CH710337A1 - Apparatus for aftertreatment of an internal combustion engine. - Google Patents

Abstract

The invention relates to an apparatus for exhaust aftertreatment of an internal combustion engine 1. The apparatus comprises an exhaust pipe 2, which connects an outlet of the internal combustion engine 1 with an inlet of an SCR catalytic converter 5, and an additive supply device 3, which is adapted between the outlet of the Internal combustion engine 1 and the inlet of the SCR catalyst 5 to introduce an additive 4 in droplet form in the exhaust pipe 2. The device is characterized in that the additive supply device 3 is designed to change an average droplet size of the additive 4. According to a preferred embodiment, the additive supply device 3 is further designed to change the mean droplet size of the additive 4 in dependence on a state of the internal combustion engine 1 or the exhaust gas aftertreatment device, preferably as a function of an exhaust gas temperature, an exhaust back pressure, a NO x mass flow and / or an additive mass flow. In a further preferred embodiment, the additive is a 32.5% aqueous urea solution.

Description

The invention relates to a device and a method for exhaust aftertreatment of an internal combustion engine.

In recent years, great efforts have been made to reduce the emissions of internal combustion engine pollutants. In part, these efforts can be attributed to the fact that in more and more countries emission limit values for pollutants emitted by internal combustion engines exist, without which it is no longer possible to position them on the market. Particular attention has been given in this context nitrogen oxides. These are regarded as precursors for ozone and in higher concentrations as harmful to health and vegetation.

It is now common practice that certain engine types are equipped with an exhaust aftertreatment system in which a selective catalytic reduction (English: Selective Catalytic Reduction, SCR) of the pollutants contained in the exhaust gas takes place. Here, by means of a selective catalytic reduction catalyst (hereinafter referred to as SCR catalyst), the nitrogen oxides, NOx, produced during the combustion process in the internal combustion engine are significantly reduced. In order to achieve satisfactory results, an additive is added to the exhaust gas emitted from the engine, so that under the effect of the exhaust gas temperature of the constituents of the additive in a chemical reaction ammonia, NH3 is generated, which in the SCR catalyst, the nitrogen oxides, NOx, in dinitrogen, N2, and water, H2O, converts. As an additive, which is supplied to the exhaust gas upstream of the SCR catalyst, an approximately 32.5% solution of ultrapure urea in demineralized water is usually used, which in European-speaking countries often with the brand name AdBlue and in the North American language area as diesel Exhaust fluid or DEF is called.

The supply of the additive to the exhaust gas flow takes place only after exceeding a certain exhaust gas temperature threshold, wherein after exceeding the exhaust gas temperature threshold with increasing exhaust gas temperatures, the amount of additive supplied is also increased until reaching a saturation value. After reaching the saturation value, the additive addition is temperature-independent. Should the additive be added to the exhaust gas prior to reaching the determined lower exhaust gas temperature threshold, the additive will not be fully converted to the necessary components for selective catalytic reduction, thereby decreasing the NOx conversion rate and damaging the SCR catalyst.

Against this background, it is the object of the invention to provide an apparatus and a method for exhaust aftertreatment of an internal combustion engine, in which or the highest possible nitrogen oxide conversion rate can be achieved at the lowest possible consumption of the additive.

This object is achieved with a device for exhaust aftertreatment according to claim 1 and with the corresponding method according to claim 8.

Accordingly, it is provided that the inventive apparatus for exhaust gas aftertreatment of an internal combustion engine comprises an exhaust pipe which connects an outlet of the internal combustion engine with an inlet of an SCR catalyst, and an additive supply means which is adapted between the outlet of the internal combustion engine and to introduce an additive in droplet form into the exhaust pipe at the inlet of the SCR catalyst. In addition, the device according to the invention is distinguished in that the additive supply device contains means with which an average droplet size of the additive can be changed.

