Patent Publication Number: US-9845738-B2

Title: Variable compression ratio piston system

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention pertains to the field of variable compression ratio systems. More particularly, the invention pertains to a variable compression ratio piston system for an engine. 
     Description of Related Art 
     Variable compression ratio (VCR) systems are known in the art. A compression ratio, as used herein, is the ratio of the volume of the cylinder chamber, or combustion chamber in the case of an engine, at its largest capacity to the volume at its smallest capacity. VCR systems for internal combustion engines are intended to be able to change the compression ratios of the pistons in their respective engine cylinders on the fly. This allows for increased fuel efficiency by varying the compression ratios in response to varying loads on the engine during operation. While VCR engine research goes back several decades and many automobile manufacturers are currently working on VCR engine designs, no current commercially-available automobiles have a VCR engine. The mechanical complexity and difficulty in controlling the system parameters to provide the desired improvement have thus far prevented commercialization of this technology in automobiles. 
     U.S. Pat. App. Pub. No. 2010/0163003, entitled “Electrohydraulic Device for Closed-Loop Driving the Control Jack of a Variable Compression Ratio Engine” by Rabhi and published Jul. 1, 2010, discloses an electrohydraulic device for controlling the compression ratio of a variable compression-ratio engine. In a first embodiment, two electrovalves are provided per control jack at an inlet and an outlet, each electrovalve being furnished with a check valve. In a second embodiment, a single electrovalve is provided and includes an electrically-controlled spool with two inlets and two outlets. In a third embodiment, a single two-way electrovalve is provided. The electrovalve is capable of opening and closing sufficiently rapidly to allow the movement of the control rack only for a few degrees of angular movement of the crankshaft. It should be noted that one of the positions seems to allow recirculation between the upper chamber and the lower chamber of the control jack. 
     U.S. Pat. App. Pub. No 2009/0320803, entitled “Control Method for a Variable Compression Actuator System” by Simpson and published Dec. 31, 2009, discloses a control system for an adjustment device for a variable compression ratio engine comprising: a jack head, a jack piston, a sprocket wheel, a movable transmission member, and a control valve. The jack piston is received within a chamber of the jack head defining first and second fluid chambers. The control valve controls the flow of fluid between the first and second fluid chambers. Based on the position of the control valve, fluid flows from the first fluid chamber to the second fluid chamber or vice versa, moving the control rack connecting the jack piston to the sprocket wheel. Reciprocating motion of the sprocket wheel adjusts the position of the cylinder of the engine. 
     The above-mentioned references are hereby incorporated by reference herein. 
     FEV, Inc. (Auburn Hills, Mich.) manufactures a two-step variable compression ratio (VCR) system. The FEV-developed 2-step VCR mechanism induces small variations in the rod length that are achieved by using the gas and mass forces for actuation. A compression ratio that is variable in two steps from 14:1-17:1 in the case of a commercial diesel version is thereby achieved. This ensures rapid and accurate actuation without the use of an expensive power actuator. Versions of the system are available for both gasoline and diesel engines and can be applied to almost all existing engines with bore diameters as low as 70 mm. In addition to increased engine efficiency, the system also offers emissions-related benefits, depending on whether applied to gasoline or diesel engines. Other potential benefits include improved cold startability and the potential to optimize performance while utilizing alternative fuels. The system can be integrated into existing engine designs due to a carry-over piston and pin design. 
     SUMMARY OF THE INVENTION 
     The variable compression ratio piston system for an engine adjusts the compression ratio of the engine piston by way of hydraulic fluid distributed between a pair of chambers formed in a pair of bores receiving control pistons mechanically coupled to the engine piston. A control valve selectively permits flow of hydraulic fluid between the high compression ratio line and the low compression ratio line. A variable force solenoid controlled by an engine control unit preferably controls the position of the control valve. The position of the control valve controls whether hydraulic fluid can flow toward the first chamber, toward the second chamber, or not at all. Flow of hydraulic fluid is actuated by alternating forces from inertial and combustion forces on a crankshaft from operation of the engine. 
