Patent Publication Number: US-8118000-B2

Title: Multi-stroke internal combustion engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application No. 12/208,205, filed Sep. 10, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND AND SUMMARY 
     Some engines operate according to a four stroke cycle comprising an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. Other engines operate according to a two stroke cycle where the intake stroke is partially shared with the compression stroke and the exhaust stroke is partially shared with the power stroke. Some engines have been developed that can be selectively operated in a two stroke cycle during some conditions and a four stroke cycle during other conditions. In this way, the engine may exhibit higher power output through selective use of the two stroke cycle while achieving increased fuel efficiency through selective use of the four stroke cycle. 
     The inventor herein has recognized that transitions between two and four stroke operation may be achieved in one approach by a method of operating an internal combustion engine including at least one combustion chamber having a piston disposed therein. As one example, the method includes: repeatedly opening a first intake valve of the combustion chamber once every four piston strokes; transitioning operation of the combustion chamber from a four stroke cycle to a two stroke cycle by activating a second intake valve of the combustion chamber to repeatedly open once every four piston strokes during a different intake stroke than the first intake valve; and transitioning operation of the combustion chamber from the two stroke cycle to the four stroke cycle by deactivating the second intake valve in a closed position. A similar approach may be applied to the exhaust valves of the engine as will be described herein. 
     In some embodiments, the above method may be performed by an internal combustion engine system for a vehicle, including: an engine body defining at least one combustion chamber having a piston disposed therein; a manifold coupled to the engine body and defining a fluid passage that communicates with the combustion chamber via a first valve and a second valve; a first cam actuator configured to open the first valve; a second cam actuator configured to open the second valve; and a control system. The first valve and the second valve may include intake valves, or the first valve and the second valve may include exhaust valves. 
     The control system may be configured to: operate the first cam actuator to repeatedly open the first valve once every four piston strokes and operate the second cam actuator to repeatedly open the second valve once every four piston strokes during a different stroke than the first valve to carry out combustion in the combustion chamber according to a two stroke cycle; and operate the first cam actuator to repeatedly open the first valve once every four piston strokes and operate the second cam actuator in an inactive lift state with the second valve so that the second valve is held closed to carry out combustion in the combustion chamber according to a four stroke cycle. 
     The intake and exhaust valves that are operated continuously during both the two and four stroke cycles may be referred to as “full time” valves, while the intake and exhaust valves that are operated during the two stroke cycle while being deactivated during the four stroke cycle may be referred to as “part time” valves. In this way, the first intake valve may continue to operate during both of the four stroke and two stroke operations, while the second intake valve may be held closed to enable the four stroke operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example internal combustion engine system. 
         FIG. 2  illustrates an example valve system. 
         FIG. 3  illustrates an example tappet for an intake or exhaust valve. 
         FIGS. 4 and 5  illustrate example process flows for controlling the internal combustion engine system. 
         FIG. 6  illustrates an example timeline depicting operation of the internal combustion engine system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example internal combustion engine system  10 . Engine system  10  includes an engine body  32  defining one or more combustion chambers (i.e. cylinders), an example of which is illustrated at  30 . Each combustion chamber may include a respective piston  36  disposed therein. Engine system  10  further includes an intake manifold  44  coupled to the engine body, which defines a first fluid passage that communicates with combustion chamber  30  (among other combustion chambers of the engine) via a first intake valve  210  and a second intake valve  220  (shown in  FIG. 2 ). Engine system  10  further includes an exhaust manifold  48  coupled to the engine body, which defines a second fluid passage that communicates with combustion chamber  30  (among other combustion chambers of the engine) via a first exhaust valve  230  and a second exhaust valve  240  (shown in  FIG. 2 ). The combustion chamber may receive intake air via the first fluid passage and may exhaust products of combustion (i.e. exhaust gases) via the second fluid passage provided by intake manifold  44  and exhaust manifold  48 , respectively. 
