INTERNAL COMBUSTION ENGINE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2008-077178 filed March 25, 2008, and Japanese Application No. 2009-018898 filed January 30, 2009, each of which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an internal combustion engine comprising a multilink-type piston crank mechanism.

2. Description of the Related Art

An internal combustion engine which uses a multilink-type piston crank mechanism for converting reciprocating motion of a piston within a cylinder bore into rotating motion of a crankshaft is disclosed, for example, in Japanese Patent Provisional Publication No. 2001-227367.

Drawbacks have been encountered in an internal combustion engine having such a multilink-type piston crank mechanism. In particular, engine performance may be improved by elongating a piston stroke, i.e., the distance of travel of the piston between top dead center and bottom dead center positions of the piston. However, it is desirable to minimize the weight of the engine block by constraining the overall height of the internal combustion engine (i.e., the distance measured from the center of the crankshaft journal to the top deck of the engine block). In prior art designs which attempt to elongate the piston stroke while constraining the overall height of the engine, interference may result between the bottom end of the cylinder bore and the path of a link pin in the multilink-type piston crank mechanism, and/or between a counterweight of the crankshaft and the engine block. When the piston is in the vicinity of the top dead center, the potential interference between the path of a link in the multilinlc-type piston crank mechanism and the bottom end of the cylinder bore becomes most significant. As used herein, the vicinity of top dead center designates a position of the piston within the cylinder bore at which the piston is within the range of approximately 45° before top dead center to approximately 45° after top

dead center, or more preferably a position of the piston within the cylinder bore at which the piston is within the range of approximately 30° before top dead center to approximately 30° after top dead center.

SUMMARY OF THE INVENTION An object of the present invention is to provide an internal combustion engine which allows an elongated piston stroke while avoiding interference between the bottom end of the cylinder bore and the path of a link of the multilink-type piston crank mechanism.

In one embodiment of the invention, an internal combustion engine is provided including a multilink-type piston crank mechanism. The mechanism includes a piston reciprocating within a cylinder bore over a piston stroke distance of reciprocation in an upward-downward direction, a crankshaft, an upper link having one end connected to the piston via a piston pin and another end connected to a lower link via an upper pin, the lower link connecting the upper link to a crankpin of the crankshaft, and a control link having one end connected to the lower link via a control pin and another end swingably supported by a main body of the engine. When the piston is in the vicinity of top dead center, the lower link moves in a lateral direction generally perpendicular to an axial direction of the crankshaft and generally perpendicular to a direction of reciprocating motion of the piston, and an upper surface of the lower link remains generally perpendicular to the direction of reciprocating motion of the piston.

In another embodiment of the invention, an internal combustion engine is provided including a multilink-type piston crank mechanism. The mechanism includes a piston, a crankshaft having a crankpin, an upper link connected at one end to the piston, a control linking swingably supported at one end by a main body of the engine, and a lower link connecting the upper link, the crankpin, and the control link. When the piston is in the vicinity of top dead center, the lower link is moved generally perpendicularly with respect to a direction of reciprocating motion of the piston, and an upper surface of the lower link remains generally parallel to the movement of the lower link when the piston is in the vicinity of top dead center.

According to the present invention, an internal combustion engine can achieve an elongated stroke while avoiding interference between the bottom end of cylinder bore and the path of a link of the multilink-type piston crank mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention.

Fig. 1 shows a first comparative example of an internal combustion engine comprising a multilink-type piston crank mechanism.

Fig. 2 shows an embodiment of an internal combustion engine according to the present invention, illustrating a positional relationship among a piston, a counterweight, and a cutout portion in the engine block.

Fig. 3 is an explanatory view showing a multilink-type piston crank mechanism according to an embodiment of the present invention.

Fig. 4 shows a second comparative example of the multilink-type piston crank mechanism.

Fig. 5 shows a third comparative example of the multilink-type piston crank mechanism. Fig. 6 shows a fourth comparative example of the multilink-type piston crank mechanism.

Fig. 7 is an explanatory view schematically showing a state in the multilink-type piston crank mechanism according to an embodiment of the present invention, the state being a 45° crank angle before the piston top dead center position. Fig. 8 is an explanatory view schematically showing a state in the multilink-type piston crank mechanism according to an embodiment of the present invention, the state being a 30° crank angle before the piston top dead center position.

Fig. 9 is an explanatory view schematically showing a state in the multilink-type piston crank mechanism according to an embodiment of the present invention, the state being a 15° crank angle before the piston top dead center position.

Fig. 10 is an explanatory view schematically showing a state in the multilink-type piston crank mechanism according to an embodiment of the present invention,' the state being the crank angle at the piston top dead center position.

Fig. 11 is an explanatory view schematically showing a state in the multilink-type piston crank mechanism according to an embodiment of the present invention, the state being a 15° crank angle after the piston top dead center position.

Fig. 12 is an explanatory view schematically showing a state in the multilink-type piston crank mechanism according to an embodiment of the present invention, the state being the crank angle at the piston bottom dead center position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

Shown in Fig. 1 is an explanatory schematic arrangement of a multilink-type piston crank mechanism in an internal combustion engine. Fig. 1 is depicts a first comparative example of a multilink-type piston crank mechanism that corresponds generally to a multilink-type piston crank mechanism according to the present invention. Therefore, corresponding components are illustrated by the same reference numerals.

