document: TECHNICAL FIELD  
           [0001]    This invention relates generally to engine compression release brakes, and more particularly to engines having engine compression release brakes for less than all engine cylinders.  
           BACKGROUND  
           [0002]    Traditional engine compression release brake systems typically include an engine brake for each engine cylinder. One such engine compression release brake system is illustrated in U.S. Pat. No. 5,647,318 which issued to Feucht et al. on Jul. 15, 1997. In braking systems such as that disclosed in Feucht et al., the braking horsepower is varied by operating less than all of the engine brakes. However, if the maximum braking horsepower required from the system does not require engine braking using all engine cylinders, the engine includes excess components. Engineers have learned that a reduction in engine components, such as by removal of excess components, can improve the overall robustness of an engine. Therefore, it should be appreciated that an engine compression release brake system including a sufficient, but reduced, number of components would be desirable.  
             [0003]    The present invention is directed to overcoming one or more of the problems as set forth above.  
           SUMMARY OF THE INVENTION  
           [0004]    In one aspect of the present invention, an engine includes an engine housing defining a plurality of engine cylinders. An engine compression release brake is provided for each of a portion of the engine cylinders, wherein the portion is less than all of the plurality of engine cylinders.  
             [0005]    In another aspect of the present invention, a method of engine braking using less than all engine cylinders includes the step of attaching an engine compression release brake to an engine housing for a portion, which is less than all, of the engine cylinders. The portion of engine cylinders is then operated in a braking mode.  
             [0006]    In yet another aspect of the present invention, an engine includes an engine housing that defines a plurality of engine cylinders. An engine compression release brake is provided for each of a portion of the engine cylinders, wherein the portion is less than all of the plurality of engine cylinders. Each engine compression release brake being operably coupled to a cam actuated exhaust valve. 
         
      
      
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]    [0007]FIG. 1 is a schematic representation of an engine including a modular engine compression release brake system according to the present invention;  
           [0008]    [0008]FIG. 2 is a sectioned front diagrammatic view of a cylinder shown in FIG. 1; and  
           [0009]    [0009]FIG. 3 is a sectioned side diagrammatic view of the modular engine compression release brake of FIG. 1. 
       
      
       DETAILED DESCRIPTION  
         [0010]    Referring now to FIG. 1 there is illustrated an engine  10  according to the present invention. A low pressure reservoir  12  is provided in engine  10  and preferably includes an amount of low pressure engine lubricating oil. While low pressure reservoir  12  is preferably an oil pan that has an amount of engine lubricating oil, it should be appreciated that other fluid sources having an amount of available fluid, such as coolant, transmission fluid, or fuel, could instead be used. A high pressure pump  13  pumps oil from low pressure reservoir  12  and delivers the same to high pressure manifold  14 . High pressure oil flowing out of high pressure manifold  14  is delivered via high pressure fluid supply line  15  to a hydraulic system provided in engine  10 , and used oil is returned to low pressure reservoir  12  via low pressure return line  16  after it has performed work in the hydraulic system. An electronic control module  17  is provided by engine  10  and is in control communication with one or more engine components via an electronic communication line  18 . Electronic control module  17  preferably controls multiple aspects of engine  10  operation, such as fuel injection timing and engine compression release brake timing. Engine  10  also provides an engine housing  11  that defines a plurality of engine cylinders  20 .  
           [0011]    Each cylinder  20  defined by engine housing  11  has a movable piston  21 . Each piston  21  is movable between a retracted, downward position and an advanced, upward position. For a typical four cycle diesel engine  10 , the advancing and retracting strokes of piston  21  correspond to the four stages of engine  10  operation. When piston  21  retracts from its top dead center position to its bottom dead center position for the first time, it is undergoing its intake stroke and air can be drawn into cylinder  20  via an intake valve. When piston  21  advances from its bottom dead center position to its top dead center position for the first time it is undergoing its compression stroke and air within cylinder  20  is compressed. At around the end of the compression stroke, fuel can be injected into cylinder  20  by a fuel injector  30 , and combustion within cylinder  20  can occur instantly, due to the high temperature of the compressed air. This combustion drives piston  21  downward toward its bottom dead center position, for the power stroke of piston  21 . However, it is known in the art that it is not always necessary, or desirable, for injection and combustion to occur during each cycle of piston  21 . Thus, for those engine cycles, engine compression release braking can occur within engine  10 , as disclosed below. Finally, when piston  21  once again advances from its bottom dead center position to its top dead center position, post combustion products remaining in cylinder  20  can be vented via a cam actuated exhaust valve  35 , corresponding to the exhaust stroke of piston  21 . While engine  10  has been illustrated as a four cycle, six-cylinder engine, it should be appreciated that any desired number of cylinders could be defined by engine housing  11 .  