The term internal combustion engine includes all internal combustion engines, especially diesel engines for various applications. The exhaust passage communicating with an exhaust of the engine conveys exhaust gas generated by the engine toward an SCR catalyst. The additive supply device introduces an additive into the exhaust pipe and is located in the flow direction of the exhaust gas upstream of the SCR catalyst. So before the exhaust gas hits the SCR catalyst, an additive is added. Due to the exhaust gas temperature, a chemical reaction of the additive produces a substance that converts the nitrogen oxides NOx into dinitrogen, N2, and water, H2O in the SCR catalyst. Further, since the additive supply device of the present invention is designed to change an average droplet size of the additive, it is possible to increase the nitrogen oxide conversion rate while simultaneously decreasing the additive amount. The mean droplet size is the mean diameter of all additive droplets that are fed downstream of the additive feed device in the exhaust pipe in the direction of the SCR catalyst.

It is particularly advantageous if the variation of the droplet size of the additive takes place as a function of a state of the internal combustion engine. Thus, the droplet size is preferably to be determined as a function of an exhaust gas temperature, an exhaust backpressure, a NOx mass flow and / or an additive mass flow.

In this case, appropriate sensors are mounted in the device for exhaust aftertreatment to measure the respective values of the states of the internal combustion engine and the exhaust aftertreatment system. The measured values obtained are passed to a central control unit, which then determines, depending on the measured values, which average droplet size is currently to be set for the exhaust gas aftertreatment in order to achieve an optimum result in the exhaust aftertreatment. The result is then communicated via the central control unit of the additive supply device, which adjusts the average droplet size of the additive accordingly.

As further variables that have an influence on the average droplet size of the additive, so of which the change in the average droplet size also depends, is to call: the speed-torque operating point of the internal combustion engine, the exhaust gas mass flow, the exhaust gas composition (in particular Nitrogen monoxide, NO, nitrogen dioxide, NO2, carbon dioxide, CO2, particulate matter contained and / or contained particulate matter), the fuel quality, the level of use of the inventive device above sea level, or the ambient air pressure, the additive supply pressure, the additive quality, the ambient air temperature, and / or the ambient humidity. All of the aforementioned quantities can be included in the determination of the droplet size.

In a particularly preferred embodiment, the additive supply means is adapted to increase the average droplet size of the additive with an increase in an exhaust gas temperature.

It is clear to those skilled in the art that introducing an additive at an exhaust gas temperature which is below a certain threshold value is detrimental to the SCR catalyst. Accordingly, the addition of the additive into the exhaust gas flow takes place only after exceeding a certain exhaust gas temperature. The threshold at which the addition of an additive begins is typically 200 ° C, preferably 250 ° C. As the temperature continues to increase, away from the threshold, the average size of the droplets of the additive is increased at the threshold as compared to an average size of the droplets of the additive.

Furthermore, it can be provided that the additive supply means a supply line for the additive, a compressor for generating compressed air, a supply line for compressed air, which is connected to the compressor, and one with the supply line for the additive and the supply line for contains the compressed air connected nozzle, which introduces the additive in droplet form in the exhaust pipe. The compressor may take any configuration of an air pump, a compressor or a similar unit. The compressor is used to generate compressed air, so that the additive can be introduced in the form of droplets into the exhaust line with the aid of the nozzle.

Alternatively, the additive supply device can introduce the additive separately from the compressed air in the exhaust pipe, wherein the average droplet size is variable by a arranged between the injection location of the additive and the SCR catalyst mixer. The mixer is the point in the flue where the flue gas, the additive and the compressed air meet. In this case, the guided into the exhaust pipe additive is brought into a certain droplet size and swirled with the exhaust gas.

In a further embodiment, the average droplet size of the additive via a volume of air generated by the compressor can be changed. This means that the droplet size of the additive is changed in an inversely proportional relationship to the change in the air volume flow. With a decreasing air volume flow of the compressor, which is led to the nozzle, the average droplet size of the introduced into the exhaust pipe from the nozzle additive increases. Conversely, the droplet size of the additive decreases with an increasing air volume flow.

