Patent Abstract:
A barrier ring for a cylinder assembly for an opposed-piston engine fits into a groove fashioned into a portion of the cylinder liner that is adjacent to the top dead center location of the end surfaces of the pistons, in a volume of the cylinder liner that defines the combustion chamber. The barrier ring and groove are part of a barrier assembly that prevents heat generated during combustion from reaching the outer wall of the cylinder assembly, reducing the need for conventional cooling systems and increasing the amount of heat retained in the combustion chamber. The barrier assembly allows for increased engine efficiency because of the combustion heat retained in the combustion chamber, as well as a reduction in the overall size of the engine because of the reduction in engine cooling needed.

Full Description:
RELATED APPLICATIONS 
       [0001]    This disclosure includes material related to the disclosure of the following commonly-owned US Patent Applications: U.S. patent application Ser. No. 13/136,402; filed Jul. 29. 2011, now U.S. Pat. No. 8,485,147; U.S. patent application Ser. No. 13/385,127, filed Feb. 2, 2012, now U.S. Pat. No. 8,851,029; U.S. patent application Ser. No. 14/255,756, filed Apr. 7, 2014, now U.S. Pat. No. 9,121,365; pending U.S. patent application Ser. No. 14/675,340, filed Mar. 31, 2015; and pending U.S. patent application Ser. No. 14/732,496, filed Jun. 5, 2015. 
     
    
     FIELD 
       [0002]    The field includes opposed-piston engines. More particularly, the field relates to a barrier assembly, which includes a barrier ring, for a cylinder assembly constructed to reduce heat rejection from the cylinder assembly in an opposed-piston engine. 
       BACKGROUND 
       [0003]    Construction of an opposed-piston engine cylinder assembly is well understood. The cylinder assembly includes a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in a cylinder block. The liner includes a bore and longitudinally displaced intake and exhaust ports, machined or formed in the liner near respective ends thereof. Each of the intake and exhaust ports includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid portion of the cylinder wall (also called a “bridge”). An intermediate portion of the liner exists between the intake and exhaust ports. In an opposed-piston engine, two opposed, counter-moving pistons are disposed in the bore of a liner with their end surfaces facing each other. At the beginning of a power stroke, the opposed pistons reach respective top dead center (TDC) locations in the intermediate portion of the liner where they are in closest mutual proximity to one another in the cylinder. During a power stroke, the pistons move away from each other until they approach respective bottom dead center (BDC) locations in the end portions of the liner at which they are furthest apart from each other. In a compression stroke, the pistons reverse direction and move from BDC toward TDC. 
         [0004]    The intermediate portion of the cylinder lying between the intake and exhaust ports bounds a combustion chamber defined between the end surfaces of the pistons when the pistons are near their TDC locations. This intermediate portion bears the highest levels of combustion temperature and pressure that occur during engine operation. The presence of openings for engine components such as fuel injectors, valves, and/or sensors in the intermediate portion diminishes the cylinder assembly&#39;s strength and makes the cylinder liner vulnerable to cracking, particularly through the fuel injector and valve openings. 
         [0005]    Heat loss through the cylinder liner is a factor that degrades engine performance throughout the operating cycle of an opposed-piston engine. Combustion occurs as fuel is injected into air compressed between the piston end surfaces when the pistons are in close mutual proximity, forming the combustion chamber. Loss of the heat of combustion through the liner reduces the amount of energy available to drive the pistons apart in the power stroke. By limiting this heat loss, fuel efficiency would be improved, heat rejection to coolant would be reduced, and higher exhaust temperatures can be realized. Smaller cooling systems and lower pumping losses are just some of the benefits of limiting heat loss through the cylinder assembly. It is therefore desirable to retain as much of the heat of combustion as possible within the cylinder assembly. 
         [0006]    An opposed-piston cylinder assembly construction according to the present disclosure satisfies the objective of heat containment, thereby allowing opposed-piston engines to operate higher heat retention than opposed-piston engines of the prior art. 
       SUMMARY 
       [0007]    The highest concentration of heat in an opposed-piston engine cylinder assembly occurs in the annular portion of the cylinder liner between the top dead center (TDC) locations of the pistons, where combustion takes place. Nearly half of the total heat flux into the liner occurs in this annular portion. Accordingly, construction of a barrier ring for insertion into the cylinder liner in such a manner as to yield a high thermal resistance will reduce heat flux through the annular liner portion. 
