Patent Publication Number: US-9422872-B2

Title: Variable compression ratio internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to a variable compression ratio internal combustion engine equipped with a variable compression ratio mechanism capable of changing an engine compression ratio. 
     BACKGROUND ART 
     The applicants of the present application have conventionally proposed a variable compression ratio mechanism that can change an engine compression ratio, utilizing a multi-link piston-crank mechanism (for instance, see Patent document 1 described later). Such a variable compression ratio mechanism is configured to control an engine compression ratio depending on an engine operating condition by changing a rotational position of a control shaft by means of an actuator such as a motor. 
     CITATION LIST 
     Patent Literature 
     Patent document 1: Japanese patent provisional publication No. 2004-257254 (A) 
     SUMMARY OF INVENTION 
     Technical Problem 
     A large combustion load and/or a large inertia load repeatedly acts on the control shaft of the variable compression ratio mechanism via the multi-link mechanism, and thus the actuator, which changes and holds the rotational position of the control shaft, requires a very large holding force as well as a very large driving force. Therefore, the applicants are studying that a speed reducer, such as a harmonic-drive speed reducer, which can provide a high reduction ratio, is interposed between the actuator and the control shaft, and hence the driving force and the holding force of the actuator can be both decreased by reducing rotation of the actuator, (i.e., by multiplying torque from the actuator) by means of the speed reducer and by transmitting the reduced rotation (the multiplied torque) to the control shaft. 
     Accordingly, in an actuator mounting structure in which an actuator and a speed reducer of a variable compression ratio mechanism are attached to a sidewall of an engine main body with a housing therebetween, it is an object of the invention to suppress undesirable mixing/entry of foreign matter (debris and contaminants) into the speed reducer and to enhance a lubricating performance. 
     Solution to Problem 
     In a variable compression ratio internal combustion engine having a variable compression ratio mechanism that enables an engine compression ratio to be changed depending on a rotational position of a control shaft driven by an actuator and a speed reducer that reduces rotation of the actuator and transmits the reduced rotation to the control shaft, the actuator and the speed reducer being attached to a sidewall of an engine main body with a housing therebetween, an oil filter, which removes contaminants from within lubricating oil, is attached to the housing, and a bypass oil passage, which supplies a portion of lubricating oil after having passed through the oil filter to lubricated parts of the speed reducer installed in the housing, is also provided. 
     Advantageous Effects of Invention 
     According to the invention, an oil filter is attached to a housing, and a bypass oil passage, which supplies a portion of lubricating oil after having passed through the oil filter to lubricated parts of a speed reducer configured in the housing, is also provided. Therefore, it is possible to feed a portion of lubricating oil, purified by means of the oil filter, through the use of the shortest route via the bypass oil passage to the lubricated parts of the speed reducer, thereby enhancing a lubricating performance and minimizing mixing/entry of foreign matter (debris/contaminants) into the speed reducer, and thus increasing the reliability and durability of the speed reducer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the configuration of one example of a variable compression ratio mechanism related to the invention. 
         FIG. 2  is a perspective view illustrating a variable compression ratio internal combustion engine according to one embodiment of the invention. 
         FIG. 3  is a side view illustrating the intake side of the internal combustion engine of the embodiment. 
         FIG. 4  is a cross-sectional view illustrating the internal combustion engine of the embodiment. 
         FIG. 5(A)  is a perspective view illustrating an auxiliary shaft and lever sub-assembly of the embodiment, whereas  FIG. 5(B)  is a perspective view illustrating an auxiliary shaft and lever sub-assembly of a comparative example. 
         FIG. 6  is a cross-section in the vicinity of a housing of the embodiment. 
         FIG. 7  is a disassembled perspective view illustrating the auxiliary shaft, a bearing sleeve (a bearing member), and the housing of the embodiment. 
         FIG. 8  is a perspective view illustrating the housing and an oil-passage-forming body in the embodiment. 
         FIG. 9  is a cross-sectional view illustrating the housing and the oil-passage-forming body in the embodiment. 
         FIG. 10  is a plan view illustrating the housing and the oil-passage-forming body in the embodiment. 
         FIG. 11(A)  is an explanatory view illustrating an oil-level height position of the auxiliary shaft at a low compression ratio, whereas  FIG. 11(B)  is an explanatory view illustrating an oil-level height position of the auxiliary shaft at a high compression ratio. 
         FIG. 12  is a side view of the auxiliary shaft, whose journal portion including two different journal sections having respective outside diameters differing from each other as viewed from the axial direction. 
         FIG. 13  is a side view illustrating a unitary structure of the auxiliary shaft of the embodiment. 
         FIGS. 14(A)-14(B)  are an explanatory views illustrating states of abutted-engagement of both side faces of a protruding portion of the auxiliary shaft with respective stopper faces of the housing. 
         FIG. 15  is a front elevation view illustrating the auxiliary shaft of the embodiment. 
         FIG. 16  is a cross-sectional view illustrating the assembled section of the bearing sleeve and the housing in the embodiment. 
