EP0431405B1 - Device for catalytic purification of exhaust gases of internal combustion two-stroke engines - Google Patents

Abstract

The invention relates to an arrangement for catalytic purification of the exhaust gases of internal combustion engines, especially two-stroke engines, consisting of a jacket tube (1) which is arranged in the exhaust path and in which there is installed a perforated inner tube (3) to form a narrow annular space (5) between the jacket tube and the inner tube. The inner tube (3) is occupied by a catalyst. <IMAGE>

Description

The invention relates to an exhaust gas purification system for the catalytic purification of the exhaust gases from internal combustion engines, in particular from two-stroke engines.

Internal combustion engines, especially according to the two-stroke principle, require a certain design of the exhaust gas routing for the purpose of a favorable sequence of the gas exchange processes (fresh gas filling, purging and exhaust gas discharge process). In addition to a suitable tuning of the silencer, certain design measures in the area between the exhaust gas outlet of the engine and the silencer play a role that is relevant to performance. So z. B. in or immediately after the exhaust manifold reflection surfaces and / or expansion spaces (so-called. Exhaust bulbs) provided to positively influence the gas dynamics.

Conventional catalytic exhaust gas cleaning systems, such as loose bed or monolith catalysts, which usually have to be arranged in the area of the exhaust system near the engine, have proven to reduce performance in vehicles with two-stroke engines due to the exhaust gas back pressure caused by them and the disturbance of the gas dynamic vibrations.

Blockages caused by coal can also occur; this happens e.g. B. sometimes with monolith catalysts under certain engine operating conditions. Conventional monolith catalysts prove to be disadvantageous at all, since such catalysts in engines with high emissions are exposed to high exotherms due to their very high conversion potential on small volumes and therefore there is a risk of thermal damage up to the melting of the catalyst. The invention has for its object to provide a catalytic exhaust gas purification system which is more suitable for such engines. This object is achieved by the arrangement described in the claims.

The arrangement according to the invention is also suitable for four-stroke engines, since these also require fine tuning of the exhaust system.

In the arrangement for exhaust gas purification according to the invention, the outer tube (1) can have any shape. The inner tube (3) is shaped accordingly in order to obtain an annular space (5) of approximately constant cross section. Tapered annular spaces are also part of the invention.

The annular space (5) takes up only a fraction of the volume of the outer tube (1) due to the dimensions provided. Annulus (5) and perforation reinforce the in the catalyst area Turbulence and thus gas exchange. In the basic configuration of the arrangement according to the invention, this effect is reinforced by the annular gap which is open on both sides and which ensures a backflow of the catalytically coated inner tube. With some motor designs, an at least one-sided closure of the annular space at any point is expedient.

With round holes as perforations, the hole diameters of the inner tube range from 0.75 to 10 mm, with a hole spacing of 1.0 to 15 mm. Hole diameters from 1.5 to 6 mm and hole spacing from 2.0 to 7.5 mm are preferred (claim 7). The inner tube can have regularly arranged holes of different diameters over the entire jacket.

In addition to the round perforations, rectangular and slot perforations and any shape can be used and their combination. Likewise, the size or number of perforations can be varied over the length of the inner tube. If necessary, the tubular body can also consist of expanded metal or coarse wire mesh.

The relative free perforated area (total perforated area) in the various embodiments is 5 to 80%, preferably 20-60%, of the outer surface of the inner tube.

The perforations in the inner tube, which act as turbulators, can be enhanced in their effect by the perforations being in the form of deep-drawn upward and / or downward bulges.

The inner tube can, if appropriate, in particular for manufacturing reasons, be formed from two half-shells which may be connected to one another. This can result in a division of the annular space into two halves.

The geometric surface can be increased by surrounding it with a wire mesh, expanded metal or another perforated tube in order to achieve increased conversion rates for the pollutants.

The arrangement according to the invention is suitable both as a main catalyst and as a starting catalyst for a downstream main catalyst (6) of any type. When used as a starting catalytic converter, partial conversion of pollutants results in an increase in temperature, which makes it easier or even possible to start a main catalytic converter which may be connected in a cooler part of the exhaust system. If high concentrations of pollutants occur and at the same time high exhaust gas temperatures, a partial conversion has a favorable effect in that it reduces the temperature load due to the exhaust gas cooling between the starting and main catalytic converters and protects the main catalytic converter from premature destruction or thermal deactivation. As the following exemplary embodiments show, the conversion rates of the arrangement according to the invention are surprisingly high when used as the main catalyst. Also at When used on two-wheelers, the main catalyst can be used to meet current and future limit values.

