Pressure limiting valve

The invention relates to a pressure limiting valve. The pressure limiting valve comprises a valve holder a valve insert connected to the valve holder a valve piston slidably supported in the valve insert and a compression spring acting on the valve piston in the closing direction. Between the valve piston and the compression spring an adjusting shim is disposed such that the compression spring is braced on one end on a bottom piece of the valve holder and on the other on a face of the adjusting shim facing away from the valve piston.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 03/03300 filed on Oct. 6, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a pressure limiting valve and more particularly to an improved pressure limiting valve suitable for use fuel injection systems.

2. Description of the Prior Art

A pressure limiting valve in conjunction with a device for injecting fuel, in particular Diesel fuel, is known from the book entitled Diesel-engine Management, Stuttgart 1999, page 273 (ISBN 0-7680-0509-4). A pressure limiting valve of this type serves to limit pressure yet at the same time furnish an emergency operation capability. Preferably, the pressure limiting valve of this type is used in conjunction with common rail systems.

Mass-produced pressure limiting valves are already known in which spring washers and valve pistons are embodied in one piece and disposed together with a compression spring in a cup-shaped valve holder. On its end piece remote from the valve piston, the valve holder has an adjusting screw with a check nut, by way of which the valve characteristic can be adjusted and production variations can also be compensated for. Aside from the complicated and expensive construction, these known valves require a relatively large installation volume, which makes it difficult to integrate them into component groups of an injection system. Moreover, these known valves have the disadvantage of regulating down to a pump feeding quantity of only about 90 liters/hour. At a lower flow rate, such a pressure limiting valve closes again. However, this causes severe restrictions in emergency operation.

SUMMARY OF THE INVENTION

The pressure limiting valve of the invention is distinguished by having only a few parts, which can also be produced economically. Because of the separation provided according to the invention between the spring washer and the piston, the possibility also exists of adjusting the opening pressure in a simple way and compensating for production variations. Moreover, the invention offers the advantage in particular that even at a comparatively low pump rpm and a slight flow rate, stable emergency operation is made possible. A pressure curve is furnished that is virtually independent of the flow rate. The pressure limiting valve equipped according to the invention is furthermore distinguished by having only a very small size. This makes it possible to install it into component groups of the injection system, such as a rail or pump, in which only little installation space is available. Because of the flat places provided on the piston of the pressure limiting valve, an adequate flow cross section for major pump feeding quantities is furnished.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows a first exemplary embodiment of a pressure limiting valve embodied according to the invention that is especially well suited to emergency operation as well. The pressure limiting valve10includes a valve holder1, which is embodied as essentially cup-shaped, and includes two subregions1a,1bof different inside diameters D1, D2. The subregions1a,1beach extend over part of the total length of the valve holder1and merge in steplike fashion with one another. The subregion1aof the valve holder1having the lesser inside diameter D1is next to the bottom of the valve holder1. The subregion1bof the valve holder having the larger inside diameter D2is oriented toward a valve insert2. The subregion1aof the valve holder1serves to receive a compression spring5. The subregion1bof the valve holder1embraces the valve insert2from outside and establishes the connection between these two parts. Preferably, the valve insert2is calked to the valve holder. This type of fastening can be done economically on a mass-production scale and has proven itself to be sufficiently reliable. Alternatively, however, it is also conceivable to provide the valve holder1and the valve insert with a suitable thread and to screw the two parts together. This type of connection will not be as economical but on the other hand would offer the advantage of a further degree of freedom for compensating for production variations. That is, by twisting the parts1,2more or less markedly, the prestressing exerted on the compression spring5could be varied. A valve piston3is slidably supported in a bore disposed centrally in the valve insert. Between the compression spring5and the valve piston3, an adjusting shim4is disposed such that the compression spring5, resting in the subregion1aof the valve holder, is braced on one end on the bottom of the valve holder1and on the other on the side face, remote from the valve piston, of the adjusting shim4. The compression spring5acts on the valve piston3in the closing direction indirectly via the adjusting shim. The adjusting shim4itself is slidably supported in a cup-shaped recess2adisposed in the valve insert2. Upon an opening or closing motion of the valve piston3, this piston is moved up and down and in the process is guided by the wall of the cup-shaped recess2a. The steplike transition of the valve holder1formed by the different diameter regions (diameters D1, D2) rests on the valve insert2and protrudes radially inward. This creates a stop for the adjusting shim4. When pressure is exerted on the valve piston3acting counter to the compression spring5, the adjusting shim4can move at most as far as the stop and thus also prevents the valve piston3from being lifted excessively far from its seat in the valve insert2. The adjusting shim4has a fluid-passable recess or opening4a, which is preferably disposed eccentrically in the adjusting shim, in order to assure good bracing of the valve piston3on the adjusting shim4. Through this recess4a, once the valve piston3has lifted from its seat, fuel can flow into the subregion1a, accommodating the compression spring5, of the valve holder. At least one outflow conduit1cis provided in the wall of the valve holder1and is expediently embodied as a bore connecting the subregion1aof the valve holder to its outer jacket face. This outflow conduit1cis preferably disposed horizontally (FIG. 3). However, as shown inFIG. 1, it may also be let into the wall of the valve holder1in such a way that it extends obliquely. That is, the axis A of the outflow conduit1cand the longitudinal axis Al of the valve holder are oriented at an angle to one another, specifically and preferably such that an acute is defined between the axis A and the direction of the longitudinal axis of the valve piston3. A sealing means, preferably an0-ring6, is also disposed on the outer jacket face of the valve holder1, resting in a seat. A protective cap7is also shown, which protects the pressure limiting valve10from damage and contamination during storage and while being transported until it is installed.

