Patent Publication Number: US-11028694-B2

Title: Valve train for opposed-piston four-stroke engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/564,044, filed on Sep. 27, 2017. The entire disclosure of the application referenced above is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to valve trains for opposed-piston four-stroke engines. 
     BACKGROUND 
     The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     An opposed-piston engine includes an engine block defining one or more cylinders, a pair of pistons disposed within each cylinder, a crankshaft connected to each of the pistons, and one or more fuel injectors that inject fuel into each cylinder. Combustion of an air/fuel mixture within the cylinder causes the pistons to translate toward one another and away from one another, which drives rotation of the crankshaft. The engine block also defines an intake port that allows intake air to enter the cylinder, and an exhaust port that allow exhaust gas to be expelled from the cylinder. 
     In an opposed-piston two-stroke (OP2S) engine, the intake and exhaust ports typically extend through the sidewall of each cylinder and are disposed near opposite ends of the cylinder. When the pistons pass the intake and exhaust ports as the pistons are moving away from each other during a combustion or power stroke, intake air is drawn through the intake port while exhaust gas is expelled through the exhaust port. When the pistons pass the intake and exhaust ports as the pistons are moving toward each other during a compression stroke, the pistons prevent flow through the intake and exhaust ports. Since movement of the pistons controls flow through the intake and exhaust ports, there is no need for intake or exhaust valves. 
     Some opposed-piston four-stroke (OP4S) engines also control flow through the intake and exhaust ports using piston movement rather than intake and exhaust valves. In such an OP4S engine, intake air is drawn into the cylinder and exhaust gas is expelled from the cylinder at different times. Intake air is drawn into the cylinder when the pistons pass the intake and exhaust ports as the pistons move away from each other during an intake stroke. Fuel is injected into the cylinder, and the air/fuel mixture is compressed as the pistons move toward each other during a compression stroke. This compression causes the air/fuel mixture to ignite, and the combustion pressure urges the pistons to move away from each other during a combustion or power stroke. The pistons once again pass the intake and exhaust ports, and exhaust gas is expelled from the cylinder as the pistons move toward each other during an exhaust stroke. 
     Controlling flow through the intake and exhaust ports using piston movement limits the ability to adjust the timing and amount of flow through the intake and exhaust ports relative to controlling flow through the intake and exhaust ports using intake and exhaust valves. While attempts have been made to design an OP4S engine that controls flow through the intake and exhaust ports using intake and exhaust valves, the attempts have resulted in cost, manufacturing, assembly, serviceability, and performance issues. 
     SUMMARY 
     An example of an opposed-piston engine according to the present disclosure includes an engine block, at least two intake valves, and at least two exhaust valves. The engine block includes a first center section and a second center section. The first center section defines a first cylinder half bore having a first longitudinal axis and a first open end. The second center section defining a second cylinder half bore having a second longitudinal axis and a second open end. The second longitudinal axis of the second cylinder half bore is offset from the first longitudinal axis of the first cylinder half bore. The second open end of the second cylinder half bore overlaps the first open end of the first cylinder half bore to form an opening between the first and second cylinder half bores. The opening places the first and second cylinder half bores in fluid communication with one another to form a single cylinder. The at least two intake valves are arranged at the first open end of the first cylinder half bore. The at least two exhaust valves are arranged at the second open end of the second cylinder half bore. 
     In one example, the at least two intake valves includes three intake valves, and the at least two exhaust valves includes three exhaust valves. 
     In one example, the size of each of the exhaust valves is equal to the size of each of the intake valves. 
     In one example, the opposed-piston engine further includes a single intake camshaft that actuates all of the intake valves, and a single exhaust camshaft that actuates all of the exhaust valves. 
     In one example, all of the intake valves have the same lift profile, and all of the exhaust valves have the same lift profile. 
     In one example, the opposed-piston engine further includes an intake rocker arm shaft, first intake rocker arms pivotally mounted on the intake rocker arm shaft and configured to open first and second ones of the intake valves when the first intake rocker arms engage first lobes on the intake camshaft, a second intake rocker arm pivotally mounted on the intake rocker arm shaft and configured to open a third one of the intake valves when the first intake rocker arms engage a second lobe on the intake camshaft, an exhaust rocker arm shaft, first exhaust rocker arms pivotally mounted on the exhaust rocker arm shaft and configured to open first and second ones of the exhaust valves when the first exhaust rocker arms engage first lobes on the exhaust camshaft, and a second exhaust rocker arm pivotally mounted on the exhaust rocker arm shaft and configured to open a third one of the exhaust valves when the first exhaust rocker arms engage a second lobe on the exhaust camshaft. 