All of the formulations containing noble metals and / or non-noble metals known from the exhaust gas cleaning of gasoline engines can be used as the catalytically active coating.

Particularly suitable coating formulations contain Pt, Pd and / or Rh as active components on a heat-resistant, oxidic carrier material (Al₂O₃, SiO₂, aluminum silicates). To activate and improve the thermal stability, the support material can be provided with oxidic additives such as CeO₂, ZrO₂, alkaline earth metal oxide and / or rare earth oxides.

If appropriate, a pretreatment by sandblasting, flame spraying, aluminizing is carried out before the application of the oxidic carrier material in order to improve the temperature resistance of the perforated tubular body and the adhesion of the coating.

example 1

A perforated, tapered pipe bend with diameters of 30 mm or 55 mm, an elongated pipe length of 345 mm, 2 mm hole diameter and 3.15 mm hole spacing made of high-temperature resistant steel 1.4841 is first tempered in air at 900 ° C for 3 h.

The tube bend is then coated with an approximately 40% dispersion of γ-Al₂O₃, (150 m² / g), cerium acetate and zirconium acetate, dried at 120 ° C. and calcined at 600 ° C. for 2 hours. The coating amount was 7.5 g Al₂O₃, 2 g CeO₂, 0.5 g ZrO₂ per part.

Then the tubular body is impregnated with a solution of Pt (NH₃) ₄ (OH) ₂ and Rh (NO₃) ₃ (weight ratio Pt: Rh = 5: 1), dried and in the forming gas (N₂: H₂ = 95: 5) at 400 ° C reduced. The amount of precious metal loaded is 0.24 g / part.

Example 2

The catalyst according to Example 1 was tested according to the experimental arrangement in FIG. 1. The annular gap between the inner tube outer diameter and outer tube inner diameter was 1 mm. A two-wheel motorcycle with an air-cooled two-stroke single-cylinder engine with a displacement of 148 cm³ served as the test vehicle. The performance of the two-wheeler was 14 KW at a nominal speed of 8100 min⁻¹. An oil / petrol mixture of 1:50 was used.

The vehicle was measured on a roller test bench in accordance with test specification ECE R40. The exhaust gas temperatures at the catalyst inlet varied in the test cycle from 250-650 ° C. The exhaust gas analysis was carried out according to the CVS principle.

The following emission values were determined: CO g / km HC + NO _x g / km without catalyst 3.25 5.86 with catalyst 1.13 2.56

The conversion types were thus 68% for CO and 56% for (HC + NO _x ).

By using this specially designed catalytic converter, it is possible to fall below the Swiss limit values applicable from October 1st, 1990 (CO = 8.0 g / km, HC + NO _x = 3.1 g / km, without causing any severe impairment of the gas dynamic processes that lead to a drop in performance of the two-stroke engine.

Example 3

A conical tube made of heat-resistant steel 1.4828 with a sheet thickness of 1 mm with a diameter of 39 mm or 61 mm and a length of 150 mm, which has a slot perforation (1.5 mm hole width x 6 mm hole length, web width 1.3 mm), is covered by flame spraying with a firmly adhering, rough oxide layer based on Al₂O₃. The metal tube pretreated in this way is immersed in a 42% suspension of La₂O₃-stabilized γ-Al₂O₃ (130 m² / g) and cerium oxide, excess coating material is removed by blowing, dried and tempered at 500 ° C. for 2 hours. After coating, there were 3.9 g of Al₂O₃, 0.2 g of La₂O₃ and 1.0 g of CeO₂ on the tube. The catalyst precursor was then impregnated with a nitric acid solution of Pt (NH₃) ₂ (NO₂) ₂ and Pd (NO₃) ₂ (weight ratio Pt: Pd = 2: 1), dried and calcined at 300 ° C in air. The amount of noble metal was 125 mg / part.

