EP3574201A1

EP3574201A1 - Method for purifying exhaust gas of an internal combustion engine

Info

Publication number: EP3574201A1
Application number: EP18701705.8A
Authority: EP
Inventors: Martin Gückelhorn; Han-Rasmus KLEINE
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 2017-01-27
Filing date: 2018-01-19
Publication date: 2019-12-04
Also published as: DE102017201292A1; WO2018138013A1

Abstract

Method for purifying exhaust gas of an internal combustion engine (2) having at least one combustion chamber (3), comprising at least the following method steps: a) burning of fuel with a first air quantity in the at least one combustion chamber (3), b) conducting of the exhaust gas to at least one catalytic converter (8), c) introducing of an additional air quantity via at least one of the combustion chambers (3) to the at least one catalytic converter (8), and d) converting of pollutants in the exhaust gas by way of chemical reactions of the exhaust gas at least with the additional air quantity in the at least one catalytic converter.

Description

Method for cleaning exhaust gas of an internal combustion engine

The invention relates to a method for purifying exhaust gas of an internal combustion engine, in particular for a motor vehicle.

It is known to clean exhaust gas of an internal combustion engine with catalysts. For this purpose, it is exploited that can be promoted by catalysts chemical reactions in which pollutants from the exhaust gas can be converted into harmless substances. However, such a conversion of fuels regularly requires a high catalyst temperature. Before this minimum temperature is reached, the corresponding chemical reactions can not or only insufficiently catalyzed by the catalyst.

In particular, immediately after switching on an internal combustion engine (starting phase), this requirement may lead to increased pollutant emissions. To counteract this, measures are known to increase the catalyst temperature. However, known measures regularly provide for unpleasant noises and / or vibrations, for increased fuel consumption, reduced efficiency and / or a non-circular running of the internal combustion engine. Also, many known processes require catalysts with a particularly large amount of catalytic material. Since noble metals are particularly suitable as catalytic materials, such catalysts can be very expensive, especially if such catalysts are designed for the reduction of exhaust emissions in a start-up phase.

In addition, in modern small volume internal combustion engines with high maximum power, occasionally additional fuel is injected to cool engine components. Occasionally, there is not enough air available to burn this extra fuel, resulting in increased emissions of hydrocarbons in the exhaust. On this basis, it is the object of the present invention to continue to solve or at least reduce the technical problems described in connection with the prior art. In particular, a method for purifying exhaust gas of an internal combustion engine will be presented, which can enable a particularly efficient and cost-effective emission reduction.

This object is achieved with a method for purifying exhaust gas of an internal combustion engine according to the features of patent claim 1. Further advantageous embodiments of the method are specified in the dependent formulated claims. The features listed individually in the claims can be combined with each other in any technologically meaningful manner and can be supplemented by explanatory facts from the description, with further embodiments of the invention being shown.

According to the invention, a method for purifying exhaust gas of an internal combustion engine having at least one combustion chamber is presented. The method comprises at least the following method steps:

a) burning fuel with a first quantity of air in the at least one combustion chamber,

b) passing the exhaust gas to at least one catalyst,

c) introducing an additional amount of air over at least one of the combustion chambers to the at least one catalyst which does not participate in the combustion in step a), and

d) reacting pollutants in the exhaust gas by chemical reactions of the exhaust gas at least with the additional amount of air in the at least one catalyst.

In the internal combustion engine, an internal combustion engine in a motor vehicle is preferred. In the combustion chambers of the internal combustion engine fuel is burned with air. The combustion chambers are preferably cylinders.

The first amount of air is used to burn the fuel. This means that the oxygen in the first volume of air burns together with the fuel. For this purpose, the first amount of air is introduced via an intake into the combustion chambers and (before or after the introduction) mixed with the fuel. The mixture of fuel and air can be ignited, for example, by a spark (for example in the gasoline engine) or by pressure and / or temperature changes (for example in the case of the diesel engine).

The resulting in such a combustion exhaust gas regularly contains pollutants. For example, these may be carbon monoxide (CO), hydrocarbons (HC) and / or nitrogen oxides (NO, NO ₂ , NO _x ). Such pollutants are harmful to the environment. Therefore, it is the aim of an exhaust gas purification to remove such pollutants from the exhaust gas before the exhaust gas is discharged into the environment of the motor vehicle.