     In one example, the first and second intake valves are located a first distance from the intake rocker arm shaft, the third intake valve is located a second distance from the intake camshaft that is different than the first distance, the first and second exhaust valves are located a third distance from the exhaust rocker arm shaft, and the third exhaust valve is located a fourth distance from the exhaust rocker arm shaft that is different than the third distance. 
     In one example, the second distance is less than the first distance, and the fourth distance is less than the third distance. 
     In one example, each of the first intake rocker arms has a first length, the second intake rocker arm has a second length that is less than the first length of each of the first intake rocker arms, each of the first exhaust rocker arms has a third length, and the second exhaust rocker arm has a fourth length that is less than the third length of each of the first exhaust rocker arms. 
     In one example, each of the first lobes on the intake camshaft has a first height, the second lobe on the intake camshaft has a second height that is greater than the first height, each of the first lobes on the exhaust camshaft has a third height, and the second lobe on the exhaust camshaft has a fourth height that is greater than the third height. 
     In one example, the intake valves include stems that are oriented parallel to the first longitudinal axis of the first cylinder half bore, and the exhaust valves include stems that are oriented parallel to the second longitudinal axis of the second cylinder half bore. 
     In one example, the opposed-piston engine further includes a first fuel injector positioned at or near the first longitudinal axis of the first cylinder half bore and a second fuel injector positioned at or near the first longitudinal axis of the second cylinder half bore, the intake valves are positioned around the first fuel injector along an outer perimeter of the first cylinder half bore, and the exhaust valves are positioned around the second fuel injector along an outer perimeter of the second cylinder half bore. 
     In one example, the first and second center sections are formed separate from one another. 
     Another example of an opposed-piston engine according to the present disclosure includes an engine block, M intake valves, and N exhaust valves. The engine block includes a first center section and a second center section. The first center section defines a first cylinder half bore having a first longitudinal axis and a first open end. The second center section defines a second cylinder half bore having a second longitudinal axis and a second open end. The second longitudinal axis of the second cylinder half bore is offset from the first longitudinal axis of the first cylinder half bore. The second open end of the second cylinder half bore overlaps the first open end of the first cylinder half bore to form an opening between the first and second cylinder half bores. The opening places the first and second cylinder half bores in fluid communication with one another to form a single cylinder. The M intake valves are arranged at the first open end of the first cylinder half bore and configured to control the flow of intake air into the cylinder. The N exhaust valves arranged at the second open end of the second cylinder half and configured to control the flow of exhaust gas out of the cylinder. M and N are integers greater than one, and N is equal to M. 
     In one example, M and N are each equal to three. 
     In one example, the size of each of the exhaust valves is equal to the size of each of the intake valves. 
     In one example, the opposed-piston engine further includes a single intake camshaft that actuates all of the intake valves, and a single exhaust camshaft that actuates all of the exhaust valves. 
     Another opposed-piston engine according to the present disclosure includes an engine block, a rocker arm shaft, first and second rocker arms, a first valve (e.g., an intake valve or an exhaust valve), and a second valve (e.g., an intake valve or an exhaust valve). The engine block includes a first center section and a second center section. The first center section defines a first cylinder half bore having a first longitudinal axis and a first open end. The second center section defines a second cylinder half bore having a second longitudinal axis and a second open end. The second longitudinal axis of the second cylinder half bore is offset from the first longitudinal axis of the first cylinder half bore. The second open end of the second cylinder half bore overlaps the first open end of the first cylinder half bore to form an opening between the first and second cylinder half bores. The opening places the first and second cylinder half bores in fluid communication with one another to form a single cylinder. At least one of the first and second center sections defining a rocker arm shaft bore. The rocker arm shaft is received in the rocker arm shaft bore. The first and second rocker arms are pivotally mounted to the rocker arm shaft. The first valve is arranged at the first open end of the first cylinder half bore, actuated by the first rocker arm, and located a first distance from the rocker arm shaft. The second valve is arranged at the first open end of the first cylinder half bore, actuated by the second rocker arm, and located a second distance from the rocker arm shaft that is different than the first distance. 
     In one example, the opposed-piston engine further includes a third rocker arm pivotally mounted on the rocker arm shaft, and a third valve arranged at the first open end of the first cylinder half bore, actuated by the second rocker arm, and located the first distance from the rocker arm shaft. 