For purpose of describing the multilink-type piston crank mechanism, directions are specified with respect to the direction of the reciprocating motion of the piston, such that the direction toward a top dead center position of the piston is designated as upward while the direction toward a bottom dead center position of the piston is designated as downward. Similarly, the upper end or top deck of the cylinder bore indicates the portion of the bore near top dead center and the lower end or bottom end of the cylinder bore indicates the portion of the bore near bottom dead center. Further, comparative designations such as higher/lower and above/below indicate the relative location of components with respect to the top of the cylinder bore when measured along a line parallel to the axis of reciprocating motion of the piston. For example, a first component being higher than or above a second component (i.e., the second component being lower than or below the first component) means that the distance between the first component and the top of the cylinder bore is less that the distance between the second component and the top of the cylinder bore, wherein

both distances are measured along lines parallel to the axis of reciprocating motion of the piston.

In addition, a lateral direction is designated as a direction that is perpendicular to both the axis of reciprocating motion of the piston and the axis of the crankshaft, wherein the axis of the crankshaft and the axis of reciprocating motion of the piston are also perpendicular to each other. The terms outer/inner and outward/inward designate radial directions or positions with respect to the center axis of the cylinder bore 18, such that a component can be disposed outward or farther from the center axis of the cylinder bore 18, or inward or close to the center axis of the cylinder bore 18. As shown in Fig. 1 , a piston 9 is disposed in a cylinder bore 18 such that the piston

9 can reciprocate upward and downward between a bottom dead center position (when the piston 9 is at a lowest point within the cylinder bore 18) and a top dead center position (when the piston 9 is at a highest point within the cylinder bore 18). The cylinder bore 18 has an upper end or top deck 30 disposed at or above a highest top dead center position of the piston 9 (i.e., when the compression ratio is at a maximum) and a lower or bottom end 32 disposed at or below a lowest bottom dead center position of the piston 9 (i.e., when the compression ration is at a minimum).

The piston 9 is linked to a crankshaft 1 by a mechanism 25 for converting the reciprocating motion of the piston 9 to rotational motion of the crankshaft 1. The crankshaft 1 is supported in the engine by a crankshaft journal 11, and the crankshaft 1 extends in an axial direction to rotate about an axis defined by the center of the journal 11. The axial direction of the crankshaft 1 is perpendicular to the reciprocating direction of the piston 9. The crankshaft 1 is a conventional crankshaft having at least one crankpin 2 extending radially outward from the crankshaft axis and at least one counterweight 16 extending radially outward from the crankshaft axis in an opposite direction from the crankpin 2.

The mechanism 25 includes an upper link 5, a lower link 3, and a control link 7, each link having two opposed ends. The lower link 3 is rotatably supported by the crankpin 2 of the crankshaft 1. One end of the lower link 3 is connected to a lower end of the upper link 5 via an upper pin 4. The other end of the lower link 3 is connected to an upper end of the control link 7 via a control pin 6. The upper end of the upper link 5 is

connected to the piston 9 via a piston pin 8. The lower end of the control link 7 is connected eccentrically relative to a control shaft 10, the control shaft 10 being supported by a main body of the internal combustion engine in such a manner as to be generally parallel with the axis of the crankshaft 1. Specifically, the lower end of the control link 7 is connected to an eccentric cam 1 Oa disposed on the control shaft 10. As a result, the control link 7 is restrained so that the control pin 6 at the upper end of the control link 7 moves in a swinging motion centered about a pivot point 21 on the eccentric cam 10a. More specifically, the fulcrum of the swinging motion of the control link 7 is arranged to be changed in accordance with a rotational position of eccentric cam 1 Oa, and as the rotational position of the eccentric cam 10a is changed, the top dead center of piston 9 is changed, resulting in a change in the compression ratio.

In a multilink-type piston crank mechanism as shown in Fig. 1, as compared with a conventional single-link type piston crank mechanism, the piston stroke can be characterized by a nearly simple oscillation, depending on the placement of the links. As a result, the maximum acceleration, and thus inertia force, in the vicinity of top dead center can be decreased by use of a multilink-type piston crank mechanism. Further, because the maximum acceleration of the piston in opposite directions at the top dead center and bottom dead center positions results in a second order component of crankshaft vibration having twice the frequency of the crankshaft rotational speed, by decreasing the maximum acceleration of the piston, a decrease in the second order crankshaft vibration can also be achieved. The amplitude of the second order vibration can be increased (i.e., worsened) by increasing the displacement of the piston, particularly by elongating the piston stroke which increases the difference in acceleration between the piston at top dead center and the piston at bottom dead center. As shown in Fig. 1, and subsequently in Figs. 3-6, coordinates are defined including a z-axis (not shown but extending into and out of the page) corresponding to the axis of the crankshaft 1, a y-axis 12 extending through the center of the crankshaft journal 11 and parallel to the reciprocating motion of the piston 9 (i.e., in an upward-downward direction) and an x-axis 26 extending through the center of the crankshaft journal 11 and perpendicular to the reciprocating motion of the piston 9 (i.e., in a lateral direction). A downward rotation region 13 is defined wherein as the crankshaft 1 rotates, a component of

the movement of the crankpin 2 is in the downward direction of the y-axis 12, and an upward rotation region 14 is defined wherein as the crankshaft 1 rotates, a component of the movement of the crankpin 2 is in the upward direction of the y-axis 12. In Figs. 1 and 7-12, the direction of crankshaft rotation is clockwise, while in Figs. 3-6, the direction of crankshaft rotation is counterclockwise.

