Abstract:
A fuel injector for an internal combustion engine, particularly of inwards or outwards opening needle injector type, includes an injector body, e.g., forming a nozzle ending in an injection office, a mechanism for closing off the injection orifice of the injector body, the closing-off mechanism including a vibrating pintle ending in a head for closing off the injection orifice, a return mechanism returning the closing-off to the position in which they close off the injection orifice, and a mechanism for setting the pintle and/or the nozzle into cyclic longitudinal vibration so as to open and close the injection orifice alternately. The fuel injector includes a selectively activatable mechanism immobilizing the pintle with respect to the body.

Description:
BACKGROUND 
     The present invention relates to a fuel injector for an internal combustion engine, in particular a diesel engine, intended in particular to be used in a motor vehicle. 
     A conventional internal combustion engine comprises at least one cylinder in which a piston slides between two maximum positions. The piston defines with the cylinder and a cylinder head a combustion chamber. In such an internal combustion engine, the purpose of an injector is to supply finely atomized fuel to the combustion chamber of the internal combustion engine. 
     From the patent FR 2,801,346 on behalf of the applicant, there is known, for example, an injection device for an internal combustion engine comprising a fuel injector  10  such as illustrated in  FIG. 6 . 
     This injector  10  comprises a body  12  including a transducer  14  which can generate vibrations in a longitudinal mode at ultrasonic frequencies. The transducer  14  terminates in the lower portion in a nozzle  16  in which the vibrations coming from the transducer  14  are amplified. 
     The assembly of the transducer  14  has a first inner cavity  18 . The first inner cavity  18  is intended to be filled with pressurized fuel. To do this, the first cavity  18  is connected to a fuel supply hole  20  which can be connected to a pressurized fuel supply circuit (not illustrated). The first cavity  18  emerges at the lower end  22  of the nozzle  16 , also called the stem of the injector, through an injection hole. 
     The injector  10  also includes a pin  24 , or needle, lying mainly along the axis y-y′. The pin  24  is installed such that it can move axially inside the nozzle  16 . The lower end of the needle  24  has a valve head  26  lying outside the nozzle  16 . This valve head  26  is designed to come into contact with the inner surface of the nozzle  16  defining the injection hole of the nozzle  16  so as to close the fuel injection hole. 
     The other end of the pin is provided with a weight  28  connected elastically by a spring  30  to the body  12  of the injector  10 . The system  32  composed of the weight  28  and the spring  30  is installed in a second cavity  34  formed in the rear portion of the body  12  of the injector  10 . 
     The pin  24  and spring  30  assembly, which is elastic, exerts an appropriate elastic return force pressing the valve head  26  of the pin  24  on the area of the nozzle  16  surrounding the injection hole. The applied preloading provides on the one hand the sealing of the injection hole made at the end of the nozzle  16  when the injector  10  is supplied with fuel at a given pressure and on the other hand the adjustment for any wear in the area of contact of the valve head  26  of the pin  24  with the nozzle  16 . 
     The weight  28  is fixed, for example, by screwing to the pin  24  so as to create a mechanical impedance break at the interface between the pin  24  and the weight  28 . 
     The value of the weight  28  and the stiffness of the spring  30  are selected to form a system having a very long response time compared with the excitation times of the transducer  14 . 
     The transducer  14  includes an area composed of a stack  36  of active piezoelectric or magnetostrictive components, which, respectively due to the application of an electric or magnetic field, change in thickness. 
     This stack  36  is clamped between two other elements  37   a ,  37   b  composed of an elastic material. The connection between the active components is provided by preloading means such as a nut  38 . The stack of several active components adds together the changes in thickness generated by each of the active components, the change in thickness resulting from the total movement of the stack of the active components remaining below the limit of elastic deformation of the preloading means. 
     Due to the application of an electric voltage to the active piezoelectric elements, these elements deform and produce an elastic deformation which is transmitted to the lower end of the nozzle  16 . 
     Preferably, the assembly  40  composed of the transducer  14  and the nozzle  16  is dimensioned to resonate at the excitation frequency of the active components to amplify the longitudinal movements right to the lower end  22  of the nozzle  16 . The pin  24 , initially closing the injection hole by means of its valve head  26 , deforms due to the pulse which is supplied to it when the nozzle  16  starts to oscillate. This deformation spreads elastically along the whole length of the pin  24  and is reflected at the interface  42  between the pin  24  and the weight  28 . 
     The characteristic responses of the pin  24  on the one hand and the nozzle  16  on the other hand make the end of the pin  24  and the opening oscillate with phase and amplitude variation. This variation results in the opening of an annular slit between the pin  24  and the end  22  of the nozzle  16 , the width of the slit depending on the phase difference and the relative difference in amplitude between the oscillation of the end  22  of the nozzle  16  and the oscillation of the valve head  26  of the pin  24 . 
