Abstract:
A pressure sensor measuring element for a pressure sensor operates to detect pressure in a combustion space of an internal combustion engine. The pressure sensor measuring element includes a separating diaphragm, a plunger for the transmission of deflections of the separating diaphragm to a force measuring element, and with a sleeve which receives the plunger. The sleeve is closed by the separating diaphragm at a first end intended to face the combustion space and is designed to hold the force measuring element at the opposite second end. Accordingly, the pressure sensor measuring element can be produced more cost-effectively. Furthermore, the plunger can be produced in one piece with the separating diaphragm as a diaphragm/plunger unit, and the sleeve and the diaphragm/plunger unit can be formed from the same metal material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119(a) to German Patent Application No. 10 2009 030 702.8, filed in Germany on Jun. 26, 2009, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to a pressure sensor measuring element for a pressure sensor for the detection of pressure in a combustion space of an internal combustion engine during the operation of the latter, with a separating diaphragm, with a plunger for the transmission of deflections of the separating diaphragm to a force measuring element, and with a sleeve which receives the plunger and which is closed by means of the separating diaphragm at a first end intended to face the combustion space and is designed for carrying the force measuring element at the opposite second end. The invention relates, moreover, to a pressure sensor, provided with such a pressure sensor measuring element, for measuring the combustion space pressure in an internal combustion engine by means of such a pressure sensor measuring element. 
     2. Background Information 
     Such a pressure sensor and such a pressure sensor measuring element are known from EP 1 255 099 81. This publication will be dealt with in more detail hereafter. 
     U.S. Pat. No. 4,382,377 A describes a pressure sensor for an internal combustion engine in order to measure the internal pressure there in a combustion space. For this purpose, the known pressure sensor has a sensor outer housing which can be screwed into a threaded bore of a combustion chamber wall (that is to say, for example, a cylinder head). At the first end intended to face the combustion space, the sensor outer housing is closed by means of a separating diaphragm which can be deflected by the combustion space pressure. The separating diaphragm is produced in one piece with the sensor outer housing. The deflection of the separating diaphragm is transmitted via a plunger, which is guided directly inside a slender threaded region of the sensor outer housing, to a second end which faces away from the combustion space and where a force measuring element converts the longitudinal movement of the plunger into pressure signals. The pressure sensor thus has a very simple set-up, the sensitive force measuring element being accommodated in a protected region outside the high combustion space temperatures. For this purpose, the plunger is produced from a ceramic having poor thermal conductivity. However, such a pressure sensor may have considerable temperature dependencies, since the sensor housing may expand differently in relation to the plunger in the case of the temperature gradients which prevail on the combustion space wall. Moreover, loads exerted upon the sensor housing when the latter is being screwed in, this being carried out with appreciable torque for the purpose of gas-tightness, may lead to a falsification of the plunger deflection and therefore to errors in pressure measurement. 
     A comparable set-up of a combustion space pressure sensor with a metal or ceramic plunger guided directly in a sensor housing is known from U.S. Pat. No. 5,703,282 A. In this case, however, the separating diaphragm is not produced in one piece with the sensor housing, but instead is welded to that end of the latter which is on the combustion space side so as to hermetically close this end. A comparable set-up is also to be found in U.S. Pat. No. 5,199,303 A. 
     In order to mitigate the abovementioned temperature dependencies, the pressure sensor for combustion spaces of internal combustion engines according to the initially mentioned EP 1 255 099 B1 was proposed. In this case, the sensor outer housing has arranged inside it a separate pressure sensor measuring element having a sleeve which forms a spacer element and in which the plunger is guided. This sleeve is fastened with its first end intended to face the combustion space to the sensor outer housing. The sleeve is not fastened at the second end lying internally and there carries a force measuring element to which the deflection of the separating diaphragm is transmitted by means of the plunger. Both the plunger and the sleeve are manufactured from ceramic in order to block the high combustion space temperatures and also temperature shocks. This combustion space sensor functions very well and has proved appropriate in practice. However, it is relatively costly to produce. 
