Abstract:
A system for varying a compression ratio of an internal combustion engine includes: at least one sensor for measuring an operating condition of the internal combustion engine; a variable compression ratio apparatus for varying the effective length of a connecting rod, the compression ratio apparatus itself comprising a bearing retainer disposed between the connecting rod and a crank pin, the bearing retainer having an inner surface in communication with the crank pin and an outer surface in communication with the connecting rod, the connecting rod being axially movable relative to the bearing retainer along a longitudinal axis of the connecting rod to effect a selective displacement of the connecting rod relative to the bearing retainer, the displacement thereby causing a change in the effective length of the connecting rod and a desired compression ratio of the internal combustion engine; and an engine controller coupled to the internal combustion engine, the sensor and the variable compression ratio apparatus for generating, based on the measured operating condition of the internal combustion engine, a control signal required to displace the bearing retainer in accordance with the desired compression ratio of the internal combustion engine.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to reciprocating piston internal combustion engines. More particularly, the invention relates to a system and method for varying the compression ratio of a reciprocating internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     The “compression ratio” of an internal combustion engine is defined as the ratio of the volume in a cylinder above a piston when the piston is at bottom-dead-center (BDC) to the volume in the cylinder above the piston when the piston is at top-dead-center (TDC). The higher the compression ratio, the more the air and fuel molecules are mixed and compressed and the better the efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the internal combustion engine. 
     In conventional internal combustion engines, however, the compression ratio is fixed and thus the engine efficiency cannot be changed to yield optimal performance. Accordingly, so-called “variable compression ratio” (VCR) internal combustion engines have been developed to vary the clearance volume of a cylinder in order to achieve improved fuel economy and increase engine power performance. Such VCR engines are designed to have a higher compression ratio during low load conditions, and a lower compression ratio during high load conditions. Conventional techniques include using “sub-chambers” and “sub-pistons” to vary the volume of the cylinder, see for example U.S. Pat. Nos. 4,246,873 and 4,286,552; varying the actual dimensions of all or a portion of a piston attached to a fixed length connecting rod, see U.S. Pat. No. 5,865,092; and varying the actual length of the connecting rod itself, see U.S. Pat. Nos. 5,724,863 and 5,146,879. 
     Other techniques include the use of eccentric rings or bushings either at the lower “large” end of a connecting rod or the upper “small” end of the connecting rod for varying the length of the connecting rod or height of the reciprocating piston. In U.S. Pat. No. 5,562,068, for example, a variable compression ratio connecting rod is disclosed wherein the effective length of the rod is varied via an eccentric ring that can be rotated and selectively locked to the crank pin and to the large end of the rod. When a working hydraulic pressure is released, the eccentric ring becomes locked with the crank pin thus resulting in a longer effective rod length and therefore a higher compression ratio mode. When the working hydraulic pressure is applied, the eccentric ring becomes locked with the rod, thus resulting in a shorter effective rod length and therefore a lower compression ratio. Transition from the higher compression ratio mode to the low compression ratio mode, or vice-versa, occurs when the piston is at BDC. 
     U.S. Pat. No. 5,960,750 similarly discloses a connecting rod having a rotatable eccentric ring in communication with a connecting rod. The connecting rod includes a mechanically actuated locking member for locking the eccentric ring in one of two positions. When actuated in a first direction, the locking member locks the eccentric ring in a position corresponding to a maximum effective length of the connecting rod, i.e., a high compression ratio mode. When actuated in a second direction, the locking member locks the eccentric ring in a position corresponding to a minimum effective length of the connecting rod, i.e., a high compression ratio mode. 
     U.S. Pat. No. 5,417,185 and Japanese Publication JP-03092552 also disclose eccentric rings, each of the rings however being disposed between the “small” end of a connecting rod and a corresponding piston pin. The rings are used to vary piston height and thus compression ratio. 
     The eccentric ring devices of the above cited references however are undesirable in that each eccentric ring must be rotated at least 180 degrees before one of the desired operating modes or positions is engaged. Locking of the ring in a proper position may not occur within an optimum period of time, for example during a single engine cycle, thus leaving the effective length of the rod and consequently the compression ratio of the cylinder in an undesired intermediate state. In an alternative embodiment, the &#39;750 device in fact requires a gear pump to assist rotation of the eccentric bushing. The &#39;068 device in addition requires the piston to be at BDC for a transition to occur from high to low compression mode. 
