EP1214551B2 - Ceramic sheathed element glow plug - Google Patents

Abstract

A ceramic sheathed-type glow plug, whose ceramic heating element is made of an electrically conductive conducting layer and an electrically insulating layer, is described. The conducting layer is made of lead layers and a heating layer. The higher specific electrical resistance of the heating layer allows the temperature of the heating layer and the combustion chamber to be determined. The electrical contact between a connector element and the heating element is established by a contacting element, which is in the form of a pellet made of an electrically conductive powder.

Description

The invention relates to a ceramic glow plug for diesel engines according to the preamble of claim 1. There are already glow plugs with external ceramic heater, for example, from Patent Application DE-OS 40 28 859 known.

Furthermore, for example DE-OS 29 37 884 metallic glow plugs are known in which the metallic filament is welded to a thermocouple. Here, the temperature in the respective cylinder can be measured during operation of the glow plug by detecting the thermoelectric voltage. In a glow plug with ceramic heating element, however, a metallic filament is not present.

Furthermore, from the DE 198 44 347 a glow plug with a connection element known which is electrically connected to the glow plug via a contacting element. This contacting element is how FIG. 1 can be seen, designed as a spring.

Further, from the Patent Abstracts of Japan vol. 013, no. 130 (M-808) & JP 63 297 924 a glow plug with a ceramic heater is known, which has a ceramic, electrically conductive, U-shaped heating layer of low electrical conductivity and ceramic lead layers of higher electrical conductivity.

Advantages of the invention

The ceramic glow plug according to the invention with the features of claim 1 has the advantage that the temperature of the glow plug is measurable. It is possible in a ceramic glow plug for the first time to measure the temperature of the glow plug directly in a selected area on the outside of the glow plug without additional equipment expense. The measurement of the temperature takes place in a small, compared to the volume of the entire glow pencil selected range, whereby the error that occurs by a temperature distribution over a large volume, can be reduced in the temperature determination. It is also advantageous that in the glow plug according to the invention, a concentration of the heating power in a selected region of the glow plug can be realized without changing the cross section of the conductive layer, so that the surface in the region in which the concentration of the heating power is to take place, constant remains and thus the interaction surface is kept constant. Another advantage is that the production of such a ceramic temperature measuring glow plug can be designed inexpensively.

The measures listed in the dependent claims advantageous refinements and improvements of claim 1 ceramic glow plug are possible.

In particular, it is ensured by a suitable choice of the ceramic materials used for the various areas of the glow plug that the mechanical stability of the heater is not impaired. A processing of the measured temperature values by a control unit allows regulation of the temperature in the selected area of the glow plug.

It is also advantageous to use the glow plug according to the invention in passive operation, after it has fulfilled the heating function, as a temperature sensor. It can thus be determined whether the combustion takes place correctly in the respective cylinder. It is advantageous that, on the basis of this information, an influencing of parameters relevant for the combustion can take place.

drawings

• Figur 1 eine erfindungsgemäße Glühstiftkerze im Längsschnitt,
• Figur 2 den vorderen Abschnitt des außenliegenden keramischen Heizers als Seitenansicht,
• Figur 3 eine Verschaltung der erfindungsgemäßen Glühstiftkerze mit den Steuergeräten,
• Figur 4 die in der erfindungsgemäßen keramischen Glühstiftkerze und in den Zuleitungen auftretenden Widerstände und
• Figur 5 eine erfindungsgemäße Glühstiftkerze im Längsschnitt.

The embodiments of the invention are illustrated in drawings and explained in more detail in the following description. Show it

• FIG. 1 a glow plug according to the invention in longitudinal section,
• FIG. 2 the front portion of the external ceramic heater as a side view,
• FIG. 3 an interconnection of the glow plug according to the invention with the control units,
• FIG. 4 the resistors occurring in the inventive ceramic glow plug and in the leads and
• FIG. 5 a glow plug according to the invention in longitudinal section.