     SUMMARY 
     Proceeding from the prior art according to EP 1 255 099 B1, the object of the invention is to improve a pressure measuring cell in such a way that more cost-effective production is possible along with uniform or improved accuracy and temperature independence. 
     A pressure sensor provided therewith for pressure measurement in combustion spaces of internal combustion engines (for example, ignition pressure sensor), which has such a pressure sensor measuring element according to the invention. 
     The invention provides a pressure sensor measuring element for a pressure sensor for the detection of pressure in a combustion space of an internal combustion engine during the operation of the latter. So that the high temperature and pressure fluctuations prevailing there can be controlled, the pressure sensor measuring element is provided with a separating diaphragm, with a plunger for the transmission of deflections of the separating diaphragm to a force measuring element, and with a sleeve which receives the plunger and which is closed by means of the separating diaphragm at a first end intended to face the combustion space and is designed for carrying the force measuring element at the opposite second end. In order to provide a simple set-up, there is provision, further, for the plunger to be produced in one piece with the separating diaphragm as a diaphragm/plunger unit, both the sleeve and the diaphragm/plunger unit being formed from metal, specifically both from the same material. 
     A pressure measuring cell with a minimal number of parts can consequently be produced. Even only two parts are sufficient for forming the pressure measuring cell. Separation points between the diaphragm and plunger are avoided, and no fastening has to take place. The selection of metal as material lowers the costs, as compared with ceramic. Surprisingly, it became apparent, during tests, that sufficient temperature shielding can be achieved in spite of the choice of metal for the sleeve and plunger. In particular, the good heat dissipation of the metal of the sleeve can be utilized to dissipate heat via an outer sensor housing. 
     For example, when the combustion space sensor is arranged near a coolant line for cooling the internal combustion engine, the cooling action can be utilized even into the interior of the pressure sensor. 
     The unity of material ensures a uniform expansion of the sleeve and diaphragm/plunger element. Thus, not only are adverse effects caused by different thermal expansion of the sleeve and plunger avoided, but also possible irregular thermal expansions of the separating diaphragm in relation to the sleeve and in relation to the plunger are avoided. 
     In an advantageous refinement of the invention, there is provision for the diaphragm/plunger unit likewise to have in one piece, at that end of the plunger which is opposite the separating diaphragm, a second diaphragm which is flush with the second end of the sleeve. This serves particularly for the greater safety and/or shielding of the force measuring sensor technology from the conditions prevailing in the combustion space. 
     According to a further advantageous refinement of the invention, there is provision for the diaphragm/plunger unit to be manufactured monolithically from one piece. Any separation points can thereby be avoided. Despite simple production, a safe and reliable component is obtained, which series both as a diaphragm and for transmitting the diaphragm movement into a region shielded from the combustion space. 
     According to a further advantageous refinement of the invention, there is provision for the pressure sensor measuring element designed, for example, as a pressure measuring cell to be manufactured overall from a uniformly identical material, to be precise metal and, in particular, steel. 
     In a further advantageous refinement, there is provision for a force measuring element to be fastened, on the one hand, to the second end of the sleeve and, on the other hand, indirectly or directly to that end of the plunger which faces away from the separating diaphragm, and to be manufactured from the same material as the sleeve and the diaphragm/plunger unit. In particular, the force measuring element is designed as a flexural beam with a strain gauge. Such force measuring elements are basically known, for example from the initially mentioned EP 1 255 099 B1. However, such a force measuring element is especially advantageous when it is used in the pressure sensor measuring element according to the invention, since it can be manufactured from the same material as the pressure sensor measuring element according to the invention, so that inaccuracies on account of a different thermal expansion are also avoided between the sleeve and the diaphragm/plunger unit, on the one hand, and the force measuring element, on the other hand. 
     In a further preferred refinement of the invention, there is provision for the sleeve to have at its first end a radially outward-extending flange and an annular projection projecting from the flange with a smaller outside diameter axially in the direction of the separating diaphragm, and for the separating diaphragm to have an outer thickened annular edge region which is flush with the flange on the outside and engages over the annular projection on the inside. As a result, it becomes easier to join the two individual parts of the pressure sensor measuring element together, and gas-tightness is increased. Moreover, a large material mass is provided exactly at the point where the sleeve is advantageously to be connected to a sensor outer housing. This increases stability and facilitates the transmission of heat from the sleeve to the sensor outer housing. In particular, the flange can also be utilized as a stop as far as which the sleeve can be introduced into the sensor outer housing. Exact positioning and sealing off of the interface between the sensor outer housing and pressure sensor measuring element are consequently facilitated. 
