DE10023049B4 - Method for determining a mounting point of a precatalyst in the exhaust line of the internal combustion engine - Google Patents

Abstract

Method for determining a mounting point of a primary catalytic converter (18) in the exhaust line (14) of a direct-injection and lean-running internal combustion engine (10) whose exhaust gas purification system (16) comprises at least one catalyst system which successively converts the primary catalytic converter (18) and an NO _x storage catalytic converter (20 ), wherein the mounting point of the precatalyst (18) is given as the average run length of the exhaust gas from the outlet of a cylinder head of the internal combustion engine (10) until entry into the precatalyst (18), characterized in that the average run length in dependence on a HC -Lightoff temperature and a cell density of the precatalyst (18) is determined, wherein the average run length is specified as ≤ 230 mm.

Description

The The invention relates to a method for determining a mounting point a precatalyst in the exhaust line of a direct injection and lean-running Internal combustion engine with the in the preamble of claim 1 mentioned features.

It is known to use so-called emission control systems for cleaning an exhaust gas from internal combustion engines. These emission control systems may include particulate filters and catalyst systems, among others. The latter include components with which on the one hand reducing agents such as incompletely burned hydrocarbons HC and carbon monoxide CO, oxidized with atmospheric oxygen and on the other hand nitrogen oxides NO _x can be converted by the reducing agents into less environmentally relevant products.

In addition to the catalytic components, the catalysts of the catalyst system used may also include storage components that allow selectively to absorb certain gas components, such as NO _x or HC. An NO _x storage component can be combined with the catalyst components to form a so-called NO _x storage catalyst. The NO _x storage catalyst stores in phases in which for a complete catalytic conversion of NO _x not the necessary reducing agent mass flows can be provided (these are preferably lean operating phases of the internal combustion engine), the NO _x as a nitrate. At regular intervals, a regeneration of the NO _x storage catalytic converter must be initiated in order to restore the original storage capacity and NO _x breakthroughs to avoid. For this purpose, a stoichiometric or rich operation of the internal combustion engine is temporarily enforced. Since the phases of stoichiometric and rich operation generally lead to an increase in fuel consumption, the shortest possible residence time of the internal combustion engine in this mode is sought. This is particularly necessary if the internal combustion engine is a direct-injection, lean-running engine, since it provides particularly large consumption advantages in lean shift operation.

In itself, an NO _x storage catalytic converter equipped with a 3-way component could already provide sufficient emission reduction across all operating modes of the internal combustion engine. However, optimal operation of each catalyst arranged in the exhaust system also depends on its minimum temperature (light-off temperature) necessary for a specific conversion reaction. If the catalyst temperature is below the light-off temperature, the specific pollutant component can only be incompletely or partially no longer reacted. In addition to this temperature window for optimal catalytic activity, the temperatures for optimum sorption behavior must also be taken into account for the NO _x storage catalyst. Furthermore, a possible thermal damage to the catalyst or storage components must be taken into account when choosing a mounting point for the NO _x storage catalytic converter. Thus, in addition to a no longer optimal activity or a sudden desorption of stored NO _x - over a material-specific temperature to an irreversible damage to the components come. In direct-injection and lean-running internal combustion engines, the exhaust gas temperature and the mass flow of reducing agents can be well above the temperatures and the reducing agent mass flow of conventional engines, so that to ensure a permanent readiness for operation of the NO _x storage catalytic converter must be arranged relatively distant from the engine in the exhaust system.

However, with a relatively remote engine arrangement of the NO _x storage catalytic converter must be taken into account that this does not yet have the necessary light-off temperature in certain operating phases and therefore there may be an increased pollutant emission. For example, in phases of low exhaust gas temperatures or after a cold start of the internal combustion engine, only insufficient pollutant conversion can occur. However, direct-injection, spark-ignited internal combustion engines usually have higher crude HC emissions, the internal composition of which is also shifted towards heavier oxidizable HC components, such as methane. The latter leads to an increase in the HC lightoff temperature. As a result of the arrangement of the NO _x storage catalytic converter remote from the engine, a considerable HC emission must therefore be accepted, at least in the cold start phase.

