Abstract:
A thermal load control assembly for a fuel injector includes a rail inlet port, a cooling inlet port and a fuel drain port. A leakage path channels leaked fuel originating from the rail inlet port to the fuel drain port. A cooling path channels fuel originating from the cooling inlet port to the fuel drain port. A fuel system using a thermal load control assembly includes a single fuel tank that supplies fuel to the rail inlet port and the cooling inlet port of a plurality of fuel injectors and collect fuel from the fuel drain port of the plurality of fuel injectors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a Divisional of U.S. patent application Ser. No. 12/287,248, filed Oct. 7, 2008, the subject matter of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to fuel injectors, and in particular to fuel injectors with a cooling feature. 
       BACKGROUND 
       [0003]    Common rail fuel systems are one of several diesel engine fuel systems used to improve diesel engine emissions and performance. Common rail fuel systems include a common rail supplying fuel to a plurality of fuel injectors. At least a part of these fuel injectors are maintained at rail pressure, while another part of the fuel injectors are kept at low pressures. The pressure differential between the various parts of the fuel injectors can create potential leakage paths. 
         [0004]    Leakage paths allow fuel to travel from high-pressure regions to low pressure regions. Any leakage of fuel that occurs at these higher fuel pressures tends to generate heat in the vicinity of the leakage path and the heat is transferred to the injector components. 
         [0005]    In addition to the increased pressures inside fuel injectors, diesel engine manufacturers have been utilizing multiple injections of fuel into the combustion chamber during any particular combustion phase to meet the increasingly stringent emissions regulations. In most cases, multiple injections are achieved by electrically energizing an actuator (e.g., solenoids, piezo-electric actuators, etc.) that controls the movement of a valve multiple times during each combustion cycle. To accomplish these multiple actuation events, more electrical energy is required. However, the increase in electrical energy supplied to the actuator often results in an increase in the heat energy that is generated. This is especially problematic in connection with the use of solenoids, which tend to be susceptible to uncertain or degraded behavior at temperatures that can be easily reached if the fuel injector is not sufficiently cooled. 
         [0006]    It has been known in the prior art that external cooling liquids may be used to cool overheated engine components. U.S. Pat. No. 4,553,059 (known as the &#39;059 patent) provides insight for cooling a piezoelectric actuator that may be degraded when the temperature of the piezoelectric element becomes higher than a Curie point. In the &#39;059 patent, the piezoelectric element experienced an increase in temperature through the repeated energization of the piezoelectric elements during injection events. The &#39;059 patent teaches the use of an external cooling liquid to cool the piezoelectric actuator by allowing the liquid to flow around the actuator. 
         [0007]    The present disclosure is directed to overcoming one or more of the problems set forth above. 
       SUMMARY 
       [0008]    In one aspect, a fuel injector comprises an injector body that defines a nozzle outlet, a common rail inlet port, a cooling inlet port and a fuel drain port. A leakage path fluidly connects the common rail inlet port to the fuel drain port. A cooling path fluidly connects the cooling inlet port to the fuel drain port. 
         [0009]    In another aspect, a common rail fuel system comprises a plurality of fuel injectors. Each of the plurality of fuel injectors includes a common rail inlet port and a cooling inlet port. A common rail is fluidly connected to the common rail inlet port. A cooling line is fluidly connected to the cooling inlet port. The common rail fuel system also includes a fuel tank for supplying fuel to the common rail and the cooling line. 
         [0010]    In yet another aspect, a method of operating a fuel system includes the steps of moving relatively small amount of fuel through a nozzle outlet of a fuel injector during a first injection event and a second injection event. The method also includes a step of moving a relatively large amount of fuel through a drain port of the fuel injector between the first injection event and the second injection event. The method also includes moving leakage fuel through the fuel drain port between the first injection event and the second injection event. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a sectioned front view of a fuel injector according to the present disclosure; 
           [0012]      FIG. 2  is an enlarged sectioned front view of a control valve of the fuel injector shown in  FIG. 1 ; and 
           [0013]      FIG. 3  is a schematic view of a fuel system having a plurality of the fuel injectors as shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The present disclosure relates to a cooling feature used in fuel injectors and fuel systems. Common rail fuel injectors include portions that are maintained under high pressures as well as other portions that are kept under low pressures. The pressure differential between the high-pressure and low-pressure portions allows for the fuel to leak from high-pressure regions to low-pressure regions. Any leakage of fuel that occurs at these higher fuel pressures tends to generate heat and the heat is transferred to the injector components. As the pressures in fuel injectors continue to increase beyond 170 MPa and soon after, beyond 200 MPa, substantially more heat is generated, which may adversely affect the performance of fuel injectors and their components. The present disclosure discusses a cooling feature that may be used in a wide variety of fuel injectors and fuel systems experiencing excess heat generation and/or insufficient heat rejection. 
