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

2002-03-15

2003-09-16

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...

C060S285000, C060S295000, C060S301000

Reexamination Certificate

active

06619035

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an exhaust emission control device for cleaning exhaust gas discharged from an internal combustion engine capable of lean burn.
BACKGROUND ART
Examples of an exhaust emission control device for cleaning exhaust gas discharged from an internal combustion engine capable of lean burn, such as a diesel engine or a lean burn gasoline engine, include NOx catalysts, such as a selective reduction type NOx catalyst and an occlusion reduction type NOx catalyst.
The selective reduction type NOx catalyst is a catalyst which reduces or decomposes NOx in an atmosphere with excess oxygen in the presence of hydrocarbon (HC). To clean NOx with the selective reduction type NOx catalyst, an appropriate amount of HC component (hereinafter referred to as reducing agent) is required.
When using the selective reduction type NOx catalyst for the exhaust emission control of an internal combustion engine as mentioned above, the amount of HC component in the exhaust gas during normal operation of the internal combustion engine is very small, so that in order to clean NOx during normal operation, it is necessary to supply as a reducing agent, for example, light oil serving as fuel to the selective reduction type NOx catalyst.
On the other hand, the occlusion reduction type NOx catalyst absorbs NOx when the air fuel ratio of the inflow exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration of the inflow exhaust gas is reduced, effecting reduction to N
2
.
When using the occlusion reduction type NOx catalyst for exhaust emission control of the internal combustion engine, the NOx in the exhaust gas will be absorbed by the NOx catalyst since the air fuel ratio of the exhaust gas during normal operation of the internal combustion engine is lean. However, when exhaust gas of lean air fuel ratio is continued to be supplied to the NOx catalyst, the NOx absorbing capacity of the NOx catalyst attains saturation, and no further NOx can be absorbed, with the result that the NOx in the exhaust gas is allowed to leak.
In view of the above, in the occlusion reduction type NOx catalyst, it is necessary to reduce the oxygen concentration by making the air fuel ratio of the inflow exhaust gas rich with a predetermined timing before the NOx absorbing capacity attains saturation, and to release the NOx absorbed by the NOx catalyst to reduce it to N
2
, thereby recovering the NOx absorbing capacity of the NOx catalyst. In the following, this operation of temporarily making the air fuel ratio of the inflow exhaust gas rich will be referred to as rich spike.
On the other hand, to recover the NOx absorbing capacity of the NOx catalyst, it is necessary to appropriately enrich the air fuel ratio of exhaust. Conventionally, the pattern in which fuel as reducing agent is added and the target fuel addition pressure are set on the basis of a map indicating the relationship between the internal combustion engine RPM and fuel injection amount obtained through experiment.
However, when supplying reducing agent at the exhaust port of an internal combustion engine, the exhaust port and the NOx catalyst are usually spaced apart from each other, and the reducing agent is not easily carried by the exhaust flow depending upon the operating condition of the internal combustion engine, resulting in a rather poor efficiency in the supply of reducing agent. Specifically, in the range where the internal combustion engine operates at low speed and with small load, the exhaust velocity is low, and the exhaust gas temperature is low, so that a part of the reducing agent added adheres to the wall surface in the exhaust passage, with the result that the degree of rich spike at the exhaust port differs from the degree of rich spike at the NOx catalyst. In other words, even if the rich spike on the exhaust port side attains the target air fuel ratio, the amount of reducing agent reaching the NOx catalyst decreases due to the adhesion to the wall surface on the NOx catalyst side, resulting in a decrease in the degree of richness. Further, the rich spike, effected instantaneously, involves a delay in reaction on the NOx catalyst side, with the result that the rich period is extended and the degree of richness becomes lower than the target air fuel ratio.
For example, even if the degree of rich spike at the exhaust port is approximately the one as shown in FIG.
6
(A), the degree of richness is lowered with the NOx catalyst as shown in FIG.
6
(B). Thus, the target air fuel ratio cannot be attained with the NOx catalyst, and the release and reduction of NOx cannot be effected to a sufficient degree. Then, in some cases, the NOx absorbing capacity of the NOx catalyst attains saturation and cannot be recovered, resulting in leakage of NOx in the exhaust.
The present invention has been made in view of the above problems. It is an object of the present invention to provide an exhaust emission control device for an internal combustion engine capable of supplying an appropriate amount of reducing agent to NOx catalyst even if the operating condition of the internal combustion engine changes.
DISCLOSURE OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine, characterized by comprising:
an NOx catalyst provided in an exhaust passage of an internal combustion engine capable of lean burn and adapted to release absorbed NOx by a reducing agent for reduction;
a reducing agent supplying means provided in the exhaust passage on the upstream side of the NOx catalyst;
a load detecting means for detecting the load of the internal combustion engine; and
a reducing agent addition control means for controlling reducing agent addition period and reducing agent addition interval on the basis of the load of the internal combustion engine.
In accordance with the present invention, it is desirable to further provide an addition determining means for determining as to whether reducing agent is to be added or not according to the operating condition of the vehicle, the addition amount of reducing agent and the reducing agent addition interval being controlled on the basis of the load detected by the load detecting means when it is determined by the addition determining means that addition is possible.
It is desirable that a judgment be made by the addition determining means as to whether the NOx catalyst is at an activation temperature or not, whether the operation range of the internal combustion engine is within the range where addition of reducing agent is possible or not, and the like, reducing agent being supplied only when release and reduction of NOx is possible, thereby preventing the reducing agent from passing through the NOx catalyst.
Further, when the load of the internal combustion engine is small, it is possible to increase the amount of reducing agent added and elongate the reducing agent addition interval as compared to the case in which the load of the internal combustion engine is large.
Further, the present invention relates to an exhaust emission control device for an internal combustion engine, characterized by comprising:
an NOx catalyst provided in an exhaust passage of an internal combustion engine capable of lean burn and adapted to release absorbed NOx by a reducing agent for reduction;
a reducing agent supplying means provided in the exhaust passage on the upstream side of the NOx catalyst;
an operating condition detecting means for detecting the load and an RPM of the internal combustion engine; and
a reducing agent addition control means for controlling reducing agent addition period and reducing agent addition interval on the basis of the detected load and the RPM of the internal combustion engine.
In the present invention, when the load and the RPM of the internal combustion engine are low, the velocity of the exhaust is low, and the reducing agent is not easily carried by the exhaust flow, so that the addition period and addition interval for the reducing agent added are controlled according to the engine load and

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