Abstract:
A fuel injector for internal combustion engines having a control valve arranged upstream of a main flow valve is configured so that the connection leading over the control chamber between the inlet and outlet, which is regulated by the valve member of the control valve, directs the function of a throttle position, among other things, by way of a limit stop of the control piston against the front wall overlapping the control chamber for the purpose of reducing control leakage.

Description:
BCKGROUND AND SUMMARY OF THE INVENTION 
   The invention concerns a fuel injector for internal combustion engines, in particular for internal combustion engines operated with diesel or heavy fuel as injection medium. 
   Considerable actuating or retention forces must be applied to some extent in fuel injectors to control the valve closing member of a main flow valve. A control valve is provided for this purpose upstream of the main flow valve, which has a control chamber that is limited to a certain extent in its volume by means of an upwardly movable control piston, whose positioning motions can be transmitted to the valve closing member of the main flow valve, for example, by means of a nozzle needle of an injection nozzle or an injection quantity control valve. The actuating forces of the control piston are dependent upon the pressure in the control chamber, in which a throttled high-pressure inlet ends, and from which a throttled and controlled outlet starts. If the outlet is open, then the pressure in the control chamber is reduced and part of the control chamber volume is displaced into the outlet by means of the control piston. Considerably greater than this quantity of injection medium also used as control medium, which is displaced into the outlet, is the leakage that occurs when the outlet is open due to the bypass between the high-pressure inlet and the low-pressure outlet, notwithstanding the two-sided throttling as control leakage. 
   This applies when the ends of the inlet and outlet are exposed toward the control chamber, regardless of which of the wall areas of the control chamber, which are not passed over or covered by the control piston, are allocated thereto. 
   A pertinent allocation can be found in European publication EP 0 907 018 A2, wherein the inlet-side opening cross section to the control chamber is allocated to its front wall and overlaps the front side of the control piston, and the outlet-side opening cross section lies radially outside of the front-side range of the control piston in the area that is not passed over by the control piston. In this way, the control member and the corresponding actuating devices of the control valve are arranged radially with respect to the control piston in accordance with the radial position of the outlet-side opening cross section, to achieve a shortened and compact design of the fuel injector. 
   In a fuel injector of the kind mentioned above, the inlet to the control chamber is radially outside of the front-side range of the control piston with respect to its opening cross section and is allocated to the outlet-side opening cross section of the front wall of the control chamber that overlaps the front side of the control piston, while the control piston has a vaulting with respect to its front side, which is part of the limit stop that overlaps the outlet-side opening cross section allocated to the front wall. In this way, by means of the limit stop with an open control valve and depressurized control chamber, a position of the control piston is produced, which more or less blocks the connection between the inlet and outlet. A solution of this kind is connected to the fact that, due to the flat-shaped vaulting of the front side of the control piston, an essential part of the front-side cross section surface is not available for a direct pressurization in connection with the changeover of the control piston from its open position into the blocked position, so that the adjustment of the actuated valve closing member, for example, the nozzle needle, is delayed in the closing direction in accordance with the delayed displacement of the control piston. A technical consideration of these facts is difficult by means of a corresponding actuation of the control valve, since already small geometrical changes in the limit stop or in the small gaps produced by this type of limit stop have a greater effect on the response characteristic of the control piston, so that a stable accurate control of the closure movement of the actuated valve closing member is made more difficult, if not impossible. 
   It is an object of the invention to configure a fuel injector of the kind mentioned above so that, starting from the control valve and its activation, the displacement of the control piston and therefore the closing motion of the valve closing member can be accurately initiated also in a stable manner with respect to the appropriate operating times. 
   This is attained in accordance with the invention in a fuel injector of the kind mentioned above in which the position of the limit stop in a front-side peripheral zone of the control piston and the end of the inlet in a gap delimited by the limit stop is given by a provided admission surface for the injection medium introduced at the inlet side. The injection medium is under high pressure and is used as control medium, and this has as a consequence that, when the control valve is activated and the control piston is displaced against the front wall of the space that is delimited by the limit stop, and when the control piston comes ever closer to the front wall, a pressure cushion is formed, whose volume is filled by means of a connection to the inlet side, and therefore to the pressurized side, with a simultaneously increasing throttling over the limit stop. As a consequence, the limit stop almost forms a pinch gap, via which namely an essential reduction of the leakage quantity that flows in the bypass from the inlet to the outlet is achieved, but which, in particular considering the short control times, allows a specific average quantity as leakage gap. In this way the starting position is created wherein, when the control valve is closed in consideration of the end of the inlet into the gap delimited by the limit stop, an abrupt pressurization of the front face of the control valve is achieved, which makes possible an accurate control of the closing motion of the valve closing member. The described pinch gap linkage ensures, at the same time, that the limit stop, as a rule, is not stressed or is stressed very little when fulfilling the function of the path-limiting seal boundary with sensitive but highly stressed bearing surfaces (in similitude to the bearing surface of the nozzle needle of a fuel injector), so that long-term stable working conditions that remain the same are also ensured from a geometrical point of view. Basically, a necessary connection, even though short on average or only temporary, that is, a very short-term sealing, is allowed within the scope of the invention, since otherwise a complete sealing over the seal boundary with open control valve would be given, but such a complete sealing would prevent a reaction of the piston to the input of the control valve. 