     The minimum opening time of the injector  10  is of the same order as the excitation period applied to the transducer, which excitation can take place at several tens of kilohertz, typically 50 kHz, which authorizes a minimum opening time of the order of 20 μs. This makes it possible to deliver quantities of fuel of the order of one microliter during a small period of time. 
     The body  12  of the injector  10  is intended to be fixed to the upper end of the cylinder head of the engine by means which are not illustrated. 
     Although the injector  10  has indirect means of setting the pin in longitudinal vibration, also known are injectors comprising direct means of setting the pin in cyclic vibration. In particular, an injector is known comprising a stack of piezoelectric ceramics or a magnetostrictive bar mounted directly in the body of the pin and which excites the pin so as to produce elastic deformations of the pin. 
     In the two types of excitation, direct or indirect, of the pin of the injector, the pin is embedded at one end in a weight. The function of this weight is to create an impedance break so that the deformation waves being propagated in the pin are reflected at the boundary between the pin and the weight. 
     Moreover, while the injector  10  is of the outward-opening valve type, injectors of the inward-opening valve type are also known. In the case of an injector of the inward-opening valve type, the pin is pressed, at rest, on the inner face of the lower end of the nozzle due to the action of a spring. The spring is mounted in the second cavity. The closing of the injection hole is thus obtained. When the body of the injector is excited, the pin is set in longitudinal vibration. The end of the pin then oscillates between its position for closing the injection hole and a position for opening this injection hole. 
     It should be noted that, depending on the type of the injector, the spring exerts, on the pin, either a tensile force (in the case of an injector of the outward-opening valve type) or a compressive force (in the case of an injector of the inward-opening valve type). 
     However, the dimensions of the injector are fixed by the space available on the engine and in the immediate area around the engine. Thus, the volume of the injector being fixed, the space occupied by the weight+spring system providing a large enough impedance break and a satisfactory sealing force at the injection hole may correspond to a spring with a stiffness such that the weight+spring system has a resonance frequency lying in the excitation range fixed by the vibrations of the engine. An excitation of the weight+spring assembly at its resonance frequency causes the injector to open randomly. 
     A known solution to this problem consists in adding damping means to the weight+spring system. However, this solution only partially solves the problem of the resonance of the weight+spring system, such an arrangement only reducing the amplitude of the oscillations of the weight+spring system excited at its resonance frequency. 
     It is also known to fix, to the body of the injector, the weight in which the needle is embedded. However, such a solution has the disadvantage that, because of the heating of the injector and therefore the expansion of the body of the injector and the needle, uncontrolled axial forces occur in the needle. These axial forces disturb the cyclic deformation of the needle and therefore the injection by the injector. 
     BRIEF SUMMARY 
     The object of the invention is to provide a fuel injector not having the aforementioned faults and which can, in particular, provide an injection of fuel in the form of fine droplets which is better controlled in relation to the constraints of the area around the injector. 
     This object of the invention is achieved by means of a fuel injector for an internal combustion engine, in particular of the inward-opening valve type or the outward-opening valve type, including:
         an injector body forming in particular a nozzle terminating in an injection hole;   means of closing said injection hole of said injector body, said means of closing including a vibrating pin terminating in a valve head for closing said injection hole;   means of returning said means of closing to the position for closing said injection hole; and   means of setting said pin and/or said nozzle in cyclic longitudinal vibration so as to alternately open and close the injection hole.       

     According to the invention, said injector includes selectively activatable means of immobilizing said pin in relation to said body. 
     Thus, as will be seen in more detail in the remainder of the description, when the means of immobilizing the pin in relation to the body of the injector are activated, the pin does not oscillate and, therefore, any risk of resonance of the pin at vibration frequencies of the engine is removed, which manages the injection. To avoid the problems connected with the differential expansion of the pin and the body of the injector, the means of immobilizing the pin can be deactivated. In that case, the means of returning the means of closing reposition the means of closing in a position where the pin is relieved of the stresses due to the differential expansion of the pin in relation to the body. 
     Preferably, said selectively activatable means of immobilizing said pin can cooperate with said pin and/or with a weight to which said pin is fixed so as to create a mechanical impedance break. 
     Preferably, said means of immobilizing include a piston which can slide in a direction generally perpendicular to said pin. 
     Preferably, the fuel injector according to the invention includes a hydraulic control chamber for controlling the movement of said piston. 
     Preferably, said hydraulic control chamber includes at least one fuel inlet hole which passes fluid to a fuel supply hole of said injector. 
     Preferably, said hydraulic control chamber also includes at least one fuel outlet hole, the total cross section of said at least one inlet hole being less than the total cross section of said at least one outlet hole. 
