Title of Invention

CERAMIC SHEATHED ELEMENT GLOW PLUG

Abstract

The invention proposes a ceramic sheathed-element glow plug, the ceramic sheathed element of which comprises an electrically conducting conductive layer and an electrically insulating insulating layer. The conductive layer comprises feed layers and a heating layer. The high~r electrical resistivity of the heating layer makes it possible to determine the temperature of the heating layer and of the combustion chamber. The electrical contact between a connection element and the sheathed element is produced by a contacting element which is formed from a pellet of an electrically conductive powder.

Full Text

Prior arfcj The invention starts from a ceramic sheathed-element glow plug for diesel engines of the generic type of the independent claims. Sheathed-element glow plugs with external ceramic heaters are already known, for example, from patent application DE-A 40 28 859. DE-A 29 37 884 has furthermore disclosed metallic sheathed-element glow plugs in which the metallic heater filament is welded to a thermocouple. Here, the temperature in the respective cylinder can be measured during operation of the sheathed-element glow plug by detecting the thermal voltage. However, there is no metallic heater filament in a sheathed-element glow plug with a ceramic heating element. Furthermore, a sheathed-element glow plug having a connection element which is electrically connected to the sheathed element via a contacting element is known from DE 198 44 347. This contacting element is designed as a spring, as can be seen from Figure 1. Advantages of the invention The ceramic sheathed-element glow plug according to the invention with the features of the first independent claim has the advantage that the temperature of the sheathed element can be measured. It is possible for the first time in a ceramic sheathed-element glow plug to measure the temperature of the sheathed element directly in a selected area on the outside of the sheathed element without additional outlay on apparatus. The temperature is measured in a selected area that is small compared with the volume of the sheathed element overall, thereby enabling the error that occurs due to temperature distribution over a large volume to be reduced when determining the temperature. It is furthermore advantageous that it is possible in the sheathed-element glow plug according to the invention to achieve a concentration of the heat output in a selected area of the sheathed element without changing the cross section of the conductive layer, with the result that the surface area in the region in which the concentration of the heat output is to occur remains constant, as does therefore the area of interaction. Another advantage is the fact that it is possible to make the manufacture of a ceramic sheathed-element glow plug of this kind, featuring temperature measurement, economical. Advantageous developments and improvements of the ceramic sheathed-element glow plug indicated in the main claim are possible by means of the measures presented in the subclaims which refer back to the first independent claim. In particular, appropriate choice of the ceramic materials used for the various areas of the sheathed-element glow plug ensures that the mechanical stability of the heater is not impaired. Processing of the measured temperature values by a control unit allows feedback control of the temperature in the selected area of the sheathed element. It is furthermore advantageous to use the sheathed-element glow plug according to the invention as a temperature sensor in the passive mode, once it has performed its heating function. In this way, it is possible to detect whether combustion in the respective cylinder is taking place correctly. It is advantageous that parameters relevant to combustion can be influenced on the basis of this information. The ceramic sheathed-element glow plug according to the invention having the features of the independent Claim 14 has the advantage over -the prior art that, on account of the larger line cross section, higher currents can be transmitted without thermal destruction of the material of the contacting element. Furthermore, the large surface area of the contacting material is advantageous since it allows good thermal conductivity. The flexible spring component ensures that thermal shifts in the surrounding parts caused by different coefficients of thermal expansion can be compensated for. The measures which are listed in the dependent claims referring back to Claim 14 allow advantageous refinements and improvements to the ceramic sheathed-element glow plug described in the main claim. In this context, it is advantageous for the contacting element to be in the form of graphite or conductive ceramic powder, since these materials are resistant to corrosion. Furthermore, it is advantageous for only the majority of the material to be provided in the form of graphite or conductive ceramic or metal powder, since it is possible to save on expensive materials while achieving approximately the same properties. Furthermore, it is advantageous to produce the