Compression ratio variable device in internal combustion engine

A compression ratio changing device in an internal combustion engine includes a piston inner element, a piston outer element slidably fitted over an outer periphery of the piston inner element for sliding movement only in an axial direction and capable of being moved between a lower-compression ratio position (L) and a higher-compression ratio position (H), a bulking member capable of being turned about axes of the piston inner and outer elements between a non-bulking position (A) and a bulking position (B), and an actuator connected to the bulking member for turning the bulking member to the non-bulking position (A) and the bulking position (B). The bulking member permits movement of the piston outer element to the lower-compression ratio position (L) when it is in the non-bulking position (A), and retains the piston outer element in the higher-compression ratio position (H), when it is turned to the bulking position (B).

FIELD OF THE INVENTION

The present invention relates to a compression ratio changing device in an internal combustion engine, and particularly, to an improvement in a compression ratio changing device in an internal combustion engine including a piston which is comprised of a piston inner element connected to a connecting rod through a piston pin, and a piston outer element which is connected to the piston inner element with an outer end face thereof exposed to a combustion chamber, the piston outer element capable of being moved between a lower-compression ratio position close to the piston inner element and a higher-compression ratio position close to the combustion chamber, so that the piston outer element is operated to the lower-compression ratio position to decrease the compression ratio of the engine and operated to the higher-compression ratio position to increase the compression ratio of the engine.

BACKGROUND ART

As conventional compression ratio changing devices in internal combustion engines, there are known (1) a compression ratio changing device in which a piston outer element is threadedly fitted over an outer periphery of a piston inner element, so that the piston outer element is advanced and retracted relative to the piston inner element to a lower-compression ratio position and a higher-compression ratio position by rotating and reversing the piston outer element (for example, see Japanese Patent Application Laid-open No.11-117779), and (2) a compression ratio changing device in which a piston outer element is axially slidably fitted over an outer periphery of a piston inner element, and an upper hydraulic pressure chamber and a lower hydraulic pressure chamber are defined between the piston inner and outer elements, so that the piston outer element is operated to a lower-compression ratio position and a higher-compression ratio position by supplying a hydraulic pressure alternately to the hydraulic pressure chambers (for example, see Japanese Patent Publication No. 7-113330).

It should be noted here that in the device (1), in order to operate the piston outer element to the lower-compression ratio position and the higher-compression ratio position, it is necessary to rotate the piston outer element. For this reason, the shape of a top face of the piston outer element cannot be determined freely in correspondence to the shape of a ceiling surface of a combustion chamber and the dispositions of intake and exhaust valves, and it is difficult to sufficiently increase the compression ratio of the engine in the higher-compression ratio position. In the device (2), particularly when the piston outer element is in the higher-compression ratio position, a large thrust load received by the piston outer element in an expansion stroke of the engine is supported by a hydraulic pressure in the upper hydraulic pressure chamber and hence, a seal withstanding a high pressure is required in the upper hydraulic pressure chamber. Moreover, when bubbles are produced in the upper hydraulic pressure chamber, the higher-compression ratio position of the piston outer element is unstable and hence, it is necessary to provide a means for removing such bubbles and as a result, an increase in cost as a whole is inevitable.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished with such circumstances in view, and it is an object of the present invention to provide a compression ratio changing device in an internal combustion engine, wherein the piston outer element can be operated simply and precisely to the lower-compression ratio position and the higher-compression ratio position without being rotated.

To achieve the above object, according to a first aspect and feature of the present invention, there is provided a compression ratio changing device in an internal combustion engine, comprising a piston inner element connected to a connecting rod through a piston pin, a piston outer element which is fitted over an outer periphery of the piston inner element for sliding movement only in an axial direction with an outer end face thereof exposed to a combustion chamber, the piston outer element capable of being moved between a lower-compression ratio position close to the piston inner element and a higher-compression ratio position close to the combustion chamber, a bulking member interposed between the piston inner and outer elements and capable of being moved between a non-bulking position where the bulking member permits the movement of the piston outer element to the lower-compression ratio position, and a bulking position where the piston outer element is retained in the higher-compression ratio position, and an actuator for retaining the bulking member alternately in the non-bulking position and the bulking position.

With the first feature, when the bulking member is moved to the non-bulking position by the actuator, the bulking member permits the movement of the piston outer element to the lower-compression ratio position and hence, the piston outer element can be moved to the lower-compression ratio position by a high pressure from the combustion chamber. When the bulking member is moved from the non-bulking position to the bulking position by the actuator, the piston outer element can be retained in the higher-compression ratio position.

During this time, the piston outer element cannot be rotated relative to the piston inner element and hence, the shape of a top face of the piston outer element exposed to the combustion chamber can be formed in correspondence to the shape of the combustion chamber to effectively increase the compression ratio in the higher-compression ratio position of the piston outer element. Moreover, in the higher-compression ratio position of the piston outer element, a large thrust force received by the piston outer element from the combustion chamber in an expansion stroke of the engine is received by the bulking member. Therefore, the application of the thrust force to the actuator is avoided and hence, it is possible to achieve a decrease in output from the actuator and in its turn, the compactness of the actuator. Even when the actuator is constructed into a hydraulic type, a high-pressure seal is not required, because the thrust force is not applied to the actuator. In addition, even if some bubbles are produced in the hydraulic pressure chamber, the higher-compression ratio position of the piston outer element cannot be made unstable.

