Abstract:
The fuel injector comprises a hollow casing, fixed on which is a nozzle having a nebulizer for the fuel under pressure, a needle axially mobile for opening and closing the nebulizer by means of a first end thereof, and a control rod, substantially coaxial with the needle and in direct engagement therewith. The needle is normally pushed into a closing position of the nozzle by the fuel under pressure acting on the rod, aided by a compression spring, which acts on the needle through a sleeve that is in axial engagement with a portion of the rod. The sleeve further comprises a surface designed to engage at the front and directly one end of the needle. The spring engages another surface of the sleeve so as to transmit its action directly onto the needle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a fuel injector for an internal-combustion engine. In particular, the invention relates to an injector comprising a casing, fixed on which is a nebulizer having a nozzle for the fuel under pressure, an axially mobile needle for opening and closing the nozzle by means of a first end thereof, and a rod for controlling the needle, which is controlled by the fuel under pressure, aided by a compression spring. 
         [0003]    2. Description of the Related Art 
         [0004]    As is known, the control rod is substantially coaxial with the needle, which is normally pushed into a closing position of the nozzle by the fuel under pressure in a control chamber, associated to a metering solenoid valve. The compression spring is set in a cavity of the casing and acts on the needle in general through a washer or other element for adjustment of the lift of the needle and/or of the pre-loading of the spring. Furthermore, in general, set between the rod and the needle is an intermediate element, which is provided in classes, such as to enable adjustment of the total axial dimensions of the ensemble formed by the needle, the intermediate element and the rod. The intermediate element presents the drawback of generally causing a certain transverse component of the action of the rod on the needle, which leads to an irregular wear and hence a faster deterioration of the injector. In order to limit this drawback, generally the intermediate element must be made with very high precision, which consequently renders it relatively costly and complex to provide. 
         [0005]    In a known injector, the needle has a second end having a conical depression, on which the rod acts. In order to guarantee a perfectly axial resultant of the mutual action of the second end of the needle by the rod, the conical depression of the needle is engaged through an intermediate ball. In a variant of this injector, the conical depression of the needle is engaged directly by one end of the rod, which is shaped so as to guarantee a perfectly axial resultant of the mutual action. This known injector is relatively costly to produce, also on account of the conformation of the two engagement ends and of the shims for the spring. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The aim of the invention is to provide a fuel injector that will present high reliability and a limited cost, and provide better performance of fuel injectors according to the known art. 
         [0007]    According to the invention, the above aim is achieved by a fuel injector for an internal-combustion engine, as claimed herein. 
         [0008]    In particular, the spring acts on the needle through a perforated intermediate member axially engaging with a portion adjacent to the end of the rod and are formed by a bushing having an area or surface of engagement with the needle with an external diameter that is smaller than or equal to that of the needle. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    For a better understanding of the invention a preferred embodiment is described herein by way of example with the aid of the annexed drawings, wherein: 
           [0010]      FIG. 1  is an axial section of a fuel injector according to the invention; 
           [0011]      FIG. 2  is a part of  FIG. 1  at an enlarged scale; and 
           [0012]      FIGS. 3 and 4  are two sections of two variants of a detail of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    With reference to  FIG. 1 , number  1  designates as a whole a fuel injector for an internal-combustion engine, in particular for a diesel engine (not illustrated). The injector  1  comprises an external hollow casing  2 , which extends along an axis  3  and has a side inlet  5 , designed to be supplied with fuel at a high pressure. The injector  1  further comprises a terminal nebulizer  7 , for injecting the fuel into a corresponding cylinder of the engine. Normally, the nebulizer  7  is kept closed by a conical end  8  of a shutter needle  9 . 
         [0014]    In particular, the nebulizer  7  is carried by a body, referred to hereinafter as nozzle  10 , which is coaxial with respect to the casing  2  and is fixed in a known way to a portion  11  of the casing  2  itself. Another portion  12  of the casing  2  is set on the opposite side with respect to the nozzle  10  and houses an electromagnetically controlled metering valve  13 , of a known type and not described in detail. The valve  13  has an outlet  14  for sending, towards the usual fuel tank (not illustrated), the fuel discharged by the valve  13  itself and the part of fuel that leaks through the internal components of the injector  1 . 