     In a further advantageous refinement of the invention, there is provision for the sleeve and the separating diaphragm to be fastened to one another in a materially integral manner, in particular by welding. Good gas-tightness and reliable fastening can thereby be achieved cost-effectively. 
     In a further advantageous refinement of the invention, there is provision for a cavity between the sleeve and the diaphragm/plunger unit to be evacuated or to have a filling with solid particles composed of a material with a higher melting point than combustion space temperatures. 
     Evacuation has the advantage that convection is prevented. Filling with, for example, refractory solid particles, such as, for example, with silica sand, has the advantage that the sensor interior and the surroundings are protected even in the event of a fracture of the front diaphragm (safety under front diaphragm fracture). 
     According to further advantageous refinement, the pressure measuring cell may also be provided with a radiation barrier in order to shield the sensor interior against heat radiation or other disturbing radiation. Preferably, the radiation barrier is slit in two parts so that it can be installed in the cavity more simply. 
     In a further advantageous refinement of the invention, there is provision for the plunger to have a longitudinal bore accessible from the second end and extending in the longitudinal direction. A measuring probe, in particular for temperature measurement, can be introduced into this bore. On account of the metal material, a temperature change can be measured relatively quickly even from inside. If, for example, a high temperature rise occurs (heat wave), this can be detected earlier by means of a probe. A rapid signal can be generated which can be used, for example, for temperature compensation for the purpose of compensating temperature dependencies possibly arising in spite of the special set-up. 
     A pressure sensor provided with such a pressure sensor measuring element can be produced relatively cost-effectively and, because of its simple set-up, is also very reliable. The fewer parts a sensor has, the less likely is the risk that a part fails. The pressure sensor is nonetheless highly suited to the detection of pressure in a conduction space of an internal combustion engine during the operation of the latter. 
     In a preferred refinement of the pressure sensor, there is provision for it to have an outer sensor housing for fastening to a component delimiting the combustion space, the sleeve being received in the sensor housing and being fastened only with an end region formed at its first end to the sensor housing. By the sleeve being independent of the sensor set-up, the pressure sensor measuring element remains free of influences, caused by installation forces. 
     Preferably, the outer sensor housing has a tool engagement point or tool engagement region with tool engagement surfaces for a tool. This may be, for example, a wrench and, in particular, an open-end wrench. The pressure sensor can consequently be screwed by means of a screw-in thread in a pressure- and vibration-resistant manner into an internal thread of a housing containing the medium to be measured, for example into an internal thread on an engine block surrounding a combustion space or into an internal thread on a pipe or a container of a hydraulic system. 
     In order to make screwing in easier, for example so that pressure can be exerted in the screw-in direction via the tool, in a preferred refinement the tool engagement region is provided, between the screw-in thread and at least one first tool engagement surface, with a projection for guiding the tool, preferably in order to prevent the latter from slipping off toward the screw-in thread. 
     On the other hand, it is desirable to be able to introduce the pressure sensor or the sensor outer housing into a holding fixture, for example a vice, for example for the purpose of machining during production or maintenance. In order to make introduction and removal possible also in the longitudinal direction of the pressure sensor, in a further-preferred refinement there is provision for at least two mutually opposite second tool engagement surfaces to be formed continuously, without a projection, in such a way that they have the outermost radial extent of the pressure sensor on an angular range, covered by the second tool surfaces, in relation to the longitudinal mid-axis of the pressure sensor. 