The control shaft 10 is located in the downward rotation region 13 in which a movement path of the center of crankpin 2 is downward. A movement path of the center of the upper pin 4 and the reciprocation axis 15 of the piston pin 8 and the piston 9 are located in the upward rotation region 14 in which the movement path of the center of the crankpin 2 is upward.

The pivot point 21 of the swinging motion of the control link 7 is located below the center of the crankshaft journal 11. A movement path of the center of the control pin 6 is an arc which is concave downward.

In an internal combustion engine having such an arrangement, it is possible to move the path of the reciprocating motion of the piston 9 upward or downward by moving the position of the pivot point 21 of the swinging motion of the control link 7 relative to the center of the crankshaft journal 11, thereby enabling the compression ratio of the internal combustion engine to be changed in accordance with the driving conditions. The position of the pivot point 21 is moved by rotating the eccentric cam 10a (i.e., by rotating the control shaft 10).

Fig. 3 is an explanatory wire-frame view illustrating link geometries of an embodiment of a multilink-type piston crank mechanism according to the present invention. In the depicted view, the crankshaft 1 is rotated counterclockwise such that the explanatory view of Fig. 3 is obtained by taking a mirror view of an arrangement such as shown in Fig. 1, or more particularly, of the arrangements according to embodiments of the present invention as shown in Figs. 7-12.

As depicted in Fig. 3, the center of the upper pin 4 is below the projection of a straight line 3 a linking the center of the crankpin 2 with the center of the control pin 6. Stated otherwise, the geometry of the lower link 3 is such that the upper pin 4 and the control pin 6 are disposed generally opposite each other with respect to the crankpin 2, and

the line 3b linking the center of the upper pin 4 and the center of the control pin 6 is disposed below the center of the crankpin 2.

In other words, the straight line 3 a is a line drawn between the center of the control pin 6, which serves as a pivot point between the control link 7 and the lower link 3, and the center of the crankpin 2, which serves as a pivot point between the crankpin 2 and the lower link 3. Also, the straight line 3b is a line drawn between the center of the control pin 6 and the center of the upper pin 4, which serves as a pivot point between the upper link 5 and the lower link 3. The lower link 3 is arranged such that the center of the upper pin 4 is located below the straight line 3 a, or alternatively such that the crankpin 2 is located above the straight line 3b.

In the multilink-type piston crank mechanism according to the embodiment of the present invention depicted in Fig. 3, the link geometries are arranged such that the magnitude of piston acceleration at top dead center of the reciprocating motion is generally equal to the magnitude of piston acceleration at bottom dead center. As a result, the inertial force in the upward-downward direction (along the y-axis 12) of the internal combustion engine approaches a harmonic vibration, which particularly allows a four-cylinder internal combustion engine to have improved vibration characteristics.

A crankpin path 2a traces the path of movement followed by the center of the crankpin 2 as the crankshaft 1 rotates about the crankshaft journal 11 and the piston 9 reciprocates upward and downward in the cylinder bore 18. The crankpin path 2a is circular. An upper link pin path 4a traces the path of movement followed by the center of the upper pin 4 as the piston 9 reciprocates upward and downward in the cylinder bore 18. The upper link path 4a is asymmetrically shaped about an upward-downward axis such that the outward displacement of the upper link pin 4 from the reciprocation axis 15 of the piston 9 is smaller during the piston downstroke (e.g., during the intake and expansion strokes of a four-stroke engine) than during the piston upstroke (e.g., during the compression and exhaust strokes of a four-stroke engine). A control pin path 6a traces the path of movement followed by the control pin 6 as the piston 9 reciprocates upward and downward in the cylinder bore 18. The control pin path 6a is shaped as an arc that is concave downward, because the control link 7 pivots about the control pivot point 21.

Fig. 4 is an explanatory wire-frame view illustrating link geometries of a conventional multilink-type piston crank mechanism corresponding to the arrangement of Fig. 1. Fig. 4 is discussed for comparison with the embodiment of the present invention illustrated in Fig. 3. In Fig. 4, components corresponding with the multilink-type piston crank mechanism as shown in Fig. 1 are illustrated by the same reference numerals. The height of the engine including the mechanism as in Fig. 1 is the same as the height of the engine including a mechanism according to the present invention as in Fig. 3, the height of the engine being measured by the distance from the center of the crankshaft journal 11 to the upper end surface or top deck 30 of the cylinder bore 18. Also, the stroke of the piston 9 in the mechanism as in Fig. 1 is the same as the stroke of the piston 9 in the mechanism according to the present invention as in Fig. 3, the piston stroke being the distance between top dead center and bottom dead center of the piston 9 within the cylinder bore 18.

