DE4117364A1 - Accelerating start-up IC engine exhaust gas purificn. catalyst - by catalytic oxidn. of carbon mon:oxide just before main catalyst and transferring heat to gas stream - Google Patents

Abstract

Reducing the start-up retardation of catalyst in an I.C. engine exhaust gas stream (I) is effected by catalytic oxidn. of CO at a point near but before the main catalyst (II) and transporting the resultant heat of combustion into (II) by (I). Oxidn. of CO is carried out in a very short section, pref. shorter than 10mm, cf. the length of (II) and the pressure drip associated with oxidn. is less than 10%, pref. less than 2%. At least 50-60% of the CO but not more than 30-40% of the NOx and hydrocarbons (CnHm) are reacted in this section, in which only Pt is used as catalytic metal. ADVANTAGE - Start-up is improved easily, without reducng the lifetime of (I) or using electric heating. In an example, (II) (1) and ancillary catalyst (III) (2) were placed in a common cylindrical housing (3) with conical inlet and outlet (6,7), fitted in the exhaust pipe (4), with (III) just before (II). (III) had a conical support of metal wire mesh (wire dia. 30mm, free area 55%), pref. of metal suitable for operating temps. above 1000 deg. C, e.g. based on Ni and Cr. It was coated with Al2O3 (pref. 1-5 mg/cm2) and then with 60 micro-g Pt/cm2. (III) touched (II) only at points, so that direct heat transfer from (III) to (II) was only slight. (II) was a commercially available catalyst with a similar support, coated with 1.5 g/l combination of Pt/Rh in 5:1 wt. ratio. It had a dia. of 10cm, length of 15cm, vol. of 1.3dm3 and cross-section of 78.5 cm2, whilst (III) had the same dia. but a vol. of only 0.002-0.003 dm3 and area of 100-150 cm2, making its wt. negligible cf. that of (II). Tests were carried out with 50000 l/l.h exhaust gas with lambda = 1 and a CO content of 1 vol. %. Using (A) (II) alone, (B) the (III)/(II) combination, the temp. which (II) needed to reach, starting from cold (20 deg. C) for 50/90% conversion was (A) 222/242, (B) 186/214 deg. C for CO, (A) 262/392, (B) 222/308 for CnHm and (A) 214/226, (B) 174/204 deg. C for NOx. In an engine test run, the overall conversion was (A) 74, (B) 81% CO, (A) 82, (B) 90% CnHm and (A) 62, (B) 70% NOx.

Description

The invention is concerned with a method for shortening the light-off delay of an in the exhaust gas stream of a combustion engine lying catalyst and is based on a Vor direction with the specified in the preamble of claim 7 Characteristics.

A catalytic converter that is exhausted from an internal combustion engine is flowing, hydrocarbons contained in the exhaust gas and Oxidize carbon monoxide and reduce nitrogen oxides. In the Germany 's statutory provisions stipulate that in a test run of the Motors, in which a certain route with certain Speeds are simulated, certain limit values for the content of carbon monoxide, hydrocarbons and stick oxides in the exhaust gas must not be exceeded. Because the test  starting with a cold engine, but catalysts their effectiveness only increases at higher temperatures fold and the components of the exhaust gas to be converted special efforts are required to put under the unfavorable conditions of the test run Achieve implementation rate. With a view to a future The exhaust gas limit values are tightened half the light-off behavior of a catalytic converter interpretation to. To improve the light-off behavior, think about using larger catalysts and thereby increasing the implementation rate. However, it is disadvantageous that the space required for the catalyst below the floor pan of the body rises that the cost of the catalyst rise and that it because of its larger Mass is heated more slowly by the exhaust gases, which one Counteracts improvement in light-off behavior.

One could also think of the catalyst closer to the Install the engine in the exhaust system and thereby become one faster heating of the catalyst to come. That has however, the serious disadvantage that the temperature load increases, which the catalyst during operation warm engine is subject, so that the life of the Catalyst is reduced; that in turn runs Forde striving for a guaranteed longer lifespan of the Contradicting catalysts.