Such cleaning of the exhaust gas can be achieved in particular by a catalyst in which the pollutants are converted into other, less or no harmful substances. The catalyst promotes chemical reactions that take place for this purpose. For this purpose, the exhaust gas is passed to the at least one catalyst according to step b). It is preferred that the entire exhaust gas passes through the catalyst, so that no unpurified exhaust gas enters the environment.

For the combustion of fuel in the combustion chambers of the internal combustion engine, a ratio of fuel to air can preferably be set. The so-called lambda value is a measure of this ratio. If the lambda value is equal to one, complete combustion of the fuel occurs (at least theoretically, with complete and uniform mixing of fuel and air). When the lambda value is less than "1", there is excess fuel, which means that the fuel can not be completely burned, and the fuel-air mixture may be referred to as "rich" when the lambda value is less than "1". If the lambda value is greater than "1", there is a lack of fuel, which means that part of the oxygen remains unused, and the fuel-air mixture may also be called lean if the lambda value is greater than "1".

For example, a (lightly) rich fuel-air mixture can bring about increased performance of the internal combustion engine. Also, by the higher fuel fanteil such a mixture cooling of particular temperature-sensitive components, such as valves, can be achieved. In a rich fuel-air mixture, however, the emission of pollutants is increased. This is due to the fact that in a rich fuel-air mixture pollutants (especially CO and HC) are already formed in increased concentration. On the other hand, this is due to the fact that the pollutants formed can not be optimally converted in the catalyst, because oxygen is missing for the catalytic reactions.

For the operation of the internal combustion engine, therefore, a (light) rich fuel-air mixture is preferred in some operating situations of the internal combustion engine, while a stoichiometric mixture is preferred for the exhaust gas purification.

The described method can help to allow combustion of a (slightly) rich fuel-air mixture with optimal pollutant conversion. In particular, according to step c), the additional amount of air is introduced into the catalyst. The oxygen contained in the additional amount of air can contribute to the catalyst in any case there is a stoichiometric ratio (ie in particular sufficient oxygen is available for the implementation of pollutants) while in the combustion in the at least one combustion chamber of the internal combustion engine is present a rich mixture.

The additional amount of air is preferably air that does not come into direct contact with combustion in the internal combustion engine. This means in particular that the additional amount of air is spatially and / or temporally separated from the burned first air quantity.

The introduction of the additional amount of air into the catalyst takes place in accordance with step c) via at least one of the combustion chambers of the internal combustion engine. This means that the additional amount of air, preferably upstream of the combustion chambers, is introduced into an intake line of the internal combustion engine. The additional amount of air is preferably passed through at least one combustion chamber of the internal combustion engine. This happens then at a time or in a period in which no combustion takes place in the affected combustion chamber. Due to the additional amount of air, a higher oxygen content in the catalyst is preferably achieved without more during combustion Oxygen must be provided (so that a rich fuel-air mixture may be present). For this purpose, the additional amount of air is preferably passed through the at least one combustion chamber, without the additional amount of air is involved in combustion in the combustion chamber.

As an alternative to passing the additional amount of air through at least one combustion chamber of the internal combustion engine, it would also be possible to introduce such an additional air quantity via a line branch directly into the catalytic converter or into an exhaust system with the catalytic converter. The introduction of the additional amount of air upstream of the at least one combustion chamber with the subsequent passage can considerably simplify the design necessary for providing the additional amount of air. Upstream of the combustion chambers are regularly far lower temperatures than downstream about in an exhaust system or on the catalyst. This can be used for the lines, valves and / or couplings of lines upstream of the combustion chambers much cheaper materials. These materials need not be able to withstand the particularly high temperatures of the exhaust gas. For this reason, it is advantageous to introduce the additional amount of air upstream of the at least one combustion chamber and to pass it through the at least one combustion chamber.

The additional amount of air is preferably provided via the (usual) intake line, via which the first quantity of air can also be introduced into the at least one combustion chamber. Thus no additional air lines are needed. Alternatively, it is also preferred that the additional amount of air is provided via its own air line, which separately (separate from the usual intake) can suck in ambient air.