     In a preferred refinement, a simple sensor element for engine pressure sensors is provided. The sensor element is preferably a two-part measuring element. This measuring element is preferably composed of a dual diaphragm plunger (plunger with two diaphragms) and of an outer sleeve. The dual diaphragm plunger has a one-part connection between a combustion space diaphragm and a measuring diaphragm. By the measuring element being formed by two diaphragms, for example, rigidity can be adjusted via the thickness of the diaphragms and deflections can be kept low. A typical thickness for the thinnest point of the combustion space diaphragm designed as an annular diaphragm is about 0.2 mm. A typical thickness of the thinnest point of the measuring diaphragm likewise designed as an annular diaphragm is about 0.32 mm. 
     The form with two diaphragms has the further advantage that the two diaphragms achieve surprisingly effective temperature partitioning for the protection of the force sensor technology lying behind them, despite that ceramics are dispensed with, along with the associated problems of composite material bonds. 
     Some advantages of a design, as above, are mentioned:
         the minimal number of parts;   a minimal error of alignment due to the one-part type of construction, thus also giving rise to good linearity and also to a useful hysteresis property;   the one-part type of construction is distortion-free;   there are, in particular, no thermal stresses when welding is carried out for mounting the plunger between two diaphragms;   there is identical thermal expansion because the same material is used for all parts of the pressure measuring cell;   the diaphragm plunger, outer sleeve, flexural beam and sensor body can be formed from the same material;   there is a greater safety because of the double diaphragm;   the measuring element is free of sensor installation forces on account of the separate type of construction       

     In a further refinement, a vacuum is provided in the interspace of the measuring cell in order to avoid convection. 
     A further refinement relates to a shadow giver or radiation barrier in the direction of the measuring diaphragm. For example, the measuring element could have a filling, for example silica sand. This serves for safety in the event of a front diaphragm fracture. 
     A further additional idea relates to a central bore for a temperature resistance probe. A temperature rise (heat wave) is thereby detected earlier, and this can be used for temperature compensation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of invention is explained in more detail below by means of the accompanying drawing in which: 
         FIG. 1  shows a side view of a pressure sensor for the measurement of combustion space pressures; 
         FIG. 2  shows a longitudinal section through the pressure sensor of  FIG. 1 ; 
         FIG. 3  shows a top view of an end, facing away from the combustion space, of a pressure sensor measuring element of the pressure sensor of  FIG. 1 ; 
         FIG. 4  shows a side view of the pressure sensor measuring element of  FIG. 3 ; 
         FIG. 5  shows a top view of an end, facing away from the combustion space, of a pressure measuring cell of the pressure sensor measuring element; 
         FIG. 6  shows a sectional view of the pressure measuring cell of  FIG. 5  along a mid-plane; 
         FIG. 7  shows a top view of an end, facing away from the combustion space, of a pressure measuring unit of the pressure sensor of  FIG. 1 ; 
         FIG. 8  shows a first sectional view through the pressure measuring unit of  FIG. 7  along a first mid-plane; 
         FIG. 9  shows a second sectional view through the pressure measuring unit of  FIG. 7  along a second mid-plane perpendicular to the first mid-plane; 
         FIG. 10  shows a perspective view of a screw-in region of the pressure sensor with a tool engagement point; 
         FIG. 11  shows a sectional view, comparable to  FIG. 6 , of a further embodiment of the pressure measuring cell with a filling; 
         FIG. 12  shows a sectional view, comparable to  FIG. 6 , of yet a further embodiment of the pressure measuring cell with a radiation barrier, and 
         FIG. 13  shows a perspective illustration of the radiation barrier used in the embodiment according to  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIGS. 1 and 2  illustrate an embodiment of a pressure sensor  10  for the measurement of pressures in a combustion space of an internal combustion engine, such as, for example, a diesel engine for ships, construction machines or motor vehicles or a gasoline engine for motor vehicles or the like. The combustion space pressure can be measured online by means of the pressure sensor  10  while the internal combustion engine is in operation. On the basis of the pressure signal, control and regulation for operating the internal combustion engine can be carried out, and the operation and functioning of the internal combustion engine can be monitored. 
     The pressure sensor  10  has a pressure measuring unit  14  at a first end  12  intended to face the combustion space and a cable  18  at the second end  16  facing away from the combustion space. The cable  18  is connected to the pressure measuring unit  14  via a cable connection  20  and a spacer sleeve  22 . 