Description of the embodiments

FIG. 1 schematically shows a longitudinal section through a ceramic glow plug according to the invention 1. At brennraumfemen end of the glow plug 1, the electrical contact via a circular connector 2, which is separated by a seal 3 from the plug housing 4 and connected to the cylindrical feed line 5. The cylindrical feed line 5 is connected via a contact pin 10, wherein the cylindrical feed line 5 can also be combined with the contact pin 10 in one component, and a suitable contacting element 12, which is preferably formed as a contact spring or as an electrically conductive powder pack or as an electrically conductive tablet with an elastic spring component, preferably made of graphite, connected to the ceramic glow plug 14. The interior of the glow plug is sealed by means of a sealing packing 15 with respect to the combustion chamber. The sealing packing 15 consists of an electrically conductive carbon compound. But the packing 15 can also by metals, a mixture of carbon and metal or a mixture of ceramic and Metal be formed. The glow plug 14 consists of a ceramic heating layer 18 and ceramic lead layers 20 and 21, wherein the two lead layers 20, 21 are connected by the heating layer 18 and together with the heating layer 18 form the conductive layer. The lead layers 20, 21 have an arbitrary shape; the heating layer 18 can also have any desired shape. Preferably, the conductive layer is U-shaped. The lead layers 20 and 21 are separated by an insulating layer 22, which is also made of ceramic material. In the in FIG. 1 illustrated embodiment, the glow plug 14 is designed such that the lead layers 20 and 21 and the heating layer 18 are arranged outside of the glow plug 14. However, it is also possible to arrange at least the lead layers 20 and 21 so that they are located within the glow plug and are still covered by an external, ceramic, insulating layer. Within the plug housing of the ceramic glow plug is isolated by a glass layer, not shown, of the other components of the glow plug 4, 8, 12, 15. In order to establish the electrical contact between the contacting element 12 and the lead layer 20, the glass layer is interrupted at the point 24. The glass layer is also interrupted for electrical contact between lead layer 21 and plug housing 4 via the packing 15 at location 26. In this embodiment, as a preferred embodiment, the heating layer 18 was placed at the tip of the glow plug. However, it is also conceivable to place this heating layer at another location of the conductive layer. The heating layer 18 should be at the point where the greatest heating effect is to be achieved.

In FIG. 2 Again, the ceramic heating element is shown in a view from the side. As in FIG. 1 For example, the embodiment in which the heating layer 18 is at the tip of the glow plug is shown. Furthermore, the lead layers 20, 21 and the insulating layer 22 can be seen. In this side view, the embodiment is shown in which the conductive layer consisting of the lead layers 20 and 21 and the heating layer 18 has a U-shaped configuration.

The operating condition in which the glow plug is heated to assist combustion in the combustion chamber, which heating occurs at engine startup, during an afterglow phase, preferably over 3 minutes, and during an intermediate annealing phase when the temperature of the combustion chamber during operation the internal combustion engine drops too much, is called active operation.

In the ceramic glow plug according to the invention, the material of the heating layer 18 is selected 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 following, the term resistance without addition understood the absolute electrical resistance In order to avoid cross-currents between the conductive layer, the resistance of the insulating layer is chosen so that it is significantly greater than the resistance of the heating layer 18 and the lead layers 20, 21.

In FIG. 3 is shown schematically, which devices communicate with the glow plug 1. This is first the engine control unit 30, which includes a computer and a memory unit. In the engine control unit 30, the motor-dependent parameters of the glow plug are stored. This can be, for example, the resistance-temperature characteristics as a function of load and speed of the motor. The engine controller memory also includes one or more temperature reference values for proper combustion. The engine controller may control parameters that affect combustion, such as the duration of injection, the beginning of injection, and the end of injection of the fuel. The control unit 32 regulates a voltage which was predetermined by the engine control unit. This voltage represents the total voltage used for the glow plug. The controller 32 also houses a current meter that measures the amount of current flowing through the glow plug. In addition, the controller 32 includes a memory and a computing unit. The engine control unit 30 and the control unit 32 can also be combined in one device.

The FIG. 4 illustrates the occurring via the glow plug resistors. The resistor 41 having a value R20 is the resistance of the ceramic lead layer 20. The resistor 43 having a value R1 includes the resistance of the heating layer. The resistor 45 with a value R21 includes the resistance of the ceramic lead layer 21. In addition, there are the resistors of the other supply and return lines, but they are all small compared to the resistors R20 and R21 and are therefore not taken into account. They are in FIG. 4 not drawn. The resistors 41, 43 and 45 are connected in series. For the basis of FIG. 4 Considerations that may be taken should neglect any crossflows that may occur. Thus, the total resistance R results from the sum of the resistances R20, R1 and R21. The resistor R1 forms the largest summands.

