Air/fuel ratio controller for internal combustion engine

Details Air/fuel ratio controller for internal combustion engine Air/fuel ratio controller for internal combustion engine

2002-09-26

2004-05-11

Tran, Binh (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

Having means analyzing composition of exhaust gas

C060S274000, C060S285000, C060S297000

Reexamination Certificate

active

06732503

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an air/fuel ratio controller for an internal combustion engine which makes use of a hydrocarbon-adsorbent exhaust purification catalyst.
2. Related Background Art
Nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), and other such substances in the exhaust gas of an internal combustion engine are purified with a three-way catalyst installed along the exhaust path. Four-way catalysts, which purifies particulate matters (PM) in addition to the above-mentioned substances, are also used with diesel engines. However, these catalysts only exhibit their purification performance at a specific activation temperature. Also, since there is a tendency for a large quantity of hydrocarbons to be emitted immediately after cold start-up, a hydrocarbon adsorbent having the property of adsorbing hydrocarbons is sometimes disposed along the exhaust path.
Hydrocarbon adsorbent adsorb hydrocarbons at low temperature, and the adsorbed hydrocarbons are then desorbed once the adsorbent reaches a certain temperature. In the desorption of hydrocarbons, the catalyst reaches its activation temperature and is able to purify the desorbed hydrocarbons. This process makes it possible to reduce the hydrocarbons released into the atmosphere after cold start-up. Hydrocarbon-adsorbent exhaust purification catalysts comprising a hydrocarbon adsorbent supported on a catalyst have also been put to practical use. Internal combustion engines featuring such hydrocarbon-adsorbent exhaust purification catalysts are known from their disclosure in Japanese Laid-Open Patent Application H11-82111 and elsewhere.
A hydrocarbon-adsorbent exhaust purification catalyst combines the properties of the above-mentioned hydrocarbon adsorbent and the properties of an exhaust purification catalyst. Even a hydrocarbon-adsorbent exhaust purification catalyst, though, does not reach its activation temperature immediately after the cold start-up, so the large quantity of hydrocarbons released after the cold start-up cannot be adequately purified. Therefore, a hydrocarbon-adsorbent exhaust purification catalyst first adsorbs hydrocarbons immediately after cold start-up and desorbs the hydrocarbons as its own temperature rises. Then it uses its own exhaust purification function to oxidize and purify the desorbed hydrocarbons. When the hydrocarbons are desorbed, the hydrocarbon-adsorbent exhaust purification catalyst reaches its activation temperature.
The apparatus disclosed in the above-mentioned publication has a start-up catalyst (exhaust purification catalyst) disposed upstream along an exhaust passage so that it will be quickly heated up to its activation temperature by the hot exhaust gas, and a hydrocarbon-adsorbent exhaust purification catalyst disposed downstream from the start-up catalyst. While hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, feedback control (lean control for facilitating hydrocarbon oxidation) is performed on the basis of the output of an air/fuel ratio sensor disposed downstream from the hydrocarbon-adsorbent exhaust purification catalyst so that it will be easier to oxidize the desorbed hydrocarbons. When no hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, the system switches over to feedback control on the basis of an air/fuel ratio sensor disposed upstream from the start-up catalyst, as in normal operation.
In the apparatus disclosed in the above publication, since feedback control is performed on the basis of the air/fuel ratio sensor disposed further downstream from the downstream hydrocarbon-adsorbent exhaust purification catalyst, it takes some time for changes in the load of the internal combustion engine, external disturbances (caused by purge gas, etc.), and so forth to be reflected in the output of the air/fuel ratio sensor. Because the feedback control is based on the output of this downstream air/fuel ratio sensor, the effect of the oxygen occlusion actions of the start-up catalyst and the hydrocarbon-adsorbent exhaust purification catalyst, for example, results in it taking longer to return to the target air/fuel ratio, which tends to delay the control. As a result, exhaust purification efficiency may suffer, and there is the danger that controllability with respect to external disturbances and so on will be poor.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an air/fuel ratio controller for an internal combustion engine, with which the air/fuel ratio can be optimized even while adsorbed hydrocarbons are being desorbed from a hydrocarbon-adsorbent exhaust purification catalyst, and deterioration of exhaust purification performance can be minimized.
The air/fuel ratio controller for an internal combustion engine of the present invention comprises a upstream catalyst disposed upstream along an exhaust passage, a hydrocarbon-adsorbent exhaust purification catalyst that is disposed downstream from the upstream catalyst and has the function of adsorbing hydrocarbons at low temperatures and releasing the adsorbed hydrocarbons as the temperature rises, upstream air/fuel ratio detection means disposed upstream from the upstream catalyst, for detecting the exhaust air/fuel ratio of exhaust gas flowing into the catalyst, and downstream air/fuel ratio detection means disposed downstream from the hydrocarbon-adsorbent exhaust purification catalyst, for detecting the exhaust air/fuel ratio of exhaust gas flowing out of the hydrocarbon-adsorbent exhaust purification catalyst. The present invention also comprises air/fuel ratio main feedback control means for performing feedback control such that the exhaust air/fuel ratio detected by the upstream air/fuel ratio detection means is kept at a specific main feedback target air/fuel ratio, and air/fuel ratio sub-feedback control means for performing sub-feedback control such that the exhaust air/fuel ratio detected by the downstream air/fuel ratio detection means is kept at a specific sub-feedback target air/fuel ratio while the adsorbed hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst (“during hydrocarbon desorption”).
Accordingly, with the present invention, air/fuel ratio main feedback control is performed on the basis of the detection result from the upstream air/fuel ratio detection means. And, during hydrocarbon desorption, air/fuel ratio sub-feedback control is performed on the basis of the detection result from the downstream air/fuel ratio detection means on the downstream side of the hydrocarbon-adsorbent exhaust purification catalyst. “Sub-feedback control” refers to feedback control performed subordinately to main feedback control, and main feedback control takes precedence in overall control. The sub-feedback control adds fine corrections to the main feedback control so that the object of feedback control will stay at the target value. The object of main feedback control here is the exhaust air/fuel ratio of the exhaust gas flowing into the upstream catalyst, while the object of the sub-feedback control during hydrocarbon desorption is the exhaust air/fuel ratio of the exhaust gas flowing out of the hydrocarbon-adsorbent exhaust purification catalyst.
During hydrocarbon desorption, that is, in a state in which desorbed hydrocarbons and the hydrocarbons contained in the injected fuel must be oxidized (purified), sub-feedback control is performed on the basis of the exhaust air/fuel ratio of the exhaust gas flowing out of the hydrocarbon-adsorbent exhaust purification catalyst, so precise exhaust purification can be achieved. Furthermore, main feedback control based on the exhaust air/fuel ratio of the exhaust gas flowing into the upstream catalyst is also performed at this time.
And, even though the sub-feedback control may take some time to feedback, the overall control of the air/fuel ratio will be carried out precisely by main feedback control. So there will be no deterioration in exhaust pur

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