The compression spring5and the hydraulically active seat diameter, which results from the contacting edge of the valve insert2and the valve piston3, are adapted to one another such that a predeterminable opening pressure P1results. This opening pressure P1can be adjusted especially advantageously by means of the adjusting shim4. By means of adjusting shims4of different thickness, unavoidable production variations can be compensated for easily. The simply constructed parts of the pressure limiting valve10can be economically produced and kept on hand. The desired pressure for the emergency operation is adapted in a simple way in cooperation between the guide diameter of the valve piston3in the valve insert2and the diameter of the valve seat. In addition, the valve piston3has flat places3adisposed on its outer circumference. In an especially advantageous embodiment of the pressure limiting valve10, a total of three flat places3a are provided, which are distributed uniformly over the circumference of the valve piston3. The limits to usability at high flow rates are set by the cross section of these flat places3a. The spring force of the compression spring5urges the adjusting shim4and thus the valve piston3in the closing direction. Beyond a defined value of the pressure, the spring force of the compression spring5no longer suffices to keep the pressure limiting valve10closed. This value for the pressure, called the opening pressure, in a second generation common rail system, is between about 1850 and 1950 bar. This pressure value can be established in a simple way by means of a suitably dimensioned adjusting shim4. Once the opening pressure is reached, the valve piston3lifts from its seat in the valve insert2, and a pressure builds up in the chamber enclosed by the valve piston3and the valve insert2. Because of the larger hydraulically active surface area, the valve piston slides so far out of the valve insert2that the flat places3amade on the valve piston3become operative, and the fuel can flow out via the open cross-sectional areas. Via the flat places or control edges machined into the valve piston3, the characteristic curve for emergency operation is regulated. Ideally, the pressure limiting valve10would not close again until at a flow rate of about 15 liters/hour. Beyond the instant of closure, the function of the pressure limiting valve10becomes unstable. If the play between the valve piston3and the valve insert2were increased, then the pressure limiting valve10would for instance already close at a flow rate of about 40 liters/hour. As a consequence, at a low rpm of an internal combustion engine with correspondingly low feeding quantities, stable injection would no longer be possible. Pressure limiting valves with a valve piston3lacking flat places3ahave the disadvantage that the pressure rises too fast.

The advantage of the construction according to the invention is illustrated by the graph shown inFIG. 2, which shows the functional relationship between pressure and flow rate. On the X axis of the rectangular coordinate system shown inFIG. 2, pressure values are plotted in arbitrary units; on the Y axis, the flow rate or volumetric flow is shown, again in arbitrary units. The curve K1represents the functional relationship between these variables in normal operation. The curve K2illustrates the relationship in emergency operation. The curve K2clearly shows that the pressure limiting valve designed according to the invention enables satisfactory emergency operation over a wide emergency operation window F.

In a further exemplary embodiment of the invention, which is shown inFIG. 3, flat places3a.1are provided on the outer jacket of the valve piston3that extend over the entire length of the valve piston3. In the exemplary embodiment shown inFIG. 1, the flat places3aextend over only a portion of the length of the valve piston3.

In a further exemplary embodiment of the invention, which is shown inFIG. 4, flat places3a.2are provided on the outer jacket of the valve piston3that are oriented at an angle relative to the longitudinal axis of the valve piston3. Depending on the position of the valve piston3, the result is a more or less large cross-sectional area for the passage of fuel. AsFIG. 4also shows, an outflow conduit1c.1can alternatively be disposed in the valve insert2as well.

In an advantageous further feature of the invention, a piston does not directly take on the sealing function of the pressure limiting valve10. Instead, the valve opening is closed by a ball, which in turn is acted upon by a spring-loaded piston. This is illustrated inFIG. 5, which shows a larger detail, in cross section, of a pressure limiting valve designed in this way. A ball50is supported in a seat in the valve insert2and thus closes a valve opening2b. The ball50is in turn acted upon a spring-loaded valve piston3, which presses the ball50into its seat.

FIG. 6shows a further exemplary embodiment of the invention. This is a simplified and therefore more-economical variant of the pressure limiting valve already described in conjunction withFIG. 1. The pressure limiting valve10shown inFIG. 6includes a valve piston3that is slidably supported in the valve insert2. The valve piston3is shaped essentially circular-cylindrically and has no flat places on its outer circumference.

An advantageous further feature of a pressure limiting valve is shown inFIG. 7. The valve piston3slidably supported in the valve insert2of the pressure limiting valve10has a plurality of flat places3a,3a.3over its circumference, whose control edges have a variable spacing [different for each control edge] from the end face, toward the compression spring5, of the valve piston3. Accordingly, upon motion of the valve piston3, the flat places3a,3a.3come into action at different times. This embodiment is possible both with flat places that extend parallel to the longitudinal axis of the valve piston3and with flat places oriented obliquely to the longitudinal axis of the valve piston3.

InFIG. 8, a further exemplary embodiment of a pressure limiting valve designed according to the invention is shown. The even sturdier construction, compared to the exemplary embodiment ofFIG. 1, of a pressure limiting valve includes a valve piston3with a piston rod3b, which is disposed concentrically to the compression spring5and protrudes into the partial chamber1aof the valve holder1. The compression spring5is embodied as a conical spring and is braced with its larger base on the cup bottom of the valve holder1. With its smaller base, it rests on an adjusting shim4, which is slipped onto the piston rod3b of the valve piston3and rests there on a step. However, this pressure limiting valve is not suitable for emergency operation.

FIG. 9shows a further variant of this pressure limiting valve, in which the valve piston3has flat places3a.