sheathed-element glow plug having a contacting element according to the invention in the manner described below, since this results in an arrangement of the components located in the plug housing which is such that short circuits are prevented. Moreover, it is ensured that the components are pressed together in such a way that, on the one hand, there is no loosening of the components and, on the other hand, the components do not burst open as a result of an excessive opposite force being exerted by resilient elements (e.g. by the contacting element). Drawings The exemplary embodiments of the invention are illustrated in drawings and explained in greater detail in the following description, in which: Figure 1 shows a sheathed-element glow plug according to the invention in longitudinal section. Figure 2 shows a side view of the front portion of the ceramic heater, which is situated on the outside. Figure 3 shows one way of interconnecting the sheathed-element glow plug according to the invention with the control units. Figure 4 shows the resistances occurring in the ceramic sheathed-element glow plug according to the invention and in the feed lines and Figure 5 shows a sheathed-element glow plug according to the invention in longitudinal section. Description of the exemplary embodiments " Figure 1 shows a schematic longitudinal section through a ceramic sheathed-element glow plug 1 according to the invention. Electrical contact is made at the end of the sheathed-element glow plug 1 remote from the combustion chamber via a circular connector 2, which is separated from the plug housing 4 by a seal 3 and is connected to the cylindrical feed line 5. The cylindrical feed line 5 is fixed in the plug housing 4 by means of a metal ring 7 and an electrically insulating ceramic sleeve 8. The cylindrical feed line 5 is connected to the ceramic sheathed element 14 by a contact pin 10, it also being possible for the cylindrical feed line 5 to be combined with the contact pin 10 in a single component, and a suitable contacting element 12, which is preferably designed as a contact spring or as an electrically - conductive powder packing or as an electrically conductive pellet with a flexible spring component, preferably composed of graphite. The interior of the glow plug is sealed off from the combustion chamber by means of packing 15. The packing 15 is composed of an "^electrically conducting carbon compound. However, the packing 15 can also be formed by metals, a mixture of carbon and metal or a mixture of ceramic and metal. The sheathed element 14 is composed of a ceramic heating layer 18 and ceramic feed layers 20 and 21, the two feed layers 20, 21 being connected by the heating layer 18 and, together with the heating layer 18, forming the conductive layer. The feed layers 20, 21 may have any shape, and the heating layer 18 may also have any shape. The conductive layer is preferably of u-shaped design. The feed layers 2 0 and 21 are separated by an insulating layer 22, which is likewise composed of ceramic material. In the exemplary embodiment illustrated in Figure 1, the sheathed element 14 is configured in such a way that the feed layers 20 and 21 and the heating layer 18 are arranged on the outside of the sheathed element 14. However, it is also possible to arrange at least the feed layers 20 and 21 in such a ^:ay that they are within the sheathed element and are additionally covered by an external ceramic insulating Layer. Within the plug housing, the ceramic sheathed alement is insulated from the remaining components of :he sheathed-element glow plug 4, 8, 12, 15 by a glass Layer (not shown). To establish electrical contact Detween the contacting element 12 and the feed layer 10, the glass layer is interrupted at location 24. The jlass layer is likewise interrupted for electrical :ontact between the feed layer 21 and the plug housing I via the packing 15 at location 26. In this exemplary embodiment, the heating layer 18 has been positioned at :he tip of the sheathed element as a preferred embodiment. However, it is also conceivable to position :his heating layer at some other point on the :onductive layer. The heating layer 18 should be iituated at the point where the greatest heating effect s to be achieved. "igure 2 shows the ceramic heating element again, in a ■lew from the side. As in Figure 1, the embodiment in which the heating layer 18 is situated at the tip of the sheathed element is illustrated. The feed layers 20, 21 and the insulating layer 22 can furthermore be seen. This side view shows the embodiment in which the conductive layer, comprising the feed layers 20 and 21 and the heating layer 18, has a u-shaped configuration. The operating state in which the sheathed element is heated to assist combustion in the combustion chamber, this heating taking place when the internal combustion engine is started, during an afterglow phase, which preferably extends over 3 minutes, and during an intermediate glow phase if the temperature of the combustion chamber falls too far during operation of the internal combustion engine, is referred to as the active mode. In the case of the ceramic