     Accordingly, the inventors herein have recognized the need to provide an improved system and method for quickly and reliably transitioning between different compression ratio modes of a variable compression ratio internal combustion engine. 
     SUMMARY OF THE INVENTION 
     The aforedescribed limitations and inadequacies of conventional internal combustion engines are substantially overcome by the present invention, in which a system is provided for selectively varying a compression ratio of an internal combustion engine, wherein the internal combustion engine includes a cylinder, a reciprocating piston disposed within the cylinder, a crank shaft having a crank pin, and a connecting rod coupled to the crank pin and the piston. In accordance with a preferred embodiment of the present invention, the system includes: at least one sensor, such as a speed or load sensor, for measuring an operating condition of the internal combustion engine, and a variable compression ratio apparatus for varying the effective length of the connecting rod. The compression ratio apparatus itself includes a bearing retainer disposed between the connecting rod and the crank pin, with the bearing retainer having an inner surface in communication with the crank pin and an outer surface in communication with the connecting rod, the connecting rod being axially movable relative to the bearing retainer along a longitudinal axis of the connecting rod to effect a selective displacement the connecting rod relative to the bearing retainer. The displacement thereby causes a change in the effective length of the connecting rod and thus the desired compression ratio of the internal combustion engine. 
     The system further includes an engine controller coupled to the internal combustion engine, the sensor and the variable compression ratio apparatus for generating, based on the measured operating condition of the internal combustion engine, a control signal required to displace the bearing retainer in accordance with the desired compression ratio of the internal combustion engine. Preferably, the system further includes at least one locking mechanism in cooperation with the bearing retainer and the connecting rod for maintaining the connecting rod at a selected position relative to the bearing retainer, the selected position corresponding to a selected compression of the internal combustion engine. 
     A principal advantage of the above-described system is that transitions between two or more compression ratio modes of an internal combustion engine can be accomplished quickly and reliably without requiring the rotation of an eccentric ring member as disclosed by the prior art. Transitions can be completed within a single cycle of the internal combustion engine by allowing the compression ratio apparatus to respond to the inertial forces acting on the connecting rod and piston. Transitions can be further assisted and the connecting rods “locked” into position using a suitable hydraulic or electromechanical system. In a preferred embodiment, the engine&#39;s oil system is used to actuate the mechanism to produce a selected compression ratio for the internal combustion engine. 
     In accordance with a related aspect of the present invention, a method is provided for varying a compression ratio of an internal combustion engine, the internal combustion engine having a cylinder, a reciprocating piston disposed within the cylinder, a crank shaft having a crank pin, a connecting rod coupled to the crank pin and the piston. The method includes the steps of: measuring at least one operating condition of the internal combustion engine; and axially moving the connecting rod relative to a bearing retainer along a longitudinal axis of the connecting rod, the bearing retainer being disposed between the connecting rod and the crank pin of the internal combustion engine, the displacement thereby causing a change in the effective length of the connecting rod and the compression ratio of the internal combustion engine. Advantageously, the method further includes the step of locking the connecting rod into place in order to maintain the connecting rod at a selected position relative to the bearing retainer, the selected position corresponding to a selected compression of the internal combustion engine. 
     In accordance with still a further aspect of the present invention, an article of manufacture is disclosed for varying a compression ratio of an internal combustion engine, the internal combustion engine having a cylinder, a reciprocating piston disposed within the cylinder, a crank shaft having a crank pin, and a connecting rod coupled to the crank pin and the piston. The article of manufacture includes: a computer usable medium; and a computer readable program code embodied in the computer usable medium for directing a computer to control the step of axially moving the connecting rod relative to a bearing retainer along a longitudinal axis of the connecting rod, the bearing retainer being disposed between the connecting rod and the crank pin of the internal combustion engine, the displacement thereby causing a change in the effective length of the connecting rod and the compression ratio of the internal combustion engine. 
     Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
     FIG. 1 is a diagram of an exemplary system for varying the compression ratio of an internal combustion engine; 
     FIGS. 2A and 2B are diagrams showing low compression ratio operation of an internal combustion engine having a variable compression ratio apparatus in accordance with a preferred embodiment of the present invention; 
     FIGS. 3A and 3B are diagrams showing high compression ratio operation of an internal combustion engine having a variable compression ratio apparatus in accordance with a preferred embodiment of the present invention; 
     FIGS. 4A and 4B are exploded and non-exploded perspective views, respectively, of a connecting rod and variable compression ratio apparatus in accordance with the present invention; 
     FIGS. 5A and 5B are exploded and non-exploded perspective views, respectively, of a connecting rod and variable compression ratio apparatus in accordance with another preferred embodiment of the present invention; 
     FIGS. 6A and 6B are diagrams showing the operation of an exemplary variable compression ratio apparatus in accordance with a preferred embodiment of the present invention; 
     FIG. 7 is a diagram showing the operation of an exemplary variable compression ratio apparatus having two locking mechanisms in accordance with a preferred embodiment of the present; 
     FIG. 8 is a diagram of an exemplary variable compression ratio apparatus having two opposing locking mechanisms and corresponding through-holes; 
     FIGS. 9A and 9B are diagrams of exemplary variable compression ratio apparatuses having two opposing locking mechanisms and corresponding channels; 
     FIG. 10 is a diagram of an exemplary variable compression apparatus having a single locking mechanism and a corresponding channel; 
     FIG. 11 is a plot showing an exemplary variable compression ratio operating strategy in accordance to a preferred embodiment of the present invention; 
     FIGS. 12 and 13 are plots of cylinder and oil pressure versus crank angle degrees during the motoring of an exemplary variable compression ratio internal combustion engine arranged and constructed in accordance with the present invention; and 
     FIGS. 14 and 15 are plots of cylinder and oil pressure versus crank angle degrees during the firing of an exemplary variable compression ratio internal combustion engine arranged and constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a diagram of a system for operating a variable compression ratio internal combustion engine in accordance with a preferred embodiment of the present invention. The engine  110  shown in FIG. 1, by way of example and not limitation, is a gasoline four-stroke direct fuel injection (DFI) internal combustion engine having a plurality of cylinders (only one shown), each of the cylinders having a combustion chamber  111  and corresponding fuel injector  113 , spark plug  115 , intake manifold  124 , exhaust manifold  132  and reciprocating piston  112 . The engine  110 , however, can be any internal combustion engine, such as a port fuel injection (PFI) or diesel engine, having one or more reciprocating pistons as shown in FIG.  1 . Each piston of the internal combustion engine is coupled to a connecting rod  114  on one end, and to a crank pin  117  of a crank shaft  116 . The connecting rod  114  is preferably a fixed-length connecting rod, but is not so limited. 
     The reciprocating piston  112  is further coupled to a compression ratio mechanism  170  that is operated by an electronic engine controller  160  to vary the compression ratio of the engine. “Compression ratio” is defined as the ratio of the volume in the cylinder  111  above the piston  112  when the piston is at bottom-dead-center (BDC) to the volume in the cylinder above the piston  112  when the piston  112  is at top-dead-center (TDC). Although the compression ratio mechanism  170  is described below as providing “high” and “low” compression ratios, the mechanism  170  can be modified as desired to provide one or more intermediate compression ratios for an internal combustion engine. Accordingly, operation of the mechanism  170  in “high” and “low” compression ratio modes is not intended to limit the scope of the claimed invention. 
     Referring again to FIG. 1, the compression ratio mechanism  170  is operated to effect a change in the engine&#39;s compression ratio in accordance with one or more parameters, such as engine load and speed, as shown by way of example in FIG.  11 . Such parameters are measured by appropriate sensors, such as a speed sensor  150 , mass air flow (MAF) sensor  130  and pedal position sensor  140 , which are electronically coupled to the engine controller  160 . The engine controller  160  includes a central processing unit (CPU)  1162  having corresponding input/output ports  169 , read-only memory (ROM)  164  or any suitable electronic storage medium containing processor-executable instructions and calibration values, random-access memory (RAM)  166 , and a data bus  168  of any suitable configuration. The controller  160  receives signals from a variety of sensors coupled to the engine  110  and/or the vehicle, and controls the operation of the fuel injector  115 , which is positioned to inject fuel into a corresponding cylinder  111  in precise quantities as determined by the controller  160 . The controller  160  similarly controls the operation of the spark plugs  113  in a known manner. 