         FIG. 17(A)  is an explanatory view illustrating a bearing sleeve of a reference example, whereas  FIG. 17(B)  is an explanatory view illustrating the bearing sleeve of the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the invention are hereinafter described in reference to the drawings. A variable compression ratio mechanism, which utilizes a multi-link piston-crank mechanism, is hereunder explained in reference to  FIG. 1 . By the way, this mechanism is publicly known as set forth in Japanese patent provisional publication No. 2004-257254 (A), and thus its construction is hereunder described briefly. 
     A piston  3  of each engine cylinder is installed in a cylinder block  1 , which constructs a part of an internal combustion engine, and slidably fitted into a cylinder  2 . Also, a crankshaft  4  is rotatably supported by the cylinder block. A variable compression ratio mechanism  10  has a lower link  11 , an upper link  12 , a control shaft  14 , a control eccentric shaft  15 , and a control link  13 . The lower link is rotatably installed on a crankpin  5  of crankshaft  4 . The upper link mechanically links the lower link  11  to the piston  3 . The control shaft is rotatably supported on the engine main body side, such as the cylinder block  1 . The control eccentric shaft is arranged eccentrically with respect to the control shaft  14 . The control link mechanically links the control eccentric shaft  15  to the lower link  11 . Piston  3  and the upper end of upper link  12  are connected together via a piston pin  16  so as to permit relative rotation. The lower end of upper link  12  and lower link  11  are connected together via a first connecting pin  17 . The upper end of control link  13  and lower link  11  are connected together via a second connecting pin  18 . The lower end of control link  13  is rotatably installed on the control eccentric shaft  15 . 
     A variable compression ratio motor  20  (for instance, see  FIG. 2 ), serving as an actuator, is connected to the control shaft  14  via a speed reducer  21  (described later). A piston stroke characteristic, including a piston top dead center (TDC) position and a piston bottom dead center (BDC) position, changes with an attitude change of lower link  11 , created by changing a rotational position of control shaft  14  by the variable compression ratio motor  20 . Hence, an engine compression ratio changes. Thus, it is possible to control the engine compression ratio depending on an engine operating condition by controlling the drive (the operation) of variable compression ratio motor  20  by a control part (not shown). By the way, the actuator is not limited to such an electric motor  20 , but a hydraulically-operated actuator may be used. 
     Referring to  FIGS. 2-3 , control shaft  14  is rotatably housed in the engine main body, constructed by the cylinder block  1  and an oil pan upper  6  or the like. On the other hand, speed reducer  21  and variable compression ratio motor  20  are attached to an outside wall of oil pan upper  6 , constructing a part of the engine main body, i.e., an intake-side sidewall  7  for details, with a housing  22 , in which speed reducer  21  is housed. In addition to the variable compression ratio motor  20 , an oil cooler  23 , which cools lubricating oil, is further attached to the housing  22 . Still further, an oil filter  24 , which removes contaminants from within the lubricating oil, is attached to the housing via an oil-passage-forming body  50  (described later). 
     By the way, in the shown embodiment, oil-passage-forming body  50 , to which oil filter  24  is attached, is constructed separately from the housing  22 , but oil-passage-forming body  50  may be configured integral with the housing  22 . 
     As shown in  FIG. 3 , an air compressor  9  is installed on the intake-side sidewall  7  of oil pan upper  6  and arranged at the front side of the engine. Also, the intake-side sidewall of the oil pan upper is provided with a fastening flange  8 , to which a transmission is fixedly connected and which is located at the rear side of the engine. Oil cooler  23 , oil-passage-forming body  50  to which oil filter  24  is attached, housing  22  in which speed reducer  21  is housed, and motor  20  are placed along the fore-and-aft direction of the engine and arranged between the fastening flange  8  and the air compressor  9 . That is, on one hand, oil cooler  23  is placed in front of a side face of housing  22 , facing the front side of the engine, in a manner so as to sandwich the oil-passage-forming body  50  between them. On the other hand, variable compression ratio motor  20  is placed in rear of a side face of housing  22 , facing the rear side of the engine. A mounting flange  25  of housing  22  is fixed to the intake-side sidewall  7  of oil pan upper  6  by means of fixing bolts  26 . 
     As shown in the drawings, in particular,  FIGS. 2, 4 , and  5 , the control shaft  14 , which is placed in the engine main body, and an auxiliary shaft  30 , which is formed integral with the output shaft of speed reducer  21  placed in the housing  22 , are connected together by means of a lever  31 . By the way, in the embodiment, auxiliary shaft  30  is integrally formed with the output shaft of speed reducer  21 . In lieu thereof, auxiliary shaft  30  may be configured separately from the output shaft of speed reducer  21  such that the auxiliary shaft and the speed-reducer output shaft rotate integrally with each other. 
     One end of lever  31  and the tip end of an arm  32  extending radially outward from the center of control shaft  14  as viewed in the axial direction are connected together via a third connecting pin  33  so as to permit relative rotation. The other end of lever  31  and auxiliary shaft  30  are connected together via a fourth connecting pin  35  so as to permit relative rotation. By the way, in  FIGS. 2 and 5 , the fourth connecting pin  35  is removed and omitted from  FIGS. 2 and 5 , and in lieu thereof a connecting-pin hole  35 A, into which the fourth connecting pin  35  is fitted, is drawn. As shown in  FIG. 4 , a lever slit  36 , into which lever  31  is inserted, is formed in the intake-side sidewall  7  of oil pan upper  6 . 