         [0015]    Referring to the drawings,  FIG. 1  shows a fuel injector  100 , which includes an injector body  11  that defines a nozzle outlet  62 , a common rail inlet port  14 , a cooling inlet port  16  and a fuel drain port  18 . The injector body  11  further includes a nozzle assembly  60 , a control valve assembly  40  and a solenoid assembly  20  that includes an armature assembly  21  and a solenoid coil  25 . 
         [0016]    In the present disclosure, the nozzle assembly  60  includes a nozzle chamber  66 , a needle control chamber  72  and a direct controlled nozzle valve  64  biased by a nozzle spring  73 . The nozzle valve  64  is movable between a first position that closes the nozzle outlet  62  and a second position that opens the nozzle outlet  62 . The nozzle valve  64  includes an opening hydraulic surface  68  exposed to fuel pressure inside the nozzle chamber  66 . The nozzle chamber  66  may receive high-pressure fuel entering through the common rail inlet port  14  via a rail supply passage  35 . In the present disclosure, high-pressure fuel is coming from a common rail and thereby the pressure inside the nozzle chamber  66  is maintained at rail pressure. The nozzle valve  64  also has a closing hydraulic surface  70  exposed to fuel pressure inside the needle control chamber  72 . 
         [0017]    Referring in addition to  FIG. 2 , the control valve assembly  40  includes a control valve member  44  that moves between an upper valve seat  56  and lower valve seat  57 . A first annular opening  58  is located above the upper valve seat  56  and a second annular opening  59  is located below the lower valve seat  57 . The rail supply passage  35  extends between the nozzle chamber  66  and the first annular opening  58  of the control valve assembly  40 . A first flow restrictor  36  extends between the rail supply passage  35  and the needle control chamber  72 . A valve supply passage  33  extends from the area between the upper valve seat  56  and the lower valve seat  57  to a second flow restrictor  37 , which is fluidly connected to the needle control chamber  72 . The second flow restrictor  37  has a larger flow area than the first flow restrictor  36 . A fuel drain passageway  34  extends between the drain port  18  and the second annular opening  59 . In  FIGS. 1 and 2 , the dotted lines representing the fuel drain passage  34  may appear disconnected because of the sectional view shown. However, the fuel drain passage  34  fluidly connects the second annular opening  59  to the drain port  18 . 
         [0018]    The control valve assembly  40  includes the control valve member  44  and a valve guide  52  disposed inside a control valve  41 . The control valve member  44  has an outer surface  46  and the valve guide  52  has an inner surface  54 . There is a guide clearance  50  (shown greatly exaggerated) between the outer surface  46  of the control valve member  44  and the inner surface  54  of the valve guide  52 , which allows the control valve member  44  to travel within the valve guide  52  without excessive wear. However, those skilled in the art may appreciate that there is a narrow guide clearance  50  between the inner surface  54  of the guide piece  52  and the outer surface  46  of the control valve member  44 , and that the guide clearance  50  runs along the length of the control valve member  44 . 
         [0019]    The injector body  10  defines a hollow cavity  12  inside which the control valve assembly  40  is positioned. The injector body  10  has a casing  11 , which has an internal surface  13  that encloses the control valve assembly  40 . Further, the control valve  41  has an external surface  42  that is adjacent the internal surface  13  of the injector body casing  11 . There is a cooling clearance  30  separating the external surface  42  of the control valve  41  and the internal surface  13  of the injector body casing  11 . Those skilled in the art will appreciate the cooling clearance  30  to extend throughout the length of the control valve  41  and throughout the distance between the internal surface  13  of the injector body casing  11  and the external surface  42  of the control valve  41 . 
