Patent Publication Number: US-9885281-B2

Title: Engine system with two pistons

Description:
BACKGROUND/SUMMARY 
     During operation of an internal combustion engine fuel can be pushed into the small gap between the piston and the cylinder wall during a compression stroke. During the next expansion stroke fuel remains trapped in the small gap and does not combust. Only during a subsequent exhaust stroke may the trapped fuel escape from the gap into the cylinder. The released fuel will flow into the exhaust system instead of being ignited during an expansion stroke. As a result, engine emissions may be increased and engine efficiency and fuel economy may be decreased. 
     As such in one approach, an engine system is provided. The engine system includes a first piston movable within a cylinder, a second piston movable within a hollow body of the first piston, the movement defining a boundary of a reservoir within the hollow body, a spring device coupled to the first piston and the second piston, and a vent passage fluidically connecting the reservoir to the cylinder. In this way, the vent passage can provide fluidic communication between the reservoir and cylinder to enable the reservoir to receive gases, such as hydrocarbons, during an exhaust stroke and release the gases during a subsequent expansion stroke. Consequently, hydrocarbons, which in previous engines would be released into the cylinder during an exhaust stroke, can be stored in the reservoir and released during desired combustion cycle phases. As a result, engine emissions are reduced. 
     The inventors herein have recognized the above issues and potential options to address them. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a view of an engine system in an engine, the engine system including a second piston moveable in a hollow body of a first piston; 
         FIGS. 2-5  the engine system of  FIG. 1  during different strokes in a combustion cycles where the relative position of the first and second pistons varies; and 
         FIG. 6  shows a method for operation of an engine system. 
     
    
    
     DETAILED DESCRIPTION 
     This description relates to systems and methods for flowing gas into and out of a reservoir in a hollow body of a piston. The gas flow pattern into and out of the piston reservoir reduces emissions caused by hydrocarbons (e.g., unburnt fuel) trapped between a cylinder wall and an outer surface of the piston during a compression stroke and released into the cylinder during a subsequent exhaust stroke. The gas flow pattern into the piston reservoir can also provide internal exhaust gas recirculation (EGR) functionality by filling the reservoir with exhaust gas during an exhaust stroke and storing the exhaust gas in the reservoir until subsequent compression and expansion strokes when it is released back into the cylinder. The gas flow between the cylinder and the reservoir is enabled by one or more vent passages fluidly connecting the reservoir to the cylinder, the vent passages outwardly extend through the first piston. Correspondingly, the gas flow into and out of the piston reservoir is generated through movement of a second piston positioned with the first piston during different combustion cylinder strokes. In particular, during an exhaust stroke the second piston moves upward, increasing the volume of the reservoir to generate gas flow into the reservoir. In this way, hydrocarbons in a gap between the cylinder wall and the piston&#39;s outer surface are flowed into and temporarily stored in the reservoir instead of remaining trapped in the gap. During a subsequent compression stroke the second piston moves downward, decreasing the volume of the reservoir to release the hydrocarbons stored in the reservoir into the cylinder. As such, the hydrocarbons are combusted during the following expansion stroke. Therefore emissions from the engine are reduced, thereby decreasing the engine&#39;s environmental impact. 
       FIG. 1  shows an engine  100  with an engine system  102  in a cross-sectional view. The engine  100  includes various components that facilitate combustion operation such as an intake valve  104  and an exhaust valve  106  coupled to a cylinder  108 . An intake conduit  110  is also provided in the engine to enable intake airflow into the cylinder  108 . Likewise an exhaust conduit  112  is provided in the engine  100  to enable exhaust gas to be expelled from the cylinder  108 . 
     An intake valve actuator  114  is coupled to the intake valve  104  and is configured to open and close the intake valve  104  at desired time intervals. Likewise, an exhaust valve actuator  116  is coupled to the exhaust valve  106  and is configured to open and close the exhaust valve  106  at desired time intervals. The intake and exhaust valve actuators  114  and  116  may be cams, electronic actuators, or other suitable devices configured to actuate each respective valve. 