Alternatively or in combination, the droplet size of the additive can also be changed via a movable element of the nozzle.

In a further preferred embodiment, the additive is an aqueous urea solution containing 31.8 to 33.2 weight percent urea, preferably 32.5 weight percent urea. The aqueous urea solution is preferably generated with demineralized water. Using the exhaust gas temperature, the components of the aqueous urea solution ammonia, NH3, obtained from a chemical reaction, which converts the nitrogen oxides, NOx, in dinitrogen, N2, and water, H2O in the SCR catalyst.

In the inventive method for exhaust aftertreatment of an internal combustion engine, an additive is introduced into an exhaust gas stream, which is present in the exhaust line in droplet form. Further, the exhaust gas additive mixture is fed to an SCR catalyst. In addition, the mean droplet size of the additive (4) present in the exhaust gas flow can be changed.

In a preferred embodiment of the method, the average droplet size of the additive is changed depending on a state of the internal combustion engine. This preferably takes place as a function of an exhaust gas temperature, an exhaust back pressure, a NOx mass flow and / or an additive mass flow.

The core of the present invention is therefore that, in contrast to the known prior art, the average droplet size of the introduced into the exhaust pipe or introduced additive is variable. As the inventors have found, it is particularly advantageous for a device or a method for the exhaust aftertreatment of an internal combustion engine if the average droplet size of an additive to be introduced or introduced into the exhaust gas line is variable.

This allows an improvement in the nitrogen oxide conversion rate while reducing the amount of additive required for it. In addition, deposits of the additive on the exhaust pipe, the SCR catalyst and other components of the exhaust system are avoided. Since such deposits reduce the nitrogen oxide conversion rate up to the inefficiency of the exhaust aftertreatment device with respect to a reduction of the nitrogen oxide, a very good result with regard to the reduction of nitrogen oxides can be achieved permanently with the device according to the invention or with the corresponding exhaust aftertreatment process. In addition, the said deposits lead to an increase in the exhaust backpressure, whereby the fuel consumption is increased and the engine dynamics is reduced. These disadvantages are also overcome by the device according to claim 1 and the method according to claim 8. Further, because of the improved nitrogen oxide conversion rate, it is possible to downsize the exhaust gas aftertreatment device compared with conventional exhaust gas aftertreatment devices. In particular, this offers a reduction of the SCR catalyst, which contains hydrolysis and thermolysis catalysts, since with an improved NOx conversion rate, the required catalyst area can be reduced.

The invention is explained in more detail with reference to embodiments shown in the drawing. Show it: <Tb> FIG. 1 <SEP> is a schematic basic structure of the exhaust gas aftertreatment device of an internal combustion engine according to an embodiment; <Tb> FIG. 2 <SEP> a relationship of an air volume flow and an average droplet size of an additive, <Tb> FIG. 3 <SEP> a relationship between an exhaust gas temperature and an airflow at respective maximum values for the nitrogen oxide conversion rate, and <Tb> FIG. 4 <SEP> a relationship between the exhaust gas temperature and the average droplet size of the additive at respective maximum values for the nitrogen oxide conversion rate.

Fig. 1 shows a schematic basic structure of an apparatus for exhaust aftertreatment of an internal combustion engine. Starting from the internal combustion engine 1, an exhaust gas (the flow direction of the exhaust gas is indicated by arrows in FIG. 1) is guided through an exhaust gas line 2 to an SCR catalytic converter 5. Between the internal combustion engine 1 and the SCR catalytic converter 5, an additive supply device 3 is provided, which introduces an additive 4 in droplet form into the exhaust gas line 2. The additive supply device contains a nozzle 8 discharging the additive 4 into the exhaust gas flow, to which compressed air generated by a compressor 6 and the additive 4 stored in a reservoir 7 flows. Via a control unit (not shown), the compressor 6 is controlled so that a variation of the air volume flow is performed to the nozzle, so that the average droplet size of the additive 4 changes accordingly. The variably operable compressor 6 therefore enables optimum mixture formation with regard to the additive.