However, as depicted in Fig. 4, the center of the upper pin 4 is above the projection of a straight line 3 a linking the center of the crankpin 2 with the center of the control pin 6. Stated otherwise, the geometry of the lower link 3 is such that the upper pin 4 and the control pin 6 are disposed generally opposite each other with respect to the crankpin 2, and the line 3 b linking the center of the upper pin 4 and the center of the control pin 6 is disposed above the center of the crankpin 2. As a result, it can be seen in Fig. 4 that the upper link path 4a overlaps the bottom end 32 of the cylinder bore 18, creating a possible interference between the cylinder bore 18 and the junction of the upper link 5 and lower link 3. One method to avoid such interference would be to form a cutaway portion near the bottom end 32 of the cylinder bore 18; however, such a cutaway portion can weaken the strength of the cylinder bore 18 when the piston 9 is in the vicinity of bottom dead center and cause the shape of the cylinder bore 18 to distort under pressure, causing a negative effect on the sliding performance between a skirt of the piston 9 and the cylinder bore 18. To avoid such interference in the present invention, the upper, link path 4a can be displaced downward such that when the piston 9 is in the vicinity of top dead center, the path of the upper link 4 does not overlap the cylinder bore 18. Thus, comparing the embodiment of the present invention of Fig. 3 with the comparative embodiment of Fig. 4, it can be seen that the entire upper pin path 4a of Fig. 3 is shifted further downward than

the entire upper pin path 4a of Fig. 4. As a result, when the piston 9 is in the vicinity of top dead center, the upper pin path 4a in Fig. 3 (the embodiment of the present invention) does not invade a space defined by the cylinder bore 18; in contrast, the upper pin path 4a in Fig. 4 (the first comparative example) invades the space defined by the cylinder bore 18. Thus, in the embodiment of the present invention, interference between the cylinder bore 18 and the junction of the upper link 5 and the lower link 3 is avoided. Accordingly, the arrangement of the multilink-type piston crank mechanism in the present invention allows the piston 9 to achieve the piston bottom dead center position while retaining the shape of the lower end 32 of the cylinder bore 18 and avoiding a negative effect on the sliding performance between the skirt of the piston 9 and cylinder bore 18, and while not raising the height of the engine block as measured between the top deck 30 of the cylinder bore 18 and the center of the crankshaft journal 11. Thus, the height of the engine block can be regulated to equal to or less than twice the length of the piston stroke, such that it becomes possible both to increase the displacement of the piston by elongating the piston stroke and to decrease the size of the internal combustion engine. Additionally, in the embodiment as shown in Fig. 3, the magnitude of piston acceleration at top dead center of the reciprocating motion of the piston 9 can be generally equal to the magnitude of piston acceleration at bottom dead center.

Further, as shown in Fig. 2, the outermost edge portion of the counterweight 16 of the crankshaft 1 can pass beside a pin boss section 17 of the piston 9 when the piston 9 is in the vicinity of the bottom dead center. Therefore, at the bottom end 32 of the cylinder bore 18, a cutout portion 19 is formed where an outermost edge of a movement path of the counterweight 16 of the crankshaft 1, or a part of the multilink-type piston crank mechanism 25, overlaps with the lower end 32 of the cylinder bore 18 while not interfering with the cylinder bore 18. With such an arrangement, it becomes possible to move the bottom dead center position of piston 9 downward thereby elongating the piston stroke more than comparative examples of the internal combustion engine having a similar overall height.

The above effects have been discussed as applied to an internal combustion engine comprising an embodiment of a multilink-type piston crank mechanism 25 as in Fig. 3, wherein the engine is formed with a cutout portion 19 at the lower end 32 of the cylinder

bore 18 to accommodate the center of the piston pin 8 being disposed lower than the outermost edge of the counterweight 16 of the crankshaft 1 when the piston 9 is at bottom dead center, wherein a principal purpose is to elongate the piston stroke. However, the advantages of preventing interference between upper link path 4a and the cylinder bore 18, and of lowering the piston bottom dead center position, can still be obtained even in an internal combustion engine comprising a multilink-type piston crank mechanism not formed with a cutout portion 19 at the lower end 32 of the cylinder bore 18 or not having an outermost edge of the counterweight 16 being disposed above the center of the piston pin 8 when the piston 9 is at the bottom dead center. For reference, the straight line 12 passing through the center of the crankshaft journal 11 and extending in the upward-downward direction of the reciprocating motion of the piston 9 is referred to as the y-axis and the straight line 26 passing through the center of the crankshaft journal 11 and extending perpendicular to the straight line 12 is referred to as the x-axis. In the link geometries as shown in Fig. 3 (the embodiment of the present invention) the distance from the center of the crankshaft journal 11 to the center axis 15 of the cylinder bore 18 is made consistent with the lateral (x-coordinate) location of the upper pin 4, as measured by the distance between the center axis 15 and an upward-do wnward line through the upper pin 4 at a timing of 15° crank angle after top dead center. In other words, the size, shape, layout, and the like of each of the links constituting the multilink- type piston crank mechanism 25 are arranged such that an offset amount in the direction of the x-axis of the center of crankshaft journal 11 from the cylinder bore 18 is consistent with the distance from the center of the crankshaft journal 11 to the center of the upper pin 4 in the direction of the x-axis at a timing of 15° crank angle after top dead center.