sheathed-element glow plug according to the invention, the material of the heating layer 18 is chosen so that the absolute electrical resistance of the heating layer 18 is greater than the absolute electrical resistance of the feed layers 20, 21. (In the text which follows, the word resistance will be taken to mean the absolute electrical resistance, without the extra words being added.) To avoid parallel-path currents between the conductive layers, the resistance of the insulating layer is chosen so that it is significantly higher than the resistance of the heating layer 18 and the feed layers 20, 21. Figure 3 illustrates schematically which devices communicate with the sheathed-element glow plug 1. These are first of all the engine control unit 30, which contains a computer unit and a memory unit. The engine-dependent parameters of the sheathed-element glow plug are stored in the engine control unit 30. These can be the resistance/temperature characteristic maps as a function of the load and speed of the engine, for example. The memory of the engine control unit also contains one or more temperature reference values for correct combustion. The engine control unit can control parameters that affect combustion, e.g. the duration of injection, the start of injection and the end of injection of the fuel. The control unit 32 regulates a voltage specified by the engine control unit. This voltage represents the total voltage used for the sheathed-element glow plug. The control unit 32 furthermore accommodates a current-measuring device, by-means of which the current flowing via the sheathed element is measured. The control unit 32 furthermore comprises a memory unit and a computing unit. The engine control unit 3 0 and the control unit 32 can also be combined in one device. Figure 4 illustrates the resistances that occur across the sheathed-element glow plug. The resistance 41 with a value R2 0 is the resistance of the ceramic feed layer 20. The resistance 43 with a value Rl comprises the resistance of the heating layer. The resistance 45 with a value R21 comprises the resistance of the ceramic feed layer 21. In addition there are the resistances of the remaining feed and return lines, although these are all small compared with the resistances R20 and R21 and are therefore not taken into account. They are not shown in Figure 4. The resistances 41, 43 and 45 are connected in series. Any parallel-path currents that occur will be ignored in the considerations made with reference to Figure 4. The total resistance R is therefore the sum of the resistances R20, Rl and R21. The resistance Rl here forms the largest summand. The engine control unit 30 specifies an effective voltage on the basis of the characteristic maps contained in it and of the desired temperature of the sheathed element, and this voltage is regulated by the control unit 32. Owing to the temperature dependence of the resistances 41, 43 and 45, a current I is established via the sheathed-element glow plug, i.e. via the resistance R, and this current is measured in the control unit 32. The temperature dependence of the total resistance R = R20 + Rl + R21 here results principally from the temperature dependence of resistance Rl since this resistance has the highest value. The temperature dependence of the resistances R20, Rl and R21 is virtually constant over the entire operating range of the sheathed-element glow plug between room temperature and a temperature of about 1400° C. The temperature of the combustion chamber is within the operating range of the sheathed-element glow plug. The current intensity I measured is converted by the control unit 32 using a stored characteristic map into a temperature resulting principally from the temperature of the heating layer 18 owing to the significantly higher resistance Rl compared with resistances R20 and R21. This temperature is fed back to the engine control unit 30, the effective voltage for the sheathed-element glow plug being respecified on the basis of the temperature determined. It is likewise possible to output the temperature of the heating layer 18 of the sheathed element in some other way, e.g. on a display. It is furthermore possible to draw conclusions on the quality of combustion for each specific cylinder on the basis of the temperature determined, taking into account one or more reference temperatures stored in the engine control unit 30, for example. In the case of incorrect combustion, the control unit can take measures specific to each cylinder to influence the process of combustion and thus reestablish correct combustion. The duration of injection, start of injection or injection pressure of the fuel could then be varied, for example. In another exemplary embodiment, it is possible to perform measurement of the temperature of the combustion chamber in the passive mode of the sheathed-element glow plug too, i.e. after the afterglow time, when the sheathed-element glow plug is no longer in active mode. In this case, a correspondingly lower effective voltage is specified and the current I established via the resistance R is measured as in active mode and thus a