Abstract:
A sheathed-element glow plug for an internal combustion engine, made up substantially of a plug housing and a glow element disposed in the plug housing, having a gas conduit for measuring the combustion chamber pressure. A gas-permeable filter element, provided according to the present invention in the gas conduit, prevents the penetration of carbon residues or combustion residues into the interior of the sheathed-element glow plug, or reduces particle entry to a tolerable level.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a sheathed-element glow plug for an internal combustion engine, made up substantially of a plug housing and a glow element disposed in the plug housing, having a gas conduit for measuring the combustion chamber pressure.  
       DESCRIPTION OF RELATED ART  
       [0002]     For good starting and warmup behavior at low temperatures, diesel engines require a heat source that preheats either the gas mixture, the intake air, or the combustion chamber. For passenger-car engines, the use of sheathed-element glow plugs is generally proposed. These encompass a plug housing and a glow element that projects out of the plug housing and, in its installed state, projects out of the cylinder head into the combustion chamber of an internal combustion engine. As a rule, the glow elements of the sheathed-element glow plugs project 4 mm into the combustion chamber of the internal combustion engine, and heat the diesel/air mixture. The glow temperature that is reached, and the total glow time, have a considerable influence on the emissions characteristics and fuel consumption of the internal combustion engine.  
         [0003]     The glow elements can be metallic heating tubes or ceramic glow elements.  
         [0004]     In order to achieve the stipulated goals of further fuel savings and reduced emissions, there is increasing interest in developing economical sensors that can deliver information about the combustion process directly from the engine&#39;s combustion chamber. Injection quantity regulation, for example, could be performed on the basis of that information. Tracking the pressure within the combustion chamber would offer substantial advantages as compared with ion current measurement, for example, which supplies only local information, since the pressure readings and their changes are larger and thus easier to sense.  
         [0005]     Sensor-integration concepts in which a pressure sensor is disposed on or in the sheathed-element glow plug have the advantage that an additional orifice does not need to be provided in the internal combustion engine. This advantage becomes that much greater given that in modern internal combustion engines, the installation space for attaching additional sensors is very limited.  
         [0006]     Solutions that provide a piezoelectric element between the housing and the threaded connection of a sheathed-element glow plug on the cylinder are proposed in the existing art, for example from EP-A 109 6141. Also proposed, in document WO-A 97 09567, is an additional pressure sensor in the sheathed-element glow plug, which sensor is interposed between a retaining member and a cylinder pressure sensing member in a body of a sheathed-element glow plug.  
         [0007]     Piezoresistive sensors such as MDS or SOI sensors, piezoelectric sensors such as GaPO 4 -quartz or langasite sensors, or optical sensors, can be used.  
         [0008]     The pressure can be directed to the sensor by way of a material displacement or elongation, which in integrated sensors is transferred via the sheathed-element glow plug.  
         [0009]     A further possibility for pressure transfer encompasses a gas conduit that, in integrated sensors, extends firstly in the gap between the glow element and cylinder head orifice, and then inside the sheathed-element glow plug. In this gas conduit, the combustion chamber pressure is transmitted via an air column to the combustion chamber pressure sensor in the engine compartment.  
         [0010]     Sheathed-element glow plugs having an integrated pressure sensor that are based on the principle of transmitting combustion chamber pressure via a gas column can be sensitive to carbon deposition, since particles that penetrate in and collect can influence the measurement.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of the invention to present a sheathed-element glow plug in which the penetration of carbon residues or combustion residues into the interior of the sheathed-element glow plug is prevented or at least reduced to a tolerable level.  
         [0012]     This and other objects of the invention are achieved by providing a gas-permeable filter element in the gas conduit.  
         [0013]     The filter element obstructs the gas conduit extending next to or within the sheathed-element glow plug so that carbon residues or combustion residues cannot reach the particularly sensitive pressure sensor.  
         [0014]     If the filter element is fitted into the combustion-chamber end of the gas conduit, the penetration of carbon residues or combustion residues into the gas conduit is also prevented or at least reduced. Any possible cross-sectional constriction resulting from deposited particles is thereby counteracted, and clogging of the gas conduit is prevented.  
         [0015]     At the same time, because of the gas permeability of the filter element, the air column for ascertaining pressure conditions in the combustion chamber is not influenced.  
         [0016]     The pressure measurement can be performed in systematically accurate fashion because the cross-sectional area of the gas conduit cannot clog and thus remains constant, and therefore no particles can reach the sensitive membrane of the pressure sensor. If movable parts are located on the pressure sensor, the use of the filter element also prevents them from being interfered with by any possible particle entry. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The invention will be described in greater detail with reference to the following drawings wherein:  
         [0018]      FIG. 1  is a schematic, enlarged, partial depiction of a first exemplary embodiment of a sheathed-element glow plug according to the present invention, in section.  
         [0019]      FIG. 2  is a schematic, enlarged, partial depiction of a second exemplary embodiment of a sheathed-element glow plug according to the present invention, in section.  
         [0020]      FIG. 3  is a schematic, enlarged, partial depiction of a third exemplary embodiment of a sheathed-element glow plug according to the present invention, in section.  