     A variable compression ratio piston system includes at least one engine piston assembly. Each engine piston assembly includes an engine piston, a first control piston, a second control piston, a high compression ratio line, and a low compression ratio line. The variable compression ratio piston system also includes a control system. The engine piston is slidingly received in an engine cylinder of an engine. The first control piston is mechanically coupled to the engine piston and actuates in a first control piston bore. The first control piston and the first control piston bore define a first chamber. The second control piston is mechanically coupled to the engine piston and actuates in a second control piston bore. The second control piston and the second control piston bore define a second chamber. The low compression ratio line supplies hydraulic fluid to the first chamber and drains hydraulic fluid from the first chamber. The high compression ratio line supplies hydraulic fluid to the second chamber and drains hydraulic fluid from the second chamber. The control system includes at least one control valve and selectively permits flow of hydraulic fluid between the low compression ratio line and the high compression ratio line. 
     When the control valve is in a first position, a first net flow of hydraulic fluid from the second chamber to the first chamber by way of the high compression line, the control valve, and the low compression line is permitted such that the first net flow raises the first control piston in the first control piston bore and lowers the second control piston in the second control bore to lower the engine piston, thereby decreasing a compression ratio of the engine piston toward a low compression ratio state. When the control valve is in a second position, a second net flow of hydraulic fluid from the first chamber to the second chamber by way of the low compression line, the control valve, and the high compression line is permitted such that the second net flow raises the second control piston in the second control piston bore and lowers the first control piston in the first control bore to raise the engine piston, thereby increasing the compression ratio of the engine piston toward a high compression ratio state. 
     A method of varying a compression ratio of at least one engine piston received in an engine cylinder of an engine includes measuring a load on the engine, calculating a compression ratio state for the at least one engine piston based on the load on the engine, adjusting the control valve to permit the variable compression ratio piston system to move toward the compression ratio state, and adjusting the control valve to a third position when the variable compression ratio piston system reaches the compression ratio state. The variable compression ratio piston system further includes a first control piston mechanically coupled to the engine piston and actuates in a first control piston bore. The first control piston and the first control piston bore define a first chamber. A second control piston mechanically coupled to the engine piston actuates in a second control piston bore. The second control piston and the second control piston bore define a second chamber. A low compression ratio line supplies hydraulic fluid to the first chamber and drains hydraulic fluid from the first chamber, and a high compression ratio line supplies hydraulic fluid to the second chamber and drains hydraulic fluid from the second chamber. A control system includes the control valve and selectively permits flow of hydraulic fluid between the low compression ratio line and the high compression ratio line. When the control valve is in a third position, the control system prevent flow of hydraulic fluid between the first chamber and the second chamber by way of the low compression line, the control valve, and the high compression line, thereby maintaining the compression ratio of the engine piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    shows a schematic of a two-position compression ratio system of a first embodiment with a control system in a first position. 
         FIG. 1 b    shows a schematic of the system of  FIG. 1 a    with the control system in a second position. 
         FIG. 2  shows a schematic of a two-position compression ratio system of a second embodiment with a control system in a first position and with bias springs. 
         FIG. 3  shows a schematic of a variable compression ratio system in a first embodiment. 
         FIG. 4 a    shows a schematic of a variable compression ratio piston with a control system in a first position. 
         FIG. 4 b    shows a schematic of the piston of  FIG. 4 a    with the control system in a second position. 
         FIG. 4 c    shows a schematic of the piston of  FIG. 4 a    with the control system in a third position. 
         FIG. 5 a    shows a schematic of a piston in an intermediate compression ratio state. 
         FIG. 5 b    shows a schematic of the piston of  FIG. 5 a    in a low compression ratio state. 
         FIG. 5 c    shows a schematic of the piston of  FIG. 5 a    in a high compression ratio state. 
         FIG. 6  shows a schematic of the variable compression ratio piston of  FIG. 4 a    with a regulated pressure control system (RPCS). 
         FIG. 7  shows a schematic of the variable compression ratio piston of  FIG. 4 a    with a differential pressure control system (DPCS). 
