Patent Publication Number: US-2005133003-A1

Title: Compression release brake system

Description:
TECHNICAL FIELD  
      The present invention relates generally to a system and method for opening at least one valve associated with a cylinder of an engine and, more particularly, to a system and method for controllably limiting the amount of lift or displacement of an engine valve during a valve opening event.  
     BACKGROUND  
      Engines, such as internal combustion engines, commonly utilize intake and exhaust valves associated with engine cylinders. In a normal running mode, intake valves may be opened to admit a fuel and air mixture into the combustion chamber of a cylinder, and exhaust valves may be opened to permit combustion byproducts to be exhausted from the cylinder.  
      It is also known to operate engines in a compression release braking mode, in which one or more engine cylinders are temporarily used to consume energy by compressing air. The energy required to compress air in the one or more engine cylinders produces a braking effect. This is accomplished by controllably opening a cylinder valve, for example, an exhaust valve, at a time when the engine has consumed energy compressing air in the cylinder, thus preventing recovery of the expended energy.  
      One common method for controllably opening an engine valve during a braking event is to use a rotating cam, for example, an injector cam, associated with the engine to provide the mechanical energy necessary to open the valve. The cam may be linked to the valve by a hydro/mechanical linkage including master and slave pistons and cylinders, such that rotation of the cam is translated into valve motion. One problem with this method is that the profile of the engine cam may not provide the optimum amount of valve displacement or lift. This is a particular problem in the braking mode of engine operation, because the engine cylinder valve is typically opened during the time that the cylinder piston is at or near top dead center. Consequently, if the valve extends too far into the cylinder it can make contact with the piston causing damage to the engine.  
      One approach to controlling the amount of valve displacement when using a fixed cam profile to indirectly actuate an engine valve is to employ some form of lost-motion linkage between the cam actuator and the engine valve. Such a linkage is designed to absorb motion caused by the fixed cam profile that exceeds the amount necessary to provide a desired valve displacement.  
      Many of the lost-motion devices employed in the past for this purpose incorporate some form of hydraulic arrangement, such that the force transmitted from the cam actuator to the engine valve is dissipated or absorbed hydraulically during at least a portion of the cam profile excursion. Such devices are sometimes referred to as hydraulic clippers, because they clip or limit excessive valve travel. For example, U.S. Pat. No. 6,415,752 illustrates a lost-motion system that includes a hydraulic accumulator that controllably absorbs hydraulic fluid in excess of that required to cause a desired engine valve displacement.  
      Known lost-motion devices used for the above described purpose tend to be complex and expensive, and often employ special and precise hydraulic fluid porting. This can lead to system failures and unreliable operation. The present invention is directed to overcoming one or more of the problems as set forth above.  
     SUMMARY OF THE INVENTION  
      In one aspect of the present invention, a compression release brake system includes a hydraulic apparatus for controllably opening at least one exhaust valve of at least one cylinder of an engine in response to motion of a cam actuator. The system includes a master piston assembly having a master piston engageable with the cam actuator. The master piston includes a compressible piston pin. A slave piston assembly is hydraulically linked to the master piston assembly and includes a slave piston engageable with the at least one exhaust valve.  
      In a second aspect of the present invention, a method for limiting travel of at least one exhaust valve of at least one cylinder of an engine during a compression release brake event is disclosed. The engine may include a cam actuator and a master piston assembly having a master piston. The master piston includes a compressible piston pin that is engageable with the cam actuator. A slave piston assembly is hydraulically linked to the master piston assembly and has a slave piston engageable with the at least one exhaust valve. A travel-limiting piston assembly has a travel-limiting piston positioned to controllably limit movement of the master piston. The method includes the steps of moving the travel-limiting piston to a position sufficient to limit movement of the master piston to a predetermined amount. The master piston is moved the predetermined amount in response to the cam actuator, and the at least one exhaust valve is responsively opened a predetermined amount. The compressible piston pin is compressed in response to further action of the cam actuator beyond that required to move the master piston the predetermined distance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of an embodiment of the present invention configured for compression release braking operation;  
       FIG. 2  is a schematic representation of an embodiment of the present invention configured for powered operation; and  
       FIG. 3  is a cross-section of a travel-limiting master piston assembly in accordance with an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION  
      Referring first to  FIGS. 1 and 2 , a compression release brake system  10  includes a hydraulic apparatus  12  for controllably opening at least one exhaust valve  14  of at least one cylinder  16  of an engine (not shown) in response to motion of a cam actuator  18 . The cam actuator  18  may be a rotating cam such as an injector cam  20  that is normally used to activate one or more fuel injectors, and may include a cam follower  22  and rocker arm arm  24  arranged to translate the rotary motion of the cam  20  into a linear driving motion, as is well known in the engine art. The engine valve  14  may include a similar cam driver  26  having an associated cam  28 , cam follower  30 , and rocker arm  32 . The rocker arm  32  normally reciprocates according to the profile of the cam  28 , in turn periodically opening the engine valve  14  by transferring motion to a push rod  34 . A valve closing spring  36  acts to close the engine valve  14  when it is not held open by the rocker arm  32 .  
