Exhaust purification system of internal combustion engine

Abstract

The exhaust purification system of an internal combustion engine is provided with an NOX selective reduction catalyst which is arranged in an engine exhaust passage and which has the function of absorbing NOX which is contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean, releasing the absorbed NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, and selectively reducing the NOX and with a fuel addition valve which feeds fuel to the NOX selective reduction catalyst. In the case of the region near the stoichiometric air-fuel ratio in the region where the air-fuel ratio of the exhaust gas flowing into the NOX selective reduction catalyst is lean, the fuel addition valve is used to feed fuel to the NOX selective reduction catalyst to selectively reduce the NOX.

Description

TECHNICAL FIELD[0001]The present invention relates to an exhaust purification system of an internal combustion engine.BACKGROUND ART[0002]The exhaust gas of a diesel engine, gasoline engine, or other internal combustion engine, for example, contains carbon monoxide (CO), unburned fuel (BC), nitrogen oxides (NOX), particulate matter (PM), or other ingredients. The internal combustion engine has an exhaust purification system attached to it to purify these ingredients.[0003]Japanese Patent No 278304 discloses an exhaust purification system provided with a device for removing nitrogen oxides constituted by an NOX absorption and release material which absorbs NOX when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NOX when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio or less. In this system, it is disclosed, at the time of releasing and treating the NOX, to make the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio...

AI Technical Summary

Benefits of technology

[0008]The present invention has as its object to provide an exhaust purification system of an internal combustion engine which suppresses a fall in purification ability of nitrogen oxides.

[0009]The first exhaust purification system of an internal combustion engine of the present invention is provided with an NOX reduction catalyst which is arranged in an engine exhaust passage and which has the function of absorbing NOX which is contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean, releasing the absorbed NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, and selectively reducing the NOX and with a reducing agent feed device which feeds a reducing agent to the NOX reduction catalyst. In the case of the region near the stoichiometric air-fuel ratio in the region where the air-fuel ratio of the exhaust gas flowing into the NOX reduction catalyst is lean, the reducing agent feed device is used to feed a reducing agent to the NC reduction catalyst to selectively reduce the NOX. Due to this configuration, ins the region near the stoichiometric air-fuel ratio, it is possible to suppress a drop in the purification ability of nitrogen oxides.

[0010]In this invention, a three-way catalyst is provided arranged downstream of the NOX reduction catalyst in the engine exhaust passage, and the reducing agent feed device includes a fuel addition valve which feeds fuel to the engine exhaust passage at the upstream side of the NOX reduction catalyst. If increasing the amount of fuel which is injected at the combustion chambers of the engine body so as to lower the air-fuel ratio of the exhaust gas flowing into the three-way catalyst to the stoichiometric air-fuel ratio or less, it is possible to feed fuel from the fuel addition valve to selectively reduce the NOX at the NOX reduction catalyst when the air-fuel ratio of the exhaust gas flowing into the NOX reduction catalyst is inside the region near the stoichiometric air-fuel ratio.

[0011]The second exhaust purification system of an internal combustion engine of the present invention is provided with an NOX reduction catalyst which is arranged in an engine exhaust passage and which has the function of absorbing NOX which is contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean, releasing the absorbed NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, and selectively reducing the NOX, a three-way catalyst which is arranged downstream of the NOX reduction catalyst, a reducing agent feed device which feeds a reducing agent to the NOX reduction catalyst, and an air-fuel ratio reducing device which reduces the air-fuel ratio of the exhaust gas flowing into the three-way catalyst. In the operating region of the internal combustion engine, there is a specific operating region in which the air-fuel ratio of the exhaust gas flowing into the NOX reduction catalyst is lean, at the NOX reduction catalyst, the NOX purification rate due to the selective reduction gradually decreases along with a temperature rise and, at the three-way catalyst, the NOX purification rate gradually increases along with a temperature rise. In this specific operating region, when the NOX purification rate of the NOX reduction catalyst becomes smaller than a predetermined judgment value, the reducing agent feed device feeds the reducing agent to the NOX reduction catalyst for selective reduction of NOX, and the air-fuel ratio reducing device makes the air-fuel ratio of the exhaust gas flowing into the three-way catalyst the stoichiometric air-fuel ratio or rich for reduction of NOX. Due to this constitution, it is possible to suppress a drop in the purification ability of nitrogen oxides.

Problems solved by technology

In this way, there was the problem that the selective reduction catalyst becomes smaller in purification rate of NOX in a predetermined operating region.

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