     In one example, the second distance is less than the first distance. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an opposed-piston four-stroke (OP4S) engine including a valve train according to the present disclosure; 
         FIG. 2  is an exploded perspective view the OP4S engine of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the OP4S engine of  FIG. 1  taken along a line  3 - 3  shown in  FIG. 1 , 
         FIG. 4  is a perspective view of a portion of the OP4S engine of  FIG. 1  including intake or exhaust valves, a fuel injector, an opening between two cylinder half bores, and an open end of one of the cylinder half bores; 
         FIG. 5  is perspective views of the valve train included in the OP4S engine of  FIG. 1 ; 
         FIG. 6  is an exploded perspective view of the valve train included in the OP4S engine of  FIG. 1 ; 
         FIG. 7  is a perspective view of a portion of a camshaft included in the valve train of the OP4S engine of  FIG. 1 ; and 
         FIG. 8  is a graph illustrating lift profiles of intake and exhaust valves in the valve train included in the OP4S engine of  FIG. 1 . 
     
    
    
     In the drawings, reference numbers may be reused to identify similar and/or identical elements. 
     DETAILED DESCRIPTION 
     In an effort to improve the volumetric efficiency of an engine and thereby improve the power output and fuel efficiency of the engine, attempts have been made to increase the number of intake and exhaust valves per cylinder. For example, in some engines that have a single piston disposed within each cylinder and a cylinder head closing off the open end of each cylinder, the number of intake and exhaust valves per cylinder has been increased to four or five valves per cylinder (e.g., two intake valves per cylinder and two exhaust valves per cylinder). However, the gas exchange in these types of engines is not as efficient as possible since intake air and exhaust gas flows into and out of each cylinder in opposite directions. Thus, the difference in volumetric efficiency between a cylinder-head engine with two intake valves per cylinder and a cylinder-head engine with three intake valves per cylinder is relatively small and may be offset by the cost and reliability concerns of adding an intake valve to each cylinder. 
     As indicated above, attempts have been made to design an OP4S engine that controls flow through the intake and exhaust ports using intake and exhaust valves. In these OP4S engines, each cylinder of the OP4S engine is split into two halves, and the two cylinder half bores are offset relative to each other to provide packaging space for valve trains. These OP4S engines provide a more efficient gas exchange process relative to cylinder-head engines since intake air and exhaust gas flows into and out of each cylinder in the same direction. However, the number and arrangement of the intake and exhaust valves in the OP4S engine have not been optimized to maximize volumetric efficiency or to provide the most improvement in the gas exchange process. 
     An OP4S engine according to the present disclosure includes three or more intake valves arranged at the open end of a first cylinder half bore, and the same number and size (e.g., disk diameter) of exhaust valves arranged at the open end of a second cylinder half bore. The first and second cylinder half bores are in fluid communication with one another to form a single cylinder. The OP4S engine may have multiple cylinders and therefore may include multiple pairs of the first and second cylinder half bores. 
     The equal number and size of the intake and exhaust valves allows for an efficient gas exchange process while using a robust valve actuation mechanism. The separation of the intake and exhaust valves on opposite sides of the OP4S engine allows flexibility in the size and position of the valves on either side. In addition, compared to cylinder-head engines, the difference in volumetric efficiency between an OP4S engine with two intake valves per cylinder and an OP4S engine with three intake valves per cylinder is significant. Thus, the difference in volumetric efficiency realized by adding an intake valve to each cylinder of an OP4S engine likely outweighs the cost and reliability concerns of adding an intake valve to each cylinder of an OP4S engine. 
     Referring now to  FIGS. 1-3 , an opposed-piston four-stroke (OP4S) engine  10  includes an engine block  12 , five pairs of first and second cylinder half liners  14  and  16 , first and second crankshafts  18  and  20 , first and second crank bearing main caps or saddles  22  and  24 , five pairs of first and second pistons  26  and  28 , intake and exhaust manifolds  30  and  32 , intake and exhaust valve trains  34  and  36 , and five pairs of first and second fuel injectors  38  and  39 . The engine block  12  includes first and second center sections  40  and  42  and first and second lower crankcases  44  and  46 . The first and second center sections  40  and  42  and the first and second lower crankcases  44  and  46  are formed (e.g., cast) separately (e.g., from iron). The first center section  40  defines five pairs of first and second cylinder half bores  48  and  50 . Each of the first cylinder half bores  48  is in fluid communication with one of the second cylinder half bores  50  through an opening  52  in the engine block  12 . Therefore, each of the pairs of the first and second cylinder half bores  48  and  50  collectively form a single cylinder within which the first and second pistons  26  and  28  reciprocate. In addition, since the OP4S engine  10  includes five pairs of the first and second cylinder half bores  48  and  50 , the OP4S engine  10  has five cylinders. However, the OP4S engine  10  may have less than five cylinders (e.g., one cylinder) or more than five cylinders. 