      A master piston assembly  40  has a master piston  42  engageable with the cam actuator  18  through the cam follower  22  and rocker arm  24  arrangement. A slave piston assembly  44  is hydraulically coupled to the master piston assembly  40  and includes a slave piston  46  engageable with the at least one exhaust valve  14  through the associated rocker arm  32  and push rod  34 . A travel-limiting piston assembly  48  is positioned relative to the master piston assembly  40  to controllably limit linear movement of the master piston  42  within the master piston assembly  40  to a predetermined amount.  
      The master, slave, and travel-limiting piston assemblies  40 ,  44 ,  48  are shown in more detail in  FIG. 3 . The master piston assembly  40  includes a piston housing  50  having a bore  52 . The master piston  42  is slidably disposed in the piston housing bore  52  and cooperates with the piston housing  50  to form a master hydraulic reservoir  54 . In the illustrated embodiment, a compressible piston pin  56  has a compressible portion  58 , a solid portion  60 , and a bore  62  extending through the compressible portion  58  into the solid portion  60 . A guide pin  64  is slidably disposed in the piston pin bore  62 , and has an end portion  66  engageable with the rocker arm  24  of the cam actuator  18 . The end portion  66  may be connected to an end of the master pin compressible portion  58  with a shear pin  65 , and may include a swivelable foot  67  adapted to maintain alignment between the guide pin  64  and the rocker arm  24 . The guide pin  64  is adapted to controllably move the master piston  42  within the piston housing bore  52  in response to motion of the cam actuator  18 . The master piston  42  also may include a pin receiving portion  68 , and a piston spring  70  arranged between the piston housing  50  and the piston pin solid portion  60 . The piston spring  70  is arranged to urge the piston pin solid portion  60  into abutting arrangement with the master piston pin receiving portion  68 , and to further urge the master piston  42  in the direction of the master hydraulic reservoir  54 .  
      The slave piston  46  of the slave piston assembly  44  is slidably disposed in a slave piston housing bore  72  to form a slave hydraulic reservoir  76 . A slave piston spring  78  is arranged to urge the slave piston  46  in the direction of the slave hydraulic reservoir  76 . The master and slave hydraulic reservoirs  54 ,  76  are connected to one another by a hydraulic line  80 .  
      The travel-limiting piston assembly  48  includes a travel-limiting piston  82  that is slidably disposed in a travel-limiting piston housing bore  84  to form a travel-limiting hydraulic reservoir  88 . The travel-limiting piston assembly  48  may be mounted integral with the master piston assembly  40 , and the end of the travel-limiting piston  82  opposite the travel-limiting hydraulic reservoir  88  may controllably extend a predetermined distance into the master hydraulic reservoir  54  in a manner sufficient to contact the master piston  22 . The predetermined distance may be established by a travel-limiting piston stop  90  as shown. This distance may be made adjustable by, for example, providing means for adjusting this stop or for moving the travel-limiting piston assembly  48  relative to the master piston assembly  40 .  
      A brake control valve  92  is controllably connectable to a pressurized hydraulic fluid supply  94 , a hydraulic fluid drain  96 , and to each of the master, slave, and travel-limiting hydraulic reservoirs  54 ,  76 ,  88 . The brake control valve  92  is movable between a power position as shown in  FIG. 2 , at which the master, slave, and travel-limiting piston reservoirs  54 ,  76 ,  88  are connected through the brake control valve  92  to the hydraulic fluid drain  96 , and a brake position, as shown in  FIG. 1 , at which the master and slave hydraulic reservoirs  54 ,  76  are connected to the hydraulic fluid supply  94  through a first check valve  98 , and the travel-limiting hydraulic reservoir  88  is connected to the hydraulic fluid supply  94  through the first check valve  98  and through a second check valve  99 .  