At an elevated exhaust gas temperature, a particularly good mode of action of the conversion of nitrogen oxides is achieved only when a certain additive droplet size is exceeded. At lower exhaust gas temperatures, however, a smaller additive droplet size is the optimum state for a particularly good mode of action. To ensure a particularly good mode of action over a larger exhaust gas temperature range, it is advantageous to change the average droplet size of the additive. In this case, by a sensor (not shown), the exhaust gas temperature measured and given the values to the control unit. This then decides on the air volume flow, the compressor 6 introduces into the nozzle 8. It is therefore changed by a variation of the nozzle 8 supplied amount of compressed air, the average droplet size of the additive in the exhaust pipe as a function of the exhaust gas temperature.

Fig. 2 shows the exemplary relationship between an air volume flow, which is sent to a nozzle 8, and the corresponding size of the dispensed through the nozzle 8 average droplet size of an additive in microns. The unit of standard liters per minute (slpm) used in the abscissa of the graph corresponds to 16.8875 millibar times liter divided by second and is a measure of the amount of molecules per unit time and under standard conditions (standard pressure P = 101 325 Pascal and standard temperature = 0 ° C) flows through a cross section. The abscissa therefore represents an increasing air volume flow starting from the origin. The ordinate of the graph in FIG. 2 indicates a droplet size of an additive brought into droplet form with the exemplary nozzle. The measurement of the droplet size was carried out by means of laser diffraction technology in a range between the additive supply device and the SCR catalyst. As the figure can be seen, it shows the inversely proportional relationship between air flow volume and droplet size. This means that with an increase in the supplied air volume flow to the nozzle, the average droplet size decreases.

FIG. 3 shows a relationship between the exhaust gas temperature in Celsius and an air volume flow in an exemplary device for exhaust gas aftertreatment. For the understanding of FIG. 2, it is important to know that there are maximum nitrogen oxide conversion rates of the exemplary device at the measurement points. For the measuring points up to 250 ° C no additive is added to the exhaust gas, which explains the constant shape of the curve for this exhaust gas temperature range. For larger temperature values, maximum NOx conversion rates result for steadily decreasing air volume flows. The air volume flow, which is still around 8.0 slpm at an exhaust gas temperature of 300 ° C, drops significantly to approx. 6.1 slpm for the exhaust gas temperature of 350 ° C.

Accordingly, it can be seen that after a certain threshold value of the exhaust gas temperature in the exemplary device for exhaust aftertreatment, in this case 250 ° C., is exceeded, a falling air volume flow, which causes an increasing mean droplet size of the additive, achieves optimum nitrogen conversion rates become.

This relationship is also shown in Fig. 4, in which the droplet size in microns is given for the exhaust gas temperature in degrees Celsius, in which there is an optimal nitrogen conversion rate. It can also be seen from FIG. 4 that, after exceeding a certain threshold value, here 250 ° C., with further increasing exhaust gas temperature, optimum nitrogen oxide conversion rates are only achieved if the droplet size also increases.

The explanatory power of FIGS. 2 to 4 is not limited to the exemplary device for exhaust-gas aftertreatment, on the basis of which the measured values on which the FIGS. 2 to 4 were based were obtained. It goes without saying that the fundamental relationships obtained can also be transferred to differently configured exhaust aftertreatment devices.

It will be apparent to those skilled in the art that it is of minor importance in the present invention by which means the mean droplet size of the additive 4 can be varied. Therefore, for the invention is also irrelevant whether the additive 4 is first mixed with the air flow of the compressor 6 and only then introduced into the exhaust pipe 2, or whether the air flow of the compressor is used after introduction of the additive 4 in the exhaust pipe 2, to fluidize the additive exhaust gas mixture to ensure effective mixing of the exhaust gas with the additive 4 at a desired average particle size of the additive.

