GB2348463A

GB2348463A - Measuring apparatus for the volumetric measuring of injection quantities

Info

Publication number: GB2348463A
Application number: GB0008144A
Authority: GB
Inventors: Oliver Mansoat; Manfred Muller; Clause Stenger
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 1999-04-03
Filing date: 2000-04-03
Publication date: 2000-10-04
Also published as: FR2791771A1; DE19915266C1; GB0008144D0

Abstract

The apparatus comprises a measuring chamber 5 into which an injection nozzle 6 injects a quantity of liquid. The injection displaces a piston 4 which is connected to travel sensor 9 by a rod 7. A gas pressure chamber 12 is provided between the travel sensor 9 and the piston 4. The piston 4 is made from plastics, eg based on PTFE, so as to be temperature resistant with good sliding properties and to expand with increasing temperature. The piston skirt 16 has a sealing zone 17 of reduced wall thickness supported radially by an expanding body in the form of a tightly wound helical spring 25 so that the piston 4 seals the measuring chamber 5 from the gas pressure chamber 12 without the need for separate sealing elements. The head 15 of the piston 4 is reinforced for fastening the rod 7 and may have axial holes 21 for radial elasticity. The head 15 may have a metallic supporting ring (23,fig.4). In a modification (figs.6,7), the expanding body is in the form of a cup 33 with a conical inner surface which receives an expanding insert 42 having a mating conical outer surface.

Description

r 2348463 1 Measuring device for the volumetric measuring of injection

quantities The invention relates to a measuring device for the volumetric measuring of injection quantities.

Measuring devices of the abovementioned type are known from DE 39 16 419 C2 and in practical use. In the case of these measuring devices, an injection nozzle injects into a measuring chamber which is sealed ofrby a gas-pressurized measuring piston guided in a measuring cylinder. The measuring piston is connected to a lifting rod, via which the piston travel corresponding to the respective injection quantity is detected by means of an inductive travel sensor.

The respective injection quantities are small; different frictional conditions occur, depending on the fuel type (petrol, diesel), and via the measuring signals the intention is not only for it to be possible to detect quantities as static values, but also injection profiles and similar dynamic variables. In association with the requirement of reliably separating the measuring chamber from the gas pressure chamber via the measuring piston, this necessitates very careful fitting of the measuring piston into the measuring cylinder, specifically with as little friction as possible between the measuring piston and the measuring cylinder. These requirements are intended to be fulfilled in spite of relatively sharp temperature fluctuations between about 20' and 160' to 20WC and in spite of relatively high gas pressures which reach up to the order of magnitude of 100 bar, in order via the proportions in the measuring chamber to approximately simulate the conditions in the combustion chamber of an internal combustion engine.

The requirement of low fliction in association with a virtually absolute, gas-tight separation of the measuring chamber from the gas pressure chamber in spite of the high gas pressures is sought to be met by the measuring piston being fitted in a seal-free manner into the measuring cylinder. Steel is used as the material for the measuring piston and measuring cylinder, so that the influence of the temperature fluctuations on the tightness of the system remains controllable.

2 The invention aims to further improve a known measuring device of this type, specifically with reduced expenditure on machining.

According to the invention there is provided a measuring device for the volumetric measuring of injection quantities comprising a measuring chamber which is sealed off by a gas-pressurized annular measuring piston with a piston skirt guided in a seal-free manner in a measuring cylinder having a first diameter, wherein the measuring piston comprises, at least in the region of a sealing zone, expandable plastic and has a diameter which is smaller than the measuring cylinder diameter and the plastic being expandable, as a function of temperature, up to a sealing diameter corresponding to the cylinder diameter, such that the piston can be fixed in the expanded state via a mechanically prestressed expanding body which follows the internal diameter of the piston skirt, which diameter enlarges up to the sealing diameter, and maintains the sealing diameter by giving radial support.

