Title of Invention

AN ELECTRICALLY HEATABLE GLOW PLUG FOR INTERNAL COMBUSTION ENGINES AND A PROCESS FOR PRODUCING AN ELECTRICALLY HEATABLE GLOW PLUG

Abstract

The invention proposes an electrically beatable glow plug (1) and a process for producing an electrically beatable glow plug (1), wbicb allows a beater coil (10) of tbe glow plug (1) to be protected from nitriding and evaporation of aluminum out of the heating conductor alloy. Tbe glow plug (1) comprises a closed-end glow tube (5), into wbicb the electrically conductive beater coil (10) is introduced, tbe heater coil (10) being formed at least in part from aluminum, in particular from an aluminum-iron-cbromium alloy. In tbe glow tube (5) there are oxygen donors in order to form an aluminum oxide layer on tbe surface of the beater coil (10) before or during the heating of tbe beater coil (10). (Fig. 1)

Full Text

The invention is based on an electrically beatable glow plug for internal combustion engines and a process for producing an electrically beatable glow plug. DE 19928037 C 1 bas already disclosed an electrically beatable glow plug for internal combustion engines which comprises a closed-end, corrosion-resistant glow tube which accommodates a filling comprising an electrically nonconductive, compacted powder in which an electrically conductive coil is embedded. The coil comprises a beater coil. This is formed from an iron-chromium-aluminum alloy. In the region of the beater coil, the electrically conductive coil is surface-hardened. As a result, the coil can withstand the mechanical loads during the compacting operation without suffering any premature damage. DE 19756988 C 1 has disclosed an electrically beatable glow plug for internal combustion engines which comprises a sheathed element comprising a corrosion-resistant metal casing. A compacted powder filling is present in the sheathed element. An electrically conductive coil is embedded in the filling. To increase the service life of the coil, a getter material for bonding the oxygen contained in the compacted powder filling is provided in the sheathed element. The getter material may be distributed through the compacted powder filling in the form of electrically nonconductive particles. These may consist of silicon or metal oxides of metals which oxidize in a plurality of oxidation states and have a higher affinity for oxygen than the coil material, the getter material, in its starting state, containing the metal oxide in their first oxidation state. EP 0079385 Al has disclosed a heater element in which a coil is arranged in a sleeve and embedded in an electrically insulating powder. The powder comprises 5 0.1 to 10% by weight of an oxide and as a result prevents oxidation of the metallic fraction of the coil. Advantages of the invention The electrically heatable glow plug and the process for producing an electrically heatable glow plug having the features as described therein, by contrast, have the advantage that oxygen donors are provided in the glow tube, in order to form an aluminum oxide layer at the surface of the heater coil before or during the heating of the heater coil. In this way, in the event of air penetrating into the glow tube, the formation of nitrides in the outer layers and thereby a local increase in the electrical resistance and premature failure of the heater coil are prevented. A further advantage consists in the fact that evaporation of aluminum out of the alloy can be substantially suppressed. The measures as described herein allow advantageous refinements and improvements to the electrically heatable glow plug and the process for producing an electrically heatable glow plug is also described herein. A less complex way of supplying oxygen donors results if the heater coil is embedded in a first insulating powder in the glow tube, the first insulating powder comprising a material which acts as an oxygen donor. It is particularly advantageous if the oxygen donor is formed as a metal oxide which can oxidize in a - plurality of oxidation states and is present in its highest oxidation state. In this way, the release of oxygen from the metal oxide is promoted to a considerable extent. The same is true if the oxidic ceramic powder comprises a metal oxide which under reducing conditions can release oxygen through the formation of defects. It is also advantageous if the oxygen donors are introduced into the glow tube in the form of oxygen molecules under pressure. In this way, the oxygen concentration in the glow tube can be increased by the pressure, and oxidation at the heater coil surface in order to form aluminum oxide can be effected by the oxygen molecules without it being necessary for the heater coil to be heated by a