Example 4

The catalyst according to Example 3 was tested in accordance with the experimental arrangement in FIG. 2. The annular gap between the inner tube outer diameter and outer tube inner diameter was 1.5 mm on average. A two-wheel motorcycle with an air-cooled two-stroke single-cylinder engine with a displacement of 134 cm³ served as the test vehicle. The engine output was 12 KW at a rated speed of 8100 min⁻¹. An oil / petrol mixture of 1:30 was used.

The vehicle was measured on a roller test bench in accordance with test specification ECE R40. The exhaust gas temperature at the catalyst inlet was 150-420 ° C. The exhaust gas analysis was carried out according to the CVS principle. The following emission values were determined: CO g / km HC g / km NO _x g / km without catalyst 6.86 10.15 0.016 with catalyst 3.16 5.96 0.014

The conversion types were thus 53% for CO, 41% for HC and 13% for NO _x .

By using this specially designed catalytic converter, it is possible to comply with the limit values applicable in Austria from September 30, 1991 (CO = 8g / km, HC = 7.5 g / km, NO _x = 0.1 g / km) for two-stroke motorcycles.

Example 5

A monolithic metal support with a diameter of 60 mm, a length of 75 mm and a cell density of 31 cells / cm² is covered by immersion in an aqueous suspension of γ-Al₂O₃, cerium acetate and zirconium acetate and excess coating material is removed by blowing out. After drying at 120 ° C and calcining for one hour at 700 ° C there are 25 g Al₂O₃, 5 g CeO₂, and 1g ZrO₂ on the support. The catalyst precursor is then impregnated with a solution of Pt (NH₃) ₄ (OH) ₂ and Rh (NO₃) ₃ and dried at 300 ° C. After the coating, 0.36 g of precious metal in the Pt: Rh = 5: 1 weight ratio were on the catalyst.

Example 6

The catalysts according to Examples 3 and 5 are integrated into the exhaust of a two-stroke motorcycle according to FIG. 3. The tests are carried out in accordance with Example 4.

The following emission values were measured: CO g / km HC g / km NO _x g / km without catalyst 6.86 10.15 0.016 with catalyst 1.10 1.45 0.014

Compared to Example 4, there were significantly increased conversion types of 84% for CO and 86% for HC, with almost the same nitrogen oxide emissions. Such a concept is therefore particularly suitable when future, stricter limit values have to be met.

If the catalyst according to Example 5 is used alone with an arrangement according to FIG. 3, the high conversion types cannot be achieved, since the temperature level is too low without a conical precatalyst.

An installation of the catalytic converter according to Example 5 closer to the engine in the area of the pre-catalytic converter (FIG. 3) would lead to a disturbance of the gas dynamic processes and thus to unacceptable power losses and is therefore not indicated.

Claims (5)

1. Exhaust gas purification system for the catalytic purification of the exhaust gases of two-stroke engines comprising an outer tube (1) and, arranged therein, a perforated inner tube (3), that is coated on one or both sides with a catalytically active coating and with the outer tube forms an annular gap (5),
   characterized in that
   the outer tube forms either a section of the exhaust pipe or an expansion chamber arranged in the exhaust pipe, that the annular gap between outer tube and inner tube is open at both ends and is formed by at least one web (4) connected with both tubes and can be interrupted by projections or beads in the inner tube, that the inner tube is perforated on its whole surface area and that the ratio of the internal diameter of the outer tube to the external diameter of the inner tube is in the range of from 1.01 to 1.20.
2. Exhaust gas purification system according to Claim 1,
   characterized in that
   the inner tube (3) is shaped in conformity with the shape of the outer tube (1), whereby the annular gap (5) has an approximately constant cross section.
3. Exhaust gas purification system according to Claim 1 or 2,
   characterized in that
   the inner tube (3) is perforated by round holes, whose diameters fall in the range between 1.5 and 6 mm and whose total hole area amounts to 5 to 80 %, preferably 20 to 60 %, of the surface area of the inner tube.
4. Exhaust gas purification system according to Claims 1 to 3,
   characterized in that
   the inner tube (3) is surrounded by and flush with a wire mesh, expanded metal or other perforated tube, also having a catalytically active coating.
5. Exhaust gas purification system according to Claims 1 to 4,
   characterized in that it is connected as precatalyst before an optionally monolithic main catalyst (6).

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