     FIGS. 2A through 3B are diagrams illustrating the operation of an internal combustion engine having the variable compression ratio apparatus of FIGS. 2A of the present invention and  2 B show the piston  212  top-dead-center (TDC) and bottom-dead-center (BDC) positions, respectively, corresponding to a “baseline” or “unextended” position of a connecting rod  218 . The compression mechanism, as shown for example in the cut-away portions of FIGS. 2A an  2 B, includes a bearing retainer  220  disposed between the connecting rod  218  and a crank pin  222 , the crank pin having a center line axis  224  extending in and out of the page and parallel to the axis of rotation  228  of a corresponding crank shaft  226 . The bearing retainer  220  has a center line axis  230  normal to the crank pin center line axis  224 , and likewise the connecting rod  218  has a center line axis (shown as  232  in FIGS.  3 A and  3 B). When the connecting rod  218  is in the baseline position as shown in FIGS. 2A and 2B, which herein corresponds to a low compression ratio mode of the internal combustion engine, the bearing retainer center line axis is  230  is coincident or substantially coincident with the connecting rod center line axis  232 . When the connecting rod is in an extended, high compression ratio mode position as shown in FIGS. 3A and 3B, the bearing retainer center line axis  230  is displaced with respect to center line axis  232  of the connecting rod. 
     As such and further shown together FIGS. 4A through 5B, the bearing retainer  220  in accordance with the present invention includes an inner surface in communication with the crank pin  222  and an outer surface in communication with the connecting rod  218 . The connecting rod  218  is move able with respect to the outer surface of the bearing retainer in a linear fashion along a longitudinal axis  234  extending between the first and second ends of the connecting rod  218 . The connecting rod center line axis is thus selectively displaced with respect to the bearing retainer center line axis, thus causing a change in the effective length of the connecting rod and the compression ratio of the internal combustion engine. Therefore, as illustrated in FIGS. 2A through 3B, the effective length of the connecting rod l L  during low compression ratio operation is equal to the baseline, unextended length l B  of the connecting rod, and the effective length of the connecting rod l H  is equal to the extended length l B +x of the connecting rod during high compression ratio operation. 
     FIGS. 4A through 5B show exploded and non-exploded perspective views of preferred embodiments of a connecting rod and compression ratio apparatus in accordance with the present invention. The preferred embodiments are provided by way of example, and are not intended to limit the scope of the invention claimed herein. Further detailed embodiments of the connecting rod and compression ratio apparatus can be found in co-pending U.S. application Ser. Nos. 09/691,668 (Attorney Docket No. 199-0483), 09/690,946 (Attorney Docket No. 200-1349), 09/691,669 (Attorney Docket No. 200-1353), 09/690,950 (Attorney Docket No. 200-1438), 09/691,306 (Attorney Docket No. 200-1439), 09/691,667 (Attorney Docket No. 200-1440) and 09/690,951 (Attorney Docket No. 200-1441), all of which are hereby incorporated by reference in their entireties. 
     Referring to FIGS. 4A and 4B, exploded and non-exploded perspective views are provided, respectively, of a connecting rod and variable compression ratio apparatus in accordance with the present invention. The connecting rod  400  includes a first or so-called “large” end  412  for journaling on a crank pin  415  of a crank shaft and a second so-called “small” end  416  for journaling on a central portion of a wrist pin (not shown) and for coupling the connecting rod  400  to a piston (not shown). A compression ratio apparatus  418  is embodied in the connecting rod at its large end for varying the effective length of the connecting rod as measured between the large and small ends  412  and  416 . 
     In accordance with the present embodiment of FIGS. 4A and 4B, the large end  412  further includes an upper cap  420  and a lower cap  422  that are fastened together around the crank pin  415 . Lower cap  22  includes parallel through-holes  426  and  428  at opposite ends of its semi-circumference. At opposite ends of its semi-circumference, upper cap  420  includes through-holes  430  and  432  that align with holes  426  and  427 , respectively, when the two caps  420  and  430  are in communication with the crank pin. 