         [0008]    In some implementations, provided herein is a barrier assembly that includes a barrier ring, a groove adjacent to the portion of the cylinder liner near the combustion chamber, and a space or gap between the barrier ring and the back wall of the groove. The combustion chamber is partially defined by a first end surface on a first piston and a second end surface on a second piston when the first and second pistons are near their top dead center positions in the cylinder assembly. In a related aspect, provided herein is a barrier ring for use in the barrier assembly. The barrier ring includes an open-ended tube with a wall defining a volume inside the tube. The tube includes a first and a second set of openings in the wall, in which the first set of openings allows for communication between engine hardware and the combustion chamber, and the second set of openings allows for pressure equalization between two volumes separated by the barrier ring. Methods of making and using the barrier ring and barrier assembly are also provided herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  shows a cross-section of a portion of a cylinder assembly from an opposed-piston engine with a compression sleeve and pistons received in a liner. 
           [0010]      FIG. 1B  shows the outer portion of the cylinder assembly of  FIG. 1A . 
           [0011]      FIG. 2A  is a three dimensional view of a portion of a cylinder liner with an installed barrier ring shown in shadow. 
           [0012]      FIG. 2B  is a schematic drawing of an opposed-piston engine with one or more cylinder assemblies according to this specification. 
           [0013]      FIG. 3  is a three-dimensional drawing of the barrier ring prior to installation into the cylinder bore. 
           [0014]      FIG. 4A  is a cross sectional view of a portion of the cylinder assembly and engine block with the opposing pistons at TDC and the barrier assembly. 
           [0015]      FIG. 4B  is an exploded partial view of the cylinder assembly and pistons of  FIG. 4A . 
           [0016]      FIG. 4C  is a variation of the cylinder assembly and barrier assembly shown in  FIG. 4B . 
           [0017]      FIGS. 5A-5C  are three exemplary configurations of a barrier ring, with the ring laid out flat prior to installation in the cylinder bore. 
           [0018]      FIG. 6  shows an exemplary barrier ring for use in a cylinder assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIGS. 1A and 1B  show an exemplary cylinder assembly for use in an opposed-piston engine. The cylinder assembly  16  includes a liner  20 , intake ports  25 , exhaust ports  29 , an external surface of the liner  42 , a compression sleeve  40 , and a bore  37 . Two pistons  35  and  36  are disposed within the bore  37 . The pistons  35  and  36  have end surfaces,  35   e  and  36   e,  respectively, that partially define the combustion chamber  41  when the pistons  35 ,  36  are at or near their respective top dead center (TDC) positions. The combustion chamber  41  is also partially defined by the cylinder liner  20  in the intermediate portion  34  of the cylinder. The intermediate portion  34  is located between the intake ports  25  and the exhaust ports  29 . Located in the intermediate portion  34 , at the periphery of the combustion chamber  41 , are openings  46  into which fuel injection components  45  and other engine components can fit. This exemplary cylinder assembly is described in detail in related U.S. patent application Ser. No. 14/675,340. 
         [0020]    The compression sleeve  40  is formed to define generally cylindrical space between itself and the external surface  42  of the liner through which a liquid coolant may flow in an axial direction from near the intake ports toward the exhaust ports. The intermediate portion  34  is reinforced by the compression sleeve  40 , as described in greater detail in U.S. patent application Ser. No. 14/675,340, and cooling fluid is circulated in the compression sleeve  40  in generally annular spaces  55  and  59 . The cooling fluid that circulates in these generally annular spaces  55 ,  59  flows to other components of the opposed-piston engine, not shown in  FIGS. 1A and 1B , that allow for heat to dissipate from the cooling fluid to the surrounding environment, such as a radiator. 