         [0015]    The nozzle  10  carries a cylindrical axial compartment  16 , which comprises a cylindrical hole  17 , axially guided in which is, in a fluid-tight way, a portion  18  of the needle  9 , which hence shares the axis  3 . The compartment  16  is engaged by a second portion  19  of the needle  9 , which terminates with the end  8  and has a diameter slightly smaller than that of the portion  18 . Defined between the portion  19  and the wall of the axial compartment  16  is a channel  20 , which, on one side, gives out into the nebulizer  7  and on the other is in communication with the inlet  5 , through a pipe  21  and an annular injection chamber  22 . 
         [0016]    The injector  1  further comprises an axial control rod  23 , which, under the control of the valve  13 , is designed to slide in a compartment  24  of the portion  11  of the casing  2 , which also shares the axis  3 . In particular, the valve  13  comprises a valve body  25  fixed to the body  2  of the injector  1 , which is provided with an axial hole  26 , guided in which is, in a fluid-tight way, a portion  27  of the rod  23 . The portion  27  terminates at the top with a surface  28  that defines a control chamber  29 . The chamber  29  is in communication with the inlet  5  for the fuel via a calibrated inlet hole  30  and with the outlet  14  via a calibrated discharge hole  31 . The latter is normally kept closed by a shutter  32  controlled in a known way by an electromagnet  33 . 
         [0017]    The rod  23  is provided with one end  34  opposite to the surface  28 , which is designed to act on a second end  35  of the needle  9  opposite to the conical end  8 . The rod  23  is thus subjected to the opposite axial thrusts of the pressure of the fuel present in the injection chamber  22  on the needle  9  and of the pressure of the fuel present in the control chamber  29 . Normally, with the valve  13  closed, the pressure of the fuel on the rod  23  prevails over the pressure on the needle  9  so that the nebulizer  7  is kept closed. 
         [0018]    Furthermore, the compartment  24  of the portion  11  of the casing  2  comprises a bottom portion  36  having a larger diameter, which forms an annular shoulder  37 . The rod  23  is provided with a portion  38  adjacent to the end  34 , set around which is a compression spring  39  housed in the portion  36  of the compartment  24  and designed to contribute to carrying the needle  9  into a closing position, as will be seen in greater detail in what follows. 
         [0019]    According to the invention, the spring  39  acts on the needle  9  through perforated intermediate means, designated as a whole by  40 , which are in axial engagement with the portion  38  of the rod  23 , the end  34  of which is designed to engage directly and at the front the second end  35  (see also  FIG. 2 ) of the needle  9 , adjacent to the portion  18 . In particular, the end  34  of the rod  23  and the end  35  of the needle  9  are represented by two corresponding front surfaces, which are in contact with one another. Preferably, the end  34  of the rod  23  has an arched shaped or is shaped like a spherical cap, whilst the end  35  of the needle  9  is preferably plane. 
         [0020]    The perforated intermediate means  40  comprise an area  43 , which is able to slide axially, with a certain amount of play, in the portion  36  of the compartment  16 . Furthermore, the intermediate means  40  comprise an area  44  having an external diameter smaller than that of the area  43  and smaller than the diameter of the portion  18  of the needle  9 . In this way, any possible displacement of the area  43  in the portion  17  of the compartment  16  of the nozzle  10  is allowed, in the case where the end  35  of the needle  9  were to be inside the portion  17  itself, ensuring that the load of the spring  39  will be transmitted only to the needle  9  excluding the nozzle  10  from said load. 
