Patent Publication Number: US-9897058-B2

Title: Fuel injector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 13/382,603 filed on Jan. 6, 2012 which is a national stage application under 35 USC 371 of PCT Application number PCT/EP2010/060576 filed on Jul. 21, 2010 which claimed the benefit of EP patent application number 09166713.9 filed on Jul. 29, 2009, the entire disclosures of each are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD OF INVENTION 
     The invention relates to a fuel injector for use in the delivery of fuel to a combustion space of an internal combustion engine. In particular, the invention relates to a fuel injector of the type intended for use in a so-called ‘common rail’ compression ignition internal combustion engine system. 
     BACKGROUND OF INVENTION 
     In a known piezoelectrically actuated fuel injector, as disclosed in EP0995901, a piezoelectric actuator is operable to control the position of a control piston which is moveable to control the pressure of fuel within a control chamber defined, in part, by a surface associated with a valve needle of the injector. Movement of the control piston to reduce the pressure of fuel within the control chamber causes the valve needle to lift away from a corresponding valve seat region in order to permit fuel to be injected through one or more nozzle outlets. Re-pressurisation of fuel pressure within the control chamber causes the valve member to be moved to re-engage the valve seating region. 
     A problem exists with the above configuration of fuel injector in that the sudden ejection of fuel through the nozzle outlets, together with movement of the internal injector components such as the piezoelectric actuator and the control piston, causes pressure wave activity within the fuel passages of the injector. The pressure wave activity can affect the quantity of fuel that is delivered through the nozzle outlets so that the actual volume of delivered fuel is not the same as the demanded fuel delivery volume as determined by the electrical driving signal provided to the piezoelectric actuator. Inaccuracy of the actual fuel delivery volume against demanded fuel delivery volume can cause excessive exhaust smoke emissions and increases fuel consumption both of which are undesirable. 
     It is an object of the invention to provide a fuel injector which does not suffer from the above problem. 
     SUMMARY OF THE INVENTION 
     To this end, the invention provides a fuel injector for use in an internal combustion engine, the fuel injector comprising a valve member that is moveable within a nozzle bore so as to be engageable with a valve seat region to control fuel delivery through one or more nozzle outlets, an injector body defining an accumulator volume for storing high pressure fuel, and housing a linear actuator that is operable to move the valve member. The invention is characterised in that the fuel injector includes a damping chamber in fluid communication with the accumulator volume through a fluid passage, the damping chamber serving to reduce pressure wave activity within the accumulator volume. 
     The invention is particularly applicable to a fuel injector in which the actuator is housed within the accumulator volume, although it should be appreciated that this is not an essential requirement of the invention and it also has utility to fuel injectors where an actuator is located away from the fuel passages within the injector. 
     In one embodiment where the actuator, preferably a piezoelectric actuator, is housed within the accumulator volume, the injector body may include a control piston coupled to the actuator to modify the pressure of fuel within a control chamber defined, in part, by a surface associated with the valve member such that movement of the valve member is coupled to fuel pressure in the control chamber. 
     In principle, the damping chamber may be defined in any part of the fuel injector provided that it communicates with the accumulator volume through the fluid passage so pressure waves can oscillate into and out of the chamber. For example, the damping chamber could be a closed volume defined in the nozzle body, or other component, having a single fluid passage in communication with the accumulator volume. However, for convenience of manufacture and space efficiency, it is preferred that the damping chamber is defined by the control piston as this uses the control piston for an additional purpose, thus avoiding the requirement for a specific component. 
     Conveniently, the damping chamber may be defined by an open-ended bore provided in the control piston, the bore being shaped to receive a free end of the actuator. 
     In order to ‘tune’ the damping chamber to be most responsive to the dominant frequency of the pressure wave activity, the dimensions of the damping chamber and the passage may be configured appropriately in accordance with the equation where:
         F is the dominant frequency of the pressure wave activity which it is desirable to reduce;   A is the speed of sound in the fluid medium (fuel) at a pressure and temperature associated with an operating condition of the injector;   S is the cross sectional area of the fluid passage of the damping chamber;   V is the volume of the damping chamber; and   L is the length of the fluid passage.       

