EXHAUST GAS SYSTEM FOR PURIFYING EXHAUST GASES OF GASOLINE ENGINE

Abstract

The 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 includes an ammonia storage function.

Claims

1. An exhaust gas purification system for purifying exhaust gases of a predominantly stoichiometrically operated internal combustion engine having a TWC1 near to the engine on a flow-through substrate, a GPF attached downstream of the TWC1 as a wall-flow filter and another TWC2 on a flow-through substrate downstream of the GPF, characterized in that the system additionally has materials for temporary storage of ammonia, wherein the materials for temporary storage of ammonia are arranged on a separate flow substrate and the latter is arranged downstream of the TWC2.

2. The system according to claim 1, characterized in that the ammonia storage capability is increased to at least 0.25 g of ammonia per L substrate volume by this additional material.

3. The system according to claim 1, characterized in that the materials for temporary storage of ammonia are present in the system in an amount of 50-350 g/L substrate volume.

4. The system according to claim 1, characterized in that the materials for temporary storage of ammonia have materials selected from the group consisting of zeolites or zeolite-like materials.

5. The system according to claim 1, characterized in that the materials for temporary storage of ammonia also have catalysts for the oxidation of NH.sub.3 to N.sub.2.

6. (canceled)

7. The system according to claim 1, characterized in that the substrate with the materials for temporary storage of ammonia accounts for a proportion of 5-30% by volume of the total volume of substrates in the exhaust gas purification system.

8. The system according to claim 7, characterized in that the substrate with the materials for temporary storage of ammonia has a greater washcoat loading in g/L than the GPF.

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

10. 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

[0040] With the exhaust system and the proposed method according to the present invention, it is possible to be able to comply with the exhaust gas limits of future, even stricter emission standards. In addition to the standard values such as HC, CO, NOx, and soot, the system according to the invention also allows the reduction of so-called secondary pollutants, e.g. NH.sub.3, N.sub.2O and others. Specifically, the arrangement of the TWC1 near the engine enables very high conversion rates for the emission-relevant pollutants CO, HC, and NOx. The additional TWC2 can optionally have a support effect and, especially at operating points with a high load and exhaust gas mass flows, can help to ensure high conversion rates of CO, HC, and NOx. By contrast, the use of the particle filter leads to significant deposition rates of soot, so that the given emission limits can be reliably met. It is well known to the person skilled in the art that classic three-way catalysts can generate certain amounts of ammonia in the corresponding temperature regimes and engine operating points. Finally, the use of materials temporarily storing ammonia, for example in a separate catalyst substrate (KAT), ensures that especially secondary pollutants, such as ammonia, can additionally be significantly reduced. Such a system is thus predestined for use in automobiles which will have to comply with future strict exhaust gas limits for an approval.

FIGURES

[0041] FIG. 1: Here the typical concentration curve of an ammonia absorption measurement is shown.

[0042] FIG. 2: Shows an embodiment of a system according to the invention with KAT downstream of the TWC2.

[0043] FIG. 3: Shows an embodiment of a system according to the invention with KAT between GPF and TWC2.

[0044] FIG. 4: Shows an embodiment of a system according to the invention with KAT upstream of the GPF.

[0045] FIG. 5: Averaged bag emissions for THC/NHC/CO/NOx of the two exhaust gas aftertreatment systems TWC-GPF-TWC and TWC-GPF-TWC+KAT in comparison.

[0046] FIG. 6: Averaged emissions for NH.sub.3 and N.sub.2O of the two exhaust gas aftertreatment systems TWC-GPF-TWC and TWC-GPF-TWC+KAT in comparison.

[0047] FIG. 7: Averaged cumulative modal curves of emissions for NH.sub.3 and N.sub.2O of the two exhaust gas aftertreatment systems TWC-GPF-TWC (black) and TWC-GPF-TWC+KAT (blue) in comparison.

Examples

Determination of Ammonia Storage Capacity

[0048] It is determined experimentally in a flow tube reactor. In order to avoid undesirable ammonia oxidation on the reactor material, a reactor made of quartz glass is used. From the region of the catalyst whose ammonia 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 600? C. in a gas atmosphere of 500 ppm nitrogen monoxide, 5% by volume of oxygen, 5% by volume of water, and remainder of nitrogen at a space velocity of 30,000 h.sup.?1 for 10 minutes. Subsequently, in a gas mixture of 0% by volume of oxygen, 5% by volume of water, and remainder of nitrogen at a space velocity of 30,000 h.sup.?1, the measuring temperature of 200? C. is started up. After stabilization of the temperature, the NH.sub.3 storage phase is initiated by switching on a gas mixture of 450 ppm ammonia, 0% by volume of oxygen, 5% by volume of water, and remainder of nitrogen at a space velocity of 30,000 h.sup.?1. This gas mixture is added until a stationary ammonia permeate concentration is registered downstream of the specimen. The mass of ammonia stored on the specimen is calculated from the recorded ammonia breakthrough curve by integrating from the start of the NH.sub.3 storage phase until stationarity is reached, taking into account the measured steady-state NH.sub.3 breakthrough concentration and the known volume flow (hatched area in FIG. 1). The ammonia storage capacity is calculated as quotient of the stored ammonia mass divided by the volume of the tested drill core.

Experimental Data

[0049] A Euro 6 gasoline vehicle with 1.5 L DI engine was tested with an exhaust system artificially aged to end-of-life 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 run on a roller 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.

[0050] This was compared to a system according to the claims mentioned herein. For this purpose, the same Euro 6 gasoline vehicle with a 1.5 L DI engine was equipped with an exhaust system artificially aged to end-of-life consisting of a first TWC near the engine with a 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 therefrom with 0.63 L catalyst volume (substrate dimensions 118.4 mm?57.2 mm) and a coating which can additionally temporarily store ammonia, with 0.11 g/L noble metal (100/0/0 Pt/Pd/Rh). It was run on a roller dynamometer in an RTS aggressive driving cycle. This system is referred to as TWC-GPF-TWC+KAT 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.

[0051] FIG. 5 shows a comparable performance of the TWC-GPF-TWC+KAT system compared to the TWC-GPF-TWC reference system. FIG. 6 shows the so-called secondary emissions of NH.sub.3 and N.sub.2O, wherein the TWC-GPF-TWC+KAT system can reduce the NH.sub.3 emissions by more than half, while N.sub.2O emissions increase only slightly. This behavior is even more evident in FIG. 7, which shows the averaged cumulative modal emissions for NH.sub.3 and N.sub.2O over the entire driving cycle. This effect is achieved at the same total substrate volume of the two systems compared, i.e., with halved TWC2 of the TWC-GPF-TWC+KAT system compared to that of the TWC-GPF-TWC reference system. Therefore, with the same volume of TWC2 in both systems, an even greater advantage can be expected for the TWC-GPF-TWC+KAT system.