         FIG. 8  shows a schematic of the variable compression ratio piston of  FIG. 4 a    with a check valve in spool as part of the control system. 
         FIG. 9  shows an exploded view of the check valve in spool of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hydraulic systems allow the compression ratio of an internal combustion engine to be varied. More specifically, a spool valve is hydraulically coupled to control piston chambers and fluid is exhausted or supplied through recirculation to these chambers as needed to alter the compression ratio. The systems use a mechanical mechanism to capture alternating forces on a connecting rod to move a piston. The alternating forces are a result of inertial and combustion forces on the crankshaft. An eccentric bearing/pivot at the top of the piston is connected to a mechanical linkage that allows the piston to move up or down. The rods from the linkage extend from the top of the piston to the bottom on both sides of the connecting rod. A control piston at the bottom of each rod rides inside a bore in the connecting rod body. Oil is supplied to the hydraulic passages at the bottom of the control piston bores by the control valve and check valves. 
     The hydraulics operate similarly to a cam torque actuated (CTA) phaser used to adjust the relative angular position of a camshaft to a crankshaft or another camshaft; the energy from the alternating forces is used to actuate the piston/linkage up and down, thereby changing the overall piston height. The alternating forces for this particular system come from inertial and combustion forces on the crankshaft. The oil in the system is controllably re-circulated back and forth between the two control pistons through the use of check valves and a control valve. Because the system is able to recirculate the oil between the control piston chambers, the oil consumption of the system is reduced compared to a conventional variable compression ratio system using oil pressure to raise or lower the control pistons, which in turn raise or lower the piston changing the compression ratio. In order to move the control pistons in the conventional system, the oil in one of the control piston chambers, depending on the direction of change, needs to be exhausted to the crank case/reservoir, while oil from crank case/reservoir is being pumped into the opposite chamber. 
     An actuator controls the position of the control valve. The actuator may be a variable force solenoid (VFS), a differential pressure control system (DPCS), regulated pressure control system (RPCS), a stepper motor, an air actuator, a vacuum actuator, a hydraulic actuator, or any other type of actuator that has force or position control. In some embodiments, the VFS is positioned in front of the control valve and moves the valve as current is applied to the VFS. In some embodiments, the control valve is a spool valve. In some embodiments, the control valve is a check valve in spool. On the opposite side of the spool is a spring, which constantly provides a counter force to the VFS and pushes the spool to a base position, when the current to the VFS is reduced to be lower than the spring force. The position of the control valve determines the position of the piston (i.e., low or high compression). Several different configurations may be used within the spirit of the present invention. In some embodiments, a DPCS uses differential oil pressure on opposite ends of the spool to control the control valve position, while a pair of opposing springs biases the spool and the piston toward each other. In other embodiments, an RPCS uses oil pressure on one end opposed by a spring on the other end to control the control valve position. 
     In a two-position system, one position produces a high compression ratio state and a second position produces a low compression ratio state. Alternatively, the positions may be flipped such that position one is low compression and position two is high compression, depending on strategy. In some two-position systems, there is one control valve, one control valve spring, two high pressure check valves, one supply check valve, and one VFS. A mechanical linkage system connects every piston. In position one, the default position, the control valve is fully extended outward with a minimum load on the control valve spring, and the VFS is fully retracted. Depending on the original equipment manufacturer (OEM) strategy, this is either the high or low compression state. Once current is applied to the VFS, the VFS pushes the control valve in to the second position, thereby changing the flow path in the hydraulic circuit, which causes the piston to move into the opposite position. In some embodiments, the two-position system includes bias springs. In some embodiments, the bias springs bias the system toward a low compression ratio state when the system is under low torsional energy. In other embodiments, the bias springs bias the system toward a high compression ratio state when the system is under low torsional energy. 
     In a variable position system, each piston on the engine has its own control system, including a control valve, a control valve spring, two high-pressure check valves, a supply check valve, a VFS, a mechanical linkage system, and a combustion sensor. With each piston having its own control system the compression ratio may be varied to any value within the mechanical range of the linkage. In order to accurately predict the movement of the mechanism, a combustion sensor is used in each cylinder to keep it properly controlled. The sensor allows each individual piston in the system to be set to a specific compression value, thereby helping to compensate for stack-ups or manufacturing defects that might result in a cylinder-to-cylinder structural difference. 
