EP1213540B1 - Sheated element glow plug for internal combustion engines - Google Patents

Abstract

The glow plug (1) comprises a dielectric resistance element (17) composed of two resistance coils (20, 21) connected in series. The coil (20) on the combustion chamber side acts as a heating element and the coil (21) on the side facing away from the combustion chamber acts as a regulating element, is made from an iron-based alloy and maintains a body-centered cubic crystal structure during all operational states. Preferred Features: The iron-based alloy has a temperature resistant coefficient of more than 6, preferably more than 7. The iron-based alloy contains 1.25-2 wt.% vanadium, 2-3.5 wt.% molybdenum and 1-2 wt.% titanium.

Description

The invention relates to a glow plug for arrangement in the combustion chamber of air-compressing internal combustion engines with two resistor coils connected in series, of which the combustion chamber side resistance coil serves as a heating element and the combustion chamber distant resistance coil acts as a result of their high positive temperature coefficient of resistance as a control element and consists of an iron-based alloy.

The basic structure and function of such a glow plug is shown for example in DE-C-28 02 625. In this glow plug, the glow plug includes an embedded in an insulating material, electrical resistance element, which is composed of two series-connected resistance coils. The combustion chamber-side resistance coil of this resistance element serves as a heating element and has a substantially temperature-independent resistance, while the combustion chamber remote resistance coil has a high positive temperature-resistance coefficient and acts as a control element. The latter resistance coil is usually made of nickel.

From DE-C-38 25 012, furthermore, a glow plug is known, which in principle has the same structure and the same function as the glow plug in the above-mentioned DE-C-28 02 625. However, the control element consists of a cobalt-iron alloy, wherein the iron content is between 20 and 35 wt.%. This alloy has a cubic-centered crystal structure at room temperature, while when heated to 1000 ° C it becomes a face-centered cubic crystal structure. Because this temperature window When operating the glow plug often has to be traversed, it comes through the thus induced phase transitions to thermal fatigue (disruptions) of the control element material. It has been shown that these latter glow plugs have only a relatively short life due to the disruption of the control element material. There are also undesirable malfunctions and, moreover, there are costs for troubleshooting.

This disadvantage was encountered in EP 0 523 062 B1 (see also document EP-A-0 607 872) by the introduction of a control element made of a cobalt-iron alloy, which maintains a cubic face-centered material structure during all operating states of the glow plug. The cobalt-base alloy used had an iron content of between 6 and 18% by weight. The use of a disruption of the control element was effectively avoided.

These cobalt-base alloys exhibit a low room temperature resistance and a high temperature factor, i. that the ratio of the resistivity at a high temperature, for example, 1000 ° C, to the room temperature resistivity is high. This causes high currents to flow at low temperatures while a steady state current sets at high temperatures. In practice, however, it has been shown that it is not always necessary to have such a high specific resistance at room temperature.

However, these alloys also have disadvantages:

As a rule, no high demands are placed on the scale resistance of these alloys. Because of this, the environment of the control element must be hermetically sealed, so that contact with oxygen can be ruled out.

Furthermore, these cobalt-based alloys must have a relatively high melting temperature, since the temperature in the control element may briefly exceed 1200 ° C., which would lead to melting of the control coil. The melting temperatures of the cobalt-base alloys are to be regarded as critical here.

As a result of the high temperatures, a coarse grain sets in, ie. H. a grain in the alloy covers the cross section. As a result, the risk of slipping on the grain boundaries exist that are perpendicular to the wire axis. This is an attempt to prevent, in which one surrounds the control coil tightly with ceramic powder and thus makes immobile. As a result, no additional measures against grain growth are needed.

Furthermore, the cobalt-base alloys known from EP 0 523 062 B1 exhibit a high degree of solidification and can therefore only be drawn with intermediate annealing to the preferred dimensions of 0.35 mm diameter and, in some cases, only with difficulty be wound into exact helices, because with the high solidification a sensitivity to internal stresses and their fluctuations is connected.

Finally, cobalt as a starting material is very expensive compared to nickel and iron, which also makes the cobalt-base alloys described in EP 0 523 062 B1 relatively expensive.

