DE102007056102B4 - Method for operating an internal combustion engine and an exhaust aftertreatment system connected thereto with a particle filter and an SCR catalytic converter - Google Patents

Abstract

Method for operating an internal combustion engine, in particular a motor vehicle, and an associated exhaust aftertreatment system with at least one particulate filter and at least one SCR catalyst, characterized in that after regeneration of the particulate filter on the passive waiting in normal operation of the internal combustion engine, an increase of the fresh air mass flow through the Internal combustion engine is made.

Description

The invention relates to a method for operating an internal combustion engine, in particular a motor vehicle, and an associated exhaust aftertreatment system with a particulate filter and an SCR catalyst according to the preamble of patent claim 1, and an exhaust aftertreatment system according to claim 4. The inventive method is particularly for diesel engines with connected Exhaust gas aftertreatment with nitrogen oxide and particle reduction function suitable.

State of the art

To reduce pollutants, in particular for the reduction of nitrogen oxides, various methods have been established in which reducing fluids (gases or liquids) are introduced into the exhaust system of an internal combustion engine.

For the reduction of nitrogen oxides, especially the SCR technology has proved successful, in which the nitrogen oxides (NOx) contained in the oxygen-rich exhaust gas are selectively reduced to nitrogen and water with the aid of ammonia or a corresponding precursor substance convertible to ammonia. Preference is given here to aqueous urea solutions. The urea solution is hydrolyzed by means of hydrolysis catalysts or directly on the SCR catalyst to ammonia and carbon dioxide. The urea solution is injected into the exhaust stream by means of special metering systems upstream of the hydrolysis catalyst or the SCR catalyst.

In addition, it is important, especially in diesel engines, to filter out the resulting soot particles from the exhaust gas. Corresponding particulate filters are known, wherein the regeneration of the loaded particulate filter can be carried out actively by burning accumulated particles with exhaust gas contained in the exhaust gas temperatures of 500 ° C or above and / or passive with nitrogen dioxide contained in the exhaust gas at lower temperatures.

In the DE 43 10 926 A1 An exhaust gas emission control method for an internal combustion engine is described, in which the exhaust gas recirculation rate and the temperature of the exhaust gas are regulated as a function of the temperature of the catalytic converter. Especially in operating phases with low load, the temperature of the exhaust gas should be increased by suitable means such as a blue flame burner, an electric heater or a particulate filter according to this document in order to achieve an optimized catalytic effect. On the other hand, exhaust gas at too high a temperature should be cooled down to a target temperature with suitable means, such as air / air heat exchangers or water coolers.

In the DE 103 00 298 A1 For example, an exhaust aftertreatment device with a nitrogen oxide storage catalyst and an SCR catalyst and an associated exhaust aftertreatment process are described. In this case, a particulate filter is provided upstream of the nitrogen oxide storage catalytic converter or between it and the SCR catalytic converter or downstream of the SCR catalytic converter and / or an NO 2 formation catalytic converter upstream of the SCR catalytic converter. The timing of regeneration operation phases of the nitrogen oxide storage catalyst may be determined depending on the nitrogen oxide content of the exhaust gas downstream of the nitrogen oxide storage catalyst or the SCR catalyst and / or its ammonia loading. In addition, a desired ammonia generation quantity can be determined for a respective regeneration operating phase. The device makes it possible to protect the SCR catalyst from high temperatures after particle filter in the thermal regeneration of the particulate filter.

The DE 34 02 692 A1 shows a method and apparatus for the regeneration of a catalytic particulate filter in the exhaust stream of a combustion source, in particular a diesel engine. During regeneration, the input air is pulsed, but preferably not necessarily, to a portion of all the cylinders of an engine when the combustion source is a multi-cylinder engine. As a result, the entire exhaust gas flow of all cylinders contains sufficient fuel, in particular in the form of carbon monoxide and a hydrocarbon fraction, which or catalytically oxidizes and thereby generates exothermic heat. By releasing heat, as with this oxidation, the filter is heated beyond the point of ignition of the particles retained in the filter, these particles are ignited and burned to clean and regenerate the filter.

