EXHAUST GAS PURIFICATION SYSTEM FOR PURIFYING EXHAUST GASES OF GASOLINE ENGINES

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 gas 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 a hydrocarbon 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 the engine on a flow-through substrate, a GPF attached downstream of the TWC1 as a wall-flow filter, and a further TWC2 on a flow-through substrate downstream of the GPF, characterized in that the system additionally comprises materials for temporarily storing hydrocarbons, and these are positioned on the downstream side of the TWC1 and behind the GPF.

2. The system according to claim 1, characterized in that by this additional material, the hydrocarbon storage capacity is increased to at least 0.020 g of hydrocarbons per L substrate volume.

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

4. The system according to claim 1, characterized in that the materials for temporarily storing hydrocarbons 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 temporarily storing hydrocarbons likewise have catalysts for the oxidation of hydrocarbons to H.sub.2O and CO.sub.2.

6. The system according to claim 1, characterized in that the materials for temporarily storing hydrocarbons are arranged on a separate flow-through substrate.

7. The system according to claim 6, characterized in that the substrate with the materials for temporarily storing hydrocarbons makes up a proportion of 5-30 vol. % of the total volume of the substrates in the exhaust gas purification system.

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

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

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

Description

[0043] 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 limit values of future, even-stricter exhaust standards. In addition to the standard values, such as HC, CO, NOx, and soot, the system according to the invention also makes it possible, at least in its advantageous embodiments, to reduce so-called secondary pollutants, e.g., NH.sub.3, N.sub.2O, and others. Specifically, the arrangement of the TWC1 close to 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 can help to ensure high conversion rates of CO, HC, and NOx most importantly at operating points with high load and exhaust gas mass flows. 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 in the corresponding temperature regimes and engine operating points cannot completely oxidize certain amounts of hydrocarbons. Finally, by using CAT, it is ensured that hydrocarbons formed in the cold start can additionally be significantly reduced. Such a system is thus predestined for use in automobiles which must comply with future strict exhaust gas limit values for approval.

[0044] FIG. 1: The typical concentration curve of an ammonia absorption measurement is shown here.

[0045] FIG. 2: shows an embodiment of a system according to the invention with CAT downstream of the TWC2.

[0046] FIG. 3: shows an embodiment of a system according to the invention with CAT between GPF and TWC2.

[0047] FIG. 4: shows an embodiment of a system according to the invention with CAT on the inflow side of the GPF.

[0048] FIG. 5: shows average bag emissions for THC/NHC/CO/NOx of the two exhaust gas aftertreatment systems TWC-GPF-TWC and TWC-GPF-TWC+CAT in comparison.

[0049] FIG. 6: shows mediated cumulative modal curves of HC, CO, and NOx during the first 200 s of the driving cycle.

EXAMPLES: EXPERIMENTAL DATA

[0050] A Euro 6 gasoline vehicle with 1.5 L DI engine was run with an exhaust system artificially aged to end of life, consisting of a first TWC close to the engine with 1.26 L catalyst volume (substrate dimensions 118.4 mm114.3 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 mm101.6 mm) and a second TWC arranged in the underbody with 1.26 L catalyst volume (substrate dimensions 118.4 mm114.3 mm) and a conventional three-way coating with 0.83 g/L noble metal (0/80/20 Pt/Pd/Rh), on an RTS roller test bench with an aggressive driving profile. 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 in this case; the measuring technology to be used for this purpose is known to the person skilled in the art. The mean value from a plurality of measurements is shown in each case.

[0051] This was compared to a system according to the claims in this application. For this purpose, the same Euro 6 gasoline vehicle with 1.5 L DI Motor was driven with an exhaust system artificially aged to end of life, consisting of a first TWC close to the engine with 1.26 L catalyst volume (substrate dimensions 118.4 mm57.2 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 mm101.6 mm), a second TWC arranged in the underbody with 0.63 L catalyst volume (substrate dimensions 118.4 mm57.2 mm) and a conventional three-way coating with 0.83 g/L noble metal (0/80/20 Pt/Pd/Rh), and a CAT, arranged downstream thereof, with a 0.63 L catalyst volume (substrate dimensions 118.4 mm57.2 mm) and a coating which can additionally temporarily store hydrocarbons, with 0.83 g/L noble metal (0/80/20 Pt/Pd/Rh) on an RTS roller test bench with an aggressive driving profile. This system is referred to as TWC-GPF-TWC+CAT 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 in this case; the measuring technology 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.

[0052] FIG. 5 shows the advantages of the TWC-GPF-TWC+CAT system compared to TWC_GPF_TWC reference system in a reduction of THC and NMHC emissions by 15% without negatively influencing the CO or NOx emissions. The advantages for the HC emissions are even more clearly shown in FIG. 6, where the averaged cumulative modal curves of HC, CO, and NOx are shown during the first 200 s of the driving cycle. Here, the HC emissions can be reduced by the TWC-GPF-TWC+CAT system in blue by more than one third compared to the TWC-GPF-TWC reference system in black, even though there is no negative influence on the CO or NOx performance. This effect is achieved with the same overall substrate volume of the two compared systems, i.e., with halved TWC2 of the TWC-GPF-TWC+CAT system compared to that of the TWC-GPF-TWC reference system. With the same volume of the TWC2 in both systems, therefore, an even more amplified advantage for the TWC-GPF-TWC+CAT system is to be expected.