JP3618092B2 - Electrically heated glow plugs or glow rods for internal combustion engines - Google Patents

Description

[0001]
The present invention relates to an electrically heated glow plug or glow rod for an internal combustion engine according to the preconditions of claim 1 as disclosed, for example, from EP 450185B1.
[0002]
The glow plug is used in the combustion chamber of a diesel engine for preheating during cold start, or in the form of a glow rod in the intake passage for preheating the intake air. The glow plug or glow rod includes one non-corrosive metal casing, one heating and control coil, and one electrically insulating compressed powder filling. The heating / control coil in the heating region is made of ferritic steel on which pure nickel wire as a control resistor is welded.
[0003]
In operation, the material of this heating coil is subject to thermal and chemical effects that can adversely affect the life of the glow plug. These influences constitute a very important parameter for at least the life of the glow plug. Due to the high operating temperature of the heating coil and the presence of oxygen in the compacted powder packing as before, the heating coil is susceptible to creeping corrosion. Specifically, this appears to be primarily intergranular corrosion promoted by the growth of crystals and the tendency to form large particles in the ferrite thermal conductor. Secondly, high temperature corrosion occurs on the free surface of the heating coil, thus leading to a weakening of the cross section of the heating coil.
[0004]
Magnesium oxide is generally used as the powder filling. In order to minimize atmospheric oxygen contained in the pores of the powder filling, this is done by upsetting the metal casing filled with powder from the outside with a centripetal driving machine. The powder is very strongly compressed, which reduces the diameter. In this powder filling, the tip end portion of the heating rod is particularly strongly compressed by compressing the tip end of the metal casing into a conical shape by upsetting.
[0005]
An object of the present invention is to improve a general-purpose glow plug related to the service life of a heating coil.
[0006]
With this universal glow plug as a starting point, this object is achieved by the features described in claim 1 of the present invention. The reason for this is that when the powder filling is compressed, the casing pipes are radially upset, which causes severe mechanical stress on the heating and control coil wires, and in this process, for example, notches and dents. This is because it has been found that the wire is damaged from the beginning without knowing it, that is, the cross section of the wire is locally contracted. This coil can withstand mechanical stress without first being significantly damaged during the compaction of the powder filling, because its surface hardness has been increased, in particular by nitriding.
[0007]
Advantageous refinements of the invention can be obtained from the respective dependent claims. The invention is also described in more detail in the following description with reference to exemplary embodiments illustrated in the drawings.
[0008]
In diesel engines, glow plugs are used in the combustion chamber for preheating during cold start, or as rod-like combustion glow plugs or combustion systems in the air inlet passage for preheating intake air used. The exemplary embodiment of the glow plug shown in FIG. 1 has a glow pipe 5 fixed to the plug body 1 and connected in electrical communication with the body. The casing of the glow pipe is generally made of a nickel-rich iron alloy or a non-corrosive nickel-based alloy, for example, Inconel 601, which is in principle electrically connected as a ground electrode, ie, the negative side. Is done. This electrical ground connection is made via a threaded screw thread 7 and / or by a conical part 7 a at the lower end of the plug body 1.
[0009]
A heating coil 8 having a control coil 9 welded via a connection weld 11 and an electrically insulating compressed powder filling 10 are arranged in the glow pipe. This powder filling has several functions, especially in the compressed state. First of all, it is ensured that the heating coil (8) and the control coil (9) are housed and fixed in place in the glow pipe, and these coils are electrically insulated and held. This means that the heat generated in the heating coil 8 by this compressed powder is inevitably transferred to the glow pipe casing as quickly as possible. Furthermore, the compaction of the powder is carried out so as to remove any air pockets to the greatest possible extent, in particular to remove a certain amount of residual oxygen. This requires a particularly strong powder compression, especially in the region of the heating coil 8.
[0010]
In the heating zone (heating coil 8), the heating and control coils 8, 9 are made of iron-chromium-aluminum alloy ferritic steel containing for example 17-22% chromium and 3-7% aluminum. Kanthal AF CrAl1225 is often used as the alloy. A coiled wire (control coil 9) made of pure nickel and functioning as a control resistor is welded onto this type of heating coil (connection weld 11). The heating coil 8 is connected to the glow pipe via a seal weld 12 within the tip of the glow pipe.
[0011]