With such a link geometry of the mechanism 25, the upper link 5 is disposed in a generally upward-downward orientation at the time of a maximum cylinder internal pressure, i.e., shortly after top dead center in the expansion stroke in a four-stroke engine. Thus, a reduction is achieved in piston side thrust, which is the force exerted on the cylinder walls by the piston 9. The greater the angle of the upper link 5 with respect to an upward-downward direction, the larger the proportion of piston side thrust. In addition, the portion of the upper pin path 4a followed by the upper pin 4 during a piston downstroke

(highlighted by a path 4b) is generally oriented in the upward-downward direction such that

the upper pin 4 travels generally parallel to a straight line extending in the direction of the reciprocating motion of the piston 9. Further, during the piston downstroke, the inclination of upper link 5 with respect to an upward-downward orientation tends not to vary significantly, and therefore it becomes possible to keep the upper link 5 generally oriented in an upward-downward direction during nearly the entire piston downstroke. Because the side thrust of the piston 9 is proportional to both the inclination of upper link 5 and the downward load applied to the piston 9, it becomes possible with such an arrangement to reduce the friction loss due to the piston side thrust during an expansion stroke. This is important because it is during the expansion stroke when the cylinder internal pressure is at its highest, due to combustion of a fuel-air mixture, such that the downward load on the piston 9 is at its maximum during the expansion stroke.

The inclination of the upper pin path 4a during the piston downstroke is determined by the length of the control link 7 and by the location of the rotational center 21 of the eccentric cam 10a, which serves as a pivot point for the swinging motion of the control link 7. In this embodiment, the pivot point 21 of the control link 7 is disposed in the downward rotation region 13 in which crankpin 2 rotates downward, and lower than center of the crankshaft journal 11.

Figs. 5 and 6 show link geometries of second and third comparative examples, each of which is different from the embodiment of Fig. 3 according to the present invention. Because the comparative examples of the multilink-type piston crank mechanism as shown in Figs. 5 and 6 are generally similar to the multilink-type piston crank mechanism of the above-discussed Figs. 1 and 3, corresponding components are illustrated by the same reference numerals, so the description is omitted for brevity.

In the multilink-type piston crank mechanism as shown in Fig. 5, the pivot point 21 of the control link 7 is disposed in the downward rotation region 13 in which the crankpin 2 rotates downward, and lower than center of the crankshaft journal. However, upper pin path 4a during the piston downstroke (as highlighted by the path 4b) slopes downward and outward with respect to the reciprocating axis 15 of the piston 9. The slope of the upper pin path 4a during the piston downstroke is determined by the position of the pivot point 21 of the control link 7. In the second comparative embodiment of Fig. 5, the pivot point 21 is disposed upward and outward as compared with the most favorable pivot point 21 in the

embodiment of Fig. 3. It can readily be seen that the inclination of the upper link 5 varies in accordance with the inward-outward displacement amount of the pivot point 21 with respect to the reciprocating axis 15 of the piston 9, such that the farther outward the pivot point 21 is displaced, the great the angle of inclination of the upper link 5 from an upward- downward orientation. Therefore, the piston side thrust during the piston downstroke is less in the most favorable embodiment of Fig. 3 as compared with the second comparative embodiment of Fig. 5.

In the multilink-type piston crank mechanism as shown in Fig. 6, the pivot point 21 of the control link 7 is disposed in upward-downward alignment with the center of the crankshaft journal 11 (i.e., directly below the crankshaft 1). The upper pin path 4a during the piston downstroke (as highlighted by the path 4b) slopes downward and inward with respect to the reciprocating axis 15 of the piston 9. In the third comparative embodiment of Fig. 6, the pivot point 21 is disposed upward and inward as compared with the most favorable pivot point 21 in the embodiment of Fig. 3. Therefore, the reduction of the piston side thrust during the piston downstroke achieved in the third comparative embodiment of Fig. 6 is less than the reduction in piston side thrust during piston downstroke achieved in the embodiment of Fig. 3. Thus, although the slope of the upper link 5 in Fig. 6 is in the opposite direction as the slope of the upper link 5 in Fig. 5, the comparative embodiment of Fig. 6 similarly does not perform as well as the embodiment of Fig. 3. Further, the embodiment of Fig. 6 has a disadvantage because the internal combustion engine must be increased in size below the center of the crankshaft journal 11 to accommodate the eccentric cam 1 Oa and the control shaft 10 being disposed directly below the crankshaft 1.

Furthermore, if the size, shape, layout, and the like of each of the links constituting the multilink-type piston crank mechanism are arranged such that the amplitude of the second order crank vibration in the reciprocating motion of the piston is equal to or less than 3% of the length of the piston stroke, the vibration characteristics of the engine operating in a vehicle can be remarkably improved.

The crank throw is defined as the distance between the center of the crankshaft journal 11 and the center of the crankpin 2. The shape and size of the lower link 3 can be arranged to elongate the piston stroke, and thus to achieve a greater engine displacement,

without proportionally increasing the crank throw. In particular, the ratio of the distance Ll from the center of the crankpin 2 to the center of the control pin 6 (i.e., the length of line 3 a) and the distance L2 from the center of the crankpin 2 to the center of the upper pin 4 can be determined by the size and shape of the lower link 3. When the ratio of L2/L1 falls within the range of about 0.9 to about 1.1 , it is possible to elongate the piston stroke while suppressing an elongation of the crank throw. Additionally, since the lower link 3 is moved in a circular motion having the crank throw as a radius (i.e., as the crankpin 2 rotates about the center of the crankshaft journal 11), a shorter crank throw results in a shorter (smaller radius) movement path of the lower link 3, enhancing the ability to avoid interference between the lower link 3 and the lower end 32 of the cylinder bore 18.