     As shown in  FIG. 5(A) , in the auxiliary shaft  30  of the embodiment, an arm length D 1 , corresponding to the distance between the rotation center of auxiliary shaft  30  and the center of connecting-pin hole  35 A into which the fourth connecting pin  35  is fitted, is set to be shorter than the radius (one-half the diameter D 2 ) of a journal portion  38  rotatably supported by a metal bearing sleeve  37  (a bearing member) mounted on the housing  22 , that is, D 1 &lt;(D 2 /2). Therefore, the fourth connecting pin  35  is located inside of the journal portion  38 . That is, the journal portion  38  is configured to include the fourth connecting pin  35  inside thereof. By the way, a slit  39  for avoiding interference with the lever  31  is formed in the journal portion  38 . In the embodiment, bearing sleeve  37  is configured as a metal integral part, but such a bearing sleeve may be constructed as a bearing member configured to have the same shape as the bearing sleeve  37  by fastening two separate parts, each of which has the same semi-cylindrical bearing surface, together with bolts. 
     On the other hand, in the auxiliary shaft  30  of the comparative example shown in  FIG. 5(B) , an arm length D 3 , corresponding to the distance between the rotation center of journal portion  38  and the center of connecting-pin hole  35 A is set to be longer than the radius (one-half the diameter D 4 ) of the journal portion  38 , that is, D 3 &gt;(D 4 /2). That is, a portion of connecting-pin hole  35 A is formed into an arm shape protruding radially outward with respect to the journal portion  38 . Thus, it is necessary to lay out the journal portion  38  at a position, which is axially offset from the portion of connecting-pin hole  35 A. Due to such an axial offset, an axial dimension D 6  of auxiliary shaft  30  tends to increase. 
     In contrast to the comparative example, in the embodiment, it is possible to place the connecting-pin hole  35 A inside of the journal portion  38 , as discussed previously. Hence, it is unnecessary to lay out both the journal portion and the connecting-pin hole at separate axial positions. In comparison with the comparative example, it is possible to greatly shorten the axial dimension D 5  of auxiliary shaft  30 . Also, regarding the journal portion  38 , for the purpose of ensuring a bearing strength, it is necessary to ensure a predetermined bearing surface area. However, in the case of the embodiment of  FIG. 5(A)  having the comparatively great diameter D 2  of journal portion  38 , it is possible to shorten the axial dimension of the journal portion  38  itself, while ensuring the same bearing surface area, in comparison with the comparative example of  FIG. 5(B)  having the comparatively small diameter D 4  of journal portion  38 . In this manner, by virtue of the shortened axial dimension of auxiliary shaft  30 , it is possible to shorten the axial dimension of housing  22  in which the auxiliary shaft  30 , together with the speed reducer  21 , can be housed. For this reason, as shown in  FIG. 3 , in particular in the case of the mounting structure in which the oil cooler  23  in front of housing  22 , the housing  22 , and the motor  20  in rear of housing  22  are placed in series with each other along the fore-and-aft direction of the engine, it is possible to improve the mountability of the engine by shortening the considerably-limited longitudinal dimension in the fore-and-aft direction of the engine. 
     The construction of speed reducer  21  is hereunder described in reference to  FIG. 6 . This speed reducer  21  utilizes a well-known harmonic drive mechanism. The speed reducer is comprised of four major component parts, namely, a wave generator  41 , a flexspline  42  arranged around the circumference of wave generator  41 , a circular spline  43  and a circular spline  44 , both circular splines being juxtaposed to each other and arranged around the circumference of the flexspline. 
     Regarding wave generator  41 , double-row ball bearings  46  are fitted onto the circumference of an ellipse-shaped cam  45  of the wave generator. Elastic deformation of the outer ring of each ball bearing  46  occurs depending on rotary motion of elliptical cam  45 , the position of the major axis of the elliptical cam wave generator is displaced in the rotation direction. Flexspline  42  is a thin-walled, ring-shaped, elastic (flexible) metal part formed with external teeth cut on its outer periphery. On one hand, circular spline  44  is formed on its inner periphery with internal teeth of the same number of teeth as the flexspline  42 . The circular spline rotates at the same speed as the flexspline  42  by a gear mesh of the circular spline with the flexspline  42 , elastically deformed into an elliptical shape, at two engagement points along the major axis of the ellipse. On the other hand, another circular spline  43  is formed on its inner periphery with two fewer internal teeth than the number of external teeth on the flexspline  42 . Similarly, a gear mesh of this circular spline with the flexspline  42  occurs at two engagement points along the major axis of the ellipse. 