conclusion is drawn about the temperature of the heating area, which then corresponds to the temperature of the combustion chamber. Just as in active mode, the temperature of the combustion chamber can be compared for each specific cylinder with one or more reference values for correct combustion which are stored in the engine control unit. If the temperature of the combustion chamber does not correspond to correct combustion, measures can be taken which reestablish correct combustion, as explained for the active mode of the sheathed-element glow plug, e.g. variation of the duration of injection, the start of injection and the injection pressure of the fuel. The value of the resistances R20, Rl and R21 and their temperature dependence is adjusted by means of the temperature dependence of the resistivity p because R = p * 1/A, where 1 is the length of the resistor and A is the cross-sectional area. The temperature dependence is obtained from: p(T) = po(To) * (1 + a(T) * (T-To)). p(T) denotes resistivity as a function of temperature T, Po denotes resistivity at room temperature To and a (T) denotes a temperature coefficient, which is temperature-dependent. To achieve a different temperature dependence of the resistances of the feed lines R20 and R21 compared with resistance Rl, the resistivity of the heating layer 18 can be chosen in such a way that Po of the heating layer is higher than Po of the feed layers. Alternatively, the temperature coefficient a of the heating layer 18 can be higher in the operating range of the sheathed-element glow plug than the temperature coefficient a of the feed layers 20, 21. It is also possible to select higher values of po and of a for the heating layer 18 for the operating range of the sheathed-element glow plug than for feed layers 20, 21. In a preferred exemplary embodiment, the composition of the heating layer 18 and of the feed layers 20, 21 is chosen so that the Po of the feed layers 20, 21 is at least 10 times less than the po of the heating layer 18. The temperature coefficient a of the heating layer 18 and of the feed layers 20, 21 is approximately the same. This ensures accuracy of temperature measurement to within 2 0 kelvin over the entire operating range of the sheathed-element glow plug. In a preferred exemplary embodiment, the resistivity of the insulating layer 22 is at least 10 times higher than the resistivity of the heating layer 18 over the entire operating range of the sheathed-element glow plug. In a preferred exemplary embodiment, the heating layer, the feed layers and the insulating layer are composed of ceramic composite structures containing at least two of the following compounds: AI2O3, MoSi2, Si3N4 and Y2O3. These composite structures can be obtained by a single-or multi-stage sintering process. The resistivity of the layers can preferably be determined by means of the MoSi2 content and/or the grain size of MoSi2, with the MoSi2 content of the feed layers 20, 21 preferably being higher than the MoSi2 content of the heating layer 18, the heating layer 18 in turn having a higher MoSi2 content than the insulating layer 22. In another exemplary embodiment, the heating layer 18, the feed layers 20, 21 and the insulating layer 22 are composed of a composite precursor ceramic containing different proportions of fillers. The matrix of this material is composed of polysiloxanes, polysilsequioxanes, polysilanes or polysilazanes, which can be doped with boron or aluminium and are produced by pyrolysis. The filler for the individual layers is formed by at least one of the following compounds: AI2O3, MoSi2 and SiC. As with the abovementioned composite structure, the MoSi2 content and/or the grain size of MoSi2 can preferably be used to determine the resistivity of the layers. The MoSi2 content of the feed layers 20, 21 is preferably set higher than the MoSi2 content of the heating layer 18, the heating layer 18 in turn having a higher MoSi2 content than the insulating layer 22. In the exemplary embodiments given above, the compositions of the insulating layer, the feed layers and the heating layer are chosen so that their thermal expansion coefficients and the shrinkage that occurs in the individual feed, heating and insulating layers during the sintering or pyrolysis process is the same, ensuring that no cracks occur in the sheathed element. Figure 5 illustrates a further preferred exemplary embodiment of the invention, on the basis of a diagrammatic longitudinal section through a sheathed-element glow plug 1 according to the invention. In this figure, reference symbols which are identical to those used in the preceding figures denote identical components, which are not explained again in the present description. In a similar manner to Figure 1, the sheathed-element glow plug illustrated in Figure 5 has a circular connector 2, which is in electrical contact with the cylindrical feed line 5. The cylindrical feed line 5 is electrically connected to the ceramic sheathed element 14 via the contact pin 10 and the contacting element 12. The cylindrical feed line 5, the contact pin 10, the contacting element 12 and the ceramic sheathed