Maintaining the gas-tight separation between the measuring chamber and gas pressure chamber, making the measuring piston as light as possible and guiding it in a manner which is as low in friction as possible, whilst reducing the expenditure on machining for the measuring piston/measuring cylinder pairing and doing all of this in spite of sharp temperature fluctuations, since the device heats up considerably during operation and temperatures of up to the order of magnitude of 200'C are achieved, are fUndamentally contradictory requirements. Above all else, in view of the different expansion behaviour of different materials, the use of different materials for the measuring piston and measuring cylinder would not appear to provide a solution. In particular, these requirements do not allow even the use of plastics for the measuring piston to appear very promising, although plastics when suitably selected, can result in a favourable frictional pairing with measuring cylinders consisting of steel. However, the different coefficients of expansion would, in view of the relatively large temperature ranges, result in quite different fitting clearances, with corresponding effects on the gas- tight separation 1 1 3 between the measuring chamber and gas pressure chamber and also on the smooth running of the measuring piston.

These fundamental disadvantages stand in the way of using plastic pistons as the measuring pistons in the pairing with measuring cylinders made of metallic materials, in particular steel, are overcome according to the invention by the measuring piston interacting in the region of its sealing zone with a corresponding expandable body. Basically, the measuring piston is first of all set, at least in the region of its sealing zone, to a constructional diameter which is below the size which would be required to result in a fit, with regard to a surrounding cylinder diameter, leading to a gas-tight separation of the gas pressure chamber and measuring chamber. This undersize takes into account the temperature-induced expansion of the measuring piston, the size being determined in this regard in such a manner that in the event of the heating up continuing into the range of the maximum operating temperature, for example due to the measuring device being operated under a high load, a state arises in which the piston diameter corresponds, at least in the region of its sealing zone, to the cylinder diameter with regard to the sought for, gas-tight separation with as little friction as possible.

This optimised fitting clearance, after it has been achieved by corresponding thermal loading, is conserved and maintained as operating clearance for the measuring device by the measuring piston being acted upon, in the region of its sealing zone, via an expandable body or an expandable device which, in the case of the temperature -induced expansion of the basic body, forming the sealing zone, of the measuring piston, follows the said basic body, on account of mecharCAJ prestress, but counteracts shrinkage, i.e. a temperature -induced constriction of tbqdiameter of the sealing zone and to the constructional diameter, on account of-ahigh radial compressional stiffness in the opposite direction to the prestress. For this purpose, use is made of the concept that a pressurized expanding body supported against the inner circumference of the sealing zone follows the sealing zone during expansion by correspondingly enlarging its circumference - with relative movement of the expanding body with respect to the sealing zone in the circumferential direction - without great ftictional resistance, i.e. it is 4 able to expand, since the prestressing forces are small and only small, radial forces are in effect, but that in the event of contraction, i.e. in the event of the sealing zone shrinking, together with a corresponding reduction in diameter, high, radial supporting forces build up between the expanding body and sealing zone, and that the relatively high friction associated therewith at least to a great extent eliminates relative displacement between the expanding body and sealing zone. The result is that via the expandable body, the diameter of the sealing zone, at which diameter the desired separation of the measuring chamber from the gas chamber was achieved, is at least to a large extent fixed.

In a preferred embodiment the expandable body may, for example, be configured as an expanding ring, or else as an annular spring, in particular a helical spring, which bears with its coils against the inner circumference of the annular body forming the sealing zone, so that frictional forces in effect in the circumferential direction counteract a reduction in the diameter of the enlarged sealing zone.

In a preferred embodiment, it has proven expedient to use, at least for the sealing zone of the measuring piston, a plastic which has a low coefficient of ffiction, is temperaturestable and sufficiently expandable and flowable that the lasting support via the expanding body ultimately results in the annular body forming the sealing zone virtually matching the internal diameter of the measuring cylinder.

The support, achieved via the expandable body and the stabilization of the supporting effect, can be further reinforced, in the case of expandable bodies having - in order to change the diameter - by relative displacement taking place in the circumferential direction between the expanding body and the annular body forming the sealing zone. If the expandable body, facing towards the annular body, is textured, coated or the like, a certain clawing action with the effect of a positive engagement is also produced over and beyond the frictionally induced fixing. A corresponding effect can be achieved, for example, by means of saw-tooth-like profiles or the like.