     Wave generator  41  is fixed to the input shaft of speed reducer  21 , which rotates integrally with the rotation axis of variable compression ratio motor  20 . Circular spline  44  is fixed to the auxiliary shaft  30 , serving as the output shaft of speed reducer  21 . Circular spline  43  is fixed to a motor cover  47 , which is fixed to the housing  22 . Hence, rotation of the input shaft of speed reducer  21  is reduced at a predetermined reduction ratio, and then the reduced rotation is transmitted to the output-shaft side. By the way, reference sign  48  denotes each ball bearing for rotatably supporting the elliptical cam  45  fixed to the input shaft of speed reducer  21 . 
     By the way, speed reducer  21  is not limited to a harmonic-drive speed reducer as described by reference to the embodiment, but another type speed reducer, such as a cycloid planetary-gear speed reducer or the like, may be utilized as the speed reducer  21 . 
     A lubrication structure for speed reducer  21  is hereunder described. 
     As shown in  FIG. 3 , the oil-passage-forming body  50  is interposed between the side face of housing  22 , facing the front side of the engine, and a side face of oil cooler  23 , facing the rear side of the engine. An oil filter  24 , in which a filter element is stored, is mounted on a filter mounting flange  50 C (see  FIGS. 7-8 ) of the oil-passage-forming body. A plurality of oil passages  51 - 58  are formed in the oil-passage-forming body  50 . 
     As shown in  FIGS. 6, and 8-10 , lubricating oil is supplied from the inside of the engine main body via a first oil passage  51  and a second oil passage  52  formed in the oil-passage-forming body  50  to the oil cooler  23 . One end of the first oil passage  51  is opened at an engine-main-body mounting face  50 A of oil-passage-forming body  50  fixed to the intake-side sidewall  7  of oil pan upper  6 . The second oil passage  52  is configured to intersect with the first oil passage  51 . One end of the second oil passage is opened at a cooler mounting face  50 B onto which oil cooler  23  is fixed. 
     Lubricating oil, discharged from the oil cooler  23 , is supplied into the oil filter  24  by way of a third oil passage  53  opened at the cooler mounting face  50 B, a fourth oil passage  54  communicating with the third oil passage  53 , and a fifth oil passage  55  communicating with the fourth oil passage  54  and formed in the filter mounting flange  50 C so as to extend in the circumferential direction. 
     Lubricating oil, discharged from the oil filter  24  immediately after having been filter-purified, is returned to the inside of the engine main body by way of a sixth oil passage  56  whose one end is opened at the filter mounting flange  50 C, and a seventh oil passage  57 , which intersects with the sixth oil passage  56  and whose one end is opened at the engine-main-body mounting face  50 A. By the way, a portion of lubricating oil, discharged from the oil filter  24  immediately after having been filter-purified, is supplied via a bypass oil passage  58  to lubricated parts configured in the housing  22 . 
     As shown in the drawings, in particular,  FIGS. 6, 11, and 13 , bypass oil passage  58  is configured at one end to communicate with the seventh oil passage  57 , and also configured to extend from the oil-passage-forming body  50  to the inside of housing  22 . The bypass oil passage has a circumferential groove  58 A formed in the circumference of the journal portion  38  of auxiliary shaft  30 , a plurality of auxiliary oil passages  58 B through which the circumferential groove  58 A and a speed-reducer accommodation chamber  64  are communicated with each other, and a communication oil passage  58 C through which the seventh oil passage  57  and the circumferential groove  58  are communicated with each other. Hence, by way of the aforementioned bypass oil passage  58 , lubricating oil, passed through the oil filter  24  immediately after having been filter-purified, is supplied to the bearing surface of journal portion  38  as well as lubricated parts of speed reducer  21  accommodated in the housing  22 , concretely, the meshed-engagement portions between flexspline  42  and each of circular splines  43 - 44 , bearing surfaces of ball bearings  46  and  48 , and the like. 
     As shown in  FIG. 8 , the internal space of housing  22  is partitioned into the speed-reducer accommodation chamber  64  and an auxiliary-shaft accommodation chamber  65  by means of a partition wall portion  61  provided inside of the housing  22  and a large-diameter portion  63  of auxiliary shaft  30 , which is rotatably loosely fitted through a slight clearance into a circular through opening  62  formed in the center of partition wall portion  61 . As discussed previously, the major component parts of speed reducer  21 , namely, wave generator  41 , flexspline  42 , circular spline  43  and circular spline  44 , and their lubricated parts are placed in the speed-reducer accommodation chamber. The major part of auxiliary shaft  30  is placed in the auxiliary-shaft accommodation chamber. Also, the auxiliary-shaft accommodation chamber is configured to face the lever slit  36  (see  FIG. 4 ) into which lever  31 , connected with the auxiliary shaft  30 , is inserted. Lubricating oil is supplied via the bypass oil passage  58  into the speed-reducer accommodation chamber  64 . Then, the lubricating oil, stored in the speed-reducer accommodation chamber  64 , is supplied via an oil hole  66  (described later) and the like into the auxiliary-shaft accommodation chamber  65 . Thereafter, the lubricating oil, stored in the auxiliary-shaft accommodation chamber  65 , is returned back to the inside of oil pan upper  6  (the engine main body) via the previously-noted lever slit  36 . 