     The pressure measuring unit  14  has an outer sensor housing  24  and a pressure sensor measuring element  26 . The sensor housing  24  is provided on its outer circumference with a thread  28  for screwing into a boundary wall  29  of the combustion space. The pressure sensor measuring element  26  is received inside the sensor housing  24 . 
     As is evident from  FIGS. 3 and 4 , the pressure sensor measuring element  26  has a pressure measuring cell  30  and a force measuring element  32  in the form of a flexural beam  34 . 
     The pressure measuring cell  30  is described below with reference to  FIGS. 5 and 6 . The pressure measuring cell  30  has only two structural elements or parts, to be precise a sleeve  36  and a diaphragm/plunger unit  38 , here in in the form of a dual diaphragm plunger  40 , received therein. 
     The sleeve  36  has an essentially cylindrical design. At the first end  12 , the sleeve  36  has on its outer circumference a radially outward-extending flange  42 . An axial annular projection  44  extends from the flange  42  further on in the axial direction toward the combustion space side, so that a step  46  is formed between the annular projection  44  and flange  42  on the outside. A sleeve wall  47  in the form of a cylindrical region  48  of the sleeve  36  is formed on the opposite side of the flange  42 , there being first a thicker region  50  with a larger diameter and then a longer thinner region  52  with a smaller wall thickness and smaller outside diameter. The sleeve  36  is produced in one piece and is worked out from a larger metal piece by material-removing shaping. 
     The diaphragm/plunger unit  38  is likewise produced in one piece. It, too, is worked out from a single metal piece by material-removing shaping. 
     The diaphragm/plunger unit  38  has as a first part region, a first diaphragm  53  in the form of a separating diaphragm  54  and as a second part region a plunger  56 . In the embodiment illustrated here, a second diaphragm  57  in the form of a measuring diaphragm  58  is also provided as a third part region, with the result that the diaphragm/plunger unit  38  is designed as a dual diaphragm plunger  40 . 
     The first diaphragm  53  has a thicker annular edge region  60 , an annular diaphragm region  62  and a central transitional region  63  which divides them in a transition-like manner by means of the plunger  56 . The thicker annular edge region  60  extends with a circumferential flange  64  in the direction of the second end  16  and is seated on the step  46  where it is connected to the sleeve  36  in a materially integral manner by means of a weld  66 . The first end  12  of the sleeve  36  is thereby closed hermetically by means of the separating diaphragm  54 . The annular diaphragm region  62  forms a thinner flexural region at which the separating diaphragm  54  can be deformed movably in the axial direction under the action of pressure. The thinnest point of the annular diaphragm region  62  of changing thickness is less than 0.3 mm, in particular about 0.2 mm thick. To assist deflection, a notch  67  may also be provided on the separating diaphragm  54 , here in the region of the transition between the annular edge region  60  and the annular diaphragm region  62 . The outside diameter of the separating diaphragm  54  corresponds to the outside diameter of the flange  42 , so that the sleeve  36  and the separating diaphragm  54  are flush at the end region  68  formed with the first end  12 . 
     The plunger  56  extends centrally inside the sleeve  36  from the separating diaphragm  54  and goes from the first end  12  in the direction of the second end  16 . In the embodiment illustrated, it connects the separating diaphragm  54  to the measuring diaphragm  58 , so that, when the separating diaphragm  54  is deflected, the measuring diaphragm  58  is likewise deflected. The plunger  56  has an orifice  70 , accessible from the second end  16 , in the form of a central bore  72 . This bore  72  may serve for receiving a temperature measuring probe (not illustrated). 
     The measuring diaphragm  58  likewise has a thicker annular edge region  74 , an annular diaphragm region  76  designed as a flexural zone and a central transitional region  78  which divides them by means of the plunger  56 . The outside diameter of the annular edge region  74  is somewhat smaller than the inside diameter of the sleeve  36 , said inside diameter being uniform over the length of the sleeve  36 , so that said annular edge region can be introduced into the sleeve  36  during the production of the pressure measuring cell  30 . At the second end  16 , the measuring diaphragm  58  is flush with the sleeve  36 . The annular edge region  74  is connected firmly to the sleeve  36  by means of a weld  80 . The annular diaphragm region  76  of the measuring diaphragm  58  is designed to be thicker than the annular diaphragm region  62  of the separating diaphragm  54 . 
