EXHAUST GAS PURIFICATION SYSTEM FOR PURIFYING EXHAUST GASES OF INTERNAL COMBUSTION ENGINES

Abstract

The present invention is directed to the purification of exhaust gases of an internal combustion engine operated predominantly with a stoichiometric fuel mixture. The exhaust system has in particular 4 purification functions in a particular order. A three-way catalyst (TWC1) near the engine is followed by a gasoline particle filter (GPF) and another three-way catalyst (TWC2) downstream thereof. The system additionally has a nitrogen oxide storage function.

Claims

1. An exhaust gas purification system for purifying exhaust gases of a predominantly stoichiometrically operated internal combustion engine, comprising a TWC1, which is near the engine and is on a flow-through substrate, a GPF, which is attached downstream of the TWC1, and an additional TWC2, which is downstream of the GPF and is on a flow-through substrate, characterized in that the system additionally comprises materials for temporarily storing nitrogen oxides in a separate coating, said additional material being selected from the group consisting of K.sub.2O, Na.sub.2O, CaO, BaO, MgO, SrO, CeO.sub.2, ZrO.sub.2, cerium mixed oxides, zeolites, or mixtures thereof, and said material being present in the coating at more than 50 wt. % and the materials for temporarily storing nitrogen oxides being arranged on a separate flow-through substrate.

2. The system according to claim 1, characterized in that the materials for temporarily storing nitrogen oxides also have catalysts for the oxidation of NO to NO.sub.2.

3. The system according to claim 1, characterized in that the coating which contains the materials which store nitrogen oxide is present in a quantity of 100-500 g/L of substrate volume.

4. (canceled)

5. The system according to claim 3, characterized in that the substrate with the materials for temporarily storing nitrogen oxides accounts for a fraction of 5-30% by volume of the total volume of the substrates in the exhaust gas purification system.

6. The system according to claim 5, characterized in that the substrate with the materials for temporarily storing nitrogen oxides has a greater washcoat loading in g/L than the GPF.

7. The system according to claim 1, characterized in that the washcoat loading in g/L of the TWC1 is greater than that of the TWC2.

8. The system according to claim 1, characterized in that the noble metal concentration in g/L of the TWC1 is greater than that of the TWC2.

9. The system according to claim 1, characterized in that at least one substrate can be electrically heated.

10. The system according to claim 1, characterized in that the GPF has a filtration-increasing coating.

11. A method for purifying exhaust gases of a predominantly stoichiometrically operated internal combustion engine, in which the exhaust gas is passed through an exhaust gas purification system according to claim 1.

Description

FIGURES

[0036] FIG. 1: Shows an according to the invention with KAT in the final position.

[0037] FIG. 2: Shows an according to the invention with KAT in the position upstream of the TWC2.

[0038] FIG. 3: Shows an according to the invention with KAT in the position near the engine.

[0039] FIG. 4: Determination of the nitrogen oxide storage capability.

[0040] FIG. 5: Averaged bag emissions for THC/NMHC/CO/NOx of the two exhaust gas aftertreatment systems TWC-GPF-TWC and TWC-GPF-TWC+KAT in comparison.

EXAMPLES

Determination of the Nitrogen Oxide Storage Capability:

