Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2001-08-20
2004-04-27
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, C060S286000
Reexamination Certificate
active
06725650
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to exhaust gas purifying systems for engines, which are provided with a catalyst displaying a three-way purification function when at least the exhaust gas is in a state corresponding substantially to a theoretical air/fuel ratio, and in which the engine is ordinarily switched between an operating state in which the air/fuel ratio in the combustion chamber is lean and an operating state in which it is substantially at the theoretical air/fuel ratio or richer than that.
Conventionally, as exhaust gas purifying systems for this kind of engine, systems are known, in which a so-called three-way catalyst is arranged in the exhaust duct of the engine, and an NOx absorbing/reducing catalyst including an NOx absorbing material is arranged nearby on the downstream side, as disclosed for example in Japanese Laid-Open Patent Publication (Tokkai) No. 11-200853. For the NOx absorbing material, it is possible to use, for example, an alkaline earth metal such as barium, and when the air/fuel ratio of the exhaust gas is in a predetermined lean state, that is, in an oxygen-rich atmosphere with an oxygen concentration of, for example, at least 4% in the exhaust gas, the NOx in the exhaust gas is oxidized, and absorbed as nitrate, whereas when the oxygen concentration drops, the absorbed nitrate undergoes a substitution reaction with the CO in the exhaust gas, and NOx is released, while the CO is absorbed as carbonate.
When the air/fuel ratio of the exhaust gas is in the lean state, the NOx in the exhaust gas is absorbed by the above-mentioned NOx absorbing material, and the exhaust gas is purified. Furthermore, when the engine is operated substantially at the theoretical air/fuel ratio, and the air/fuel ratio of the exhaust gas is in a state corresponding substantially to the theoretical air/fuel ratio, that is, when the oxygen concentration is about 0.5 to 1% or lower, almost all of the HC, CO and NOx in the exhaust gas is purged due to the three-way purification function of the catalyst, and the NOx released from the NOx absorbing material reacts with the HC and CO, and reduction purification is performed.
Moreover, the NOx absorbing material has the property that, as the amount of absorbed NOx increases, the ability of the NOx absorbing material to absorb NOx decreases, so that in the above conventional example, when the engine shifts from an operating state with a lean air/fuel ratio to an operating state at substantially the theoretical air/fuel ratio, the air/fuel ratio is enriched considerably for a short time, that is, in spike-form, thus promoting the release of NOx.
Like the NOx absorbing/reducing catalyst, the so-called three-way catalyst, which displays a three-way purification function when the air/fuel ratio of the exhaust gas is in a state corresponding substantially to the theoretical air/fuel ratio, generally includes an oxygen absorbing material, such as ceria (CeO
2
). This oxygen absorbing material has the property that it absorbs oxygen in the exhaust gas when the oxygen concentration is larger than a certain level (for example 0.5%), and releases oxygen when the oxygen concentration is lower than that value, so that the variations of the oxygen concentration in the exhaust gas are dampened, and the three-way purification function can be increased.
However, when a three-way catalyst and an NOx absorbing/reducing catalyst are arranged in series in the exhaust duct of the engine, as in the above conventional example, and the air/fuel ratio of the exhaust gas is changed from a lean state to a state corresponding substantially to the theoretical air/fuel ratio, then oxygen is released from the oxygen absorbing material included in the catalysts, so that the air/fuel ratio shifts locally toward the lean side in the vicinity of the two catalysts, and the HC and CO in the exhaust gas are used up by reacting with the oxygen, and eventually it becomes impossible to release NOx efficiently from the NOx absorbing/reducing catalyst located downstream.
Regarding this aspect, the above conventional example estimates the amount of oxygen released by the three-way catalyst by considering the operation history of the engine and the oxygen absorbing capability in the three-way catalyst, anticipates that HC or CO are consumed by reacting with the oxygen, and enriches the air/fuel ratio in the combustion chamber considerably so that a sufficient amount of HC and Co are supplied to the downstream NOx absorbing/reducing catalyst.
However, in the above-described conventional exhaust gas purifying system, when the air/fuel ratio in the combustion chamber of the engine is switched from the lean state to the state of substantially the theoretical air/fuel ratio, the amount of injected fuel increases spike-like for a short time, enriching the air/fuel ratio considerably, so that as described above, even when increasing the HC and CO in the exhaust gas to match the amount of oxygen released by the catalyst, in actuality, a lot of the HC and CO are blown through the catalyst and are emitted into the atmosphere.
Thus, the HC and CO emitted into the atmosphere increases sharply, even if only temporarily, and the condition of the exhaust gas deteriorates. Moreover, the oxygen released by the catalysts may not react sufficiently with the HC and CO, whereby in the vicinity of the catalysts, the air/fuel ratio of the exhaust gas shifts to the leaner side, leaving the appropriate air/fuel ratio range for displaying the three-way purification function, and there is the risk that the catalysts' property of purifying the exhaust gas drops.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an exhaust gas purifying system for an engine, in which a catalyst having a three-way purification function is provided in an exhaust duct, and in which the engine is usually switched between an operating state of lean air/fuel ratio and an operating state with an air/fuel ratio of substantially the theoretical air/fuel ratio or richer than that, wherein a temporary deterioration of the state of the exhaust gas caused by emission of oxygen from the catalyst can be prevented, while suppressing deterioration of the total fuel efficiency when considered for all operating regions.
SUMMARY OF THE INVENTION
In order to attain these objects, when the engine has shifted from an operating state with lean air/fuel ratio to an operating state with richer air/fuel ratio, the control target value of the air/fuel ratio is corrected to be slightly richer than the theoretical air/fuel ratio, so as to match the resulting enrichment of the air/fuel ratio with the pace in which oxygen is released from the catalyst.
More specifically, in accordance with a first aspect of the present invention, an exhaust gas purifying system for an engine includes a catalyst displaying a three-way purification function when an air/fuel ratio state of at least an exhaust gas corresponds substantially to a theoretical air/fuel ratio; a first air/fuel ratio control means for controlling an air/fuel ratio in a combustion chamber of the engine by switching to either a state that is leaner than the theoretical air/fuel ratio or a state that is richer than that, depending on the operating state of the engine; and an air/fuel ratio correction means for correcting the air/fuel ratio in the combustion chamber temporarily to a state that is even richer, when the first air/fuel ratio control means switches the air/fuel ratio in the combustion chamber from a lean state to a richer state. When the air/fuel ratio in the combustion chamber has been switched from a lean state to a richer state, the air/fuel ratio correction means sets the air/fuel ratio in the combustion chamber to a state that is slightly richer than the theoretical air/fuel ratio, so as to match a release of oxygen from the catalyst.
With this configuration, first, when the engine is in an operating state with lean air/fuel ratio, the exhaust gas from the engine is in a state of high oxygen concentration, so
Denion Thomas
Mazda Motor Corporation
Nguyen Tu M.
Nixon & Peabody LLP
Studebaker Donald R.
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