Variable compression ratio mechanism for reciprocating internal combustion engine

#The present invention relates to a variable compression ratio mechanism for a reciprocating internal combustion engine, and particularly to the improvements of a lower link of a multi-link type reciprocating internal combustion engine, rotatably installed on a crankpin.

#In recent years, there have been proposed and developed various variable compression ratio mechanisms for reciprocating internal combustion engines, in which a compression ratio is variable depending upon engine operating conditions, such as engine speed and load. One such variable compression ratio mechanism has been disclosed in Japanese Patent Provisional Publication No. 2000-73804 (hereinafter is referred to as “JP2000-73804”). JP2000-73804 discloses a multi-link type variable compression ratio mechanism that a piston and a crankshaft are mechanically linked to each other via a plurality of links. Briefly explaining, the multi-linked variable compression ratio mechanism of JP2000-73804 includes an upper link, a lower link, and a control link. One end of the upper link is rotatably connected to a piston via a piston pin. The other end of the upper link is rotatably pin-connected to the lower link by means of a first connecting pin. The lower link is rotatably installed onto a crankpin of an engine crankshaft. One end of the control link is rotatably connected to the lower link by means of a second connecting pin. The other end of the control link is rotatably connected onto an eccentric cam of a control shaft. The position of the axis of the eccentric cam relative to the axis of the control shaft, that is, the center (pivot axis) of oscillating motion of the control link shifts or displaces relative to the engine body (a cylinder block) by rotating the control shaft by means of an actuator such as an electric motor. As a result of this, a condition of restriction of motion of the lower link via the control link changes, and thus a crank angle versus piston stroke characteristic (containing the position of TDC), that is, a compression ratio varies. Generally, the lower link has a two-split structure composed of a main lower-link portion and a lower-link bearing cap portion separable from each other, so that the lower link can be installed onto or removed from the crankpin. The main lower-link portion and the lower-link bearing cap portion are integrally connected by means of bolts. The substantially half-round section of the main lower-link portion and the substantially half-round section of the lower-link bearing cap provide or form a cylindrical crankpin bearing, when these two halves are assembled to each other with bolts. The main lower-link portion is also formed with a first connecting-pin bearing portion into which the first connecting pin is inserted and a second connecting-pin bearing portion into which the second connecting pin is inserted. As viewed in a direction perpendicular to the axial direction of the crankpin, each of the first and second connecting-pin bearing portions is formed as a forked end, so that each connecting pin is supported at its both axial ends by means of the forked end composed of a pair of axially-spaced connecting-pin supports or a pair of axially-spaced connecting-pin bearings.

#In the multi-linked variable compression ratio mechanism of JP2000-73804, input load is transferred from the upper link and/or the control link and then acts on the lower link via the first connecting pin and/or the second connecting pin. At this time, the input load is further transferred from the two axially-spaced connecting-pin bearings of the forked end of each connecting-pin bearing portion, and acts directly on axial ends of the cylindrical crankpin bearing (see FIG. 9A). There is a possibility that two axial ends of the cylindrical crankpin bearing are remarkably deformed due to the input load. The crankpin bearing is a slide bearing that supports the load by virtue of the films of lubricating oil. In such slide bearings, there is a tendency that the pressure of the lubricating oil film in the crankpin bearing is relatively high at the axial central portion of the crankpin bearing. On the other hand, the pressure of the lubricating oil film in the crankpin bearing is released at the axial end of the crankpin bearing and thus the pressure of the lubricating oil film is relatively low at the axial end. For the reasons discussed above, if the two axial ends of the cylindrical crankpin bearing deform owing to the input load, the input load may not be satisfactorily supported by virtue of the pressure of the oil film. Therefore, there is a possibility of metal-to-metal contact between the axial ends of the crankpin bearing and the outer peripheral wall surface of the crankpin (the bearing journal portion). This results in extremely rapid wear and increased friction.

#FIG. 1 is a cross-sectional view showing a first embodiment of the variable compression ratio mechanism of the invention.

#FIG. 2 is a perspective view showing only a lower link of the first embodiment.

#FIG. 3 is a disassembled perspective view of the lower link of the first embodiment.

#Referring now to the drawings, particularly to FIG. 1, there is shown the detailed multi-link structure of the variable compression ratio mechanism of the first embodiment for a reciprocating internal combustion engine, in a state that an upper link 11, a lower link 13A (13), and a control link 15 are assembled to each other. A piston 1 is slidably fitted to a cylinder liner or a cylinder 6 formed in a cylinder block 5. Piston 1 is attached to one end of upper link 11 via a piston pin 2, to permit adequate freedom for movement between the piston and pin. The other end of upper link 11 is rotatably connected to lower link 13A by way of a first connecting pin 12. Lower link 13A is installed on the outer periphery of a crankpin 4 of an engine crankshaft 3. Piston 1 receives combustion pressure from a combustion chamber defined above its piston crown. Crankshaft 3 is rotatably installed onto cylinder block 5 by means of crankshaft bearing brackets 7. One end of control link 15 is rotatably connected to lower link 13A by means of a second connecting pin 14. The other end of control link 15, that is, the center (pivot axis) 16 of oscillating motion of control link 15 is pivotably supported by an engine body such as the cylinder block so as to permit a displacement of the center 16 of oscillating motion of control link 15 relative to the engine body. When changing a compression ratio of the engine, the center 16 of oscillating motion of control link 15 is shifted or displaced relative to the engine body by means of a support-position control mechanism (a support position changing means) 17. Support position changing means 17 includes a control shaft 18 that is driven or rotated about its axis when changing the compression ratio and a disk-shaped control cam 19 that is fixed to control shaft 18 and whose rotation axis is eccentric to the axis of control shaft 18. The other end of control link 15 is rotatably fitted to the outer periphery of control cam 19. Control shaft 18 is parallel to crankshaft 3 and extends in the cylinder-row direction. Control shaft 18 is rotatably supported by means of crankshaft bearing brackets 7 and control-shaft bearing brackets 8.

