Patent Publication Number: US-10767611-B2

Title: Fuel injector

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fuel injector suitable for controlling the delivery of two different fuels into an internal combustion engine. In particular, but not exclusively, the invention relates to a fuel injector suitable for injecting a gaseous fuel and a liquid fuel. 
     BACKGROUND TO THE INVENTION 
     Internal combustion engines for heavy-duty and industrial applications are typically fuelled by diesel. However, the use of natural gas as an alternative to diesel is of increasing interest. Natural gas is relatively abundant and relatively cheap, and can, in principle, provide similar levels of power to diesel whilst producing lower particulate and nitrogen oxide (NOx) emissions. 
     Natural gas can be used in place of diesel to fuel a compression-ignition engine, in which combustion of the fuel occurs as a result of compression of the air-fuel mixture in the cylinder. However, because natural gas has a higher auto-ignition temperature than diesel, it can be necessary to initiate combustion with a pilot injection of diesel fuel before introducing the natural gas to the combustion chamber. 
     In one type of natural gas-powered engine, known as a high-pressure direct injection (HPDI) engine, both natural gas and diesel are injected directly into the combustion chamber. Due to the space constraints in an engine cylinder head, it is desirable to inject both fuels using one fuel injector per cylinder. This requires a fuel injector that is specially adapted to keep the two fuels separate within the injector, and to deliver independently the respective fuel at the appropriate time. 
     One such ‘dual fuel’ injector is described in International Patent Application Publication No. WO 00/15956. In this example, a fuel injector with a concentric twin nozzle arrangement is provided. Inner and outer valve needles are engageable at their lower ends with respective valve seats to control the flow of fuel through respective inner and outer sets of outlets. The outer valve needle controls the injection of natural gas through the outer set of outlets, and the inner valve needle controls the injection of diesel through the inner set of outlets. The outer valve needle is tubular to accommodate the inner valve needle, and the inner set of outlets is formed at a tip of the outer valve needle. 
     The inner and outer valve needles are controlled independently by two electromagnetic control valves, which are configured to control the pressure of a control fluid (normally diesel fuel) within respective control chambers for the inner and outer valve needles. The control chambers receive the upper ends of the respective needles, so that changing the pressure of the control fluid in each control chamber changes the downward (closing) force on the corresponding needle. Gas or diesel fuel pressure acts on downwardly-facing thrust surfaces of the respective needles to generate an upward (opening) force on the needle. When the pressure of the control fluid in a control chamber is relatively high, the downward force is greater than the upward force and the respective needle remains seated, and when the pressure of the control fluid is relatively low, the upward force overcomes the downward force and the respective needle opens to permit fuel injection through the respective set of outlets. 
     Each control chamber is connected to a source of control fluid at relatively high pressure. Each control valve is operable to connect the respective control chamber to a low-pressure drain for the control fluid. In this way, opening of each control valve causes a reduction in the pressure of the control fluid in the corresponding control chamber, resulting in opening of the corresponding valve needle. 
     The valve needles are typically housed in a nozzle body of the injector, with the fuel outlets disposed at a tip of the nozzle body. The nozzle body is attached to a nozzle holder or injector body by a cap nut. In use, the injector is mounted in a bore in the cylinder head of the engine. The injector extends through the bore, so that the tip of the nozzle body protrudes into the respective combustion chamber. The maximum diameter of the cylinder head bore, and hence the maximum diameters of the cap nut, the nozzle body and the injector body of the injector, are constrained by the limited space available in the cylinder head. 
     The restricted maximum diameter of the nozzle body can be particularly limiting in the design of dual fuel injectors. The nozzle body must accommodate supply passages to deliver the two fuels to the tip region for injection, and may also accommodate service passages to connect with one or both of the control chambers. Accordingly, it would be desirable to maximise the amount of space available within the nozzle body. 