The other end of the control coil 9 is connected to a connection bolt 2 embedded in the insulating washer 4, electrically insulated, sealed with a sealing material 6, and taken out of the plug body 1. This connection bolt is connected to the positive pole of the current source via a nut 3, and this nut firmly contacts the terminal of the cable with the connection bolt. Further, the connecting bolt 2 seals the open end of the upper part of the glow pipe with a flexible insulating sealing material 6 'for the purpose of reliably preventing oxygen in the atmosphere from entering the compressed powder filling. .
[0012]
In general, magnesium oxide is used as the powder filling 10. In order to compress this powder filling, as mentioned above, the glow pipe filled with powder is upset radially from the outside by a concentric acting driving machine, which reduces the diameter, and It becomes possible to achieve a conical shape. The powder filling is particularly strongly compressed and is particularly strongly compressed in the radial direction by a metal casing at the tip of the glow pipe.
[0013]
Due to the strong compression of the powder filling, the entire coils 8, 9 and in particular the heating coil 8 are mechanically stressed. When the glow pipe 5 is upset in the radial direction, the casing thereof is not only plastically deformed, but also the coils 8 and 9 attached inside are further deformed. The compacted and compacted powder filling 10 also conveys the action of forces from the driving machine that acts in a balanced manner on the swivel of the heating and control coils 8, 9 from the outside, during which the coil Reduce the diameter to an appropriate thickness. However, since this powder filling is not completely uniform and involves some variation, the force acting on the coil through the powder filling varies in size depending on the location of the powder density.
[0014]
This results in a different degree of plastic upset of the coil depending on the location in the case of a coil that has not been processed. The degree of upset varies from part to part, as shown in FIG. 3 using the example of a conventional glow plug with an unprocessed coil 8 ', and a stochastic dent on the surface of the coil 8'. It can be a cause. Even when new, the surface 15 of the coil can be dented after the powder is compressed. According to the findings of the present invention, a certain amount of damage is initially observed in this surface dent. This appears to be the result of this type of stochastic indentation as a result of local cross-section compression of the coil cross-section. This local cross-sectional compression results in an increase in local electrical resistance, which causes more intense heating of the coil during glow plug operation. This means that the thermal and chemical aging process proceeds rapidly since this compression is at a higher temperature than elsewhere. Initially, there is only a small amount, and the compression of the coil conductor cross-section caused by the dent can be a factor that determines the service life, i.e. a factor that shortens the service life.
[0015]
The present invention seeks to increase the useful life of the coil, in particular the heating coil 8 which is subjected to particularly severe mechanical stresses during compression and also to particularly high thermal stresses during operation. In order to increase the service life, according to the invention, the conductive coils 8, 9, at least the particularly dangerous heating coil 8, are surface hardened. Specifically, a diffusion treatment such as nitriding is recommended as advantageous, said treatment resulting in increased hardness by forming nitrides in the diffusion region, and as a result of diffusion these processes are hardened A gradual hardness transition occurs from the region to the soft center. The diffusion region 13 of the coils 8 and 9 preferably has a depth t of approximately 5 to 10 μm.
[0016]
These coils are merely hardened by the end face layer 13 near the surface, but they can still be plastically deformed as a whole. On the other hand, when the radius of the powder filling is reduced, the occurrence of significant dents on the wire surface is avoided by curing the end face layer. Even after compression of the powder filling, the coil has a smooth surface 14. As a result, initial mechanical damage to the conductor is avoided. As a result, a long service life is expected for the conductors and thus for the entire glow plug.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view through a glow plug.
FIG. 2 is an enlarged view of a detail II portion of FIG.
FIG. 3 is a cross-sectional view through the coil in a detail II section where the surface of the coil of a conventional glow plug was first damaged by mechanical stress during compression of the powder filling.

Claims (4)

An electrically heated glow plug or glow glow for an internal combustion engine having a corrosion resistant glow pipe with a closed end and an embedded conductive coil and containing a non-conductive compressed powder filling Glow plug or glow rod, characterized in that the conductive coil (8, 9) is surface hardened. 2. Glow plug or glow rod according to claim 1, characterized in that at least the heating coil (8) is surface hardened. Glow plug or glow rod according to claim 1, characterized in that the conductive coil (8, 9) is surface hardened over at least a part of the longitudinal extent by diffusion treatment, in particular nitriding treatment. Glow plug or glow rod according to claim 3, characterized in that the hardened diffusion region (13) of the conductive coil (8, 9) has a depth (t) of about 5 to 10 µm.

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