heating current to achieve this effect. Therefore, even before first operation, i.e. before first heating by a heating current, the heater coil can be protected from nitriding by a layer of oxide. A further advantage consists in the fact that a control coil which adjoins the heater coil is embedded in a second insulating powder, which is as far as possible free of oxygen donors and/or comprises getter material for bonding oxygen. Therefore, the material used for the control coil can be a material which does not form a protective oxide layer under the influence of oxygen donors, as is the case, for example, with cobalt-iron alloys. Therefore, corrosion to the control coil can be prevented or at least significantly delayed by the use of the second insulating powder, which is as far as possible free of oxygen donors. If getter material is used in the second insulating powder, disruptive oxygen molecules in the region of the control coil can be bonded. Drawing Exemplary embodiments of the invention are illustrated in the accompanying drawing and explained in more detail in the description which follows. In the drawing, Figure 1 shows a first exemplary embodiment of an electrically beatable glow plug according to the invention, and Figure 2 shows a second exemplary embodiment of an electrically beatable glow plug according to the invention. Description of the exemplary embodiments In Figure 1, 1 denotes a glow plug, designed as a sheathed-element glow plug, or an internal combustion engine. The sheathed-element glow plug 1 comprises a plug housing 40 with a screw thread 45 for screwing into a cylinder head of the internal combustion engine. The plug housing 40 also comprises a hexagon head 50, by means of which the sheathed-element glow plug or the plug housing 40 can be screwed into or out of the cylinder head by means of a turning tool, for example a hexagon-socket spanner. A glow tube 5 is pressed into the plug housing 40, which is of tubular design, and projects out of the plug housing 40 on the combustion chamber side, i.e. at the opposite end of the plug housing 40 from the hexagon head 50. On the combustion chamber side, the glow tube 5 is closed at its end. As in the present example, the cross section of the glow tube 5 may be reduced in a region 20 at the combustion-chamber-side tip 55 of the glow tube 5 formed in this way. However, it is not imperative that this cross section be reduced. Only the region 20 of reduced cross section of the sheathed-element glow plug 1 projects into the combustion chamber. In the region of reduced cross section, the glow tube 5 comprises a heater coil 10, which is welded to the tip 55 of the glow plug 5 which is on the combustion chamber side. The heater coil 10 is adjoined by a control coil 60 which is arranged in the region of the glow tube 5 and the cross section of which is not reduced. At that end of the glow tube 5 which is remote from the combustion chamber, the control coil 60 makes contact with a connection bolt 65 which can be connected to the positive terminal of a vehicle battery. In the direction toward the opening in the plug housing 4 0 which is remote from the combustion chamber, the glow tube 5 is still sealed off from environmental influences inside the plug housing 40 by a Viton ring 70. A further sealing ring 75 seals off the connection bolt 65 which projects out of the plug housing 40 at the end remote from the combustion chamber with respect to the plug housing 40. An insulating plate 80 which adjoins the sealing ring 7 5 on the side remote from the combustion chamber is used to electrically insulate the connection bolt 65 from the plug housing 4 0 and therefore electrically insulate the connection bolt 65 from the plug housing 40, the electrical potential of which is at vehicle ground. An annular nut 85 presses the insulating plate 80 onto the plug housing 4 0 and presses the sealing ring 7 5 into the plug housing 40. The glow tube 5 is of metallic design and, on account of being pressed into the plug housing 40, is likewise at vehicle ground in terms of its electrical potential. The heater coil 10 is welded to the control coil 60 at a connecting point 90. The function of the Viton ring 70 is very important, since it consists of a soft, insulating material and therefore not only seals off the connection bolt 65 from the plug housing 40 in an electrically insulating manner at its end which projects into the glow tube 5 in order to make contact with the control coil 60, but also prevents air from penetrating into the glow tube 5, which is open at the end remote from the combustion chamber. This seal should be as reliable as possible. The heater coil 10 consists, for example, of a ferritic steel with an