     The nozzle body typically defines both the seating region and a needle guide region for the outer valve needle. To mitigate wear or other damage to the seating region due to the closing impact of the outer valve needle, the nozzle body is usually made from a high-strength steel, such as a tool steel. Tool steel is relatively expensive and difficult to machine, which makes the component cost of the nozzle body relatively high. It would therefore also be desirable to reduce the cost of the nozzle body. 
     SUMMARY OF THE INVENTION 
     Against that background, from a first aspect, the present invention resides in a fuel injector for an internal combustion engine, comprising a generally tubular outer valve needle, an inner valve needle slidably received in the outer valve needle, and a nozzle body assembly comprising a tip part and a needle guide part. The tip part defines a seating region for the outer valve needle, and the needle guide part comprises a guide bore for slidably receiving the outer valve needle. 
     Advantageously, because the nozzle body assembly is formed from two parts, the different materials can be selected to improve manufacturability, reduce cost and optimise performance. Preferably, therefore, the tip part and the needle guide part are made from different materials. For example, the tip part may be made from a high-grade steel with good wear resistance and very good machinability. The needle guide part may be made from a lower-grade steel. Alternatively, or in addition, the tip part and the needle guide part may be made from materials that have been subjected to different heat treatments. 
     The design of the nozzle tip of an injector is typically tailored to suit a particular application. In particular, the number, position, angle and shape of the fuel outlet orifices in the nozzle tip can be selected to optimise performance of the injector for a particular combustion chamber design. In the present invention, only the tip part of the nozzle body assembly needs be designed to suit a particular application. The needle guide part of the nozzle body assembly, and the remaining components of the injector, can be of a common design across multiple applications. Therefore the present invention provides an injector design that can be readily tailored for different applications at minimum cost. 
     The needle guide part may have a larger outer diameter than the tip part. The needle guide part may comprise a recess for receiving an end region of the tip part. The recess may be formed in an end face of the needle guide part. 
     The injector may comprise a cap nut arranged to clamp the tip part to the needle guide part. In one embodiment, the tip part comprises a shoulder, and the cap nut is arranged to engage with the shoulder to clamp the tip part to the needle guide part. Preferably, the shoulder is disposed at an end of the tip part, opposite the seating region. In this way, the needle guide part need not include a shoulder for engagement with the cap nut, so that the diameter of the cap nut can be maximised along the length of the cap nut, creating more space within the cap nut for fuel passages and the like. 
     The inner and outer valve needles may be arranged along a common injector axis. The needle guide part may comprise one or more fuel supply passages that extend parallel to the injector axis. 
     The injector may include biasing means to bias the outer valve needle into engagement with the seating region defined by the tip part. Preferably, the biasing means is housed, at least in part, in the nozzle body assembly. For example, the needle guide part may comprise a spring chamber for receiving the biasing means. Alternatively, or in addition, the biasing means may be housed, at least in part, in the tip part. The two-part construction of the nozzle body assembly enables the biasing means for the outer valve needle to be inserted in the nozzle body assembly during assembly of the injector. 
     In one embodiment, the biasing means comprises a spring disposed around the outer valve needle. The outer valve needle may include a spring seat for the spring, for example in the form of a collar that is press-fitted to the outer valve needle. 
     The injector may further comprise sealing means to guard against leakage of fuel between the outer valve needle and the guide bore. For example, the sealing means may comprise an annular seal disposed around the outer valve needle. The sealing means is preferably disposed at or adjacent to an end of the guide bore. 
     When the fuel injector also includes biasing means for the outer valve needle, the biasing may be arranged to retain the sealing means. For example, the biasing means may bias the sealing means against a surface of the needle guide part. 
     The fuel injector is preferably a dual fuel injector. Accordingly, the outer valve needle may be arranged to control the injection of a first fuel from the fuel injector, and the inner valve needle may be arranged to control the injection of a second fuel from the fuel injector. Preferably, the first fuel is a gaseous fuel, such as natural gas, and the second fuel is a liquid fuel, such as diesel. 