There are also vehicles that are in addition to one as usual Head located in the rear section of the exhaust  catalytic converter in the exhaust manifold, so very close to the engine arranged smaller ballast, which by its installation close to the engine is warmed up more quickly than the main one catalyst and therefore earlier in the warm-up phase of the engine starts up as the main catalyst. Since this Vorschaltkataly sator, however, because of the installation close to the engine and the result called higher thermal load a significantly ge has a shorter lifespan than the main catalytic converter again strayed from being such ballasts use.

Another possibility, the light-off behavior of a Kataly sators is to improve it with the help of the vehicle to heat the battery electrically (DE-PS 39 20 159). A disadvantage there is, however, that the battery in particular in the cal season, when their performance is least, strongest is loaded and that by preheating the catalyst the vehicle can only be started with a delay, what is undesirable.

The present invention has for its object that Starting behavior of a catalytic converter to be as simple as possible Way to improve without affecting the Lifespan of the catalyst goes and without that electrical heating of the catalyst would be necessary.

This problem is solved by a method with the in the An claim 1 specified features and by a device with the features specified in claim 7. Beneficial  Developments of the invention are the subject of the dependent Expectations.

According to the invention, at a point close to the Kataly sator, but at a distance from the catalytic converter, the exhaust gas temperature increased by catalyzed oxidation processes, and mainly due to oxidation of carbon monoxide Carbon dioxide. For this purpose, close to the catalyst, which is referred to here as the main catalyst, an auxiliary arranged catalyst, the mass of which is very small against the Mass of the main catalyst. Because of its small mass the auxiliary catalyst is much faster than the exhaust gases the main catalyst warms up to its ignition temperature. With the beginning implementation of carbon monoxide by the auxiliary catalyst increases the temperature of the main catalyst flowing exhaust gas vigorously, so that the main catalyst its ignition temperature reaches earlier than without one Auxiliary catalyst. Although the catalytic conversion in the main catalytic converter the exhaust gas temperature and thus the temperature rature of the main catalyst itself increased, but because the comparatively large mass of the main catalytic converter much slower than with the auxiliary catalytic converter and only when a noteworthy implementation takes place on the main catalyst itself finds.

The additional effort for the auxiliary catalyst is low, since its mass is very small compared to the mass of the main catalyst sators is. The life of the main catalyst is determined by the provision of the auxiliary catalyst is not reduced since the  The temperature load in continuous operation does not increase. Because others on the side of the auxiliary catalytic converter close to the main catalytic converter is ordered, so it is not much closer to the engine than this, its temperature load is not greater than that of the Main catalyst, so that for the auxiliary catalyst with a Can be expected that is not greater than that of the main catalyst. Since the auxiliary catalyst in low Ab stood in front of the main catalytic converter, you have the further front part that the reaction heat generated there by catalysis from Exhaust gas stream is transported directly into the main catalytic converter, without significant heat loss on pipes, flanges, cones, Exhaust housing and other components.

The method according to the invention is limited to to implement the auxiliary catalyst mainly carbon monoxide. The catalytic oxidation of carbon monoxide takes place very much quickly, so that for extensive implementation of coal monoxides not a long way of exhaust gas through the catalyst is needed. Rather, you come with a very thin one Catalyst from, whose mass is therefore very small against the Mass of the main catalyst can be. Preferably, the Auxiliary catalyst shorter than 1-5 millimeters. So short Catalyst does not generate any significant pressure loss in the exhaust gas flow and therefore causes no loss of performance of the Motors. The catalyst can be kept so short that the associated pressure drop in the exhaust gas flow is less than 1% remains.  

