having the above-described double piston structure requires a structural improvement for response and durability. 
         [0012]    The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
       BRIEF SUMMARY 
       [0013]    Various aspects of the present invention are directed to providing a variable compression ratio device capable of reducing the weight and cost thereof through a simplified structure and improving response, operation reliability, and durability, and an internal combustion engine including the same. 
         [0014]    In an aspect of the present invention, a variable compression ratio device may include a piston assembly having double pistons of which the volume is varied, a lifter relatively moving one piston of the double pistons with respect to the other piston of the double pistons, a guide unit engaged with the piston assembly and guiding a movement of the one piston with respect to the other piston, and a locking unit selectively coupling the one piston to the other piston. 
         [0015]    The double piston may include an outer piston, and an inner piston slidably coupled to the outer piston so as to protrude from the outer piston to the outside of the outer piston. 
         [0016]    The outer piston may include an opening formed at a top thereof, and a housing groove formed in the outer piston, wherein the inner piston is inserted through the opening and mounted in the housing groove. 
         [0017]    The guide unit may include a pinion gear rotatably mounted on an exterior circumference of the inner piston, and a rack gear mounted in the outer piston so as to be engaged with the pinion gear. 
         [0018]    The lifter may include a wax or bimetal which pushes the inner piston upward through the opening in receiving an electrical signal. 
         [0019]    The wax or bimetal is formed in a plate shape, and the inner piston is supported by the wax or bimetal. 
         [0020]    The locking unit may include a solenoid installed in the outer piston and selectively protruding toward the housing groove in receiving an electrical signal, and a locking groove formed in the inner piston such that the solenoid is selectively coupled to the locking groove to lock the outer piston and the inner piston. 
         [0021]    The variable compression ratio device may further include an engine control unit (ECU) detecting an operation state of an engine and applying an electrical signal to the lifter and the locking unit such that the volume of the piston is varied according to the operation state of the engine. 
         [0022]    The ECU varies a compression ratio by controlling a magnitude of the electrical signal applied to the lifter according to the operation state of the engine. 
         [0023]    An internal combustion engine may include a cylinder block having the piston assembly, the piston assembly being housed to reciprocate along the cylinder block, and a cylinder head coupled to the top of the cylinder block and forming a combustion chamber with the cylinder block. 
         [0024]    In the variable compression ratio device and the internal combustion engine including the same according to the embodiments of the present invention, as the flexible member is extended in the axial direction of the connecting rod according to the control signal of the ECU, the internal piston is lifted to increase the compression ratio. When the flexible member is contracted in the axial direction of the connecting rod, the inner piston is lowered to decrease the compression ratio. 
         [0025]    Thus, the structure may be simplified to reduce the weight and cost of a vehicle and improve the durability and operation reliability. 
         [0026]    Furthermore, since the variable compression ratio device is operated according to an electrical signal, the response thereof may be improved. The variable compression ratio device may control the compression ratio according to the operation situation of the engine, thereby improving the output and fuel efficiency of the engine. 
         [0027]    The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a perspective view illustrating that a piston to which a variable compression ratio device according to an exemplary embodiment of the present invention is applied is coupled to a connecting rod and a crankshaft. 
           [0029]      FIG. 2  is a cross-sectional view of the piston which realizes a low compression ratio state and a high compression ratio state through the variable compression ratio device according to the exemplary embodiment of the present invention. 
       
    
    
       [0030]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
         [0031]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       DETAILED DESCRIPTION 
       [0032]    Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0033]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0034]    Referring to  FIGS. 1 and 2 , a cylinder head  20  is mounted over a cylinder block  10  constituting an engine, thereby forming a combustion chamber. 
         [0035]    A piston  30  is inserted into the cylinder block  10  so as to reciprocate vertically along the cylinder block  10 . One end of a connecting rod  40  is connected to the piston  30  through a piston pin or the like so as to move as one unit, and the other end of the connecting rod  40  is coupled to a crankshaft  50 . 
         [0036]    When the piston  30  reciprocates along the cylinder block  10 , the connecting rod  40  converts the reciprocating operation of the piston  30  into a rotating operation to rotate the crankshaft  50 , and the crankshaft  50  generates a rotation power while rotated in one direction. 
         [0037]    When the piston  30  rises or drops along the cylinder block  10 , the volume of the combustion chamber is varied by the piston  30 . 
         [0038]    When the volume of the combustion chamber formed by the cylinder  10 , the cylinder head  20 , and the piston  30  is varied at the time at which the piston  30  is moved toward the top of the combustion chamber and positioned at the top dead center, the compression ratio of the engine is varied. 