     The injector may comprise a first control chamber associated with the outer valve needle, and a second control chamber associated with the inner valve needle. Movement of the outer and inner valve needles may be controllable by varying the fuel pressure in the first and second control chambers respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like reference numerals are used for like features, and in which: 
         FIGS. 1( a ) and 1( b )  are sectional views of part of a fuel injector according to one embodiment of the present invention; 
         FIGS. 2( a ) and 2( b )  are sectional views of part of a fuel injector according to another embodiment of the present invention; 
         FIG. 3( a )  is a side view of a nozzle body of the fuel injector of  FIGS. 2( a ) and 2( b ) ; and 
         FIG. 3( b )  is a perspective view of the nozzle body of the fuel injector of  FIGS. 2( a ) and 2( b ) . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIGS. 1( a ) and 1( b )  show a nozzle region of a fuel injector  10  according to a first embodiment of the present invention. The injector  10  is elongate to define an injector axis 
     A.  FIG. 1( a )  is a sectional view taken on a first plane that contains the injector axis A, and  FIG. 1( b )  is a sectional view taken on a second plane that also contains the injector axis A and is perpendicular to the first plane. 
     The injector  10  comprises a nozzle body assembly  12 , an injector body  14 , and a cap nut  16 . Only the lower ends of the injector body  14  and the cap nut  16  are illustrated in  FIGS. 1( a ) and 1( b ) . 
     The nozzle body assembly  12  comprises a needle guide part  18  and a tip part  20 . The needle guide part  18  comprises an upper, relatively large diameter cylindrical region  18   a  and a lower, smaller-diameter cylindrical region  18   b . The upper and lower regions  18   a ,  18   b  are separated by a frustoconical region  18   c . The tip part  20  is generally tubular, and comprises an upper end region  20   a  and a tip region  20   b  opposite the upper end region  20   a . The upper end region  20   a  has an increased diameter compared to the remainder of the tip part  20 , so as to define a shoulder  20   c  at the upper end of the tip part  20 . The lower end face of the lower region  18   a  of the needle guide part  18  includes a recess  18   d  that receives, in part, the upper end region  20   a  of the tip part  20 . 
     The cap nut  16  is generally tubular, and comprises an inwardly-directed flange  16   a  at its lowermost end. The flange  16   a  engages with the shoulder  20   c  of the tip part  20 , and the remainder of the tip part  20  protrudes through the circular opening  16   b  in the cap nut defined by the flange  16   a . A relatively small-diameter, tubular first region  16   c  of the cap nut is disposed adjacent to the flange  16   a . The upper region  20   a  of the tip part  20  and the lower region  18   b  of the needle guide part  18  are received within the first region  16   c  of the cap nut  16   c.    
     A relatively large-diameter, tubular second region  16   d  of the cap nut  16  is connected to the first region  16   c  by an intermediate frustoconical region  16   e . The upper region  18   a  of the needle guide part  18  is disposed in the second region  16   d  of the cap nut  16 , and the frustoconical region  18   c  of the needle guide part is disposed in the frustoconical region  16   e  of the cap nut  16 . There is a clearance between the cap nut  16  and all of the parts of the needle guide part  18 . 
     The upper end of the second region  16   d  of the cap nut  16  includes an internally threaded region (not shown), which engages with an externally-threaded region of an injector cap part (not shown), which is located at the top end of the injector body  14 , remote from the nozzle body assembly  12 . In this way, the cap nut serves to clamp together the tip part  20  and the needle guide part  18  of the nozzle body assembly  12 , and to hold the nozzle body assembly  12  in engagement with the injector body  14 . The clamping force applied by the cap nut  16  is sufficient to create a fluid-tight seal between the injector body  14  and the needle guide part  18  and between the needle guide part  18  and the tip part  20 . 
     The nozzle body assembly  12  houses two concentrically arranged valve needles for controlling the delivery of two different fuels from the injector. A first or outer valve needle  30  is slidably received within a guide bore  18   e  that extends axially through the needle guide part  18  of the nozzle body assembly  12 . The outer valve needle  30  extends into the bore  20   d  of the tubular tip part  20  of the nozzle body assembly  12 . The outer valve needle  30  is a clearance fit within the bore  20   d , so as to define a first annular accumulator volume  32  for a first fuel between the outer valve needle  30  and the bore  20   d . The accumulator volume  32  is fed with the first fuel by way of suitable passages (not shown) formed in the needle guide part  18 . 
