Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2002-03-28
2003-11-11
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C123S14500A, C313S141000, C313S143000
Reexamination Certificate
active
06646231
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a ceramic heater employed in a glow plug or the like for preheating a Diesel engine and having an ion detecting electrode, and a method for manufacturing the ceramic heater, and a glow plug using the aforementioned ceramic heater and an ion current detecting device using said glow plug.
RELATED ART
From the view point of environmental protection of recent years, not only a gasoline engine but also a Diesel engine is desired for reducing the exhaust gas discharged from the engine and the noxious substance in the exhaust smoke. Especially the Diesel exhaust particle (DEP) to be discharged as a main cause of an incomplete combustion in the engine is being recently regulated in Japan. In order to satisfy such desire, moreover, there have been made various proposals on the construction of the engine, the improvement in the combustion control, the exhaust gas treatment using a catalyzer and the improvement in the fuel or lubricant.
In some engine combustion control system of recent years, moreover, there is mounted a mechanism for detecting the engine combustion state as control information. The specific parameters to be measured are exemplified by the internal cylinder pressure, the combustion light or the ion current. Especially the detection of the engine combustion state in terms of the ion current is accepted useful because the chemical reaction situation accompanying the combustion can be directly grasped. Thus, there have been proposed a variety of ion current detecting methods. On the gasoline engine, there has been adopted a detection method, in which the ignition interval is utilized to use the spark discharge gap of the spark plug in the ion current generating portion. However, this method cannot be adopted in the Diesel engine because this engine does not employ the spark plug, as known in the art.
On the Diesel engine, on the other hand, there is mounted a glow plug for warming up the engine. Therefore, the ion current detecting method utilizing the glow plug has been disclosed, for example, in (unexamined) Japanese Patent Kokai Publication Nos. JP-10-89223A, 10-89686A, 10-89687A and 10-122114A. The summary of the principle will be described in the following.
Specifically, the glow plug is provided with a resistance heating heater arranged in the combustion chamber. This heater is continuously fed with an electric power to heat till the warm-up of the engine is completed, but is not basically used after the warm-up was ended. Therefore, the glow plug is used as an ion current detecting probe. In order to add the ion detecting function to the glow plug, more specifically, an additional structure is made such that an ion current detecting electrode portion is so mounted on the resistance heating element of the heater that a portion of the electrode surface is exposed to the heater surface. At the time of starting the engine, moreover, the warm-up is performed by connecting the resistance heating element with the heating power source to energize it for the heating action. After the end of the warm-up, on the other hand, the power source and the conduction passage are switched for the ion current so that the ion current may be produced between the inner face of the combustion chamber in the grounded engine block and the ion current detecting electrode portion. In case a waveform reflecting the situation of an incomplete combustion is detected in a signal of the ion current, for example, the connection can be switched again to the heating power source to cause the resistance heating element to heat thereby to assist the combustion.
For example, the glow plug disclosed in Japanese Patent Kokai Publication No. JP-10-89686A employs a ceramic heater in which a resistance heating element made of ceramic is buried in an insulating ceramic substrate. As the materials for the resistance heating element and the ion current detecting electrode portion, there are enumerated conductive inorganic compounds such as molybdenum disilicate (MoSi
2
), pentamolybdenum trisilicate (Mo
5
Si
3
), molybdenum silicon carbide (MoxSi
3
Cy), molybdenum boride (MOB), tungsten carbide (WC) and TiN, etc.
SUMMARY OF THE DISCLOSURE
However, the conductive inorganic compound of the above-specified material is relatively satisfactory in the electric characteristics when employed as the resistance heating element, but has the following problems as a material for the ion current detecting electrode portion to contact directly with a hot combustion gas. Specifically, Mo or W or a cation component of those inorganic conductive compounds is defective in that it is easily oxidized in contact with the hot combustion gas, and in that an oxide such as MoO
3
or WO
3
produced is, because of the trivalence, so volatile that it is seriously exhausted at a high temperature to significantly shorten the lifetime of the ion current detecting electrode portion. Here, Japanese Patent Kokai Publication No. JP-10-89686A has also disclosed a mode in which the exposed surface portion of the ion current detecting electrode portion is coated with a precious metal such as Pt, Ir, Rh, Ru or Pd. However, the precious metal is expensive and is complex in the manufacture steps so that it is not economical. Moreover, the contact with the conductive inorganic compound making the substrate for the coating and the separation or cracking of the precious metal coating portion due to the difference in the linear expansion coefficient are liable to raise problems so that the coating is not preferred from the view point of durability.
It is an object of the present invention, according to one aspect, is to provide a ceramic heater which is better in the durability of an ion current detecting electrode portion and which can be manufactured at a low cost.
It is another object, according to another aspect of the invention, to provide a method for manufacturing the ceramic heater. It is a further object, according to a further aspect of the invention to provide a glow plug and an ion current detecting device employing the ceramic heater, respectively.
Further objects and aspects of the invention will become apparent in the entire disclosure, claims and drawings.
According to a first aspect of the present invention, there is provided a first construction of a ceramic heater comprising:
an insulating ceramic substrate;
a resistance heating element made mainly of conductive ceramic and buried in the insulating ceramic substrate; and
an ion current detecting electrode portion made mainly of conductive ceramic and integral with the resistance heating element in the insulating ceramic substrate and having a portion of its own surface exposed as an ion current detecting face to the surface of the insulating ceramic substrate,
characterized in that the ion current detecting electrode portion is constructed such that a portion including at least a portion of the ion current detecting face is made of a nonmetallic conductive ceramic having a cation component of at least one nonmetallic element.
According to the aforementioned construction of the ceramic heater, the ion current detecting electrode portion is constructed such that a portion including at least a portion of the ion current detecting face is made of a nonmetallic conductive ceramic having a cation component of a nonmetallic element or elements. The nonmetallic conductive ceramic is superior in oxidation resistance to the metallic conductive ceramic which is generally employed as a material for a ceramic resistance heating element and which has a cation component made of a metallic element(s), and also hardly generates high-temperature volatile oxides. By adopting the nonmetallic conductive ceramic as a material for constructing the ion current detecting face, therefore, it is possible to elongate the lifetime of the ion current detecting electrode portion.
In the aforementioned first construction, the nonmetallic conductive ceramic can be made mainly of one kind or two kinds or more of silicide, carbide, nitride or boride of nonmetallic cat
Hotta Nobuyuki
Okinaka Manabu
Yoshikawa Takaya
Dahbour Fadi H.
Morrison & Foerster / LLP
NGK Spark Plug Co. Ltd.
Walberg Teresa
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