1 1 In a preferred embodiment the difference between the ftictional forces counteructing a relative displacement of the expanding body with respect to the annular body of the sealing zone, is used in order, via the expanding body, to stabilize the diameter of the annular body acting as the sealing zone. In this case, the ffictional forces counteracting a contraction of the annular body result in the expandable body acting virtually as a supporting ring of the annular body.

Preferably an expanding body of this type, is, in particular, a helical spring whose external diameter exhibits oversize with regard to the constructional diameter of the annular body, which helical spring is thus first of all pretensioned by pulling up, i.e. rotating its ends with the effect of slimming the spring, and which, in conjunction with the temperature-induced expansion of the annular body, can correspondingly enlarge its diameter, and forms a supporting-ring-like bearing. If the diameter of the annular body contracts, a corresponding, radial load on the helical spring is produced, and the spring, in the case of a helical spring having a plurality of coils, is unable to follow the said load, by means of a reduction in diameter, because of the other application of force - radial pressure on the spring, and non-contraction of the spring by rotation of the spring ends about the spring axis - since the spring has a high, radial compressional stiffness. If fewer coils are provided, the ffiction between the- spring and annular body comes more in useful, and the frictional forces in effect between the annular body and spring coil counteract a relative displacement in the circumferential direction, with the result that the annular body is only able to contract to a very limited extent, if at all.

Alternatively, a dowel-like clamping device can also be used as the expanding body. In a preferred embodiment, one such clamping device can be formed, in particular, by an expandable dowel outer surface against which an expandable insert can be adjusted. The dowel outer surface can thus, for example, be of slotted design, in particular slotted in the longitudinal direction, and can interact with the expandable insert, the dowel outer surface and expanding insert having, on their mutually facing surfaces, run-on regions which as they increasingly overlap cause the outer surface to expand. The mutually facing surfaces can thus preferably be of conical design, so that - in the case of an even 6 rise - this results in, for example due to resilient prestress in the axial direction, an enlargement of the dowel outer surface, which enlargement follows an enlargement of the diameter of the piston skirt in its sealing zone and which is fixed by pushing the conical expanding insert from behind, in particular when securing the latter by self- locking.

In a preferred embodiment, it is particularly expedient to design the dowel outer casing such that it is spherical in the region of its outer circumferential region lying over the sealing zone of the piston skirt, with a corresponding convexity also being of advantage for the expandable insert.

In order to secure the expandable body with respect to the measuring piston in its skirt region containing the sealing zone and formed by the piston body, it has proven expedient to form the dowel outer surface by the longitudinally slotted wall region of a cup body, at least one, preferably a plurality of whose fingers, which are produced by the slotting. gripping the piston skirt on the inside from behind in the region of assigned latches. It being possible for these latches to be formed by raised latching projections, for example a circumferential rib or latching notches, or also an annular groove. In this case, the cup bottom, which is opposite the piston head, can at the same time form the support for the spring elements acting upon the expandable inserts.

Preferably, the measuring piston is preferably formed as a whole by a plastic body, the sealing zone of the piston lying in that region of the piston body which is remote from the piston head.

Preferably, a reinforced design of the piston head also makes it possible, in particular in a simple manner, to secure the lifting rod, which is assigned to the piston and via which the piston interacts with an inductive travel sensor, in the piston head. It is possible for a threaded region of the lifting rod to be pressed or screwed into a corresponding receiving hole in the piston head, with the result that a complete pair of threads produced by machining can be omitted.

1 7 In a preferred embodiment, in order to eliminate the build-up of excessively large radial compression forces on the measuring cylinder in the solid head region of the piston, it has proven expedient to provide at least part of the rear side of the head region of the piston with axial recesses, resulting in a capacity for yielding. In particular, it has proven advantageous to provide such capacity for yielding, in the form of axial holes or the like, in ring form in the vicinity of the piston wall, with the result that the latter is supported against the core forming the holder for the lifting rod, only via the webs lying between the holes, and therefore exhibits comparatively high flexibility.