     A tip region  30   a  of the outer valve needle  30  is slidably received in an aperture  20   e  in the end of the tip region  20   b  of the tip part  20  of the nozzle body assembly  12 . A frustoconical valve seat  30   b  is provided on the outer valve needle  30  upstream of its tip region  30   a . The valve seat  30   b  is engageable with a frustoconical seating region  20   f  formed at the tip region  20   b  of the tip part  20  to control the delivery of the first fuel from the first accumulator volume  32  through a plurality of outlets  34  (one of which is shown in  FIG. 1( b ) ), disposed in the tip part  20  downstream of the seating region  20   f.    
     An upper end of the outer valve needle  30 , opposite the tip region  30   a , is closed by a cap  36 . The cap  36  is slidably received in a bore  14   a  formed in the injector body  14 . The bore  14   a  and the end face of the cap  36  together define a first control chamber  38  for the outer valve needle. The first control chamber  38  is connected, by way of a service passage  40 , to a first control valve (not shown) which is operable to vary the pressure of a control fluid in the first control chamber  38  in a manner known in the art. When the pressure of control fluid in the first control chamber  38  is relatively high, the outer valve needle  30  is urged into engagement with the seating region  20   f . When the first control valve is operated to reduce the pressure of control fluid in the first control chamber  38  to a relatively low level, the outer valve needle  30  is caused to lift away from the seating region  20   f , thereby allowing the injection of the first fuel from the outlets  34 . 
     A second, inner valve needle  40  is slidably received within the bore  30   c  of the outer valve needle  30 . A tip  40   a  of the inner valve needle  40  is engageable with a seating region  30   b  formed at the tip region  30   a  of the outer valve needle  30  to control the release of a second fuel from a second annular accumulator volume  42 , disposed between the inner valve needle  40  and the bore  30   c  of the outer valve needle  30 , through a plurality of outlets formed in the tip region  30   a  of the outer valve needle  30 , downstream of the seating region  30   b.    
     The second accumulator volume  42  is supplied with the second fuel by way of a drilling  42   a  through the wall of the outer valve needle  30  that connects the second accumulator volume  42  to an annular gallery  42   b  formed between the outer valve needle  30  and the wall of the guide bore  18   e . As shown in  FIG. 1( a ) , a further passage  42   c  extends through the needle guide part  18  to connect the gallery  42   b  to a second fuel supply passage  42   d  in the injector body  14 . 
     The inner valve needle  40  includes a guide region  40   b  which is in sliding contact with the wall of the outer valve needle bore  30   c . The guide region  40   b  includes grooves, flutes or other formations to allow the second fuel to flow freely past the guide region  40   b . Upstream of the guide region  40   b , the inner valve needle  40  is formed into a piston region  40   c , which is also in sliding contact with the wall of the outer valve needle bore  30   c . The second fuel cannot flow freely past the piston region  40   c , so that the piston region  40   c  separates the second accumulator volume  42  from a second control chamber  44  for the inner valve needle  40 . The second control chamber  44  is disposed at the upper end of the outer valve needle bore  30   c , and is defined by the cap  36 , the wall of the outer valve needle bore  30   c , and an upper end of the inner valve needle  30 . 
     The second control chamber  44  is connected, by way of a second service flow path  46 , to a second control valve (not shown) which is operable to vary the pressure of a control fluid in the second control chamber  44  in a manner known in the art. As shown most clearly in  FIG. 1( b ) , the second service flow path  46  includes a passage  46   a  that extends through the wall of the outer valve needle  30  to connect the second control chamber  44  to an annular gallery  46   b  formed between the outer valve needle  30  and the guide bore  18   e . The second service flow path  46  also includes a passage  46   c  that extends through the guide part  18  to connect the gallery  46   b  to a further passage  46   d  in the injector body  14 . 
