1. Field of the Invention
The present invention generally relates to an apparatus for detecting deterioration of a three way catalytic converter (hereinafter also referred to simply as the catalyst) disposed in an exhaust pipe of an internal combustion engine system on the basis of air-fuel ratios detected at locations upstream and downstream, respectively, of the catalytic converter within the exhaust pipe. More particularly, the present invention is concerned with the catalyst deterioration detecting apparatus for an internal combustion engine which apparatus is improved such that erroneous detection of deteriorated state of the catalyst ascribable to variations of the operation state thereof such as variations of the environmental conditions can be suppressed and thus high reliability can be ensured for the detection or decision as to deterioration of the catalyst. Furthermore, the present invention is concerned with a catalyst deterioration detecting method which can ensure significantly enhanced reliability and a recording medium which stores the deterioration detecting method in the form of a program or programs executable with a computer such as a microcomputer.
2. Description of Related Art
In general, in the fuel injection control system for the internal combustion engine of a motor vehicle or car, it is known to control the fuel injection quantity on the basis of a basic fuel injection quantity which is arithmetically determined in dependence on operation state parameters of the engine such as intake-air quantity, engine rotation number (rpm) and/or the like and which is correctively modified on the basis of detection signal(s) outputted from at least one air-fuel ratio sensor such as an O.sub.2 -sensor disposed in the exhaust pipe for detecting concentration of oxygen contained in the exhaust gas so that the air-fuel ratio can ultimately converge to a desired or stoichiometric value.
By controlling the fuel injection quantity such that the stoichiometric air-fuel ratio can be realized in operation of the internal combustion engine, the purifying capability of the three way catalytic converter can be sustained at a high level and thus CO (carbon monoxide), NO.sub.x (nitrogen oxides) and HC (hydrocarbon) can be converted to harmless components through oxidation-reduction reaction.
Further, there have heretofore been proposed and developed for practical application a variety of schemes for detecting deterioration of the catalyst or deteriorated state by making use of the detection signals outputted from a pair of air-fuel ratio sensors disposed within the exhaust pipe at locations upstream and downstream, respectively, of the three way catalytic converter, as is described, for example, in Japanese Unexamined Patent Application Publication No. 98949/1993 (now matured to Japanese Patent No. 2611070).
For better understanding of the present invention, description will first be made of the conventional catalyst deterioration detecting apparatus known heretofore.
FIG. 6 is a block diagram showing generally an arrangement of a conventional catalyst deterioration detecting apparatus for an internal combustion engine which is designed to detect deterioration of the catalyst on the basis of air-fuel ratio detection signals obtained at upstream and downstream sides of the catalyst, respectively.
Referring to the figure, an engine 1 which constitutes a main part of the internal combustion engine system is provided with an intake pipe 2 and an exhaust pipe 3 in an arrangement known per se in the art.
The intake pipe 2 is provided with a fuel injector (not shown) for charging an air-fuel mixture A containing the fuel as injected into the engine 1 from the intake pipe 2. The exhaust gas B resulting from combustion of the air-fuel mixture in the engine 1 is discharged through the exhaust pipe 3.
A three way catalytic converter 4 for purifying the exhaust gas B is disposed in the exhaust pipe 3. Ordinarily, the catalytic converter 4 is disposed at a bottom plane of the car body together with the exhaust pipe 3.
Installed on the exhaust pipe 3 are a pair of air-fuel ratio sensors 5 and 6 at positions upstream and downstream of the catalytic converter 4, respectively. Detection signals outputted from the air-fuel ratio sensors 5 and 6 are designated by reference characters D1 and D2, respectively.
For the purpose of monitoring or detecting operation states of the engine 1, various types of sensors denoted collectively by reference numeral 7 are provided. As the sensors 7, there may be mentioned typically a crank angle sensor, an engine rotation number sensor, a temperature sensor, a throttle opening degree sensor and an intake-air quantity sensor all of which are known in the art. Thus, information indicative of operation states of the engine 1 can be obtained on the basis of the output signals of the various sensors mentioned above.
An ECU (Electronic Control Unit) 10 which may be constituted by a microcomputer is composed of a deterioration parameter arithmetic means 11, a decision reference value setting means 12 and a catalyst deterioration decision means 13 in addition to a fuel injection control means and an ignition control means (not shown) which are known in the art. The ECU 10 receives as inputs thereto the air-fuel ratio detection signals D1 and D2 and the detection information derived from the outputs of the various sensors 7. Parenthetically, the various means mentioned just above can be realized in the form of program(s) which is executable with a computer.
The deterioration parameter arithmetic means 11 is so designed or programmed as to arithmetically determine a deterioration parameter P for making decision as to degree of deterioration of the catalyst on the basis of the result of comparison of the air-fuel ratio detection signals D1 and D2.
More specifically, the deterioration parameter arithmetic means 11 may be designed to determine arithmetically at least one of the number of polarity inversions and the waveform areas (integral values) of the detection signals D1 and D2 as the basic values, respectively, and then determine arithmetically the ratio between the basic values as the deterioration parameter P (arithmetically determined value) indicating the degree of deterioration of the catalyst.
The decision reference value setting means 12 is so designed or programmed as to set a decision reference value a serving as the comparison reference value for the deterioration parameter P.
The catalyst deterioration decision means 13 is designed or programmed to compare the deterioration parameter P with the decision reference value a to thereby generate an abnormality signal E indicative of deteriorated state of the catalyst when the condition that the deterioration parameter P is greater than the decision reference value .alpha., i.e., P&gt;.alpha., is satisfied.
In response to the abnormality signal E, an alarm device (not shown) is actuated to inform the operator or driver of the deteriorated state of the catalyst to prompt him or her exchange of the catalytic converter 4.
Parenthetically, the catalyst deterioration decision means 13 is designed to compare the deterioration parameter P with the reference value .alpha. to thereby make decision as to the deteriorated state of the catalyst 4 every time the detection signals D1 and D2 are inputted to the ECU 10.
At this juncture, it should be mentioned that the purifying function or capability of the catalyst is likely to be affected by variation of environmental or operating condition and may temporarily be degraded when the temperature lowers. As mentioned previously, the catalyst 4 is disposed on the bottom plane of the car body. Accordingly, the catalytic converter 4 may be sputtered with water from the puddles on the roads in the course of running the motor vehicle or car to be thereby cooled temporarily. Under the circumstances, there arises such possibility that the catalyst deterioration decision means 13 may make erroneous decision as to the deteriorated state or level of the catalyst due to unexpected factors such as temperature lowering as encountered during operation of the internal combustion engine.
As will now be appreciated from the foregoing, the conventional catalyst deterioration detecting apparatus for the internal combustion engine suffers a problem that the deteriorated state of the catalyst is likely to be erroneously determined when the function of the catalyst becomes temporarily lowered due to e.g. temporal temperature lowering mentioned above, since the deterioration decision processing is executed every time the deterioration parameter is arithmetically determined.