     In the embodiment shown and described herein, the oil hole  66  (see  FIGS. 4 and 11 ), through which speed-reducer accommodation chamber  64  and auxiliary-shaft accommodation chamber  65  are communicated with each other, is formed as a through hole that penetrates the large-diameter portion  63  (the rotating body) of auxiliary shaft  30  that partitions the interior space of housing  22  into the speed-reducer accommodation chamber  64  and the auxiliary-shaft accommodation chamber  65 . That is, oil hole  66  is formed in the large-diameter portion  63  constructing a part of the wall surface of speed-reducer accommodation chamber  64 . As shown in  FIGS. 4 and 11 , oil hole  66  is located at a given position radially spaced apart from the rotation center of large-diameter portion  63 . The level (the height position) of the oil hole changes depending on the rotational position of auxiliary shaft  30  that rotates in synchronism with rotation of control shaft  14 . By the way, as shown in the drawings, in particular,  FIGS. 5 and 11 , regarding the auxiliary shaft  30 , the radial dimension of large-diameter portion  63  is dimensioned to be greater than that of journal portion  38 . 
     Additionally, as shown in  FIGS. 4 and 11 , an auxiliary oil hole  67  is formed in the bottom wall of housing  22 . Speed-reducer accommodation chamber  64  and auxiliary-shaft accommodation chamber  65  (or the inside of the engine main body) are communicated with each other via the auxiliary oil hole, in a similar manner to the previously-noted oil hole  66 . The auxiliary oil hole  67  is dimensioned and configured as an orifice passageway having a smaller inside diameter and a smaller opening area than the previously-noted oil hole  66 . The auxiliary oil hole is located at a given position lower than the oil hole  66  in the vertical direction, concretely, arranged at the lowermost end of housing  22 . 
       FIG. 11  shows the position (the level) of the oil hole  66  depending on a rotational position of auxiliary shaft  30  (that is, a state of setting of the engine compression ratio).  FIG. 11(A)  shows a state of setting of a low compression ratio, used in a high-temperature high-load range, whereas  FIG. 11(B)  shows a state of setting of a high compression ratio, used in a low-temperature low-load range. Two-dotted lines G 1 -G 3  indicated in these drawings represent respective oil-level heights. That is, these two-dotted lines G 1 -G 3  correspond to respective oil-level horizontal lines parallel to each other in the horizontal direction under a state where the actuator has been mounted on the vehicle. 
     Under a condition where the engine is operating, lubricating oil is always supplied to the speed-reducer accommodation chamber  64  via the bypass oil passage  58 . Thus, a slight amount of lubricating oil tends to flow out from the speed-reducer accommodation chamber  64  through the auxiliary oil hole  67  and the like, but most of the lubricating oil flows from the speed-reducer accommodation chamber  64  through the oil hole  66  into the auxiliary-shaft accommodation chamber  65 . Therefore, the respective oil-level height positions G 1 , G 2  of lubricating oil, stored in the speed-reducer accommodation chamber  64 , become near the lowermost end of oil hole  66 . In the embodiment, during a low compression ratio setting shown in  FIG. 11(A) , the position of oil hole  66  is higher than that of a high compression ratio setting shown in  FIG. 11(B) . The position of oil hole  66  is set such that the oil-level height position G 1  within the speed-reducer accommodation chamber  64  during a low compression ratio becomes higher than the oil-level height position G 2  within the speed-reducer accommodation chamber  64  during a high compression ratio. 
     Therefore, in a state of setting of a low compression ratio, used in a high-temperature high-load range, by raising the oil-level height position G 1  within the speed-reducer accommodation chamber  64  and by increasing the amount of lubricating oil in the speed-reducer accommodation chamber  64 , it is possible to improve the lubricating performance and the cooling performance of speed reducer  21  in a high-temperature high-load range, thus enhancing both the durability and the reliability. On the other hand, in a state of setting of a high compression ratio, used in a low-temperature low-load range, by relatively lowering the oil-level height position G 2  within the speed-reducer accommodation chamber  64  and by reducing the amount of lubricating oil in the speed-reducer accommodation chamber  64 , it is possible to reduce a resistance to oil agitation, occurring owing to rotation of speed reducer  21 . For the reasons discussed above, for instance during acceleration with an engine load increase, the engine compression ratio has to be rapidly reduced from a high compression ratio (e.g., approximately 14) to a middle compression ratio (e.g., approximately 12) needed for knocking avoidance, but, according to the embodiment, it is possible to reduce the resistance to oil agitation, occurring owing to rotation of speed reducer  21 , by adjusting the oil-level height position G 2  to a relatively lower level. For instance, the response time to a compression ratio decrease can be shortened by several ten milliseconds. In this manner, by improving the response to a compression ratio decrease from a high compression ratio to a low compression ratio, it is possible to alleviate a limit for knocking avoidance to a compression ratio change to high compression ratios. Hence, it is possible to improve fuel economy by virtue of a compression ratio change to high compression ratios. 
     Additionally, in the embodiment, such an oil-level height adjustment based on the engine compression ratio is realized by forming the oil hole  66  in the auxiliary shaft  30 , serving as a rotating body that rotates in synchronism with rotation of control shaft  14 , and thus it is possible to provide the previously-discussed operation and effects by a simple construction. 