[0041] The nitrogen oxide storage capability/capacity is determined experimentally in a flow tube reactor. From the region of the catalyst substrate whose nitrogen oxide storage capacity is to be determined, a drill core is taken as specimen. Preferably, a drill core 1 inch in diameter and 3 inches long is taken as the specimen. The drill core is inserted into the flow tube reactor and conditioned at a temperature of 650? C. rich/lean cycles of 10 seconds each with in a gas atmosphere composed of 500 ppm nitrogen monoxide, 7 vol. % oxygen, 10 vol. % water, 10 vol. % carbon dioxide, 50 ppm hydrocarbons (propane/propene 17/33) and the remainder nitrogen in the lean phases and a gas atmosphere composed of 500 ppm nitrogen monoxide, 55000 ppm carbon monoxide, 1 vol. % oxygen, 10 vol. % water, 10 vol. % carbon dioxide, 50 ppm hydrocarbons (propane/propene 17/33) and the remainder nitrogen in the rich phases with a space velocity of 50000 h.sup.?1 for 15 minutes. Subsequently, there are 10 rich/lean cycles of 10 seconds each at 650? C. with a space velocity of 30000 h.sup.?1 and with a gas atmosphere composed of 0 ppm nitrogen monoxide, 1 vol. % oxygen, 0 vol. % water, 0 vol. % carbon dioxide, 0 ppm hydrocarbons and the remainder nitrogen in the lean phases and with a gas atmosphere composed of 0 ppm nitrogen monoxide, 20000 ppm carbon monoxide, 0 vol. % oxygen, 0 vol. % water, 0 vol. % carbon dioxide, 0 ppm hydrocarbons (propane/propene 17/33) and the remainder nitrogen in the rich phases. Subsequently, cooling to a temperature of 350? C. under nitrogen at a space velocity of 50000 h.sup.?1 is carried out. Subsequently, there is a conditioning of 3 rich/lean cycles of 20 seconds each at 350? C. with a space velocity of 35000 h.sup.?1 and with a gas atmosphere composed of 0 ppm nitrogen monoxide, 1 vol. % oxygen, 0 vol. % water, 0 vol. % carbon dioxide, 0 ppm hydrocarbons and the remainder nitrogen in the lean phases and with a gas atmosphere composed of 0 ppm nitrogen monoxide, 20000 ppm carbon monoxide, 0 vol. % oxygen, 0 vol. % water, 0 vol. % carbon dioxide, 0 ppm hydrocarbons (propane/propene 17/33) and the remainder nitrogen in the rich phases. Subsequently, cooling to a temperature of 250? C. under nitrogen at a space velocity of 50000 h.sup.?1 is carried out and, after stabilization of the temperature, the nitrogen oxide storage capability is determined by switching on a gas mixture composed of 500 ppm nitrogen monoxide, 8 vol. % oxygen, 10 vol. % water, and 10 vol. % carbon dioxide at a space velocity of 30000 h.sup.?1. This gas mixture remains switched on until the NOx conversion via the specimen is less than 10%. This sequence is also shown in FIG. 4. The value thus determined represents the maximum storage quantity of the nitrogen storage catalyst. This maximum storage quantity is set in relation to the total substrate volume in the system which is aimed at.

Experimental Data:

[0042] A Euro 6 gasoline vehicle with 1.5 L DI engine was driven with an exhaust system artificially aged to end-of-life and consisting of a first TWC near the engine with 1.26 L catalyst volume (substrate dimensions 118.4 mm?114.3 mm) and a conventional three-way coating with 1.77 g/L noble metal (0/92/8 Pt/Pd/Rh), an uncoated GPF arranged downstream with 1.39 L catalyst volume (substrate dimensions 132.1 mm?101.6 mm), and a second TWC arranged in the underbody with 1.26 L catalyst volume (substrate dimensions 118.4 mm?114.3 mm) and a conventional three-way coating with 0.83 g/L noble metal (0/80/20 Pt/Pd/Rh) and on a chassis dynamometer in an RTS aggressive driving cycle. This system is referred to as a TWC-GPF-TWC reference system and has a total substrate volume of 3.9 L. The emissions THC, NNHC, CO, NOx, NH.sub.3 and N.sub.2O were measured, the measuring technique to be used for this purpose is known to a person skilled in the art. The mean value from a plurality of measurements is shown in each case.

[0043] This was compared to a system according to the claims mentioned herein. For this purpose, the same Euro 6 gasoline vehicle with 1.5 L DI engine was driven with an exhaust system artificially aged to end-of-life and consisting of a first TWC near the engine with 1.26 L catalyst volume (substrate dimensions 118.4 mm?114.3 mm) and a conventional three-way coating with 1.77 g/L noble metal (0/92/8 Pt/Pd/Rh), an uncoated GPF arranged downstream with 1.39 L catalyst volume (substrate dimensions 132.1 mm?101.6 mm), a second TWC arranged in the underbody with 0.63 L catalyst volume (substrate dimensions 118.4 mm?57.2 mm) and a conventional three-way coating with 0.83 g/L noble metal (0/80/20 Pt/Pd/Rh), and a KAT arranged downstream thereof with 0.50 L catalyst volume (substrate dimensions 105.7 mm?57.2 mm) and a coating which can additionally temporarily store nitrogen oxides, with 1.34 g/L noble metal (79/8/13 Pt/Pd/Rh), and on a chassis dynamometer in an RTS aggressive driving cycle. TWC-GPF-TWC+ KAT system compared to that of the TWC-GPF-TWC reference system. With the same volume of the TWC2 or TWC2/KAT in both systems, there is an advantage for the system according to the invention with regard to the nitrogen oxide conversion and surprisingly also the hydrocarbon emissions.