     Intake manifold  44  may receive intake air from an intake passage  42 , which may include one or more of a throttle  62  and a compressor  121 . Compressor  121  forms a compressor stage of a boosting device, which may include a supercharger or a turbocharger that further includes an exhaust gas turbine  123  arranged along the exhaust passage downstream of the exhaust valves. Throttle  62  may include a throttle plate  64 , the position of which may be electronically controlled. Intake passage  42  may also include a mass airflow sensor  120  and intake manifold  44  may include a manifold pressure sensor  122  that can provide an indication of boost pressure downstream of compressor  121 . 
     Combustion chamber  30  may include a fuel injector  66 , which is configured to inject fuel directly into the combustion chamber in this particular example. However, in other examples, fuel injector  66  may be arranged upstream of intake valve  210 , such as along intake manifold  44  or intake passage  42 . A fuel injector driver  68  may be provided to control the fuel injection according to a prescribed timing, which may depend on whether the engine is operating in a two stroke cycle or a four stroke cycle. Combustion chamber  30  may optionally include a spark plug  92  for igniting an air and fuel mixture within the combustion chamber. An ignition system  88  may initiate ignition within the combustion chambers of the engine via the spark plugs according to a prescribed timing, which may depend on whether the engine is operating in a two stroke cycle or a four stroke cycle. In other embodiments, spark plug  92  may be omitted, such as with compression ignition engines or diesel engines. Specifically, in one example, engine  10  may operated with homogeneous charge compression ignition (in one or both of two-stroke and/or four-stroke operation. Alternatively, compression ignition diesel combustion may be used. Further still, a first combustion mode may be used with a first number of strokes (e.g., gasoline HCCl in two stroke) and a second, different combustion mode may be used with a second number of strokes (e.g., gasoline spark ignition in four stroke mode). 
     Engine body  32  may include coolant passages  114  that at least partially surround the combustion chamber. A coolant temperature sensor  112  can provide an indication of engine coolant temperature. Piston  36  may be operatively coupled with a crankshaft  40  via a crank arm. Crankshaft  40  may be similarly coupled to other pistons of the engine. An engine speed and position sensor  118  may be provided at crankshaft  40 . As one example, sensor  118  may comprise a hall effect sensor. 
     An exhaust gas composition sensor  126  may be provided along an exhaust passage that is fluidly coupled with exhaust manifold  48 . Sensor  126  may provided an indication of oxygen concentration in the exhaust gas produced by the engine, thereby providing an indication of air/fuel ratio combusted at the engine. An exhaust aftertreatment device  70  may be provided along the exhaust passage for processing products of combustion. 
     Engine system  10  may include an electronic controller  12 . Electronic controller  12  in combination with ignition system  88  and driver  68 , among other suitable control components may comprise a control system of the engine. Controller  12  may include a processing subsystem (CPU)  102 , which may comprise one or more processors. Controller  12  may include memory that comprises instructions that may be executed by the processing subsystem. This memory may include read only memory (ROM)  106 , random access memory (RAM)  108 , and keep alive memory (KAM)  110 . Processing subsystem  102  may communicate with the various sensor and actuators described herein via an input/output (I/O) interface  104 . For example, controller  12  may receive sensory feedback from in the form of mass air flow (MAF) via sensor  120 , intake manifold pressure (MAP) via sensor  122 , throttle position (TP) from throttle  62 , engine coolant temperature (ECT) via sensor  112 , exhaust composition (EGO) such as exhaust gas oxygen content via sensor  126 , and engine position (PIP) via sensor  118  which may be used to calculate engine speed. Additionally, controller  12  may receive an indication of operator requested engine output (e.g. torque and/or speed) from a user input device  130  via a sensor  134 . As one example, user input device  130  may include an accelerator pedal that may be depressed by a vehicle operator  132 . Sensor  134  may comprise a pedal position sensor, for example. Further, controller  12  may control one or more of the following: the position of throttle plate  64  via throttle  62 , a state of compressor  121  to vary boost pressure, a state of turbine  123  to vary boost pressure, spark timing via ignition system  88  through spark advance signal (SA), fuel injection timing and amount via driver  68  through fuel pulse-width (FPW) signal, and valve timing via valve system  200 . 