Overall, therefore, the desire to provide alternative alloys for glow plugs, which in addition to a price advantage over the known from EP 0 523 062 B1 cobalt-base alloys, a higher melting point, a better Have processability in wire and Wendelherstellen and a better scaling resistance.

According to the invention this object is achieved by a glow plug of the type mentioned above, which is characterized in that the serving as a control element resistance coil consists of an iron-based alloy, which maintains a cubic interior-centered crystal structure during all operating conditions of the resistance coil.

The strong temperature dependence of the electrical resistance of these iron-based alloys is related to the phenomenon of ferromagnetism. The temperature dependence is extreme for metallic alloys with the highest saturation magnetization. This is usually accompanied by a high Curie temperature. The Curie temperature determines the anomalous temperature range of the resistor and also contributes to a high temperature coefficient of resistance.

For this reason, in addition to the cobalt-base alloys of the prior art only iron and its alloys in question. But pure iron shows at elevated temperature in a window between 900 ° C and 1400 ° C, a phase transition from α-iron to γ-iron, ie, from a cubic-centered crystal structure to a cubic face-centered crystal structure. It is therefore out of the question, because it causes the disruption phenomena. By alloying, this γ-area can be "pinched off", so that the resulting iron-based alloy according to the invention remains single-phase over the entire temperature range, ie has a cubic-centered crystal structure. This crystal structure per se has a lower solidification compared to a cubic face-centered crystal structure and is therefore per se in comparison to the cubic mentioned at the outset face-centered cobalt-based alloys easier to process.

Typically, the alloys according to the invention have a temperature factor greater than 6 and, if the requirements for the scale resistance are not so high, a temperature factor greater than 7.

Suitable metals to be added to the iron-based alloy are preferably aluminum and / or chromium and / or titanium and / or vanadium and / or molybdenum. However, binary alloys are preferred for ease of manufacture.

In preferred embodiments, discussed in detail below, the iron-based alloy contains either between about 1.25 and about 2.00 weight percent vanadium or between about 2.00 and about 3.50 weight percent molybdenum or between about 1.00 and about 2.00% by weight of titanium.

An embodiment of a glow plug according to the invention is shown in the drawing and explained in more detail in the following description. The figure shows a longitudinal section through the combustion chamber side region of a glow plug in an enlarged view.

The alloys according to the present invention were prepared by melting ARMCO-iron as starting material. ARMCO iron is understood to mean a technically pure iron produced in large-scale operation, ie an iron with an iron content of 99.80 to 99.90% by weight. The melts were then respectively alloyed with the metals aluminum, chromium, titanium, vanadium or molybdenum, so that the alloys shown in the table were produced. Of course, the alloys produced on the production-related impurities.

The alloy designated in the table with "CF8" forms a prior art alloy according to EP 0 523 062 B1 and is a cobalt-based alloy which has a content of 8% by weight of iron. This alloy was used as a reference to compare the alloys of the present invention with the prior art. The table shows the resistivity at 1000 ° C and at 20 ° C, the scale thickness dz in a heat treatment of a duration of 1 hour at 1100 ° C in air, the melting point Tm, the Curie temperature Tc and the change in resistance at room temperature by alloying 1.00% by weight of the corresponding alloying metal.

After the starting material was melted, the melt was poured into a mold and the casting was hot-rolled to a thickness of 6 mm. Thereafter, the hot-rolled wire was pulled, surface-finished and drawn to a diameter of 0.35 to 0.5 mm.

In this condition, spirals were wound. After a first heating above 1000 ° C., the so-called "soft" state is present. The values listed in the table refer to this condition.

In cases in which the scale thickness is not specified, it must be assumed that a non-uniform scaling and thus an oxide penetration in large parts of the cross section, which adversely affects service life and longer-term resistance behavior.