         [0021]      FIG. 2A  is a three dimensional view of a portion of a cylinder liner  20  with a barrier ring  200  installed. The barrier ring  200  is shown in shadow. The barrier ring  200  is located in the intermediate portion  34 , overlapping with the portion of the intermediate portion that includes one or more openings  46  for injection components, as well as the portion of the intermediate portion that encircles the combustion chamber.  FIG. 2B  illustrates an opposed-piston engine  100  with three cylinder assemblies  101 , in which each cylinder comprises a cylinder tunnel  103  in a cylinder block  105  and a cylinder liner  107  (reference number  20  in  FIGS. 1A-1C ) according to this specification seated in the cylinder tunnel. Of course, the number of cylinders is not meant to be limiting. In fact, the engine  100  may have fewer, or more, than three cylinders. Each cylinder assembly  101  has a barrier ring  200  installed in the intermediate portion of the cylinder assembly  101 . The barrier ring  200  is shown in shadow, as in  FIG. 2A . 
         [0022]    The barrier ring  200  discussed herein is a part of a barrier assembly (e.g., a heat barrier assembly) that is inserted into, or located in, the bore of a cylinder assembly and that prevents heat incident upon the barrier ring from the combustion chamber from passing to other parts of the opposed-piston engine. The barrier ring can be thin compared to the walls of the cylinder assembly, and numerous openings, perforations, or holes, can be present in the ring. The materials of the barrier ring, barrier ring shape, openings in the barrier ring, and combination of the barrier ring with insulation or air gaps influence the ability of the barrier assembly to keep heat from escaping to other volumes in the engine. 
         [0023]      FIG. 3  is a three-dimensional drawing of an exemplary barrier ring  200  (e.g., heat screening ring) prior to installation into the cylinder bore. The barrier ring  200  is a thin-walled tube or ring with folded edges  210 , openings  220  for communication between injection/combustion hardware and the combustion chamber, and openings  215  to allow for pressure equalization between the space inside and the cylinder environment outside of the barrier ring  200 . The barrier ring  200  sits in a circumferential groove on the inside of the cylinder liner. The groove is located at, or adjacent to, the combustion chamber. The barrier ring  200  is formed so that the folded edges  210  allow the inside surface of the barrier ring  200  to lie substantially flush with inside wall of the cylinder liner when inserted into the groove. The barrier ring  200  and the circumferential groove, along with a gap between the ring and groove back wall, are part of a barrier assembly. 
         [0024]      FIG. 4A  is a cross sectional view of a portion of the cylinder assembly  16  and engine block with the opposing pistons  35 ,  36  at TDC, forming the combustion chamber  41 , and with the barrier ring  200  installed into a groove  225 , as described above. The barrier ring  200  has a width that is approximately the height of the combustion chamber  41 , as measured along the central axis of the cylinder, from one piston end surface  36   e  to another piston end surface  35   e.    FIG. 4B  is an exploded partial view of the cylinder sleeve and pistons of  FIG. 4A  that shows the barrier assembly, including the groove  225  and the barrier ring  200 , in greater detail. The openings  215  in the barrier ring for equalizing pressure between the gap  230  in the groove  225  and the combustion chamber  41  are also shown in  FIG. 4B . The gap  230  helps to prevent the flow of heat away from the combustion chamber  41 . In  FIG. 4B , the barrier ring  200  is situated in the groove  225  and is shown as flush with the sides  226  of the groove; the folded edges  210  of the barrier ring  200  are up against the groove sides  226 . The main portion of the barrier ring, the barrier ring wall that includes the openings  215 , is spaced away from the back wall  227  of the groove  225  by the folded edges  210  of the barrier ring. The barrier ring  200  may have the configuration shown in  FIG. 4B  after the engine has warmed up and the barrier ring  200  has expanded. When the engine is cold, there can be a clearance of between  10  microns and  100  microns in the interface  240  between the groove sides  226  and the folded edges  210  of the barrier ring. In some implementations, the clearance in a cold engine between the groove sides  226  and the edges  210  of the barrier ring can be less than  10  microns, or alternatively, the clearance can be  100  microns or greater. Alternatively, or additionally, the material at or around the groove sides  226  can be compliant enough or be constructed to accommodate any expansion of the barrier ring  200  in the axial direction of the cylinder assembly  16 . 
         [0025]    In most engines, a circumferential clearance space between pistons and the inner wall of the cylinder liner is provided to allow for thermal expansion. After long hours of operation carbon builds up in this clearance space, on the top land of a piston, which can result in increased friction and ring wear; at worst it can cause ring jacking. It is preferable that carbon removal not occur where the ports are located. Carbon debris near the ports can contaminate charge air entering the bore or be swept into the gas stream exiting the cylinder assembly after combustion, degrading the performance of the engine. 