     Engine system  10  may include a valve system  200  shown in greater detail in  FIG. 2 . The valve system may include a plurality of cam actuators for controlling the position of the various intake and exhaust valves. Controller can obtain valve timing and/or cam actuator state information for the intake and exhaust valves from sensors  54  and  58 . In some embodiments, the engine system may include a first cam actuator and a second cam actuator configured to open the first intake valve and the second intake valve, respectively. In some embodiments, first cam actuator and second cam actuator may be arranged on a common cam shaft. In other embodiments, first cam actuator and second cam actuator may be arranged on different cam shafts. 
     In some embodiments, the engine system may further include a third cam actuator and a fourth cam actuator configured to open the first exhaust valve and the second exhaust valve, respectively. In some embodiments, third cam actuator and fourth cam actuator may be arranged on a common cam shaft. In other embodiments, third cam actuator and fourth cam actuator may be arranged on different cam shafts. In some embodiments, the first, second, third, and fourth cam actuators may be arranged on a common cam shaft. While in other embodiments, the first and third cam actuators may be arranged on a first cam shaft, and the second and fourth cam actuators may be arranged on a second cam shaft. As such, various embodiments of the valve system are possible. 
     Regardless of the particular cam actuator configuration relative to the above described cam shafts, these cam shafts may be mechanically coupled to a crankshaft of the engine so that the cam shaft rotates in relation to the rotation of the crankshaft by a prescribed speed ratio. As one example, where the various cam actuators each include a single cam lobe, the cam shafts may be mechanically coupled to the crank shaft in a manner that causes the cam shaft to rotate at half the speed of the crank shaft. In other words, the cam shafts may be configured to rotate only one revolution for every two revolutions of the crank shaft. In this way, the cam lobe of each cam actuator may engage its respective valve once per every four strokes of the piston to cause the valve to open. 
     In other embodiments where the cam actuators each include two or more cam lobes, the cam shafts may be configured to rotate at different speeds relative to the crank shaft. For example, where the cam actuators each include two cam lobes, the cam shafts may be configured to rotate at one quarter the speed of the crank shaft. In other words, the cam shafts may be configured to rotate only one revolution for every four revolutions of the crank shaft. In this way, one of the two cam lobes of each cam actuator may be engage its respective valve once per every four strokes of the piston to cause the valve to open. 
       FIG. 2  illustrates a non-limiting example of valve system  200 . In this example, combustion chamber  30  which was previously described with reference to  FIG. 1 , is depicted with engine body  32  defining intake ports  262  and  264 , and exhaust ports  266  and  268 . While the combustion chamber is depicted with two intake ports and two exhaust ports, it should be appreciated that combustion chamber  30  may include another suitable number of intake and exhaust ports. Note that in the embodiment of  FIG. 2 , the intake and exhaust valves are configured as poppet valves that are arranged in the intake and exhaust ports located at or near the top of the combustion chamber. 
     Each of the intake and exhaust ports include an associated valve that may be moved (e.g. translated) relative to the port to open and close communication with the combustion chamber and the intake or exhaust manifold. For example, intake air may be admitted to combustion chamber  30  via one or more of intake ports  262  and  264  when their respective intake valves  210  and  220  are opened. Intake valve  210  is depicted in  FIG. 2  in an open position that enable intake air to flow into the combustion chamber from the intake manifold, while intake valve  220  is depicted in a closed position, thereby inhibiting intake air from flowing into the combustion chamber via intake port  264 . Similarly, exhaust gases may be exhausted from combustion chamber  30  via one or more of exhaust ports  266  and  268  when their respective exhaust valves  230  and  240  are opened. 
     In this particular embodiment, intake valves  210  and  220  are actuated by cam actuators mounted on a common cam shaft  252  and exhaust valves  230  and  240  are actuated by cam actuators mounted on a common cam shaft  256 . In other embodiments, the intake and/or exhaust valves may be actuated by cam actuators that are mounted on separate cam shafts. Cam shafts  252  and  256  may be driven to rotate at a rotational speed that is proportional to the rotational speed of the crank shaft of the engine (i.e. engine speed). In some embodiments, crank shafts  252  and  256  may be operatively coupled to the crankshaft via a 2:1 gear ratio, which causes the cam shafts to rotate at one half the rotational speed of the crank shaft. Thus, the cam shafts may be driven to rotate through 360 cam angle degrees for every 720 crank angle degrees. It should be appreciated that other suitable gear ratios may be used. 