Since it depends on the invention, by the auxiliary mainly to implement carbon monoxide, be one does not need one as a catalytically active metal Mixture of platinum and rhodium in the weight ratio 5: 1, which is common in the main catalytic converter, for an Ab gas with the composition λ = 1 for all three harmful components in the exhaust gas (carbon monoxide, hydro substances and nitrogen oxides) to achieve maximum implementation chen (λ = 1 characterizes an exhaust gas from the combustion a fuel-air mixture, which fuel and Contains oxygen in a stoichiometric ratio). A lot more is needed for the auxiliary catalytic converter with a single one catalytically active metal, preferably platinum, which is excellent for the oxidation of carbon monoxide is suitable. The combination platinum / rhodium would also be very suitable, but is more expensive than platinum. The use formation of palladium as a catalytically active metal of Auxiliary catalyst is also possible, but is palladium less effective than platinum, but cheaper than it Platinum.

The usual regulation of the exhaust gas composition by means of a sensor (λ-probe) arranged in the exhaust gas flow by using the auxiliary catalyst according to the invention not affected and can be done unchanged, what a The advantage of the invention is.  

To achieve the effect of the auxiliary according to the invention it is important that the catalyst is only a small one Has heat capacity and therefore - unlike the main catalyst sator - can be heated up quickly by the exhaust gas. In order to this effect is not nullified, should be a warmth derivation from the auxiliary catalyst to the main catalyst be disabled as much as possible. That's why between Main catalyst and the auxiliary catalyst a distance pre see. However, the auxiliary catalytic converter must be in some way in the exhaust unit, on the housing of the main catalytic converter or on the inlet connector or attached to the main catalyst itself will. The required mechanical connection preferably follows only point by point in order to heat transfer hinder.

Otherwise, it is sufficient if the distance of the auxiliary catalyst of the main catalyst is only a few millimeters, preferred wise between 2 millimeters and 20 millimeters; because of the Stability is spherical, in particular a hollow cone Construction favors.

Since it is sufficient according to the invention, through the auxiliary catalyst mainly oxidize carbon monoxide and carbon monoxide can be implemented very quickly, the helper can catalyst not only be very short, but also one have a large free flow cross section, which is preferred is between 50% and 95%. So the pressure drop, which is caused by the auxiliary catalyst, so low that it is practically not measurable. The auxiliary catalytic converter therefore, no loss of performance of the internal combustion engine,  on the contrary: since it is the effectiveness of the main catalyst in the decisive according to the legal provisions Warm-up phase of the engine increased, there is even the possibility speed, the main catalytic converter and thereby the gently reduce the power loss of the internal combustion engine.

A particularly suitable support body for the auxiliary catalyst a metallic net, sieve or sheet, which according to Art of an expanded metal is broken, in each case in extension across the exhaust gas flow. Can achieve the necessary stability the carrier body must be corrugated. In the space between one so trained auxiliary catalyst and the main catalyst there is some turbulence in the exhaust gas flow, which is desirable since it transfers the heat of oxidation favored the main catalyst.

Instead of a sieve, net or sheet, a shin can also be used ben-shaped auxiliary catalyst can be used, depending but expediently over a length of no more than 10 Extends millimeters.

The cross section of the main catalyst is usually a multiple of the cross section of the exhaust pipe, which is why the main catalytic converter is in a special housing, which with inlet opening and outlet opening between two from sections of the exhaust pipe is inserted. So that the Ab gas readily across the entire cross-section of the head distributed catalyst, has its housing between its on  opening and the main catalyst an expanding Section (cone); in this expanding housing the auxiliary catalyst is preferably placed, so that there is no additional space for him.