         [0020]    At some point along the valve guide  52 , the valve guide  52  may define a weep annulus  48 . The weep annulus  48  accumulates the fuel that leaks up along the guide clearance  50 . A weep annulus passage  49  may allow fuel to flow from the weep annulus  48  to the cooling clearance  30 . The weep annulus passage  49  may be a bore drilled inside the control valve  41  or may be an internal passage made from ordinary machining methods. Those skilled in the art may appreciate that the location of the weep annulus  48  may affect the amount of heat transfer between the fuel and the solenoid coil  25  inside the armature assembly  21 . As the leakage fuel gets closer to the solenoid coil  25 , the greater heat transfer there may be between the coil  25  and the surrounding fuel. Therefore, those skilled in the art may select a position on the valve guide  52 , which is far enough from the armature assembly  21  to inhibit the leaked fuel from entering into the armature assembly  21 . Also, the location at which the weep annulus passage  49  joins the cooling clearance  30  may vary. In one embodiment, fuel that leaks out of the guide clearance  50  into the weep annulus passage  49  may join the cooling clearance  30  as close as possible to the fuel drain port  18 . The fuel that leaks out of the guide clearance  50  into the weep annulus passage  49  is defined as the leakage fuel. In one embodiment, the leakage fuel also includes any fuel that enters the fuel injector through the common rail inlet port  14  and leaves the fuel injector through the fuel drain port  18 . 
         [0021]    The injector body  10  also includes the armature assembly  21 , which further includes an armature  22  disposed in an armature cavity  26 . The armature cavity  26  has a cooling inlet opening  27  through which fuel enters the armature assembly  21 . The cooling inlet opening  27  is connected to the cooling inlet port  16  via a cooling supply passage  32 . It may be appreciated by those skilled in the art that the cooling inlet port  16  may be located at various locations inside the fuel injector  100 . The cooling supply passage  32  may be a bore drilled inside the injector body  10  and may have a diameter sized to allow fuel to flow into the fuel injector  100  at varying desired flow rates. 
         [0022]    A load screw  38  may be located inside the injector body  10  and may secure components of the fuel injector  100  to the injector body  10  while containing the pressure inside the injector body  10 . The load screw  38  may include at least one load screw bore  39  passing through it, allowing fuel to travel between the different portions of the injector  100 , including fuel from the armature cavity  26  to the cooling clearance  30 . 
         [0023]    Referring still to  FIGS. 1 and 2 , the fuel injector  100  also includes the fuel drain port  18 . The fuel drain port  18  is fluidly connected to a fuel drain passage  34 , allowing fuel to flow from inside the fuel injector  100  to the fuel drain port  18 . Because the fuel drain port  18  and the fuel drain passage  34  are at low pressure, high pressure fuel that leaks from the valve guide  52  and fuel that enters from the cooling inlet port  16  will travel towards the fuel drain port  18 . For the sake of simplicity, cooling fuel is defined to mean any fuel that enters into the fuel injector  100  through the cooling inlet port  16  and leaves the fuel drain port  18 , and leakage fuel is the fuel that leaks out of the guide clearance  50  into the weep annulus passage  49 . However, those skilled in the art will appreciate that during the multiple cycles of operation, the cooling fuel and the leakage fuel may mix inside the fuel injector  100  and therefore, the cooling fuel and leakage fuel may not be discernable during the actual operation of the fuel injector  100 . 
         [0024]    A leakage path is defined as the flow path of the leakage fuel beginning at the point it enters the common rail inlet port  14  and leaves the fuel injector  100  through the fuel drain port  18 . The leakage path includes the area defined by the guide clearance  50  and the area defined by the weep annulus  48  and the weep annulus passage  49 . Similarly, the flow path of the cooling fuel defines a cooling path. The cooling path is the flow path of the fuel entering in from the cooling inlet port  16  and leaving the fuel injector  100  through the fuel drain port  18 . The cooling path also includes the load screw passage  39 , the cooling clearance  30 , the armature cavity  26  and the area inside the solenoid assembly  20 . In one embodiment, the leakage fuel merges with the cooling fuel before exiting the fuel drain port  18 . 