     The cylinder  108  includes a cylinder wall  118  with a first piston  120  positioned therein. The first piston  120  is included in the engine system  102 . The first piston  120  may be coupled to a crankshaft  122  through a piston rod  123  so that reciprocating motion of the first piston is translated into rotational motion of the crankshaft. It will be appreciated that the piston rod  123  and first piston  120  may be constructed out of different materials, in one example. The crankshaft  122  may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to the crankshaft  122  via a flywheel to enable a starting operation of engine  100 . 
     The engine system  102  also includes a second piston  124  moveable within a hollow body  125  of the first piston  120 . The pressure differential between the cylinder  108  and the reservoir  126  affects the movement of the second piston  124 . The movement of the second piston  124  generates gas flow into and out of a reservoir  126  of the hollow body  125 . The reservoir  126  can have a cylindrical shape or other suitable geometries. 
     A spring device  128  is coupled to the first piston  120  and the second piston  124  and is included in the engine system  102 . The spring device  128  is embodied as a coil spring in  FIG. 1 . However, other types of spring devices have been contemplated, such as leaf springs, wave springs, gas springs, etc. Additionally, the spring device  128  is positioned within the reservoir  126  in  FIG. 1 . Positioning the spring device  128  within the reservoir  126  can enable the compactness of the engine system  102  to be increased. However spring devices positioned external to the reservoir may be used, in other examples. 
     The spring device  128  exerts a return force on the second piston  124  when there is greater amount of pressure exerted on a top surface  130  of the second piston  124  than a bottom surface  132  of the second piston  124 . As such, the spring device  128  enables the second piston  124  to move up and down, changing the volume of the reservoir  126  during different combustion cycles phases. The spring device  128  is in a neutral position in  FIG. 1 . Specifically in the depicted example, the spring device  128  is in the neutral position when the top surface  130  of the second piston being parallel to a top surface  134  of the first piston  120 . However, other neutral position arrangements of the first and second pistons may be used. 
     The top surface  130  of the second piston  124  is open to the cylinder  108  while the bottom surface of the second piston is open to the reservoir  126 . Specifically, the top surface  130  of the second piston  124  defines a portion of a boundary of the cylinder  108  while the bottom surface  132  of the second piston defines a portion of a boundary of the reservoir  126 . Another portion of the boundary of the cylinder  108  is also defined by the cylinder wall  118 . Additionally, interior surfaces  136  of the first piston  120  define another portion of the boundary of the reservoir  126 . It will be appreciated that the volume and therefore boundary of the reservoir  126  changes due to the movement of the second piston  124 . As such, gas (e.g., fuel vapor, exhaust gas, etc.,) can be flowed into and out of the reservoir  126  during desired phases of the combustion cycle to reduce emissions. The gas flow pattern into and out of the reservoir  126  is discussed in greater detail herein with regard to  FIGS. 2-5 . 
     The engine system  102  also include vent passages  138  providing fluidic connection between the cylinder  108  and the reservoir  126 . It will be appreciated that the cutting plane of the cross-sectional view in  FIG. 1  extends through the vent passages  138 . In such a cross-sectional view, the vent passages  138  appear to form a boundary between two sections of the first piston  120  that are spaced apart from one another. However, this is not in fact the case. The first piston  120  is formed from a solid piece of material and therefore has a continuous shape. In one particular example, the first piston  120  can have the shape of a partially hollow cylinder with cylindrical vent passages extending through selected regions with material of the first piston extending around and defining boundaries of the vent passages. The vent passages  138  each include a first end  140  opening into the reservoir  126  and a second end  142  opening into a gap  144  between the cylinder wall  118  and an outer surface  119  of the first piston  120 . The vent passages  138  extend upward and outward through the first piston  120 , in the illustrated example. In other examples, the vent passages  138  may only extend outward through the first piston in a horizontal direction. Vertical and horizontal axes are provided for reference. However, other relative orientations of the engine system  102  have been contemplated. Although, two vent passages  138  are depicted in  FIG. 1 , an alternate number of vent passages  138  may be included in the engine system  102 , such as a single vent passage or three or more vent passages extending through the first piston  120  from the reservoir  126  to the outer surface  119  of the first piston  120 . Additionally in one example, the vent passages  138  may be evenly distributed around the first piston  120  and may be cylindrical in shape. However, other relative positions and profiles of the vent passages have been contemplated. 