     Preferably, the fuel injector according to the invention includes means for controlling the filling or the emptying of said hydraulic control chamber of the magnetostrictive or electromagnetic or electrostrictive or piezoelectric type. 
     Preferably, said means of setting said pin and/or said nozzle in cyclic vibration are of the piezoelectric and/or magnetostrictive and/or electromagnetic type. 
     Preferably, said means of setting said pin and/or said nozzle in cyclic vibration can cause elastic deformations of said pin and/or said nozzle at ultrasonic frequencies. 
     Preferably, said means of setting said pin and/or said nozzle in cyclic vibration are solidly mounted on said body and/or said pin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will emerge from an examination of the description of preferred embodiments which follows, presented only as non-limiting examples, with reference to the attached figures in which: 
         FIG. 1  illustrates a longitudinal cross-section view of an injector according to a first embodiment of the invention; 
         FIG. 2  illustrates a cross-section view along the cross-section plane A-A of the injector in  FIG. 1 ; 
         FIG. 3  illustrates a longitudinal cross-section view of an injector according to a second embodiment of the invention; 
         FIG. 4  illustrates a longitudinal cross-section view of an injector according to a third embodiment of the invention; 
         FIG. 5  illustrates a longitudinal cross-section view of an injector according to a fourth embodiment of the invention; and 
         FIG. 6  illustrates a longitudinal cross-section view of an injector according to the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     On the figures, the elements which are the same or have the same function are shown with the same reference number. 
     A first embodiment of the injector  44  according to the invention is illustrated in longitudinal cross section in  FIG. 1 . 
     The elements of the injector  44  according to the first embodiment of the invention which are the same as the elements of the injector of the prior art described above with reference to  FIG. 6  are not described again hereinafter. 
     According to the invention, the injector  44  includes means of immobilizing the weight  28  in relation to the body  12  which are selectively activatable. 
     These means of immobilizing include a piston  46  mounted such that it is free to move translationally in relation to the body  12  of the injector  44  along an axis x-x′ generally perpendicular to the axis y-y′ of the pin  24 . The means of immobilizing also include a bearing part  48  which can cooperate with the piston  46  so as to stop translation of the weight  28  along the axis y-y′. 
     Preferably, the piston  46  and/or the bearing part  48  has/have a face for bearing on the weight  28  having a shape which is complementary to that of the weight  28 . Thus, the weight  28  being, in this case, cylindrical, the piston  46  and the bearing part  48  have a bearing face which can cooperate with the weight  28  of concave, generally cylindrical, shape. Thus, advantageously, the immobilizing force exerted by the means of immobilizing is optimized for a given pressure. 
     Preferably, the bearing part  48  is made of hard steel. 
     Also preferably, the piston  46  and the bearing part  48  are generally of the same height as the weight  28  so as to provide the largest possible contact surface between the piston, the weight and the bearing part. Thus, advantageously, the immobilizing force exerted by the means of immobilizing is optimized for a given pressure. 
     So as to control the translation of the piston  46 , the injector  44  according to the first embodiment of the invention has a hydraulic control chamber  50 . This hydraulic control chamber  50  is defined on the one hand by the body  12  of the injector  44  and, on the other hand, by the piston  46 . The hydraulic control chamber  50  has a fuel inlet hole  52 . A fuel bypass channel  53  is connected to this fuel inlet hole  52 . The bypass channel  53  is connected at the other end to the supply channel  54  of the injector  44 , preferably between the supply hole  20  and the first cavity  18 . 
     The hydraulic control chamber  50  has moreover a hydraulic fluid outlet hole  56  of which the cross section is, preferably, larger than the cross section of the inlet hole  52 . This outlet hole  56  is connected to the second cavity  34 . The second cavity  34  is closed by a plug  58 . This plug  58  has a low pressure fuel discharge channel  60 . 
     Moreover, so as to control the filling or the emptying of the hydraulic control chamber  50 , the injector  44  according to the first embodiment of the invention includes a valve  62 , in this case of the electrical control type, preferably of the magnetostrictive, electromagnetic or electrostrictive type. This valve  62  can cut off the passage of fluid between the hydraulic control chamber  50  and the second cavity  34 . 
     The operation and the advantages of the fuel injector  44  according to the first embodiment of the invention result directly from the description of it which has just been given. 
     When the pressurized fuel enters the body  12  of the injector  44  via the supply hole  20 , it spreads through the first cavity  18  and the hydraulic control chamber  50 . 
     When the valve  62  is not electrically fed, it is closed and the passage of fluid between the second cavity  34  and the hydraulic control chamber  50  is interrupted. The fuel is not therefore discharged toward the second cavity  34 . The pressure of the fuel in the hydraulic control chamber  50  therefore remains high, that is to say higher than the pressure of the fuel located in the second cavity  34 . That is why the fuel pushes the piston  46  along the axis x-x′ in the direction of the weight  28 . Thus, the piston  46  holds the weight  28  in its initial position by pressing the weight  28  against the bearing part  48 . This initial position of the weight  28  and the initial tension in the pin  24  are obtained by construction, in particular by means of the spring  30  disposed in the second cavity  34 . 