Exemplary embodiments of the invention will now be described in greater detail with reference to the drawing which shows:

Fig. I shows, in section, a measuring device having a measuring chamber into which injection via an injection nozzle takes place, and which is delimited in a volume -changeable mariner by means of a gas-pressurized measuring piston, Fig. 2 shows an enlarged sectional illustration of the measuring piston according to Figure I with a first embodiment of an expanding body, Fig. 3 shows a schematic sectional illustration in a section running along 1114H in Figure 2, Fig. 4 shows a further measuring piston, Fig.5 shows a view of the measuring piston according to Figure 4 in the direction of the arrow V in Figure 4, Fig. 6 shows an illustration corresponding to Figure 2 with a finiher embodiment of an expandable body, and Fig. 7 shows a schematic sectional representation in a section running along VH-VII in Figure 6.

The figures relate to a measuring device whose principle of functioning is disclosed, for example, in DE 39 16 419 C2.

8 The measuring device denoted overall by 1 includes a measuring cylinder 2 in whose cylinder hole 3 a measuring piston 4 is arranged. The measuring piston 4 is arranged in the measuring cylinder 2 in a seal-free manner and forms a boundary for a volumechangeable measuring chamber 5 into which an injection nozzle 6 opens out, the intention being for a volumetric measurement of the injection quantity injected via the injection nozzle 6 to be carded out via the measuring device 1. For this purpose, the measuring piston 4 is connected coaxially to a lifting rod 7 which is assigned, in the region remote from the measuring piston 4, the sensor core 8 of a travel sensor 9 which has sensor coils 10 in the axial overlapping region to the sensor core 8. The travel sensor 9 is assigned overall to a head part 11 covering the measuring cylinder 2, a gas pressure chamber 12 being provided in the transition region between the travel sensor 9 and measuring piston 4, into which chamber a supply hole 13 for setting the respectively required gas pressure opens out.

Furthermore, the measuring chamber 5 provided in the measuring cylinder 2 is connected via an outlet hole 14 to the return flow, with (which is not illustrated here) the outlet hole 14 being controlled via a valve which is activated and opened either after every injection or after a certain number of injections, depending on which type of measurements are to be carTied out.

The measuring piston 4, which is illustrated on an enlarged scale in Figures 2 and 3, is a plastic piston, temperature-resistant plastic materials having good sliding properties and expanding as a function of temperature as the temperature rises. Preferably plastics based on PTFE which also have a certain flowability are used, in particular, for the measuring piston 4.

A measuring piston 4 of this type has a closed head region 15 assigned to the measuring chamber 5, and a piston skirt 16 which forms a sealing zone 17 in its subregion remote from the head 15, this sealing zone 17 being designed as an annular body of reduced wall thickness which, in the exemplary embodiment shown, forms an integral component 1 1 9 together with the measuring piston 4, the said piston-skirt region, which serves as the scaling zone 17 of the measuring piston 4 and is of reduced wall thickness, being assigned an expanding body 18 which, in the exemplary embodiment, is formed by a relatively tightly wound helical spring 25 having a small wire thickness. As is explained in further detail below, the spring 25 supports the wall region, serving as the scaling zone 17, mdially in such a manner that via the sealing zone 17 of the piston 4, the gas pressure chamber 12 is sealed oE in a gas-tight manner from the measuring chamber 5 without separate sealing elements having been provided.

In the exemplary embodiment, the helical spring 25 serving as the expanding body 18 has a relatively large number of coils, 10 coils here in total, while having a wire thickness of 0.3 mm, with the result that the wire thickness corresponds approximately to one tenth to one twentieth of the spring radius, the distance between the individual spring coils being of the order of magnitude of the spring-wire thickness, so that a closely joined, tightly meshed support is achieved.

In the region of the sealing zone 17, which makes up the rear part of the piston 4, which part is remote from the piston head 15, the piston skirt 16 is of reduced wall thickness by virtue of the fact that the internal diameter of the piston skirt 16 is enlarged in the part holding the expanding body 18, with the result that an undercut annular region is produced on the inside. As a result, the expanding body 18, which is designed as a helical spring 25, is secured axially with respect to the piston 4.