     It should be appreciated that cam shafts  252  and  256  may include cam actuators for opening valves associated with other combustion chambers of the engine. As such, intake cams actuators associated with other combustion chambers of the engine may be mounted on cam shaft  252  and exhaust cams actuators associated with other combustion chambers of the engine may be mounted on cam shaft  256 . 
     Intake valve  210  may include a tappet  212  and a valve spring  214 . Valve spring  214  may be configured to urge intake valve  210  to a closed position with respect to intake port  262  until opened by an actuation received from a cam actuator (e.g. cam actuator  216 ) via tappet  212 . For example, in  FIG. 2 , cam actuator  216  which includes a cam lobe  217  is depicted actuating valve  262  via tappet  212  to cause valve  262  to open to admit intake air to the combustion chamber. Note that valve spring  214  is depicted in a compressed state relative to the other valve springs shown in  FIG. 2 . 
     Similarly, exhaust valve  230  may include a tappet  232  and a valve spring  234 . Valve spring  234  may be configured to urge exhaust valve  230  to a closed position with respect to exhaust port  266  until opened by an actuation received from a cam actuator (e.g. cam actuator  236 ) via tappet  232 . For example, in  FIG. 2 , cam actuator  236  which includes a cam lobe  237  is depicted in a position relative to tappet  232  that does not cause exhaust valve  230  to open. This position may be referred to as the base circle of the cam actuator. As further depicted in  FIG. 2 , cam actuator  216  may be at a different rotational position relative to tappet  212  than cam actuator  236  relative to tappet  232 , thereby causing intake valve  210  and exhaust valve  230  to open at different timings relative to the crank angle of the engine. An example timing diagram is depicted in  FIG. 6 . 
     Intake valve  220  may be associated with a tappet  222 , valve spring  224 , and a cam actuator  226  having a cam lobe  227  that causes intake valve  220  to open. As depicted in  FIG. 2 , cam actuators  216  and  226  may be configured to open their respective intake valves at different timings as illustrated by their different orientations on cam shaft  252 . As one example, cam actuator  216  may be orientated at approximately 180 cam angle degrees (e.g. 360 crank angle degrees) relative to cam actuator  226 . Intake valve  220  may also be associated with a non-lifting cam actuator  228 . The non-lifting cam actuator  228  may be the equivalent of the base circle of cam actuator  226 . By varying which one of cam actuators  226  and  228  that engage tappet  222 , an activated or a deactivated state of valve  220  may be selected. For example, during the activated state, cam actuator  227  may engage tappet  222 , whereby valve  220  may be opened according to the lift profile of cam actuator  226  as defined by cam lobe  227 . During deactivated state, valve  226  may not engage tappet  222 , whereby valve  220  may remain closed (e.g. held closed by the valve spring) through an entire revolution of the cam shaft. 
     There are at least two ways in which intake valve  220  may be transitioned between the deactivated state and the activated state. As a first example, the crank shaft may be translated relative to tappet  222 , thereby causing one of cam actuator  226  or cam actuator  228  to engage the tappet. As a non-limiting example, a variable valve timing device  270  may be configured to translate cam shaft  252  between at least two different positions, which correspond to the engagement and disengagement of cam actuator  226  from tappet  222 . When cam actuator  228  engages the tappet, the valve is not lifted as a result of the non-lifting aspect of cam actuator  228 . By contrast, when cam actuator  226  engages the tappet, the valve is lifted according to the profile of cam lobe  227 . Where translation of the cam shaft is used to select between two or more different cam actuators, cam actuator  216  may optionally be configured with a width that enables cam actuator  216  to engage tappet  212  during each of the activated and deactivated positions of intake valve  220 . In this way, intake valve  210  may continue to open according to the profile of cam actuator  216  regardless of the state of intake valve  220 . 
