Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-06-09
2003-03-18
Warden, Sr., Robert J. (Department: 1744)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S429000, C073S023310
Reexamination Certificate
active
06533911
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for measuring combustible-gas concentration and a method for measuring combustible-gas concentration by use of the device. More particularly, the invention relates to such devices and methods capable of measuring the concentration of a combustible gas such as hydrocarbon gas contained in an exhaust gas emitted from a lean-burn engine including a diesel engine.
The present device enables measurement of the combustible gas concentration with low dependency on the high concentration of oxygen contained in such a lean-burn engine.
2. Description of the Related Art
Conventionally, in order to optimize the combustion efficiency of an internal combustion engine and to maximize combustion of combustible gases contained in a combustion exhaust gas, a percentage of a gas component, particularly oxygen, contained in the combustion exhaust gas is measured, and the measured data is fed back to an engine control system. However, in order to cope with a trend toward tightening of exhaust gas regulations and a goal of attaining fuel economy in recent years, there have been demands for more sensitive detection of a combustible gas component contained in the exhaust gas in addition to apparatus and methods for accurately measuring concentration of the combustible gas component.
For example, since the lean-bum engine can promote fuel economy but emits a large amount of nitrogen oxides, a system has been studied as shown in
FIG. 1
for eliminating nitrogen oxides, in which system the fuel is added into the exhaust gas by an injector placed at a position upstream of a catalytic converter. Also, there have been demands for attachment of a catalyst to the diesel engine that is conventionally unequipped with a catalytic converter. Thus, a sensor system as shown in
FIGS. 2
or
3
has been studied, which is capable of measuring the concentration of a combustible gas and/or detecting deterioration of the catalytic converter used therein.
However, a lean-bum engine involves great variations in oxygen concentration as compared to a conventional stoichiometricly controlled engine and exhibits a particularly high oxygen concentration under lean conditions. Thus, there have been demands for a device for measuring combustible-gas concentration capable of detecting a hydrocarbon that contains a relatively large number of carbon atoms (about 2-15 carbon atoms), without dependence on oxygen concentration.
Techniques for detecting such a combustible gas are disclosed in PCT Application Laid-open No. WO 95/25277 and Japanese Patent Application Laid-Open (kokai) No. 82763/1998. According to the former publication, a detection electrode may contain gold, silver, platinum, or bismuth. The latter publication does not particularly describe a detection electrode. These publications do not mention techniques that decrease a measurement problem such as dependence on oxygen concentration and/or dependence on temperature of the exhaust gas.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the above-mentioned problems of the prior art, and to provide a device for measuring combustible-gas concentration and a method for measuring combustible-gas concentration using of the same. This device and method enables sensitive detection of a combustible gas such as a hydrocarbon gas, and accurate detection of concentration of such a combustible gas while suppressing dependence on oxygen concentration in a combustion exhaust gas and/or dependence on temperature of a gas to be measured, and as well exhibiting a stable response.
A device of a first aspect of the invention for measuring combustible-gas concentration comprises an oxygen-ion-conductive solid electrolyte element, a reference electrode and a detection electrode formed on the surface of the solid electrolyte element, and a heater element for heating the solid electrolyte element. The device is characterized in that the detection electrode contains gold and a metallic oxide. The “metallic oxide” may be an oxide of a transition metal. Preferably, the detection electrode contains an oxide of at least one element selected from among In, Fe, Ta, Ga, Sr, Eu, W, Ce, Ti, Zr and Sn. Most preferably, the detection electrode contains one of indium oxide and iron oxide. The metallic oxide greatly reduces dependence on oxygen concentration of the device for measuring combustible-gas concentration, according to the invention.
Examples of the above-mentioned “combustible gas” include hydrocarbons contains 2-15 carbon atoms, hydrogen, carbon monoxide and ammonia. The above-mentioned “solid electrolyte element” may be a known oxygen-ion-conductive solid electrolyte body such as a zirconia-based ceramic body and an LaGaO
3
-based ceramic body.
The above-mentioned “reference electrode” is an electrode which is brought into contact with a reference gas or an oxygen atmosphere of constant pressure established by means of an oxygen pump. The reference electrode shows a higher electric potential than does a detection electrode upon contact with a combustible gas component contained in the gas being measured.
The above-mentioned “detection electrode” is an electrode exposed to the gas to be measured.
The above-mentioned “heater element” is an element for heating the solid electrolyte element so as to maintain the solid electrolyte element at a predetermined temperature, and may assume a conventional form and may be formed from a known material. In the case where the above-mentioned “solid electrolyte element” assumes the form of a closed-bottomed cylinder with a hollow inside, the heater element may assume a rod form, such as the form of a round bar or strap, so as to be placed in the hollow . In the case where the device for measuring combustible-gas concentration assumes the form of a laminate, the heater element may be an integral portion of the laminate or may be disposed separately in the vicinity of the solid electrolyte element.
A device of a second aspect of the invention for measuring combustible-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element and a second electrode layer formed on the surface of the first electrode layer; the first electrode layer containing at least one of platinum and gold; and the second electrode layer containing at least one of gold and a metallic oxide (except for a case where the first electrode layer is identical in composition with the second electrode layer).
The above-mentioned “combustible gas,” “reference electrode,” “detection electrode,” “heater element,” “solid electrolyte element,” and “metallic oxide” are similar to those of the first aspect of the invention. By employing the detection electrode structure comprising the “first electrode layer” and the “second electrode layer,” sensor output can be reduced (or rather lowered) for a gas to be measured which does not contain a combustible gas (hereinafter, this sensor output is called merely “offset”). Also, adhesion of the detection electrode to the solid electrolyte element can be improved in this structure.
The offset is apt to increase when an electrode contains a metallic oxide is formed in direct contact with the solid electrolyte element. An increase in the offset is not preferred, since the sensor output relatively decreases for a combustible gas contained in the gas to be measured. By contrast according to the invention, by forming the first electrode layer containing at least one of platinum and gold, between the solid electrolyte element and the second electrode layer that contains a metallic oxide, the offset can be suppressed to a low level such that a combustible gas can be detected and measured more accurately.
By employing a detection electrode contains gold and a metallic oxide, a device for measuring combustible-gas concentration can suppress offset to 0-40 mV and further to 0-10 mV. The device for measuring combustible-gas concentration equipped with
Fujita Hiroki
Fuma Tomohiro
Inoue Ryuji
Kato Kenji
Kitanoya Shoji
NGK Spark Plug Co. Ltd.
Olsen Kaj K.
Sughrue & Mion, PLLC
Warden, Sr. Robert J.
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