According to a second aspect and feature of the present invention, in addition to the first feature, the bulking member and the actuator are constructed so that the piston outer element is permitted to be moved, during reciprocal movements of the piston inner and outer elements, between the lower-compression ratio position and the higher-compression ratio position by natural external forces applied to the piston inner and outer elements to move these elements axially away from and toward each other. The natural external forces include a friction resistance received from an inner surface of a cylinder bore by the piston outer element, an inertia force of the piston outer element, an intake negative pressure applied to the piston outer element and the like.

With the second feature, the natural external forces can be utilized to move the piston outer element from the lower-compression ratio position to the higher-compression ratio position or from the higher-compression ratio position to the lower-compression ratio position. Therefore, if the actuator exhibits an output enough to merely move the bulking member between the non-bulking position and the bulking position, it suffices and hence, it is possible to provide reductions in capacity and size of the actuator.

According to a third aspect and feature of the present invention, in addition to the first or second feature, the bulking member is interposed between the piston inner and outer elements so as to be capable of turning about axes of the piston inner and outer elements between the non-bulking position and the bulking position, and a first cam and a second cam are formed into a convex shape on axially opposed surfaces of the bulking member and one of the piston inner and outer elements, and have slants for slipping on each other axially away from each other, when the bulking member is turned from the non-bulking position to the bulking position, and flat top faces for abutting against each other, when the bulking member has reached the bulking position.

With the third feature, when the bulking member is turned from the non-bulking position to the bulking position, the first and second cams are moved axially away from each other, while their slants are slipped on each other. Therefore, the piston outer element can be pushed up to the higher-compression ratio position. Moreover, when the bulking member has reached the bulking position, the flat top faces of the first and second cams are put into abutment against each other and hence, a large thrust force received from the combustion chamber by the piston outer element is applied vertically to the flat top face during an expansion stroke of the engine and can be reliably prevented from being applied as a turning torque to the bulking member.

According to a fourth aspect and feature of the present invention, in addition to the second feature, the bulking member is interposed between the piston inner and outer elements so as to be capable of turning about axes of the piston inner and outer elements between the non-bulking position and the bulking position, and a first cam and a second cam are formed into a convex shape on axially opposed surfaces of the bulking member and one of the piston inner and outer elements, and have flat top faces for abutting against each other, when the bulking member has reached the bulking position, and precipice faces extending downwards substantially vertically from circumferentially opposite side edges of the top faces to roots of the cams.

With the fourth feature, it is possible to set the operational stroke angle of the bulking member at a small value and to form each of the top faces of the cams in a large extent by forming the opposite sides of the first and second cams as the precipice faces. Thus, it is possible to enhance the responsiveness of the bulking member and to reduce the surface pressure applied to the top faces to enhance the durability of the top faces.

Moreover, in order to move the piston outer element between the lower and higher-compression ratio positions, the natural external forces for moving the piston inner and outer elements axially away from and toward each other are utilized and hence, the turning movement of the bulking member between the non-bulking position and the bulking position cannot be hindered.

According to a fifth aspect and feature of the present invention, in addition to any of the first to fourth features, a piston outer element locking means is provided between the piston inner and outer elements for locking the piston outer element relative to the piston inner element, when the piston outer element has reached the lower-compression ratio position.

With the fifth feature, when the piston outer element has reached the lower-compression ratio position, the operations of the piston inner and outer elements in unison with each other can be guaranteed.

According to a sixth aspect and feature of the present invention, in addition to any of the first to fifth features, a piston outer element restricting means is provided between the piston inner and outer elements for restricting the movement of the piston outer element relative to the piston inner element toward the combustion chamber, when the piston outer element has reached the higher-compression ratio position.

With the sixth feature, even when the piston outer element has reached the higher-compression ratio position, the operations of the piston inner and outer elements in unison with each other can be guaranteed.

According to a seventh aspect and feature of the present invention, in addition to any of the first to sixth features, the actuator comprises a hydraulically operating means operated by a hydraulic pressure from a hydraulic pressure source to operate the bulking member to the bulking position, and a return spring for biasing the bulking member toward the non-bulking position.

With the seventh feature, a single hydraulic pressure chamber suffices in the hydraulic operating means and hence, the construction of the hydraulically operating means can be simplified.

According to an eighth aspect and feature of the present invention, in addition to any of the first to seventh features, the piston outer element locking means comprises a locking member supported on the piston inner element to be moved between an operated position where the locking member is in engagement in a locking groove in an inner peripheral surface of the piston outer element and a retracted position the locking member is out of engagement in the locking groove, an operating spring for biasing the locking member toward the operated position, and a hydraulically returning means operated by the hydraulic pressure from the hydraulic pressure source to operate the locking member toward the retracted position. With the eighth feature, a single hydraulic pressure chamber suffices even in the piston outer element locking means and hence, the construction of the piston outer element locking means can be simplified.