     Each of the first cylinder half bores  48  has an open end  49 , and each of the second cylinder half bores  50  has an open end  51 . The open end  51  of each of the second cylinder half bores  50  overlaps the open end  49  of one of the first cylinder half bores  48  to form the opening  52 . In this regard, the opening  52  may be defined by the first and second cylinder half liners  14  and  16  rather than the engine block  12 . 
     A longitudinal axis  54  of the first cylinder half bore  48  is disposed within the same plane as a longitudinal axis  56  of the first crankshaft  18 . Similarly, a longitudinal axis  58  of the second cylinder half bore  50  is disposed within the same plane as a longitudinal axis  60  of the second crankshaft  20 . In addition, the longitudinal axes  54 ,  58  of the first and second cylinder half bores  48  and  50  are aligned with one another in a longitudinal direction X that is parallel to the longitudinal axes  56 ,  60  of the first and second crankshafts  18  and  20 . Further, the longitudinal axes  54 ,  58  of the first and second cylinder half bores  48  and  50  are offset from one another in a vertical direction Z to provide packaging space for the intake and exhaust valve trains  34  and  36 . 
     The first and second center sections  40  and  42  are joined to one another using first threaded rods  62 , which extend through the first and second center sections  40  and  42  and have first nuts (not shown) threaded onto opposite ends thereof. Each of the first and second center sections  40  and  42  defines a camshaft bore  70 , a rocker arm shaft bore  72 , a plurality of fastener bores  74 , a plurality of valve train openings  76 , and a plurality of manifold openings  78 . The camshaft bore  70  in the first center section  40  receives an intake camshaft  80  of the intake valve train  34 , and the camshaft bore  70  in the second center section  42  receives an exhaust camshaft  82  of the exhaust valve train  36 . The rocker arm shaft bore  72  in the first center section  40  receives an intake rocker arm shaft  84  in the intake valve train  34 , and the rocker arm shaft bore  72  and the second center section  42  receives an exhaust rocker arm shaft  86  in the exhaust valve train  36 . 
     Each of the valve train openings  76  in the first and second center sections  40  and  42  receives a valve cover (not shown). In addition, the valve train openings  76  in the first and second center sections  40  and  42  can receive fuel lines (not shown) that provide fuel to the first and second fuel injectors  38  and  39 . The manifold openings  78  place the intake and exhaust manifolds  30  and  32  in fluid communication with intake and exhaust ports  114  and  118 , respectively. 
     The first crank bearing saddles  22  and the first lower crankcase  44  are joined to the first center section  40  using second threaded rods  94  and second nuts  96  threaded onto the second threaded rods  94 . Similarly, the second crank bearing saddles  22  and the second lower crankcase  46  are joined to the second center section  42  using the second threaded rods  94  and the second nuts  96 . The first crank bearing saddles  22  are joined to the first lower crankcase  44  using third threaded rods  98  and third nuts  100  threaded onto the third threaded rods  98 . Similarly, the second crank bearing saddles  24  are joined to the second lower crankcase  46  using the third threaded rods  98  and the third nuts  100 . The first threaded rods  62  have a first length, the second threaded rods  94  have a second length that is less than the first length, and the third threaded rods  98  have a third length that is less than the second length. In various implementations, the second threaded rods  94 , the second nuts  96 , the third threaded rods  98 , and/or the third nuts  100  may be replaced with screws or bolts. 
     The first crank bearing saddles  22  and the first lower crankcase  44  form a first crankcase assembly, while the second crank bearing saddles  22  and the second lower crankcase  46  form a second crankcase assembly. The first crankshaft  18  is positioned between the first crank bearing saddles  22  and the first lower crankcase  44  before the second and third threaded rods  94  and  98  are inserted through the first crankcase assembly. Thus, the first crankshaft  18  is captured between the first crank bearing saddles  22  and the first lower crankcase  44 . Similarly, the second crankshaft  20  is positioned between the second crank bearing saddles  24  and the second lower crankcase  46  before the second and third threaded rods  94  and  98  are inserted through the second crankcase assembly. Thus, the second crankshaft  20  is captured between the second crank bearing saddles  24  and the second lower crankcase  46 . 