#With the previously-noted arrangement, when control shaft 18 of support position changing means 17 is driven by an actuator (not shown) in order to change the compression ratio, the axis (rotation center) of control cam 19, corresponding to the center 16 of oscillating motion of control link 15, shifts relative to the engine body. As a result of this, a condition of restriction of motion of lower link 13A via control link 15 changes, and thus a crank angle versus piston stroke characteristic (containing the position of TDC and the position of BDC), that is, a compression ratio varies.

#Referring to FIGS. 2 through 10, there is shown the detailed structure of lower link 13A incorporated in the variable compression ratio mechanism of the first embodiment. Lower link 13A of the first embodiment has a three-split structure. Concretely, lower link 13A is mainly comprised of a first member 71, a second member 72, and a third member 73. Lower link 13A is formed with a substantially cylindrical crankpin bearing portion 74 into which crankpin 4 is fitted or inserted, a forked first connecting-pin bearing portion 75, whose axis is parallel to the axis of crankpin bearing portion 74 and into which first connecting pin 12 is inserted or fitted, and a substantially cylindrical second connecting-pin bearing portion 76, whose axis is parallel to the axis of crankpin bearing portion 74 and into which second connecting pin 14 is inserted or fitted. As best shown in FIGS. 2 and 3, crankpin bearing portion 74 is divided into two bearing halves, namely a first bearing half-round section (a lower half-round section in FIG. 4) 74a and a second bearing half-round section (an upper half-round section in FIG. 4) 74b. First member 71 is integrally formed with first bearing half-round section 74a and second connecting-pin bearing portion 76. Second member 72 is integrally formed with second bearing half-round section 74b. Third member 73 is integrally formed with first connecting-pin bearing portion 75. The first, second, and third members 71, 72, and 73 are integrally tightened or connected to each other in a direction normal to the axial direction of crankpin 4 by means of a first mounting bolt 77, a second mounting bolt 78, and an auxiliary mounting bolt (a third mounting bolt) 79, so that second member 72 is sandwiched by first and third members 71 and 73 as viewed from the direction normal to the axial direction of crankpin 4. First and second members 71 and 72 are formed with a central connecting portion 80 having a substantially constant axial length L1 (see FIGS. 5 and 6). Central connecting portion 80 corresponds to a central thick-walled portion that annularly surrounds the axial central portion of crankpin bearing portion 74. Concretely, central connecting portion 80 (the central thick-walled portion) is formed by radially increasing partly the thickness of the axial central portion of crankpin bearing portion 74. Central connecting portion 80 is integrally formed with crankpin bearing portion 74. As can be appreciated from the cross sections of FIGS. 5 and 6, the axial length L1 of central connecting portion or central thick-walled portion 80 is dimensioned to be shorter than each of an axial length L2 of crankpin bearing portion 74, an axial length L3 of first connecting-pin bearing portion 75, and an axial length L4 of second connecting-pin bearing portion 76. In the shown embodiment, please note that each of first and second connecting-pin bearing portions 75 and 76 is connected to crankpin bearing portion 74 via only the central connecting portion or central thick-walled portion 80. In more detail, as shown in FIGS. 5 and 9B, the axial central portion of crankpin bearing portion 74 and the axial central portion of second connecting-pin bearing portion 76 are connected to each other via only the central connecting portion 80. That is to say, only the central connecting portion 80 exists between crankpin bearing portion 74 and second connecting-pin bearing portion 76. In other words, crankpin bearing portion 74 and second connecting-pin bearing portion 76 cannot be connected to each other except via the central connecting portion 80. Due to connection between crankpin bearing portion 74 and second connecting-pin bearing portion 76 via central connecting portion 80, as appreciated from the analytical mechanics shown in FIG. 9B, the load transferred from second connecting-pin bearing portion 76 mainly acts on the axial central portion of crankpin bearing portion 74 via central connecting portion 80. Therefore, in the lower link structure of the first embodiment, there is a less possibility that the load is locally concentrated at both axial ends of crankpin bearing portion 74. Thus, as indicated by the phantom line or one-dotted line 74a in FIG. 9B, it is possible to adequately effectively suppress an undesired deformation of each of the axial ends of crankpin bearing portion 74. As a result, it is possible to avoid or suppress direct contact (metal-to-metal contact) between the axial ends of crankpin bearing portion 74 and crankpin 4, thus preventing extremely rapid wear and reducing friction. Actually, the load acting on the axial central portion of crankpin bearing portion 74 can be effectively reliably supported by way of the pressure of the lubricating oil film in the crankpin bearing portion, which pressure is relatively high at the axial central portion of crankpin bearing portion 74. In contrast to the above, in the first comparative example shown in FIG. 9A that a central connecting portion having a relatively short axial length does not exist between a crankpin bearing portion 74′ and a connecting-pin bearing portion 76′, the load applied to both axial ends of connecting-pin bearing portion 76′ acts directly on both axial ends of crankpin bearing portion 74′. As a result, as indicated by the phantom line or one-dotted line 74a′ in FIG. 9A, there is an increased tendency for the axial ends of crankpin bearing portion 74′ to be undesirably remarkably deformed. The undesirable deformation of each axial end of crankpin bearing portion 74′ may lead to direct contact (metal-to-metal contact) between the axial ends of crankpin bearing portion 74′ and crankpin 4.