For the purpose of fastening the lifting rod 7, the piston head 15 is of reinforced design, at least in the central region towards the rear piston end. This reinforcement is formed by a head part 15 which is overall of corresponding thickness and is provided with a central mount for the holding rod 7, the mount being formed by a hole 19 provided in the rear side of the piston head 15. On account of the piston 4 being designed as a plastic piston, the fastening of the lifting rod 7 in the hole 19 can take place by the lifting rod 7 being provided with a threaded section 20 which cuts into the hole 19, with the result that an additional means of securing the lifting rod 7 can be omitted, particularly also because of the resilient properties of the plastic used for the piston 4.

In order, despite the solid design of the piston 4 in the head region 15, to avoid excessively high nadial contact-pressure forces of the piston 4 with respect to the cylinder hole 3, which are caused by tolerances and/or, for example, because of expansions caused by temperature and which could impair the easy displaceability of the piston, the piston head 15 is provided, from the rear side in the vicinity of the outer circumference, with a ring of axial holes 21 by means of which compensating spaces are provided, the ring of holes according to Figure 3 being configured in such a manner that only comparatively narrow webs 22 remain between the individual axial holes 2 1, with the result that the piston in this region is comparatively elastic radially and is non-rigid in form over and beyond the elasticity of the material.

In view of the provision of a gas-tight sealing boundary in the region of the sealing zone 17 at comparatively low, radial bracing of the piston 4 with respect to the wall of the cylinder hole 3, and therefore also at comparatively low firiction, which would impair the smooth-nmning of the piston 4 in the cylinder 3, use is preferably first of all made of a plastic material, for example, PTFE having particularly favourable ffictional properties, for the piston 4. Compared to known steel pistons which have to be fitted into the measuring cylinder which is also present and consists of steel, with microfinishing, for example by lapping in, this material additionally has a lower specific weight, which proves to be expedient in view of the impairment of measuring results by the measuring piston having a high mass. Moreover, in the case of the solution according to the invention, microfinishing of this type is omitted; use is made to a certain extent of the material properties of the plastic piston combined with expansion of the said piston in the region of the sealing zone 17 in order to achieve the required, gas-tight separation between the measuring chamber 5 and gas pressure chamber 12, the compressed gas acting upon the piston 4 from the rear side and also having an effect on the inside of the piston skirt 16, with the result that the piston skirt 16 is loaded to a certain extent radially towards the cylinder wall.

1 1 I I However, the measuring piston 4 is not only exposed to corresponding pressure loads reaching up to the order of magnitude of approximately 100 bar, in order to obtain a pressure level in the measuring chamber 5 which comes as close as possible to the motive conditions corresponding to practical operation, but is also exposed to considerable thermal loads, it being possible for temperatures of up to the order of magnitude of 150 to 200'C to be set in the operating mode.

With regard to the seal, to be achieved via the piston 4, between the measuring chamber 5 and gas pressure chamber 12, this means that deviations in diameter caused b., temperature have to be compensated for to the greatest possible extent if, firstly, excessively high ftictional values, particularly also in the region of the sealing zone 17, and secondly, leakages are to be avoided.

Therefore the annular body forming the sealing zone 17 and of reduced wall thickness. with regard to the wall thickness of the rest of the piston skirt 16, is configured to a constructional size, i.e. to a constructional diameter, at which there is still an under-size with respect to the cylinder diameter - with regard to the sought for, gas- tight bearing of the piston skirt with respect to the wall of the cylinder hole. The piston 4 therefore still has overall a radial clearance with respect to the cylinder hole.

If the measuring device is put into operation and correspondingly heated up. for example by a correspondingly high load, because of the material properties of the plastic used, the piston 4 expands to a greater extent than the measuring cylinder 2 receiving the cylinder hole. However, this expansion does not lead to any impermissible bracing of the piston 4 within the measuring cylinder, since the piston skirt has a comparatively thin wall, in particular in the region of the sealing zone, and since high radial supporting forces can be avoided by a corresponding capacity for yielding (axial holes 21) in the piston-head region 15 too.

