Exhaust Purification System of Internal Combustion Engine

Abstract

An engine exhaust passage is provided with a fuel addition valve (14) for adding fuel in the form of atomized drops, an oxidation catalyst (11), and an NOx storage and reduction catalyst (12) in that order. The oxidation catalyst (11) carries as a precious metal not only platinum Pt, but also palladium Pd. A molar ratio of the platinum Pt with respect to the sum of the platinum Pt and palladium Pd carried on the oxidation catalyst (11) is set to between about 50 percent to about 80 percent.

Description

TECHNICAL FIELD [0001] The present invention relates to an exhaust purification system of an internal combustion engine. BACKGROUND ART [0002] Known in the art is an internal combustion engine provided with an NOx storage and reduction catalyst storing the NO contained in exhaust gas when the air-fuel ratio of the exhaust gas flowing into the engine exhaust passage is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. This NOx storage and reduction catalyst includes a precious metal catalyst comprised of platinum Pt and an NOx absorbent. When the air-fuel ratio is lean, the NOx contained in the exhaust gas, that is, the NO, is oxidized on the platinum Pt to NO2, then is stored in the form of nitrate ions NO3− in the NOx absorbent. [0003] On the other hand, when releasing and reducing the NOx absorbed from the NOx absorbent, the air-fuel ratio of the exhaust gas flowing into the NOx storage and red...

AI Technical Summary

Benefits of technology

[0009] The present invention provides a ratio of platinum Pt and palladium Pd enabling an excellent NOx release action to be secured even if the platinum Pt is covered by the fuel drops when the fuel is added in the form of drops so as to release NOx from the NOx storage and reduction catalyst.

Problems solved by technology

However, if the platinum Pt is covered by the fuel drops, the oxygen in the exhaust gas cannot reach the surface of the platinum Pt and as a result the oxidation reaction of the fuel drops on the platinum Pt can no longer be performed well.

If the oxidation reaction of the fuel drops cannot be performed well, the oxygen in the exhaust gas will not be sufficiently consumed and as a result the oxygen concentration in the exhaust gas will not sufficiently fall, so NOx will no longer be released well from the NOx absorbent.

Further, the fuel drops will insufficiently vaporize, so the amount of the gaseous state unburned HC contained in the exhaust gas will not be sufficient and therefore the released NOx will not be able to be sufficiently reduced.

In this case, if increasing the amount of palladium Pd and decreasing the amount of platinum Pt, the heat of the oxidation reaction of the oxygen stored in the palladium Pd will promote the vaporization of the fuel drops on the platinum Pt, but due to the small amount of platinum Pt, the release action of NOx will be weak and as a result an excellent NOx release action will not be able to be obtained.

As opposed to this, if reducing the amount of palladium Pd and increasing the amount of platinum Pt, the vaporization of the fuel drops on the platinum Pt will not be sufficiently promoted due to the heat of the oxidation reaction of the stored oxygen of the palladium Pd, so even if increasing the amount of platinum Pt, only a weak NOx release action will be obtained and, therefore, in this case as well, an excellent NOx release action will not be obtained.

However, with this molar ratio, the amount of palladium Pd is too great compared with the platinum Pt and as a result an excellent NOx release action cannot be obtained.

Images