EXHAUST GAS PURIFICATION SYSTEM FOR STOICHIOMETRIC-COMBUSTION ENGINES

Abstract

The present invention relates to a stoichiometric-combustion spark-ignition engine comprising a specific exhaust gas system for reducing harmful exhaust gases resulting from the combustion process. The exhaust gas system consists in the through-flow direction of a three-way catalytic converter close to the engine, an oxidation catalyst and a gasoline particulate filter.

Claims

1. A stoichiometrically operated spark-ignition engine comprising an exhaust-gas system for reducing harmful exhaust gases resulting from fuel combustion, wherein the exhaust-gas system has a three-way catalyst close to the engine and a gasoline particulate filter installed in the under-floor, wherein the exhaust gas coming from the three-way catalyst close to the engine is passed through an oxidation catalyst before filtration, said oxidation catalyst being capable of oxidizing NO to NO.sub.2 in the presence of excess air, at temperatures of 250 C.-500 C.; characterized in that the oxidation catalyst comprises platinum group metals on a temperature-resistant metal oxide with a large surface area, and the metal oxide of the oxidation catalyst coating has an average pore volume (Q3 distribution) of 0.7 ml/g-2 ml/g.

2. The spark-ignition engine according to claim 1, characterized in that the Pt:Pd weight ratio in the oxidation catalyst is 1.

3. The spark-ignition engine according to claim 2, characterized in that the oxidation catalyst is designed as a two-layer catalyst in which, in the lower layer, Pd and an oxygen storage material are deposited on the temperature-resistant metal oxide with a large surface area and, in the upper layer, Pt is deposited on the temperature-resistant metal oxide with a large surface area.

4. The spark-ignition engine according to claim 1, characterized in that the temperature-resistant metal oxide with a large surface area is selected from the group consisting of silicon dioxide, aluminum dioxide, zeolite, cerium oxide, cerium/zirconium oxide, titanium dioxide, zirconium dioxide, mixed oxides, composite materials and mixtures of the aforementioned.

5. The spark-ignition engine according to claim 1 any one of the preceding claims, characterized in that the loading with platinum group metals in the oxidation catalyst is 0.035-4.0 g/L.

6. The spark-ignition engine according to claim 1, characterized in that the oxidation catalyst is arranged as a separate component before the catalytically coated or uncoated gasoline particulate filter.

7. The spark-ignition engine according to claim 1, characterized in that the oxidation catalyst is designed as a coating on and/or in the gasoline particulate filter.

8. The spark-ignition engine according to claim 1, characterized in that the average pore volume (Q3 distribution) of the metal oxides used in the oxidation catalyst increases in the direction of the exhaust-gas flow.

9. The spark-ignition engine according to claim 1, characterized in that the ratio of the average pore volume (Q3 distribution) of the metal oxide of the three-way catalyst close to the engine to the metal oxide of the oxidation catalyst is 0.25-1.

10. A method for purifying the exhaust gas of a stoichiometrically operated spark-ignition engine by means of an exhaust-gas system for reducing harmful exhaust gases resulting from fuel combustion, wherein the exhaust-gas system comprises a three-way catalyst close to the engine and a gasoline particulate filter installed in the under-floor, characterized in that the exhaust gas coming from the three-way catalyst close to the engine is passed through an oxidation catalyst before filtration, said oxidation catalyst being capable of oxidizing NO to NO.sub.2 in the presence of excess air, at temperatures of 250 C.-500 C.

Description

Example 1 According to the Invention

[0038] Stabilized aluminum oxide was suspended in water. The aluminum oxide used has an average pore volume (Q3 distribution) of 1.25 ml/g. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a platinum nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, the coating being introduced into the porous filter wall on the inlet side over 100% of the substrate length. The total load of this filter amounted to 10 g/L; the total precious metal load amounted to 0.35 g/L with a 1:12 ratio of palladium to platinum. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as EGPF1.

Example 2 According to the Invention

[0039] Stabilized aluminum oxide was suspended in water. The aluminum oxide used has an average pore volume (Q3 distribution) of 1.25 ml/g. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a platinum nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, the coating being introduced into the porous filter wall on the inlet side over 100% of the substrate length. The total load of this filter amounted to 10 g/L; the total precious metal load amounted to 0.35 g/L with a 1:2 ratio of palladium to platinum. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as EGPF2.

Comparative Example 1

[0040] Stabilized aluminum oxide was suspended in water. The aluminum oxide used has an average mean pore volume (Q3 distribution) of 0.5 ml/g. The suspension thus obtained was subsequently mixed with a palladium nitrate solution and a platinum nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available wall-flow filter substrate, the coating being introduced into the porous filter wall on the inlet side over 100% of the substrate length. The total load of this filter amounted to 10 g/L; the total precious metal load amounted to 0.35 g/L with a 1:12 ratio of palladium to platinum. The coated filter thus obtained was dried and then calcined. It is hereinafter referred to as VGPF1.

[0041] Performance:

[0042] The resulting filters EGPF1, EGPF2, VGPF2 and an uncoated wall-flow substrate VGPF2 were first loaded with 4 g/L of soot on the engine test bench and then subjected to a soot burn-off test. The burn-off behavior of the filters was investigated at a constant exhaust-gas temperature of 500 C. before the filter and with a lean exhaust-gas composition at lambda=1.1, by calculating the times t50 and t75 after which the exhaust-gas back pressure of the soot-loaded filters decreased by 50 and 75%, respectively. It was found (Table 1) that the filters according to the invention better catalyze the soot oxidation, which is reflected by a faster decrease in the back pressure. In particular, the uncoated filter VGPF2 does not show any soot burn-off at the 500 C. test temperature.

TABLE-US-00001 t50 t75 EGPF1 707 sec 885 sec EGPF2 654 sec 837 sec VGPF1 725 sec 907 sec VGPF2 (uncoated) No burn-off No burn-off

[0043] In a further test, a system according to the invention consisting of a commercially available three-way catalyst close to the engine and an EGPF1 arranged in the under-floor, was tested for particle filtration efficiency against a system not according to the invention consisting of a commercially available three-way catalyst close to the engine and an uncoated VGPF2 arranged in the under-floor, in the WLTP test on a current gasoline engine with direct injection and turbocharging (Table 2). It was found here that after a conditioning test, the system according to the invention has a significantly increased filtration performance than the comparative system.

TABLE-US-00002 Filtration efficiency of the system according to Filtration efficiency of the invention including the comparative system EGPF1 [%] including VGPF2 [%] WLTP test 2 86 80 WLTP test 3 91 80 WLTP test 4 93 80