     A first piston ring  146  is coupled the first piston  120 . The first piston ring  146  provides a seal between the first piston  120  and the cylinder wall  118 . The first piston ring  146  is positioned vertically below the second end  142  of each of the vent passages  138  and vertically above the first end  140  of each of the vent passages  138 . Positioning the first piston ring  146  below the second end  142  of each of the vent passages  138  enable gas to flow through the gap  144  into or out of the vent passages  138 . Specifically in the depicted example, the first piston ring  146  is positioned directly below the second end  142  of each vent passages  138 , to reduce the likelihood of hydrocarbons (e.g., fuel) becoming trapped in a lower section of the gap  144 . Additionally, positioning the first piston ring  146  vertically above the first end  140  of each of the vent passages  138  and below the second end  142  enables the exhaust gas to flow upward through the first piston  120 . This can potentially enable the hollow body  125  to extend further into the first piston  120 , increasing the size of the hollow body  125 , if desired. 
     A second piston ring  148  is coupled to an outer surface  150  of the second piston  124 . The second piston ring  148  provides a seal between the second piston  124  and a portion of the interior surfaces  136  of the first piston  120 . 
     The engine system  102  also includes a fuel injector  152  which in the depicted example is a direct fuel injector. Additionally or alternatively the engine  100  may include a port fuel injector configured to inject fuel upstream of the cylinder  108 . The fuel injector  152  is configured to deliver a metered amount of fuel to the cylinder  108  at selected time intervals. A fuel delivery system including a fuel tank, pumps, fuel conduits, etc., may be provided in the engine  100  to supply fuel to the fuel injector  152 . 
     The engine  100  may also include an ignition device  154  coupled to the cylinder  108 , in the case of a spark ignition engine. However in other instances, the engine may be configured to perform compression-ignition. 
     A controller  151  may be configured to receive signals from sensors in the engine  100  and engine system  102  as well as send command signals to components such as the ignition device  154 , fuel injector  152 , intake valve actuator  114 , exhaust valve actuator  116 , etc., to adjust operation of the components. Various components in the engine system  102  may be controlled at least partially by a control system including the controller  151  and by input from a vehicle operator  153  via an input device  155 . The control system may also include actuators and/or other component for adjusting injectors, valves, etc., and sensors described herein. In this example, input device  154  includes an accelerator pedal and a pedal position sensor  156  for generating a proportional pedal position signal PP. The controller  151  is shown in  FIG. 1  as a microcomputer, including processor  160  (e.g., microprocessor unit), input/output ports  162 , an electronic storage medium for executable programs and calibration values shown as read only memory  164  (e.g., read only memory chip) in this particular example, random access memory  166 , keep alive memory  168 , and a data bus. 
     Storage medium read-only memory  164  can be programmed with computer readable data representing instructions executable by processor  160  for performing the methods described below as well as other variants that are anticipated but not specifically listed. 
     During operation, the cylinder  108  typically undergoes a four stroke combustion cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. The four stroke combustion cycle is described herein with regard to  FIGS. 2-5  showing the cylinder  108  and engine system  102  during different phases of a single four stroke combustion cycle. Specifically,  FIG. 2  shows the engine system  102  during an exhaust stroke,  FIG. 3  shows the engine system  102  during an intake stroke,  FIG. 4  shows the engine system  102  during a compression stroke, and  FIG. 5  shows the engine system  102  during an expansion stroke. Various arrows depicting gas flow are provided in  FIGS. 2-5  to aid in the understanding of the general direction of gas flow during the combustion cycles. However it will be appreciated that the gas flow patterns can have greater complexity than what is illustrated. Additionally,  FIGS. 2-5  are shown in the cross-sectional view similar to  FIG. 1 . 