     In some embodiments, the variable position system includes a biasing spring added between the control piston and control piston bore to push the linkage to a default or start-up position or to balance the mean torque of the system. 
       FIG. 1 a    shows a four-cylinder, two-position variable compression ratio system  10  with a control system in a first position. Each piston includes an engine piston  11 ,  21 ,  31 ,  41  rotatably connected to an eccentric bearing  12 ,  22 ,  32 ,  42 , and a connecting rod  13 ,  23 ,  33 ,  43 , a first linking rod  14 ,  24 ,  34 ,  44  coupling the engine piston to a first control piston  15 ,  25 ,  35 ,  45 , which is slidingly received in a first control piston bore  16 ,  26 ,  36 ,  46 , and a second linking rod  17 ,  27 ,  37 ,  47  coupling the engine piston to a second control piston  18 ,  28 ,  38 ,  48 , which is slidingly received in a second control piston bore  19 ,  29 ,  39 ,  49 . The engine pistons actuate in engine cylinders (not shown). 
     The compression ratios of the engine pistons  11 ,  21 ,  31 ,  41  are simultaneously controlled by a single control system. An actuator  51 , in combination with a control valve spring  52 , controls the position of a spool  54  in a control valve bore of the control valve  53 . A vent  53 ′ through the control valve body to the atmosphere minimizes air pressure fluctuations in the back end of the spool valve bore when the spool  54  moves back and forth in the spool valve bore. An engine control unit (ECU)  8  controls the actuator  51 . When the actuator  51  is a variable force solenoid, an engine control unit (ECU)  8  energizes the variable force solenoid  51  to control the position of the spool  54  within the control valve  53 . The spool  54  of the control valve  53  is shown in a first position in  FIG. 1 a   . With the control valve  53  in the first position, the spool  54  connects the high compression ratio lines  57  to the central line  9  while a first land of the spool  54  blocks the low compression ratio lines  58  from the central line  9 . The first high-pressure check valve  55  permits flow of hydraulic fluid from the central line  9  to the low compression ratio lines  58  as indicated by the arrows, while the second high-pressure check valve  56  and the spool  54  prevent flow of hydraulic fluid to the high compression ratio lines  57  and from the low compression ratio lines  58 , respectively. This circuit achieves the net effect of decreasing the amount of hydraulic fluid in the chambers formed by the second control pistons  18 ,  28 ,  38 ,  48  and increasing the amount of hydraulic fluid in the chambers formed by the first control pistons  15 ,  25 ,  35 ,  45 , thereby moving the control pistons and the engine pistons  11 ,  21 ,  31 ,  41  toward a low compression ratio position. A supply check valve  59  in a supply line  60  permits flow of hydraulic fluid into the system and prevents flow of the hydraulic fluid back to a hydraulic fluid source to maintain hydraulic pressure in the system. 
       FIG. 1 b    shows the four-cylinder, two-position compression ratio system  10  of  FIG. 1 a    with the control system in a second position. The spool  54  of the control valve  53  is shown in a first position in  FIG. 1 a   . With the control valve  53  in the second position, the spool  54  connects the low compression ratio lines  58  to the central line  9  while a second land of the spool  54  blocks the high compression ratio lines  57  from the central line  9 . The second high-pressure check valve  56  permits flow of hydraulic fluid from the central line  9  to the high compression ratio lines  57  as indicated by the arrows, while the first high-pressure check valve  55  and the spool  54  prevent flow of hydraulic fluid to the low compression ratio lines  58  and from the high compression ratio lines  57 , respectively. This circuit achieves the net effect of decreasing the amount of hydraulic fluid in the chambers formed by the first control pistons  15 ,  25 ,  35 ,  45  and increasing the amount of hydraulic fluid in the chambers formed by the second control pistons  18 ,  28 ,  38 ,  48 , thereby moving the control pistons and the engine pistons  11 ,  21 ,  31 ,  41  toward a high compression ratio position. This is also the default position of the spool  54  when the VFS  51  is not energized. 