aluminum component, for example of an iron-chromium-aluminum alloy. The control coil, may, for example, be formed from pure nickel or a cobalt-iron alloy comprising from 6-18% by weight of cobalt and has the function of a control resistor with a positive temperature coefficient. Furthermore, an electrically insulating powder filling 25, 30, which is compacted after ramming of the glow tube 5 and ensures that the heater coil 10 and the control coil 60 are accommodated and fixed in fixed positions in the interior of the glow tube 5 and are electrically insulated with respect to the glow tube 5 apart from the tip 55 of the glow tube 5, is provided in the glow tube 5. The powder filling used is generally magnesium oxide. Moreover, the powder filling is responsible for thermally linking the glow tube 5 and the heater coil 10 or the control coil 60. Given a sufficient supply of oxygen, the alloy of the heater coil 10 is soon protected against further corrosion by the formation of a thin layer of AI2O3. However, in the sheathed-element glow plug 1, this condition is not present, on account of a lack of oxygen which generally occurs initially. During the cyclical thermal loading of the sheathed-element glow plug when used in a cylinder head, air can penetrate into the glow tube 5 despite the sealing ring 75 and Viton ring 70. This leads to a simultaneous reaction of the material of the heater coil 10 with oxygen and nitrogen. Nitrogen, unlike oxygen, which forms a protective aluminum oxide layer in the surface of the heater coil 10, leads to internal nitriding, i.e. to the formation of aluminum nitride in the material of the heater coil 10. The result is a local rise in the electrical resistance of the heater coil 10, which leads to a higher voltage drop and therefore greater heating at the heater coil 10 and can lead to premature failure of the heater coil 10. For this reason, a material which acts as an oxygen donor, releasing oxygen at high temperatures and thereby promoting the formation of a protective layer of aluminum oxide on the heater coil 10, is added to the insulating powder filling. As a result, in the event of air penetrating into the glow tube 5, the formation of nitrides in the outer layers of the heater coil 10 is prevented. In this case, the aluminum oxide layer is at least partially produced as early as during the first heating of the heater coil 10 by a heating current, at which temperatures of over 1000°C are reached. If the material of the control coil 60 does not include any aluminum and also does not include any silicon, as in the example described here, it does not form a protective layer of oxide with the oxygen released by the oxygen donors, but rather is corroded. This needs to be prevented. Therefore, in this case, the material in the insulating powder filling which acts as an oxygen donor should only be added in the region 20 at the tip 55 of the glow tube 5 in which the heater coil 10 is located. The material which acts as an oxygen donor should therefore only be present in the region of the heater coil 10 and not in the region of the control coil 60. For this purpose, during assembly of the sheathed-element glow plug 1, first of all the insulating powder which includes the material which acts as an oxygen donor is introduced into the glow tube 5 until the heater coil 10 is embedded therein as completely as possible, and the control coil 60, even after ramming of the glow tube 5, does not come into contact with the material which acts as an oxygen donor. The insulating powder filling enriched with the material which acts as an oxygen donor is denoted by reference numeral 25 in Figure 1 and is referred to below as the first insulating powder. The insulating powder which is then introduced into the glow tube 5 and in which the control coil 60 is embedded should in this example not contain any material which acts as an oxygen donor and should be formed, for example from pure magnesium oxide. In this way, oxidation is promoted only in the region of the heater coil 10, with the result that both nitriding of the heater coil 10 and corrosion of the control coil 60 can be prevented. The insulating powder, which is free of materials which act as an oxygen donor, is denoted by reference numeral 30 in Figure 1 and represents a second insulating powder. As an alternative or in addition, the second insulating powder 30 may comprise a getter material for bonding oxygen, such as for example Si, Ti, Al or reduced metal oxides, such as for example FeO, Ti203. In the case of electrically conductive getter material, such as for example Si, Ti, Al, the second insulating powder 30 must contain electrically insulating material, such as for example MgO, in a