           [0012]    Each cylinder  20  is operably connected to a number of hydraulically and/or mechanically actuated devices. In addition to hydraulically actuated fuel injector  30  and cam actuated exhaust valve  35  illustrated in FIG. 1, other devices could be operably connected to each cylinder  20 , such as an intake valve. Fuel injector  30  is fluidly connected to a fuel source  31  via a fuel supply line  32  and delivers fuel to cylinder  20  for combustion while exhaust valve  35  controls release of combustion remnants after each injection event. In addition, as illustrated in FIG. 1, a portion, but not all, of cylinders  20  each include a hydraulically actuated engine compression release brake  40  that is operably connected to the exhaust valve  35  for the cylinder  20 . While engine  10  has been illustrated having engine compression release brakes  40  connected to four cylinders  20 , it should be appreciated that engine compression release brakes  40  could instead be connected to any suitable number of engine cylinders  20  that is less than the total number of cylinders  20  defined by engine housing  11 .  
           [0013]    Referring now to FIG. 2, a cam  29  is provided which is positioned to mechanically engage exhaust valves  35 , preferably via a rocker arm assembly  23 . As cam  29  rotates, a lifter assembly  27  is moved upward about lifter group shaft  28 . Lifter assembly  27  acts upon rocker arm assembly  23 , which includes a rocker arm  24  mounted to pivot about pivot  25  corresponding to rotating movement of cam  29  via a connector rod  26 . Thus, cam  29  can mechanically engage an exhaust valve actuator  37  movably positioned within each exhaust valve  35  via rocker arm assembly  23 . With each exhaust stroke of piston  21 , exhaust valve actuator  37  is driven downward to open cylinder  20  to an exhaust manifold  39  via an exhaust passage  38  defined by exhaust valve body  36 . In addition, for those cylinders  20  having engine brakes  40 , exhaust valve actuator  37  can also be opened during the compression stroke of piston  21  by engine brake  40 , as disclosed below.  
           [0014]    Referring in addition to FIG. 3, each engine brake  40  has a brake body  41  and provides an electrical actuator  42  that is preferably a solenoid. However, it should be appreciated that any suitable electrical actuator, such as a piezoelectric actuator, could instead be provided. Solenoid  42  includes a biasing spring  43 , a coil  44  and an armature  45 . Armature  45  is attached to move with a valve member  46 . When solenoid  42  is de-energized, such as when engine braking is not desired, valve member  46  is biased toward its downward position by biasing spring  43 . When valve member  46  is in this position, it opens a high pressure seat  47  defined by brake body  41  and closes a low pressure seat  48 , also defined by brake body  41 . Thus, high pressure fluid can flow around valve member  46  and into a pressure communication passage  52  from a high pressure passage  49 . When solenoid  42  is energized, such as to initiate an engine braking event, valve member  46  is pulled to an upward position by armature  45  against the force of biasing spring  43 . When valve member  46  is in this position, high pressure seat  47  is closed to block pressure communication passage  52  from high pressure passage  49 . Low pressure seat  48  is opened such that pressure communication passage  52  is fluidly connected to a low pressure passage  50 .  
           [0015]    Also positioned in brake body  41  is a spool valve member  55  that is movable between an upward, retracted position as shown, and a downward, advanced position. Spool valve member  55  is biased toward its retracted position by a biasing spring  63 . Spool valve member  55  defines a high pressure annulus  57  that is always open to high pressure passage  49  and is positioned such that it can open an actuation fluid passage  67  to high pressure passage  49  when spool valve member  55  is in its advanced position. A low pressure annulus  60  is also provided on spool valve member  55  that can connect actuation fluid passage  67  to a low pressure passage  61  defined by brake body  41  when spool valve member  55  is in its retracted position as shown. Spool valve member  55  has a control surface  64  that is exposed to fluid pressure in a spool cavity  65 , and a high pressure surface  56  that is continuously exposed to high pressure in high pressure passage  44  via a number of radial passages defined by spool valve member  55 . Surfaces  56  and  64  preferably are about equal in surface area, but could be different. Spool cavity  65  is fluidly connected to pressure communication passage  52 .  
