Abstract:
An electronic unit injector comprising a spray tip including a valve seat, a needle valve arranged to close on the seat to prevent discharge of fuel from the spray tip or to open off the seat to dispense fuel from the spray tip, a spring biasing the needle valve to a closed position, a spring seat between the spring and the needle valve, the needle valve overcoming the biasing force when the pressure reaches a predetermined level, the spring and seat being disposed in a cage having port areas circumferentially arranged about said spring and spring seat to supply low pressure fuel to the area occupied by said spring and spring seat to reduce the risk of cavitation in said spring cage.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to improvements in fuel injectors for diesel engines. 
     PRIOR ART 
     A common arrangement for diesel injector assemblies has a needle valve immediately upstream of the injector orifices biased closed by a spring. The needle valve is cyclically opened by an impulse of high pressure fuel operating on an area of the needle valve that opposes the biasing spring. The spring resides in a space, typically in a part of the injector assembly referred to as a spring cage that is exposed to fuel at low pressure levels. Exposing the spring space to fuel is done to avoid a need and the practical difficulty to completely seal it from the necessarily high injection pressures. A persistent and seemingly complex problem in an electronically controlled injector is cavitation in the valve spring space. This cavitation can lead to degradation of the spring and ultimate failure. 
     U.S. Pat. No. 6,811,092 is directed to the problem of cavitation in the spring cage of an electronic fuel injector. Experience has shown the solution proposed in this patent is not effective, at least in certain applications, in satisfactorily eliminating cavitation in the spring cage. The patent indicates an earlier described arrangement of a fuel injector assembly with a spring cage vented to a low pressure region of the injector to avoid a hydraulic lock had a potential for cavitation. 
     SUMMARY OF THE INVENTION 
     The invention relates to the discovery that cavitation in a spring cage of an electronic fuel injector can be effectively eliminated by affording a sufficient, positive supply of fuel to a critical area of the spring cage. Where the spring cage, as is conventional, is a hollow cylinder, it has been found effective to port the cage walls with an area that is at least a significant fraction of the area of the spring seat and, preferably, to provide this port area in an arrangement generally surrounding the spring seat. Additionally, it is desirable to provide a port area adjacent the end of the spring cage remote from the spring seat. By porting the spring cage at opposite ends, fuel more readily circulates in and out of the spring cage area thereby improving heat transfer, lowering temperature of fuel in the spring cage and reducing the risk of cavitation. 
     In the disclosed embodiment, the spring cage is arranged to be used with an original equipment manufactured nozzle nut or a duplicate thereof. As such, in its preferred embodiment, the spring cage of the invention is a hollow cylindrical body with an outside diameter sized to provide a large functional clearance with the inside diameter of the surrounding portion of the nozzle nut. The spring cage can be concentrically located on the axis of the nozzle nut bore, for example, by indexing it to a spray tip at a lower end and at an upper end to a spacer fitted to the nozzle nut bore. In their assembled state, the spring cage and nozzle nut form an annular fuel plenum surrounding the spring cage which freely communicates with all of the ports in the spring cage wall. The annular plenum serves as a local reservoir that can supply fuel and thereby reduce the tendency for cavitation to occur within the spring cage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an injector assembly taken in a longitudinal plane of its central axis; 
         FIG. 2  is an exploded side view, partially in section, of elements of a kit including the novel spring cage (sectional in the planes indicated at the lines  2 - 2  in  FIG. 3 ) of the invention for use in the assembly of  FIG. 1 ; 
         FIG. 3  is a view of the upper end of the spring cage; 
         FIG. 4  is a view of the lower end of the spring cage; and 
         FIG. 5  is a longitudinal cross-sectional view of the spring cage taken in the plane indicated in  FIG. 3  at the lines  5 - 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An injector assembly  10  for introducing fuel to the cylinder of a diesel engine such as used in a railroad locomotive is illustrated in  FIG. 1 . The injector assembly  10  is installed on an engine in a known manner. The injector assembly  10  has a general construction like that of the prior art units shown in U.S. Pat. No. 6,811,092, the disclosure of which is incorporated herein by reference. As is common, a separate injector assembly  10  is provided for each cylinder of the engine. 
     Most of the components of the injector  10  are centered about an axis indicated at  11 . At an upper end, the assembly  10  includes a plunger socket  12  that receives a lever mechanically operated in synchronization with the engine&#39;s crankshaft. The socket  12  drives a cylindrical plunger  13  down into a fuel pressurizing chamber  14  formed in a main body or housing  16  of the injector  10 . A spring  17  encircling the top of the plunger  13  and operating through a retainer  18  returns the plunger from its fuel pressurizing stroke. Fuel is delivered into the chamber  14  by a distribution rail fed by a fuel supply pump in a known manner. The supply pressure of the fuel is relatively low, being typically in the range of about 105 psi. An electronically operated control valve  21  on the housing  16  is normally open and allows fuel being displaced from the chamber  14  by downward movement of the plunger  13  to be vented at low pressure to a return circuit. When the control valve  21  is closed by electrically energizing the coil of its armature, downward movement of the plunger  13  is immediately reflected in high pressurization of the fuel remaining in the chamber  14 . 