     As a second example, tappet  222  may be adjusted to selectively engage cam actuator  226  during the activated state of valve  220  or disengage from cam actuator  226  during the deactivated state of valve  220 . For example, tappet  222  may include lost motion functionality whereby lift provided by cam lobe  227  does not translate to valve  220  via tappet  222 . Note that with each of the first and second examples, cam actuator  228  may be optionally omitted since no lift may be provided during the deactivated state of the valve. This lost motion functionality will be described in greater detail with reference to  FIG. 3 . 
     Exhaust valve  240  may be associated with a tappet  242 , valve spring  244 , and a cam actuator  246  having a cam lobe  247  that causes intake valve  240  to open. As depicted in  FIG. 2 , cam actuators  236  and  246  may be configured to open their respective exhaust valves at different timings as illustrated by their different orientations on cam shaft  256 . As one example, cam actuator  236  may be orientated at approximately 180 cam angle degrees (e.g. 360 crank angle degrees) relative to cam actuator  246 . Intake valve  240  may also be associated with a non-lifting cam actuator  248 . The non-lifting cam actuator  248  may be the equivalent of the base circle of cam actuator  246 . 
     By disengaging cam actuator  246  from tappet  242  and/or by varying which one of cam actuators  246  and  248  that engage tappet  242 , an activated or a deactivated state of valve  240  may be selected. For example, during the activated state, cam actuator  246  may engage tappet  242 , whereby valve  240  may be opened according to the lift profile of cam actuator  246  as defined by cam lobe  247 . During the deactivated state, valve  246  may be disengaged from tappet  242 , whereby valve  240  may remain closed (e.g. held closed by the valve spring) through an entire revolution of the cam shaft. There are at least two ways in which exhaust valve  240  may be transitioned between the deactivated state and the activated state as previously described with reference to intake valve  220 . For example, a variable valve timing device  280  may be configured to translate cam shaft  256  between at least two positions, which correspond to a state where cam actuator  246  engages tappet  242  and a state where cam actuator  246  is disengaged from tappet  242 . As another example, tappet  242  may provide lost motion functionality as described with reference to  FIG. 3 . 
     Note that variable valve timing devices  270  and  280  may comprise a combined unit in some embodiments. It should be appreciated that these variable valve timing devices may be actuated by any suitable approach to cause translation of the cam shafts, including hydraulic actuation or electromechanical actuation that may be initiated by controller  12 . Furthermore, variable valve timing devices  270  and  280  may be configured to cause rotation of the camshafts relative to the position of the piston. In this way, the intake and exhaust valve timing may be adjusted (e.g. advanced or retarded) as will be described in greater detail with regards to  FIGS. 4-6 . It should be appreciated that these valve timing adjustments may be initiated by the controller via the variable valve timing devices using any suitable actuation approach, including hydraulic actuation or electromechanical actuation. 
     Intake valve  210  and exhaust valve  230  may be referred to as “full time” valves, since they may be operated during both four stroke and two stroke operation of combustion chamber  30 , while intake valve  220  and exhaust valve  240  may be referred to as “part time” valves, since they may be operated only during the two stroke operation of combustion chamber  30 , at least in some embodiments. However, as will be described with reference to  FIG. 5 , the full time valves and part time valves may be periodically switched in some embodiments, where the part time valves become the full time valves and vice-versa. Further still, it should be appreciated that cam shaft  252  may include a non-lifting cam actuator for valve  210  as was previously described with reference to cam actuator  228  for valve  220 . Similarly, it should be appreciated that cam shaft  256  may include a non-lifting cam actuator for valve  230  as was previously described with reference to cam actuator  248  for valve  240 . 
       FIG. 3  depicts a non-limiting example of a tappet  310  that exhibits lost motion functionality. It should be appreciated that other suitable tappets may be used to enable the deactivation or activation of a particular valve. Tappet  310  may refer to one of the previously described tappets. For example, where valves  210  and  230  are always operated as full time valves, tappet  310  may refer to the previously described tappets  222  and  242  of the part time valves  220  and  240 . However, where valves  210  and  230  may transition between full time status and part time status as described with reference to  FIG. 5 , tappet  310  may refer to any of the previously described tappets. 