     When the pressure of control fluid in the second control chamber  44  is relatively high, the inner valve needle  40  is urged into engagement with the seating region  30   b . When the second control valve is operated to reduce the pressure of control fluid in the second control chamber  44  to a relatively low level, the inner valve needle  40  is caused to lift away from the seating region  30   b , thereby allowing the injection of the second fuel from the outlets in the tip region  30   a  of the outer valve needle  30 . 
     A first biasing spring  48  for the outer valve needle  30  is housed in the first control chamber  38 . A second biasing spring  50  for the inner valve needle  40  is housed in the second control chamber  44 . The biasing springs  48 ,  50  serve to provide an additional closing force to the respective outer and inner valve needles  30 ,  40  that helps to keep the needles  30 ,  40  seated when the injector is not in use. 
     In one embodiment of the invention, the first fuel is a gaseous fuel, such as natural gas, and the second fuel is a liquid fuel, such as diesel. Diesel fuel can also be used as the control fluid in the first and second control chambers  38 ,  44 . Advantageously, when the second fuel and/or the control fluid is diesel, the sliding interfaces between the valve needles  30 ,  40  and the nozzle holder assembly  12  are self-lubricated by the diesel fuel. 
     Because the nozzle holder assembly  12  is made from two parts, rather than a single part as known in the prior art, in the present invention the needle guide part  18  and the tip part  20  of the nozzle holder assembly  12  can be made from different materials. In a particularly advantageous arrangement, the tip part  20  is made from a steel with good wear resistance, such as a tool steel, whereas the needle guide part  18  is at lower risk of wear or other damage and so can be made from a cheaper material, such as a mild steel. 
     A further advantage of using the two-part nozzle holder assembly  12  of the present invention is that the needle guide part  18  and the tip part  20  can be manufactured separately, for example by machining each part from tube stock of a starting size selected according to the required maximum diameter of each part. In this way, material wastage can be minimised. 
     It will also be appreciated that, because the cap nut  16  engages with the shoulder  20   c  of the tip part  20  of the nozzle holder assembly  12 , and not with the needle guide part  18 , the diameter of the needle guide part  18  can be maximised within the envelope of the cap nut  16 . In particular, it is not necessary to reduce the diameter of the needle guide part  18  to form a step or shoulder with which the cap nut  16  can engage. 
     Part of a fuel injector  100  according to a second embodiment of the invention is shown in  FIGS. 2( a ) and 2( b ) , which are sectional views taken in perpendicular planes that intersect along the injector axis A. The second embodiment of the invention is generally similar to the first embodiment of the invention described above, and only the differences between the embodiments will be described in detail. In  FIGS. 2( a ) and 2( b ) , the cap nut and the injector body are omitted for clarity. 
     As in the first embodiment of the invention, in this second embodiment the tip part  120  of the nozzle body assembly  112  is clamped against the needle guide part  118  of the nozzle body assembly  112  by the cap nut (not shown), which engages with a shoulder  120   c  of the tip part  120 . The needle guide part  118  includes a recess  118   d  for receiving the upper end of the tip part  120 . 
     In this second embodiment, the first biasing spring  148  for the outer valve needle  130  is housed within the nozzle body assembly  112 , instead of in the injector body. To accommodate the first biasing spring  148 , the needle guide part  118  of the nozzle body assembly  112  includes a spring cavity  152  that is defined by an enlarged diameter lower bore region  118   f  that connects at its uppermost end with the guide bore  118   e  and at its lowermost end with the recess  118   d.    
     A lower end of the first biasing spring  148  seats upon a collar  154  that is press-fitted or otherwise attached to the outer surface of the outer valve needle  130 . In this way, the first biasing spring  148  acts through the collar  154  to bias the outer valve needle  130  into engagement with the seating region  120   f  of the tip part  120 . An upper end of the first biasing spring  148  serves to retain a sealing assembly  160  in position against a surface defined by the uppermost end  152   a  of the spring cavity  152 . 