         [0026]    In the configuration shown in  FIG. 4A , the barrier ring  200  is shown as contacting the pistons  35 ,  36  and bridging the gap  450  between the cylinder bore and the sides of the pistons  35 ,  36 . By protruding beyond the groove  225 , the barrier ring  200  can contact and scrape the sidewalls of the pistons  35 ,  36  as the pistons approach and/or leave TDC in the cylinder. This contacting and scraping can remove carbon buildup on the sidewalls of the pistons  35 ,  36  while avoiding the possibility of fouling incoming air with the scraped carbon or adding to exhaust emissions. 
         [0027]    Alternatively, in some implementations, the barrier ring  200  may be flush with the sides  226  of the groove when the engine is cool. When the engine warms up, the barrier ring  200  can bow away from the cylinder liner, into the combustion chamber. The bowing portion of the barrier ring can rub against the sidewalls of the pistons  35 ,  36  as the pistons move through the cylinder, toward or away from TDC. In such implementations, the clearance in the interface  240  between the barrier ring edge and groove sidewall when the engine is cold, discussed above, may or may not be present. 
         [0028]      FIG. 4C  shows an alternate configuration for the barrier ring  200  and groove  225 . The barrier ring  200  shown in  FIG. 4C  lacks folded edges, and the barrier ring  200  and back wall  227  of the groove  225  are separated by a spacer  228 . The spacer  228  is shown as a pair of ledges that protrude from the groove sidewalls  226  and back wall  227 . This spacer  228  replaces the folded edges  210  of the barrier ring shown in  FIG. 4B . The clearance between the barrier ring  200  and the groove sidewall  226  at the interface between the two  240 , when the engine is cold, could have the characteristics of the clearance discussed with respect to the configuration shown in  FIG. 4B . A barrier ring  200  with folded edges  210  could be used with a liner whose groove  225  includes a spacer  228 , however, doing so may lead to a configuration in which the barrier ring  200  protrudes too far into the volume of the cylinder, and not only scrapes the top lands of the pistons, but may in fact hinder the movement of the pistons. 
         [0029]    In any case, whether the spacer  228  is present as a ledge, as in  FIG. 4C , or as folded edges  210  of the barrier ring, or in some other fashion, the barrier ring  200  is separated from the back wall  227  of the groove  225  by a distance ranging from about 0.5 mm to about 3 mm. In some implementations, the gap separating the barrier ring from the back wall of the groove can be about 0.5 mm to about 2.5 mm, such as about 0.75 mm to about 2 mm, including about 1.0 mm to about 1.5 mm. 
         [0030]    The barrier ring  200  can be made from any suitable material that can withstand repeated exposure to the temperatures and pressures experienced in the combustion chamber, as well as that can quickly dissipate heat. In some implementations, the material used to make the barrier ring will be different from the material used to form the cylinder liner or bore. Suitable materials for the barrier ring include high temperature nickel-chromium-based alloys such as Inconel®, a cobalt-chromium alloy such as Stellite® Alloy 6, stainless steel, and the like. The thickness of the barrier ring  200  is selected, along with the material used to fabricate the barrier ring and the pattern of openings made in the barrier ring, so that the barrier ring  200  is robust enough to withstand mechanical failure when exposed to the temperatures and pressures of the cylinder assembly interior while the engine is running. The thickness of the barrier ring can range from about 0.5 mm to about 3.0 mm, such as from about 1.0 mm to about 2.5 mm, including from about 1.0 mm to about 2.0 mm. 
         [0031]    As described above, openings in the barrier ring can allow engine components to contact the interior of the combustion chamber and/or allow for equalization in pressure between the volumes in the cylinder that are separated by the barrier ring. The barrier ring is sized to fit into a groove in the bore of a cylinder liner where the combustion chamber is formed when the pistons are near their TDC positions. Together the barrier ring and the groove, including the space between the barrier ring and back wall of the groove, form the barrier assembly that prevents heat loss from the combustion chamber to the surrounding cylinder assembly and engine. 