In order to reduce the cold start emissions, it is known to precede the NO _x storage catalytic converter, a pre-catalyst, which ensures a rapid heating to its light-off temperature due to its close-coupled arrangement. For example, describes DE 198 44 082 C1 an exhaust gas purification system of lean running, optionally equipped with a direct fuel injection internal combustion engine, which has a NO _x storage catalytic converter and a close to the engine pre-catalyst. Out DE 196 40 161 A1 is a self-igniting internal combustion engine with a NO _x storage catalytic converter is known, which is preceded by a converter, wherein the distance between the Ab Gas outlet of the internal combustion engine and the converter is about 20 cm.

task The present invention is to provide a method available with which an optimum mounting point of a precatalyst be set in the exhaust system of an internal combustion engine can, so that the pollutant emission even at low exhaust gas temperatures, especially shortly after a cold start of the internal combustion engine, preferably can be kept low and at the same time the precatalyst is protected against high temperature loads.

According to the invention this object is achieved by the method for determining the mounting point of the precatalyst in the exhaust system according to the features mentioned in claim 1. The method is initially distinguished by the fact that upstream of the NO _x storage catalyst, a precatalyst is mounted in the exhaust system of the internal combustion engine and that a mounting point of the precatalyst is selected so that an average run length of the exhaust gas from the exit of a cylinder head of the internal combustion engine until entering the Pre-catalyst is ≤ 230 mm. As a result of this arrangement of the pre-catalytic converter, which is very close to the engine, it very quickly reaches the HC light-off temperature necessary for HC conversion. Further, according to the method, the average run length is determined as a function of the HC Lightoff temperature and a cell density of the precatalyst on the premise that the mean run length is at least ≤ 230 mm.

To A preferred embodiment of the method becomes the cell density dependent on from exhaust back pressure and a desired emission reduction determined by HC in the cold start phase of the internal combustion engine. Although lets reduce pollutant emissions with increasing cell density, However, due to a rising exhaust gas back pressure increased consumption be accepted. An optimized for technical realization relationship Both parameters can then preferably given by a motor specific Characteristic can be determined.

Of the Pre-catalyst is preferably an oxidation catalyst, in particular a 3-way catalyst. He can preferably be used as trimetal catalyst based on Platinum, palladium and rhodium, as here especially high conversion rates can be achieved. Further, the catalyst components preferably applied to a metallic carrier due to its thermal conductivity a particularly fast and uniform heating of the precatalyst allows. there is preferably a HC-Lightoff temperature of the precatalyst in a temperature range of 220 to 350 ° C. If appropriate high temperature resistant materials for the Pre-catalyst can be used, the average run length preferably to an amount ≤ 190 mm shortened become.

at The internal combustion engine can be a spark ignited or self-igniting internal combustion engine act.

Further preferred embodiments of the invention will become apparent from the others, in the subclaims mentioned features.

The Invention will be described below in an embodiment with reference to the accompanying drawings explained in more detail. It demonstrate:

1 a schematic perspective view of a direct-injection, lean-running internal combustion engine with an emission control system and

2 a ratio of relative emission of HC to a cell density of a precatalyst.

The 1 shows a schematic perspective view of a lean-running, direct-injection internal combustion engine 10 , The internal combustion engine 10 sucks over a suction pipe 12 Fresh air on. The air-fuel mixture to be combusted is formed directly by injection of fuel into the cylinder chamber with the aid of an injection system, not shown here. Such injection systems and methods for operating these systems can be found in the prior art and are therefore not explained in detail at this point. It should be noted that in this way not only a composition of the air-fuel mixture but also its distribution in the combustion chamber of the cylinder can be influenced (flow profile) and so, for example, a particularly low-consumption lean shift operation can be achieved.

The resulting exhaust gas flows through an exhaust line 14 in which an emission control system 16 is installed. The emission control system 16 includes a precatalyst 18 and a NO _x storage catalyst 20 , On the presentation of other components of the emission control system 16 has been omitted for reasons of clarity. These components can include, for example, particulate filters but also sensor elements with which an exhaust gas composition, an exhaust gas temperature or a content of specific gas components in the exhaust gas can be detected and / or modeled in selected regions. Also, such measurement and evaluation and control and regulatory procedures, the discontinuous operation of the emission control system 16 for optimum operation of the NO _x storage catalyst 20 he possible, can be found in the prior art.

The precatalyst 18 is in this case already in the exhaust manifold 22 - So immediately downstream of the internal combustion engine 10 - assembled. This can - as shown here - using an intermediate flange connection 24 be accomplished. Alternatively, however, methods are also conceivable in which the precatalyst is integrated directly into the exhaust manifold, without such a flange connection 24 necessary is. Such a solution has the advantage that an ingress of air via a possibly leaky flange connection 24 is avoided. Furthermore, an end face 26 of the precatalyst 18 in the exhaust system 14 adjusted prevailing flow profile, so that the most uniform flow through the precatalyst 18 is guaranteed.