     The invention also relates to an internal combustion engine having a fuel injector in accordance with the invention therein. 
     It will be understood that by the term “nozzle outlets” it is meant the holes (or apertures) through which fuel is injected from the injection nozzle of the fuel injector and into an associated engine cylinder (in use), which may also be referred to as injection holes, spray holes or similar terms known in the art. By “a set of nozzle outlets” it is meant the one or more nozzle outlets through which fuel is injected when a valve needle is disengaged from an associated seating region. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In order that the invention may be better understood it will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectioned view of a portion of a fuel injector in accordance with an embodiment of the invention; and 
         FIG. 2  is an enlarged sectioned view of a fuel injector similar to that in  FIG. 1  but which shows a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to  FIG. 1 , a fuel injector  2  is generally elongate in form and includes an injector body  4 , a lower end of which is attached to an injection nozzle  6 . A bucket-shaped cap nut  8  includes a tubular side wall  10  that carries an internal threaded section  12  and a base part  14  which defines a central aperture  16  through which a tip  18  of the injection nozzle  6  extends. A shoulder  20  of the cap nut  8  defined by the aperture  16  bears against a flange portion  22  of the injection nozzle  6  by which means the injection nozzle  6  is secured to the injector body  4  by rotating the cap nut  8  with respect to the injector body  4  so that the threaded section  12  cooperates with a complimentarily threaded section  24  of the injector body  4 . 
     The injector body  4  is generally tubular in form so as to define an internal chamber  30  that constitutes an accumulator volume for storing pressurised fuel and also for housing a linear actuator  32 . Internal chamber  30  is hereinafter referred to as accumulator volume  30 . Although not shown in  FIG. 1 , a fuel inlet passage is provided at the upper end of the injector body  4  to supply fuel to the accumulator volume  30  from a source of pressurised fuel, which may be a high pressure fuel pump or a common rail fuel volume of an internal combustion engine system. The upper end of the injector body  4  also includes electrical connections for supplying power to the actuator, but it should be noted that the exact configuration is not essential to the invention. Note that EP0995901 exemplifies a known fuel injector having a fuel inlet connector and electrical power supply connectors provided at its uppermost end. 
     The actuator  32  takes the form of a stack-type piezoelectric actuator, the general structure of which would be known to the skilled person and is described in EP0995901, the entire contents of which are incorporated herein by reference. The actuator  32  includes an end region  34  that is coupled to a valve member  40  of the injection nozzle  6  by a motion amplifier arrangement, as generally indicated by  42 , and as will be described in more detail hereafter. 
     The valve member  40  is generally needle-shaped and is slidably received within a blind bore  43  provided in the injection nozzle  6 . A tip end  44  of the valve member  40  is shaped for engagement with a valve seat region  46  defined adjacent the blind end of the bore  43 . 
     The valve member  40  is of stepped form and includes a relatively large diameter region  40   a  which has a diameter substantially equal to that of the adjacent part of the bore  43  and so is guided for sliding movement within the bore  43 . The lower end of the valve member  40  includes a smaller diameter region  40   b  which defines with the bore  43  an annular delivery chamber  48  for fuel around the smaller diameter region  40   b . Although not shown in the Figures, it should be noted that the injector body  4  and the injection nozzle  6  include fuel passages to convey fuel from the accumulator volume  30  to the delivery chamber  48 . Engagement between the tip end  44  of the valve member  40  and the valve seat region  46  controls fluid communication between the delivery chamber  48  and a set of nozzle outlets  50  that are provided in the injection nozzle  6  in a position downstream of the valve seat region  46 . 
     In addition, the valve member  40  defines an angled step  52  at the interconnection of the relatively large region and relatively small diameter regions  40   a ,  40   b  which forms a thrust surface on which pressurised fuel within the delivery chamber  48  acts to provide a force on the valve member  40  urging it away from the valve seat region  46 . Similarly, the tip end  44  of the valve member  40  provides a further thrust surface via which a further force is applied. 