In the region of the sealing zone 17, the piston skirt 16, however, additionally surrounds the expanding body or the expanding element 18 which is formed by an annular body in 12 the form of a flat spinal spring 25. This flat spiral spring 25 is configured with regard to its external diameter in such a manner that, in the region of the sealing zone 17, it has a certain oversize, with respect to the structural internal diameter of the piston skirt 16, and is therefore bmced radially. If the piston skirt 16 now expands, the spring 25 follows it on account of the previously given, radial bracing. The spring 25 then forms, as it were, a supporting corset for the comparatively thin wall region of the piston skirt 16 in the region of the sealing zone 17. The wall thickness in the region of the sealing zone 17 corresponds preferably approximately to half to two thirds of the wall thickness in the adjoining axial regions, and is approximately twice the wire thickness.

Since the piston skirt is enlarged because of temperature, since the spring 25 used as the expanding body or expanding element simply follows this enlargement, and since the prestress achieved by pulling up the spring 25 is also only small, the frictional forces which are effective in the circumferential direction between the spring 'Aire and the inner surface of the skirt are only small and scarcely impair the enlarging of the spring 25.

When the full operating temperature is reached, the piston skirt 16 lies in the region of the sealing zone 17 in a virtually clearance-fi-ee manner, and therefore bears against the inner surface of the cylinder hole 3 in a gas-tight manner, specifically because of the thermal expansion, supported by the spring 25 as the expanding body or expanding element, the supporting force applied as prestress by the spring 25 being able to be determined by the configuration of the spring 25.

However, the sealing fit thus achieved is also maintained as the temperature drops. If the temperature drops, the piston skirt 16 has a tendency to constrict, i.e. to shrink. The consequence is a corresponding, radial load on the expanding body. If the latter, as in the exemplary embodiment, is designed as a helical spring 25 having a plurality of coils, the diameter of the said spring can be changed, i.e. can also be prestressed, with comparatively little effort by alternate twisting of its ends about the spring axis. However, on the other hand, it is relatively stiff with regard to radial forces, i.e. forces acting perpendicularly to the spring axis, and can accordingly also support high radial forces 13 without a substantial change in diameter. This effect is used to support the sealing zone 17, the high radial forces occurring during shrinking of the skirt 16 additionally causing increased friction between the expanding body or the coils of the spring 25 and the piston skirt 16, with the consequence that the expanding body, by being displaced in the circumferential direction with respect to the piston skirt 16, is unable to undergo any constriction, but rather acts as a supporting formation which is rigid to a great extent. By means of this the piston skirt 16 is kept in the region of the sealing zone 17 to the diameter which was set as the sealing diameter. The support via the expanding body ultimately also results in a certain flowing of the material of the piston skirt 16 in the region of the sealing zone, with the result that a durable contact pressure and seal are achieved with clamping forces which are to be applied via the expanding body and which decrease over time. The constructional diameter of the expanding body, i.e. in particular of the helical spring 25, is selected in such a manner that, with regard to the sealing diameter, a clearance-free bearing against the piston skirt is ensured.

The helical spring 25 constitutes a particularly simple and adaptable refinement of an annular body. However, within the context of the invention, use can also be made of other annular bodies, for example,overlapping rings in the region of the ends, it also possibly being expedient in particular in the case of annular expanding elements of this type which have a relatively large surface area to texture their outer surface which faces the skirt surface in such a manner that with a corresponding radial load, an approximately positive -locking connection with respect to the piston skirt is produced, which connection at least virtually eliminates mutual displacement of the annular body with respect to the skirt in the circumferential direction, and thereby leads to fixing of the set sealing diameter.

The piston 26 illustrated in Figures 4 and 5 largely corresponds to that according to Figures 2 and 3. Accordingly, reference can be made to the description relating thereto. In addition, the piston 26 according to Figures 4 and 5 is taken back around the circumference in its head region 15, i.e. is reduced in diameter, so that in the event of an operation in which sharp edges are brushed over axially, no damage to the piston 26

14 occurs. In order to prevent the piston 4 from being damaged as a consequence of axial loads, in particular in the event of possibly striking against the bottom of the measuring chamber 5, and from possibly being enlarged radially in this region, in the refinement according to Figures 4 and 5, the head region 15 is surrounded, a metallic supporting ring denoted by 23 being provided. The sealing function of the piston is not impaired by such a refinement, since the said sealing function is undertaken in the region of the sealing zone 17.