The precatalyst 18 is an oxidation catalyst, in particular a 3-way catalyst. To ensure the highest possible conversion rate, it has catalyst components based on platinum, palladium and rhodium. Furthermore, it has a metallic support, which ensures rapid temperature compensation within the precatalyst 18 and guaranteed with the exhaust gas. An HC lightoff temperature of the precatalyst 18 lies in a temperature interval of 220 to 350 ° C at λ = 1.02 to 1.10 and a space velocity sv <80,000 h ^-1 .

Are form and materials of the precatalyst 18 known, so can a mounting point in the exhaust manifold 22 be determined. The mounting point is then the average run length of the exhaust gas from the outlet of a cylinder head of the internal combustion engine 10 until entry into the precatalyst 18 specified. The mean run length is specified as at least <230 mm. If necessary, this value can be reduced to ≤ 190 mm, if sufficient high-temperature stability for the pre-catalyst 18 used materials is ensured. The mean run length now depends on the HC-Lightoff temperature and a cell density of the precatalyst 18 calculated.

The influence of the cell density Z on a relative emission of HC can be determined by the 2 remove. Starting from a low cell density Z, the relative emission initially drops sharply. With increasing cell density Z, however, the incoming emission advantage changes only slightly. Since, on the other hand, with increasing cell density Z, the exhaust gas backpressure also increases and thus fuel consumption of the internal combustion engine 10 increases, can be predefined on the basis of this characteristic a range that represents a meaningful compromise between the two parameters. In this case, the cell density of the precatalyst should be 18 in the range of 200 to 800 cpsi, preferably 300 to 500 cpsi. The characteristic here must be specific to the internal combustion engine used 10 and the conditions within the emission control system 10 be adjusted.

10

Internal combustion engine

12

suction tube

14

exhaust gas line

16

emission control system

18

precatalyzer

20

NO _x storage catalyst

22

exhaust manifold

24

flange

26

face

Claims (12)

Method for determining a mounting point of a precatalyst ( 18 ) in the exhaust line ( 14 ) of a direct-injection and lean-running internal combustion engine ( 10 ), their emission control system ( 16 ) comprises at least one catalyst system which successively separates the precatalyst ( 18 ) and an NO _x storage catalyst ( 20 ), wherein the mounting point of the precatalyst ( 18 ) as the average run length of the exhaust gas from the outlet of a cylinder head of the internal combustion engine ( 10 ) until entry into the precatalyst ( 18 ), characterized in that the average run length as a function of an HC light-off temperature and a cell density of the precatalyst ( 18 ), the average run length being specified as ≤ 230 mm. Method according to claim 1, characterized in that that the mean run length as ≤ 190 mm is specified. Method according to one of claims 1 or 2, characterized in that the cell density as a function of the exhaust gas back pressure and a desired emission reduction of HC in a cold start phase of the internal combustion engine ( 10 ) is determined. Method according to one of claims 1 to 3, characterized in that it is in the internal combustion engine ( 10 ) is a spark-ignited version. Method according to one of claims 1 to 3, characterized in that it is in the internal combustion engine ( 10 ) can be a self-igniting design. Method according to one of the preceding claims, characterized in that as a precatalyst ( 18 ) an oxidation catalyst, in particular a 3-way catalyst, is used. A method according to claim 6, characterized in that as a precatalyst ( 18 ) a trimetal catalyst based on platinum, palladium and rhodium is used. Method according to one of the preceding claims, characterized in that the precatalyst ( 18 ) has a metallic carrier. Method according to one of the preceding claims, characterized in that an HC light-off temperature of the precatalyst ( 18 ) in a temperature interval of 220 to 350 ° C at λ = 1.02 to 1.10 and a space velocity sv <80,000 h ^-1 . Method according to one of the preceding claims, characterized in that the cell density of the precatalyst ( 18 ) in the range of 200 to 800 cpsi, preferably in the range of 300 to 500 cpsi. Method according to one of the preceding claims, characterized in that the precatalyst ( 18 ) is arranged such that its engine-side end face is applied as uniformly as possible from the exhaust gas. Method according to one of the preceding claims, characterized in that the precatalyst ( 18 ) in an exhaust manifold ( 22 ) of the internal combustion engine ( 10 ) is integrated.