To direct the additional amount of air through the at least one combustion chamber, without the additional amount of air is involved in combustion, for example, a fuel supply for the at least one combustion chamber (temporarily) are turned off. In that case, air (as the additional amount of air) would pass through the combustion chamber without being involved in combustion. Such shutdown of the fuel supply may also be referred to as cylinder deactivation. In particular, in a low-load phase, a cylinder deactivation in particular in internal combustion engines with, for example, eight or twelve cylinders to save fuel and pollutant emissions contribute.

Alternatively, in one embodiment of the method, it is preferred that the introduction of the additional amount of air into the at least one catalyst in step c) takes place at least for the duration of a phase in which at least one of the combustion chambers, a respective inlet valve and a respective outlet valve are opened simultaneously. Such phases may be provided during operation of the valves of a combustion chamber of an internal combustion engine. Preferably, a delivery device is provided for introducing an additional amount of air, with which the additional amount of air can be promoted in such phases targeted by the combustion chamber of the internal combustion engine.

In this embodiment, the additional amount of air may preferably be introduced into the catalyst without having to shut down a cylinder. Therefore, the method described in this embodiment can also be applied at full load or at least at medium load.

It is preferable that each of the combustion chambers has an intake valve and an exhaust valve, respectively. The intake valves and the exhaust valves may be opened and closed via a camshaft, for example. The opening and closing of the valves is preferably carried out according to a cyclic operation of the internal combustion engine. Preferably, the internal combustion engine is operated in four-stroke operation (or is preferably in the internal combustion engine preferably a four-stroke engine). In an intake stroke, when the intake valve is open, air is introduced into the combustion chamber and mixed with fuel (before or after introduction). In a subsequent compression stroke, the fuel-air mixture is compressed with the valves closed. In a subsequent power stroke, for example, when the valves are closed, an explosion of the fuel-air mixture drives a piston. This is followed by an exhaust stroke in which the exhaust gas is led out of the combustion chamber when the exhaust valve is open. After the exhaust stroke follows a new intake stroke.

The phase in which a respective inlet valve and a respective outlet valve are open at the same time in at least one of the combustion chambers is preferably between the exhaust stroke and the intake stroke. If the exhaust valve is closed after the end of the exhaust stroke and / or the intake valve is opened before the start of the intake stroke, both valves are opened simultaneously. In this phase, air (as the additional amount of air) from the intake pipe through the inlet valve into the combustion chamber, flow through the exhaust valve immediately back out of the combustion chamber and so get into the catalyst.

In a further preferred embodiment of the method, at least one exothermic chemical reaction takes place in step d) to convert the pollutants in the exhaust gas, which catalyzes with a catalyst material (in particular a catalytically active coating) of the at least one catalyst.

For optimal conversion of the pollutants in the catalyst, it may be necessary for the catalyst to have a minimum temperature which, depending on the catalyst material and the chemical reaction for the conversion of pollutants, may vary. In order to keep pollutant emissions as low as possible, it is desirable to achieve the required minimum temperature as soon as possible after a start of the internal combustion engine. In this embodiment, the effluent, exothermic chemical reactions may contribute to the warming of the catalyst. An exothermic chemical reaction is a chemical reaction that proceeds with the release of energy. Such chemical reactions on the catalyst can be specifically promoted by the method described here. Combustion in the internal combustion engine may take place under rich conditions (with lambda less than 1). The exhaust gas is supplied with an additional amount of air to cause an exothermic reaction with the remaining hydrocarbons from the first amount of air to a catalyst. In particular, a particularly small amount of catalyst material can be provided in the catalyst by the method described. Because costly precious metals are particularly suitable as catalyst material, costs can be saved.

In a further preferred embodiment of the method, a mass flow of the additional amount of air introduced into the at least one catalyst in step c) is regulated at least as a function of a temperature of the at least one catalyst. The temperature of the catalyst is preferably determined with a temperature sensor in or on the catalyst. Also, the temperature of the catalyst may be calculated from an exhaust gas temperature before and / or after the catalyst. The regulation of the mass flow of the additional amount of air is preferably carried out via an (automatic) valve of the air line, via which the additional amount of air can be preferably introduced into the intake manifold of the internal combustion engine. The control is preferably carried out in a control unit which is connected at least to the valve.