     The lower end of the cylindrical bore or chamber  14  is closed by a cylindrical spacer  22 . Below the spacer  22  is a cylindrical spring cage  23  and below that is a circular spray tip  24 . The spacer  22 , spring cage  23 , and spray tip  24  are held together and against the housing  16  by a nozzle nut  26  threaded onto the bottom of the housing. Aligned drilled passages  27 ,  28  and  29 , through the spacer  22 , spring cage  23 , and circular spray tip  24  communicate with one another to deliver fuel from the pressure chamber  14  to a cavity  31  in the spray tip. While only one passage in each of these components is illustrated in  FIG. 1 , it will be understood that two identical passages exist in each of these components as is suggested in  FIGS. 3 and 4 . The angular orientation of the spacer  22 , spring cage  23 , and spray tip  24  relative to one another is maintained by axially oriented pins  34  received in aligned blind holes  35  at their interfaces. A needle valve  36  having a precision sliding fit in a central bore  37  in the spray tip  24  has a tapered end  38  that seals on a seat  39  in the spray tip  24  and controls discharge of fuel out of the spray tip through orifices  41  and into a combustion chamber. 
     The spring cage  23  is a cylindrical tube having an outer cylindrical surface  46  and an inner cylindrical surface  47  forming a boundary of the interior space  48  of the spring cage. Assembled in the space  48  are a helical compression spring  51 , a spring seat  52  at the lower end of the spring, and a shim  53  at its upper end. The spring seat  52  has a blind bore in which a reduced diameter stub of the needle valve fits. At its upper side, the spring seat  52  has a cylindrical shank  54  sized to fit into the inside diameter of the helical spring  51 . When the spray tip  24 , spring cage  23 , and spacer  22  are held in place by the nozzle nut  26 , the spring  51  is compressed to hold the needle valve  36  closed on the seat  39  with a predetermined force. 
     An annular chamber  56 , formed between the nozzle nut  26  and body  16  receives pressurized fuel from the supply rail, e.g. at about 105 psi. This pressurized fuel communicates with an annular chamber  57  around the spacer through a flat  58  on a threaded area at the bottom of the housing  16 . Similarly, flats  59  on diametrally opposite outer sides of the spacer communicate rail pressure fuel to the outer periphery of the spring cage  23 . 
     Both the spray tip  24  and spacer  22  have outside diameters that produce a close fit with respective surrounding internal surfaces of the nozzle nut  26  so as to hold these elements concentric with the axis  11 . The outside diameter of the spring cage  23 , however, is significantly smaller than the inside diameter of the respective part of the nozzle nut  26 . The axial locating pins  34  serve to hold the spring cage concentric with the axis  11 . 
     In operation, the plunger  13  is driven downwardly with the force developed on the socket  12  by the engine&#39;s camshaft. Fuel in the chamber  14  below the plunger  13  is discharged through a side port in the chamber wall and through an internal passage to the control valve  21  and beyond to a return to the fuel tank. When the control valve  21  closes, fuel in the chamber  14  is immediately pressurized. This pressure is transmitted through the passages  27 - 29  to the cavity  31 . The resulting high fuel pressure in the cavity  31  lifts the needle valve  36  against the force of the spring  51  whereupon fuel is injected into the engine cylinder through the spray tip orifices  41 . A shoulder  64  on an upper end of the needle valve  36  abuts the spring cage  23  to limit opening movement of the needle valve. When the control valve  21  opens, the fuel pressure in the injector assembly  10  drops, the needle valve  36  closes and injection stops. This process repeats cyclically as the engine operates. 
     As a practical matter, pressurized fuel migrates along the needle valve  36  from the cavity  31  into the interior space  48  of the spring cage  23 . The very rapid movement of the needle valve  36  and the spring seat  52  has been found to result in destructive cavitation producing erosion and failure of the needle valve spring in prior art electronic unit injectors. With reference to  FIGS. 2 and 5 , the spring cage  23  has a plurality of ports  61  through its cylindrical wall that have been found, surprisingly, to effectively eliminate cavitation with the spring cage particularly in the area around the spring seat  52 . In one preferred arrangement, the ports  61  are distributed around the circumference of the spring cage  23  at four equally spaced locations in a plane perpendicular to the axis  11  and passing through the spring seat shank  54 . Thus, the ports  61  are at the lower end of the spring cage  23  adjacent the spring seat  52 . Supplementing these lower ports  61 , is at least one additional port  62  in the spring cage wall adjacent the upper end of the spring  51 . It is theorized that the tendency for fuel to cavitate in the area of the spring seat  52  is the result of sudden closing motion of the needle valve  36  caused by the requisite high force applied by the spring when the pressure in the cavity  31  drops following opening of the control valve  21 . This jerk-like motion of the spring seat  52  requires a similar movement of fuel directly behind it. By locating the ports  61  at or adjacent the plane of the spring seat  52  and maintaining the fuel at these ports above atmospheric pressure, i.e. at the level of the fuel supply rail, it is believed that a sufficient quantity of fuel at a sufficient positive pressure is maintained behind the space vacated by the spring seat as it drives the needle valve closed. An annular space  60  between the nozzle nut  26  and spring cage  23  serves as a fuel reservoir to instantaneously feed fuel to the space  48  or interior of the spring cage  23  through the ports  61  should a localized low pressure condition occur behind the spring seat  52  as the spring  51  snaps the needle valve  36  closed. A factor in effective avoidance of cavitation is the collective cross-sectional area of the ports  61  being at least a significant fraction of the cross-sectional area of the spring seat  52 . In the illustrated arrangement, the spring seat  52  has a nominal diameter of 0.392″ and the collective area of the ports  61  is at least about ¼ the cross-sectional area of the spring seat. Further, the ID of the nozzle nut is nominally 0.965″ and the OD of the spring cage is nominally 0.933″ leaving a cross-sectional area of the reservoir space between these surfaces approximately 4/10 of the area of the spring seat  52 . The upper port  62  can have the same diameter as that of the lower ports  61 . The reciprocating motion of the spring seat  52  as it follows the motion of the needle valve  36  can induce currents in the fuel in the spring cage  23  through the ports  61 ,  62  with the result of an improvement in heat transfer, thereby reducing temperature and, therefore, the risk of cavitation of fuel in the spring cage. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.