           [0016]    When pressure communication passage  52  is fluidly connected to high pressure manifold  14 , such as when pilot valve member  46  is in its downward position, pressure within spool cavity  65  is high and spool valve member  55  is preferably hydraulically balanced and maintained in its retracted position by biasing spring  63 . When spool valve member  55  is in this position, actuation fluid passage  67  is blocked from fluid communication with high pressure passage  49  but fluidly connected to low pressure passage  61  via low pressure annulus  60 . Conversely, when pressure communication passage  52  is fluidly connected to low pressure reservoir  12 , such as when pilot valve member  46  is in its first position, pressure within spool cavity  65  is sufficiently low that the high pressure acting on high pressure surface  56  can to overcome the force of biasing spring  63 , and spool valve member  55  can move to its advanced position. When spool valve member  55  is in this advanced position, actuation fluid passage  67  is blocked from low pressure passage  61  but high pressure fluid can flow into actuation fluid passage  67  via high pressure annulus  57  and high pressure passage  49 .  
           [0017]    As best illustrated in FIG. 3, a piston  70  is movably positioned in brake body  41  above rocker arm  24  and provides a hydraulic surface  71  that is exposed to fluid pressure in actuation fluid passage  67 . In addition, a lash adjuster  73  is operably coupled to piston  70  via a lash screw  75 . Lash adjuster  73  is preferably sized and positioned to provide sufficient lash to accommodate thermal expansion of the various components when engine  10  warms up, such as from a cold start. When actuation fluid passage  67  is open to low pressure passage  61 , such as when engine braking is not desired, piston  70  remains in its upward, retracted position. However, when actuation fluid passage  67  is open to high pressure passage  49 , high pressure acts on hydraulic surface  71  to move piston  70  toward its downward, advanced position. When piston  70  advances, lash screw  75  comes into contact with exhaust valve actuator  37  and exerts a downward force on an exhaust valve actuator  37 , causing the same to move to an open position against the pressure in cylinder  20 .  
           [0018]    Industrial Applicability  
           [0019]    Prior to the intake stroke for cylinder  20 , electronic control module  17  has determined if engine braking, rather than fuel injection, is desirable from one or more cylinders  20 . Once it has been determined that engine braking is desirable, a determination is made by electronic control module  17  regarding how much braking horsepower is required. Thus, electronic control module  17  will determine if all cylinders  20  having engine brakes  40  should be operated in a braking mode. Recall, however, that engine  10  according to the present invention provides for a number of cylinders  20  having engine brakes  40  that is less than all engine cylinders  20 . Thus, regardless of the desired braking horsepower a number of cylinders, two for engine  10  as illustrated in FIG. 1, will not be capable of being placed in an engine braking mode. Instead, each cylinder  20  not having an engine brake  40  will under go typical intake and compression strokes of piston  21  during engine braking, but with no fuel injection from fuel injector  30 . Finally, each of the cylinders  20  not having an engine brake  40  can undergo a typical exhaust stroke of piston  21 , wherein exhaust valve  35  is opened by rocker arm.  
           [0020]    For illustrative purposes, the operation of only one engine brake  40 , and its respective cylinder  20 , will be described. However, it should be appreciated that each engine brake  40  will operate in a similar manner. Prior to activation of engine brake  40 , solenoid  42  is de-energized such that pilot valve member  46  is in its downward position opening pressure communication passage  52  to high pressure passage  49 . Spool valve member  55  is in its retracted position opening actuation fluid passage  67  to low pressure passage  61  and piston  70  and plunger  75  are in their retracted positions. As piston  20  is retracting for its intake stroke, an amount of air is introduced into cylinder  20  via an intake valve (not shown). As piston  21  reaches its bottom dead center position and begins to advance, air within cylinder  20  is compressed. During typical diesel engine operation, when cylinder  20  was operating in a power mode, fuel would be injected into cylinder  20  at some point during the compression stroke of piston  21 . For instance, for a traditional engine  10 , fuel injection would occur as piston  21  nears the top dead center position for its compression stroke. Conversely, for a homogeneous charge compression engine, fuel injection would occur much sooner during the advance of piston  21 , such as when piston  21  is closer to its bottom dead center position than its top dead center position. However, when cylinder  20  is to be operated in a braking mode, engine brake  40  is activated by electronic control module  17  during the compression stroke of piston  21 .  