         [0021]    The two areas  43  and  44  each have an axial hole  45  of a diameter corresponding to the diameter of the portion  38  of the rod  23  so that the means  40  are guided axially by the portion  38  of the rod  23  through said hole  45 . The portion  38  has a diameter smaller than that of the portion  18  of the needle  9  so that also the end  34  of the rod  23  has a diameter smaller than that of the end  35  of the needle  9 . According to the variant of  FIGS. 1 and 2 , the two areas  43  and  44  are made of a single piece and form a single bushing  40 . The two areas  43  and  44  are cylindrical and have two external annular, plane, surfaces  47  and  48 , which are opposite and perfectly parallel to one another. The two areas  43  and  44  form between them an annular shoulder  49 , which is also external. The compression spring  39  can act directly on the plane surface  47  of the bushing  40 , whilst the plane surface  48  is designed to act directly against an annular portion of the end or surface  35  of the needle  9 . 
         [0022]    In use, when the electromagnet  33  causes opening of the metering valve  13 , the pressure in the control chamber  29  drops rapidly so that the pressure in the injection chamber  22  acting on the needle  9  prevails over the resultant of the reduced pressure acting on the rod  23  and of the spring  39  acting on the bushing  40 . The needle  9  is hence displaced upwards thus opening the nebulizer  7  and compressing the spring  39  against the shoulder  37  of the compartment  16 . When then the electromagnet  33  is no longer energized and the shutter  32  closes under the action of elastic contrast means, in themselves known, the pressure of the fuel in the control chamber  29  is restored so that, on the one hand, the end  34  of the rod  23  pushes the needle  9  towards the nebulizer  7  and, on the other hand, the spring  39  acts on the bushing  40 , which, by means of its surface  48 , contributes to the thrust of the needle  9  in the direction of the nebulizer  7 . 
         [0023]    According to the variant of  FIG. 3 , the two areas  43   a  and  44   a  also form a single bushing  40   a , but the area  44   a  has the shape of a truncated cone instead of being cylindrical. The surface  48  of the area  44   a  acts also against the end  35  of the needle  9  (see also  FIG. 2 ), but its external diameter and the angle of opening of the conical area  44   a  are such that, when the needle  9  is in the position for closing the nebulizer  7 , the external surface shaped like a truncated cone of the area  44   a  will not touch the edge of the cylindrical hole  17  of the nozzle  10 . 
         [0024]    According to the variant of  FIG. 4 , the intermediate means  40   b  comprise two areas formed by two separate bushings  43   b  and  44   b , which both have cylindrical external lateral surfaces. In this case, the two plane surfaces in contact with the two bushings  43   b  and  44   b  must be machined with a precision sufficient to guarantee the parallelism of the two external plane surfaces  47  and  48 . 
         [0025]    From the above description, it is evident that, in all the variants described of the intermediate means  40 ,  40   a ,  40   b , the surfaces  47  and  48  are external to the hole  45  and are hence in sight. In particular, they are without any projection in the bottom area so that machining thereof is simpler and more precise. Furthermore, there is no need to have any adjustable element between the spring  39  and the intermediate means  40 ,  40   a  and  40   b  nor a resting element between the spring  39  itself and the shoulder  37 . However, such an adjustment or resting element does not modify operation of the injector  1 . 
         [0026]    It is understood that various modifications and improvements can be made to the injector described herein without departing from the scope of the claims. For example, the surfaces of contact of the ends  34 ,  35  of the rod  23  and of the needle  9  can both be plane or curved in a complementary way. In turn, the bushing  40  of  FIG. 2  can be provided with a groove made between the shoulder  49  and the external lateral surface of the area  44 , for example, for machining requirements. It is moreover possible to provide between the cylindrical areas  43  and  44  a linked area of transition different from the conical one. The external surface of the area  44   a  of the bushing  40   a  can also be shaped differently, or the areas  43   a  and  44   a  can be englobed in a single conical surface between the plane surfaces  47  and  48 . Also the external surface of the bushing  44   b  can be conical or with a shaped profile. Finally, the bushing  40  can have a constant diameter, whereas the portion  18  of the nozzle  9  can be provided with an undercut for enabling its displacement, or else said portion  18  can have a length such that its end  35  will remain always outside the hole  17 . 
         [0027]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
         [0028]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.