According to a ninth aspect and feature of the present invention, in addition to any of the first to eighth features, the actuator comprises a hydraulically operating means operated by the hydraulic pressure from the hydraulic pressure source to operate the bulking member to the bulking position, and a returning spring for biasing the bulking member toward the non-bulking position, and the piston outer element locking means comprises a locking member supported on the piston inner element to be moved between an operated position where the locking member is in engagement in a locking groove in an inner peripheral surface of the piston outer element and a retracted position where the locking member is out of engagement in the locking groove, an operating spring for biasing the locking member toward the operated position, and a hydraulically returning means operated by the hydraulic pressure from the hydraulic pressure source to operate the locking member toward the retracted position, so that the hydraulic pressure in the hydraulic pressure source is supplied simultaneously to the hydraulically operating means and the hydraulically returning means.

With the ninth feature, the actuator and the piston outer element locking means can be operated rationally by the common hydraulic pressure, thereby providing the simplification of a hydraulic pressure circuit.

According to a tenth aspect and feature of the present invention, in addition to the first feature, the actuators are disposed in a plurality of sets in a circumferential direction of the bulking member.

With the tenth feature, the actuators are disposed in the plurality of sets in the circumferential direction of the bulking member and hence, operating forces of the actuators can be applied to the bulking member at a plurality of circumferential points to reliably turn the bulking member from the non-bulking position to the bulking position or from the bulking position to the non-bulking position. Moreover, it is possible to provide a reduction in size of the actuator and it is easy to dispose the actuators in narrow internal spaces in the piston.

According to an eleventh aspect and feature of the present invention, in addition to the tenth feature, the actuators are disposed in the plurality of sets at equal distances in the circumferential direction of the bulking member.

With the eleventh feature, during operation of the plurality of sets of actuators, the bulking member can be turned smoothly without application of an unbalanced load to the bulking member.

According to a twelfth aspect and feature of the present invention, in addition to the tenth or eleventh feature, the actuators are disposed in two sets on opposite sides of the piston pin.

With the twelfth feature, the two sets of actuators can be disposed at equal distances in the circumferential direction of the bulking member without being interfered by the piston pin, and the disposition of the actuators in narrow internal spaces in the piston can be achieved more simply.

According to a thirteenth aspect and feature of the present invention, in addition to the first feature, the actuator comprises an operating member and a returning member which are slidably disposed in the piston inner element on the same axis extending in a direction of turning of the bulking member and are opposed to each other on opposite sides of a pressure-receiving portion of the bulking member, so that the bulking member is turned alternately to the non-bulking position and the bulking position by alternately operating the operating member and the returning member.

With the thirteenth feature, the actuator comprises the operating member and the returning member which are slidably disposed in the piston inner element on the same axis extending in the direction of turning of the bulking member and are opposed to each other on the opposite sides of the pressure-receiving portion of the bulking member and hence, it is possible to provide a reduction in size of the actuator, and it is easy to dispose the actuator in a narrow internal space in the piston.

According to a fourteenth aspect and feature of the present invention, in addition to the thirteenth feature, the operating member and the returning member comprise an operating plunger and a returning plunger, respectively, which are slidably received in a common cylinder bore defined in the piston inner element and are opposed to each other on opposite sides of the pressure-receiving portion.

With the fourteenth feature, the cylinder bore is used commonly for the operating plunger and the returning plunger, leading to a simplification of the working for provision of the cylinder bore and a simplification of the construction.

According to a fifteenth aspect and feature of the present invention, in addition to the thirteenth or fourteenth feature, the operating member and the returning member are disposed on the same axis intersecting, at substantially right angles, a radial line of the bulking member extending through the center of the pressure-receiving portion.

With the fifteenth feature, the operating force of the operating member and the returning force of the returning member can be transmitted efficiently to the bulking member through the pressure-receiving portion and hence, it is possible to provide reductions in capacity and size of the actuator.

According to a sixteenth aspect and feature of the present invention, in addition to any of the thirteenth to fifteenth features, the actuators are disposed in a plurality of sets at equal distances in a circumferential direction of the bulking member.

With the sixteenth feature, the bulking member can be turned smoothly by the operation of the plurality of sets of actuators without application of an unbalanced load to the bulking member.

The above-described piston outer element restricting means corresponds to a stop ring18,118in embodiments of the present invention which will be described hereinafter. The above-described hydraulically operating means corresponds to an operating plunger23,123and a first hydraulic pressure chamber25,125which will be described hereinafter, and the above-described hydraulically returning means corresponds to a second hydraulic pressure chamber37,137and a piston38,138which will be described hereinafter.

Further, according to a seventeenth aspect and feature of the present invention, in addition to any of the thirteenth to sixteenth features, the actuators are disposed in two sets on opposite sides of the piston pin.

With the seventeenth feature, the two sets of actuators can be disposed at equal distances in the circumferential direction of the bulking member without being interfered by the piston pin, and the disposition of the actuators in narrow internal spaces in the piston can be achieved easily.

The above and other objects, features and advantages of the invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of the present invention shown inFIGS. 1 to 10will first be described.