           [0021]    Just prior to the start of engine braking by cylinder  20 , solenoid  42  is activated by electronic control module  17  and armature  45  pulls poppet valve member  46  upward against the force of biasing spring  43  to close high pressure seat  47 . Pressure communication passage  52  is now blocked from high pressure passage  49  and fluidly connected to low pressure passage  50 . With low pressure fluid acting on control surface  64  in spool cavity  65  via pressure communication passage  52 , the high pressure acting on high pressure surface  56  is now sufficient to move spool valve member  55  downward toward its advanced position against the force of biasing spring  63 . Actuation fluid passage  67  is now blocked from low pressure passage  61  and opened to high pressure passage  49  via high pressure annulus  57 . High pressure in actuation fluid passage  67  acts on hydraulic surface  71  to move piston  70  downward toward its advanced position. As piston  70  advances, lash screw  75  comes into contact with exhaust valve actuator  37 , which is pushed toward its open position against the pressure in cylinder  20 . Compressed air within cylinder  20  can now be vented via exhaust valve  35 .  
           [0022]    Once engine brake  40  has been activated for a sufficient amount of time to provide the desired engine braking, electrical actuator  42  is de-energized. Pilot valve member  46  is returned to its biased position opening high pressure seat  47  by biasing spring  43 . Pressure communication passage  52  is now blocked from low pressure passage  50  and opened to high pressure passage  49 . With high pressure again acting on control surface  64  in spool cavity  65 , spool valve member  55  is once again hydraulically balanced, and is returned to its retracted position by biasing spring  63 . Actuation fluid passage  67  is again blocked from high pressure passage  49  and reopened to low pressure passage  61  via low pressure annulus  60 . With low pressure acting on hydraulic surface  71 , piston  70  is returned to its upward, retracted position, allowing exhaust valve actuator  37  to close under the force of biasing spring  71  and the pressure within cylinder  20 . While the various components of engine brake  40  reset themselves, piston  21  continues its reciprocating movement. Piston  21  retracts for its power stroke and then advances for its exhaust stroke. Exhaust valve actuator  37  is reopened by rocker arm to allow removal of the contents of cylinder  20  via exhaust valve  35 .  
           [0023]    It should be appreciated that a number of modifications could be made to the present invention. For instance, the poppet and spool valve assembly of engine brake  40  could be positioned above piston  70 , as opposed to the orientation that has been illustrated herein. However, it should be appreciated that the disclosed orientation would find particular applicability where height of engine brake  40  is a concern or limitation. In addition, while engine brake  40  has been illustrated with piston  70  positioned above rocker arm  24 , such that it contacts exhaust valve actuator  37  to move the same to an open position for engine braking, it should be appreciated that alternate orientations are possible. For instance, engine brake  40  could be positioned such that piston  70  is positioned below rocker arm  24  and is capable of lifting rocker arm  24  to an upward position in which exhaust valve actuator  37  is opened for engine braking. It should be appreciated, however, that for this embodiment, modifications to rocker arm assembly  23  might be desirable to prevent rocker arm  24  from disconnecting from connector rod  26  when rocker arm  24  moves independent of cam  29 . Further, while the present invention has been illustrated having four engine brakes  40  utilized with a six cylinder engine  10 , it should be appreciated that it could be used with an engine having any number of cylinders and could include any number of engine brakes that is less than the total number of cylinders and that is capable of providing sufficient engine braking horsepower for engine  10 .  
           [0024]    In addition to the above listed modifications, it should be appreciated that any suitable compression release brake structure having, or being modifiable to include, modular characteristics could be substituted for the hydraulically actuated brake that has been illustrated. In addition, the compression release brake could be separate from the exhaust valve, and instead utilize a separate valve member. Indeed, the modularity of the present invention can allow customers to choose, and only pay for, the amount of braking horsepower they desire for a specific application.  
           [0025]    It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Thus, those skilled in the art will appreciate that other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.
summary: Traditional engine compression release brake systems include an engine brake that is associated with each cylinder of the engine. However, if the maximum braking horsepower required by the engine is less than that produced using all engine cylinders, the engine includes excess components. In an effort to reduce the number of engine components, and therefore increase engine robustness, the present invention includes an engine compression release brake system that provides a number of engine brakes that is less than the total number of engine cylinders.