     As has been mentioned above, the actuator  32  is coupled to the valve member  40  by way of an amplifier arrangement  42  the primary purpose of which is to convert, or transmit, expansion of the actuator, that is to say axial movement of the end region  34  of the actuator, into axial movement of the valve member. In this embodiment, the amplifier arrangement  42 , hereinafter referred to as sleeve  42 , is formed from a generally cylindrical sleeve member or piston which is H-shaped in cross section having an upper tubular wall region  42   a  and a lower tubular wall region  42   b , both wall regions defining a respective opening. 
     The opening of the lower wall region  42   b  of the sleeve  42  receives an enlarged head region  60  of the valve member  40  such that an upper end  60   a  of the head region  60  opposes a base of the lower wall region  42   b  to define a control chamber  62  for fuel. It should be noted that it is not essential for the enlarged head region  60  to be connected to the valve member  40 , and it may be a separate component. 
     At the other end of the sleeve  42 , the opening of the upper wall region  42   a  receives the end region  34  of the actuator in a press-fit such that linear movement of the end region  34  due to a change in energisation state of the actuator  32  is coupled mechanically to the sleeve  42 . 
     In use, with the accumulator volume  30  supplied with fuel under high pressure and with the actuator  32  having an energisation state which causes it to adopt an extended position, as shown in  FIG. 1 , the sleeve  42  occupies a position in which the fuel in the control chamber  62  is pressurised to an extent sufficient to ensure that the force applied to the valve member  40  by pressurised fuel in the control chamber  62  holds the valve member  40  in engagement with the valve seat region  46  against the action of fuel pressure in the delivery chamber  48  acting on the thrust surfaces  52 . In this circumstance, fuel injection through the nozzle outlets  50  does not take place. In practice, a spring would be housed within the control chamber  62  to provide an additional closing force to the valve member  40  in order to ensure that the valve member remains seated when the injector is not operating. However, such a spring is not shown here for the purpose of clarity. Also, it should be noted that although not shown in  FIG. 1  the sleeve member is provided with a restricted passage to permit pressurised fuel to flow into the control chamber at a restricted rate. 
     In order to initiate fuel delivery through the nozzle outlets  50 , the actuator  32  is operated to a second energisation state (in this case, de-energised) in which its axial length is reduced. Since the upper end of the actuator  32  is held in a fixed position relative to the injector body  4 , the change in the energisation state of the actuator  32  to reduce its axial length results in an upward movement of the end region  34  of the actuator  32 . The upward movement of the end region  34  is therefore transmitted to the sleeve  42 , which lifts by a corresponding amount. This upward movement of the sleeve  42  acts to increase the volume of the control chamber  62  thereby reducing the pressure of fuel therein acting on the valve member  40 . The reduction in fuel pressure will continue to a point beyond which the downward force on the valve member  40  is insufficient to keep the valve member  40  in engagement with the valve seat region  46  so the valve member  40  will move axially away from the valve seat region  46  and, as a result, fuel is able to flow from the delivery chamber  48  through the nozzle outlets  50 . 
     To terminate fuel injection, the actuator  32  is returned to its initial energisation state such that the sleeve  42  is pushed in a downward direction returning it to the original position. As a result, fuel pressure within the control chamber  62  increases, applying a greater force to the valve member  40 , such that a point is reached beyond which the fuel pressure in the control chamber  62  is able to urge the valve member  40  back into engagement with the valve seat region  46 . 
     It will be appreciated that the fuel injector forms part of an internal combustion engine, in use, and that whilst the engine is operating the fuel injector performs many injection operations every second. When the valve member  40  is lifted away from the valve seat region, the delivery chamber  48  undergoes rapid depressurisation as fuel rushes out of the delivery chamber  48  through the nozzle outlets  50  and into an associated combustion cylinder of the engine. This depressurisation of the delivery chamber  48  causes a negative pressure wave to travel through the fuel passages of the injection nozzle and into the accumulator volume  30  of the injector body  4 . The pressure waves are made worse by the rapid movement of the actuator  32  and the sleeve  42 . 
     The pressure wave activity generated by operation of the injector causes the fuel pressure in the delivery chamber to oscillate. Since the quantity of fuel delivered through the nozzle outlets is a function of the time period that the valve member is ‘open’ and the pressure of fuel, it will be appreciated that the pressure wave activity has a detrimental affect on the fuel delivery accuracy. 
