Abstract:
A unit injector assembly is provided that helps to ensure maximum injection flow while minimizing the harmful affects of pressure waves within the unit injector assembly. The unit injector assembly has a first fuel passageway extending from the plunger piston pumping chamber to the tip check stem injection chamber and valve seat. The unit injector assembly has a second fuel passageway extending from the tip check stem injection chamber to a fuel control valve assembly. A flow restricting fuel passageway connects one of the pumping chamber and the first fuel passageway with the second fuel passageway and acts to minimize cavitation at the valve seat and effectively eliminates pressure waves within the unit injector assembly.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to a unit fuel injector for an internal combustion engine and more particularly to a unit injector having a stabilized pilot injection.  
         BACKGROUND  
         [0002]    Unit fuel injectors are well known in the art for controlling the timing and volume of fuel being injected into respective combustion chambers of an engine. Typically, many of these unit injectors are mechanically or hydraulically actuated. In most applications currently used, the timing of the respective injections are controlled electronically based on various system parameters. In one example of these known unit injectors, fuel is delivered to a pumping chamber through an electrically controlled valve assembly and a plunger piston acts in response to rotation of a cam arrangement moving the plunger piston to force the fuel from the pumping chamber. When the electrically controlled valve assembly is closed, the fuel is forced towards a pressure responsive check valve and subsequently to a fuel nozzle for injection into the combustion chamber of the engine. When the electrically controlled valve is open during the movement of the plunger piston, the fuel is forced to flow back into the low-pressure fuel gallery. In many of these known systems, unstable pilot injection may occur. These unstable pilot injections may be a result of fluctuations in the pressure in the system due to the fluid dynamics therein. An example of such as system is illustrated in U.S. Pat. No. 5,494,220 which issued on Feb. 27, 1996 to R. D. Shinogle et al. This patent attempts to offset pressure variations around the periphery of the valve seat in order to prevent or minimize weakening of the fluid seal at the valve seat. The pressure variations are typically a result of sudden changes in fluid flow within the unit injector that result in undesirable pressure fluctuation (spikes). Likewise, these pressure spikes may also cause cavitation at the nozzle tip during injection of fuel into the combustion chamber.  
           [0003]    The subject invention is directed to overcoming one or more of problems as set forth above.  
         SUMMARY OF THE INVENTION  
         [0004]    In one aspect of the subject invention, a unit injector assembly is provided for controllably delivering fuel to a combustion chamber of an engine. The unit injector assembly includes an injector body having a plunger piston bore and an injection chamber bore defined therein. A valve assembly is disposed in the injector body and is selectively movable between a flow passing position and a flow blocking position. A plunger assembly is disposed in the injector body and has a plunger piston disposed in the plunger piston bore of the injector body to define a pumping chamber therein. An injector tip assembly is disposed in the injector body and has a tip check stem disposed in the injection chamber bore of the injector body and operative to control the flow of injection fuel from the injection chamber bore to the combustion chamber. A first fuel passageway is defined in the injector body between the pumping chamber and the injection chamber bore and a second fuel passageway is defined in the injector body between the injection chamber bore and the valve assembly. A flow restricting fuel passageway is defined in the injector body interconnecting the first and second fuel passageways.  
           [0005]    In another aspect of the present invention, a method is provided for controlling pressure spikes in a unit injector assembly. The method includes the step of providing a plunger assembly connected to an injector tip assembly through a first fuel passageway; providing a valve assembly connected to the injector tip assembly through a second fuel passageway; and providing a flow restricting fuel passageway between the first and second fuel passageways. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a partial diagrammatic and partial schematic representation of an embodiment of the subject invention in one mode of operation; and  
         [0007]    [0007]FIG. 2 is a partial diagrammatic and partial schematic representation of the embodiment of the subject invention in another mode of operation. 
     
    
     DETAILED DESCRIPTION  
       [0008]    Referring to FIGS. 1 &amp; 2, a unit injector assembly  10  is illustrated in cooperation with a cam arrangement  12  of an engine (not shown), a source of fuel  14 , and a combustion chamber  16  of the engine (not shown). The cam arrangement  12 , in a well known manner, has a cam lobe  17  disposed thereon.  
         [0009]    The unit injector assembly  10  includes an injector body  18  having a plunger piston bore  20  and an injection chamber bore  22  defined therein. A plunger assembly  23  has a plunger piston  24  that is slideably disposed in the plunger piston bore  20  and defines a pumping chamber  26  in the plunger piston bore  20 . The plunger piston  24  extends from the injector body  18  and is in mating contact with the cam arrangement  12 . The plunger piston  24  is biased towards the cam arrangement by a spring  28 . It is recognized that the plunger piston  24  could be composed of two or more elements without departing from the essence of the subject invention.  
         [0010]    A valve assembly  30  is operatively disposed in the injector body  18  and is connected to the source of fuel  14  by a conduit  32 . The valve assembly  30  is selectively movable between a closed, flow blocking position, as shown in FIG. 1 and an open, flow passing position, as illustrated in FIG. 2. The valve assembly  30  is movable, in a known manner, between its open and closed position in response to an electrical signal delivered through an electrical line  34 . It is recognized that the valve assembly  30  could be separate from the injector body  18  and connected to the injector body  18  VIA a conduit (not shown).  
         [0011]    An injector tip assembly  36  is disposed in the injector body  18  and includes a nozzle tip  38  disposed in the injector body  18 . The nozzle tip  38  has a valve seat  40  disposed at one end of the injection chamber bore  22 . A plurality of passages  41  are defined in the nozzle tip  38  between the valve seat  40  and the combustion chamber  16 . A tip check stem  42  is disposed in the injection chamber bore  22  and is biased by a spring  44  towards the valve seat  40 . A differential area  46  is defined on a portion of the tip check stem  42  within the injection chamber bore  22 . In a well known manner, the differential area  46  is operative, in response to pressurized fuel in the injection chamber bore  22 , to urge the tip check stem  42  away from the valve seat  40  against the bias of the spring  44 .  
