Valve control unit for a fuel injection valve

A valve control unit for a fuel injection valve, has a housing body, in which two valve control chambers that communicate continuously with one another are provided. The first valve control chamber communicates fuel with an inflow conduit for fuel pressure on a terminal member of a valve control piston which is displaceable in the housing body and the second valve control chamber communicates with a closable outlet conduit. A mechanical stop for limiting the mobility of the valve control piston in the direction of the second valve control chamber is embodied on the housing body. The quantity of preinjected fuel can be minimized by the volume of fuel in the first valve control chamber.

BACKGROUND OF THE INVENTION
 The invention is based on a valve control unit for a fuel injection valve,
 in particular for a common rail injector.
 A valve control unit of this kind for a fuel injection valve is known for
 instance from European Patent Disclosure EP 0 661 442 A1.
 In the known valve control unit, there are two valve control chambers
 communicating continuously with one another in the housing body. The first
 valve control chamber communicates with an inflow conduit for fuel, which
 is connected to a high-pressure reservoir (common rail). The second valve
 control chamber has a passage to an outlet conduit, which can be opened
 and closed via a magnet valve. When the valve control unit is triggered,
 the outlet conduit is opened. As a result, the pressure in the second
 valve control chamber and thus in the first valve control chamber as well
 drops, so that the hydraulic imposition of pressure on one end of the
 valve control piston is also reduced. The other end of the valve control
 piston is connected to a nozzle needle for performing the injection. As
 soon as the hydraulic pressure imposition drops below the pressure
 imposition of the nozzle needle, the nozzle needle opens, so that the fuel
 can emerge through the injection opening into a combustion chamber.
 Manipulating the pressure ratios of the valve control chambers is done so
 as to control the valve control piston.
 The terminal member of the valve control piston can be displaced in the
 injection event inside the first valve control chamber as far as a
 hydraulic stop (fuel cushion), which forms in the passage region between
 the first and second valve control chambers. This hydraulic stop is
 determined essentially by the size of the volume of the first valve
 control chamber. The known valve control unit has a first valve control
 chamber with a small volume, since only a small volume of the first valve
 control chamber assures that the hydraulic stop will not cause any
 vibration of the valve control piston and will have adequate rigidity.
 Because of the volumetric proportions of the first and second valve
 control chamber to one another, however, a considerable pressure gradient
 develops between the first and second valve control chambers. In the
 preinjection, the valve control piston can consequently move with a long
 valve stroke, so that a larger quantity of fuel is injected into the
 combustion chamber in the preinjection.
 It would also be desirable to reduce the outer diameter of the terminal
 member of the valve control piston, so that this terminal member would
 have a lesser positive displacement cross section for fuel from the first
 valve control chamber. Reducing the outer diameter, however, necessarily
 increases the free volume of the valve control chamber, so that designing
 first valve control chamber in this way in turn leads to increased
 vibration on the part of the valve control piston at the hydraulic stop.
 Disadvantageously, the outer diameter of the terminal member of the valve
 control piston can therefore not be reduced, even though such a reduction,
 because of its reduced positive displacement cross section, would make an
 increased speed of motion of the terminal member possible.
 OBJECT AND SUMMARY OF THE INVENTION
 To minimize the preinjection quantity of fuel, the valve control unit for a
 fuel injection valve according to the invention has a mechanical stop for
 limiting the mobility of the valve control piston in a direction of a
 second valve control chamber.
 The motion of the valve control piston is limited by a mechanical stop,
 which is embodied inside the housing body of the valve control unit. This
 mechanical stop can be embodied in various ways. In a simple variant of
 the invention, the valve control piston can change over in steps from a
 larger to a smaller outer diameter, and a counterpart step complementary
 to this step can be present on the housing body. To uncover the injection
 opening, the valve control piston can be moved with a stroke which is
 determined by the spacing of the two steps, when the injection opening is
 closed and the terminal member of the valve control piston is impinged by
 pressure.
 Because of the embodiment of the mechanical stop, the hydraulic stop can be
 dispensed with, so that the first valve control chamber can be embodied
 with a larger volume. The first valve control chamber can advantageously
 have a volume of up to 60 mm.sup.3, while it is still assured then that no
 vibration with regard to the stop of the valve control piston will occur.
 If the volume of the second valve control chamber is kept small in
 comparison with this volume of the first valve control chamber, then the
 pressure gradient upon opening of the outlet conduit between the first and
 second valve control chambers is reduced substantially. The resultant
 shorter stroke of the valve control piston means that initially only a
 small quantity of fuel is preinjected. The main injection is unaffected.
 In the larger volume of the first valve control chamber, a pressure can
 always be embodied which is essentially equivalent to the pressure inside
 the high-pressure reservoir (common rail). The pressure ratios inside the
 first valve control chamber are thus virtually constant both when the fuel
 injection valve is open and when it is closed. The small volume of the
 second valve control chamber is pressure-relieved through the opening of
 the outlet conduit in the injection event. Upon reclosure of the fuel
 injection valve because of the closure of the outlet conduit by the magnet
 valve, only the small volume of the second valve control chamber has to be
 brought to a higher pressure level, and as a result the closing process
 can be accomplished faster.