Moreover, the center of the crankshaft journal 11 is disposed outward from the reciprocating axis 15 of the piston 9 so that the overall height of the internal combustion engine can be restrained while suppressing interference with the cylinder bore 18.

The positional relationship between the upper pin path 4a and the cylinder bore 18 has been discussed with reference to Fig. 3 by using the wire-frame diagram. However, in actuality, members such as the lower link 3 require a wall thickness sufficient to support loads to which they are subjected. Therefore, it is necessary to consider the interference between such actual-thickness members.

Figs. 7 to 12 are views of the multilink-type piston crank mechanism whose members are provided with a wall thickness in consideration of a load which is to be applied to each of member. Figs. 7 to 11 show states in 15° increments of crank angle, including 45° before top dead center (Fig. 7), 30° before top dead center (Fig. 8), 15° before top dead center (Fig. 9), top dead center (Fig. 10), and 15° after top dead center (Fig. 11). Fig. 12 shows a state of bottom dead center. The crankshaft rotates clockwise as indicated by an arrow in the figures.

In the multilink-type piston crank mechanism 25, the piston stroke is elongated. More specifically, in order to increase the length of the upper pin path 4a in an upward- downward direction parallel to the piston stroke reciprocating axis 15, it is preferable to widely swing or oscillate the lower link 3 about the control pin 6 while the crankshaft 1 is rotated. In this case, there is a limit to the amount of swing or degree of oscillation of the swinging motion of the lower link 3 when the piston 9 is at the bottom dead center

position, as shown in Fig. 12, because of the interference between the control link 7 and the lower link 3 due to their thickness.

More specifically, the links must be arranged in such a manner as to not overlap with each other (except for the pin boss section where each pin is disposed) since the center lines of the upper link 5, the lower link 3 and the control link 7 are generally aligned in the same plane so as not to impose the overturning moment viewed from the side of engine (or from the direction perpendicular to the axis of the crankshaft) on each of the links.

For example, in the embodiment of the present invention, consider the straight line 3 a drawn from the center of the control pin 6 to the center of the crankpin 2. As the crankshaft 1 rotates in the vicinity of bottom dead center, the inclination of the line 3 a can vary with a span of about 30° to 40° with respect to the lateral direction. The span of this angle is difficult to increase.

Similarly, the inclination of the straight line 3 a as the crankshaft 1 rotates in the vicinity of top dead center side also can vary with a span of about 30 to 40° with respect to the lateral direction. When the straight line 3 a is further inclined at the top dead center position (without altering other conditions) it is necessary, for example, to enlarge the crank throw (i.e., to set the center of the crankshaft journal 11 and the center of the control pin 6 further apart from each other). However, enlargement of the crank throw increases the possibility of interference between the link members at the bottom dead center position such that it is necessary to decrease the inclination of the straight line 3 a with respect to the lateral direction at bottom dead center (or to reduce the oscillation of the lower link 3), thereby shortening the piston stroke.

Additionally, elongation of the distance between the center of crankshaft journal 11 and the center of the control pin 6 reduces a lever ratio of the lower link 3 (or the ratio of the distance between the center of the upper pin 4 and the center of the control pin 6 to the distance between the center of the crankpin 2 and the center of the control pin 6). By reducing the lever ratio of the lower link 3, the effect of elongating the piston stroke is balanced out by a longer crank throw. Therefore, the inclination of the lower link 3 (or the straight line 3 a) in the embodiment of Fig. 3 can be regarded as a preferred inclination obtained as a result of taking the above into account. In this embodiment, the lower link 3

is thus widely swung about the control pin 6 within a sufficient range that interference between the members does not occur, and thereby the piston stroke can be lengthened. Meanwhile, when the respective inclinations of the straight line 3 a at top dead center and bottom dead center are significantly different from each other, the upper pin path 4a of the upper pin 4 is sloped (as shown in Figs 5 and 6) so as not to achieve a preferred piston stroke. Therefore, it is preferable that the respective inclinations of the straight line 3 a with respect to the lateral direction at top dead center and bottom dead center are nearly equal to each other, resulting in a condition in which the lower link 3 symmetrically oscillates in the lateral direction. If the inclination of the straight line 3 a drawn between the center of the control pin

6 and the center of the crankpin 2 in the vicinity of the top dead center is thus increased by swinging the lower link 3 as widely as possible, the upper pin 4 will be swung further upward. In this embodiment, the center of the upper pin 4 is disposed downward in the piston stroke direction from the straight line 3 a drawn between the center of the control pin 6 and the center of the crankpin 2, as discussed above. Therefore, the interference between the lower link 3 and the cylinder bore 18 is avoided.

