Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
1999-10-15
2001-09-18
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S276000, C060S277000, C060S274000
Reexamination Certificate
active
06289673
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates technique of air-fuel ratio control for purification of exhaust gases from an engine.
For efficient and simultaneous purification of noxious emissions HC, CO and NOx, a three way catalyst calls for an atmosphere of a stoichiometric air-fuel ratio. A catalyst having a capability of oxygen storage can keep such a stoichiometric atmosphere of stoichiometric oxygen concentration by absorbing an excess of oxygen in an exhaust gas mixture flowing into the catalyst and releasing oxygen corresponding to an excess of reducing agents (HC, CO) in the exhaust gas mixture. When a lean exhaust gas mixture leaner than the stoichiometry flows into the catalyst, the catalyst absorbs an excess of oxygen instantly and maintains the stoichiometric atmosphere until the oxygen storage amount of the catalyst reaches saturation. When a rich exhaust gas mixture richer than the stoichiometry flows into the catalyst, the catalyst desorbs oxygen instantly to remedy the deficiency in oxygen and maintain the stoichiometric atmosphere until the stored oxygen is fully desorbed.
Thus, the oxygen storage type catalyst can hold its atmosphere at the stoichiometric state by compensating for any excess or deficiency of oxygen caused by temporary air-fuel ratio deviations. However, in the saturated state in which the oxygen storage amount reaches a saturation level or in the empty state in which the catalyst has no stored oxygen, the catalyst cannot efficiently purify HC, CO and NOx any more, so that the exhaust emission degrades.
Japanese Patent Kokai Publications No. H5(1993)-195842 and No. H7(1995)-259602 propose feedback control systems for controlling an oxygen storage amount of a catalyst to prevent degradation of exhaust emission.
SUMMARY OF THE INVENTION
Conventional assumption is that the oxygen storage amount of a catalyst reaches a greatest possible oxygen storage amount when a sensed downstream air fuel ratio on the downstream side of a catalyst turns lean. This is not always accurate, however.
As a result of experiments of the inventor of this application, it has been first found out that a catalyst absorbs oxygen even after a transition of the downstream air-fuel ratio to lean.
FIG. 2
shows the results (experimental results) of measurement of an upstream air-fuel ratio (F-A/F) on the upstream side of a catalyst, and a downstream air-fuel ratio (R-A/F) on the downstream side of the catalyst when the air-fuel ratio of an exhaust gas mixture is changed from a rich level of about 13 to a lean level of about 16. During an A period shown in
FIG. 2
, the catalyst absorbs oxygen at a fast rate. Therefore, excess oxygen is totally absorbed in the catalyst, and the downstream air-fuel ratio does not become lean (being held at the stoichiometric ratio) despite the upstream air-fuel ratio being lean. During a B period following the A period, the catalyst cannot absorb the whole of inflowing excess oxygen, and the downstream air-fuel ratio turns lean. Even in the B period during which the downstream air-fuel ratio is lean, the catalyst absorbs oxygen (or oxide such as NO) though its absorbing rate is slow. After the transition of the downstream air-fuel ratio to lean, the amount of oxygen absorbed at the slow absorbing rate (referred to as a slow reaction oxygen absorbing amount) is added to a maximum effective oxygen storage amount (a saturation amount of oxygen absorbed at a fast rate) which is an oxygen storage amount when the downstream air-fuel ratio turns lean. Thus, the oxygen storage amount increases beyond the maximum effective oxygen storage amount specifically in the case of fuel cutoff and lean clamp (the term “fuel cutoff” is hereinafter used to refer to both cases).
Disregard of the slow reaction oxygen absorbing amount in the B period as in a conventional system can cause errors when the fuel cutoff is canceled and the control is returned to an air-fuel ratio control to control the oxygen storage amount under the maximum effective oxygen storage amount.
It is therefore an object of the present invention to provide control devices and/or processes for accurately estimating and controlling an oxygen storage amount of a catalyst in consideration of oxygen absorption at a fast rate and a slow rate.
According to one aspect of the present invention, an air-fuel ratio control device for an engine, comprises a catalyst, a memory and a microprocessor.
The catalyst is disposed in an exhaust passage of the engine. The catalyst absorbs oxygen when an inflowing exhaust gas mixture flowing into the catalyst is excessive in oxygen as compared with a stoichiometric exhaust gas mixture of a stoichiometric air-fuel ratio, and releases oxygen stored in the catalyst when the inflowing exhaust gas mixture is deficient in oxygen as compared with the stoichiometric exhaust gas mixture.
The memory stores an oxygen storage capacity corresponding to an amount of oxygen stored in the catalyst when the air-fuel ratio of an outflowing exhaust gas mixture flowing out of the catalyst changes from a ratio substantially equal to the stoichiometric ratio to a lean air-fuel ratio.
The microprocessor is programmed to:
calculate a current oxygen storage amount based on an oxygen absorbing rate of the catalyst which is lower when the air-fuel ratio of the outflowing exhaust gas mixture is lean than when the air-fuel ratio of the outflowing exhaust gas mixture is substantially stoichiometric; and
control the air-fuel ratio of the inflowing exhaust gas mixture flowing into the catalyst, based on the current oxygen storage amount so as to make the current oxygen storage amount smaller than the oxygen storage capacity when a predetermined air-fuel ratio control condition is satisfied.
An air-fuel ratio control process according to one aspect of the present invention comprises: storing an oxygen storage capacity; calculating a current oxygen storage amount based on an oxygen absorbing rate of the catalyst; and controlling the air-fuel ratio of the inflowing exhaust gas mixture flowing into the catalyst, based on the current oxygen storage amount so as to make the current oxygen storage amount smaller than the oxygen storage capacity.
An air-fuel ratio control device according to another aspect of the present invention comprises: a catalyst; a first linear air-fuel ratio sensor sensing the air-fuel ratio of the inflowing exhaust gas mixture flowing into the catalyst in a wide air-fuel ratio range; a second linear air-fuel ratio sensor sensing the air-fuel ratio of an outflowing exhaust gas mixture flowing out of the catalyst in a wide air-fuel ratio range; a memory storing an oxygen storage capacity; and a microprocessor programmed to calculate a current oxygen storage amount based on a ratio difference between the air-fuel ratio sensed by the first linear air-fuel ratio sensor and the air-fuel ratio sensed by the second linear air-fuel ratio sensor both when the outflowing exhaust gas mixture is stoichiometric and when the outflowing exhaust gas mixture is lean, and to control the air-fuel ratio of the inflowing exhaust gas mixture flowing into the catalyst, based on the current oxygen storage.
An air-fuel ratio control device according to the present invention may comprises: first means for monitoring a sensed upstream air-fuel ratio on an upstream side of the catalyst, and a sensed downstream air-fuel ratio on a downstream side of the catalyst; second means for determining an oxygen absorbing rate in accordance with the sensed upstream and downstream air-fuel ratios in such a manner that the oxygen absorbing rate is equal to a lower value when the air-fuel ratio of the outflowing exhaust gas mixture is in a lean region, and equal to a higher value when the air-fuel ratio of the outflowing exhaust gas mixture is in a stoichiometric region, and for calculating a current oxygen storage amount in accordance with the oxygen absorbing rate; third means for determining an effective oxygen storage capacity from a value of the oxygen storage amount calculated at a transition of the sense
Kanetoshi Kazuhiko
Okada Keiji
Tayama Akira
Tsuchida Hirofumi
Denion Thomas
Foley & Lardner
Nissan Motor Co. Ltd
Tran Binh
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