These proposals for optimizing the temperature of the exhaust gas before entering the exhaust gas treatment system are complicated and require additional facilities and tools that make the exhaust treatment system in addition complicated. The continuous adjustment of the exhaust gas temperature requires additional sensors and control, which are further potential sources of error and can affect the life of the entire exhaust aftertreatment system.

Furthermore, there is the problem of a high exhaust gas temperature for the SCR catalyst when the high temperature is caused by an active or passive regeneration of the particulate filter. There are no suitable measures except the shutdown or bypass of the SCR catalyst known that for this operating condition Provide optimization. This leads to shortened lifecycles of the installed SCR catalysts whose surfaces age prematurely. At the same time, no optimal working of the nitrogen oxide reduction can be ensured for the operating state during and after the particle filter regeneration.

task

The object of the present invention is therefore to provide a method for operating an internal combustion engine, in particular a motor vehicle, and an exhaust aftertreatment system connected thereto with a particulate filter and an SCR catalyst, which enables a simple optimization of the SCR catalyst temperature, in particular after the regeneration of the particulate filter.

This is inventively achieved by a method according to claim 1 and an exhaust aftertreatment system according to claim 4. In the dependent claims each preferred developments of the invention are given.

According to the invention, a method for operating an internal combustion engine, in particular a motor vehicle, and an associated exhaust aftertreatment system with at least one particulate filter and at least one SCR catalyst is proposed in which, after a regeneration of the particulate filter via passive waiting in normal operation of the internal combustion engine, an increase of the fresh air mass flow is made by the internal combustion engine.

The inventive increase of the fresh air mass flow through the internal combustion engine cooler exhaust gas is generated. By doing so, the SCR catalyst temperature can be actively lowered so that, compared to the previous operating methods of such systems, a faster return of the SCR catalyst temperature from the areas which are detrimental to the coating of the catalyst, and also from the areas with limited effect and storage capacity of the SCR catalyst is made possible.

This inventive optimization of the temperature behavior of the SCR catalyst after a regeneration of the particulate filter advantageously requires no internals for the active cooling of the exhaust gas in the exhaust gas purification system. Furthermore, an additional complicated control with a variety of parameters is avoided.

Due to the fact that at high temperatures in general an increased nitrogen oxide emission from the side of the internal combustion engine is observed, in the case of a specification for the lowest possible total nitrogen oxide production exhaust gas recirculation should continue. In this way, by means of a compromise specification, rapid cooling by means of raising the fresh air mass flow through the internal combustion engine on the one hand is associated with a minimum nitrogen oxide raw emission.

The increase of the fresh air mass flow through the internal combustion engine can take place in various ways known to the person skilled in the art. Preferably, such an increase can take place with the aid of turbocharging in turbocharged internal combustion engines.

In a further embodiment of the method according to the invention, a reduction in the exhaust gas recirculation rate can be carried out simultaneously to increase the fresh air mass flow through the internal combustion engine.

In this way, an even faster cooling rate can be achieved. The reduction of the exhaust gas recirculation rate may even include a complete shutdown of the exhaust gas recirculation. Then the fresh air mass flow is further increased and the cooling of the SCR catalyst temperature is particularly fast. As already stated above, this measure can affect the engine-side nitrogen oxide emissions, but this can be favorable in consideration of the other operating parameters.

With the method according to the invention in the embodiments presented, an exhaust gas recirculation rate between normal rate and complete shutdown can accordingly be set according to the operating state at regeneration end of the particulate filter, which achieves optimal temperature recycling of the SCR catalyst on the one hand and a desired engine-side nitrogen oxide crude emission on the other hand.

Preferably, the transitions in the method according to the invention between the operating adjustments to the particle filter regeneration, to increase the fresh air mass flow through the engine and to reduce the exhaust gas recirculation rate via setpoint ramps can be performed.

As a result, impairments in ride comfort can be avoided.

The invention further relates to an exhaust gas aftertreatment system for carrying out an aforementioned method, preferably for a motor vehicle comprising at least one particle filter and at least one SCR catalytic converter, wherein the exhaust gas aftertreatment system has means for controlling the increase of the fresh air mass flow through the internal combustion engine.