     In the case that a negative pressure occurs in the variable compression ratio motor  20  owing to a fall in internal temperature in the motor  20  on the assumption that the oil-level height within the housing  22  is a position higher than a seal part of the motor input shaft of variable compression ratio motor  20 , lubricating oil is sucked from the seal part of the motor input shaft into the inside of the motor and thus there is a possibility for oil to enter into the inside of the motor. Therefore, in the embodiment, the oil-level height positions G 1 , G 2  based on the engine operating condition are set at positions further lower than the lower end of the seal part of the motor input shaft of variable compression ratio motor  20 . Hence, it is possible to suppress or avoid oil from entering the inside of the motor. 
     When the engine has stopped running, lubricating oil is gradually drained from the speed-reducer accommodation chamber  64  via the auxiliary oil hole  67  having a smaller flow passage area, and then returned via the lever slit  36 , facing the auxiliary-shaft accommodation chamber  65 , back to the inside of the engine main body. Therefore, as shown in  FIG. 11 , the oil-level height position G 3  within the speed-reducer accommodation chamber  65  during a stop of the engine tends to become near the lowermost end of housing  22  in the vicinity of the auxiliary oil hole  67 , irrespective of the engine compression ratio setting. Also, as shown in  FIG. 4 , an oil-level height position G 4  within the auxiliary-shaft accommodation chamber  65  becomes near the lowermost end of housing  22 . Hence, housing  22  comes to a state where most of lubricating oil in the housing has been drained. 
     When the engine has stopped running, foreign matter, such as iron, aluminum and the like, existing in the lubricating oil, becomes deposited on the bottom of housing  22 , but, according to the embodiment, it is possible to drain the foreign matter or contaminants deposited on the bottom of housing  22 , together with the lubricating oil, by forming the auxiliary oil hole  67  in the bottom of housing  22 , thus suppressing wear of speed reducer  21 . Additionally, during the maintenance, such as during disassembling or assembling of the speed reducer  21  and/or the variable compression ratio motor  20 , housing  22  has been brought into a state where lubricating oil has already been drained out from within the housing. Thus, it is possible to suppress an oil leakage or the like during the maintenance. This is superior in maintainability. 
     The construction, operation and effects, peculiar to the shown embodiment, are hereunder enumerated. 
     [1] As shown in the drawings, in particular,  FIGS. 2, 3, and 6 , oil filter  24  is attached via the oil-passage-forming body  50  to the housing  22 , in which speed reducer  21  is housed. Additionally, bypass oil passage  58 , which supplies a portion of lubricating oil passed through the oil filter  24  immediately after having been filter-purified to lubricated parts of speed reducer  21  placed in the speed-reducer accommodation chamber  64  of housing  22 , is provided. Therefore, it is possible to feed the lubricating oil, immediately after having been purified by means of the oil filter  24 , through the use of the shortest route via the bypass oil passage  58  to the lubricated parts of speed reducer  21 , thereby minimizing mixing/entry of foreign matter (debris/contaminants) into the speed-reducer accommodation chamber  64 , and thus increasing the reliability and durability of the speed reducer. 
     [2] As shown in the drawings, in particular,  FIGS. 2 and 3 , housing  22 , in which variable compression ratio motor  20  and speed reducer  21  are housed, is attached to the intake-side sidewall  7  of oil pan upper  6 , constructing a part of the engine main body, for the purpose of protecting them against exhaust heat. 
     [3] However, in the case that housing  22  and the like are arranged on the intake-side sidewall  7  as discussed above, as shown in  FIG. 3 , the respective component parts have to be installed in a limited space sandwiched between the air compressor  9  arranged at the front side of the engine and the fastening flange  8  to which the transmission is fixedly connected and which is located at the rear side of the engine, and thus a limitation on the longitudinal dimension in the fore-and-aft direction of the engine becomes severe. Also, from the relevance to the layout of an oil pump and a main oil gallery on the intake-side sidewall  7  of cylinder block  1  above the oil pan upper  6 , oil cooler  23  and oil filter  24  have to be arranged on the intake side. Thus, it is more difficult to ensure the mounting space. 
     For the reasons discussed above, in the embodiment, oil cooler  23 , which cools the lubricating oil, together with the oil filter  24 , is attached to the housing  22 . Thus, oil cooler  23  and oil filter  24  are gathered around the housing  22 , and thus it is possible to improve the mountability of the engine, thus realizing simplification and shortening of the oil passages. 
     [4] Concretely, oil cooler  23  is fixedly connected to the housing  22  with the oil-passage-forming body  50 , whose thickness is thinner than the oil filter  24 , therebetween. Oil filter  24  is attached to the oil-passage-forming body  50 . Additionally, oil passages  51 - 58 , through which the lubricating oil flows, are formed in the oil-passage-forming body. Therefore, in addition to the operation and effect of the above-mentioned item [3], by virtue of offset arrangement of the oil filter  24  at a position, which is offset from the oil cooler  23 , the oil-passage-forming body  50 , and the housing  22 , all placed in series with each other in the fore-and-aft direction of the engine, it is possible to shorten the longitudinal dimension in the fore-and-aft direction of the engine, thus improving the mountability of the engine. 