     With specific reference to  FIG. 3 , each first cylinder half liner  14  is placed within one of the first cylinder half bores  48 , and each second cylinder half liner  16  is placed within one of the second cylinder half bores  50 . Each first piston  26  reciprocates within one of the first cylinder half liners  14 , and each second piston  28  reciprocates within one of the second cylinder half liners  16 . Each of the first and second pistons  26  and  28  includes a connecting rod  108 , a piston head  110 , and a wristpin  112 . The connecting rod  108  of the first piston  26  connects the piston head  110  of the first piston  26  to the first crankshaft  18 . Similarly, the connecting rod  108  of the second piston  28  connects the piston head  110  of the second piston  28  to the second crankshaft  20 . The wristpins  112  join the piston heads  110  to the connecting rods  108  while allowing the connecting rods  108  to pivot with respect to the piston heads  110 . 
     The first center section  40  defines the intake ports  114  and intake valve bores  116 , the second center section  42  defines the exhaust ports  118  and exhaust valve bores  120 , and each of the first and second center sections  40  and  42  defines a fuel injector bore  122 . Each of the intake ports  114  is in fluid communication with the intake manifold  30  via the manifold openings  78 , and each of the exhaust ports  118  is in fluid communication with the exhaust manifold  32  via the manifold openings  78 . Each of the fuel injector bores  122  receives one of the first and second fuel injectors  38  and  39 . 
     Referring now to  FIGS. 4-7 , the intake valve train  34  includes the intake camshaft  80 , the intake rocker arm shaft  84 , first and second intake rocker arms  124  and  126 , intake valves  128 , and intake rocker arm springs  130 . As best shown in  FIG. 4 , the intake valve train  34  includes three of the intake valves  128  for each of the first cylinder half bores  48 . Two of the three intake valves  128  are actuated by two of the first intake rocker arms  124 , and one of the thee intake valves  128  is actuated by one of the second intake rocker arms  126 . The three intake valves  128  are positioned around the outer perimeter of the corresponding first cylinder half bore  48 , which provides space for each first fuel injector  38  and/or a spark plug to be located at or near the longitudinal axis  54  of the corresponding first cylinder half bore  48 . Each of the intake valve bores  116  in the first center section  40  receives one of the intake valves  128 . 
     The intake camshaft  80  is driven by the first crankshaft  18 . The first intake rocker arms  124  pivot about the intake rocker arm shaft  84  when first rollers  132  on the first intake rocker arms  124  engage first lobes  134  on the intake camshaft  80 . Similarly, the second intake rocker arms  126  pivot about the intake rocker arm shaft  84  when second rollers  136  on the second intake rocker arms  126  engage second lobes  138  on the intake camshaft  80 . The intake rocker arm springs  130  bias the first and second intake rocker arms  124  and  126  into engagement with the first and second lobes  134  and  138  on the intake camshaft  80 . When the first and second intake rocker arms  124  and  126  pivot about the intake rocker arm shaft  84  due to engagement with the first and second lobes  134  and  138  on the intake camshaft  80 , the intake valves  128  unseat from the intake ports  114  and move further into the corresponding first cylinder half bore  48 . This allows intake air to be drawn into the first cylinder half bore  48  and the corresponding second cylinder half bore  50 . 
     The intake valves  128  actuated by the first intake rocker arms  124  are located a first distance D 1  from the intake rocker arm shaft  84 , and the intake valves  128  actuated by the second intake rocker arms  126  are located a second distance D 2  from the intake rocker arm shaft  84 . In addition, the intake rocker arm shaft  84  may extend through the center of gravity of each of the first and second intake rocker arms  124  and  126 . Thus, each of the first intake rocker arms  124  has a first length L 1  and each of the second intake rocker arms  126  has a second length L 2 . The second length L 2  of each of the second intake rocker arms  126  is less than the first length L 1  of each of the first intake rocker arms  124 , which allows both the first and second intake rocker arms  124  and  126  to be mounted to the intake rocker arm shaft  84  and to be actuated by the intake camshaft  80 . In other words, the difference between the first and second length L 1  and L 2  of the first and second intake rocker arms  124  and  126  enables the first and second intake rocker arms  124  and  126  to be mounted to a common rocker pivot shaft and to be actuated by a single camshaft. The difference between the first and second lengths L 1  and L 2  may be two times the difference between the first and second distances D 1  and D 2 . 