         [0025]    Those skilled in the art may recognize that the present disclosure may be implemented in numerous possible ways. For instance, instead of having one cooling inlet port  16 , a fuel injector  100  may have more than one cooling inlet port  16  that enters at various locations within the injector body  10 . Similarly, a fuel injector  100  may have more than one fuel drain port  18  and the drain ports may be located at different locations within the injector body  10  as well. However, the present disclosure is not intended to limit the scope of the disclosure to the embodiments discussed herein. Instead, the present disclosure intends to include all embodiments that fall within the spirit of the disclosure. 
         [0026]    Referring also to  FIG. 3 , a fuel system schematic is shown. A fuel system  500  including a plurality of fuel injectors  200  includes a first injector  101  and a second injector  102  where the first and second fuel injectors  101  and  102  could be any of the plurality of fuel injectors  200 . The fuel system  500  further includes a common rail  80  fluidly connected to the common rail inlet port  14  of each of the plurality of identical fuel injectors  200 . A cooling line  82  may be fluidly connected to the cooling inlet port  16  of each of the plurality of fuel injectors  200 . A fuel return line  72  may fluidly connect the fuel drain port  18  of each of the plurality of fuel injectors  200  to a fuel tank  90 . 
         [0027]    In a different version of the disclosure, the cooling line  82  may be connected to the first fuel injector  101 . The fuel drain port  18  of the first fuel injector  101  may be fluidly connected to the cooling inlet port  18  of the second fuel injector  102 . Similarly, in a fuel system  500  with more than two fuel injectors  100 , the fuel drain port  18  of a preceding fuel injector may be fluidly connected to the cooling inlet port  16  of the succeeding fuel injector, such that the fuel injectors are sequentially arranged. 
         [0028]    The fuel tank  90  has at least one inlet port  88  and at least one outlet port  89 . The at least one inlet port  88  is fluidly connected to the fuel return line  86  of the plurality of fuel injectors  200 . However, it is conceivable that each fuel injector  100  may be fluidly connected to a respective inlet port  88  of the fuel tank  90 . The outlet port  89  of the fuel tank  90  is fluidly connected to an inlet port  93  of a fuel transfer pump  92 , which moves fuel from the fuel tank  90  to the cooling line  82  and an inlet port  97  of a common rail pump  96 . The common rail pump  96  has an outlet port  98  that is fluidly connected to the common rail  80 . 
         [0029]    In one embodiment of the disclosure, the fuel system  500  may have a first filter  83  that filters the fuel between the fuel tank  90  and the fuel transfer pump  92  and a second filter  84  that filters the fuel from the fuel transfer pump  92  to the cooling line  82  and common rail  80 . In another embodiment, a pressure regulator  85  between the fuel return line  86  and the fuel tank  90  may control the flow of fuel. In another embodiment of the disclosure, an electronic controller  76  may be in communication with a temperature sensor  77  positioned between the plurality of fuel injectors  200  and the fuel tank  90 . The electronic controller  76  may execute a cooling control algorithm that has an input signal from the temperature sensor  77  to control the cooling function of the fuel system  500 . 
         [0030]    Although the embodiments disclosed in the disclosure discuss common rail fuel injectors, it remains within the scope of the disclosure to include other embodiments not limited to common rail fuel injectors or common rail fuel systems. Further, it may be appreciated by those skilled in the art that fuel injectors come in various shapes and forms and different embodiments of a fuel injector should not limit the scope of the disclosure in any way. All fuel injectors having one of a variety of nozzle assemblies, control assemblies and armature assemblies, including those using or not using solenoid actuators lie within the spirit of the present disclosure and are thus within the intended scope of the present disclosure. 
       INDUSTRIAL APPLICABILITY 
       [0031]    The present disclosure finds potential application in fuel injectors and fuel systems in any engine or machine. The present disclosure has a general applicability in fuel injectors having an actuator that generates heat during operation and fuel injectors operating under high pressures, and a particular applicability in common rail fuel injectors. 