     [5] Formed in the oil-passage-forming body  50  are oil passages  51 - 52 , which supply the lubricating oil from the engine main body to the oil cooler  23 , oil passages  53 ,  54 , and  55 , which supply the lubricating oil from the oil cooler  23  to the oil filter  24 , oil passages  56 - 57 , which supply the lubricating oil from the oil filter  24  to the engine main body, and bypass oil passage  58 , which supplies the lubricating oil from the oil filter  24  to the lubricated parts of the speed reducer. In this manner, the oil passages, which are provided for respectively supplying the lubricating oil to the oil cooler  23 , the oil filter  24 , and the lubricated parts of speed reducer  21 , are concentrated at the oil-passage-forming body  50 , and thus it is possible to realize shortening of the oil passages and compactification of the device/system. 
     [6] Also, as shown in  FIG. 4 , control shaft  14 , which is placed in the engine main body, and auxiliary shaft  30 , which is rotatably supported in the housing  22  and rotates integrally with the output shaft of speed reducer  21 , are connected together by means of the lever  31 , which is inserted through the lever slit  36  formed in the sidewall  7  of the engine main body. One end of lever  31  and auxiliary shaft  30  are connected together by the fourth connecting pin  35  so as to permit relative rotation. 
     By the way, assume that, for the purpose of the previously-discussed demand for shortening of the longitudinal dimension in the fore-and-aft direction of the engine, the axial dimension of auxiliary shaft  30  is simply shortened. In such a case, the width of the bearing surface of the journal portion  38  of auxiliary shaft  30 , which is rotatably supported in the housing  22 , becomes shortened, and thus the bearing surface pressure tends to increase, and as a result there is a possibility for wear to develop. Therefore, in the embodiment, as shown in  FIG. 5(A) , connecting-pin hole  35 A, into which the connecting pin is inserted, is configured to be located inside of the journal portion  38 . That is, the arm length D 1  between the center of journal portion  38  and the center of connecting-pin hole  35 A is set to be shorter than the radius (D 2 /2) of journal portion  38 , and thus the journal portion  38  is configured to include the connecting-pin hole  35 A inside thereof. Hence, it is possible to suppress or reduce the axial dimension D 5  of auxiliary shaft  30 , while ensuring a bearing surface area by enlarging the radial dimension of journal portion  38 , thus improving the mountability of the engine. 
     [7] Concretely, as shown in  FIG. 5(A) , the axial dimension D 5  of auxiliary shaft  30  containing the journal portion  38 , is set to be shorter than the radial dimension (i.e., the diameter) D 2  of journal portion  38 . Thus, it is possible to provide the sufficiently shortened axial dimension. 
     [8] In a modification shown in  FIG. 12 , the radial dimension (i.e., the diameter)  38 A of an actuator-side journal section of journal portion  38  is set to be greater than the radial dimension (i.e., the diameter)  38 B of an anti-actuator-side journal section. The actuator-side journal section, on which motor  20  and speed reducer  21  are installed, tends to oscillate, since motor  20  as well as speed reducer  21  serves as a vibrating weight. Therefore, the input load of the actuator-side journal section tends to become greater than that of the anti-actuator-side journal section. For the reasons discussed above, it is possible to effectively reduce the bearing surface pressure by setting the dimension (i.e., the diameter)  38 A of the actuator-side journal section to be relatively greater than the anti-actuator-side. 
     [9] As shown in  FIG. 13 , a partially axially protruding portion  70  is provided at a part of journal portion  38  on which the maximum combustion load acts. Hence, an axial dimension  38 C of this part is set to be greater than an axial dimension  38 D of a part of the journal portion on which the maximum combustion load does not act. Thus, by virtue of the increased bearing surface area of the journal on which the maximum combustion load acts, it is possible to effectively reduce the bearing surface pressure. 
     [10] As shown in  FIGS. 5(A) ,  13 , and  14 (A)- 14 (B), journal portion  38  is provided with the partially axially protruding sector portion  70  located at the portion of connecting-pin hole  35 A. Additionally, both circumferential side faces  70 A,  70 B of the protruding portion  70  are configured to permit abutted-engagement with respective stopper faces  71 A,  71 B formed at the side of housing  22 . 
     Therefore, it is possible to mechanically limit the range of rotation of control shaft  14 , that is, the variable range of the engine compression ratio by limiting the movable range of auxiliary shaft  30  within a given range determined by abutted-engagement of both side faces  70 A,  70 B with respective stopper faces  71 A,  71 B. Additionally, part of the maximum combustion load can be received by the abutting portions of these two components, and thus it is possible to reduce the maximum bearing pressure acting on the bearing surface. Also, the axial dimension of the protruding portion  70 , at which connecting-pin hole  35 A is placed, becomes increased, and thus the rigidity of the bearing area of connecting-pin hole  35 A can be enhanced. Furthermore, a snap-ring groove, into which a connecting-pin anti-loose snap ring is fitted, can be easily formed in the protruding portion  70  without increasing the axial dimension. 