considerably greater concentration than the getter material. The material which acts as an oxygen donor may be formed, for example, as oxidic ceramic powder. In this case, the ceramic powder may be a metal oxide of a metal which can oxidize in a plurality of oxidation states. To promote the release of oxygen, in a starting state this metal oxide may be present in its highest oxidation state. In this case, by way of example, the oxygen donor used may be Ti02. A further possibility consists in the oxygen donor used being an oxidic ceramic powder or metal oxide which under reducing conditions, as are present in the region 20 at the tip 55 of the glow tube 5 on account of the aluminum content of the heater coil 10, release oxygen. so that a defect results in the crystal lattice of the metal oxide in question as a result of the absence of oxygen atoms. By way of example, ZrO2 can be selected for an oxygen donor of this type. To initiate the oxidation at the heater coil 10 during heating, it is proven sufficient for the material which acts as an oxygen donor to form only approximately 0.1% by weight to approximately 20% by weight of the first insulating powder 25, while the remainder of the first insulating powder 25 may be formed, for example, by magnesium oxide. Figure 2 illustrates a second exemplary embodiment of a glow plug according to the invention, in which identical reference numerals denote the same components as in Figure 1. Unlike in the first embodiment shown in Figure 1, in the second embodiment shown in Figure 2, the glow tube 5 does not include a control coil, but rather an electronic control element 95 which is protected from oxidation, may comprise, for example, a temperature sensor and a means for determining the current fed to the heater coil 10 as a function of the temperature detected and need not be described in more detail in the present context. It is also possible to dispense with a control coil or control element altogether. Moreover, the first insulating powder 25 and the second insulating powder 30 are replaced throughout the glow tube 5 by a third insulating powder 15 which is formed from an electrically insulating material, for example from magnesium oxide, and is free of oxygen donors. The heater coil 10 is connected to the connection bolt 65 via the control element 95, it also being possible for the control element 95 to be arranged as far away from the combustion chamber as possible, in order not to be heated excessively. It is now possible for an opening 35 to be drilled into the glow tube 5 before the sheathed-element glow plug 1 operates for the first time; the opening 35 should lie outside the region 20 at the tip 55 of the glow tube 5 with the heater coil 10, since this region could be too sensitive to drilling on account of its reduced cross section. However, if there are no stability problems in the region 20 at the tip 55 of the glow tube 5, it is also conceivable for the drilled hole 35 to be arranged there, i.e. in the immediate vicinity of the heater coil 10. The opening 35 is only made after the heater coil 10 and if appropriate the control element 95 have been introduced into the region 20 at the tip 55 of the glow tube 5 and the glow tube 5 has been filled with the third insulating powder 15. Only then is the opening 35 drilled into the glow tube 5. Then, oxygen molecules are introduced into the glow tube 5 through the opening 35 under a gas atmosphere with a controlled partial pressure. This operation may, for example, last for between about 1 hour and about 20 hours, although the limits of this time range may also be shifted upward or downward. Then, the opening 35 which has been formed by the drilling operation is closed again. It can be closed up, for example, by welding. The oxygen concentration in the glow tube 5 is increased by the controlled partial pressure. The higher the partial pressure, the higher the concentration of oxygen in the glow tube 5. On account of the high concentration of oxygen and above all the presence of pure oxygen molecules, it is possible to accelerate oxidation of the surface of the heater coil 10, so that even before or during first operation of the sheathed-element glow plug 1 in the internal combustion engine, the heater coil 10 can be passivated within a short time by the formation of a thin layer of AI2O3 at the surface of the heater coil 10, the AI2O3 layer having a protective function and preventing the formation of nitrides at the heater coil 10 in the event of small quantities of air penetrating into the glow tube while the sheathed-element glow plug is operating* In this way, the service life of the sheathed-element glow plug 1 can be increased. In this case, this is achieved by