         [0032]    The openings in the barrier ring that allow engine components to reach into the combustion chamber can be located where fuel injection nozzles, compression release engine breaking valves, and sensors project from the cylinder into the combustion chamber (e.g.,  46  in  FIG. 2 ). These pressure-equalizing openings (e.g.,  220  in  FIG. 3 ) are sized to just allow engine components (e.g., nozzles and sensors) through; openings that are too large are undesirable, as will be explained further below. The barrier ring is then about 2 mm-20 mm wider (taller) than the diameter of the largest opening. In some implementations, the barrier ring has a height about 4.0 mm to about 20.0 mm wider than the diameter of the largest opening in the barrier ring wall, including a height about 2.0 mm to 4.0 mm wider than the diameter of the largest opening, about 5.0 mm to about 20.0 mm wider than the diameter of the largest opening, about 6.0 mm to about 19.0 mm, about 7.0 mm to about 18.0 mm, and about 8.0 mm to about 16.0 mm wider than the diameter of the largest opening in the barrier ring wall. 
         [0033]    There are various possible configurations for the openings in the barrier ring that are meant to allow for equalization in pressure between the spaces on either side of the barrier ring (e.g.,  215  in  FIG. 3 ). These openings allow for movement of gas between the space in the combustion chamber enclosed by the barrier ring and the gap between the barrier ring and the cylinder liner in the groove. This allows for equalization of pressure, which in turn prevents excessive deformation of the barrier ring due to high mechanical stresses. While larger openings will allow for rapid equalization of pressure across the barrier ring, openings that are too large will not provide the heat screening properties that are desired. Openings that are too large will allow heat to escape through the cylinder liner and the rest of the cylinder assembly, while openings that are too small will lead to inequality in pressure across the ring and in turn mechanical stresses in, and deformation of, the barrier ring. 
         [0034]    The size and shape of all of the openings in the barrier ring are optimized to achieve maximum heat-loss reduction while maintaining an acceptable pressure difference across the barrier ring. Pressure-equalizing openings can have any shape, such as circular, elliptical, triangular, rectangular, square, slit-like, and the like. Fillets can be used to eliminate stress concentration in the barrier ring. The arrangement of pressure-equalizing openings can vary to maximize heat-loss reduction and pressure equalization across the barrier ring. Groupings of pressure-equalizing openings can be used to vary the density of the openings. In some implementations, the selected opening locations can produce a ring with no pressure-equalizing openings along the center, or midline, of the barrier ring. Alternatively, the selected opening locations can produce a barrier ring with openings exclusively along the midline of the ring, or a barrier ring with openings along the midline and off the midline of the ring. Also, the location of the openings can be targeted to a particular angular pitch (e.g., frequency of openings along the ring). The angular pitch of the pressure-equalizing openings can be between 30° and 45°. Pressure-equalizing openings can be located randomly or have a definite pattern. These openings can all have similar sizes and shapes, or the sizes and shapes of the pressure-equalizing openings can vary, so long as the barrier ring maximizes the heat-loss reduction of the cylinder while minimizing mechanical stresses in the ring that can cause failure. 
         [0035]    In general, the total surface area of the barrier ring can be made up of between 1% and 5% openings. In some implementations, the barrier ring can have a surface area that is less than 1% openings. In some implementations, openings can make up 5% or more of the surface area of the barrier ring. 
         [0036]      FIGS. 5A-5C  show exemplary barrier ring configurations with the barrier ring laid out flat prior to installation in the cylinder bore.  FIG. 5A  is a barrier ring  200  with folded edges  210 , openings for injection nozzles and other components  220 , and pressure-equalizing openings  215 . In the barrier ring shown in  FIG. 5A , the pressure-equalizing openings  215  are circular and are grouped so that these types of openings are not located along the midline  260   a  of the ring.  FIG. 5B  shows a barrier ring  200   b  with folded edges  210   b,  openings for injection nozzles and other components  220   b,  and slit-like pressure-equalizing openings  215   b.  The slit-like openings  215   b  are spaced evenly in pairs on either side of the midline  260   b  of the ring.  FIG. 5C  shows a barrier ring  200   c  with folded edges  210   c,  openings for engine components  220   c,  and circular pressure-equalizing openings  215   c.  Like the slit-like openings  215   b,  the circular openings  215   c  are located in a pattern that avoids placing any openings  215   c  along the midline  260   c  of the barrier ring. The openings  215   c  are grouped in alternating pairs and single openings. As described above, though the barrier ring configurations shown in  FIGS. 5A-5C  do not have openings along the midline of the rings, in some implementations, the barrier rings can include openings along the midline. 