An embodiment of the invention is shown schematically in the attached drawing, which shows a longitudinal section through an arrangement of a main catalyst 1 and an auxiliary catalyst 2 in a common housing 3 , which has a cylindrical portion in which chem the main catalyst 1 is arranged. The connection to an exhaust pipe 4 is made through a conical housing section 5 with inlet opening 6 on the inlet side and a conical housing section 7 with outlet opening 8 on the outlet side. In the housing section 5 is located just in front of the main catalyst 1, the auxiliary catalyst 2 , which has a metallic network as the support body, which has a hollow cone-shaped shape to achieve high mechanical stability, the cone tip being opposite to the exhaust gas flow (arrow 9 ). The auxiliary catalyst 2 is only selectively connected to the main catalyst 1 , so that the direct heat transfer from the auxiliary to the main catalyst is only slight.

The main catalyst 1 can be a commercially available catalyst, e.g. B. with a diameter of 10 cm, a length of 15 cm, a volume of 1.3 dm ³ and a cross-sectional area of 78.5 cm ² .

The auxiliary catalyst 1 has a corresponding diameter, a volume of only 0.002 to 0.003 dm ³ and an area of 100 to 150 cm ² . Accordingly, its weight is negligibly small compared to the weight of the main catalyst 1 .

For the support bodies of the auxiliary catalytic converters, it is expedient to use metallic materials which are sufficiently stable at the high operating temperatures, which can exceed 1000 ° C. Heating conductor materials based on nickel and chrome, e.g. B. the material No. 1.4868 according to DIN .... An aluminum oxide layer is applied to the metallic support body in a manner known per se, preferably with an area coverage of 1 to 5 mg / cm ^2, and the catalytically active noble metal is then anchored on this aluminum oxide layer , expediently with an area coverage of 5 to 500 µg / cm ² . As far as the prior art is concerned, reference is made to DE-OS 34 36 400 and DE-OS 37 35 033.

So far for the auxiliary catalyst, a net or sieve as a carrier body is selected, the wire thickness is selected first Line depending on the desired stability. The Wire thickness is advantageously between 0.05 and 2 mm, the hole diameter between 0.1 and 5 mm and the resulting free opening of the auxiliary catalyst between 50% and 95%.

The light-off behavior of a commercially available main catalytic converter (as stated above) with a fiction arranged in front Auxiliary catalyst was tested in a laboratory and on a Engine test bench examined and with the light-off behavior of the same main catalyst, but without using the inventive Auxiliary catalyst compared. The main catalyst  contained 1.5 g / (l cataly volume) of the combination platinum / rhodium in weight ratio Ratio 5: 1.

In the laboratory test, the main catalyst was with an exhaust gas the composition λ = 1 and a CO content of 1% by volume operated, the exhaust gas flow rate 50,000 l / (l catalyst volume and per hour).

A 60 g platinum / cm ² coated network with a diameter of 30 mm and a free opening of 55% was used as the auxiliary catalyst.

The temperatures are given in Table I below give that the main catalyst starting from the cold Zu level (20 ° C) must be reached in the accumulated exhaust gas volume for the individual pollutants a 50% (T 50) or 90% (T 90) implementation is achieved.

It can be seen that with the help of the aid according to the invention catalyst the 50% and 90% implementation essential can be achieved earlier than without the auxiliary catalyst.

2nd example

The same commercially available catalyst as in the first example was tested on a motor test bench in accordance with the statutory test regulations (cite reference), once without an auxiliary catalyst and once with an auxiliary catalyst in the form of a network with a diameter of 100 mm, with a platinum coating of 60 µg / cm ² and with a free opening of 55%.

In both cases, the proportion of the damage was checked substances in the exhaust gas from the entire test run have been converted. The result is shown in Table II below. It can be seen that the reaction is carried out by the auxiliary catalyst increases significantly.