All alloys showed an almost hysteresis-free resistivity when heating from room temperature to 1200 ° C and then cooling from 1200 ° C to room temperature. Therefore, it can be assumed that the alloys listed in the table were single-phase throughout the temperature range or that other phases were negligible. The highest temperature coefficient of resistance was found to be an iron-based alloy containing 1.50 weight percent titanium followed by an iron-base alloy containing 1.25 weight percent vanadium and an iron-based alloy containing 2.00 weight percent molybdenum.

It has been shown that the elements aluminum and chromium compared to the elements titanium, vanadium and molybdenum had less favorable properties. This was partly due to the fact that aluminum had too high a change in resistance and secondly because the addition of chromium requires too much chromium in order to achieve the requirement for single-phase.

To achieve a limited scale resistance, the molybdenum-based iron-base alloys are particularly favored because their scale thicknesses are only about 1/10 to 1/25 of the pure iron or cobalt-based alloy containing 8 wt% iron (CF8). The highest melting temperatures were found in the vanadium-based iron-based alloys. Their melting temperatures were about 1530 ° C.

All the iron-base alloys according to the present invention can be processed in principle in the same way as pure iron and thus have the low solidifications known for ferritic materials. All manufactured alloys did not have to be annealed during processing of the 6 mm thick wire into wires with 0.3 to 0.5 mm diameter. The strengths achieved were similar to the strengths that can be achieved with a cobalt-based alloy with 8 wt.% Iron, ie at strengths ≤ 1000 N / mm ² . As a result, the coils wound in the hard state can be. Due to the lower solidification, there were no problems when winding the coils.

Overall, it can be stated that although the alloys according to the present invention have a coefficient of thermal resistance which is lower by a factor of about 1.5 compared with the cobalt-base alloys of EP 0 523 062 B1 forming the prior art, this is not yet to look critically. However, they show several physical-technically advantageous properties compared to the prior art alloys and are overall much cheaper.

Thus, the alloys have a lower risk of melting due to the up to 50 ° C higher melting point. Furthermore, they have a much higher resistance to scaling, so that lower demands can be placed on the hermetic seal within the glow plug. Further, they have better processability in making wires and winding these wires into helices.

The following will discuss the exact structure of the glow plugs.

The figure shown is an original reproduction of Figure 1 of EP 0 523 062 B1 and represents a glow plug 10, which is provided for arrangement in a combustion chamber of air-compressing internal combustion engines, not shown. This glow plug 10 has a tubular metal housing 11, in the longitudinal bore 12, a glow plug 13 is fixed sealingly with a portion of its length. This glow plug 13 has a corrosion-resistant, thin-walled glow tube 14 which is closed at its combustion chamber end with a bottom 15. In the interior 16 of the glow tube 14 extends an electrical resistance element 17 which extends in the axial direction. The resistance element 17 is embedded in an insulating material 18. Furthermore, the electrical resistance element 17 is provided with a connection part 19 for the electric current away from the combustion chamber and is connected electrically conductively and fixedly connected to the bottom 15 of the glow tube 14 on the combustion chamber side.

The electrical resistance element 17 consists of two connected in series resistance coils 20 and 21. The combustion chamber side resistance coil 20 serves as a heating element and the brennraumferne resistance coil 21 acts as a result of their high positive temperature coefficient of resistance known as a control element, whereas where serving as a heating element resistance coil 20th In a known manner consists of a wire material with substantially temperature-independent resistance behavior, the acting as a control element resistance coil 21 is selected from an iron-based alloy, which maintains a cubic interior-centered crystal structure during all operating states of the glow plug 10.

An interior-centered structure of the iron-base alloy of such a control element resistance element 21 is given, for example, when the alloy between about 1.25 and 2.0 wt.% Vanadium or between 2.00 and 3.50 wt.% Molybdenum or between approx 1.00 and 2.00 wt% titanium. However, mixtures of these additives are also possible. If the admixtures of alloy metals are not reached, there would be no cubic interior-centered structure and / or the alloy would not be single-phase in the temperature interval of interest. However, if the admixtures of alloying metals are exceeded, the specific resistances become too high and thus the temperature resistance coefficients for use in the control coils not suitable. Of course, these two statements apply only to binary alloy systems.