     Connecting rod  412  further includes a part  434  containing a connecting rod portion  435 . One end of part  434  includes the small end  416 , and the opposite end is coupled through the compression ratio mechanism  418  with large end  412 . The coupling of the compression ratio mechanism and the large end  412  is preferably implemented using through-holes  436  and  438  that align with through-holes  430  and  432 , respectively, fasteners  440  and  442 , and nuts  441  and  443 . Through-holes  436  and  438  are disposed mutually parallel, and are disposed in free ends of curved arms  445  that extend from connecting rod portion  435 . 
     Each fastener  440  and  442  includes a head  444  disposed at a proximal end and a screw thread  446  disposed at a distal end. Intermediate proximal and distal ends, each fastener includes a circular cylindrical guide surface  448 . The parts are assembled in the manner indicated by FIG. 4A with the respective fastener shanks passing though respective aligned through-holes  436  and  430 ,  438  and  432 , and  426  and  428 ; and threading into respective nuts  441  and  443 . The diameters of through-holes  436  and  438  are larger than those of through-holes  430  and  432  to allow shoulders  450  at the ends of guides  448  to bear against the margins of through-holes  430  and  432 . As the fasteners and nuts are tightened, such as by turning with a suitable tightening tool, the two caps  420  and  422  are thereby forced together at their ends, crushing the crank pin bearing in the process and thereby forming a bearing retainer structure around the crank pin. 
     The axial length of each guide surface  448 , as measured between head  444  and shoulder  450 , is slightly greater than the axial length of each through-hole  436  and  438 , and the diameters of the latter are slightly larger than those of the former to provide sliding clearance. In this way, it becomes possible for the rod part  434  to slide axially, i.e., the outer surface of the combined  420 / 430  assembly is axially movable relative to the connecting rod, over a short range of motion relative to the large end  412  along a longitudinal axis  234  extending between the large and small ends of the connecting rod. The range of motion is indicated in FIG. 4B by the displacement x of a connecting rod center line  232  with respect to a center line  230  of the assembled caps  420  and  430 . The displacement x of the two center line axes thus translates into a change x in length of the connecting rod assembly  400 . When arms  445  abut part  420  around the margins of through-holes  30  and  32 , the connecting rod assembly  400  has a minimum or “baseline” length corresponding to a low compression ratio mode of operation for the internal combustion engine. When arms  445  abut heads  444 , the connecting rod assembly  400  has a maximum or extended length corresponding to a high compression ratio operation of the internal combustion engine. 
     As further shown in FIGS. 4A and 4B, channels  454  may be assembled at the sides of the connecting rod assembly  400  to provide additional bearing support for the axial sliding motion of the connecting rod. Mechanism  418  may include passive and/or active elements for accomplishing overall length change, and resulting compression ratio change. 
     FIGS. 5A and 5B are exploded and non-exploded perspective views, respectively, of another preferred embodiment of a connecting rod and compression ratio mechanism in accordance with the present invention. As shown in FIGS. 5A and 5B, a connecting rod  500  comprises a large end  564  for journaling on a crank pin  415  of a crank shaft (not shown) and a small end  566  for journaling on a central portion of a wrist pin (not shown) for coupling the connecting rod  500  to a piston (not shown). The compression ratio mechanism  568  is embodied in this case entirely within the large end  564  of the connecting rod  500  to provide for variation in the overall length between the large and small ends of the connecting rod. 
     Mechanism  568  in accordance with the present invention is provided by a single-piece bearing retainer  570  which is captured between a cap  572  and one end of a rod part  574 . Opposite ends of the semi-circumference of cap  572  contain holes  576  and  578  that align with threaded holes  580  and  582  in rod part  574 . Fasteners  584  and  586  fasten the cap to the rod part. The cap and rod part have channels  588  and  590  that fit to respective portions of a flange  592  of bearing retainer  570 . The channel and flange depths are chosen to allow the assembled cap and rod part to move axially a short distance on the bearing retainer, thereby changing the overall length, as marked by x in FIG.  5 B. Mechanism  568  may comprise passive and/or active elements for accomplishing overall length change and corresponding compression ratio change. The channels form the groove, and the flange the tongue, of a tongue-and groove type joint providing for sliding motion that adjusts the length of the connecting rod assembly. 
