Abnormality diagnosis system and method for oxygen sensor

Details Abnormality diagnosis system and method for oxygen sensor Abnormality diagnosis system and method for oxygen sensor

2002-06-24

2004-03-30

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Instrument proving or calibrating

Gas or liquid analyzer

C073S023310, C073S023320, C073S031050

Reexamination Certificate

active

06711932

ABSTRACT:

INCORPORATION BY REFERENCE
This disclosure of Japanese Patent Application No. 2001-203592 filed on Jul. 4, 2001 including the specification, drawing and abstract is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to an abnormality diagnosis system and method for an oxygen sensor. More particularly, the invention relates to an abnormality diagnosis system and method which performs a diagnosis on an abnormality of the oxygen sensor caused by its fractured detector.
2. Description of Related Art
In an internal combustion engine which includes an exhaust purification system utilizing a catalyst, it is essential to accurately control an air-fuel ratio of an air-fuel mixture which is combusted in the internal combustion engine such that the catalyst effectively purifies exhaust gas emission. The air-fuel ratio is defined as a weight ratio of the air to the fuel contained in the air-fuel mixture for combustion. In the internal combustion engine requiring the aforementioned highly accurate control of the air-fuel ratio, an oxygen sensor for detecting a partial pressures of oxygen (hereinafter referred to as oxygen partial pressure) in exhaust gas is provided in an exhaust system of the engine. The air-fuel ratio obtained on the basis of the detected partial pressures of oxygen is used for executing feedback control of the air-fuel ratio.
By way of an example, a tubular oxygen sensor utilizing a solid electrolyte will be described.
FIG. 1
shows a conceptual structure of the tubular oxygen sensor (hereinafter referred to as an “oxygen sensor”). As shown in
FIG. 1
, the oxygen sensor includes a tubular detector that extends into an exhaust passage. An inside of the detector is exposed to the atmosphere whereas an outside thereof is exposed to the exhaust gas that flows into the detector through a sensor cover that surrounds the detector. The detector is made of a solid electrolyte and the inside and outside of the detector are covered with electrodes as shown in FIG.
1
. The solid electrolyte is a solid substance that allows movement of an ionized oxygen. For example, zirconia is typically used as the solid electrolyte for the oxygen sensor.
As shown in
FIG. 1
, a difference of the oxygen partial pressure is caused between the atmosphere (inside of the detector) and the exhaust gas (outside of the detector) which has been isolated via the detector. In this state, the oxygen in a high oxygen partial pressure side (normally the atmosphere side) is ionized and the resultant oxygen ion moves to a low oxygen partial pressure side (normally the exhaust gas side) so as to reduce the difference of the oxygen partial pressure. The oxygen molecule receives four electrons into an ionized state, and releases four electrons into an unionized state. In the course of the movement of the oxygen molecules, electrons move between the electrodes formed on the inside and outside of the detector, generating an electromotive force. Accordingly, the oxygen sensor generates a voltage corresponding to the difference in the oxygen partial pressure between the atmosphere and the exhaust gas.
The oxygen partial pressure of the exhaust gas varies with a change in the air-fuel ratio of a combusted air-fuel mixture. For example, when the air-fuel mixture is combusted at a stoichiometric ratio or in a fuel rich condition, most of the oxygen contained in the air-fuel mixture will be burnt out. As a result, the oxygen partial pressure of the exhaust gas becomes substantially 0. Conversely, when the air-fuel mixture is combusted in a fuel lean condition, an excess amount of oxygen is kept unburned. That is, the oxygen partial pressure of the exhaust gas becomes higher as the air-fuel ratio becomes leaner. Meanwhile, the oxygen partial pressure of the ambient air remains generally constant. Accordingly, the air-fuel ratio of the air-fuel mixture combusted in the internal combustion engine may be obtained on the basis of the output voltage of the oxygen sensor corresponding to the oxygen partial pressure relative to that of the atmosphere.
Various type of oxygen sensors may be employed as well as the aforementioned tubular oxygen sensor. The oxygen sensor including a strip-type detector, the oxygen sensor including a detector made of zirconia, or the like may be applied to the diagnosis system. The aforementioned oxygen sensor is constructed to detect an oxygen partial pressure of the exhaust gas in the same manner as described above. That is, the detector of the oxygen sensor outputs the detection signals in accordance with the difference of the oxygen partial pressure between the exhaust gas and the reference gas. Most oxygen sensors use the atmosphere as the reference gas relative to the exhaust gas in the same manner as in the oxygen sensor shown in FIG.
1
.
An internal combustion engine requiring an air-fuel ratio control for the purpose only of a stoichiometric combustion tends to employ an oxygen sensor having its output voltage considerably increased or decreased at a point of the stoichiometric air-fuel ratio. The aforementioned type of oxygen sensor may satisfy the requirement of the stoichiometric combustion in spite of a low resolving power that only indicates whether the air-fuel ratio corresponding to the detected output voltage is richer or leaner than the stoichiometric air-fuel ratio. On the contrary, the internal combustion engine in which the air-fuel mixture is combusted at a wider range of the air-fuel ratio, for example, a lean-burn combustion, the oxygen sensor is required to have a relatively higher resolving power that allows the output voltage to be linearly changed in accordance with the oxygen partial pressure of the exhaust gas.
When the detector of the aforementioned oxygen sensors has a fracture as shown in
FIG. 2A
, the exhaust gas flows into the detector such that the inside of the detector is filled with the exhaust gas. Accordingly the oxygen partial pressures in the inside and outside of the detector become equal. The oxygen sensor becomes incapable of generating the electromotive force.
When identifying an output pattern in which detection signals indicating no difference of the oxygen partial pressure between the inside and the outside of the detector are continuously generated by monitoring the output of the oxygen sensor, it is determined that the detector is fractured.
The time period taken from the start of the output of the detection signal indicating the lean air-fuel ratio (lean signal) compared with the stoichiometric air-fuel ratio to output the signal indicating rich the air-fuel ratio (rich signal) compared with the stoichiometric air-fuel ratio is constantly measured during the engine operation. It is determined that the detector is fractured when the measured time exceeds a predetermined time period.
The aforementioned diagnosis system, however, may not always detect the fracture of the detector accurately in the case as described below. In the internal combustion engine for a vehicle, a fuel-cut control for temporarily cutting the fuel injection to the internal combustion engine is frequently performed. During the fuel-cut operation, air is supplied to the exhaust passage. As a result, both the inside and the outside of the detector are filled with air. When the fuel injection resumes in the aforementioned state, the exhaust gas resulting from combustion of the fuel may flow into the exhaust passage.
As shown in
FIG. 2B
, when the detector is fractured in the aforementioned state, there is substantially no difference in the oxygen partial pressure between the inside and the outside of the detector even if the air-fuel ratio becomes rich as the air-fuel combustion resumes. As a result, the oxygen sensor continues outputting the lean signal. In this state, however, a certain period of time is taken for the exhaust gas reaching the outside of the detector to enter into the inside of the detector through the fracture. Therefore, in the state immediately after the stop of the fuel-cut operation,

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