It goes without saying that in the alloys according to the invention impurities or processing surcharges, as used in the production of iron-based alloys, have been neglected in the above statements. <B> Table </ b> material CF 8 Fe Fe-2, OMo Fe-2.5Mo Fe 3, OMo Fe-1.25V Fe-1.50V Fe-1.75V Rho 1000 = 0.81 1,124 1,146 1,126 1,152 1,136 1,141 1.143 [ΜΩm] Rho 20 = 0,071 0,099 0.156 0.168 0.18 0.145 0,157 0.166 [ΜΩm] Delta Rho /% Zu leg. = 0,029 0.028 0.027 0.037 0,038 0,038 [ΜΩm] Temp. Fac. (1000120) = 11.41 11.354 7,346 6,702 6,400 7,834 7,268 6,886 Tm approx. = 1480 1534 1520 1517 1514 1529 1527 1525 ° C Tc approx. = 1005 769 765 764 763 780 783 786 ° C dz = 0.1-0.25 <0.1 <12:03 <00:02 <00:01 mm material Fe 1,1Al Fe-1.5C 1.5Al Fe 2 Al-2Cr Fe 13,5Cr Fe 1,0Ti Fe 1,5Ti R 1000 = 1,188 1,225 1,247 1.21 1.16 1,166 [ΜΩm] R 20 = 0.229 0,378 0.463 .4307 .1439 0.168 [ΜΩm] Delta Rho /% Zu leg. = 0.118 0.093 0.091 0,025 0,045 0.046 [ΜΩm] Temp. Fak. (1000/20) = 5,188 3,241 2,693 2.80 8.06 6.94 Tm approx. = 1530 1525 1520 1500 1514 1504 ° C Tc approx. = 759 755 750 760 734 717 ° C dz = <0.01 mm

Claims (10)

1. A sheathed element glow plug (10) for installation in the combustion space of air compressing internal combustion machines including an electrical resistance element (17), which consists of two series connected resistive coils (20,21), of which the resistive coil (20) on the combustion chamber side serves as a heating element and the resistive coil (21) remote from the combustion chamber acts as a control element as a consequence of its high positive temperature-resistance coefficient and consists of an alloy based on iron, in which the resistive coil (21), which serves as a control element and consists of an alloy based on iron, maintains a body centred cubic crystal structure under all operating conditions of the sheathed glow plug (10).
2. A sheathed element glow plug (10) as claimed in claim 1, characterised in that the alloy based on iron has a temperature-resistance coefficient greater than 6.
3. A sheathed element glow plug (10) as claimed in claim 2, characterised in that the alloy based on iron has a temperature factor greater than 7.
4. A sheathed element glow plug (10) as claimed in one of claims 1 to 3, characterised in that the iron alloy contains between ca. 1.25 and 2.00% by wt. Vanadium.
5. A sheathed element glow plug (10) as claimed in one of claims 1 to 3, characterised in that the iron alloy contains ca. 2.00 and 3.50% by wt. Molybdenum.
6. A sheathed element glow plug (10) as claimed in one of claims 1 to 3, characterised in that the iron alloy contains between ca. 1.00 and 2.00% by wt. Titanium.
7. A sheathed element glow plug (10) as claimed in one of claims 1 to 6, characterised in that the sheathed glow plug is surrounded by a tubular metal housing (11), in the longitudinal bore (12) of which a portion of the length of the glow plug (13) is sealingly fixed in position, wherein the glow plug (13) includes a thin walled glow tube (14), which is closed at its end closest to the combustion space with a base (15) and in whose interior the electrical resistive element (17) extends in the axial direction.
8. A sheathed element glow plug (10) as claimed in claim 7 characterised in that the electrical resistive element (17) is embedded in an insulating material (18).
9. A sheathed element glow plug (10) as claimed in claim 7, characterised in that the electrical resistive element (17) is provided remotely from the combustion chamber with a connector member (19) for the electric current.
10. A sheathed element glow plug (10) as claimed in claim 7, characterised in that the electrical resistive element (17) is firmly and electrically conductively connected to the base (15) of the glow tube (14) on the combustion chamber side.

Images