     FIGS. 6A and 6B are schematic diagrams showing the operation of an exemplary compression ratio mechanism  600  in accordance with a preferred embodiment of the present invention. In FIGS. 6A and 6B, the compression ratio mechanism  600  includes a unitary bearing retainer  602  having post portions  621  and  622  disposed on opposite ends of the main bearing retainer along the longitudinal axis  234  of the connecting rod. Note, only a cut-out, inner profile  606  of the connecting rod is shown in FIGS. 6A and 6B. When the compression ratio mechanism of the present invention is assembled within the inner profile of the connecting rod, the mechanism is actuated from a low compression ratio position as shown in FIG. 6A to a high compression ratio position as shown in FIG.  6 B and vice-versa by actuating the bearing retainer via a hydraulic or electromechanical system coupled to and/or within the connecting rod. A hydraulic system having openings  612  and conduits  614  are provided for enabling the flow of oil or other suitable fluid to and from each of the post regions so as to move the entire bearing retainer from one position to another. A check valve  616  is also provided for controlling the flow of oil used to position the connecting rod relative to the bearing retainer. 
     In order for the connecting rod to move from an extended state to the baseline state, the rod must be in compression, e.g., during the combustion stroke of a four-stroke internal combustion engine, and the check valve  620  must be positioned so as to allow the flow of oil into the lower reservoir  632  formed between the inside of the connecting rod and the bearing retainer. The check valve allows oil to move from the upper reservoir  634  to the lower reservoir  632 . In this manner, the connecting rod is locked in the baseline position until the check valve is moved. 
     In order for the VCR to move back to the extended position, the rod must be in tension, e.g., during the intake stroke of a four-stroke internal combustion engine, and the check valve  620  must be positioned so as to allow the flow of oil from the lower reservoir  632  to the upper reservoir  634 . In this manner, the connecting rod remains locked in the extended, high compression ratio position. 
     In the present embodiment, a positive oil pressure combined with inertial forces on the connecting rod are used to extend or retract the connecting rod as required to yield the desired compression ratio. Further, the positive oil pressure is used to maintain or “lock” the connecting rod in the desired position. FIGS. 7 through 10 discussed below show alternative embodiments of the compression ratio mechanism having one or more hydraulically or electromechanically actuated locking mechanisms for maintaining the effective length of the connecting rod as required. 
     FIG. 7 is a diagram showing the operation of an exemplary compression ratio apparatus having two locking mechanisms  722  and  732  in accordance with a preferred embodiment of the present. The mechanism further includes a bearing retainer having a main body portion  702  in contact with a corresponding crank pin, an upper post portion  708 , a lower post portion  710  and oil conduits  704  and  706  for providing passageways for a high pressure oil line  740  and a low pressure oil line  750 . The elements or portions thereof shown within boxes  720  and  730  are preferably housed within the large end of the connecting rod adjacent to the corresponding post portions  708  and  710  of the bearing retainer. 
     The locking mechanisms shown in FIG. 7 are held in their current positions using the low “lubrication” oil pressure line  750  and transitioned to the next position using the high pressure oil line  740 . The high pressure line  740 , which is represented in FIG. 7 as a solid line, is used for transitioning the connecting rod to the next position. This is accomplished using high-pressures pulses on the line  740  that cause the elements of the locking mechanisms  722  and  732  either to compress or move part so as to allow compression or tension forces on the connecting rod to transition the rod to a high compression ratio mode position or low compression ratio mode position. The low oil pressure line  750 , in contrast, is used to maintain the locking pins  722  and  732  in their positions after corresponding high-pressure pulses have been provided to displace the center line axis of the connecting rod. Preferably, a single high-pressure pulse on high pressure line  740  causes the lock pin already in the “locked” position, for example mechanism  722  shown in FIG. 7, to expand and thus unlock while at time causing the opposing lock mechanism  732  to compress and remain in a locked position after the connecting rod shifts in the direction away from the piston. As shown in FIG. 7, the operation of the compression ratio apparatus thus corresponds to a transition from high compression ratio mode to low compression ratio mode. 
     Note, as with all of the preferred embodiments of the present invention, is understood that the compression ratio apparatus of the present invention can be adapted accordingly to transition between more than two compression ratio states. For example, the compression ratio apparatus can be designed accordingly to transition between three or more compression ratio states, i.e., high, medium and low compression ratio states. 