Figures 6 and 7 show, in conjunction with a measuring piston 30 corresponding in its basic design to the measuring pistons 4 and 26 of the preceding exemplary embodiments, a further refinement of an expanding body 31 in the form of a dowel-like clamping device. The expanding body 31 comprises a dowel outer surface 32 which is formed by the wall region of a cup body 33 which has a bottom 34, with the result that the expanding body 31 comprises a dowel outer surface 32 which is formed by the wall region of a cup body 33 which has a bottom 34, with the result that the expanding body 3 1, when inserted into the rear, hollow piston interior 35, seals off the said piston interior via the cup body 33, the lifting rod 7 passing with a clearance through the bottom 34 of the cup body 33. As can be seen in particular in Figure 7, the dowel outer surface 32 is of slotted design, the slots 36 running in the axial direction of the piston 30 and the outer-surface fingers 37, produced by the division of the dowel outer surface 32, being axially secured with respect to the measuring piston 30 in the region of their free ends via a latching connection. The latching connection comprises a latching ring 38 which is provided projecting into the piston interior 35 and assigned to the measuring piston 30, and which is provided with a run-on slope 39 in the insertion direction of the expanding body 3 1, and with a latching shoulder 40 in the opposite direction. The latching shoulder 40 is grasped from behind by latching hooks 41 which are assigned radially outwards to the free end regions of the outer-surface fingers 37. As the axial holes 21 are made in the piston head 15, the latching ring 38 begins to be cut into via the respective holes, so that the latching ring 38 is ultimately formed by a number of individual projections.

1 1 The cup body 33 holds an expanding insert 42, the mutually facing circumferential surfaces of the expanding insert 42 and of the dowel outer surface 32 tapering - vAth approximately corresponding conicity - towards the head 15 of the measuring piston 30 with the result that by displacement of the expanding insert 42 towards the head 15 of the measuring piston 30, the dowel outer surface 32 is enlarged and placed against the wall of the measuring piston 30 and, in the bearing region indicated here at 17 - forms the scaling zone.

As can be seen from the drawing, it is expedient if the outer circumference of the dowel outer surface 32 is of spherical design, specifically with the apex lying in the region of the sealing zone 17, and furthermore, a corresponding convexity of the expanding insert 421 also turns out to be expedient. with the result that the support of the expanding insert 42 with respect to the dowel outer surface 32, and of the dowel outer surface 3-1 with respect to the body of the measuring piston 30, is concentrated in the sealing zone 17.

The expanding insert 42 is elastically compliant towards the piston head 15, and is supported here by a spring 43, with the result that when the interior 35 of the measuring piston 30 is enlarged, the expanding insert 42 is automatically pushed fi-om behind, and the dowel outer surface 32 is enlarged, in a manner matching the inner circumference of the body of the measuring piston 30, if a corresponding enlargement of the skirt of the measuring piston 30 to adapt to the internal diameter of the measuring cylinder takes place due to temperature. In the opposite direction, a virtually rigid support results, since because of the flat cone angle a self-locking efrect occurs which virtually eliminates displacement of the expanding insert 42 with respect to the dowel outer surface 32 under the influence of radial forces.

Suitable materials for the expanding body 31 are likewise preferably plastics, although it is possible here for plastics of lower quality than for the measuring piston 30 to be used, but in particular plastics of lower elasticity and without flow properties can also be used.