         [0032]    The present disclosure is directed towards fuel injectors and fuel systems, which include a plurality of fuel injectors. For the sake of clarity, this disclosure will describe a common rail fuel system in terms of one of its solenoid actuated fuel injectors. Further, the present disclosure is not limited to common rail fuel systems but include other fuel systems as well. Similarly, all types of fuel injectors including solenoid actuated, piezoelectric actuated, and cam actuated fuel injectors fall within the scope of this disclosure. 
         [0033]    To better understand the cooling feature of the present disclosure, a general understanding of the operation of a fuel injector during an entire injection event is described. Before an injection event, the solenoid coil  25  is in a de-energized state. When the solenoid coil  25  is de-energized, the armature assembly  21  biases the control valve assembly  40  to a first configuration, where the control valve member  44  is at the lower valve seat  57 . When the control valve assembly  40  is in the first configuration, the first annular opening  58  establishes a fluid connection between the needle control chamber  72  and the high-pressure nozzle chamber  66  via the rail supply passage  35  and the valve supply passage  33 . In this configuration, high-pressure fuel from the common rail inlet port  14  passes through the rail supply passage  35  in to the nozzle chamber  66  and the first annular opening  58  of the control valve assembly  40 . Because the control valve member  44  is seated at the lower valve seat  57 , a fluid connection between the first annular opening  58  and the valve supply passage  33  is established. Because the valve supply passage  33  is fluidly connected to the needle control chamber  72  via the second flow restrictor  37 , high-pressure fuel also passes into the needle control chamber  72  from the valve supply passage  33 . Also, high-pressure fuel from the rail supply passage  35  passes into the needle control chamber  72  through the first flow restrictor  36 . The high-pressure fuel in the needle control chamber  72  acts on the closing hydraulic surface  70  of the nozzle valve  64 . The pressure exerted on the closing hydraulic surface  70  combined with the preload of the nozzle spring  73  is greater than the pressure acting on the opening hydraulic surface  68 , thereby biasing the nozzle valve  64  towards the nozzle outlet  62  and keeping the nozzle outlet  62  closed. 
         [0034]    When the control valve member  44  is at the lower valve seat  57 , there is high pressure inside the nozzle chamber  66 , the pressure communication passage  35 , the first annular opening  58 , the valve supply passage  33 , the first and second flow restrictors  36  and  37 , and the needle control chamber  72 . Because there is high pressure within these passages, the fuel may find its way into lower pressure regions inside the fuel injector  100 . For instance, leakage fuel may travel up the guide clearance  50  between the valve guide  52  and the control valve member  44  into the weep annulus  48  and through the weep annulus passage  49  into the cooling clearance  30 . The rate at which leakage fuel enters into the cooling clearance is defined as the leakage rate. This rate may be determined by calculating the difference between the rate of flow of fuel entering the cooling inlet port and the rate of flow of fuel leaving the fuel drain port  18 . The rate of flow of fuel entering through the cooling inlet port  16  into the fuel injector  100  is defined as the cooling flow rate and is about an order of magnitude greater than the leakage rate of the fuel injector  100 . The term about means that when a number is rounded to a like number of significant digits, the numbers are equal. Thus both 0.5 and 1.4 are about equal. The term “order of magnitude greater” means an exponential change of plus 1 in the value of quantity or unit. Therefore, the term “about an order of magnitude greater” means an exponential change of plus 0.5 to plus 1.4 in the value of quantity or unit. Therefore, for instance, if the leakage rate is 1 unit and the cooling rate is about an order of magnitude greater than the leakage rate, the cooling rate could lie anywhere from 3.2 to 25.1 units. 
         [0035]    When the leakage fuel flows from a high-pressure region to a low pressure region, some heat is generated. As the rail pressure is increased to higher levels, and the pressure difference increases, more heat is generated and this heat is dissipated along the leakage path. The heat dissipated is transferred to the injector components causing the temperature of the injector components and the leakage fuel to rise. 
         [0036]    Independent of whether the solenoid coil  25  is in a de-energized state or an energized state, fuel from a cooling line  82  of the fuel system  500  enters into the fuel injector  100  through the cooling inlet port  16 . The fuel that comes from the cooling line  82  is the same fuel that enters the common rail inlet port  14 , although it may enter at a lower pressure. The cooling fuel travels from the cooling inlet port  16  through the cooling supply passage  32  into the armature cavity  26 . As the pressure of the cooling fuel is greater than the pressure of fuel at the fuel drain port  18 , the cooling fuel will travel from the higher-pressure region to the lower pressure region. Further, the armature cavity  26  may be fluidly connected to the solenoid assembly  20  allowing cooling fuel to cool the area around the solenoid coil  25 . 