         [0012]    A first fuel passageway  50  is defined in the injector body  18  by a first passage  52  that is disposed between the pumping chamber  26  and the injection chamber bore  22 . A second fuel passageway  54  is defined in the injector body  18  by a second passage  56  that is disposed between the valve assembly  30  and the injection chamber bore  22 .  
         [0013]    A flow restricting fuel passageway  58  is defined in the injector body  18  by a connecting passage  60  that is disposed between the pumping chamber  26  and the second passage  56 . The connecting passage  60  has an orifice  62  disposed therein. It is recognized that the flow restricting fuel passageway  58  could also be connected between the first and second passageways  50 ,  54  without departing from the essence of the subject invention. It is recognized that the connecting passage  60  could be of a size sufficient to provide the needed flow restriction without having to provide the orifice  62 .  
         [0014]    It is recognized that the subject invention could be utilized in various injector valve arrangements without departing from the essence of the subject invention. For example, the fuel being supplied to the pumping chamber  26  could be supplied directly to the pumping chamber without going through the valve assembly  30 . In this type of arrangement, the fuel is directed to the pumping chamber  26  through another conduit having a one-way check valve therein. Likewise, in other fuel injector assemblies, the tip check stem  42  of the subject disclosure could be replaced with a direct-operated check valve (needle valve) in which the needle valve is directly controlled and not controlled by the injection pressure within the injection chamber bore  22 . Other possible ways of utilizing the subject invention would be known to one skilled in the art.  
       INDUSTRIAL APPLICABILITY  
       [0015]    The unit injector assembly  10  of FIG. 1 is illustrated in its fuel injection mode of operation with the valve assembly  30  in its flow blocking position. As the cam lobe  17  engages the plunger piston  24 , the plunger piston  24  is moved against the bias of the spring  28  and forces fuel from the pumping chamber  26 . Prior to the lifting portion of the cam lobe  17  engaging the plunger piston  24 , the valve assembly  30  is in its flow passing position, as illustrated in FIG. 2. With the valve assembly  30  open, fuel from the source of fuel  14  is delivered through the valve assembly  30 , the second fuel passageway  54 , the injection chamber bore  22 , and the first fuel passageway  50  to the pumping chamber  26 . The valve assembly  30  is closed prior to the lifting portion of the cam lobe  17  contacting the plunger piston  24 . As the cam lobe  17  urges the plunger piston  24  against the bias of the spring  28 , fuel is forced through the first fuel passageway  50  into the injection chamber bore  22 . Since the valve assembly  30  is in its flow blocking position, the pressure of the fuel within the injection chamber bore  22  quickly increases. The pressurized fuel in the injection chamber bore  22  acts on the differential area  46  of the tip check stem  42  creating a force that urges the tip check stem  22  against the bias of the spring  44 .  
         [0016]    Once the force being generated by the pressurized fuel acting on the differential area  46  of the tip check stem  42  reaches a predetermined value, the tip check stem  42  moves away from the valve seat  40 . The predetermined value of the force is reached when the pressure in the injection chamber bore  22  acting on the differential area  46  overcomes the force of the spring  44 . As the tip check stem  42  moves away from the valve seat  40 , fuel is passed therethrough and injected across the plurality of passages  41  into the combustion chamber  16 .  
         [0017]    Due to the overall length of the first and second fuel passageways  50 , 54 , pressure fluctuations (spikes) may occur within the injection chamber bore  22 . These pressure fluctuations may cause the tip check stem  42  to operate in an erratic manner by opening and closing prematurely during the injection mode. Such pressure fluctuations can result in cavitation of the fuel at the valve seat  40 . Pressure fluctuations in the injection chamber bore  22  are primarily offset by the flow restricting fuel passageway  58  interconnecting one of the pumping chamber  26  and the first fuel passageway  50  with the second fuel passageway  54 . The restricted flow of fuel across the flow restricting fuel passageway  58  acts to more quickly pressurize the fuel in the second fuel passageway  54 . This eliminates the tendency of a pressure wave (water hammer effect) being generated within the first and second fuel passageways  50 , 54  and the injection chamber bore  22 . The pressure wave (increase and decrease in pressure) tends to move back and forth within the first and second fuel passageways  50 , 54  and the injection chamber bore  22 . This event causes the tip check stem  42  to become erratic and bounce which adversely affects the quality of the injection cycle.  
         [0018]    Once the cam lobe  17  permits the plunger piston  24  to retract, the pressure in the injection chamber bore  22  quickly reduces and the force of the spring  44  urges the tip check stem  42  against the valve seat  40  thus closing the fuel injection cycle. At the same time, the valve assembly  30  is moved to its flow passing position. With the valve assembly  30  in the flow passing position, fuel is once again delivered through the first and second fuel passageways  54 , 50  to fill the pumping chamber  26  as the plunger piston  24  retracts. At the close of the fuel injection cycle, the flow restricting passageway  58  ensure that the tip check stem  42  quickly and positively seats against the valve seat  40  by providing an additional path of fuel flow to more quickly reduce the pressurized fuel in the pumping chamber  26 .  
         [0019]    From the foregoing, it is readily apparent that the subject unit injector assembly  10  provides maximum injection flow to the combustion chamber  16  free of cavitation at the valve seat  40  while maintaining stable, consistent movement of the tip check stem  42 .  
         [0020]    Other aspects, objects and advantages of the invention can be obtained from a study of the drawings, the disclosure and the appended claims.