16

Claims

Claims

I. Measuring device for the volumetric measuring of injection quantities comprising a measuring chamber which is sealed off by a gas-pressurized annular measuring piston with a piston skirt guided in a seal-free manner in a measuring cylinder having a first diameter, wherein the measuring piston comprises, at least in the region of a sealing zone, expandable plastic and has a diameter which is smaller than the measuring cylinder diameter and the plastic being expandable, as a function of temperature, up to a sealing diameter corresponding to the cylinder diameter, such that the piston can be fixed in the expanded state via a mechanically prestressed expanding body which follows the internal diameter of the piston skirt, which diameter enlarges up to the sealing diameter, and maintains the sealing diameter by giving radial support.
2. Measuring device according to Claim 1, wherein the sealing zone is formed by a region of the piston skirt.
3. Measuring device according to Claim I or 2, wherein the sealing zone is provided in that region of the piston skirt which is remote from the piston head.
4. Measuring device according to one of the preceding claims, wherein the sealing zone is formed by a region of reduced wall thickness of the piston skirt.
5. Measuring device according to Claim 4, wherein the sealing zone is formed by a region of the piston skirt, which region is notched out annularly in the inner circumference.
6. Measuring device according to one of the preceding claims, wherein the measuring piston has, on the rear side of the piston head, a central mount for a lifting rod connected to the piston.

1 1 17
7. Measuring device according to Claim 6, wherein the lifting rod can be rotated into or inserted into a hole which is provided in the central mount and is situated in the piston a-xis.
8. Measuring device according to one of the preceding claims, wherein the piston has a solid head region encompassing the piston head.
9. Measuring device according to Claim 8, wherein the mount for the lifting rod is provided in the head region of the measuring piston.
10. Measuring device according to Claim 8 or 9, wherein adjacent to the piston wall, the head region of the piston is provided with axial material recesses.
11. Measuring device according to Claim 10, wherein the material recesses are of cylindrical design.
12. Measuring device according to Claim 10 or 11, wherein the material recesses are designed as holes.
13. Measuring device according to one of Claims 10 to 12, wherein the material recesses form a ring adjacent to the piston wall.
14. Measuring device according to one or more of the preceding claims, wherein the expanding body is designed as a spring element.
15. Measuring device according to one or more of the preceding claims, wherein the expanding body is an annular body.
16. Measuring device according to Claim 15, wherein the annular body has overlapping ends.

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17. Measuring device according to one of Claims 14 to 16, wherein the expanding body is designed as a spring element which is prestressed radially outwards in the structurally installed state.
18. Measuring device according to one of Claims 14 to 16, wherein the expanding body is compressionally stiff with regard to radially inwardly directed compression forces counter to its radial, outwardly directed prestress.
19. Measuring device according to Claims 14 to 18, wherein the expanding body is a helical spring.
20. Measuring device according to one of Claims 14 to 19, wherein the expanding body is an axially constrained helical spring.
21. Measuring device according to one of Claims 19 or 20, wherein the helical spring, which is provided as the expanding body, has a plurality of coils.
22. Measuring device according to one or more of Claims 19 to 21, wherein the helical spring has a diameter which is of the order of magnitude of its axial length.
23. Measuring device according to one of Claims 19 to 22, wherein the wire thickness of the helical spring is substantially one tenth to one fitlieth of the diameter of the spring.
24. Measuring device according to Claim 23, wherein the wire thickness is one thirtieth to one fortieth of the diameter of the spring.
25. Measuring device according to one of Claims 19 to 23, wherein the distance between the spring coils is substantially equal to the thickness ofthe spring wire.

1 1 19
26. Measuring device according to one of Claims 19 to 23, wherein the distance between the spring coils is smaller than the wire thickness.
27. Measuring device according to one or more of Claims I to 13, wherein the expanding body is a dowel-like clamping device.
-)8.

Measuring device according to Claim 27, wherein the dowel-like clamping device has a slotted dowel outer surface and an expandable insert which is longitudinally displaceable with respect to the latter.
29. Measuring device according to Claim 27 or 28, wherein the expandable insert is loaded in an elastically compliant manner in the expansion direction.
30. Measuring device according to one of Claims 27 to 29, wherein the expandable insert and the dowel outer surface taper conically in the direction of the resiliently elastic support of the expandable insert.
31. Measuring device according to one or more of the preceding claims, wherein the measuring piston is produced in one piece from plastic.
32. Measuring device according to Claim 31,,vherein the measuring piston comprises a thermally stable, expandable and flowable plastic which has a low coefficient of friction.
33. Measuring device according to Claim 31 or 32, wherein the piston consists ofPTFE
34. Measuring device substantially as described herein, with reference to and as illustrated in the accompanying drawings.