Exhaust emission control device of internal combustion engine

Details Exhaust emission control device of internal combustion engine Exhaust emission control device of internal combustion engine

2001-07-24

2003-01-07

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...

C060S295000, C060S301000

Reexamination Certificate

active

06502391

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an exhaust emission purifying device capable of removing nitrogen oxides (NOx) from the exhaust emitted from an internal combustion engine capable of lean burning.
BACKGROUND ART
Recently, as a vehicle-mounted internal combustion engine, a lean-burn type internal combustion engine is being developed which is capable of burning an air-fuel mixture with excessive oxygen. Along with this, a technology for removing harmful gas components, in particular, nitrogen oxides (NOx), contained in the exhaust gas from a lean-burn type internal combustion engine is being developed.
As a technology for purifying the exhaust emitted from a lean-burn type internal combustion engine, a technology is known according to which an NOx absorbing material, such as an occlusion reduction type NOx catalyst, is provided in the exhaust passage of an internal combustion engine.
The NOx absorbing material absorbs the NOx in the exhaust when the air-fuel ratio of the exhaust flowing therethrough indicates excessive oxygen (that is, in the case of a lean air-fuel ratio), and releases the NOx it has absorbed when the oxygen concentration of the exhaust flowing therethrough is reduced. The occlusion reduction type NOx catalyst, which is an example of the NOx absorbing material, is a catalyst which absorbs the NOx in the exhaust when the air-fuel ratio of the exhaust flowing in is lean, and reduces the NOx to nitrogen (N
2
) while releasing the NOx it has absorbed when the oxygen concentration of the exhaust flowing in is reduced.
When the occlusion reduction type NOx catalyst is arranged in the exhaust passage of a lean-burn-type internal combustion engine, the NOx in the exhaust is absorbed by the occlusion reduction type NOx catalyst when the air-fuel ratio of the exhaust is lean, and the NOx which has been absorbed by the occlusion reduction type NOx catalyst is released as NO
2
when the air-fuel ratio of the exhaust is stoichiometric or rich, the NO
2
reacting with the reduction components in the exhaust, such as hydrocarbon (HC) and carbon monoxide (CO), to be thereby reduced to nitrogen (N
2
)
In some cases, the fuel of an internal combustion engine contains sulfur content. If such a fuel is burnt in an internal combustion engine, the sulfur content in the fuel is oxidized to generate sulfur oxides (SOx), such as SO
2
and SO
3
. The occlusion reduction type NOx catalyst absorbs the SOx in the exhaust on the same principle as the absorption of NOx, so that, when the occlusion reduction type NOx catalyst is arranged in the exhaust passage of an internal combustion engine, the occlusion reduction type NOx catalyst absorbs not only NOx but also SOx.
The SOx absorbed by the occlusion reduction type NOx catalyst forms a stable sulfate with passage of time, so that, in the same conditions for effecting release/reduction of NOx from the occlusion reduction type NOx catalyst, it is not easily decomposed or released and tends to be accumulated in the occlusion reduction type NOx catalyst. When the accumulation amount of SOx in the occlusion reduction type NOx catalyst increases, the NOx absorption capacity of the occlusion reduction type NOx catalyst decreases, and it becomes impossible to remove the NOx in the exhaust to a sufficient degree, that is, so-called SOx poisoning occurs.
To cope with this problem, there has conventionally been proposed an exhaust emission purifying device in which an SOx absorbing material for absorbing the SOx contained in the exhaust is provided in the exhaust passage on the upstream side of the occlusion reduction type NOx catalyst. When the air-fuel ratio of the exhaust flowing in is lean, the SOx absorbing material absorbs the SOx in the exhaust, and when the air-fuel ratio of the exhaust flowing in is stoichiometric or rich, it releases the SOx it has absorbed as SO
2
.
In this exhaust emission purifying device, the SOx in the exhaust is removed on the upstream side of the occlusion reduction type NOx catalyst, and it is possible to prevent the SOx poisoning of the occlusion reduction type NOx catalyst.
However, there is a limitation to the SOx absorption capacity of the SOx absorbing material, so that it is necessary to perform a processing for releasing the SOx absorbed by the SOx absorbing material, that is, a regeneration processing, before the SOx absorption capacity of the SOx absorbing material has been saturated.
An example of the SOx absorbing material regeneration technology is disclosed in Japanese Patent No. 2605580. According to this patent official gazette, to release the SOx absorbed by the SOx absorbing material, it is necessary for the air-fuel ratio of the exhaust flowing in to be stoichiometric or rich. Further, the higher the temperature of the SOx absorbing material, the easier it is for the SOx to be released.
In the exhaust emission purifying device disclosed in the above-mentioned official gazette, to prevent the SOx released from the SOx absorbing material from being absorbed by the occlusion reduction type NOx catalyst, there are provided an bypass path branching off from the exhaust pipe connecting the SOx absorbing material and the occlusion reduction type NOx catalyst and bypassing the occlusion reduction type NOx catalyst, and an exhaust switching valve for selectively switching the exhaust flow between the occlusion reduction type NOx catalyst and the bypass path. When executing the regeneration process of the SOx absorbing material, the exhaust switching valve is controlled so as to cause all the exhaust from the SOx absorbing material to flow through the bypass path.
Further, in the exhaust emission purifying device disclosed in the above-mentioned official gazette, when the regeneration process of the SOx absorbing material is not being performed, in other words, when the absorption or releasing of NOx is to be performed by the occlusion reduction type NOx catalyst, the exhaust switching valve is controlled so as to cause all the exhaust to flow through the occlusion reduction type NOx catalyst.
As is known in the art, the sealing property of the exhaust switching valve used in the above exhaust emission purifying device cannot be regarded as perfect, and the valve allows leakage of approximately 1 to 10% of the exhaust. Thus, in the exhaust emission purifying device disclosed in the above official gazette, if the exhaust switching valve is controlled so as to allow the exhaust to flow through the occlusion reduction type catalyst and as to prevent the exhaust from flowing into the bypass path, some exhaust is allowed to leak through the exhaust switching valve to the bypass path, with the result that the NOx contained in the exhaust leaking through the exhaust switching valve to the bypass path is released to the atmosphere without being removed from the exhaust.
As a result of the recent progress in the catalyst technology, the NOx purifying ratio by the occlusion reduction type NOx catalyst is over 90%. Thus, the deterioration in exhaust emission control due to the leakage through the exhaust switching valve cannot be neglected.
Further, the above-described conventional exhaust emission purifying device for an internal combustion engine is not provided with a means for reducing the hydrocarbon (HC) in the exhaust when the internal combustion engine is started when the ambient temperature is low (that is, at the time of low-temperature startup), so that there is the danger of the hydrocarbon (HC) in the exhaust being released to the atmosphere without being purified from the exhaust. Thus, the conventional exhaust emission purifying device leaves room for improvement.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of the above various problems. It is an object of the present invention to prevent a deterioration in exhaust emission purifying due to leakage of the exhaust into the bypass path when the bypass path is closed by the exhaust flow switching means.
Another object of the present invention is to reduce the hydrocarbon concentration in the exhaust when the inter

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