         [0037]    The armature cavity  26  may also be fluidly connected to the external surface  42  of the control valve  41  through at least one load screw bore  39  located on the load screw. At least one load screw bore  39  may be drilled through or threaded to allow cooling fuel to enter into contact with the external surface  42  of the control valve  41 . Because the control valve assembly  40  is positioned inside the hollow cavity  12  formed by the injector body casing  11 , cooling fuel enters into the cooling clearance  30 . The cooling fuel flows through the cooling clearance  30 , which is fluidly connected to the fuel drain passage  34 . There is a portion of the cooling path where the cooling fuel flows through the cooling clearance  30 . This portion of the cooling path includes a heat exchange interface with the external surface  42  of the control valve  41 . Therefore, there is heat exchange between the cooling fuel and the control valve  41 , thereby reducing the temperature of the control valve  41 . 
         [0038]    In the present disclosure, the weep annulus  48  allows leakage fuel to flow through the weep annulus passage  49  into the cooling clearance  30 , where the leakage fuel merges with the cooling fuel. The merged cooling fuel and leakage fuel then flow together into the fuel drain passage and out of the fuel injector  100  through the fuel drain port  18 . The amount of fuel leaving the fuel drain port  18  is a combination of the cooling fuel supplied and the leakage fuel. 
         [0039]    When the solenoid coil  25  is energized, the armature assembly  21  no longer exerts a force on the control valve assembly  40  and the control valve assembly  40  moves towards a second configuration. The control valve assembly  40  remains in this configuration until the solenoid coil  25  is de-energized again. An injection event begins when the solenoid coil  25  is energized from a de-energized state and ends when the solenoid coil  25  is de-energized from the energized state. Upon energizing the coil  25 , the control valve member  44  moves and becomes seated at the high-pressure valve seat  56 , blocking the fluid connection passing through the first annular opening  58 . Instead, the second annular opening  59  is open and the second annular opening  59  fluidly connects the needle control chamber  72  to the fuel drain passage  34  via the valve supply passage  33 . Because the fuel drain passage  34  is at a lower pressure than rail pressure, the pressure difference allows fuel, which was at high pressure inside the needle control chamber  72 , to flow through the second flow restrictor  37  and the valve supply passage  33  and into the fuel drain passage  34  via the second annular opening  59 . The second flow restrictor  37  has a greater flow rate than the flow rate of the first flow restrictor  36 . Therefore, more fuel can leave the needle control chamber  72  via the second flow restrictor  37  than the fuel that can enter the needle control chamber  72  via the first flow restrictor  36 . Hence, the pressure inside the needle control chamber  72  becomes lower as more fuel is leaving the needle control chamber  72 . As the pressure inside the needle control chamber  72  drops, the pressure acting on the closing hydraulic surface  70  also drops. Eventually, the pressure acting on the opening hydraulic surface  68  exceeds the combined force of the pressure acting on the closing hydraulic surface  70  and the preload of the nozzle spring  73 , causing the direct controlled nozzle valve  64  to move away from the nozzle outlet  62 . The nozzle outlet  62  is now open and a small amount of fuel moves through the nozzle outlet  62 . The amount of fuel that moves through the nozzle outlet  62  is relatively small compared to the relatively large amount of fuel that moves through the fuel drain port  18 . 
         [0040]    Because the cooling fuel may be entered through the cooling line  82  during and between injection events, there may always be a relatively large amount of fuel leaving the fuel drain port  18 . In one embodiment of the present disclosure, the cooling fuel may be controlled to flow through the cooling inlet port  16  when the solenoid coil  25  is de-energized, or in other words, between injection events. Similarly, leakage fuel flows between injection events and may also flow during injection events as well. 
         [0041]    In one embodiment of the disclosure, a relatively small amount of fuel may flow through the nozzle outlet  62  during a first injection event and a second injection event. Between the first and second injection events, the nozzle outlet  62  is closed and there is high-pressure fuel inside the fuel injector  100 . Inherently, some fuel around the control valve member  44  may begin to leak into the weep annulus  48 , and down the weep annulus passage  49  towards the drain port  18 . Therefore, in between the first and second injection events, a relatively large amount of fuel as well as leakage fuel may flow through the fuel drain port  18  of the fuel injector  100 . Furthermore, it is possible that leakage fuel may move through the guide clearance  50  up to the weep annulus  48  during the first and second injection events and between the first and second injection events. Because there is leakage fuel moving through the guide clearance  50  both during and between the first and second injection events, this leakage fuel along with the cooling fuel, which is a relatively large amount of fuel may flow through the drain port  18 , both during and between the first and second injection events. 
         [0042]    Referring to the fuel system as shown in  FIG. 3 , the fuel system  500  includes the fuel tank  90  containing fuel that is supplied to the common rail inlet port  14  and the cooling inlet port  16  of each of the plurality of fuel injectors  200  in the fuel system  500 . Fuel from the fuel tank  90  is pumped to the cooling line  82  and inlet port  97  of the common rail fuel pump  96  by the fuel transfer pump  92 . The fuel flows through the outlet port  89  of the fuel tank  90  into the inlet port  93  of the fuel transfer pump  92 , which may be passively controlled. The fuel flowing from the outlet port  94  of the fuel transfer pump  92  may pass through a series of filters  83  and  84  before entering the plurality of fuel injectors  200 , to remove any particles that may affect the performance of the fuel injectors  100 . The outlet port  94  of the fuel transfer pump  92  may connect to the cooling line  82  and the inlet port  97  of the common rail pressure pump  96 , which may be controlled by the electronic controller  76 . The fuel then enters the common rail  80  at rail pressure and flows into each of the fuel injectors  100  through their respective common rail inlet ports  14 . Fuel from the cooling line  82  flows into the fuel injectors  100  through their respective cooling inlet ports  16 . During each engine cycle, relatively small amounts of fuel are injected through the nozzle outlets  62 , while relatively large amounts of fuel leave the fuel drain ports  18  and return to the fuel tank  90  via the fuel return line  86 , even if the cooling line  82  is kept closed during injection events. In between injection events, no fuel in injected through the nozzle outlets  62  of the fuel injectors  100 , but relatively large amounts of fuel continue to leave the respective fuel drain ports  18  and return to the fuel tank  90  via the fuel return line  86 . The pressure regulator  85  may be positioned along the fuel return line  86  to regulate the circulation of flow of the fuel. 
         [0043]    Those skilled in the art will appreciate the scope of this disclosure and will realize the scope is not limited to the embodiments described herein. Therefore, changes made to the fuel system and the addition or removal of components that control the flow of the fuel in the fuel system  500  fall within the scope of the present disclosure. For instance, in one embodiment, an engine controller configured to execute a cooling control algorithm may be used. A temperature sensor  77  may be used to provide information to the cooling control algorithm regarding the temperature inside the fuel injectors. If the temperature is higher than a predetermined high-temperature marker, the cooling control algorithm may send a signal to a fuel transfer pump  92  to increase the cooling flow rate into the fuel injectors. Similarly, if the temperature is lower than a predetermined low-temperature marker, the cooling control algorithm may send a signal to the fuel transfer pump  92  to reduce the cooling flow rate of the fuel system  500 . In another embodiment, the cooling flow rate may be increased when the engine speed is increased. An electronic controller  76  may control the cooling flow rate by determining the speed of the engine and adjusting the cooling flow rate accordingly. Furthermore, a back-pressure regulator  85  may also regulate the flow of fuel. The cooling line  82  may be supplied at rail pressure or fuel entering the cooling line  82  may flow through a step down pump to reduce the pressure inside the cooling line  82 . Further, the fuel drain port  18  of each injector  100  may be fluidly connected to the cooling line  82  or the fuel tank  90  directly. All other embodiments that are within the spirit of the disclosure are intended to fall within the scope of this disclosure. 
         [0044]    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 disclosure in any way. Thus, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure, and the appended claims.