   In one configuration of the invention, the gap space can be formed by a front-side depression of the control piston and/or a depression in the side of the front wall, wherein the boundary of the gap can be configured as running contrary to the limit stop or can also be configured by steps, whereby, aside from the production-related possibilities, also the flow conditions can be influenced with respect to the pinch gap formation. 
   It was also shown to be practical to provide an annular-shaped free space between the control piston and the receiving bore in the limit stop of the neighboring axial area and allocating the outlet with its opening cross section o this free space, wherein the free space is formed in the axial end area allocated to the front wall preferably by means of a widening of the bore for receiving the control piston, but can also be formed by a specific diameter reduction of the control piston adjacent to its front face. This free space in the form of an annular space can be utilized in accordance with the invention to form the pinch gap, in that its front-side boundary is axially offset with respect to the front wall and forms a small step, so that the piston axially overlaps the step in its upper limit position adjacent to the front wall in the area of the seal boundary, whereby a particularly intensive damping of the control piston results when traveling into this upper limit and/or stop position of the control piston. 
   Further details and features of the invention result from the claims, and the invention is explained in the following with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic overall view of a fuel injector in section for the purpose of clarifying its overall design and the function of the control valve, and 
       FIGS. 2 and 3  show highly schematized and enlarged cutout illustrations of a control valve with the configuration according to the invention, wherein  FIG. 2  shows the control valve in its open position and  FIG. 3  shows the control valve in its closed position. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The fuel injector  1  shown in the schematized overall view according to  FIG. 1  consists essentially of a nozzle part  2  and a control and actuator part  3 , which also forms functionally the nozzle holder and to which the supply connection symbolically illustrated by the arrow  4  for the injection medium under high pressure is allocated, in particular diesel or heavy fuel. On the feedback side, the corresponding feedback-side connection is symbolized by the arrow  5 . 
   The control and actuator part  3  comprises a magnetic disk  6 , by means of which the control valve  7  is actuated, which comprises the valve member  8  acted on by the magnetic disk  6 , by means of which the pressurization of a control chamber  9  is controlled. 
   The control chamber  9  is overlapped in the illustration according to  FIG. 1 , in a modular design of the injector, on the one hand, by the front wall  10  of an intermediate plate  11  and, on the other hand, by a bore  12 , and is delimited by a control piston  13  guided therein, which acts on the nozzle needle  15 . The bore  12  is provided in the illustration according to  FIG. 1  in a sleeve  14 , which in turn is connected coaxially to a nozzle needle  15  on the control piston  13 . The nozzle needle  15  forms the valve member of a main flow valve, whose seat is allocated to the nozzle element  16 , which at the same time also forms the guide for the nozzle needle  15 , and has a pressure chamber  17 , on which the supply of injection medium on the high-pressure side takes place by means of the line connection  18  shown with the dashed line. The nozzle needle  15  rests in a sealing manner in the closed position, which is shown, in the nozzle seat  19  and is charged by means of the pressure existing in the pressure chamber  17  by actuating its pressure shoulder  20  in the opening direction. In the opposite direction, the nozzle needle  15  is acted on by means of the spring  21 , as well as also by the pressure existing in the control chamber  9 , if, as shown in  FIG. 1  with a dashed line, the control chamber  9  is supplied from the side of the high pressure line connection  18 , which is also shown with a dashed line, by the throttled inlet  22  shown with a dashed line, and a throttled outlet  23  by means of which the valve member  8  is blocked in its connection to the feedback (arrow  5 ) indicated by the line  24 . If the valve member  8  is transferred by flooding the magnetic actuator  6  into an open position (not shown in  FIG. 1 ), then the outlet  23  is connected to the line  24 , and the control chamber  9  is depressurized, so that the nozzle needle  15  lifts off the nozzle seat  19  actuated by the pressure shoulder  20  in the opening direction and the injection is released. 
   In accordance with the described arrangement with throttled high-pressure inlet  22  and throttled and controlled outlet  23  toward the low-pressure side, when the connection of the outlet  23  to the feedback is released according to arrow  5  in connection with the depressurization in the control chamber  9 , the control chamber volume is reduced by means of the upward motion of the control piston  13  connected with the opening of the nozzle needle  15  and with the same direction, and a corresponding volume is pushed toward the feedback  5 . Otherwise, the bypass connection created by the opening of the valve member  8  remains open until the valve member  8  is reversed, regardless of the throttling in the inlet  22  and in the outlet  23 . The open throughflow connection causes considerable leakage losses. 