     The difference between the first and second lengths L 1  and L 2  of the first and second intake rocker arms  124  and  126  enables the intake valves  128  actuated by the first and second intake rocker arms  124  and  126  to be located at the different distances D 1  and D 2  from the intake rocker arm shaft  84 . Thus, the difference between the first and second lengths L 1  and L 2  of the first and second intake rocker arms  124  and  126  makes it possible to arrange three or more of the intake valves  128  around the outer perimeter of each first cylinder half bore  48  as shown in  FIG. 4  without mounting the first and second intake rocker arms  124  and  126  to different rocker arm shafts. As a result, the volumetric efficiency of the OP4S engine  10  is improved relative to OP4S engines that have a fewer number of intake valves per cylinder. To this end, a cylinder having three intake valves arranged as shown in  FIG. 4  may have 30 percent more available area for intake air to flow through relative to a cylinder that only has two intake valves. In addition, the packaging space required by the intake valve train  34  is less compared to intake valve trains that have multiple rocker arm shafts. Further, the stems of the intake valves  128  can be oriented parallel to the longitudinal axis  54  of the first cylinder half bore  48  (e.g., when the OP4S engine  10  is a compression-ignition engine) or inclined relative to the longitudinal axis  54  (when the OP4S engine  10  is a spark-ignition engine). 
     In addition, the intake valves  128  actuated by the first and second intake rocker arms  124  and  126  may have different lever ratios so that all of the intake valves  128  have the same lift profile despite the difference between the first and second lengths L 1  and L 2 . A lever ratio of an intake or exhaust valve is a ratio of cam lobe lift to valve lift. The intake valves  128  actuated by the first intake rocker arms  124  may have a first lever ratio between 1.5 and 2 (e.g., 1.75), and the intake valves  128  actuated by the second intake rocker arms  126  may have a second lever ratio between 1 and 1.5 (e.g., 1.1). Further, to achieve these different lever ratios, the first and second lobes  134  and  138  on the intake camshaft  80  may have different outer profiles. For example, the first lobes  134  on the intake camshaft  80  may have a first height H 1 , and the second lobes  138  on the intake camshaft  80  may have a second height H 2  that is greater than the first height H 1 . 
     The exhaust valve train  36  includes the exhaust camshaft  82 , the exhaust rocker arm shaft  86 , first and second exhaust rocker arms  140  and  142 , exhaust valves  144 , and exhaust rocker arm springs  146 . The exhaust valve train  36  includes three of the exhaust valves  144  for each of the second cylinder half bores  50 . Two of the three exhaust valves  144  are actuated by two of the first exhaust rocker arms  140 , and one of the three exhaust valves  144  is actuated by one of the second exhaust rocker arms  142 . The three exhaust valves  144  are positioned around one of the second fuel injectors  39  similar to the way in which three of the intake valves  128  are positioned around one of the first fuel injectors  38  in  FIG. 4 . Each second fuel injector  39  is located at or near the longitudinal axis  56  of the corresponding second cylinder half bore  50 , and three of the exhaust valves  144  are positioned around each second fuel injector  39  along the outer perimeter of the corresponding second cylinder half bore  50 . Each of the exhaust valve bores  120  in the second center section  42  receives one of the exhaust valves  144 . 
     The exhaust camshaft  82  is driven by the second crankshaft  20 . The first exhaust rocker arms  140  pivot about the exhaust rocker arm shaft  86  when first rollers  148  on the first exhaust rocker arms  140  engage first lobes  150  on the exhaust camshaft  82 . Similarly, the second exhaust rocker arms  142  pivot about the exhaust rocker arm shaft  86  when second rollers  152  on the second exhaust rocker arms  142  engage second lobes  154  on the exhaust camshaft  82 . The exhaust rocker arm springs  146  bias the first and second exhaust rocker arms  140  and  142  into engagement with the first and second lobes  150  and  154  on the exhaust camshaft  82 . When the first and second exhaust rocker arms  140  and  142  pivot about the exhaust rocker arm shaft  86  due to engagement with the first and second lobes  150  and  154  on the exhaust camshaft  82 , the exhaust valves  144  unseat from the exhaust ports  118  and move further into the corresponding second cylinder half bore  50 . This allows exhaust gas to be expelled from the first and second cylinder half bores  48  and  50 . 
     The exhaust valves  144  actuated by the first exhaust rocker arms  140  are located a third distance D 3  ( FIG. 3 ) from the intake rocker arm shaft  84 , and the exhaust valves  144  actuated by the second exhaust rocker arms  142  are located a fourth distance D 4  ( FIG. 3 ) from the exhaust rocker arm shaft  86 . In addition, the exhaust rocker arm shaft  86  may extend through the center of gravity of each of the first and second exhaust rocker arms  140  and  142 . Thus, each of the first exhaust rocker arms  140  has a third length L 3  and each of the second exhaust rocker arms  142  has a fourth length L 4 . The fourth length L 4  of each of the second exhaust rocker arms  142  is less than the third length L 3  of each of the first exhaust rocker arms  140 , which allows both the first and second exhaust rocker arms  140  and  142  to be mounted to the exhaust rocker arm shaft  86  and to be actuated by the exhaust camshaft  82 . In other words, the difference between the third and fourth length L 3  and L 4  of the first and second exhaust rocker arms  140  and  142  enables the first and second exhaust rocker arms  140  and  142  to be mounted to a common rocker pivot shaft and to be actuated by a single camshaft. The difference between the third and fourth length L 3  and L 4  may be two times the difference between the third and fourth distances D 3  and D 4 . 