The shape of the lower link 3 suitable for elongation of the piston stroke must be determined in view of the required strength of the lower link 3. The control pin 6 and the upper pin 4 are disposed generally symmetrically to each other with respect to the crankpin 2 in such a manner as not to be largely different from each other in distance from the crankpin 2. Without considering inertial forces, which have relatively small effect on the lower link 3 at a top dead center position as depicted in Fig. 10, a force balance on the lower link 3 can be done in the direction of the piston stroke when a combustion pressure is applied. The force balance indicates that the lower link 3 receives generally equal downward loads from the upper pin 4 and the control pin 6, and a corresponding upward load from the crankpin 2 for balancing with the downward loads of the upper pin 4 and the control pin 6, such that the upward load from the crankpin 2 is about twice as large as either of the downward loads from the pins 4, 6. Therefore, it is necessary to impart a sufficient strength to portion of the lower link 3 disposed about the periphery of the crankpin 2. Accordingly, the wall thickness of the lower link 3 is increased in the region near the crankpin 2. In particular, the thickness of lower link 3 above crankpin 2 when the

mechanism 25 is in the vicinity of top dead center must be capable of withstanding a high loading, such that the distance between an upper edge portion 23 of the lower link 3 and a portion of the lower link 3 in contact with an uppermost section of crankpin 2 in Fig. 10, is increased. On the other hand, the thickness of the lower link 3 above the upper pin 4, and more specifically the distance between a portion of the lower link 3 in contact with an uppermost section of the upper pin 4 and the upper edge portion 23 in the region of the upper pin 4 need not be as thick as the thickness in the region above the crankpin 2, because of the smaller load imposed on the lower link above the upper pin 4. If the center of the upper pin 4 is disposed aligned with or above the straight line 3a when the lower link 3 is widely and upwardly swung in the vicinity of top dead center, in order to achieve the piston stroke as discussed above, the upper edge portion 23 of the lower link 3 disposed above upper pin 4 may be located above an upper section of the crankpin 2 such that the upper edge portion 23 of the lower link 3 will be higher above the upper pin 4 than above the crankpin 2. (In contrast, note that in the embodiment of Fig. 10, the upper edge portion 23 of the lower link 3 is shown as being lower above the upper pin 3 than above the crankpin 2.)

The movement path of the center of the control pin 6 is an arc which is concave downward, and therefore the control pin 6 performs a generally lateral motion rather than a generally upward-downward motion. In addition, the direction of the motion of the crankpin 2 is generally perpendicular to the direction of the reciprocating motions of the piston 9, or is generally lateral, in the vicinity of the top dead center. Accordingly, the lower link 3 moves substantially in the lateral direction (or in the rightward direction in Figs. 8 to 11) while generally keeping the same attitude or inclination (as compared to the motion of the lower link 3 at other crank angles). Therefore, interference between the lower link 3 and the cylinder bore 18 may occur if the lower link 3 is so inclined that upper edge portion 23 ascends toward the upper pin 4 such that the upper edge portion 23 above upper pin 4 is located higher than the upper edge portion 23 above the crankpin 2. However, in this embodiment, the center of the upper pin 4 is disposed lower than the straight line 3 a drawn between the center of the crankpin 2 and the center of the control pin 6, and additionally the upper edge portion 23 of the lower link 3 is arranged such that the

upper edge portion 23 above upper pin 4 is not located higher than the upper edged portion 23 above the crankpin 2 in the vicinity of the top dead center. With this configuration, interference between the upper edge portion 23 of the lower link 3 and cylinder bore 18 can be avoided while keeping a sufficient wall thickness for the lower link 3 to sustain the loads to which it is subjected.

Further if the center of the upper pin 4 is located excessively downward from the straight line 3 a, the upper pin path 4a will be sloped so as to be reduced in height in the upward-downward direction of the path, thereby decreasing the piston stroke. Therefore, it is preferable that the upper edge portion 23 of the lower link 3 in the vicinity of top dead center has little variations in height in the lateral direction as the crankshaft 1 rotates in the vicinity of top dead center, thereby preventing the upper pin 4 from deviating far from the straight line 3 a, while ensuring sufficient thicknesses of the lower link 3 above the crankpin 2 and above the upper pin 4. In other words, the lower link 3 moves substantially in the direction generally perpendicular to the direction of the reciprocating motions of the piston 9 in the vicinity of the top dead center while generally keeping the same attitude when viewed from the axial direction of the crankshaft 1. Additionally the shape of the upper edge portion 23 of the lower link 3 at the end near the piston 9 is characterized by having little variation in height as the lower link 3 moves in the lateral direction.

In this embodiment, the upper edge portion 23 of the lower link 3 forms a generally straight line from above the upper pin 4 to above the crankpin 2, as shown in the drawings. Such an arrangement can be achieved when the center of the upper pin 4 is located lower than the straight line 3 a drawn from the center of the crankpin 2 and the center of the control pin 6. Accordingly, the lower end 32 of cylinder bore 18 (i.e., the right side lower end of Figs. 7-12) under which the moving lower link 3 passes also has little variations in height in the direction perpendicular to the direction of the reciprocating motions of the piston 9 and is capable of ensuring the strength of the cylinder bore 18 and does not require an increase in the overall size of the engine.