         [0039]    According to the exemplary embodiment of the present invention, when the piston  30  is positioned at the top dead center, the volume of the piston  30  may be varied to control the compression ratio of the engine. 
         [0040]    That is, the piston  30  is a volume variable piston. 
         [0041]    The volume variable piston  30  has a double piston structure to vary the volume thereof. 
         [0042]    That is, the piston  30  includes an outer piston  32  and an inner piston  34 . 
         [0043]    The outer piston  32  has an opening  55  formed at the top thereof and a housing groove  32   a  formed therein to communicate with the opening  55 . The housing groove  32   a  is expanded in a radial direction to have a larger diameter than that of the inner piston  34 , and extended in an axial direction. 
         [0044]    The inner piston  34  is inserted through the upper opening of the outer piston  32  and housed in the housing groove  32   a.    
         [0045]    The inner piston  34  is installed to relatively move in an axial direction with respect to the outer piston  32  in a state in which the inner piston  34  is housed in the housing groove  32   a.    
         [0046]    A guide unit is provided between the inner piston  34  and the outer piston  32  so as to guide the movement of the inner piston  34 . 
         [0047]    The guide unit includes a pinion gear  60  mounted on the exterior circumference of the inner piston  34  positioned in the housing groove  32   a  and a rack gear  62  mounted on the exterior circumference of the housing groove  32   a  facing the exterior circumference of the inner piston  34  and engaged with the pinion gear  60 . 
         [0048]    When the inner piston  34  is relatively moved with respect to the outer piston  32 , the pinion gear  60  is moved along the rack gear  62  in a state where the pinion gear  60  is engaged with the rack gear  62 , thereby guiding the movement of the inner piston  34 . 
         [0049]    A lifter  70  is provided to relatively move the inner piston  34  with respect to the outer piston  32 . 
         [0050]    The lifter  70  is installed at the bottom of the inner piston  34 , and may have a plate shape. 
         [0051]    The lifter  70  may include a bimetal plate or wax plate which is deformed to protrude upward in the axial direction when receiving an electrical signal, and pushes the inner piston  34  upward in the axial direction toward the combustion chamber. 
         [0052]    A locking unit is provided to maintain the position of the inner piston  34  in a state where the inner piston  34  is pushed upward by the lifter  70 . 
         [0053]    The locking unit may include a solenoid  80  installed in the outer piston  32  so as to protrude toward the housing groove  32   a  and a locking groove  34   a  formed on the exterior circumference of the inner piston  34 . The locking groove  34   a  may have a depressed shape such that the solenoid  80  is detachably coupled to the locking groove  34   a.    
         [0054]    In  FIG. 2 , a low compression ratio state corresponds to a state in which the inner piston  34  is inserted into the housing groove  32   a  of the outer piston  32  when the outer piston  32  is positioned at the top dead center. In this state, the combustion chamber has a volume V 1  formed by the cylinder head  20 , the cylinder block  10 , the outer piston  32 , and the top surface of the inner piston  34 . 
         [0055]    When an engine control unit (ECU) applies an electrical signal to the lifter  70  at the time at which the outer piston  32  is positioned at the top dead center, the lifter  70  is elastically deformed upward in the axial direction toward the combustion chamber, and the inner piston  34  supported by the lifter  70  is protruded from the housing groove  32   a  of the outer piston  32  upward in the axial direction toward the combustion chamber through the elastic deformation of the lifter  70 . 
         [0056]    Thus, the volume V 2  of the combustion chamber decreases by a volume (V 1 -V 2 ) corresponding to the protrusion height H of the inner piston  34 . Then, a high compression ratio state is realized. 
         [0057]    The ECU applies an electrical signal to the solenoid  80  in a state where the inner piston  34  is protruded by the lifter  70 , and the solenoid  80  is inserted and locked to the locking groove  34   a  of the inner piston  34  toward the housing groove  32 , thereby maintaining the high compression ratio state. 
         [0058]    When the electrical signal applied by the ECU is blocked, the lifter  70  and the solenoid  80  are recovered to the original shapes thereof, and the inner piston  34  is housed in the housing groove  32   a  by the weight thereof and the pressure of the compression chamber, thereby realizing the low compression ratio state. 
         [0059]    According to the magnitude of the electrical signal applied to the lifter  70  from the ECU, the deformation of the lifter  70  may be varied to control the protrusion height H of the inner piston  34 . 
         [0060]    For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner” and “outer”, are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
         [0061]    The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.