     The difference between the third and fourth length L 3  and L 4  of the first and second exhaust rocker arms  140  and  142  enables the exhaust valves  144  actuated by the first and second exhaust rocker arms  140  and  142  to be located at the different distances D 3  and D 4  from the exhaust rocker arm shaft  86 . Thus, the difference between the third and fourth length L 3  and L 4  of the first and second exhaust rocker arms  140  and  142  makes it possible to arrange three or more of the exhaust valves  144  around each second cylinder half bore  50  similar to the arrangement of  FIG. 4  without mounting the first and second exhaust rocker arms  140  and  142  to different rocker arm shafts. As a result, the volumetric efficiency of the OP4S engine  10  is improved relative to OP4S engines that have a fewer number of exhaust valves per cylinder. To this end, a cylinder having three exhaust valves arranged as shown in  FIG. 4  may have 30 percent more available area for exhaust gas to flow through relative to a cylinder that only has two exhaust valves. In addition, the packaging space required by the exhaust valve train  36  is less compared to exhaust valve trains that have multiple rocker arm shafts. Further, the stems of the exhaust valves  144  can be oriented parallel to the longitudinal axis  58  of the second cylinder half bore  50  (e.g., when the OP4S engine  10  is a compression-ignition engine) or inclined relative to the longitudinal axis  58  (when the OP4S engine  10  is a spark-ignition engine). 
     The exhaust valves  144  actuated by the first and second exhaust rocker arms  140  and  142  may have different lever ratios so that all of the exhaust valves  144  have the same lift profile despite the difference between the third and fourth length L 3  and L 4 . The exhaust valves  144  actuated by the first exhaust rocker arms  140  may have a third lever ratio between 1.5 and 2 (e.g., 1.75), and the exhaust valves  144  actuated by the second exhaust rocker arms  142  may have a fourth lever ratio between 1 and 1.5 (e.g., 1.1). In addition, to achieve these different lever ratios, the first and second lobes  150  and  154  on the exhaust camshaft  82  may have different outer profiles. For example, the heights of the first and second lobes  150  and  154  on the exhaust camshaft  82  may differ just as the heights of the first and second lobes  134  and  138  on the intake camshaft  80  may differ. 
     Referring now to  FIGS. 3 and 5 , all of the intake valves  128  are arranged on one side of a centerline  156  of the OP4S engine  10 , and all of the exhaust valves  144  are arranged on the other side of the centerline  156 . This arrangement improves the flow of intake air and exhaust gas through cylinders of the OP4S engine  10 , and reduces the complexity of the OP4S engine  10  relative to an engine that has both intake and exhaust valves arranged on one side thereof. To this end, an engine that has both intake and exhaust valves arranged on one side thereof required both intake and exhaust runners to be routed to that side. In contrast, the OP4S engine  10  only requires one of an intake runner or an exhaust runner to be routed to each side thereof. 
     Referring again to  FIGS. 1-3 , the OP4S engine  10  further includes a gear train  158 , a turbocharger  160 , an intake line  162 , a first side cover  164 , a second side cover  166 , a first end cover  168 , and a second end cover (not shown). The gear train  158  includes first and second crankshaft gears  170  and  172 , first and second camshaft gears  174  and  176 , first and second idler gears  178  and  180 , an output shaft gear  182 , a fuel pump gear  184 , and oil scavenge pump gears  186 . The first and second crankshaft gears  170  and  172  are coupled to the first and second crankshafts  18  and  20 , respectively. The first and second camshaft gears  174  and  176  are coupled to the intake and exhaust camshafts  80  and  82 , respectively. The output shaft gear  182 , the fuel pump gear  184 , and the oil scavenge pump gears  186  may be coupled to an output shaft (not shown), a fuel pump (not shown), and oil scavenge pumps (not shown), respectively. 