Referring toFIGS. 1 and 2, an engine body1of an internal combustion engine E comprises a cylinder block2having a cylinder bore2a, a crankcase3coupled to a lower end of the cylinder block2, and a cylinder head4coupled to an upper end of the cylinder block2and having a combustion chamber4aleading to the cylinder bore2a. A connecting rod7is connected at its smaller end7athrough a piston pin6to a piston5slidably received in the cylinder bore2aand at its larger end7bto a crank pin9aof a crankshaft9rotatably carried on a crankcase3with a pair of left and right bearings8and8′ interposed therebetween.

The piston5comprises a piston inner element5aconnected to the smaller end7aof the connecting rod7through the piston pin6, and a piston outer element5bslidably fitted over an outer peripheral surface of the piston inner element5aand to an inner peripheral surface of the cylinder bore2awith its top face exposed to the combustion chamber4a. A plurality of piston rings10ato10care mounted around an outer periphery of the piston outer element5band slidably put into close contact with the inner peripheral surface of the cylinder bore2a.

As shown inFIGS. 2 and 3, a plurality of spline teeth11aand a plurality of spline grooves11bare formed on slidably fitted surfaces of the piston inner and outer elements5aand5brespectively to extend in an axial direction of the piston5and engaged with each other, so that the piston inner and outer elements5aand5bcannot be rotated about their axes relative to each other.

Referring toFIGS. 2 and 6, an annular bulking member14is placed on an upper surface of the piston inner element5aand turnably fitted over a pivot12integrally and projectingly provided on such upper surface. The pivot12is divided into a plurality of (two inFIGS. 2 and 6) blocks12a,12ato receive the smaller end7aof the connecting rod7.

The bulking member14is capable of being turned between first and bulking positions A and B established about its axis, and a cam mechanism15is provided between the bulking member14and the piston outer element5band moves the piston outer element5balternately to a lower-compression ratio position L (seeFIGS. 2 and 10A) close to the piston inner element5aand a higher-compression ratio position H (seeFIGS. 7 and 10C) close to the combustion chamber4ain response to the reciprocal turning movement of the bulking member14.

As best shown inFIGS. 10A to 10C, the cam mechanism15comprises a plurality of convex first cams16formed on an upper surface of the bulking member14, and a plurality of convex second cams17formed on a lower surface of a top wall of the piston outer element5b. The first and second cams16and17are formed so that they are arranged circumferentially alternately with each other to permit the movement of the piston outer element5bto the lower-compression ratio position L, when the bulking member14is in the non-bulking position A. Each of the first cams16and each of the second cams17are provided respectively with slants16aand17awhich are slipped on each other axially away from each other, when the bulking member14is turned from the non-bulking position A to the bulking position B, and flat top faces16band17bwhich abut against each other to retain the piston outer element5bat the higher-compression ratio position H, when the bulking member14has reached the bulking position B. A stop ring18capable of abutting against a lower end face of the piston inner element5ais locked to an inner peripheral surface of a lower end of the piston outer element5b, and serves as a restricting means for inhibiting the further movement of the piston outer element5bbeyond the higher-compression ratio position H toward the combustion chamber4a, when the piston outer element5bhas reached the higher-compression ratio position H.

An actuator20for turning the bulking member14to the first and bulking positions A and B is mounted between the piston inner element5aand the bulking member14. The actuator20will be described below with reference toFIGS. 2,5and6.

First and second bottomed cylinder bores22are provided in the piston inner element5aon opposite sides of the piston pin6to extend in parallel to the piston pin6, and first and second plungers23and24are slidably received in the cylinder bores21and22. Tip ends of the operating and returning plungers23and24protrude in the same direction from the first and second cylinder bores21and22, and first and second pressure-receiving pieces14aand14bare projectingly provided on a lower surface of the bulking member14and disposed to abut against such tip ends.

A first hydraulic pressure chamber25is defined in the first cylinder bore21and faced by an inner end of the operating plunger23, so that when a hydraulic pressure is supplied to the first hydraulic pressure chamber25, the operating plunger23receives the hydraulic pressure to turn the bulking member14through the first pressure-receiving piece14ato the bulking position B. A spring chamber25is defined in the second cylinder bore22and faced by an inner end of the returning plunger24, and a return spring27is accommodated in the spring chamber25, so that the returning plunger24biases the bulking member14to the non-bulking position A through the second pressure-receiving piece14bby a force of the return spring27. The non-bulking position A of the bulking member14is defined by the abutment of the first pressure-receiving piece14aagainst the tip end of the operating plunger23abutting against a bottom surface of the first cylinder bore21(seeFIG. 5), and the bulking position B of the bulking member14is defined by the abutment of the second pressure-receiving piece14bagainst the tip end of the returning plunger24abutting against a bottom surface of the second cylinder bore22(seeFIG. 9).

The bulking member14and the actuator20permit the movement of the piston outer element5bbetween the lower-compression ratio position L and the higher-compression ratio position H by natural external forces applied to the piston inner and outer elements5aand5bto move the elements5aand5baxially away from and toward each other, such as an inertia force of the piston outer element5b, a friction resistance received from the inner surface of the cylinder bore2aby the piston outer element5b, an intake negative pressure applied to the piston outer element5band the like.