     In the embodiment of the pressure measuring cell  30 , as depicted in  FIG. 6 , a cavity  82  formed inside the pressure measuring cell  30  evacuates. 
     As is evident from  FIGS. 3 and 4 , the flexural beam  34  is welded with its outer region  84  to the second end  16  of the sleeve  36 . The flexural beam  34  is connected by means of a middle region  86  to the outside of the transitional region  78  of the measuring diaphragm  58  likewise by welding. In between, the flexural beam  34  has thinner deflection regions  88  which are provided with strain gauges  90 , so that their deflection can be used for generating an electrical signal. 
     The entire pressure measuring cell  30  with the two parts of the pressure sensor measuring element  26  and with the flexural beam  34  is manufactured from uniform material. For this purpose, in the exemplary embodiment, a suitable steel is used. 
     As is evident from  FIGS. 7 to 9 , the pressure measuring cell  30  is introduced into an end, open on the combustion space side, of the sensor housing  24 , so that the flange  42  butts against the end edge  92  of the sensor housing  24 . This end edge  92  is connected firmly to the flange  42  and to the thicker region  50  by means of a weld  94 . 
     The sensor housing  24  has on a shank region  96  the thread  28 . This is followed toward the second end  16  by a tool engagement region in the form of a hexagon  98 . 
     The hexagon  98  has four first tool engagement surfaces  102 , in each case a projection  104  for guiding a tool (not illustrated), for example an open-end wrench or the like, and for preventing it from slipping off being provided between the first tool engagement surfaces  102  and the thread  28 . Two mutually opposite second tool engagement surfaces  99  are formed continuously, without a projection, in such a way that the pressure sensor  10  can be introduced overall in the longitudinal direction, for example, between two clamping jaws (not illustrated), even if the clamping jaws are loosened only slightly. The second tool engagement surfaces  99  are designed as parallel continuous wrench engagement surfaces. 
     The sensor housing  24  is provided toward the second end  16  with a larger recess  100  for the reception of electronics (for example, a chip or ASIC, not illustrated in any more detail here). The sole firm mechanical connection between the pressure measuring cell  30  and the sensor housing  24  is found in the weld  94  at the end region  68 . The pressure measuring cell  30  and the sensor housing  24  can expand differently inward in the axial direction in the event of temperature changes. 
     Even though the pressure sensor  10  is described by way of example of use for online measurement of combustion space pressures and is especially suitable for this purpose, other types of use are, of course, also possible. The pressure sensor  10  is, for example, likewise eminently suitable for the use of pressures in hydraulic systems, even in the case of hot or aggressive hydraulic media. 
     Further exemplary embodiments of the pressure measuring cell  30  for use in the pressure sensor  10  are described below by means of the illustration of  FIGS. 11 to 13 . In this case, only the differences in relation to the embodiment of the pressure measuring cell  30 , as illustrated in  FIGS. 5 and 6 , are explained. All other constituents and features are identical to those of the embodiment explained above. 
     In the embodiment according to  FIG. 6 , the cavity  82  of the pressure measuring cell  30  is evacuated inside it. The purpose of this is to cause as little heat as possible to be conducted through the cavity  82 . By contrast, in the second embodiment of the pressure measuring cell  30  according to  FIG. 11 , the cavity  82  is filled with a filling  106  composed of thermally separating material. In the exemplary embodiment illustrated, the material is in the form of flowable granular material. In actual fact, in this example, silica sand  108  is used. 
     In the embodiment according to  FIG. 12 , a radiation barrier  109 , which is individually illustrated in more detail in  FIG. 13 , is provided in the cavity  82  in the region of the first end  12 . The radiation barrier  109  has two individual elements  110  and  111  which are separated from one another and are adapted to the shape of the cavity  82  and which can be introduced more simply into the cavity  82  individually. Any material which attenuates heat radiation or other harmful radiation will be considered as material for the radiation barrier  109 .