Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-11-24
2002-01-15
Warden, Sr., Robert J. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C205S783500, C205S787000, C156S089160
Reexamination Certificate
active
06338783
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensor and a gas sensor system using the same.
Resistance-type sensors are known for measuring the concentration of a combustible gas component such as hydrocarbon (hereinafter may be referred to as HC) or CO contained in exhaust gas from an automobile or the like. For example, an oxide semiconductor (n type) such as SnO
2
or the like is used as a sensing element for measuring the concentration of a combustible gas component such as HC or CO. Specifically, oxygen in the atmosphere adsorbs on the sensing element through an effect induced by negative charges. When the atmosphere contains a combustible gas component such as HC or CO, the combustible gas component undergoes a combustion reaction with the adsorbing oxygen, thereby causing oxygen to be desorbed from the sensing element. Since a change in an electric resistance of the sensing element associated with the oxygen desorption depends on the combustible gas component concentration of the atmosphere, the combustible gas component concentration of the atmosphere can be obtained through measurement of the change of the electric resistance. However, such a resistance-type sensor has a drawback that an output from the sensing element formed of an oxide semiconductor is likely to vary depending on the concentration of oxygen or water vapor contained in an exhaust gas. Accordingly, even when the combustible gas component concentration remains unchanged, a detection output value varies depending on, for example, the oxygen concentration of the exhaust gas.
In order to solve the above problem, an apparatus for measuring a combustible gas component concentration having the following structure is disclosed in Japanese Patent Application Laid-Open No. 8-247995. In the apparatus, a sensing element has two processing zones. An exhaust gas is introduced into a first processing zone via first diffusion-controlling means. Oxygen is pumped out from the first processing zone by a first oxygen concentration adjustment pump element so as to reduce the oxygen concentration of the first processing zone to a low value at which a combustible gas component is not substantially burned. Next, the gas having the thus-reduced oxygen concentration is introduced into a second processing zone via second diffusion-controlling means. Oxygen is pumped into the second processing zone by a second oxygen concentration adjustment pump element so as to burn the combustible gas component. The combustible gas component concentration is determined based on a value of current flowing through or voltage built up across the second oxygen concentration adjustment pump element.
The apparatus disclosed in the above-described patent publication employs a structure such that oxygen is pumped into the second processing zone by the second oxygen pump element in order to burn the combustible gas component on the electrode in the second processing zone. The oxygen concentration in the second processing zone is detected by the oxygen concentration cell element, and the second oxygen pump element is operated such that the detected electromotive force becomes constant. The concentration of the combustible gas component is detected on the basis of the pumping current of the second oxygen pump element. However, this structure has the following drawback. When the oxygen concentration changes as a result of combustion of the combustible gas component, this change is detected by the oxygen concentration cell element and is fed back to a control section for controlling the pumping current, so that the response in detecting the combustible gas component decreases accordingly.
Further, in the above disclosed apparatus, the oxygen concentration of the exhaust gas introduced into the first processing zone is reduced by the first oxygen concentration adjustment pump element to “a low value at which a combustible gas component is not substantially burned.” According to the publication, the low value is not higher than 10
−14
atm, preferably not higher than 10
−16
atm, and is normally about 10
−20
atm. However, when the oxygen concentration of the first processing zone is set at such a low value, there arises the following problem related to accuracy in measuring the combustible gas component concentration.
Specifically, an exhaust gas generally contains a fair amount of water vapor in addition to combustible gas components such as hydrocarbon, carbon monoxide, and hydrogen. Generally, the amount of water vapor varies according to operating conditions of an internal combustion engine. According to studies conducted by the inventors of the present invention, when the oxygen concentration of such an exhaust gas is reduced to the above-mentioned value, a portion of water vapor is decomposed into hydrogen and oxygen. The thus-generated oxygen is pumped out from the first processing zone by the first oxygen concentration adjustment pump element, whereas the thus-generated hydrogen is not pumped out, but introduced into the second processing zone, where the hydrogen induces combustion. If such a state occurs during measurement of a gas to be examined which mainly contains hydrocarbon as a combustible gas component, the accuracy in measuring hydrocarbon concentration is greatly affected by combustion of hydrogen generated through decomposition of water vapor. Notably, measurement examples disclosed in the above publication are all conducted under the condition that the water vapor concentration of the gas to be examined is constant, and do not refer to the influence of a variation in water vapor concentration on measurement of a combustible gas component concentration.
As disclosed in the above publication, a proton pump may be additionally used in order to pump out the thus-generation hydrogen from the first processing zone, so that only HC is selectively burned to thereby improve measurement accuracy. However, this method merely employs the proton pump as a means of last resort for coping with hydrogen generation associated with decomposition of water vapor. Addition of the proton pump makes a sensor structure and a sensor control mechanism complex, causing an increase in apparatus cost. Further, residual hydrogen which the proton pump has failed to pump out may induce a measurement error.
Also, the following problem is involved. With the recent tendency to tighten exhaust gas regulations for air pollution control, internal combustion engines such as gasoline engines, diesel engines, and like engines tend to shift to the lean-burn type in order to suppress generation of HC associated with incomplete combustion. An exhaust gas produced under lean-burn conditions has an oxygen concentration higher than that produced under stoichiometric or rich conditions. When the above-mentioned conventional apparatus is applied to such an exhaust gas, an oxygen concentration adjustment pump element carries a significant burden in order to reduce the oxygen concentration to the above-mentioned low value. As a result, the service life of the oxygen concentration adjustment pump element is shortened. Further, since the operating power of the oxygen concentration adjustment pump element must be increased, a peripheral control circuit must be of high output, causing an increase in apparatus cost.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a gas sensor which can detect the concentration of a combustible gas component in a measurement gas, such as exhaust gas, with high accuracy even when the oxygen concentration of the measurement gas or the element temperature changes and which has an excellent response in detecting the combustible gas component, as well as to provide a gas sensor system using the gas sensor. An second object of the present invention is to provide a gas sensor in which accuracy in measuring a combustible gas component concentration is less susceptible to decomposition of water vapor and which is suitably applicable to lean-burn conditions, as well as to provide a gas sensor syst
Fuma Tomohiro
Inoue Ryuji
Kato Kenji
Kitanoya Shoji
Oshima Takafumi
NGK Spark Plug Co. Ltd.
Olsen Kaj K.
Warden, Sr. Robert J.
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