Preferably, the mass flow of the additional amount of air is controlled so that the temperature of the catalyst as far as possible above the minimum temperature (which is required for a reaction of the pollutants), but below a maximum temperature. The maximum temperature is preferably selected such that aging phenomena of the catalyst due to thermal stress are at least largely avoided. For example, the mass flow of the additional amount of air can be controlled so that the temperature of the catalyst is within a range of 300 ° C to 1000 ° C, in particular in a range of 400 ° C to 800 ° C. The greater the mass flow of the additional amount of air, the sooner can exothermic chemical reactions take place in the catalyst. If the temperature of the catalyst is thus below the conversion temperature, the mass flow of the additional amount of air is preferably increased. However, if the temperature of the catalyst is already above the maximum temperature, the supply of the additional amount of air is preferably stopped, so that the exothermic chemical reactions are at least not further promoted.

In a further preferred embodiment of the method, a mass flow of the additional amount of air introduced into the at least one catalyst in step c) is regulated at least as a function of a fuel-air ratio of the internal combustion engine.

The fuel-air ratio (ie the lambda value) is preferably measured via one or more lambda probes, which are connected to the control unit like the (automatic) valve for setting the mass flow of the additional air quantity. Preferably, a stoichiometric mixture is present in the catalyst. This requires in particular a certain oxygen content. Depending on the lambda value of the combustion, there is already a residual oxygen content in the exhaust gas (which is formed during combustion with the first air quantity). The mass flow of the additional amount of air is preferably set such that the oxygen from the additional amount of air together with the (residual) oxygen from the exhaust gas gives the required amount of oxygen, which is suitable for the desired chemical reactions to convert the pollutants in the exhaust gas.

In a further preferred embodiment of the method, a mass flow of the additional quantity of air introduced into the at least one catalyst in step c) is regulated downstream of the at least one catalyst, at least as a function of a pollutant content of the exhaust gas.

In particular, carbon monoxide (CO), hydrocarbons (HC) and / or nitrogen oxides (NO, NO ₂ , NO _x ) come into consideration as pollutants. The respective proportions of these pollutants can be measured or modeled, for example, with a corresponding pollutant sensor downstream of the at least one catalytic converter. The pollutant sensors are preferably connected to the control unit. For monitoring the nitrogen oxides, in particular a nitrogen oxide sensor or a method for modeling can be used.

In this embodiment, it is preferably determined from the remaining pollutant amounts (or pollutant proportions) in the exhaust gas flow direction behind the catalyst whether a stoichiometric mixture was present in the catalyst or not.

In a further preferred embodiment of the method, the additional amount of air is compressed prior to introduction into the at least one catalyst.

By compressing the additional amount of air whose mass flow can be adjusted over a particularly wide range. In particular, a particularly large mass flow can be achieved due to the compression, which is promoted in a short time by the at least one combustion chamber of the internal combustion engine. This can be advantageous, for example, if the additional amount of air in one Phase is passed through the combustion chambers, in which at least one of the combustion chambers, a respective inlet valve and a respective outlet valve are opened simultaneously. In that case, the compression of the additional air volume makes it possible to keep the corresponding phase particularly short, so that the operation of the internal combustion engine is not adversely affected (by a particularly long duration of this phase).

The compression of the air is preferably carried out by means of a conveying device arranged for this purpose, which is in particular an electrical and / or mechanical compressor or a blower. Alternatively, however, it is also preferred that the compression takes place by means of a turbocharger. In particular, the additional amount of air can be diverted from an otherwise compressed for combustion air quantity.

The method is particularly preferred if the internal combustion engine has at least two combustion chambers and steps a) and b) take place at least partially parallel to one another in different combustion chambers.

The invention finds particular application in a motor vehicle comprising at least one internal combustion engine, which is intended and set up to operate the described method for purifying exhaust gas of the internal combustion engine.

The particular advantages and design features described above for the method are applicable to the motor vehicle described and transferable.