From the engine control unit 30 is determined based on the maps contained therein and the desired temperature of the glow plug an effective voltage, which is controlled by the controller 32. Due to the temperature dependence of the resistors 41, 43 and 45, a current I via the glow plug, so via the resistor R, which is measured in the control unit 32. The temperature dependence of the total resistance R = R20 + R1 + R21 results mainly from the temperature dependence of the resistor R1, since this resistor has the largest value. The temperature dependence of the resistors R20, R1 and R21 are almost constant over the entire operating range of the glow plug between room temperature and a temperature of approx. 1400 ° C. The temperature of the combustion chamber is in the operating range of the glow plug.

The measured current intensity I is converted by the control unit 32 on the basis of a stored map into a temperature, which results mainly due to the significantly higher resistance R1 against the resistors R20 and R21 from the temperature of the heating layer 18. This temperature is returned to the engine control unit 30, wherein due to the determined temperature, the effective voltage for the glow plug is redefined.

It is also possible to otherwise output the temperature of the heating layer 18 of the glow plug, for example on a display. Furthermore, it is possible, based on the determined temperature, for example, taking into account one or more, stored in the engine control unit 30, reference temperatures deduce conclusions about the quality of combustion cylinder specific. In the event of improper combustion, cylinder-specific measures can be taken by the control unit, which influence the combustion process and can thus ensure correct combustion again. For example, the duration of injection, the start of injection or the injection pressure of the fuel could then be varied.

In a further embodiment, it is possible, even in the passive mode of the glow plug, i. After the afterglow time, when the glow plug is no longer in active mode, make a measurement of the temperature of the combustion chamber. Here, a correspondingly lower rms voltage is given and, analogously to the active operation of the current I, which adjusts itself via the resistor R, is measured and thus closed to the temperature of the heating region, which then corresponds to the temperature of the combustion chamber. As in active operation, the temperature of the combustion chamber can be compared cylinder-specifically with one or more reference values stored in the engine control unit for correct combustion. If the temperature of the combustion chamber does not correspond to a correct combustion, as explained for the active operation of the glow plug, measures can be taken to ensure correct combustion again, for example a variation of the duration of injection, the start of injection and the injection pressure of the fuel.

wobei 1 die Länge des Widerstandes und A die Querschnittsfläche darstellt,
durch die Temperaturabhängigkeit des spezifischen Widerstandes ρ eingestellt. Dabei ergibt sich die Temperaturabhängigkeit aus $$\mathrm{\rho }\left(\mathrm{T}\right)\mathrm{=}{\mathrm{\rho }}_{\mathrm{0}}\left({\mathrm{T}}_{\mathrm{0}}\right)\mathrm{*}\left(\mathrm{1}\mathrm{+}\mathrm{\alpha }\left(\mathrm{T}\right)\mathrm{*}\left(\mathrm{T}\mathrm{-}{\mathrm{T}}_{\mathrm{0}}\right)\right)\mathrm{.}$$

The value of the resistors R20, R1 and R21 and their temperature dependency is due to $$\mathrm{R}\mathrm{=}\mathrm{\rho }\mathrm{*}\mathrm{1}\mathrm{/}\mathrm{A}\mathrm{.}$$

where 1 is the length of the resistor and A is the cross-sectional area,
adjusted by the temperature dependence of the specific resistance ρ. This results in the temperature dependence $$\mathrm{\rho }\left(\mathrm{T}\right)\mathrm{=}{\mathrm{\rho }}_{\mathrm{0}}\left({\mathrm{T}}_{\mathrm{0}}\right)\mathrm{*}\left(\mathrm{1}\mathrm{+}\mathrm{\alpha }\left(\mathrm{T}\right)\mathrm{*}\left(\mathrm{T}\mathrm{-}{\mathrm{T}}_{\mathrm{0}}\right)\right)\mathrm{,}$$

It denotes p (T) the resistivity as a function of temperature T, ρ ₀ the resistivity at room temperature T ₀ and α (T) a temperature coefficient which is temperature dependent.

In order to achieve a different temperature dependence of the resistances of the supply lines R20 and R21 with respect to the resistor R1, the specific resistance of the heating layer 18 can be chosen so that ρ _{0 of} the heating layer is greater than ρ _{0 of} the supply layers. Or the temperature coefficient α of the heating layer 18 may be greater than the temperature coefficient α of the lead layers 20, 21 in the operating range of the glow plug. It is also possible to choose both ρ ₀ and α for the heating layer 18 for the operating range of the glow plug larger than for Lead layers 20, 21.