      In a preferred embodiment, the compressible piston pin  56  compressible portion  58  may be implemented as a machined helical spring. Such machined springs are commercially available from Helical Products Company, Inc. However, other structural arrangements that result in a compressible piston pin may be substituted for the preferred machined spring without deviating from the scope of the present invention. For example, the piston pin  56  could be implemented in two or more discreet pieces incorporating a solid portion with a more conventional coil spring. It is also foreseeable that the solid portion could be dispensed with and the entire compressible pin could be implemented in the form of a machined or coil spring. Thus, it is intended that the present invention be construed to cover these and other modifications that come within the scope of the appended claims and their legal equivalents.  
     Industrial Applicability  
      During powered operation of an engine incorporating the present invention, the brake control valve  92  is electrically shifted to the power position described in  FIG. 2 . In this mode, the hydraulic reservoirs  54 ,  76 ,  88  of the master piston assembly  40 , the slave piston assembly  44 , and the travel-limiting piston assembly  48  are all connected through the control valve  92  to the hydraulic fluid drain or sump  96 . Consequently, the master and slave return springs  70 ,  78  associated with the respective piston assemblies  40 ,  44  force the respective pistons  42 ,  46  to retract, and the master and slave pistons  42 ,  46  are removed from engagement with the cam actuator  18  and exhaust valve  14 , respectively.  
      The master piston spring  70  and master piston  42  also act on the travel-limiting piston  82  causing it to reset to a retracted position. Hydraulic fluid in the master, slave, and travel-limiting hydraulic reservoirs  54 ,  76 ,  88  is returned through the control valve  92  to the hydraulic fluid drain  96 . Consequently, the exhaust valve  14  is not influenced by the hydraulic apparatus  12  and the engine operates in a conventional manner.  
      During braking operation of the engine, the control valve  92  is shifted to the braking position as depicted in  FIG. 1 . Hydraulic fluid is supplied under pressure from the hydraulic fluid supply  94  through the check valves  98 ,  99  to the master hydraulic reservoir  54 , the slave hydraulic reservoir  76 , and the travel-limiting hydraulic reservoir  88 . In turn, the master piston  42 , compressible piston pin  56 , and guide pin  64  are engaged through the rocker arm  24  and cam follower  22  with the cam  20  of the cam actuator  18 . Likewise, the slave piston  46  is engaged with the exhaust valve  14  through the rocker arm  32  and push rod linkage  34 . Finally, the travel-limiting piston  82  is extended to its predetermined position and is hydraulically locked by the second check valve  99 . With the hydraulic reservoirs  54 ,  76 ,  88  filled and hydraulically locked by the check valves  98 ,  99 , the guide pin  64  and responsively the compressible piston pin  56  and master piston  42  will attempt to follow the motion induced by rotation of the cam actuator  18 .  
      As the master piston  42  moves upward in the piston housing bore  52 , hydraulic fluid in the master hydraulic reservoir  54  is displaced through the hydraulic line  80  to the slave hydraulic reservoir  76 . This forces the slave piston  46  to extend and the exhaust valve  14  to open. However, once the master piston  42  makes contact with the travel-limiting piston  82 , further linear movement of the master piston  42  within the piston housing bore  52  is blocked and further extension of the slave piston  46  cannot occur. Consequently, the amount of exhaust valve  14  displacement or lift is limited in accordance with the preset travel-limiting piston assembly  48 .  
      However, the cam actuator  18  continues to exert upward force on the guide pin  64  and the compressible piston pin  56 , causing the piston pin compressible portion  58  to compress. The guide pin  64  is free to continue to move within the piston pin bore  62  as compression occurs, so the additional motion imparted by the cam actuator  18  is absorbed by the compressible piston pin  56 . As the cam actuator  18  continues to revolve and the direction of linear motion is reversed, the piston pin compressible portion  58  again extends to follow the cam  20  profile. Once fully extended, continued rotation of the cam actuator  18  permits the exhaust valve closing spring  36  to close the exhaust valve  14 , forcing hydraulic fluid out of the slave hydraulic reservoir  76  and back into the master hydraulic reservoir  54 . The cycle continues until the control valve  92  is returned to the engine power position depicted in  FIG. 2 .  
      The above described embodiments of the invention discuss a simple system for controllably actuating a single valve associated with one cylinder of an engine. However, the described system can be readily multiplied in a straightforward manner to include multiple valves in multiple engine cylinders, as would be typically employed for compression release braking without departing from the scope of the present invention. Likewise, other modifications of the described embodiment may be made by one skilled in the art without deviating from the scope of the appended claims and legal equivalents.