     The first and second idler gears  178  and  180  connect the first and second crankshaft gears  170  and  172  to the first and second camshaft gears  174  and  176  and the output shaft gear  182 . Each of the first idler gears  178  is engaged with one of the first and second crankshaft gears  170  and  172 , one of the second idler gears  180 , and the output shaft gear  182 . Each of the second idler gears  180  is engaged with one of the first idler gears  178  and one of the first and second camshaft gears  174  and  176 . Thus, the first and second crankshafts  18  and  20  drive the intake and exhaust camshafts  80  and  82  via the gear train  158 . In various implementations, the first and second crankshafts  18  and  20  may drive the intake and exhaust camshafts  80  and  82  using mechanisms other than gears, such as belts or chains. 
     The turbocharger  160  is fluidly coupled to the exhaust manifold  32  and is driven by exhaust gases flowing through the exhaust manifold  32 . The turbocharger  160  compresses intake air and provides the compressed intake air to the intake manifold  30  via the intake line  162 . The first side cover  164  encases one side of the OP4S engine  10 , and the second side cover  166  encases the other side of the OP4S engine  10 . The first end cover  168  encases one end of the OP4S engine  10 , and the second end cover encases the other end of the OP4S engine  10 . 
     With continued reference to  FIGS. 1-3 , operation of the OP4S engine  10  will now be described. As its name indicates, the OP4S engine  10  operates using four strokes—an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. During the intake stroke, the intake valves  128  are open (i.e., unseated from the intake ports  114 ), and the first and second pistons  26  and  28  move from the positions shown in  FIG. 4  in a direction away from each other. The motion of the first and second pistons  26  and  28  creates a vacuum that draws intake air through the intake ports  114  and into the first and second cylinder half bores  48  and  50 . In addition, the first and second fuel injectors  38  and  39  may inject fuel into the first and second cylinder half bores  48  and  50 , respectively, to yield an air-fuel mixture. 
     During the compression stroke, the intake valves  128  are closed (i.e., seated against the intake ports  114 ), and the first and second pistons  26  and  28  move toward one another to the positions shown in  FIG. 4 , which are commonly referred to as top dead center. When the first and second pistons  26  and  28  are near top and center, the pressure within the first and second cylinder half bores  48  and  50  causes the air-fuel mixture to ignite (i.e., if the OP4S engine  10  is a compression-ignition engine) or a spark produced by a spark plug (not shown) causes the air-fuel mixture to ignite (i.e., if the OP4S engine  10  is a spark-ignition engine). During the power stroke, a rapid pressure increase within the first and second cylinder half bores  48  and  50  resulting from combustion of the air-fuel mixture causes the first and second pistons  26  and  28  to move away from one another. As the first and second pistons  26  and  28  move away from one another, the first and second pistons  26  and  28  drive the first and second crankshaft  18  and  20 , respectively. 
     During the exhaust stroke, the exhaust valves are open (i.e., unseated from the exhaust ports  118 ), and the first and second pistons  26  and  28  move toward each other to the positions shown in  FIG. 4 . The motion of the first and second pistons  26  and  28  increases the pressure within the first and second cylinder half bores  48  and  50 , which forces exhaust gas out of the first and second cylinder half bores  48  and  50  through the exhaust ports  118 . A gasket  188  seals the interface between the first and second center sections  40  and  42  to prevent exhaust gas from escaping the first and second cylinder half bores  48  and  50  through a path other than the exhaust ports  118 . 
     Referring now to  FIG. 8 , an intake lift profile  190  and an exhaust lift profile  192  are plotted with respect to an x-axis  194  that represents crank angle in degrees and a y-axis  196  that represents valve lift in millimeters. As shown in  FIG. 8 , the intake lift profile  190  is different than the exhaust lift profile  192 . However, the intake valves  128  actuated by the first intake rocker arms  124  and the intake valves  128  actuated by the second intake rocker arms  126  may both have the intake lift profile  190 . Similarly, the exhaust valves  144  actuated by the first exhaust rocker arms  140  and the exhaust valves  144  actuated by the second exhaust rocker arms  142  may both have the exhaust lift profile  192 . The difference between the lever ratios of the first and second intake rocker arms  124  and  126  enables all of the intake valves  128  to have the same lift profile despite the difference between the lengths of the first and second intake rocker arms  124  and  126 . Similarly, the difference between the lever ratios of the first and second exhaust rocker arms  142  and  144  enables all of the exhaust valves  144  to have the same lift profile despite the difference between the lengths of the first and second intake rocker arms  124  and  126 . 
     The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     None of the elements recited in the claims are intended to be a means-plus-function element within the meaning of 35 U.S.C. § 112(f) unless an element is expressly recited using the phrase “means for,” or in the case of a method claim using the phrases “operation for” or “step for.”