     The sealing assembly  160  comprises an annular sealing member in the form of an O-ring  162   a  and a dynamic shaft seal  162   b  disposed between the O-ring  162   a  and the outer valve needle  130 . The O-ring  162   a  forms a static fluid-tight seal between the uppermost end  152   a  of the spring cavity  152  and the dynamic shaft seal  162   b . The first biasing spring  148  bears upon a steel washer  164 , which in turn presses the O-ring  162   a  against the uppermost end  152   a  of the spring cavity  152 . The compression of the O-ring  162   a  causes a radial sealing force to be applied to the dynamic shaft seal  162   b  to maintain a sealing force against wall of the outer valve needle  130 . 
     A cap  136  of the outer valve needle  130  is exposed to fuel pressure in the first control chamber (not shown in  FIG. 2 ). Advantageously, by housing the first biasing spring  148  in the nozzle body assembly  112  instead of in the first control chamber, the volume of the first control chamber can be minimised, which helps to optimise control of the movement of the outer valve needle  130 . 
     As shown in  FIG. 2( b ) , two supply passages  166  for the first fuel extend from the top face of the needle guide part  118  to communicate with an annular gallery  168  that, in turn, communicates with the spring cavity  152 . The collar  154  is dimensioned to allow free flow of the first fuel from the spring cavity  152  into the annular accumulator volume  132  defined between the outer valve needle  130  and the bore  120   d  of the tip part  120 . 
     Referring back to  FIG. 2( a ) , a supply passage  142   c  for the second fuel is formed in the needle guide part  118  to connect with an annular gallery  142   b  formed in the needle guide bore  118   e . Radial drillings  142   a  in the outer valve needle  130  connect the annular gallery  142   b  to the accumulator volume  142  for the second fuel. The annular accumulator volume  142  for the second fuel is disposed between the inner valve needle  140  and the outer valve needle  130 . A piston region  140   c  of the inner valve needle  140  separates the accumulator volume  142  for the second fuel from the second control chamber  144 . 
     A service passage  146   c  is also formed in the needle guide part  118 , to connect the second control valve (not shown) with a further annular gallery  146   b  formed in the needle guide bore  118   e . Further radial drillings  146   a  in the outer valve needle  140  connect this annular gallery  146   b  to the second control chamber  144 , in which the second biasing spring  150  is housed. 
       FIG. 3 a    shows a side view of the nozzle holder assembly  112 .  FIG. 3 b    shows a perspective view of the nozzle holder assembly  112 , in which the service passage  146   c , second fuel supply passage  142   c  and first fuel supply passages  166  are visible where they emerge from the top face of the needle guide part  118  of the nozzle holder assembly  112 . 
     Referring back to  FIGS. 2( a ) and 2( b ) , the sealing assembly  160  serves to guard against mixing of the first and second fuels, by reducing or preventing the flow of fuel from the spring chamber  152  into the guide bore  118   e  past the sealing member  162 , or vice versa. This arrangement means that the pressure of the liquid second fuel can exceed the pressure of the gaseous first fuel by a considerable amount without the risk of mixing of the fuels, therefore improving the atomisation of the second fuel upon injection. 
     Advantageously, because it is not necessary to form a step in the diameter of the needle guide part  118  of the nozzle body assembly  112  to engage with the cap nut, the supply passages  166  for the first fuel can be oriented parallel to the injector axis A, which makes the formation of the supply passages  166  during manufacture substantially easier. 
     It will be appreciated that, in this second embodiment of the invention, the use of a two-part nozzle body assembly  112  allows the sealing assembly  160  and the first biasing spring  148  to be inserted in the spring cavity  152  during assembly of the injector  100 . If the nozzle body assembly  112  were a single-piece component, it would not be possible to house the first biasing spring  148  in the nozzle body assembly  112 , and nor would it be possible to provide a sealing assembly  160 . 
     Further modifications and variations not explicitly described above could also be contemplated by a person skilled in the art without departing from the scope of the invention as defined in the appended claims.