Table I

Table II

Implementation (%)

Claims (21)

1. A method for shortening the light-off delay of a catalyst lying in the exhaust gas stream of an internal combustion engine, characterized in that mainly carbon monoxide (CO) is catalytically oxidized at one point, but at a distance from the main catalyst, and the resulting combustion heat is generated by the exhaust gas stream in the Catalyst is transported. 2. The method according to claim 1, characterized in that the oxidation of CO in one compared to the length of the main catalyst very short section of the exhaust gas flow he follows. 3. The method according to claim 2, characterized in that that section is shorter than 10 mm. 4. The method according to any one of the preceding claims, characterized characterized in that with the catalytic oxidation of the CO in front of the main catalyst linked pressure loss in the exhaust gas flow less than 10%, preferably less than 2% ge will hold. 5. The method according to any one of the preceding claims, characterized in that at that point close to the main catalyst already at least 50-60% of the CO, but not yet more than 30-40% of the nitrogen oxides (NO _x ) and hydrocarbons contained in the exhaust gas stream (H _m C _n ) are implemented. 6. The method according to any one of the preceding claims, characterized characterized in that for catalytic conversion on that Only place platinum as catalyze in front of the main catalytic converter effective metal is used. 7. Device for the catalytic conversion of exhaust gases from internal combustion engines with a main catalyst ( 1 ) which is housed in a housing ( 3 ) which is inserted with an inlet opening ( 6 ) and outlet opening ( 8 ) in an exhaust pipe ( 4 ), the lights Cross section is smaller than that of the housing ( 3 ), and with an auxiliary catalytic converter ( 2 ) lying in the exhaust gas flow at a distance from the main catalytic converter ( 7 ), the mass of which is smaller than that of the main catalytic converter ( 1 ), characterized in that the auxiliary catalytic converter ( 2 ) un directly in front of or in the housing ( 3 ) of the main catalyst ( 1 ) and its mass is very small compared to the mass of the main catalyst ( 1 ). 8. The device according to claim 7, characterized in that the mass of the auxiliary catalyst ( 2 ) is at most 5% of the mass of the main catalyst ( 1 ). 9. Apparatus according to claim 7 or 8, characterized in that the mass of the auxiliary catalyst ( 2 ) is at most 75 g, preferably not more than 50 g. 10. Device according to one of claims 7 to 9, characterized in that the auxiliary catalyst (2) only pointwise housing with the main catalyst (1) and / or the Ge is connected (3). 11. Device according to one of claims 7 to 10, characterized in that the distance of the auxiliary catalyst ( 2 ) from the main catalyst ( 1 ) is between 2 mm and 20 mm. 12. Device according to one of claims 7 to 11, characterized in that the free opening of the auxiliary catalyst ( 2 ) is between 50% and 95%. 13. Device according to one of claims 7 to 12, characterized in that the auxiliary catalyst ( 2 ) as a support body has a mesh, sieve or formed in the manner of an expanded metal, perforated sheet metal which extends transversely to the exhaust gas flow. 14. The apparatus according to claim 13, characterized in that the carrier body is corrugated. 15. The device according to one of claims 7 to 14, characterized in that the auxiliary catalyst ( 2 ) is disc-shaped. 16. The device according to one of claims 7 to 14, characterized in that the auxiliary catalyst ( 2 ) has convex surfaces opposite to the exhaust gas flow and the concave surface faces the main catalyst ( 1 ). 17. The apparatus according to claim 16, characterized in that the auxiliary catalyst ( 2 ) has the shape of a hollow spherical section or a hollow cone. 18. Device according to one of claims 7 to 17, characterized in that the auxiliary catalyst ( 2 ) extends in the direction in which the exhaust gas flows through it over a length of at most 10 mm, preferably not more than 5 mm. 19. Device according to one of claims 7 to 18, characterized in that the housing ( 3 ) of the main catalyst ( 1 ) between its inlet opening ( 6 ) and the main catalyst ( 1 ) has an expanding section ( 5 ) and that the auxiliary catalyst ( 2 ) in this section of the housing ( 5 ). 20. The apparatus according to claim 19, characterized in that the auxiliary catalyst ( 2 ) has a correspondingly widening cross section in the direction of the exhaust gas flow. 21. Device according to one of claims 7 to 20, characterized in that the auxiliary catalyst ( 2 ) contains only platinum as the catalytically active metal.