In a preferred embodiment, the motor vehicle furthermore has at least one air line for introducing the additional amount of air into the at least one catalytic converter, wherein the at least one air line opens into an intake line of the internal combustion engine.

The invention and the technical environment will be explained in more detail with reference to the figure. The figure shows a particularly preferred embodiment, to which the invention is not limited. In particular, it should be noted that the Figure and in particular the illustrated proportions are only schematically. It shows:

Fig. 1: a schematic representation of a motor vehicle, which is intended and set up for the described method.

1 shows a motor vehicle 1 comprising an internal combustion engine 2. The internal combustion engine 2 has four cylinders as combustion chambers 3. Each combustion chamber 3 has a respective inlet valve 4 and a respective outlet valve 5. In particular, a first amount of air can be passed through the intake valves 4 into the combustion chambers 3 via an intake line 6. In the combustion chambers 3, the first amount of air can be burned with fuel. Resulting exhaust gas can be passed via an exhaust pipe 7 in a catalyst 8 and from there into the environment of the motor vehicle 1. In the catalytic converter 8 pollutants from the exhaust gas can be converted by chemical reactions of the exhaust gas at least with the additional amount of air. In this case, at least one exothermic reaction can take place for the conversion of the pollutants, which is catalyzed with a catalyst material of the catalyst 8.

The motor vehicle 1 furthermore has an air line 9 for introducing an additional amount of air into the catalytic converter 8. The air line 9 opens into the suction line 6 a. Thus, the additional amount of air can be passed through the combustion chambers 3 to the catalyst 8. This can be done by the additional amount of air for the duration of a phase is passed through the combustion chambers 3, in which at least one of the combustion chambers 3, the respective inlet valve 4 and the respective outlet valve 5 are opened simultaneously.

In the air line 9, a compression device 10 is integrated. With the compression device 10, a particularly large mass flow of the additional amount of air can be achieved. Motor vehicle reference

Internal combustion engine

combustion chamber

intake valve

outlet valve

suction

exhaust pipe

catalyst

air line

compacting device

Claims

claims

1 . Method for purifying exhaust gas of an internal combustion engine (2) with at least one combustion chamber (3) comprising at least the following method steps:

a) burning fuel with a first amount of air in the at least one

Combustion chamber (3),

b) passing the exhaust gas to at least one catalyst (8),

c) introducing an additional amount of air over at least one of the combustion chambers

(3) to the at least one catalyst (8) which does not participate in the combustion in step a) and

d) reacting pollutants in the exhaust gas by chemical reactions of the

At least with the additional amount of air in the at least one catalyst (8).

2. The method of claim 1, wherein the introduction of the additional amount of air in the at least one catalyst (8) in step c) takes place at least for the duration of a phase in which in at least one of the combustion chambers (3) a respective inlet valve (4) and a respective outlet valve (5) are opened simultaneously.

3. The method according to any one of the preceding claims, wherein a mass flow of in step c) in the at least one catalyst (8) introduced additional amount of air is controlled at least in response to a temperature of the at least one catalyst (8).

4. The method according to any one of the preceding claims, wherein a mass flow of in step c) in the at least one catalyst (8) introduced additional amount of air is controlled at least in dependence on a fuel-air ratio of the internal combustion engine (2).

5. The method according to any one of the preceding claims, wherein a mass flow of in step c) in the at least one catalyst (8) introduced additional amount of air is controlled at least in dependence on a pollutant content of the exhaust gas downstream of the at least one catalyst (8).

6. The method according to any one of the preceding claims, wherein the additional amount of air is compressed prior to introduction into the at least one catalyst (8).

7. The method according to any one of the preceding claims, wherein the internal combustion engine (2) has at least two combustion chambers (3) and the steps a) and b) take place at least partially parallel to each other in different combustion chambers (3).

8. Motor vehicle (1) comprising at least one internal combustion engine (2), which is intended and set up for operating a method for purifying exhaust gas of the internal combustion engine (2) according to one of the preceding claims.

9. motor vehicle (1) according to claim 8 further comprising at least one air line (9) for introducing the additional amount of air in the at least one catalyst (8), wherein the at least one air line (9) in a suction line (6) of the internal combustion engine (2) opens.