     FIGS. 2 and 3  show in a highly simplified schematic illustration sections of a configuration according to the invention of the area A, wherein regardless of the deviations from the design of the corresponding parts, the same reference numerals as in  FIG. 1  are used, and wherein the correspondingly described functions and designs are not bound to the design of the fuel injector  1  according to  FIG. 1 , for example, the modular design of the injector  1  or the like. 
   The valve member of the control valve is therefore identified with the numeral  8 , and the control piston is identified with the numeral  13 . The control piston  13  is guided in the bore  12  with an upward motion and delimits with the bore  12  and the front wall  10  covering the bore  12  the control chamber  9 , while the control chamber  9  adjacent to the front wall  10  is enlarged in diameter by a radial widening  27  of the bore  12 , so that an annular free space, in particular a cylindrical annular chamber, is provided around the front-side end of the control piston  13  opposite to the front wall  10 . In the area that radially overlaps the front side  28  of the piston  13 , the opening cross section  29  of the inlet  22  lies on the control chamber  9 . In the inlet  22  lies a throttle  30 . The outlet  23  has an opening cross section  31  on the side of the control chamber. A throttle  32  is allocated to the outlet  23 , which in the design example is formed by a bore in a seat disk  33 , which covers the valve member  8  in the closed position provided in the design example with a flat blocking surface  34  and is locked. 
   Between the control piston  13  and the front wall  10  a limit stop  35  is formed when the control chamber  9  is depressurized, and the control piston  13  is displaced upward against the front wall  10 , which is allocated to the edge zone  36  of the control piston  13  in the transition between the front wall  10  and the front side  28 , while the same is formed, for example, by reverting the piston  13  on the front side within the edge zone  36 , as shown in  FIGS. 2 and 3 . In a similar way, a corresponding configuration could also be allocated to the front wall  10 . The axially reverted configuration of the front-wall of the control piston  13  opposite to the edge zone  36  leads in the upper limit position of the control piston  13  corresponding to the opening position of the valve member  8  shown in  FIG. 2  to the formation of a flat gap  37  enclosed at its periphery by a limit stop  35  formed by the periphery of the piston  13 , which is also blade-shaped, if required, whose depth is shown drawn over in the figures, and which is stepped radially outwardly or runs into the peripheral zone  36 . 
   This design has, in connection with the end (opening cross section  29 ) of the inlet  22 , the consequence on the annular space enclosed by the limit stop  35  that, when the outlet  23  is opened by means of the valve member  8  against the feedback (arrow  5 ), and the control piston  13  is consequently displaced upward against the front wall  10  as a consequence of pressurization in the nozzle needle  15  in the opening direction, the control piston  13  runs against a pressure cushion fed through the still open inlet  22 , so that even with the desired almost abrupt opening motions of the nozzle needle  15 , the same is intercepted in a damped manner in the end phase because the limit stop  35  has the function of a pinch gap. According to this function, the limit stop  35  forms as a rule also no absolute seal boundary, but rather a throttle gap, which first reduces considerably the leakage when the valve member is open. Furthermore, it is also ensured in this way that, when the valve member  8  is closed, the gap volume is increased almost abruptly to the pressure level of the inlet  22 , while the configuration according to the invention also creates the prerequisites that the front face  25  of the control piston  13  is acted on completely without noticeable time delay. In addition, the solution in accordance with the invention prevents that, in the opening phase of the valve member  8 , the injection medium under high pressure flowing between the front wall  10  and the front face (corresponding to the hydrodynamic paradox) would lead to an adhesion of the control piston  13  with its front face  25  on the front wall  10 , which would have as a consequence a delay of the desired almost abrupt closure of the nozzle needle  15  by a corresponding displacement of the control piston  13  when the valve member  8  is reversed from the opening into the closing position. 
   In this way, the invention creates a solution with simple means, which also reduces the leakage as well as also increases the operational safety by a reduction of the abrasion. 
   The desired “pinch gap formation” and damping function can also be achieved or improved within the scope of the invention in that the annular free space formed by the radial widening  27  is not pulled through up to the front wall  10 , so that an annular step  38  is produced, into which the control piston  13  plunges when its end position is reached. In this way, despite the only small axial overlap, a radial narrow throughflow gap and a corresponding damping result. The annular step  38 , shown as an example in  FIG. 3 , is illustrated schematically for the purposes of clarity in a way that deviates from  FIGS. 1 and 2 , and the element that receives the cylinder bore  12  is shown shaded, but is for practical purposes configured as one piece with this element, for example, with reference to an overall view according to  FIG. 1 , as one piece with the intermediate plate  11 .