element 14 are arranged one behind the other, in this order, as illustrated in Figure 5, in the direction of the combustion chamber. In the preferred embodiment illustrated in Figure 5, the ceramic sheathed element 14 has a pin 11 at the end which is remote from the combustion chamber. An extension of the sheathed element 14 in the direction of the end which is remote from the combustion chamber through the ceramic feed layers 20, 21 and the insulating layer 22 being led out in a cylindrical fashion forms the pin 11, the pin 11 having a smaller external diameter than that part of the sheathed element 14, namely the collar 13, which adjoins it in the direction of the combustion chamber. Furthermore, it is not necessary for the sheathed element 14 to have a heating layer 18 at the end which lies on the side of the combustion chamber. In a preferred exemplary embodiment, the two feed layers 20 and 21 can simply be connected to that end of the sheathed element which lies on the combustion chamber side in the same way as that which is effected by the heating element 18. The cylindrical feed line 5 and the contact pin 10 together form the connection element, which may likewise be of single-part design. At that end of the connection element which is on the side of the combustion chamber, there is a flange which, together with the pin 11, delimits the contacting element 12 in the direction of the axis of the sheathed-element glow plug. The contacting element 12, which comprises a pellet made from electrically conductive powder, is preferably in the form of graphite or a metal powder or an electrically conductive ceramic powder. In a further preferred embodiment, the pellet made from electrically conductive powder may also comprise at least predominantly graphite or the metal powder or the electrically conductive ceramic powder. Forming the contacting element 12 as electrically conductive powder means that the contacting element 12 ensures flexible contacting, which is able to carry high currents without being thermally destroyed. The large surface area of the powder ensures good thermal conductivity. For the same reason, it is also possible to produce a low contact resistance with good conductivity. Moreover, graphite and ceramic conductive materials are resistant to corrosion. The flexible spring component of the pellet made from electrically conductive powder ensures that the pellet compensates for thermal movements of the parts caused by different coefficients of thermal expansion. The pellet of electrically conductive powder is laterally delimited by a cylindrical clamping sleeve 9, which in this case is present as an independent component instead of the ceramic sleeve 8 illustrated in Figure 1. The clamping sleeve 9 is provided, in a similar manner to the ceramic sleeve 8, as an insulating component, and in a preferred exemplary embodiment it consists of ceramic material. During production of the sheathed-element glow plug, the pellet of electrically conductive powder is pressed securely between the flange of the connection element on the end side which is remote from the combustion chamber, the pin 11 of the sheathed element 14 on the end side which faces the combustion chamber, and the clamping sleeve 9. The clamping between these fixed components, in particular the fixed abutment of the clamping sleeve 9 on the ceramic sleeve 8, i.e. the limited compression level, prevents the surrounding clamping sleeve 9 from being torn by an excessive build-up of internal pressure on account of the pressing action of the contacting element 12. The axial prestressing of the flexible spring component which is achieved by the clamping of the pellet of electrically conductive powder enables thermal expansions, settling and vibrational loads in the event of shaking of the sheathed-element glow plug to be compensated for. A sheathed-element glow plug as illustrated in Figure 5, having a pellet of electrically conductive powder as contacting element 12, is produced in the following way. First of all, the packing 15 is guided over the ceramic sheathed element 14 from the tip of the ceramic sheathed element 14 which lies on the combustion chamber side, and this assembly is introduced into the plug housing 4 from the end which is remote from the combustion chamber. Then, the contacting element 12, the clamping sleeve 9, the connection element 5, 10, the ceramic sleeve 8 and the metal ring 7 are arranged in a holding element and are then likewise introduced into the plug housing 4 from the end which is remote from the combustion chamber. Then, an axial force which is exerted on that end of the metal ring 7 which is remote from the combustion chamber causes the parts which are located in the plug housing to be pressed together, and in particular the contacting element 12, which comprises a pellet of electrically conductive powder, and the packing 15 are compressed. In the process, a force is only exerted on the contacting element 12 until the contact pin 10 of the connection element 5, 10 has been pressed fully into the clamping sleeve 9 and the end side of the ceramic sleeve 8 is resting on the end side of