A piston outer element locking means is provided between the piston inner element5aand the piston outer element5bto lock the piston outer element5bto the piston inner element5a, when the piston outer element5bhas reached the lower-compression ratio position L. The piston outer locking means30will be described with reference toFIGS. 2 and 4.

A plurality of locking grooves31are defined at equal distances in the inner peripheral surface of the piston inner element5ato extend circumferentially, and a plurality of locking levers32are swingably mounted on the piston inner element5athrough pivots33, so that they are brought into and out of engagement in the locking grooves31when the piston outer element5bhas reached the lower-compression ratio position L. Namely, the locking levers32are capable of being swung between an operated position (seeFIG. 4) where they are in engagement in the locking grooves31and a retracted position D (seeFIG. 8) where they are out of engagement in the locking grooves31.

Each of the locking levers32comprises a long arm portion32awhich is brought into and out of engagement in the locking groove31, and a short arm portion32bextending in a direction opposite from the long arm portion32awith the pivot33interposed therebetween. An operating spring34for biasing the long arm portion32ain a direction to engage in the locking groove31is mounted under compression between the long arm portion32aand the piston inner element5a. In this case, a positioning projection35is formed on the long arm portion32aand fitted to an inner periphery of the operating spring34to retain the operating spring34in place. On the other hand, a plurality of cylinder bores36are defined in the piston inner element5ain correspondence to the short arm portions32b, and a plurality of pistons38are slidably received in the cylinder bores36and disposed with their tip ends abutting against tip ends of the short arm portions32b. A second hydraulic pressure chamber37is defined in each of the cylinder bores36and faced by an inner end of the corresponding piston38, so that when a hydraulic pressure is supplied to the second hydraulic pressure chamber37, the piston38receives such hydraulic pressure to move the locking lever32away from the locking groove31against the force of the operating spring34.

As shown inFIGS. 4 and 5, a cylindrical oil chamber41is defined between the piston pin6and a sleeve40press-fitted into a hole in the piston pin6, and first and second distributing oil passage42and43are provided to extend within the piston pin6and the piston inner element5aand to connect the oil chamber41to the first and second hydraulic pressure chambers25and37. The oil chamber41is connected to an oil passage44provided to extend within the piston pin6, the connecting rod7and the crankshaft9, as shown inFIG. 1, and the oil passage44is connected switchably to an oil pump46as a hydraulic pressure source and an oil reservoir47through a solenoid switchover valve45.

The operation of this embodiment will be described below.

To provide a lower-compression ratio state to avoid the knocking, for example, in the rapidly accelerating operation of the internal combustion engine E, the solenoid switchover valve45is brought into a non-energized state as shown inFIG. 1to put the oil passage44into communication with the oil reservoir47. This causes both of the first hydraulic pressure chamber25and the second hydraulic pressure chamber37to be opened to the oil reservoir47through the oil chamber41and the oil passage44. Therefore, in the actuator20, the returning plunger24pushes the second pressure-receiving piece14bunder the action of the biasing force of the return spring27to turn the bulking member14to the non-bulking position A, as shown inFIG. 5. As a result, the first cam16and the second cam17of the cam mechanism15are disposed in positions in which their tops are misaligned from each other, as shown inFIG. 10Aand hence, when the piston outer element5bhas been pushed relative to the piston inner element5aby a pressure in the combustion chamber4ain an expansion stroke or a compression stroke of the engine, when the piston outer element5bhas been pushed relative to the piston inner element5aby a friction resistance produced between the piston rings10ato10cand the inner surface of the cylinder bore2ain an upstroke of the piston5, or when the piston outer element5bhas been pushed relative to the piston inner element5aby its inertia force with the deceleration of the piston inner element5ain the latter half of a downstroke of the piston5, the piston outer element5bcan be lowered relative to the piston inner element5ato reach the lower-compression ratio position L, while allowing the first cam16and the second cam17to be meshed with each other. At that time, in the piston outer element locking means30, the locking lever32pivotally supported on the piston inner element5aand the locking groove31in the piston outer element5bare opposed to each other and hence, the locking lever32is swung by the biasing force of the operating spring34, so that the long arm portion32ais brought into engagement in the locking groove31, and the piston outer element5bis retained in the lower-compression ratio position L by the engagement of the long arm portion32aand the locking groove31. Thus, plays in the cam mechanism15are eliminated, and the piston inner and outer elements5aand5bcan be lifted and lowered together with each other within the cylinder bore2a, while decreasing the compression ratio.