         [0037]    Though  FIG. 2  shows the barrier ring  200  as a continuous ring, with the ends, as shown in  FIGS. 5A-5C , adhered to each other, the ends may actually not be sealed or adhered. This can facilitate installation of the barrier ring  200  into the cylinder liner, as well as to allow for changes in the dimensions of the ring with changes in temperature in the cylinder assembly. The barrier ring  200  can be fabricated as a strip of material, as shown in  FIGS. 5A-5C , with the openings and folded edges machined or cast into the material. The strip of material can then be worked to conform to a certain radius of curvature. The radius of curvature can be equal to that of the groove or slightly larger, to that when the barrier ring  200  is placed into the groove  225 , the barrier ring  200  pushes against the edges of the groove and is secured into place. Alternatively, the barrier ring  200  can be fabricated without folded edges, and the barrier ring can hold a radius of curvature worked into it because the ring is sufficiently thick. Barrier rings without folded edges can maintain a gap in the groove, between the ring and the cylinder liner, by using a spacer, such as a lip or step (i.e., a ledge  228  in  FIG. 4C ) in the groove that supports the edges of the barrier ring and keeps the edges away from the back wall of the groove. 
         [0038]    Additionally, or alternatively, cylinder assemblies for opposed-piston engines that use liners with a barrier ring can be used in conjunction with pistons that each have a barrier layer at their end surface. The barrier layer at the end surface of such pistons can allow for higher temperatures to be reached in the combustion chamber without diminishing performance. Such a combination of pistons with a heat-loss preventing barrier layer and the cylinder assemblies described herein can allow for reductions in conventional thermal management systems, better engine efficiency, and/or reductions in emission levels. 
         [0039]    During a combustion event in an opposed-piston engine, a first piston and a second piston will move in a cylinder assembly, through the bore of an annular cylinder liner, in a direction along the long axis of the cylinder liner, from bottom dead center (BDC) towards top dead center (TDC). As the first and second pistons move axially, and both pistons are near their top dead center locations, they will eventually create a combustion chamber between their end surfaces. The air that is in the cylinder assembly between the end surfaces of the pistons heats up as the pistons move towards each other to form the combustion chamber. Fuel is injected into the combustion chamber, and the fuel mixes with the heated air. Combustion takes place between the end surfaces of the first and second pistons, releasing heat and creating pressure. The pressure pushes the first and second pistons apart. A barrier assembly, including a barrier ring as described herein and a groove in the cylinder liner, that is located inside the bore of the annular cylinder liner, on the periphery of the combustion chamber (e.g., between the TDC locations in the bore for the first and second pistons) prevents some of the combustion heat from reaching the outside of the cylinder assembly. 
         [0040]    Cylinder assemblies for opposed-piston engines that use liners with barrier ring, as described herein, can be used with conventional thermal management systems to dissipate heat lost through the cylinder walls. By using cylinder liners with a barrier ring, as described above, the conventional cooling systems may not have to dissipate as much heat from cylinder assembly, around the combustion chamber. As a result of this, the cooling systems can be smaller in size, resulting in an overall more compact and efficient engine. 
       EXAMPLE 1 
       [0041]      FIG. 6  shows an exemplary barrier ring  600  for a cylinder liner of an opposed piston engine. The barrier ring  600  fits into a groove in a cylinder liner. The cylinder liner for which the barrier ring is made has a 98.25 cm internal diameter. The barrier ring  600  has pressure-equalizing openings  615  of 2.5 mm diameter and 45° angular pitch that are formed along the centerline of the barrier ring. The barrier ring  600  also has folded edges  610  and has openings  620  to allow for nozzles injecting fuel into the combustion chamber that is surrounded by the barrier ring  600 . 
         [0042]    The scope of patent protection afforded these and other barrier ring embodiments that accomplish one or more of the objectives of durability and thermal resistance of an opposed-piston engine according to this disclosure are limited only by the scope of any ultimately-allowed patent claims.

Technology Classification (CPC): 5