     FIGS. 8 through 10 show alternative embodiments of the locking mechanisms for the compression ratio apparatus of the present invention. FIG. 8 is a diagram of an exemplary variable compression apparatus having two opposing locking mechanisms  824  and  826  and corresponding through-holes  814  and  816  formed through post portions  804  and  806 . Lock mechanism  814 , shown in FIG. 8 as a shaded region, is shown to be in a locked position. Preferably, both mechanisms are cylindrically shaped pins suitably designed to withstand the inertial forces exerted via the connecting rod during operation of the engine. 
     FIG. 9A shows a similar embodiment as shown in FIG. 8 except that locking mechanisms  924  and  926  are arranged and constructed to cooperate with corresponding channels  914  and  916  formed on the upper and lower sides of the post portions  904  and  906 , respectively. An additional embodiment is also shown in FIG. 9B, except that the locking mechanisms are flattened cylindrical pins  974  and  976  having correspondingly shaped channels  964  and  966  formed on post portions  954  and  956 . FIG. 10 shows an embodiment similar to the embodiment of FIG. 9B except that only one post  1004  and corresponding locking mechanism/channel  1024 / 1014  are provided. 
     FIG. 11 is a plot showing an exemplary compression ratio map  1100  for use with the various compression ratio apparatuses described above. The map  100  shows the operating strategy for a variable compression ratio internal combustion engine, and is implemented in accordance with a preferred embodiment of the present invention by the electronic engine controller of FIG.  1 . The mapping, which is embodied in computer readable program code and corresponding memory means, is used to operate an internal combustion engine in accordance with high and low compression ratio modes  1102  and  1104 , respectively, depending on the detected operating speed and load of the internal combustion engine. The mapping determines when the compression modes are to be switched, and is provided solely by way of example and is intended to limit the scope of the present invention in any way. 
     FIGS. 12 through 15 are plots of cylinder and oil pressure versus crank angle degrees for a three-cylinder, four-stroke variable compression ratio gasoline internal combustion engine. FIGS. 12 and 13 correspond to low-to-high and high-to-low compression mode transitions, respectively, and show plots of cylinder and oil pressure during motoring. FIGS. 14 and 15 also correspond to low-to-high and high to-low compression mode transitions, respectively, and show plots of cylinder and oil pressure during firing. All of FIGS. 12 through 15 show pressure plots  1201 - 1203 ,  1301 - 1303 ,  1401 - 1403  and  1501 - 1503  for each of the cylinders (plots also labeled “1”, “2” and “3”) and “galley” oil pressure plots  1204 ,  1304 ,  1404  and  1504 . Operating conditions include a nominal engine speed of 1500 rpm (1500 rpm, 2.62 bar brake mean effective pressure (BMEP) for firing cylinders) with an oil temperature of approximately 120 degrees F and an engine coolant temperature of approximately 150 degrees F. 
     The plots  1200  through  1500  shown in FIGS. 12 through 15 correspond to an engine having compression ratio apparatuses requiring a relatively high oil pressure, nominally greater than 100 psi, for maintaining the connecting rods in a low compression ratio operating mode, and a relatively low oil pressure, nominally less than 100 psi, for maintaining the connecting rods in a high compression ratio operating mode. The actual values of the oil pressure levels and relation to compression ratio modes however is not intended to limit the scope of the present invention. As indicated by the plots, once the galley oil pressure reaches a threshold level, the connecting rods transition within a single engine cycle to the commanded position. The transitions in FIGS. 12 and 14 result in high compression mode operation, and the transitions in FIGS. 13 and 15 result in low compression mode operation. 
     Accordingly, a system for varying the compression ratio of an internal combustion engine has been described having a bearing retainer in cooperation with a connecting rod wherein the center line axis of the connecting rod is displaced quickly and reliably with respect to the center line axis of the bearing retainer to effect a change in the length of the connecting rod, thereby selectively causing a change in the compression ratio of the internal combustion engine. The transition from one compression ratio mode to another is accomplished in a linear fashion without requiring the rotation of an eccentric ring member as shown by the prior art. 
     Although the present invention has been described in connection with particular embodiments thereof, it is to be understood that various modifications, alterations and adaptations may be made by those skilled in the art without departing from the spirit and scope of the invention. It is intended that the invention be limited only by the appended claims.