To provide a higher-compression ratio state in order to provide an increase in output, for example, during the high-speed operation of the internal combustion engine E, electric current is supplied to the solenoid switchover valve45to connect the oil passage44to the oil pump46. This causes the hydraulic pressure discharged from the oil pump46to be supplied through the oil passage44and the oil chamber41to the first hydraulic pressure chamber25and the second hydraulic pressure chamber37. Therefore, first, in the piston outer element locking means30, the piston38receives the hydraulic pressure in the second hydraulic chamber37to swing the locking lever32to the retracted position D against the biasing force of the operating spring34, thereby disengaging the long arm portion32afrom the locking groove31in the piston outer element5b, as shown inFIG. 8. When the locking lever32has been disengaged from the locking groove31, the movement of the piston outer element5bto the higher-compression ratio position H is permitted. Therefore, in the actuator20, the operating plunger23receives the hydraulic pressure in the first hydraulic pressure chamber25to push the first pressure-receiving piece14a, thereby turning the bulking member14from the non-bulking position A to the bulking position B, as shown inFIG. 9. In the cam mechanism15, the first cam16and the second cam17are axially moved away from each other with the turning of the bulking member14, while their slants16aand17aare slipped on each other (seeFIG. 10B). When the bulking member14has reached the bulking position, as is shown inFIG. 7, the cams16and17reach states in which their flat top faces16band17bare in abutment against each other (seeFIG. 10C), thereby pushing the piston outer element5bup to the higher-compression ratio position H. At this time, the stop ring18on the piston outer element5bis put into abutment against the lower end face of the piston inner element5ato inhibit the further movement of the piston outer element5btoward the combustion chamber4aand hence, the higher-compression ratio position H of the piston outer element5bis maintained by the abutment of the top faces16band17bof the cams16and17against each other and the abutment of the stop ring18against the lower end face of the piston inner element5a. Thus, plays in the cam mechanism15are eliminated, and the piston inner and outer elements5aand5bcan be lifted and lowered together with each other within the cylinder bore2a, while increasing the compression ratio.

When the piston outer element5bis moved between the lower-compression ratio position L and the higher-compression ratio position H, the rotation of the piston outer element5brelative to the piston inner element5ais restrained by the spline teeth11aand the spline grooves11bformed in fitted surfaces of the piston inner element5aand the piston outer element5band slidably engaged with each other. The shape of the top face of the piston outer element5bfacing to the combustion chamber4acan be formed in correspondence to the shape of the combustion chamber4ato effectively increase the compression ratio in the higher-compression ratio position H of the piston outer element5b. Moreover, in the higher-compression ratio position H of the piston outer element5b, in the expansion stroke of the engine, a large thrust force received from the combustion chamber4aby the piston outer element5bis applied vertically to the flat top faces16band17bof the first cam16and the second cam17, which abut against each other. Therefore, the bulking member14cannot be turned by such thrust force and hence, the hydraulic pressure supplied to the first hydraulic pressure chamber25is not required to be as high as it opposes the thrust force, and even if bubbles exist in a small amount in the first hydraulic pressure chamber25, the piston outer element5bcan be retained stably in the higher-compression ratio position H, thereby causing no hindrance.

It should be noted here that when the locking lever32has been disengaged from the locking groove31, natural external forces which will be described below assist in movement of the piston outer element5bto the high-compression ratio position H. Namely, when the piston outer element5bhas been pulled toward the combustion chamber4aby an intake negative pressure in the intake stroke of the engine, when the piston outer element5bhas been left behind from the piston inner element5aby a friction resistance produced between the piston rings10ato10cand the inner surface of the cylinder bore2ain the downstroke of the piston5, or when the piston outer element5bis about to be floated from the piston inner element5aby its inertia force with the deceleration of the piston inner element5ain the latter half of the upstroke of the piston5, the piston outer element5bcan be lifted from the piston inner element5ato easily reach the higher-compression ratio position H. As a result, the movement of the piston outer element5bto the higher-compression ratio position H can be conducted quickly in cooperation with the operation of the actuator20. This can contribute to an enhancement in responsiveness.

Among the natural external forces contributing to the switchover of one of the lower-compression ratio position L and the higher-compression ratio position H of the piston outer element5bto the other, the friction resistance between the piston rings10ato10cand the inner surface of the cylinder bore2aand the inertia force of the piston outer element5bare particularly effective. The variation in friction resistance is relatively small, as compared with the variation in rotational speed of the engine, but the inertia force of the piston outer element5bis increased in a secondary curve in accordance with an increase in rotational speed of the engine. Therefore, for the switchover of the position of the piston outer element5b, the friction resistance is dominant in a lower rotational speed range of the engine, and the inertia force of the piston outer element5bis dominant in a higher rotational speed range of the engine.

The actuator20is comprised of the operating plunger23capable of being operated by the hydraulic pressure in the first hydraulic pressure chamber25to turn the bulking member14from the non-bulking position A to the bulking position B, and the returning plunger24capable of being operated by the biasing force of the return spring27to return the bulking member14from the bulking position B to the non-bulking position A in the release of the hydraulic pressure in the first hydraulic pressure chamber25. Therefore, the single hydraulic pressure chamber25suffices and hence, the construction can be simplified.

The piston outer element locking means30is comprised of the locking lever32which is moved between the operated position C where it is pivotally supported on the piston inner element5aand engaged in the locking groove31in the piston outer element5b, and the retracted position D where it is disengaged from the locking groove31, the operating spring34for biasing the locking lever32toward the operated position C, and the piston38operated by the hydraulic pressure in the second hydraulic pressure chamber37to operate the locking lever32to the retracted position D. Therefore, even in the locking means30, the single hydraulic pressure chamber37suffices and hence, the construction can be simplified.