     The first piston  120  moves from bottom dead center (BDC) to top dead center (TDC) during the exhaust stroke depicted in  FIG. 2 . Thus, the first piston  120  moves in an upward direction, indicated by arrow  202 , to generate the exhaust gas flow past the exhaust valve  106 , during the exhaust stroke. Again, vertical and horizontal axes are provided for reference. The point at which the first piston  120  is at the end of its exhaust stroke (e.g., when cylinder  108  is at its smallest volume) is typically referred to by those of skill in the art as TDC. On the other hand, the position at which the first piston  120  is at the beginning of its exhaust stroke (e.g., when the cylinder  108  is at its largest volume) is typically referred to by those of skill in the art as BDC. Furthermore, during the exhaust stroke illustrated in  FIG. 2  the exhaust valve  106  is opened so as to enable exhaust gas to flow from the cylinder  108  into the exhaust conduit  112 . The general direction of exhaust gas flow past the exhaust valve  106  is illustrated via arrows  200 . 
     During the exhaust stroke, the second piston  124  is moving in an upward direction, indicated by arrow  204 . As such, the volume of the reservoir  126  increases during the exhaust stroke. The increase in volume of the reservoir  126  generates gas flow from the cylinder  108  to the reservoir  126  through the vent passages  138 . Arrows  206  indicate the gas flow from the cylinder  108  into the gap  144  and arrows  208  indicate the gas flow from the vent passages  138  into the reservoir  126 . Generating gas flow in this pattern enables hydrocarbons (e.g., unburnt fuel) in the gap  144  to be flowed into the reservoir  126  rather than remaining trapped in the gap  144  during subsequent combustion cycle phases. In this way, the reservoir  126  can store hydrocarbons, such as unburnt fuel, to achieve a reduction in emissions. It will be appreciated that exhaust gas from the cylinder  108  also flows through the vent passages  138  into the reservoir during the exhaust stroke. In this way, the reservoir  126  also acts as an internal EGR chamber, enabling a further reduction in emissions to be achieved. Specifically, exhaust gas is flowed into and stored in the reservoir  126  until subsequent compression and expansion strokes, providing EGR functionality. 
     The first piston  120  moves from TDC to BDC during the intake stroke depicted in  FIG. 3 . Also during the intake stroke the intake valve  104  is opened and the exhaust valve  106  is closed to enable intake air flow from the intake conduit  110  to the cylinder  108 . The general direction of exhaust gas flow past the intake valve  104  is illustrated via arrows  300 . During the intake stroke the first piston  120  moves in a downward direction, indicated by arrow  302 , to generate intake air flow past the intake valve  104  from the intake conduit  110  into the cylinder  108 . Again, vertical and horizontal axes are provided for reference. 
     During the intake stroke, the second piston  124  is moving in an upward direction, indicated by arrow  304 . As such, the volume of the reservoir  126  further increases during the intake stroke. Therefore, it will be appreciated that the volume of the reservoir  126  increases both during the exhaust and intake strokes. The increase in volume of the reservoir  126  generates additional gas flow from the cylinder  108  to the reservoir  126  through the vent passages  138 . Specifically, intake air is flowed into the reservoir  126  from the cylinder  108  during the intake stroke. Arrows  306  indicate the gas flow from the cylinder  108  into the gap  144  and arrows  308  indicate the gas flow from the vent passages  138  into the reservoir  126 . As mentioned above, generating gas flow in this pattern enables hydrocarbons (e.g., unburnt fuel) trapped in the gap  144  to be flowed into the reservoir  126 . Furthermore, intake air may also be flowed into the reservoir  126  during the intake stroke. 
     The first piston  120  moves from BDC to TDC in an upward direction, indicated by arrow  402 , during the compression stroke depicted in  FIG. 4 . During the compression stroke both the intake valve  104  and the exhaust valve  106  are closed. It will be appreciated that the fuel injector  152  also injects fuel into the cylinder  108  during the compression stroke. However, fuel may also be introduced into the cylinder during the intake stroke depicted in  FIG. 3 . 
     Continuing with  FIG. 4 , during the compression stroke, the second piston  124  is moving in a downward direction, indicated by arrow  404 . As such, the volume of the reservoir  126  decreases during the compression stroke. The decrease in reservoir volume generates gas flow from the reservoir  126  to the cylinder  108  through the vent passages  138 . Arrows  406  indicate the gas flow from the reservoir  126  into the vent passages  138 . Arrows  408  indicate the gas flow from the gap  144  above the second end  142  of each of the vent passages  138 . In this way, gas (e.g., fuel vapor, exhaust gas, intake air, etc.,) stored in the reservoir  126  during the exhaust stoke and intake stroke can be released into the cylinder  108  during the compression stoke. 
