Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-02-19
2003-10-21
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S406000, C204S408000, C205S781000, C073S023310
Reexamination Certificate
active
06635161
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an NOx sensor control circuit unit and an NOx sensor system including the control circuit unit.
BACKGROUND OF THE INVENTION
NOx-concentration measuring apparatus using an NOx sensor for detecting the concentration of nitrogen oxides (hereinafter called NOx) in exhaust gases from internal combustion engines and the like are disclosed, for example, in European Patent Application Laid-Open No. 0678740A1 and SAE Paper No. 960334, pp. 137-142, 1996. An NOx sensor used in such a conventional NOx-concentration measuring apparatus is composed of oxygen-ion conductive solid electrolyte layers that form a first processing chamber and a second processing chamber. The first processing chamber communicates with the gas to be measured (hereinafter called “a measurement gas”) via a first diffusion-controlling passage, and the second processing chamber communicates with the first processing chamber via a second diffusion-controlling passage. The solid electrolyte layer of the first processing chamber is sandwiched between porous electrodes so as to form a first pump element and an oxygen-concentration-measuring cell. The solid electrolyte layer of the second processing chamber is sandwiched between porous electrodes so as to form a second pump element.
In the thus-configured NOx-concentration measuring apparatus, a current is made to flow through the first pump element such that an output voltage from the oxygen-concentration-measuring cell attains a predetermined value, thereby controlling the concentration of oxygen in the first processing chamber to a constant level. At the same time, a constant voltage is applied to the second pump element to thereby pump out oxygen from the second processing chamber. As a result, the NOx concentration of a measurement gas can be obtained from the current flowing through the second pump element.
A measurement gas, e.g., exhaust from an internal combustion engine or the like, contains gas components other than NOx, such as oxygen, carbon monoxide and carbon dioxide. Thus, in the aforementioned NOx-concentration measuring apparatus, first, the oxygen concentration of the first processing chamber is controlled to a very low level by means of the first pump element. Then, in the second processing chamber which receives the measurement gas controlled to a low oxygen concentration, a constant voltage is applied to the second pump element in a direction such that oxygen is pumped out from the second processing chamber. As a result, NOx contained in the measurement gas is decomposed into nitrogen and oxygen by means of the catalyzing function of the porous electrodes of the second pump element, and the thus-generated oxygen is then pumped out from the second processing chamber. Thus, the NOx concentration of the measurement gas can be obtained from the current flowing through the second pump element without influence of other gas components contained in the measurement gas.
In the NOx-concentration measuring apparatus, in order to accurately detect the NOx concentration, the NOx sensor must be heated to a predetermined activation temperature (for example, about 850° C.) so that the cells are activated. Therefore, the apparatus is provided with a heater for heating the NOx sensors.
On the other hand, in a lean burn engine, for example, which is run at a lean air-fuel ratio (namely, a high ratio of air to fuel), the resulting exhaust gas tends to contain a relatively large amount of NOx. Accordingly, in order to suppress NOx emissions, the above-mentioned NOx-concentration measuring apparatus is often used to monitor the state of a reducing catalyst installed in an exhaust line. Specifically, the NOx sensor is installed in an exhaust passage of an internal combustion engine downstream of a reducing catalyst in order to measure the NOx concentration of the exhaust gas. When NOx emissions increase, the air-fuel mixture supplied to the internal combustion engine is temporarily controlled to a rich air-fuel ratio so as to emit unburnt gas from the internal combustion engine. The unburnt gas reacts with NOx accumulated in the catalyst, thereby suppressing NOx emissions.
Since the above-mentioned NOx-concentration measuring apparatus cannot measure the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine, the realization of the above NOx control requires a separate air-fuel ratio measuring apparatus for obtaining an air-fuel ratio from the oxygen concentration of the exhaust. That is, in order to carry out the above NOx control, air-fuel ratio control must also be carried out simultaneously. To meet this end, both the NOx sensor and an oxygen concentration sensor (a so-called air-fuel ratio sensor) must be provided in the exhaust system of the internal combustion engine.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide an NOx sensor control circuit unit which enables an existing NOx sensor to serve not only as an NOx sensor but also as an oxygen concentration sensor by attachment to the existing NOx sensor and which can thus simplify the configuration of a detection system for detecting NOx concentration and oxygen concentration (or air-fuel ratio). It is also an object of the present invention to provide an NOx sensor system including the control circuit unit.
The above objects of the present invention are achieved by providing an NOx sensor control circuit unit (hereinafter also referred to as a control circuit unit) adapted for use by connection to an NOx sensor. The NOx sensor for connection to the control circuit unit of the present invention comprises:
First processing chamber: The first processing chamber is an internal chamber of the NOx sensor and is isolated from the surrounding atmosphere. A measurement gas is introduced into the first processing chamber via a first diffusion-controlling passage.
Second processing chamber: The second processing chamber is an internal chamber of the NOx sensor and is isolated from the surrounding atmosphere. A gas contained in the first processing chamber is introduced into the second processing chamber via a second diffusion-controlling passage.
Oxygen concentration detection element: The oxygen concentration detection element is formed of an oxygen-ion conductive solid electrolyte, sandwiched between porous electrodes, and measures the oxygen concentration of gas in the first processing chamber.
First pump element: The first pump element is formed of an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes, and pumps out oxygen from the first processing chamber.
Second pump element: The second pump element is formed of an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes, and pumps out oxygen from the second processing chamber.
Heating element: The heating element heats the first pump element, the oxygen concentration detection element, and the second pump element.
The control circuit unit of the present invention comprises:
First pump element control circuit: The first pump element control circuit controls voltage applied to the first pump element to thereby control the partial pressure of oxygen in the first processing chamber such that an output voltage from the oxygen concentration detection element becomes substantially constant.
First pump current detection circuit: The first pump current detection circuit detects current flowing through the first pump element (first pump current) and outputs a detection signal indicative of the detected current (hereinafter referred to as a first pump current detection signal).
Second pump element control circuit: The second pump element control circuit applies a constant voltage to the second pump element in a direction so as to pump out oxygen from the second processing chamber.
Second pump current detection circuit: The second pump current detection circuit detects current flowing through the second pump element (second pump current) and outputs a detection signal indicative of the detected current
NGK Spark Plug Co. Ltd.
Olsen Kaj K.
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