prior oxidation of the heater coil 10 before the sheathed-element glow plug 1 first commences operation. A protective layer of defined composition, in this example in the form of an aluminum oxide layer, can be produced on the heater coil 10 by suitably presetting the partial pressure for introduction of oxygen into the glow tube 5 and by suitably presetting the time for which the oxygen is introduced into the glow tube 5. If the oxygen which is introduced into the glow tube 5 in this way is also distributed outside the region where the heater coil 10 is located in the glow tube 5, it is not recommended to use a control coil which is susceptible to oxidation and corrosion in the second exemplary embodiment, but rather a control element which is resistant to oxidation and corrosion, as has been described by way of example with reference to the control element 95, should be used, or a control coil or control element should be omitted altogether- ■ WE CLAIM: 1. An electrically heatable glow plug (1) for internal combustion engines, having a closed-end glow tube (5) into which an electrically conductive heater coil (10) is introduced, the heater coil (10) being formed at least in part from aluminum, in particular from an aluminum-iron-chromium alloy, characterized in that in the glow tube (5) there are oxygen donors in order to form an aluminum oxide layer on the surface of the heater coil (10) before or during the heating of the heater coil (10). 2. The glow plug (1) as claimed in claim 1, wherein in the glow tube (5) the heater coil (10) is embedded in a first insulating powder (25), and in that the first insulating powder (25) comprises a material which acts as an oxygen donor. 3. The glow plug (1) as claimed in claim 2, wherein the material is an oxidic ceramic powder. 4. The glow plug (1) as claimed in claim 3, wherein the ceramic powder comprises a metal oxide of a metal which is capable of being oxidize in a plurality of oxidation states, in particular TiO2. 5. The glow plug (1) as claimed in claim 4, wherein in a starting state the metal oxide is in its highest oxidation state. 6. The glow plug (1) as claimed in any one of claims 3 to 5, wherein the oxidic ceramic powder comprises a metal oxide,in particular ZrO2, which under reducing conditions is capable of being releasing oxygen through the formation of defects. 7. The glow plug (1) as claimed in any one of claims 2 to 6, wherein the material which acts as an oxygen donor forms from 0.1% by weight to 20% by weight of the first insulating powder (25). 8. The glow plug (1) as claimed in claim 1, wherein the oxygen donors are introduced into the glow tube (5) in the form of oxygen molecules. 9. A process for producing an electrically beatable glow plug (1) for internal combustion engines, in which an electrically conductive heater coil (10), which is formed at least in part from aluminum, in particular from an aluminum-iron-chromium alloy, is introduced into a closed-end glow tube (5), characterized in that prior to operation of the glow plug (1) oxygen donors are introduced into the glow tube (5), in order to form an aluminum oxide layer on the surface of the heater coil (10) before or during the heating of the heater coil (10). 10. The process as claimed in claim 9, wherein after the heater coil (10) has been introduced into the region (20) of the tip of the glow tube (5), a first insulating powder (25) is introduced into the glow tube (5), this insulating powder comprising a material which acts as an oxygen donor, so that the heater coil (10) is embedded as completely as possible in this first insulating powder (25). 11. The process as claimed in claim 10, wherein a second insulating powder (30), in particular based on MgO, which is as far as possible free of oxygen donors and/or comprises getter material for bonding oxygen and in which a control coil (60), which adjoins the heater coil (10) and is formed in particular from a cobalt-iron alloy, is embedded in this way, is then introduced. 12. The process as claimed in claim 9, wherein, after the heater coil (10) has been introduced into the region (20) of the tip of the glow tube (5) and after the glow tube (5) has been filled with a third insulating powder (15), an opening (35) is drilled into the glow tube (5), in that oxygen molecules are introduced into the glow tube (5) under pressure through the opening (35) in the glow tube (5), and in that the opening (35) formed by the drilling operation is then closed again, preferably by welding. 13. The process as claimed in claim 12, wherein the oxygen molecules are introduced into the glow tube (5) for a predetermined time, preferably between lhand20h.

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