     The first piston  120  moves from TDC to BDC in a downward direction, indicated by arrow  502 , during the expansion stroke depicted in  FIG. 5 . During the expansion stroke both the intake valve  104  and the exhaust valve  106  remain closed. It will be appreciated that the ignition device  154  may deliver a spark to the cylinder  108  during the expansion stroke, in the case of a spark ignition engine. 
     During the expansion stroke, the second piston  124  is moving in a downward direction, indicated by arrow  504 . As such, the volume of the reservoir  126  further decreases. Thus, the volume of the reservoir  126  decreases during both the compression stroke and the expansion stroke. The additional decrease in the volume of the reservoir  126  generates additional gas flow from the reservoir  126  to the cylinder  108  through the vent passages  138 . Arrows  506  indicate the continued gas flow from the reservoir  126  into the vent passages  138 . Additionally, arrows  508  indicate the continued gas flow from the gap  144  above the second end  142  of each of the vent passages  138 . This gas flow pattern enables gas (e.g., fuel vapor, exhaust gas, intake air, etc.,) stored in the reservoir  126  during the exhaust stoke and intake stroke to continue to be released into the cylinder  108  during the expansion stroke following the gas release in the compression stroke. In this way, unburnt fuel vapor previously stored in the reservoir  126  can be combusted and oxidized during the expansion stroke to reduce emissions. Additionally, exhaust gas previously stored in the reservoir  126  is flowed back into the cylinder  108  during the compression stroke and compression stroke to provide an internal EGR flow pattern to reduce peak temperature during combustion to further reduce emissions. 
     Note that the above is shown merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
       FIG. 6  shows a method  600  for operating of an engine system. The method  600  may be implemented by the engine system described above with regard to  FIGS. 1-5  or another suitable engine system. 
     At  602  the method includes moving a second piston within a hollow body of a first piston away from a piston rod of the first piston in an upward direction to flow gas into a reservoir between the first piston and the second piston from a cylinder during an exhaust stroke. Next at  604  the method includes moving the second piston in the upward direction to flow gas into the reservoir from the cylinder during an intake stroke. In this way, the reservoir may be filled with exhaust gas, intake air, and hydrocarbons (e.g., unburnt fuel vapor) trapped between a cylinder wall and an outer surface of the first piston during both the exhaust and intake strokes. 
     Next at  606  the method includes moving the second piston down towards the piston rod in a downward direction to flow combustion gas from the reservoir to the cylinder during a compression stroke. At  608  the method includes moving the second piston in the downward direction to flow gas from the reservoir to the cylinder during an expansion stroke. In this way, the exhaust gas, intake air, and hydrocarbons stored in the reservoir during the exhaust and intake strokes can be released into the cylinder and combusted during the expansion stroke. It will be appreciated the exhaust stroke, intake stroke, compression stroke, and expansion stroke are successively implemented during a single combustion cycle. 
     Further in one example, the gas flows into and out of the reservoir through a vent passage including a first end opening into the reservoir and a second end opening into a gap between a cylinder wall and an outer surface of the first piston. In an additional example, the first end is positioned vertically below a piston ring coupled to an outer surface of the first piston. In such an example, the amount of hydrocarbons trapped between the cylinder wall and the outer surface of the piston can be reduced. 
     The figures show example configurations with relative positioning of the various components. If shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. As yet another example, elements shown above/below one another, at opposite sides to one another, or to the left/right of one another may be referred to as such, relative to one another. Further, as shown in the figures, a topmost element or point of element may be referred to as a “top” of the component and a bottommost element or point of the element may be referred to as a “bottom” of the component, in at least one example. As used herein, top/bottom, upper/lower, above/below, may be relative to a vertical axis of the figures and used to describe positioning of elements of the figures relative to one another. As such, elements shown above other elements are positioned vertically above the other elements, in one example. As yet another example, shapes of the elements depicted within the figures may be referred to as having those shapes (e.g., such as being circular, straight, planar, curved, rounded, chamfered, angled, or the like). Further, elements shown intersecting one another may be referred to as intersecting elements or intersecting one another, in at least one example. Further still, an element shown within another element or shown outside of another element may be referred as such, in one example. 