     In order to attenuate the pressure waves that propagate through the fuel injector, and particularly the accumulator volume, the amplifier arrangement includes a damping chamber  70  that is provided by the closed volume, or chamber, defined between the upper wall region  42   a  of the sleeve  42  and the end region  34  of the actuator  32 . It should be noted at this point that it is not essential that the end region  34  of the actuator closes the chamber, and other configurations may occur to the skilled person. For example, the chamber could be closed by a ceiling defined by the sleeve, the actuator being secured to the sleeve  42  by another method—for example by a screw thread, or a press fit within a further opening defined by the sleeve  42 . 
     The damping chamber  70  is provided with a fluid passage, or neck,  72  in the form of a single drilling located approximately mid-way along the upper wall region  42   a  such that the damping chamber  70  is in fluid communication with the accumulator volume  30 . The neck  72  opens into the accumulator volume  30  at an annular groove  74  provided in the upper wall region  42   a  that provides an unobstructed entrance to the neck  72  and also provides some flexibility to select the length of the neck  72 , the purpose of which will be described. 
     The damping chamber  70  provides a volume in which the pressure waves propagating through the accumulator volume  30  can resonate and reduce over time. Conveniently, the dimensions of the damping chamber  70  are selected so that the chamber is most responsive at damping a certain frequency according to the formula: 
             F   =       A     2   ⁢           ⁢   π       ⁢       S   VL               
where:
         F is the frequency that needs to be damped;   A is the speed of sound in the fluid medium (fuel) at a suitable pressure and temperature associated with an operating condition of the injector;   S is the cross sectional area of the neck of the damping chamber;   V is the volume of the damping chamber; and   L is the length of the neck.       

     Therefore, the volume of the damping chamber  70  and/or the dimensions of the neck  72  are configurable to match the frequency response of the damping chamber  70  to the dominant frequency of the pressure waves. 
     It would be apparent to the skilled person that various modifications could be made to the above described embodiment without departing from the inventive concept, as defined by the claims. For example, although the neck  72  is shown in  FIG. 1  to extend from approximate mid-way along the length of the damping chamber, this need not be the case. For example,  FIG. 2  shows an alternative embodiment in which similar parts to those in  FIG. 1  are denoted by the same reference numerals. In  FIG. 2 , the damping chamber  70  is provided with a relatively small sub-chamber  80  and a neck  82  that extends from the sub-chamber  80  to the outer edge of the sleeve  42 . Since the neck  82  in this embodiment extends through a relatively thick walled section of the sleeve  42  from the sub-chamber  80 , there is a greater freedom to configure the length of the neck appropriately to fine tune the frequency response of the damping chamber  70 . For example, to reduce the length of the neck  82 , the sub-chamber  80  can simply be formed in a part of the base closer to the outer edge of the sleeve  42 . 
     In the embodiments of  FIGS. 1 and 2 , the damping chamber is formed in the sleeve member that couples movement of the actuator to the valve member and it should be noted that this particular configuration provides a space efficient solution to the pressure wave problem since the creation of the chamber does not require additional components to those already part of the actuator. However, the general principle of the invention is applicable to other injector structures: for example if an injector includes a piezoelectric actuator located outside the fluid passage of the injector, the damping chamber could be formed in another component, or in part of the injector body, or in the injection nozzle. However, it is preferable if the neck of the damping chamber is located in close proximity to the injection nozzle since the pressure waves propagate from this region into the fuel passages of the injector body. 
     It should be mentioned at this point that the accumulator volume in the embodiments of  FIGS. 1 and 2  comprises a relatively large volume defined by the injector body that houses the piezoelectric actuator. However, the term ‘accumulator volume’ also encompasses other fuel passages of injectors, even those which do not house piezoelectric actuators, for example drillings, slots and flutes since it is the combination of the fuel passages and volumes that function together to store pressurised fuel. 
     Similarly, the presence of a piezoelectric actuator is not essential to the invention and a suitable damping chamber could also be formed in an electromagnetically operated fuel injector.