Preferably, the exhaust aftertreatment system may additionally comprise means for reducing the exhaust gas recirculation rate. With active control of the exhaust gas recirculation rate, in conjunction with the control of the fresh air mass flows of the internal combustion engine, a compromise setting can be achieved between a very rapid cooling of the SCR catalyst temperature and a low engine-side nitrogen oxide emissions.

The invention is explained below by way of example with reference to several embodiments in conjunction with the drawing, without being limited thereto.

In these shows:

1 a schematic representation of the temperature profile in an SCR catalyst and for the engine-side nitrogen oxide emission course.

1 shows a schematic representation of the temperature profile in an SCR catalyst, starting from the temperature in normal operation 1. In the time course, shown along the X-axis, follows in the diagram after a heating period 2, a regeneration of the particulate filter 3. In this phase, both an increase in the SCR catalyst temperature in the upper part of the schematic diagram and an increase in the engine-side nitrogen oxide emissions at the bottom of the schematic diagram are shown, which are inevitably associated with an active or passive regeneration of the particulate filter. After completing the particulate filter regeneration phase 3, three different curves are shown in the diagram for the subsequent cooling phase 4, the upper curve representing no other measure for cooling the exhaust gas or the SCR catalyst than passive waiting (prior art). In the upper SCR temperature profile, the temperature development due to the inventive increase of the fresh air mass flow through the internal combustion engine is shown by the mean alternative curve. The reduction of the SCR catalyst temperature is significantly faster than without action.

The lower temperature curve shows the fastest cooling rate of the SCR catalyst for a likewise combined increase of the fresh air mass flow rate by the internal combustion engine together with a complete interruption of the exhaust gas recirculation.

Conversely, the corresponding in this Phase 4 Stickoxidrohemissionskurve on a reverse behavior. While here, too, the decrease in nitrogen oxide emissions shows the slowest decline without any further measures (state of the art), the increase in the fresh air mass flow according to the invention with still existing exhaust gas recirculation shows the best results, as can be seen from the lower curve. A very fast in cooling behavior combination of the increase of the fresh air mass flow by the internal combustion engine with a total interruption of exhaust gas recirculation leads to an improved reduction of nitrogen oxide emissions in this phase as shown in the middle curve, but this combination does not make the optimum in terms of variable nitrogen oxide emission of the engine. In the last phase 5, only the curve behavior for the SCR temperature and the raw emission is shown without any measures.

In summary, therefore, a method for operating an internal combustion engine, in particular a motor vehicle, and an associated exhaust aftertreatment system with at least one particulate filter and at least one SCR catalyst is proposed, in which after a regeneration of the particulate filter, an increase of the fresh air mass flow is made by the internal combustion engine. In this way, without complex internals for actively cooling the exhaust stream and without expensive controls depending on a variety of variables, active cooling of the SCR catalyst temperature after regeneration of the particulate filter can be achieved resulting in rapid return of the SCR catalyst temperature below the thresholds cause damage to the coating and the optimum operating state.

In addition to an extension of the life of the SCR Katalysatorbeshichtung can be achieved with the erfindugnsgemäßen process improved exhaust gas purification after regeneration of the particulate filter.

Claims (5)

A method for operating an internal combustion engine, in particular a motor vehicle, and an associated exhaust aftertreatment system with at least one particulate filter and at least one SCR catalyst, characterized in that after regeneration of the particulate filter on the passive waiting in normal operation of the internal combustion engine, an increase of the fresh air mass flow through the Internal combustion engine is made. A method according to claim 1, characterized in that at the same time to increase the fresh air mass flow through the internal combustion engine, a reduction of the exhaust gas recirculation rate is made. A method according to claim 1 or 2, characterized in that the transitions between the operating adjustments to the particle filter regeneration, to increase the fresh air mass flow through the engine and to reduce the exhaust gas recirculation rate via setpoint ramps are performed. Exhaust after-treatment system for carrying out a method according to one of the preceding claims 1 to 3 preferably for a motor vehicle comprising at least one particulate filter and at least one SCR catalyst, characterized in that the exhaust aftertreatment system comprises means for controlling the increase of the fresh air mass flow through the internal combustion engine. Exhaust after-treatment system according to claim 4, characterized in that it additionally comprises means for reducing the exhaust gas recirculation rate.

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