In a preferred embodiment, the composition of the heating layer 18 and the lead layers 20, 21 is selected so that the ρ _{0 of} the lead layers 20, 21 is at least 10 times smaller than the ρ _{0 of} the heating layer 18. The temperature coefficient α of the heating layer 18 and the lead layers 20, 21 is approximately equal. Thus, an accuracy of the temperature measurement of 20 Kelvin in the entire operating range of the glow plug is realized.

In a preferred embodiment, the specific resistance of the insulating layer 22 is at least 10 times greater than the specific resistance of the heating layer 18 over the entire operating range of the glow plug.

In a preferred embodiment, the heating layer, the lead layers and the insulating layer consist of ceramic composite structures containing at least two of the compounds Al ₂ O ₃ , MoSi ₂ , Si ₃ N ₄ and Y ₂ O ₃ . These composite structures are available by a single or multi-stage sintering process. The specific resistance of the layers can be determined preferably by the MoSi ₂ content and / or the grain size of MoSi ₂ , preferably the MoSi ₂ content of the feed layers 20, 21 is higher than the MoSi ₂ content of the heating layer 18, wherein the Heating layer 18 in turn has a higher MoSi ₂ content than the insulating layer 22.

In a further embodiment, heating layer 18, lead layers 20, 21 and the insulation layer 22 consist of a composite precursor ceramic with different proportions of fillers. The matrix of this material consists of polysiloxanes, Polysilsequioxanen, polysilanes or polysilazanes, which may be doped with boron or aluminum and which are produced by pyrolysis. The filler forms at least one of the compounds Al ₂ O ₃ , MoSi ₂ and SiC for the individual layers. Analogously to the above-mentioned composite structure, the MoSi ₂ content and / or the grain size of MoSi ₂ may preferably determine the specific resistance of the layers. Preferably, the MoSi ₂ content of the feed layers 20, 21 is set higher than the MoSi ₂ content of the heating layer 18, wherein the heating layer 18 in turn has a higher MoSi ₂ content than the insulating layer 22.

The compositions of the insulating layer, the lead layers and the heating layer are chosen in the above-mentioned embodiments so that their thermal expansion coefficients and the shrinkage occurring during the sintering or pyrolysis process of the individual supply, heating and insulating layers are the same, so that no cracks arise in the glow plug.

In FIG. 5 is a further preferred embodiment of the invention with reference to a schematic longitudinal section through a glow plug 1 according to the invention. Here, the same reference numerals used in the preceding figures mean the same components, which will not be explained again here. Analogous to FIG. 1 has the in FIG. 5 shown glow plug on a circular connector 2, which is in electrical contact with the cylindrical feed line 5. The cylindrical feed line 5 is electrically connected via the contact pin 10 and the contacting element 12 with the ceramic glow plug 14. The cylindrical feed line 5, the contact pin 10, the contacting element 12 and the ceramic glow plug 14 are successively in this order, as in FIG. 5 represented, arranged in the direction of the combustion chamber. The ceramic glow plug 14 has in the in FIG. 5 illustrated preferred embodiment at the combustion chamber remote end a pin 11. The pin 11 forms an extension of the glow plug 14 in the direction of the combustion chamber distal end by a cylindrical lead-out of the ceramic lead layers 20, 21 and the insulating layer 22, wherein the pin 11 has a smaller outer diameter than the adjoining in the direction of the combustion chamber part of the glow plug 14, It is also not necessary that the glow plug 14 has a heating layer 18 at the combustion chamber end. In a preferred embodiment, the two feed layers 20 and 21 may be connected only at the combustion chamber end of the glow plug, as is done via the heating element 18.

The cylindrical lead 5 and the contact pin 10 together form the connecting element, which may also be integrally formed. At the combustion chamber end of the connecting element, a flange is provided, which limits the contacting element 12 in the direction of the axis of the glow plug together with the pin 11.