       FIG. 3  illustrates a cam shaft which may include one of the previously described cam shafts  252 / 256  and its associated cams  228 / 248  and  226 / 246 . In this particular embodiment, tappet  310  has an actuation member  320  which may be translated relative to the tappet to engage or disengage the lifting cam actuator (e.g. cam actuator  226  or  246 ). Tappet  310  may include a contact surface  312  that interfaces with a non-lifting cam (e.g.  228 / 248 ) when the valve is set to the deactivated state. When the valve is to be set to the activated state, actuation member  320  may be translated toward the lifting cam so that surface  322  contacts the lifting cam causing the valve to open according to the profile of the lifting cam. Note that in some embodiments, a second non-lifting cam  330  may be provided to balance the forces applied to surface  312  of the tappet. In other embodiments, such as where the lifting cam is arranged on the outside of the tappet and the non-lifting cam is arranged on the inside of the tappet, a second lifting cam may be provided to balance the forces applied to the surface of the tappet. Note that  FIG. 3  provides merely one non-limiting example of how activation and deactivation of a valve may be performed by a tappet having lost motion functionality. 
       FIGS. 4 and 5  depict example process flows that may be used to control valve system  200 . These process flows may represent instructions residing on computer readable media that may be performed by a processing subsystem of the control system. Referring to  FIG. 4 , at  410 , operating conditions of the engine system may be identified. For example, the control system may receive feedback from the various sensors that were previously described with reference to  FIG. 1 . These operating conditions may include, but are not limited to: current engine speed, current engine load, current engine torque, an operator requested output (e.g. as indicated by pedal position) such as desired engine torque or desired engine speed to be produced by the engine, boost pressure, ambient conditions, etc. 
     At  412 , if one or more combustion chambers of the engine system are to be transitioned from the two stroke cycle to the four stroke cycle, the process flow may proceed to  414 . For example, the control system may judge that a transition is to be performed from the two stroke cycle to the four stroke cycle responsive to the operating conditions identified at  410 . As a non-limiting example, the control system may transition the engine to the four stroke cycle at higher engine speeds or when a lower engine output torque is requested by the vehicle operator. 
     At  414 , the part time valves (e.g. valves  220  and  240 ) may be deactivated (i.e. set to the deactivate state) while operation of the full time valves (e.g. valves  210  and  230 ) is maintained. For example, referring also to  FIG. 6 , the full time (FT) valves may continue to operate in the active state while the part time (PT) valves may be deactivated at the closed valve state (e.g. as assisted by the valve springs). 
     Optionally, at  416 , the timing of the full time intake valve (e.g. valve  210 ) may be retarded and the timing of the full time exhaust valve (e.g. valve  220 ) may be advanced responsive to the transition from the two stroke cycle to the four stroke cycle. For example, referring also to  FIG. 6 , the full time (FT) exhaust valve timing may be retarded to a later timing during the four stroke cycle as indicated at  630  relative to its timing during the two stroke cycle, and the full time (FT) intake valve timing may be advanced to an earlier timing during the four stroke cycle as indicated at  640  relative to its timing during the two stroke cycle. 
     From  416  or if the answer at  412  is judge no, the process flow may proceed to  418 . If a transition from the four stroke cycle to the two stroke cycle is to be performed, the process flow may proceed to  420 . For example, the control system may judge whether the transition from four stroke operation to two stroke operation is to be performed responsive to the operating conditions identified at  410 . As a non-limiting example, the control system may transition the engine to the two stroke cycle at lower engine speeds or when a higher engine output torque is requested by the vehicle operator. It should be appreciated that the valve control approaches described herein are not limited to this particular example. 
     At  420 , the part time valves may be activated to transition to two stroke operation while maintaining operation of the full time valves (e.g. valves  210  and  230 ). During two stroke operation, the spark and fuel may be provided to the combustion chambers approximately every 360 crank angle degrees as depicted in  FIG. 6 . By contrast, during four stroke operation, the spark and fuel may be provided to the combustion chambers approximately every 720 crank angle degrees as also shown in  FIG. 6 . 