       FIG. 2  shows a four-cylinder, two-position variable compression ratio system  110  with a control system in a first position. The system of  FIG. 2  operates similarly to the system of  FIG. 1 a    and  FIG. 1 b   , except that in this system, the second control piston  18 ,  28 ,  38 ,  48  is biased upward by a control piston bias spring  20 ,  30 ,  40 ,  50 . The control piston bias springs  20 ,  30 ,  40 ,  50  on the second control pistons  18 ,  28 ,  38 ,  48  bias the engine pistons  11 ,  21 ,  31 ,  41  toward a high compression ratio state. 
       FIG. 3  shows a four-cylinder variable compression ratio system  210  with a separate control system for each of the four pistons  11 ,  21 ,  31 ,  41 . As in the system of  FIG. 1 a    and  FIG. 1 b   , each piston includes an engine piston  11 ,  21 ,  31 ,  41  rotatably connected to an eccentric bearing  12 ,  22 ,  32 ,  42 , and a connecting rod  13 ,  23 ,  33 ,  43 , a first linking rod  14 ,  24 ,  34 ,  44  coupling the engine piston to a first control piston  15 ,  25 ,  35 ,  45 , which is slidingly received in a first control piston bore  16 ,  26 ,  36 ,  46 , and a second linking rod  17 ,  27 ,  37 ,  47  coupling the engine piston to a second control piston  18 ,  28 ,  38 ,  48 , which is slidingly received in a second control piston bore  19 ,  29 ,  39 ,  49 . 
     The compression ratios of the engine pistons  11 ,  21 ,  31 ,  41  are independently controlled by separate control systems. For each piston, an actuator  61 ,  71 ,  81 ,  91  in combination with a control valve spring  62 ,  72 ,  82 ,  92 , controls the position of the control valve  63 ,  73 ,  83 ,  93 . A vent  63 ′,  73 ′,  83 ′,  93 ′ through each control valve body to the atmosphere minimizes air pressure fluctuations in the back end of the spool valve bore when the spool  64 ,  74 ,  84 ,  94 , respectively, moves back and forth in the spool valve bore. A single engine control unit preferably controls all of the actuators  61 ,  71 ,  81 ,  91 , although a separate engine control unit for each actuator may be used within the spirit of the present invention. The spool  74  of the control valve for the second engine piston  21  is shown in a first position. The spools  64 ,  94  for the control valves for the first engine piston  11  and the fourth engine piston  41 , respectively, are shown in a second position. The spool  84  of the control valve for the third engine piston  31  is shown in a third position. 
     With the control valve  73  in the first position, the high-pressure check valves  75 ,  76  permit flow of hydraulic fluid in the direction indicated by the arrows from the chamber formed by the second control piston  28  to the chambers formed by the first control piston  25  by way of the high compression ratio lines  77  and the low compression ratio lines  78  toward a low compression position. With the control valves  64 ,  94  in the second position, the high-pressure check valves  65 ,  66 ,  95 ,  96  permit flow of hydraulic fluid in the direction indicated by the arrows from the chamber formed by the first control pistons  15 ,  45  to the chambers formed by the second control pistons  18 ,  48  by way of the high compression ratio lines  67 ,  97  and the low compression ratio lines  68 ,  98  toward a high compression position. With the control valve  84  in the third position, the control valve  84  and the high-pressure check valves  85 ,  86  prevent flow of hydraulic fluid between the chamber formed by the first control piston  35  and the chambers formed by the second control piston  38  by way of the high compression ratio line  87  and the low compression ratio line  88  to maintain the current compression position. Supply check valves  69 ,  79 ,  89 ,  99  in a supply line  100  permits flow of hydraulic fluid into the system and prevents flow of the hydraulic fluid back to the hydraulic fluid source to maintain hydraulic pressure in the system. In this system, each control system has its own individual supply check valve  69 ,  79 ,  89 ,  99 , but alternatively, a single supply check valve could be used upstream for all four control systems. 
     Although not shown with respect to the systems of  FIG. 1 a   ,  FIG. 1 b   , and  FIG. 2 , the control valves  54  may be held in a third position similar to the control valve  84  in  FIG. 3  such that the control valve  54  and the high-pressure check valves  55 ,  56  prevent flow of hydraulic fluid between the chamber formed by the first control piston  15 ,  25 ,  35 ,  45  and the chambers formed by the second control piston  18 ,  28 ,  38 ,  48  by way of the high compression ratio line  57  and the low compression ratio line  58  to maintain the current compression position. 
       FIG. 4 a   ,  FIG. 4 b   , and  FIG. 4 c    show a single piston system  310  controlled by an individual control system in a first position, a second position, and a third position, respectively. In these systems, the second control piston  18  is biased upward by a control piston bias spring  20 . The control piston bias spring  20  on the second control piston  18  biases the engine piston  11  toward a high compression ratio state. 
       FIG. 5 a   ,  FIG. 5 b   , and  FIG. 5 c    show a single piston system  410  in an intermediate compression ratio state, a low compression ratio state, and a high compression ratio state, respectively, with the individual control system in a third position to prevent flow of hydraulic fluid between the chamber formed by the first control piston  15  and the chamber formed by the second control piston  18  by way of the high compression ratio line  67  and the low compression ratio line  68 . The actuator is not shown in  FIG. 5 a   ,  FIG. 5 b   , and  FIG. 5 c    for clarity only. In  FIG. 5 a    both control pistons  15 ,  18  are at intermediate positions in their respective control piston bores  16 ,  19 . This positions the engine piston  11  at an intermediate height at top dead center for an intermediate compression ratio state in its cylinder (not shown). In  FIG. 5 b   , the first control piston  15  is at a top position in its control piston bore  16 , and the second control piston  18  is at a bottom position in its control piston bore  19 . This positions the engine piston  11  at a minimum height at top dead center for a low compression ratio state in its cylinder (not shown). In  FIG. 5 c   , the first control piston  15  is at a bottom position in its control piston bore  16 , and the second control piston  18  is at a top position in its control piston bore  19 . This positions the engine piston  11  at a maximum height at top dead center for a high compression ratio state in its cylinder (not shown). In this system, the first control piston  15  is biased upward by a control piston bias spring  20 . The control piston bias spring  20  on the first control piston  15  bias the engine piston  11  toward a high compression ratio state. 
     Although the systems of  FIG. 1 a   ,  FIG. 1 b   ,  FIG. 2 , and  FIG. 3  are shown as four-cylinder/four-piston systems and the systems of  FIG. 4 a   ,  FIG. 4 b   ,  FIG. 4 c   ,  FIG. 5 a   ,  FIG. 5 b   , and  FIG. 5 c    are shown as one-cylinder/one-piston systems, a variable compression ratio system of the present invention may have any number of cylinders/pistons within the spirit of the present invention. Any of the disclosed systems may have any number of cylinders/pistons, including, but not limited to, one, two, three, four, five, six, and eight. 
     Although the systems of  FIG. 1 a    through  FIG. 5 c    are described with a hydraulic control system with a two-land spool controlled by a variable force solenoid as the actuator and a check valve in each of the hydraulic lines, other control systems may be used within the spirit of the present invention. Other actuators include, but are not limited to, a differential pressure control system (DPCS), regulated pressure control system (RPCS), a stepper motor, an air actuator, a vacuum actuator, a hydraulic actuator, or any other type of actuator that has force or position control. 
     In some embodiments, a regulated pressure control system (RPCS), such as disclosed in U.S. Pat. App. Pub. no. 2008/0135004, entitled “Timing Phaser Control System”, by Simpson et al. and published Jun. 12, 2008, hereby incorporated by reference herein, is used.  FIG. 6  shows a single piston system  510  controlled by a RPCS  520  with the control valve  563  in a first position. A vent  563 ′ through the control valve body to the atmosphere minimizes air pressure fluctuations in the back end of the spool valve bore when the spool  64  moves back and forth in the spool valve bore. The RPCS  520  receives a signal from a control unit  508 , based on a set point, that causes a regulated pressure control valve or a direct control pressure regulator valve  561  to adjust an input oil pressure to a regulated control oil pressure in a biasing channel  560  that biases the end of the spool  64  of the control valve  563 , in proportion to the signal and the pressure in the main oil gallery. The other end of the spool  64  of the control valve  563  is preferably biased in the opposite direction by a spring  62 . Although a RPCS is shown only in the embodiment of  FIG. 7 , a RPCS may be used as the valve control system in any of the embodiments disclosed herein. 
     In some embodiments, a differential pressure control system (DPCS), such as disclosed in U.S. Pat. No. 6,883,475, entitled “Phaser Mounted DPCS (Differential Pressure Control System) to Reduce Axial Length of the Engine”, issued Apr. 26, 2005 to Simpson, hereby incorporated by reference herein, is used.  FIG. 7  shows a single piston system  610  controlled by a solenoid DPCS  620  with the control valve  663  in a first position. The position of the spool  64  of the control valve  663  is influenced by the solenoid DPCS  630  that is fed by oil pressure  622  from the engine. The solenoid DPCS  630  is controlled by a control unit  608 . The solenoid DPCS  630  utilizes engine oil pressure to control the position of a piston  632  against one end of the spool  64 , while oil pressure in a second line  624  opposes the piston  632 . The piston  632  is biased toward the spool  64  by a piston spring  634  and the spool  64  is biased toward the piston  632  by a spool spring  62  to maintain contact between the piston  632  and the spool  64  at low oil pressures. The oil pressure in the second line  624  is preferably unregulated engine oil at engine oil pressure but the oil pressure may alternatively be regulated. The solenoid  636  is preferably controlled by an electrical current applied to a coil in response to a control signal, preferably coming directly from the engine control unit  608 . Although a DPCS is shown only in the embodiment of  FIG. 7 , a DPCS may be used as the valve control system in any of the embodiments disclosed herein. 
     In some embodiments, a check valve in spool control valve, such as disclosed in PCT patent publication no. WO2012/135179, entitled “Using Torsional Energy to Move an Actuator”, by Pluta et al. and published Oct. 4, 2012, hereby incorporated by reference herein, is used.  FIG. 8  shows a single piston system  710  with a control valve  763  containing check valves, commonly referred to as a check valve in spool control valve, in place of the control valve shown in the previous figures. A vent  763 ′ through the control valve body to the atmosphere minimizes air pressure fluctuations in the back end of the spool valve bore when the spool  729  moves back and forth in the spool valve bore. The check valves  728   a ,  728   b  are visible in the exploded view of the valve assembly  720  in  FIG. 9 . The actuator piston  762  is also shown in  FIG. 9 . Although a check valve in spool is shown only in the embodiment of  FIG. 8 , a check valve in spool may be used as the control valve in any of the embodiments disclosed herein. 
     The check valve assembly  720  includes a spool  729  with two lands  729   a  and  729   b  separated by a central spindle  740 . Within each of the lands  729   a  and  729   b  are plugs  737   a  and  737   b  that receive the check valves  728   a  and  728   b . Each check valve  728   a ,  728   b  includes a disk  731   a ,  731   b  and a spring  732   a ,  732   b . Other types of check valves  728   a ,  728   b  may be used, including, but not limited to, band check valves, ball check valves, and cone-type. The spool  729  is biased outwards from the control shaft by a spring  736 . An actuator  761 , controlled by a control unit  708 , controls the position of the control valve  763 . In the position shown, fluid flows from the high compression ratio line  67  to the second port  738   b , through the central spindle hole  740   a  of the central spindle  740 , through the first land  729   a , through the first check valve  728   a , and through the first port  738   a  to the low compression ratio line  68 . The second check valve  728   b  prevents fluid flow in a reverse direction. The check valves  728   a ,  728   b  obviate the need for a central line  9  and check valves  65 ,  66  controlling flow between the central line  9  and the high compression ratio line  67  and low compression ratio line  68 . 
     Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.