Exhaust gas purification system for internal combustion engine

Details Exhaust gas purification system for internal combustion engine Exhaust gas purification system for internal combustion engine

2002-02-05

2003-09-09

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...

C060S278000, C060S280000, C060S289000, C060S290000, C060S297000

Reexamination Certificate

active

06615579

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine, more specifically to an exhaust gas purification system for an internal combustion engine comprising an NOx (Nitrogen Oxide) adsorber catalyst which absorbs NOx when exhaust gas inflow is lean in its air fuel ratio, and releases NOx when the concentration of oxygen of exhaust gas inflow becomes low.
2. Description of Related Art
It has been conventionally known, to lessen the content of NOx in exhaust gas discharged from an internal combustion engine (e.g., diesel engine), an exhaust gas purification system is introduced in connection with the internal combustion engine.
The exhaust gas purification system comprises a NOx adsorber catalyst which absorbs NOx when exhaust gas inflow is lean in its air fuel ratio, and releases NOx when the concentration of oxygen of exhaust gas inflow becomes low, the NOx adsorber catalyst being placed in an exhaust passage from the internal combustion engine.
The exhaust gas purification system includes one proposed in the Japanese Patent Publication No. 2586739.
This exhaust gas purification system comprises an NOx adsorber catalyst as above; an NOx mass estimating means for estimating the mass of NOx absorbed by the NOx adsorber catalyst; and a release means which, whenever the mass of NOx the estimating means estimates to be absorbed by the NOx adsorber catalyst exceeds a predetermined value, causes the NOx adsorber catalyst to release NOx. With this system, the release means causes the NOx adsorber catalyst to release NOx by lowering the oxygen concentration of exhaust gas inflow to be in contact with the NOx adsorber catalyst after having increased the supply of fuel to the cylinder of the internal combustion engine so as to make the engine run nearly on stoichiometric air fuel ratio.
According to the exhaust gas purification system as described above, whenever the NOx mass the NOx mass estimating means estimates to be absorbed by the NOx adsorber catalyst exceeds a predetermined value, the release means, regardless of the current load of the internal combustion engine, causes the NOx adsorber means to release NOx by increasing the fuel supply to the cylinder.
In other words, the release means increases the fuel supply to the cylinder as long as the same criterion is met, regardless of whether the engine works at a high load range or at a low load range.
However, when the internal combustion engine works at a high load range, the pressure within the cylinder (cylinder pressure) is high. Thus, if the amount of fuel supplied to the cylinder which is in such a sate as above is increased, the temperature within the cylinder will become considerably high. Because of this, if such an episodic increase of fuel supply to the cylinder is frequently repeated while the internal combustion engine runs in a high load range, it will encroach on the durability of the engine itself.
Particularly, if the internal combustion engine is equipped with a super charger, the amount of air supplied to the cylinder becomes so high that the temperature within the cylinder will be elevated which may further affect the durability of the engine. Moreover, then, the amount of air discharged from the cylinder will also become so high that, to keep the air fuel ratio close to stoichiometric value, a large amount of fuel must be supplied to the cylinder, which will lower the engine's rate of fuel consumption, and worsen the driving cost.
SUMMARY OF THE INVENTION
This invention has following features, in order to provide an exhaust gas purification system for an internal combustion engine by which it is possible not only to reduce the NOx content of exhaust gas, but also to restrict aggravation of the durability and the rate of fuel consumption of the internal combustion engine to a minimum limit.
In one aspect, this invention provides an exhaust gas purification system for an internal combustion engine being equipped with an NOx adsorber catalyst which absorbs NOx when an exhaust gas lean in the air fuel ratio flows in, and releases NOx when the oxygen concentration of exhaust gas inflow becomes low comprising an NOx mass estimating means for estimating the mass of NOx depositing in the NOx adsorber catalyst; a first air fuel ratio controlling means which, whenever the NOx deposition estimated by the NOx mass estimating means reaches a predetermined value (reference NOx level), causes the NOx adsorber catalyst to release NOx by altering the air fuel ratio of exhaust gas coming in contact with the NOx adsorber catalyst to a value close to stoichiometric one; and a reference NOx level varying means which varies the reference NOx level according to the load and engine speed (rate of revolution) of the internal combustion engine.
According to this invention, the first air fuel ratio controlling means changes the air fuel ratio of exhaust gas from the internal combustion engine to a value close to stoichiometric one, and lowers the concentration of oxygen of an exhaust gas coming in contact with the NOx adsorber catalyst, thereby causing the latter to release NOx. Specifically, the means in question causes the catalyst to release NOx deposited thereupon as N
2
which contributes to the lowered concentration of NOx of exhaust gas.
The first air fuel ratio controlling means, whenever the NOx deposition (the amount of NOx accumulated in the NOx adsorber catalyst) estimated by the NOx mass estimating means reaches a predetermined value (reference NOx level), alters the air fuel ratio of an exhaust gas coming in contact with the NOx adsorber catalyst to a value close to stoichiometric one, and the reference NOx level is also varied by the reference NOx level varying means in accordance with the load of the internal combustion engine, or with the load and engine speed of the internal combustion engine. If, for example, the engine runs with a high load, or with a high load at a high engine speed, that is, the amounts of fuel and air supplied to a cylinder of internal combustion engine are high, the reference NOx level may be set high. Then, it will be possible to lessen the number of occasions in which the air fuel ratio of exhaust gas must be altered to a value close to stoichiometric one which would be otherwise required when the internal combustion engine works at a high load range. This will restrict aggravation of the rate of fuel consumption and durability of the internal combustion engine to a minimum limit.
With this invention, the reference NOx level setting means may preferably set the reference NOx level close to the maximum attainable level of NOx deposition for the NOx adsorber catalyst when the engine runs at a high speed in a high load range, while it may preferably set the reference NOx level at a level lower than the maximum attainable level of NOx deposition for the NOx adsorber catalyst when the engine runs at a low speed in a low load range.
According to this invention, when the internal combustion engine runs in a high load range where the engine would be impaired in its durability and rate of fuel consumption if recourse to the driving at about stoichiometric air fuel ratio were frequently introduced, the reference NOx level setting means sets the reference NOx level close to the maximum attainable level of NOx deposition for the NOx adsorber catalyst, and thus can reduce to a least possible limit such frequent recourse to the driving at about stoichiometric air fuel ratio, thereby restricting aggravation of the durability and rate of fuel consumption of the internal combustion engine to a minimum limit.
In contrast, when the engine runs in a low load range, the means in question sets the reference NOx level below the maximum attainable level of NOx deposition for the NOx adsorber catalyst, and thus allows the NOx adsorber catalyst to keep a room to accept the deposition of new NOx. Through this arrangement, the need for the immediate alteration of the reference NOx level to stoichiometric value can be e

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