     The subject matter of the present disclosure is further described in the following paragraphs. According to one aspect, an engine system is provided. The engine system includes a first piston movable within a cylinder, a second piston movable within a hollow body of the first piston, the movement defining a boundary of a reservoir within the hollow body, a spring device coupled to the first piston and the second piston, and a vent passage fluidically connecting the reservoir to the cylinder. 
     In another aspect, a method for operating an engine system is provided. The method includes moving a second piston within a hollow body of a first piston away from a piston rod of the first piston in an upward direction to flow gas into a reservoir between the first piston and the second piston from a cylinder during an exhaust stroke and moving the second piston down towards the piston rod in a downward direction to flow combustion gas from the reservoir to the cylinder during a compression stroke. 
     In another aspect, an engine system is provided. The engine system includes a first piston movable within a cylinder, a second piston movable within a hollow body of the first piston, the movement defining a boundary of a reservoir within the hollow body, a spring device coupled to the first piston and the second piston, and a vent passage fluidically connecting the reservoir to the cylinder and including a first end opening into the reservoir and a second end opening into a gap between a cylinder wall and an outer surface of the first piston. 
     In any of the aspects described herein or combinations of the aspects, the vent passage can include a first end opening into the reservoir and a second end opening into a gap between a cylinder wall and an outer surface of the first piston. 
     In any of the aspects described herein or combinations of the aspects, the engine system may further include a piston ring coupled to the outer surface of the first piston vertically below the second end of the vent passage. 
     In any of the aspects described herein or combinations of the aspects, the first end may be positioned vertically below the piston ring. 
     In any of the aspects described herein or combinations of the aspects, a volume of the reservoir may increase during an exhaust stroke and an intake stroke of the first piston, the increase in reservoir volume generating gas flow from the cylinder to the reservoir through the vent passage. 
     In any of the aspects described herein or combinations of the aspects, a volume of the reservoir can decrease during a compression stroke and a expansion stroke of the first piston, the decrease in reservoir volume generating gas flow from the reservoir to the cylinder through the vent passage. 
     In any of the aspects described herein or combinations of the aspects, the vent passage may extend in an upward direction away from a piston rod of the first piston. 
     In any of the aspects described herein or combinations of the aspects, the spring device may be positioned within the reservoir. 
     In any of the aspects described herein or combinations of the aspects, the engine system may further include a direct fuel injector extending into the cylinder. 
     In any of the aspects described herein or combinations of the aspects, the method may further include moving the second piston in the upward direction to flow gas into the reservoir from the cylinder during an intake stroke. 
     In any of the aspects described herein or combinations of the aspects, the method may further include moving the second piston in the downward direction to flow gas from the reservoir to the cylinder during an expansion stroke. 
     In any of the aspects described herein or combinations of the aspects, the gas flowing into and out of the reservoir can include unburnt fuel vapor. 
     In any of the aspects described herein or combinations of the aspects, the gas can flow into and out of the reservoir through a vent passage including a first end opening into the reservoir and a second end opening into a gap between a cylinder wall and an outer surface of the first piston. 
     In any of the aspects described herein or combinations of the aspects, the first end may be positioned vertically below a piston ring coupled to an outer surface of the first piston. 
     In any of the aspects described herein or combinations of the aspects, a volume of the reservoir can increase during an exhaust stroke and an intake stroke of the first piston, the increase in reservoir volume generating gas flow from the cylinder to the reservoir through the vent passage. 
     In any of the aspects described herein or combinations of the aspects, a volume of the reservoir can decrease during a compression stroke and an expansion stroke of the first piston, the decrease in reservoir volume generating gas flow from the reservoir to the cylinder through the vent passage. 
     In any of the aspects described herein or combinations of the aspects, the vent passage may extend in an upward direction away from a piston rod of the first piston. 
     In any of the aspects described herein or combinations of the aspects, the engine system may further include a piston ring coupled to the outer surface of the first piston vertically below the second end of the vent passage. 
     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. Further, one or more of the various system configurations may be used in combination with one or more of the described diagnostic routines. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein.