Abstract:
A lubricating configuration in a two-stroke cycle, opposed-piston engine for a piston wristpin minimizes losses in oil pressure at the wristpin as the piston approaches bottom center and reduces the required oil supply pressure to the engine. The wristpin is constructed to absorb and store oil pressure energy when oil pressure at the wristpin is high, and to release that stored energy to pressurize the oil at the wristpin when connecting rod oil pressure is low.

Description:
RELATED APPLICATIONS 
     This application contains subject matter related to the subject matter of U.S. patent application Ser. No. 13/136,955, filed Aug. 15, 2011 for “Piston Constructions for Opposed-Piston Engines,” published as US 2012/0073526 on Mar. 29, 2012, and U.S. patent application Ser. No. 13/776,656, filed Feb. 25, 2013 for “Rocking Journal Bearings for Two-Stroke Cycle Engines”. 
     BACKGROUND 
     The field is lubrication management for two-stroke cycle engines. More specifically the application relates to implementation of a wristpin oil pressure recovery device for pistons of a two-stroke cycle, opposed-piston engine. 
     Wristpins in reciprocating engines must be lubricated to mitigate the risk of highly loaded asperity contact in the joint. If asperity contact in the joint is sustained at high loads, excessive friction, wear and even catastrophic failure is possible. The applied load that causes this asperity contact is constantly changing as engine speed and load change. 
     In some aspects of two-stroke cycle opposed-piston engine operation, the nature of the cycle presents two distinct threats to wristpin durability: continuous compression loading and oil pressure variation. 
     Continuous compression loading results because load reversal on the wristpin bearings of a two-stroke engine may never occur during the normal speed and load range operation of the engine. During operation of a two-cycle engine, a combustion event occurs every cycle and there is nearly always a gas pressure loading the crown of a piston near top center (TC), which, even at high piston speeds, is still greater than the inertial force of the piston assembly on a wristpin bearing. At the other end of the cycle, at bottom center (BC), the inertial force of the piston assembly keeps the bearing loaded as well. As a result, the bearing is nearly always under positive load throughout the cycle, and it is difficult to replenish it with oil. Furthermore, given limited angular oscillation of the bearing, oil introduced between the bearing surfaces does not completely fill the bearing. Eventually the bearing begins to operate in a boundary layer lubrication mode which leads to excess friction, wear, and then bearing failure. 
     Solutions to the first problem include bearing constructions that cause separation of bearing parts in response to bearing rotation. One such solution is disclosed in related U.S. application Ser. No. 13/776,656: wristpins coupling the pistons of an opposed-piston engine are constructed with rocking journal bearings that provide biaxial rotation of bearing parts, which separates the parts to allow introduction of oil between the bearing surfaces. This bearing construction includes a reservoir in the rocking journal that acts as an accumulator to receive and maintain a volume of pressurized oil that is delivered to the bearing parts via outlet passages through the journal. An inlet passage in the journal for delivering oil to the accumulator is fed from a high-pressure oil passage in the associated connecting rod. Pressurized oil is transported to the oil passages of the connecting rods from a main oil gallery in an engine block. Here, the second problem becomes apparent. 
     As a pair of pistons move in opposition in a cylinder bore, the pressurized oil fed to their respective wristpins undergoes inertial loading that is most pronounced when the pistons change direction in the reversal zones at their top center (TC) and bottom center (BC) locations. Given the direction of motion from TC to BC during a power stroke, the inertial load can cause the oil pressure in the connecting rod oil passages to drop below a minimum level for effective wristpin lubrication as the pistons reverse direction at BC. Providing adequate oil pressure to lubricate the wristpins throughout the operating cycle of an opposed-piston engine, especially in the face of non-reversing loads, may require that the supply pressure to the main oil gallery increase with engine speed to overcome inertial forces on the oil column in the connecting rod. Typically, the main oil gallery is fed from a positive pressure pump, and it is possible to control the pump so as to vary the supply pressure with the speed of the engine. However, increasing engine-wide oil pressure solely for wristpin lubrication, as the speed of the engine increases, may result in oil pressure in excess of that required for the rest of the engine lubrication system. This will result in higher parasitic loads for the lubrication system and a higher friction-mean-effective-pressure (FMEP) for the engine. 
     Accordingly, there is a need for maintaining oil pressure in the oil reservoir of the wristpin of a two-stroke cycle, opposed-piston engine during engine operation. It is particularly desirable that the oil pressure be maintained at levels that guarantee effective lubrication throughout the operating cycle, at all engine speeds, without imposing excessive pumping losses on engine performance. 
     SUMMARY 
     In order to minimize losses in oil pressure at the wristpin as the piston approaches BC and reduce the required oil supply pressure, a wristpin is constructed to absorb and store oil pressure energy when oil pressure at the wristpin is high, and to release that stored energy to pressurize the oil at the wristpin when connecting rod oil pressure is low. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematic drawing of a prior art opposed-piston engine with a pump-supplied oil gallery, and is appropriately labeled “Prior Art” 
         FIG. 2  is a graph showing wristpin oil supply pressures through one revolution from TC-to-BC-to-TC of a compression-ignition, two-stroke cycle, opposed-piston engine for various gallery oil pressures. 
         FIG. 3  is a side view, in perspective, of a piston/connecting rod assembly for a two-stroke cycle, opposed-piston engine according to the detailed description. 
         FIG. 4A  is an axial sectional view taken along line A-A of the piston and connecting rod of  FIG. 3  showing a wristpin lubricating configuration.  FIG. 4B  is a sectional view taken along line B-B of  FIG. 3 . 
         FIG. 5  is an exploded view of the piston and connecting rod showing elements of the wristpin lubricating configuration. 
         FIG. 6  is an enlarged sectional view, in perspective, showing the wristpin partially disassembled. 
         FIG. 7  is an exploded view of the piston and connecting rod showing elements of a pressure-responsive control device configured to couple an oil delivery passage in the connecting rod with an oil inlet passage in the wristpin 
     
    
    
     DETAILED DESCRIPTION 
     A two-stroke cycle engine is an internal combustion engine that completes a power cycle with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which a pair of pistons is disposed in opposition in the bore of a cylinder. 
     As seen in  FIG. 1 , an opposed-piston engine  49  has at least one ported cylinder  50 . For example, the engine may have one ported cylinder, two ported cylinders, three ported cylinders, or four or more ported cylinders. For purposes of illustration, the engine  49  is presumed to have a plurality of ported cylinders. Each cylinder  50  has a bore  52 : exhaust and intake ports  54  and  56  are formed in respective ends thereof. The exhaust and intake ports  54  and  56  each include one or more circumferential arrays of openings. Exhaust and intake pistons  60  and  62  are slidably disposed in the bore  52  with their end surfaces  61  and  63  opposing one another. The exhaust pistons  60  are coupled to a crankshaft  71 , and the intake pistons are coupled to a crankshaft  72 . Each of the pistons is coupled to its associated crankshaft by a wrist pin  74  and a connecting rod  76 . When the pistons  60  and  62  of a cylinder  50  are at or near respective TC locations, a combustion chamber is defined in the bore  52  between the end surfaces  61  and  63  of the pistons. Fuel is injected directly into the combustion chamber through at least one fuel injector nozzle  100  positioned in an opening through the cylinder&#39;s sidewall. 
     A lubrication system that supplies oil to lubricate the moving parts of the engine  49  includes an oil reservoir  80  from which pressurized oil is pumped by a pump  82  to a main gallery  84 . The main gallery supplies pressurized oil to the crankshafts  71  and  72 , typically through drillings to the main bearings (not seen). From grooves in the main bearings, pressurized oil is provided to grooves in the big end bearings of the connecting rods  76 . From there, pressurized oil flows through drillings  77  in the connecting rods to the wristpins  74 . 
     The engine  49  is equipped with an air management system  51  that includes a supercharger  110  and a turbocharger  120 . The turbocharger has a turbine  121  and a compressor  122  rotating on a common shaft  123 . The turbine  121  is coupled to the exhaust subsystem and the compressor  122  is coupled to the charge air subsystem. Exhaust gas emptied into the conduit  125  from the exhaust port  54  rotate the turbine  121 . This rotates the compressor  122 , causing it to generate charge air by compressing intake air. The charge air output by the compressor  122  flows through a conduit  126 , whence it is pumped by the supercharger  110  to the openings of the intake port  56 . 
     The operational cycle of an opposed-piston engine is well understood. In response to combustion occurring between their end surfaces  61 ,  63  the opposed pistons  60 ,  62  move away from their TC locations in the cylinder. While moving from TC, the pistons keep their associated ports closed until they approach respective BC positions. The pistons may move in phase so that the exhaust and intake ports  54 ,  56  open and close in unison; alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times. As the pistons move through their BC locations exhaust products flowing out of the exhaust port  54  are replaced by charge air flowing into the cylinder through the intake port  56 . After reaching BC, the pistons reverse direction and the ports are again closed by the pistons. While the pistons continue moving toward TC, the charge air in the cylinder  50  is compressed between the end surfaces  61  and  63 . As the pistons advance to their respective TC locations in the cylinder bore, fuel is injected through the nozzles  100  into the charge air, and the mixture of charge air and fuel is compressed between the pistons  60  and  62 . When the mixture reaches an ignition temperature, the fuel ignites. Combustion results, driving the pistons apart, toward their respective BC locations. 
       FIG. 2  is a graph that shows the oil pressures at the wristpins of a representative opposed-piton engine during a full cycle of the engine from a crank angle (CA) of 0° at TC to BC (180° CA) and back to TC (360° CA) for several main gallery oil pressures. As can be seen in  FIG. 2 , for a given gallery pressure, the oil pressure at the wristpins is at its highest at TC and at its lowest at BC. If main gallery pressure is below an engine-specific threshold, the pressure of the column of oil in the connecting rod drilling supplying the wristpin will go negative and the wristpin will not have pressurized oil available until such time in the cycle that the column pressure is positive again. As per  FIG. 2 , for a given engine geometry and crankshaft speed a main gallery pressure below five bar results in the oil pressure at the wristpin being negative at 180° CA (BC). At a main gallery oil pressure of five bar, the wristpin oil pressure is slightly positive and at six bar the wristpin oil pressure is at slightly above one bar which is the desired minimum pressure. A three bar main gallery oil pressure may be sufficient for all other lubrication systems in the engine, but a gallery oil pressure twice that amount is required to adequately lubricate the wristpin assembly during an entire engine cycle. With reference to  FIG. 2 , it is desirable that the lubrication system of an opposed-piston engine be configured to deliver lubricating oil in an amount sufficient to guarantee oil availability to the wristpins during the entire two-stroke engine cycle. 
       FIG. 3  is a perspective view of a piston assembly used in an opposed-piston engine that shows the piston  200  and its associated connecting rod  210 . The piston  200  has a crown  206  attached to a skirt  207 . An end surface  208  of the crown is configured to form a combustion chamber in cooperation with the end surface of an opposing piston, when both pistons are at or near TC. See, for example, the piston configurations described and illustrated in US publication 2011/0271932 and WO publication 2012/158756, and the piston configurations described and illustrated in U.S. application Ser. No. 13/843,686 and U.S. application Ser. No. 14/026,931. The connecting rod  210  has a large end  212  for coupling to a crank throw of a crankshaft (not seen). An oil groove  214  in the large end transports oil to a drilling in the shaft of the connecting rod. As per  FIGS. 4A ,  4 B, and  5 , the piston  200  further includes a bearing support structure  219  fixed to the lower surface of the crown  206  and disposed in the recess formed by the skirt  207 . The bearing support structure  219  includes a generally cylindrical bearing surface  220  that receives a wristpin  221  (also called a “journal”) mounted to the small end  215  of the connecting rod  210 . The wristpin  221  includes an internal oil reservoir  222  in communication with one or more oil outlet passages  225  drilled through the wristpin  221  and operative to pass oil from the reservoir, through the wristpin and into an interface between the wristpin and the bearing surface. In some aspects, best seen in  FIG. 4A , the oil reservoir  222  can be configured as a cylindrical recess with opposing ends and an axis that corresponds to the axis on which the wristpin  221  oscillates. Pressurized oil is transported from the groove  214  for delivery to the oil reservoir  222  through an oil delivery passage  216  in the connecting rod  210 . 
     Lubricating Configuration for Maintaining Wristpin Oil Pressure 
     A lubricating configuration for maintaining wristpin oil pressure at a level sufficient to guarantee oil availability to the wristpins of an opposed-piston engine during the entire two-stroke engine cycle is illustrated by an embodiment shown in  FIGS. 4A ,  4 B,  5 , and  6 . However, it should be understood that no aspect of the embodiment is meant to be specifically limiting. 
     In this example, a lubrication configuration includes at least one oil pressure absorber  250  disposed in the oil reservoir  222 , a wristpin oil inlet passage  223  in communication with the oil reservoir  222 , the oil delivery passage  216  in the connecting rod  210 , and a pressure-responsive control device  260 . The pressure-responsive control device  260  can be configured to couple the oil delivery passage  216  with the wristpin oil inlet passage  223  for transport of oil into the reservoir  222  while oil pressure in the oil delivery passage  216  slightly exceeds the oil pressure in the oil reservoir  222 , and to decouple the oil delivery passage  216  from the wristpin oil inlet passage  223  so as to block the transport of oil from the reservoir  222  to the oil delivery passage when oil pressure in the oil delivery passage  216  decreases with respect to the oil pressure in the oil reservoir  222 . Thus, while the piston  200  moves from BC to TC and inertial forces cause the oil pressure in the oil delivery passage  216  to rise and peak, the oil delivery passage  216  is coupled with the wristpin oil inlet passage  223  and the energy in the incoming surge in oil pressure is absorbed by the oil pressure absorber  250 . In response to the surge, the oil pressure absorber  250  absorbs energy in such a manner as to create additional space in the oil reservoir  222  for more pressurized oil. While the piston  200  returns from TC to BC, the inertial forces on the oil column in the oil delivery passageway  216  reverse, whereby the oil pressure in the oil delivery passageway  216  drops. When the oil pressure in the oil delivery passage  216  begins to fall with respect to the oil pressure in the reservoir, the oil delivery passage  216  is decoupled from the wristpin oil inlet passage  223  so as to block transport of oil from the reservoir to the oil delivery passage  216 . The energy stored in the oil pressure absorber  250  is released, causing the absorber to act on the oil stored in the oil reservoir  222  by reducing the additional space, which maintains oil pressure in the reservoir at a level sufficient to continue separating and lubricating the wristpin/bearing surface interface until the piston  200  reverses direction and the oil delivery passage  216  is again coupled with the wristpin oil inlet passage. 
     In a preferred embodiment, the oil pressure absorber  250  is constructed to absorb energy by compressing, and to release stored energy by expanding. In some aspects, seen in  FIGS. 4A ,  5 , and  6 , an oil pressure absorber  250  is configured as a piston  251  with a closed end  253  and a spring  256  disposed in the axial space of the piston  251 . The cylindrical recess forming the reservoir  222  includes a ledge  272  with an annular surface  273  opposing an end  274  of the wristpin  221  that is closed by a press-fit plug  276 . The annular surface  273  lies in a plane perpendicular to the axis  270 , which is shared by the wristpin and the cylindrical recess. The piston  251  is disposed between the annular surface  273  and the end  274  for being moved therebetween, along the axis  270 , in response to a change in oil pressure in the reservoir  222 . Movement of the piston  251  is controlled by the spring  256 , which acts between the closed end  253  and the plug  276 . When the parts  221 ,  251 ,  256 , and  276  are assembled, the spring  256  is compressed enough to urge the piston toward, if not against, the annular surface  273 . As oil pressure in the reservoir increases and exceeds the pressure of the spring  256 , the piston  251  is urged by the oil pressure away from the surface  273 , toward the plug  276 , further compressing the spring  256 ; as the oil pressure drops, the spring compressed  256  pushes the piston  251  once again back toward the surface  273 . 
     Two oil pressure absorbers are shown in  FIGS. 4A ,  5 , and  6 , one at each end of the wristpin  221 . However, it should be understood that this number is not meant to be specifically limiting; instead, one absorber  250  can be provided at either end of the wristpin. Further, the oil pressure absorber is configured as a piston/spring combination. However, it should be understood that this configuration is not meant to be specifically limiting. Instead, the absorber can be configured as a bladder, a bellows, or another equivalent structure capable of absorbing, storing, and releasing energy in response to changes in oil pressure. 
     With reference to  FIGS. 4B and 7 , the pressure-responsive control device  260  can be configured to couple the oil delivery passage  216  with the wristpin oil inlet passage  223  in response to a difference in oil pressures in the passages. For example, the device can be configured as a check valve  282  seated in a recess  284  in the connecting rod  210  by a threaded or press-fit plug  286  which may be separate from or integral with the check valve. The check valve  282  is positioned between the oil delivery passage  216  and an auxiliary oil passage  288  formed in the connecting rod  210  in alignment with the oil inlet passage  223 . The check valve  282  is a passive valve that reacts to the pressure of the oil in the oil delivery passage  216 . When the oil pressure in the passage  216  is high, as when the piston  200  approaches TC, the valve ball depresses the spring, which opens the check valve  282  to allow pressurized oil into the wristpin oil reservoir  222 . As the piston  200  reverses and accelerates during the combustion cycle, oil in the wristpin reservoir  222  will start migrating down the oil delivery passage  216  which results in a pressure differential across the check valve  282 . This pressure differential causes the check valve  282  to close thereby closing the auxiliary oil passage  288 . Therefore, the pressurized oil in the reservoir  222  stays at its high pressure level for lubricating the wristpin interfaces. 
     The pressure-responsive control device  260  is configured as a ball check valve. However, it should be understood that this configuration is not meant to be specifically limiting. Instead, the pressure-responsive control device  260  can be configured as a diaphragm valve, a swing or tilting disc valve, a lift or an in-line valve, a reed valve, or another one-way device that that normally allows oil or lubricant to flow through it in only one direction. 
     During any engine cycle at TC there is an abundance of galley oil pressure available at the wristpin assembly. By capturing this high oil pressure at the wristpin when it is available and storing it until it is needed during the engine cycle when the available galley oil pressure is at a minimum, a more constant oil pressure can be maintained for the wristpin during an entire engine cycle. 
     The lubricating configuration embodiments that are described herein, and the devices and procedures with which they are implemented, are illustrative and are not intended to be limiting.