     [11] As shown in the drawings, in particular,  FIGS. 4, 7, and 16 , bearing sleeve  37 , which rotatably supports the journal portion  38  of auxiliary shaft  30 , is formed separately from the housing  22 . The bearing sleeve is configured to be fixed to the housing  22  with two bolts  72 . The difference in the coefficient of thermal expansion between the auxiliary shaft  30  and the bearing sleeve  37  is set to be less than the difference in the coefficient of thermal expansion between the bearing sleeve  37  and the housing  22 . For instance, in the case that the material of housing  22  is aluminum, the material of bearing sleeve  37  is iron, and the material of auxiliary shaft  30  is iron, the difference in the coefficient of thermal expansion between the auxiliary shaft  30  and the bearing sleeve  37  can be decreased, and hence it is possible to suppress a clearance change of the bearing area occurring owing to the thermal expansion. Therefore, it is possible to suppress a deterioration in noise/vibration performance owing to a clearance increase of the bearing area. Also, it is possible to suppress an increase in friction, occurring owing to an excessive decrease in clearance. 
     [12] As shown in  FIGS. 7 and 16 , bearing sleeve  37  is comprised of a cylindrical portion  73  that rotatably supports the journal portion  38  of auxiliary shaft  30 , and a mounting base  74  having a housing-mounting flat face  74 A that is fitted or fixed onto one sidewall of housing  22  with the two bolts  72 . The cylindrical portion and the mounting base are integrally molded or formed of an iron material. The cylindrical portion  73  is formed with the slit  36  through which the lever  31  is inserted. 
     As shown in  FIG. 16 , the bearing sleeve  37  is set or configured such that the maximum combustion load acts on a given part (a given position) of the inner circumferential surface positioned on the side of the mounting base  74  of bearing sleeve  37  and sandwiched between the two bolts  72 . Thus, it is possible to suppress the force acting in the direction of the opening, facing apart from the bolting face, by fastening the bearing sleeve with the bolts on the side of the bearing sleeve on which the maximum combustion load acts. This is because the tensile load (the inertia load), produced by the inertia force acting on the bolt  72 , is comparatively smaller, that is, approximately 50% of the combustion load. The load is distributed through the bearing sleeve  37  formed of iron having a rigidity higher than aluminum into the light-weight housing  22  formed of aluminum, and thus it is possible to suppress the deformation of the aluminum housing  22 . Accordingly, it is possible to suppress fluctuations in the engine combustion ratio. 
     [13]  FIG. 17(A)  shows a bearing sleeve  37 A of the reference example, which is formed into a cylindrical shape, and whose bearing thickness is uniform around the entire circumference. In contrast, as shown in  FIG. 17(B) , in the embodiment, the rigidity of a thin-walled central portion  743  of the mounting base  74  of bearing sleeve  37 , on which the maximum combustion load acts, is set to be less than the rigidity of thick-walled both-side bolted portions  74 C through which the bearing sleeve is fastened with the two bolts. Hence, when the combustion load acts, the greatest contact portions with the bearing sleeve  37  become two points near the previously-noted bolted portions of bearing sleeve  37 . In this manner, the bearing sleeve is configured such that the load is supported mainly by these two points, and thus the friction tends to increase approximately 1 to 1.4 times greater than the reference example of  FIG. 17(A)  in which the maximum combustion load is supported by one point. Therefore, when the maximum combustion load acts, by virtue of the increased friction, it is possible to reduce the holding torque of control shaft  14 . 
     On the other hand, when the combustion load is small, the amount of elastic deformation is also small. Thus, the strong contact tends to occur at one point on which the combustion load acts, in the same manner as the previously-discussed reference example. Hence, an increase in friction can be suppressed, and therefore it is possible to suppress a deterioration in the response to a compression ratio change, which may occur owing to such an increase in friction. 
     [14] As shown in  FIGS. 6 to 10 , a connecting-pin assembling window  75 , facing the fourth connecting pin  35 , is formed in the oil-passage-forming body  50  of oil filter  24  so as to penetrate the oil-passage-forming body. Therefore, when assembling, under a state where oil-passage-forming body  50  has been assembled on the housing  22  in advance as a unit, the housing  22  is bolted to the intake-side sidewall  7  of oil pan upper  6 . After this, the fourth connecting pin is installed through the connecting-pin assembling window  75 . In this manner, lever  31  and auxiliary shaft  30  can be connected together so as to permit relative rotation. 
     Thereafter, as shown in  FIG. 6 , oil cooler  23  is fixedly connected to the cooler mounting face  50 B of oil-passage-forming body  50 . As a result, oil passages  52 ,  53 , which are opened at the cooler mounting face  50 B of oil-passage-forming body  50 , are communicated with respective oil passages (not shown), which are opened at a mounting face  23 A of oil cooler  23 , and at the same time the previously-discussed connecting-pin assembling window  75  is sealed by the mounting face  23 A of oil cooler  23  in a fluid-tight fashion, thereby avoiding oil leakages from occurring.