         [0021]      FIG. 4  is a schematic, enlarged, partial depiction of a fourth exemplary embodiment of a sheathed-element glow plug according to the present invention, in section. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     In an advantageous embodiment, the gas conduit of a sheathed-element glow plug is made up of two sub-segments: a first conduit that is formed by the open space between the glow element and the cylinder head orifice, and a second conduit that is located in a region between the glow element and the plug housing.  
         [0023]     The filter element can be disposed in the first conduit or in the second conduit.  
         [0024]     The filter element is preferably located in a region of the sheathed-element glow plug that is either sufficiently hot, or sufficiently heatable, that carbon residues in the filter element are collected or combusted. The filter element is placed for that purpose in the immediate vicinity of the glow element, and preferably forms a direct or at least positively engaged connection to the glow element. This can be effected, for example, by insertion, press-fitting, or plug-fitting.  
         [0025]     In an advantageous embodiment, the filter element is joined to a sealing element that ensures a seal between the plug housing and the sealing seat in the cylinder head orifice.  
         [0026]     The filter element and sealing element can be either produced integrally from one material that is suitable for both purposes, or embodied in two parts, the two parts being secured to one another.  
         [0027]     For the combination between filter element and sealing element, the filter element is preferably mounted into the gas conduit in the vicinity of the transition between the first and second conduit, since it is precisely at the transition between the first and second conduit that the sealing seat between the cylinder head and plug housing is placed. The sealing element and filter element are in this case located physically close to one another.  
         [0028]     A tight-mesh fabric, such as a nonwoven fabric, made of metal, or porous ceramic, or a metal having very small orifices, is suitable for the material of the filter element.  
         [0029]      FIG. 1  schematically depicts a first exemplary embodiment of a sheathed-element glow plug  1  according to the present invention, in section. Sheathed-element glow plug  1  possesses a plug housing  2  and a ceramic glow element  3 . The latter is joined via a contact element  4  to a support tube  5 , which creates an electrical connection to plug housing  2 . At combustion-chamber end  6  of contact element  4 , a contact is likewise provided with glow element  3  to a retaining sleeve  7 . The casing of glow element  3 , made up of retaining sleeve  7 , support tube  5 , and plug housing  2 , possesses at combustion-chamber end  6  a conically shaped termination  8  with which it presses in sealing fashion against orifice  9  in cylinder head  10 .  
         [0030]     In order to transmit the pressure in the combustion chamber (not depicted in the Figure) to the pressure sensor (likewise not depicted in the Figure), sheathed-element glow plug  1  has a gas conduit  11  that is made up of two sub-regions: a first conduit  12   a  that is located between cylinder head  10  and glow element  3 , and a second conduit  12   b  that extends between support tube  5  and plug housing  2 .  
         [0031]     Located in second conduit  12   b  at combustion-chamber end  6  is a filter element  13 . This prevents combustion residues and carbon residues, created in the combustion chamber (not depicted in the Figure), from penetrating into second conduit  2   b  and reaching the pressure sensor (likewise not depicted). Filter element  13  can be installed during the production of sheathed-element glow plug  1 .  
         [0032]      FIG. 2  schematically depicts a second exemplary embodiment of a sheathed-element glow plug  1  according to the present invention, in section. Sheathed-element glow plug  1  possesses substantially the same construction as in the previous example: it has a plug housing  2 , a ceramic glow element  3 , and a two-part gas conduit  11  made up of a first conduit  12   a  and a second conduit  12   b.    
         [0033]     In this exemplary embodiment, a filter element  213  is located in first conduit  12   a . Filter element  213  can be placed between cylinder head  10  and glow element  3  upon assembly of sheathed-element glow plug  1 .  
         [0034]      FIG. 3  schematically depicts a third exemplary embodiment of a sheathed-element glow plug  1  according to the present invention, in section. This exemplary embodiment shows a filter element  313  that represents a combination of the first two exemplary embodiments.  
         [0035]     Filter element  313  is located both in first conduit  12   a  and in second conduit  12   b , and thus closes off the connection between the two gas conduits  12   a ,  12   b  with respect to particles.  
         [0036]      FIG. 4  schematically depicts a fourth exemplary embodiment of a sheathed-element glow plug  1  according to the present invention, in section. Sheathed-element glow plug  1  possesses substantially the same construction as in the previous examples. It has a plug housing  2 , a ceramic glow element  3 , and a two-part gas conduit  11  made up of a first conduit  12   a  and a second conduit  12   b.    
         [0037]     A filter element  413  covers end  414 , facing away from combustion-chamber end  6 , of first conduit  12   a , and is joined to a sealing element  415 . The latter fits between the conically shaped termination  8  of sheathed-element glow plug  1  and orifice  9  in cylinder head  10 , and thus ensures sealing of the combustion-chamber pressure with respect to the external environment.  
         [0038]     Filter element  413  and sealing element  415  can be produced integrally from a material that is suitable for both filtering and sealing purposes. Sealing element  415  can also be manufactured from a material specifically designed for sealing purposes, e.g. as a copper washer, and a further component having a filtering function is secured thereto. Upon assembly, sealing element  415  is pressed between cylinder head  10  and sheathed-element glow plug  1 .  
         [0039]     Filter element  413  can also (although this is not depicted in the Figure) project into first and/or second conduit  12   a ,  12   b , as in examples 1 to 4.