Further, the oil pump46and the oil reservoir47are switchably connected to the first and second hydraulic pressure chambers25and37through the common solenoid switchover valve45and hence, the actuator20and the piston outer element locking means30can be operated rationally by the common hydraulic pressure, whereby the hydraulic pressure circuit can be simplified, and the compression ratio changing device can be provided at a low cost.

A second embodiment of the present invention shown inFIGS. 11 to 21will be described below.

Referring toFIGS. 11 and 12, a piston105comprises a piston inner element105aconnected to a smaller end107aof a connecting rod107through a piston pin106, and a piston outer element105bslidably fitted over an outer peripheral surface of the piston inner element105aand to an inner peripheral surface of a cylinder bore102awith its top face exposed to a combustion chamber104a. A plurality of piston rings110ato110care mounted around an outer periphery of the piston outer element105band slidably put into close contact with the inner peripheral surface of the cylinder bore102a.

As shown inFIGS. 12 and 13, a plurality of spline teeth111aand a plurality of spline grooves111bare formed on slidably fitted surfaces of the piston inner and outer elements5aand5brespectively to extend in an axial direction of the piston105and engaged with each other, so that the piston inner and outer elements105aand105bcannot be rotated about their axes relative to each other.

Referring toFIGS. 12 and 17, an annular bulking member114is placed on an upper surface of the piston inner element105aand turnably fitted over a pivot12integrally and projectingly provided on such upper surface, and a retaining ring150is secured to an upper surface of the pivot112by a machine screw151for retain an upper surface of the bulking member114to inhibit the removal of the bulking member114from the pivot112. The pivot12is divided into a plurality of (four inFIGS. 12 and 17) blocks112a,112ato receive the smaller end107aof the connecting rod107.

The bulking member114is capable of being turned between first and bulking positions A and B established about its axis, and a cam mechanism115is provided between the bulking member114and the piston outer element105band moves the piston outer element105balternately to a lower-compression ratio position L (seeFIGS. 12 and 21A) close to the piston inner element105aand a higher-compression ratio position H (seeFIGS. 18 and 21C) close to the combustion chamber104ain response to the reciprocal turning movement of the bulking member114.

As best shown inFIGS. 21A to 21C, the cam mechanism115comprises a plurality of convex first cams116formed on an upper surface of the bulking member114, and a plurality of convex second cams117formed on a lower surface of a top wall of the piston outer element105b. The first and second cams116and117are formed so that they are arranged circumferentially alternately with each other to permit the movement of the piston outer element105bto the lower-compression ratio position L, when the bulking member114is in the non-bulking position A. Each of the first cams116and each of the second cams117have opposite sides arranged in a circumferential direction of the bulking member114, which are precipice faces116aand117astanding up substantially vertically from roots of the cams116and117, and flat top faces116band117beach of which connects both of upper edges of the precipice faces116a,117ato each other, and which are put into abutment against each other to retain the piston outer element105bin the higher-compression ratio position H, when the bulking member114has reached the bulking position B. Since the opposite sides of the first and second cams116and117are the precipice faces116aand117a, as described above, the spacing between the adjacent cams116,117arranged circumferentially can be narrowed, and the total area of the top faces116b,117bof the cams116,117can be set remarkably larger than that in the first embodiment.

A stop ring118capable of abutting against a lower end face of the piston inner element105ais locked to an inner peripheral surface of a lower end of the piston outer element105b, and serves as a restricting means for inhibiting the further movement of the piston outer element105bbeyond the higher-compression ratio position H toward the combustion chamber104a, when the piston outer element105bhas reached the higher-compression ratio position H.

As best shown inFIGS. 12,15and16, a plurality of sets (two sets in the illustrated embodiment) of actuators120for turning the bulking member114to the first and bulking positions A and B are mounted between the piston inner element105aand the bulking member114. The structure in which the actuators120are disposed in two sets will be described below.

A pair of bottomed cylinder bores121,121are provided in the piston inner element105aon opposite sides of the piston pin106to extend in parallel to the piston pin106, and elongated bores154,154are also provided in the piston inner element105ato extend through upper walls of intermediate portions of the cylinder bores121,121. A pair of pressure-receiving pins114a,114aare integrally and projectingly provided on a lower surface of the bulking member114and arranged in a diametrical line on the bulking member114, so that they face to the cylinder bores121,121through the elongated bores154,154. The elongated bores154,154are arranged so that they do not disturb the movement of the pressure-receiving pins114a,114abetween the non-bulking position A and the bulking position B along with the bulking member114.

Operating plungers123,123and bottomed cylindrical returning plungers124,124are slidably received in the cylinder bores121,121on opposite sides of the corresponding pressure-receiving pins114a,114a. In this case, the operating plungers123,123and the returning plungers124,124are disposed point-symmetrically with respect to an axis of the piston105.

A first hydraulic pressure chamber125is defined in a bottom of the cylinder bore121, and an end of the operating plunger23opposite from the pressure-receiving pin114afaces to the first hydraulic pressure chamber125, so that when a hydraulic pressure is supplied to the chamber125, the operating plunger23receives such hydraulic pressure to turn the bulking member114to the bulking position B through the corresponding the pressure-receiving pin114a. The first hydraulic pressure chamber125is connected to an oil passage144(seeFIG. 11) through a first distributing oil passage142and an oil chamber141, and the oil passage144is connected switchably to an oil pump-146as a hydraulic pressure source and an oil reservoir147through a solenoid switchover valve145.