The contacting element 12, which consists of a tablet of electrically conductive powder, is preferably formed as graphite or a metal powder or an electrically conductive ceramic powder. In a further preferred embodiment, the tablet of electrically conductive powder can also consist of at least a predominant proportion of graphite or of the metal powder or of the electrically conductive ceramic powder. Due to the formation of the contacting element 12 as an electrically conductive powder, the contacting element 12 ensures a resilient contact, which is able to carry high currents without thermal destruction. The large surface of the powder ensures a good thermal conductivity. For the same reason, a low contact resistance with good conductivity can be realized. Graphite and ceramic conductive materials are also corrosion resistant. The elastic spring portion of the tablet of electrically conductive powder ensures that the tablet compensates thermal movements of the components by different coefficients of thermal expansion.

Laterally, the tablet of electrically conductive powder is limited by a cylindrical clamping sleeve 9, which instead of the in FIG. 1 shown ceramic sleeve 8 is present as an independent component. The clamping sleeve 9 is provided analogously to the ceramic sleeve 8 as an insulating component, it consists in a preferred embodiment of ceramic material. In the manufacture of the glow plug, the tablet of electrically conductive powder is pressed firmly between the flange of the connecting element on the combustion chamber remote end side, the pin 11 of the glow plug 14 on the combustion chamber side end and the clamping sleeve 9. The clamping between these fixed components, in particular the fixed stop of the clamping sleeve 9 on the ceramic sleeve 8, ie the limited Verpreßhöhe prevents the surrounding clamping sleeve 9 is not torn by a too large internal pressure buildup due to the compression of the contacting element 12. The axial preload of the elastic spring component achieved by the clamping of the tablet of electrically conductive powder can be compensated for thermal expansions, setting behavior and vibration stress during shaking stress of the glow plug.

A glow plug after FIG. 5 with a tablet of electrically conductive powder as the contacting element 12 is prepared as follows. First, the seal pack 15 is guided by the combustion chamber-side tip of the ceramic glow plug 14 on the ceramic glow plug 14 and introduced as a composite in the plug housing 4 from the combustion chamber remote end. Subsequently, the contacting element 12, the clamping sleeve 9, the connecting element 5, 10, the ceramic sleeve 8 and the metal ring 7 are arranged in a holding element and then also introduced from the combustion chamber remote end into the candle housing 4. Then, by means of an axial force exerted on the combustion chamber remote end of the metal ring 7, the components contained in the plug housing is pressed, in particular the contacting element 12, which consists of a tablet of electrically conductive powder, and the sealing packing 15 is pressed. In this case, a force is exerted on the contacting element 12 only until the contact pin 10 of the connection element 5, 10 has completely pressed into the clamping sleeve 9 and the end face of the ceramic sleeve 8 rests on the end face of the clamping sleeve 9. The compression of the tablet of electrically conductive powder also ensures that the elastic spring portion of the tablet is biased. Subsequently, the metal ring 7 is caulked by means of a force applied radially from the outside to the plug housing 4. Thereafter, the seal 3 and the circular connector 2 are mounted and also caulked by means of a radially applied externally to the plug housing 4 force.

Claims (12)

1. Sheathed-element glow plug with a ceramic heating device, which has a ceramic, electrically conducting conductive layer and a ceramic, electrically insulating insulating layer, the conductive layer comprising 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), characterized in that 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.
2. Sheathed-element glow plug according to Claim 1, characterized in that 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. Sheathed-element glow plug according to Claim 1, characterized in that 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).
4. Sheathed-element glow plug according to Claim 1, characterized in that the electrical resistivity of the material of the heating layer at room temperature is at least 10 times higher than the higher of the electrical resistivities of the feed layers (20, 21) at room temperature.
5. Sheathed-element glow plug according to one of Claims 1 to 4, characterized in that the heating layer is situated at the tip of the sheathed element.
6. Sheathed-element glow plug according to one of Claims 1 to 5, characterized in that 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: Al₂O₃, MoSi₂, Si₃N₄ and Y₂O₃.
7. Sheathed-element glow plug according to one of Claims 1 to 5, characterized in that 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: Al₂O₃, MoSi₂ and SiC.
8. Sheathed-element glow plug according to one of Claims 1 to 7, characterized in that the temperature of the heating layer (18) is determined on the basis of its resistance R1.
9. Sheathed-element glow plug according to Claim 8, characterized in that 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.
10. Sheathed-element glow plug according to Claim 8, characterized in that 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.
11. Sheathed-element glow plug according to Claim 8, characterized in that 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.
12. Sheathed-element glow plug according to either of Claims 10 or 11, characterized in that the parameters relevant to combustion are: the duration of injection, the start of injection and the injection pressure of the fuel.

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