the clamping sleeve 9. Moreover, the compression of the pellet of electrically conductive powder ensures that the flexible spring component of the pellet is prestressed. Then, the metal ring 7 is jammed into place by means of a force which is applied radially from the outside to the plug housing 4. Next, the seal 3 and the circular connector 2 are fitted and are likewise jammed in place by means of a force which is applied radially from the outside to the plug housing 4. WE CLAIM: 1. A sheathed-element glow plug with a ceramic heating device, which has a ceramic, electrically conducting conductive layer and a ceramic, electrically insulating insulating layer, characterized in that the conductive layer comprises feed layers (20, 21), which are connected by a heating layer (18), the electrical resistivity of the material of the heating layer (18) in the operating temperature range of the sheathed-element glow plug being temperature-dependent and being higher than the electrical resistivity of the material of the feed layers (20, 21) and lower than the electrical resistivity of the insulating layer (22). 2. The sheathed-element glow plug according to Claim 1, wherein the electrical resistivity of the heating layer (18) at room temperature is higher than the electrical resistivity of the feed layers (20, 21) at room temperature. 3. The sheathed-element glow plug according to Claim 1, wherein the temperature coefficient of the feed layers (20, 21) is lower than the temperature coefficient of the heating layer (18) over the entire operating range of the sheathed-element glow plug. 4. The sheathed-element glow plug according to Claim 1, wherein the electrical resistivity at room temperature and the temperature coefficient of the feed layers (20, 21) is lower than the electrical resistivity at room temperature and the temperature coefficient of the heating layer (18). 5. The sheathed-element glow plug according to Claim 1, wherein the electrical resistivity of the material of the heating layer at room temperature is at least times higher than the higher of the electrical resistivities of the feed layers (20, 21) at room temperature. 6. The sheathed-element glow plug according to any one of Claims 1 to 5, wherein the heating layer is situated at the tip of the sheathed element. 7. The sheathed-element glow plug according to any one of Claims 1 to 6, wherein the heating layer (18), the feed layers (20, 21) and the insulating layer (22) are composed of ceramic composite structures which can be obtained by a single or multi-stage sintering process from at least two of the following compounds: AI2O3, MoSi2, Si3N4 and Y2O3. 8. The sheathed-element glow plug according to any one of Claims 1 to 6, wherein the heating layer (18), the feed layers (20, 21) and the insulating layer (22) are composed of a composite precursor ceramic, the matrix material being composed of polysiloxanes, polysilsequioxanes, polysilanes or polysilazanes, which can be doped with boron or aluminium and have been produced by pyrolysis, the filler being formed by at least one of the following compounds: AI2O3, MoSi2 and SiC. 9. The sheathed-element glow plug according to any one of Claims 1 to 8, wherein the temperature of the heating layer (18) is determined on the basis of its resistance Rl. 10. The sheathed-element glow plug according to Claim 9, wherein the temperature value determined is passed to an engine control unit (30), whereupon the engine control unit (30) compares the temperature value with a reference value and performs readjustment of the voltage specified by the control unit (32) for the sheathed-element glow plug. 11. The sheathed-element glow plug according to Claim 9, wherein the temperature value determined is passed to an engine control unit (30), whereupon the engine control unit (30) compares the temperature value with one or more reference values for correct combustion and performs readjustment of variables that are relevant to combustion. 12. The sheathed-element glow plug according to Claim 9, wherein temperature measurement, comparison with one or more reference values for correct combustion and readjustment of variables that are relevant to combustion take place in the passive mode of the sheathed-element glow plug. 13. The sheathed-element glow plug according to either of Claims 11 or 12, wherein the parameters relevant to combustion are; the duration of injection, the start of injection and the injection pressure of the fuel.

Documents:

in-pct-2002-0389-che abstract duplicate.pdf

in-pct-2002-0389-che abstract.pdf

in-pct-2002-0389-che claims duplicate.pdf

in-pct-2002-0389-che claims.pdf

in-pct-2002-0389-che correspondence-others.pdf

in-pct-2002-0389-che correspondence-po.pdf

in-pct-2002-0389-che description (complete) duplicate.pdf

in-pct-2002-0389-che description (complete).pdf

in-pct-2002-0389-che drawings duplicate.pdf

in-pct-2002-0389-che drawings.pdf

in-pct-2002-0389-che form-1.pdf

in-pct-2002-0389-che form-19.pdf

in-pct-2002-0389-che form-26.pdf

in-pct-2002-0389-che form-3.pdf

in-pct-2002-0389-che form-5.pdf

in-pct-2002-0389-che pct.pdf

in-pct-2002-0389-che petition.pdf