     When the valve  62  is electrically fed, it opens. The fuel is then discharged toward the second cavity  34 . The pressure in the hydraulic control chamber  50  then drops and the piston  46  relaxes its hold. The weight  28  is released and the tension in the pin  24  resumes the value that the spring  30  imparts to it. 
     This activation of the opening of the valve  62  occurs at regular intervals (for example every minute) and for very short times, of the order of a few hundred milliseconds, in order to enable the tension in the pin  24  to resume the value imparted by the spring  30 , and eliminate excess tension which can occur in the pin  24  due to differential expansions of the body  12  of the injector  44  and the pin  24 . The opening of this valve  62  can, for example, be carried out between two successive injections. 
     It should be noted that the fuel, which is always supplied under pressure by a pump, continues to exert a pressure on the piston  46 . Therefore, in spite of the opening of the valve  62 , the fuel can tend to maintain a pressure, in the hydraulic control chamber  50 , higher than the pressure in the second cavity  34 . This problem is solved by the fact that the arrival of the fuel in the hydraulic control chamber  50  takes place via a narrow bypass channel  53 , and that the discharging of the fuel from the hydraulic control chamber  50  is carried out by means of a discharge hole  56  and a discharge channel  60  of larger diameter than the diameter of the bypass channel  53 . Thus, the pressure drop when discharging the fuel from the control chamber  50  is less than the pressure drop when filling this control chamber  50 . It is thus possible to facilitate the discharging of the fuel from the control chamber  50  so as to reduce, very rapidly, the pressure of the fuel in the control chamber  50 . 
     The valve  62  preferably gives a small pressure drop in order that the pressure in the hydraulic control chamber  48  drops rapidly. The bypass channel  53  adequately prevents the pressure rise in the hydraulic control chamber  50  from rising again. Thus, the pressure of the fuel in the hydraulic control chamber  50  does not have time to rise to prevent the release of the weight  28 . 
     When the valve  62  is no longer fed, it closes so as to stop the passage of fluid between the hydraulic control chamber  50  and the second cavity  34 . The pressure in the hydraulic control chamber  50  then increases. The piston  46  is then pressed on the weight  28  against the bearing part  48  so as to immobilize the weight  28 , as illustrated in  FIG. 2 . Immediately after immobilization, the force in the pin  24  has the value that the spring  30  imparts, the pin  24  being relieved of additional forces which could have been created as a result of differential expansions. 
       FIG. 3  shows a second embodiment of the injector according to the invention. The injector  66  illustrated in  FIG. 3  is different from the injector  44  according to the first embodiment of the invention in that it is an injector of the inward-opening valve type. Thus, in order to close the injection hole  68 , the pin  24  is pressed, at rest, on the inner face of the lower end  22  of the nozzle  16  due to the action of the spring  30  which is mounted in the second cavity  34 . 
       FIG. 4  shows a third embodiment of the injector according to the invention. The injector  70  illustrated in  FIG. 4  is different from the injector  44  according to the first embodiment in that it does not have a stack  36  of active components, for example piezoelectric or magnetostrictive components, mounted on the body of the injector. In fact, a stack  72  of active components which can deform due to the action of an electric or magnetic field, preferably piezoelectric or magnetostrictive components, is solidly mounted on the pin  24  so that the deformation of this stack  72  of active components directly causes the setting in longitudinal vibration of the pin  24 . 
       FIG. 5  shows a fourth embodiment of the injector according to the invention. The injector  74  illustrated in  FIG. 5  is different from the injector  66  according to the second embodiment in that it does not have a stack  36  of active components, for example piezoelectric or magnetostrictive components, mounted on the body of the injector. In fact, as has been described for the injector  70  according to the third embodiment, a stack  72  of active components which can deform due to the action of an electric current, for example piezoelectric or magnetostrictive elements, is solidly mounted on the pin  24  so that the deformation of this stack  72  of active components directly causes the setting in longitudinal vibration of the pin  24 . 
     Of course, the present invention is not restricted to the embodiments presented above as illustrative and non-limiting examples and many modifications are possible without departing from the scope of the invention. 
     Thus, the immobilizing piston and the bearing part can cooperate directly with the pin, the weight then possibly being able to be omitted. 
     Moreover, although the device formed of the piston  46  and bearing part  48  is an advantageous embodiment of selectively activatable means of immobilizing, these elements can be replaced by any selectively activatable device effectively able to carry out the immobilizing of the weight and/or the pin. In particular, as examples, an electric or hydraulic actuator or a system of immobilizing by an electromagnet can be mentioned.