Technical ideas of the present invention, grasped from the above embodiment, will be discussed together with advantages obtained thereby. In the embodiment, an internal combustion engine is provided with a multilink-type piston crank mechanism 25 having the upper link 5 connected at one end to the piston pin 8 of the piston 9, the lower link 3 for

connecting the upper link 5 to the crankpin 2 of the crankshaft 1, and the control link 6 having one end swingably supported by a main body of the engine and the other end connected to the lower link 3. The lower link 3 is arranged such that the connection point between the upper link 5 and the lower link 3 is disposed below the straight line drawn from the connection point between the control link 7 and the lower link 3 to the center of the crankpin 2. As a result, the upper pin path 4a can be disposed lower than a corresponding upper pin path 4a in a conventional multilink-type piston crank mechanism, thereby preventing the upper pin 4 from invading the space defined by the cylinder bore 18 in the vicinity of the top dead center so as to prevent interference between the path of the link members 3, 5 and the lower end 32 of the cylinder bore 18 in the vicinity of top dead center.

The mechanism 25 in the internal combustion engine can be arranged such that an amplitude of second order crank vibration in the reciprocating motion of the piston 9 is equal to or less than 3% of the piston stroke. Thus, the vibration characteristics of the engine can be substantially improved. Further, the center of the piston pin 8 can be arranged, in the vicinity of bottom dead center, to be disposed below an outer edge the counterweight 16 of the crankshaft 1. With this arrangement, the piston stroke can be elongated without increasing the overall height of the internal combustion engine, while avoiding interference between the lower end 32 of the cylinder bore 18 and the link path 4a. Still further, the cylinder bore 18 can be formed with a cutout portion 19 for preventing interference against the crankshaft 1 or the multilink-type piston crank mechanism 25, in the vicinity of the lower end 32 of the cylinder bore 18, to make it possible to elongate the piston stroke without increasing the overall height of the internal combustion engine while avoiding interference between the lower end 32 of the cylinder bore 18 and the link path 4a.

The upper pin path 4a traced by the upper pin 4 during a piston downstroke is generally parallel to a straight line extending in the direction of the reciprocating motion of the piston 9. Thus, it is possible to keep the inclination of the upper link 5 in a generally upward-downward orientation with little variations during an expansion stroke of the internal combustion engine during which a high cylinder internal pressure is applied to the piston 9, thereby reducing a friction loss due to the piston side thrust. In addition, the

multilink-type piston crank mechanism 25 can be arranged such that the center of the one end of the control link 7 is located below and in the downward rotation region 13 with respect to the crankshaft 2.

In the embodiment, the distance from the rotational center of the crankshaft 2 to an upper end surface 30 of the cylinder bore 18 can arranged so as not to be more than twice the piston stroke, to allow elongation of the piston stroke without increasing the overall height of the internal combustion engine. Further, the value obtained by L2/L1 is within a range of from about 0.9 to about 1.1, Ll representing the distance from the crankpin 2 to the control pin 6 and L2 representing the distance from the crankpin 2 to the upper pin 4. Within this range of L2/L1, it is possible to elongate the piston stroke while suppressing an elongation of the crank throw. Moreover, since the lower link 3 performs circular motions having a radius set by the crank throw while performing swinging motions about the crankpin 2, the movement path of the lower link 3 becomes shorter by suppressing the elongation of the crank throw, thereby further helping to avoid interference between the lower link 3 and the lower end 32 of the cylinder bore 18.

In the embodiment, a y-axis can be defined by a straight line passing the center of the crankshaft journal 11 and extending in the direction of the reciprocating motion of the piston 9, and an x-axis can be defined as a straight line passing the center of the crankshaft journal 11 and extending perpendicular to the y-axis and perpendicular to the axis of the crankshaft 2. The center of the cylinder bore 18 can be offset from the center of the crankshaft 2 by an amount generally equal to the distance from the center of the crankshaft 2 to the upper pin 4 at a timing of 15° crank angle after top dead center, wherein both distances are measured parallel to the x-axis. In this arrangement, the upper link 5 is disposed in a generally upward-downward direction parallel to the y-axis during a range of the timing of a maximum cylinder internal pressure in a gasoline engine (i.e., crank angles after top dead center in the expansion stroke of the engine), thereby significantly reducing the piston side thrust.

Further, the lower link 3 can formed such that the upper edge portion 23 of the lower link 3 is generally laterally disposed (i.e., generally parallel to the x-axis) in the vicinity of the top dead center such that the upper edge portion 23 moves generally in the lateral direction along the lower end 32 of the cylinder bore 18 in the vicinity of the top

dead center. This facilitates the prevention of interference between the lower link 3 and the lower end 32 of the cylinder bore 18. In other words, it is not necessary to form a cutout portion in the lower end 32 of the cylinder bore 18 through which the lower link 3 can pass, thereby avoiding a weakening of the cylinder bore 18. As discussed previously, for readily understanding of the description of the embodiment of the present invention, the direction of the reciprocating motion of the piston has been discussed as an upward-downward direction and the direction perpendicular to the direction of the reciprocating motion of the piston has been discussed as a lateral direction, However, it is understood that the engine itself can be oriented in any direction, so that upward, downward, and laterally with respect to the engine are not necessary upward, downward, and laterally with respect to gravity.

Also, when two surfaces are indicated as being "generally parallel" it is understood that the angle between the two surfaces is equal to or less than about 5°. Further, "a generally upward-down direction" or "a generally lateral direction" means that the angle formed with respect to the reciprocating motion of the piston or a perpendicular to the reciprocating motion of the piston, respectively, is equal to or less than about 5°. While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and equivalents thereof. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.