 The invention will be better understood and further objects and advantages
 thereof will become more apparent from the ensuing detailed description of
 preferred embodiments taken in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 It can be seen from FIG. 1 that a valve control unit 1 has a housing body
 2, in which a valve control piston 3 is displaceably supported. The valve
 control unit 1 is suitable for controlling the fuel injection into a
 combustion chamber. FIG. 1 shows the state of repose with the injection
 opening closed. The valve control piston 3 is shown only in part in FIG. 1
 and extends as far as a nozzle needle 4. The nozzle needle 4 can be moved
 in the direction of the arrow 5, so that an injection opening 6 can be
 uncovered so that the fuel can be injected.
 The triggering of the valve control piston 3 is effected via a hydraulic
 pressure imposition on a terminal member 7 of the valve control piston 3.
 A first valve control chamber 8 communicates continuously with a second
 valve control chamber 9. Fuel from a high-pressure reservoir (common rail)
 can reach the first valve control chamber 8 with the aid of an inflow
 conduit in the form of an inflow throttle 10. The second valve control
 chamber 9 is connected to an outlet conduit in the form of an outlet
 throttle 11. If a valve ball 12 of a magnet valve, not shown in further
 detail in FIG. 1, opens the outlet conduit, fuel can flow out in the
 direction of the arrow 13. The change in pressure inside the valve control
 chamber 8 causes the valve control piston 3 to move inwardly in the
 direction of the arrow 5. The stroke of the valve control piston 3 is
 limited, because a mechanical stop is provided on the valve control piston
 3, or on the housing body 2. The valve control piston 3 changes over in
 steps to its terminal member 7. A housing step 14 is also embodied on the
 housing body 2. Edge faces 15 of the valve control piston 3 can therefore
 come to rest against counterpart faces 16 of the housing body 2. Because
 of the embodiment of the mechanical stop, the motion of the terminal
 member, 7 in the first valve control chamber 8 is limited. The terminal
 member 7 can move in the direction of the second valve control chamber 9,
 so that a flow conduit (gap) for fuel from the first valve control chamber
 8 into the second valve control chamber 9 becomes smaller in terms of its
 free flow cross section. Because of the mechanical stop, the volume of the
 first valve control chamber 8 can be designed to be as great as possible.
 Compared with the volume of the second valve control chamber 9, the volume
 of the first valve control chamber 8 is substantially greater. By the
 design of the volume of the valve control chamber 8, it can be attained
 that when the outlet conduit 11 opens, only a slow pressure loss occurs in
 the valve control chamber 8. The terminal member 7 can also be embodied
 with a reduced outer diameter, so that the free volume, accessible to
 fuel, in the first valve control chamber 8 is increased still further.
 Vibration of the valve control piston 3, of the kind that can occur when
 there is a hydraulic stop and an increased volume of the first valve
 control chamber 8, is prevented by the mechanical stop.
 The outer diameter of the nozzle needle 4 can be reduced while maintaining
 the same quantity of preinjected fuel as before, for instance to an outer
 diameter of 3 to 3.7 mm. In that case, the speed of motion of the nozzle
 needle can be increased (small positive displacement cross section). An
 increased speed of motion of the nozzle needle can be attained without
 requiring a larger or faster magnet valve to open or close the outlet
 conduit. Also because of the increased speed of motion of the nozzle
 needle, a faster traversal through critical-tolerance stroke regions can
 become possible.
 FIG. 2 shows a further possible design for a first valve control chamber 20
 and a terminal member 21. The other components of FIG. 2 correspond to the
 components of the valve control unit 1 shown in FIG. 1 and are provided
 with the same reference numerals. The first valve control chamber 20 has a
 substantially larger volume than the second valve control chamber 9. A
 valve control piston, not shown, is connected to the terminal member 21,
 whose outer diameter is reduced still further to enable moving the
 terminal member 21 in the first valve control chamber 20 and to displace
 as little fuel as possible. The volume of the first valve control chamber
 20 can additionally be varied by installing an adjusting ring 23, in order
 to adapt the volume of the first valve control chamber 20 optionally in
 such a way that vibration of the valve control piston contacting the stop
 cannot occur.
 By means of the volumetric ratios of the first and second valve control
 chambers 20 and 9, the development of a great pressure gradient upon the
 opening of the outlet conduit 11 is prevented. This prevents an overly
 large quantity of fuel from being injected in the preinjection when the
 valve ball 12 is triggered. The speed of the motion of the valve control
 piston in the main injection is effected in the way employed in
 conventional valve control units as well.
 FIGS. 3 and 4 show how the stroke of the valve control piston and the
 pressure gradient between the first and second valve control chambers can
 be varied by varying the volume in the first valve control chamber. Solid
 lines represent a smaller volume of the first valve control chamber, and
 dashed lines represent a larger volume of the first valve control chamber.
 The symbol VE stands for the range of the preinjection, and HE stands for
 the range of the main injection.
 In the range HE, the lines come to be superimposed. It can be seen from
 FIG. 3 that the stroke of the valve control piston is shorter in the
 region VE if the volume of the first valve control chamber is larger. As
 FIG. 4 shows, the pressure inside the first valve control chamber drops
 more slowly, if there is a larger volume, to the level of the outlet
 conduit (P.sub.A) after opening of the outlet throttle and then rises
 again to the level of the common rail (P.sub.CR). Less fuel is
 preinjected. The main injection remains unchanged.
 The foregoing relates to preferred exemplary embodiments of the invention,
 it being understood that other variants and embodiments thereof are
 possible within the spirit and scope of the invention, the latter being
 defined by the appended claims.