     Optionally, at  422 , the timing of the full time intake valve (e.g. valve  210 ) may be advanced and the timing of the full time exhaust valve (e.g. valve  220 ) may be retarded responsive to the transition from four stroke operation to two stroke operation. For example, referring also to  FIG. 6 , the full time (FT) exhaust valve timing may be advanced to an earlier timing during the two stroke cycle as indicated at  610  relative to its timing during the four stroke cycle, and the full time (FT) intake valve timing may be retarded to a later timing during the two stroke cycle as indicated at  620  relative to its timing during the four stroke cycle.  FIG. 6  illustrates how the part time (PT) intake valve (e.g. valve  220 ) and part time (PT) exhaust valve (e.g. valve  240 ) may be operated during the two stroke cycle.  FIG. 6  also depicts how the relative timing of the intake and exhaust valves (i.e. the valve overlap) may be adjusted during the two stroke cycle responsive to operating conditions identified at  410 . For example, the overlap between a full time valve and a part time valve may be adjusted according to boost pressure, engine speed, engine load, engine torque output, operator requested engine torque, etc. From  422  or if the answer is no at  418 , the process flow may return to the start. 
     Referring to  FIG. 5 , at  510 , it may be judged whether a valve&#39;s status as a “full time” valve should be switched to a part time operation. As one example, the control system may periodically operate full time valves as part time valves and part time valves as full time valves in order to reduce asymmetry in valve wear among the various valves of the engine. As a non-limiting example, the control system may record the number of opening events provided by each valve or the amount of time each valve has been activated, whereby the control system may assign “full time” and “part time” status to the valves in a manner that more closely balances the number of opening events or activation time of the various valves. If the answer at  510  is yes, the process flow may proceed to  512 . Alternatively, if the answer at  510  is no, the process flow may return to the start. 
     At  512 , it may be judged whether the engine system or a particular combustion chamber of the engine system is currently operating in the four stroke cycle. If the answer at  512  is judged yes, the process flow may proceed to  514 , where operation of the current full time valves may be maintained in the active full time state at  514 . Alternatively, if the answer at  512  is judged no, the process flow may proceed to  516 . 
     At  516 , it may be judged whether the engine system or a particular cycle of the engine system is currently operating in the two stroke cycle. If the answer at  516  is judged yes, the process flow may proceed to  518 . Alternatively, if the answer at  516  is judged no, the process flow may return to the start. At  518 , during the next transition of the engine system or combustion chamber from the two stroke cycle to the four stroke cycle, the control system may deactivate the previous full time valves (rather than the previous part time valves) while maintaining operation of the previous part time valves. In this way, the part time valves may obtain the status of full time valves while the previous full time valves are deactivated in accordance with their new status as part time valves. Note that the valve timings of the previous part time valves (i.e. new full time valves) may be adjusted responsive to the transition as described with reference to  FIG. 4 . In this way, valve system wear may be balanced more evenly among the various valves of the engine system. 
       FIG. 6  illustrates how the cam actuator arrangement of  FIG. 2  may cause a full time intake valve to repeatedly open once every four strokes during the four stroke cycle and a full time exhaust valve to repeatedly open once every four strokes during the four stroke cycle. During the two stroke cycle, the full time valves may continue to operate at the same frequency relative to piston position (although valve timing adjustments may be performed responsive to the transition as indicated at  610 ,  620 ,  630 , and  640 ), while the part time intake valve is repeatedly opened once every 4 strokes during a different intake stroke than the full time intake valve, and the part time exhaust valve is repeatedly opened once every 4 strokes by a during a different exhaust stroke than the full time exhaust valve. For example, during two stroke operation, the full time intake valve and the part time intake valve may be opened approximately 360 crank angle degrees apart and the full time exhaust valve and the part time exhaust vale may be opened approximately 360 crank angle degrees apart (excluding the transient valve timing adjustments that may be performed in response to changes in operating conditions as demonstrated by the latter half of the two stroke cycle shown in  FIG. 6 ). 
     Note that the example control and estimation routines that are depicted by the above process flows can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and subcombinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.