Spring-retaining rings152,152are locked in open ends of the cylinder bores121,121by stop rings153,153, and return springs127,127comprising coil springs are mounted under compression between the spring-retaining rings152,152and the returning plungers124,124for biasing the returning plungers124,124toward the pressure-receiving pins114a,114a, respectively. Thus, the returning plungers124,124can turn the bulking member114to the non-bulking position A through the pressure-receiving pins114a,114aby biasing forces of the return spring127,127.

Each of the operating plungers123is formed into a hollow shape by a cup-shaped plunger body123aand a cap123bmade of a hard material and press-fitted into and secured in an open end of the plunger body123ain order to reduce the weight of the operating plunger123. The operating plunger123is disposed so that the cap123bthereof is in abutment against the pressure-receiving pin114a. Each of the returning plungers124is also of a cap-shape in order to reduce the weight of the returning plunger124and disposed so that its bottom wall is in abutment against the pressure-receiving pin114a.

Each of the spring-retaining rings152has a cylindrical skirt portion152alocated inside the return spring127and extending into the returning plunger124, whereby the buckling of the return spring127can be prevented.

The non-bulking position A of the bulking member114is defined by the abutment of the pressure-receiving pins114a,114aagainst tip ends of the operating plungers123,123abutting against bottom surfaces of the cylinder bores121,121(seeFIG. 15), and the bulking position B of the bulking member114is defined by the abutment of the pressure-receiving pin114aagainst the tip end of the returning plunger24abutting against the skirt portion152aof the spring-retaining ring152(seeFIG. 20). Thus, in the non-bulking position A of the bulking member114, the side contact of the adjacent first and second cams116and117can be avoided, and the smooth movement of the piston outer element105btoward the higher-compression ratio position H can be achieved.

The constructions of other members such as a piston outer element locking means130are the same as those in the first embodiment and hence, portions or components inFIGS. 11 to 21Ccorresponding to those in the first embodiment are designated by like reference characters comprising100added to the numerals used in the first embodiment, and the description of them is omitted.

In the second embodiment, the movements of the piston outer element105bfrom the lower-compression ratio position L to the higher-compression ratio position H and from the higher-compression ratio position H to the lower-compression ratio position L are carried out by utilizing only the above-described natural external forces applied to the piston inner and outer elements105aand105bto move them axially away from and toward each other during the reciprocal movement of the piston105(seeFIG. 21B). Therefore, if the actuator120merely exhibits an output enough to move the bulking member114between the non-bulking position A and the bulking position B, as shown inFIG. 21C, it suffices, and hence, reductions in capacity and size of the actuator120can be provided.

In each of the first and second cams116and117, its opposite sides arranged in a sliding direction can be formed as precipice faces116a,117a, and it is possible to set the operational stroke angle of the bulking member114at a small value and to form the top faces116band117bof the cams116and117in a large extent in correspondence to that the slants16aand17aare not provided as in the first embodiment. In addition, it is possible to enhance the responsiveness of the bulking member114and to reduce the surface pressures applied to the top faces116band117bto enhance the durability thereof.

As shown inFIGS. 15 and 16, the plurality of sets of actuators120for operating the bulking member114are disposed at equal distances and hence, the bulking member114can be turned smoothly about the pivot112without application of an unbalanced load thereto. Moreover, a total output from the plurality of sets of actuators120is large and hence, it is possible to provide a reduction in capacity and in its turn, a reduction in size of the actuator120in each set.

In addition, the operating plunger123and the returning plunger124which are components for the actuator120in each set are received in the common cylinder bore121defined in the piston inner element105aand hence, the structure is simple, and the provision of the bore by working is simple, which can contribute to a reduction in cost.

When the actuators120are disposed in two sets, the respective cylinder bore121,121are defined in the piston inner element105ain parallel to the piston pin106. Therefore, the two sets of actuators120,120can be disposed at equal distances in the circumferential direction of the piston105without being interfered by the piston pin106.

The axes of the operating and returning plungers123and124are disposed to intersect, at substantially right angles, a radial line of the pivot112traversing the axis of each pressure-receiving pin114a. Therefore, pushing forces of the operating and returning plungers123and124can be transmitted efficiently to the bulking member114through the pressure-receiving pins114ato contribute to the compactness of the actuators120.

Each of the end faces of the operating and returning plungers123and124and the cylindrical outer peripheral surface of each of the pressure-receiving pins114aare in line contact with each other and hence, the contact area is wide, as compared with that in the first embodiment, thereby providing a reduction in surface pressure and contributing to an enhancement in durability.

It will be understood that the present invention is not limited to the above-described embodiments, and various modifications in design may be made without departing from the spirit and scope of the invention defined in claims. For example, the operating mode of the solenoid switchover valve45,145may be reverse from that in each of the above-described embodiments. More specifically, in the non-energized state of the switchover valve45,145, the oil passage44,144may be connected to the oil pump46,146, and in the energized state of the switchover valve45,145, the oil passage44,144may be connected to the oil reservoir47,147.