In a further embodiment, the average droplet size of the additive to be introduced into the exhaust gas line is changed as a function of a state of the internal combustion engine or of the device for exhaust gas aftertreatment. Thus, the droplet size of the additive can be determined as a function of an exhaust gas temperature, an exhaust gas back pressure, a NOx mass flow and / or an additive mass flow. This means that, based on the instantaneous states of at least one of the aforementioned parameters, the optimum average droplet size of the additive to be introduced into the exhaust gas line is determined.

As further states which have an influence on the average droplet size of the additive, ie of which the change of the average droplet size also depends and which can also be used to determine the mean droplet size of the additive to be used, the following may be mentioned: the rotational speed Torque operating point of the internal combustion engine, the exhaust gas mass flow, the exhaust gas composition (in particular with respect to nitrogen monoxide, NO, nitrogen dioxide, NO2, carbon dioxide, CO2, particulate matter contained and / or contained particulate matter), the fuel quality, the level of use of the inventive device above sea level, or the ambient air pressure , the additive feed pressure, the additive quality, the ambient air temperature, and / or the ambient humidity.

It is therefore conceivable to use at least one of the abovementioned states for determining the optimum droplet size of the additive. It is not necessary that the above parameters are measured by appropriate measuring devices. It is conceivable to deposit some of the states within a software country setting, such as the fuel quality, and, for example, enter the amount of operation by the operator of the device for exhaust gas aftertreatment of an internal combustion engine by hand in a control unit.

On the basis of the state values available to it, this control unit can then control the additive supply means 3 in such a way that the optimum mean particle size of the additive 4 determined for this purpose is changed accordingly.

Claims (10)

1. A device for exhaust aftertreatment of an internal combustion engine (1), comprising: an exhaust pipe (2) connecting an outlet of the internal combustion engine (1) to an inlet of an SCR catalyst (5), and an additive supply device (3) which is designed to supply an additive (4) in droplet form between the outlet of the internal combustion engine (1) and the inlet of the SCR catalytic converter (5) to the exhaust gas line (2), characterized in that the additive supply means (3) is further adapted to change an average droplet size of the additive (4). 2. Device according to claim 1, wherein the additive supply device (3) is further adapted to change the mean droplet size of the additive (4) in dependence on a state of the internal combustion engine (1) or the device for exhaust aftertreatment, preferably in response to a Exhaust gas temperature, an exhaust back pressure, a NOx mass flow and / or an additive mass flow. 3. Device according to one of the preceding claims, wherein the additive supply means (3) is further adapted to increase the average droplet size of the additive (4) with an increase in exhaust gas temperature. 4. Device according to one of the preceding claims, wherein the additive supply device (3) contains: a feed line (71) for the additive (4), a compressor (6) for generating compressed air, a compressed air supply line (61) connected to the compressor (6), and a nozzle (8) connected to the supply line (71) for the additive (4) and the supply line (61) for the compressed air; to introduce the additive (4) in droplet form in the exhaust pipe (2). 5. Apparatus according to claim 4, wherein an air flow generated by the compressor (6), which is directed to the nozzle (8), is variable, so that the average droplet size of the additive (4) changes in accordance with a change in the air volume flow. 6. Apparatus according to claim 4 or 5, wherein the nozzle (8) includes a movable element, with which the average droplet size of the additive (4) is variable. 7. Device according to one of the preceding claims, wherein the additive (4) is an aqueous urea solution whose urea content is in the range of 31.8 to 33.2 weight percent, preferably 32.5 weight percent. 8. A method for exhaust aftertreatment of an internal combustion engine (1), wherein in the method: an additive (4), preferably a 32.5 percent aqueous urea solution, is introduced in droplet form into an exhaust gas stream, and an exhaust gas additive mixture is fed to an SCR catalytic converter (5), characterized in that the average droplet size of the additive (4) in the exhaust gas additive mixture is variable. 9. The method of claim 8, wherein the mean droplet size of the additive (4) is changed depending on a state of the internal combustion engine (1) or an apparatus for exhaust aftertreatment, preferably in dependence on an exhaust gas temperature, an exhaust back pressure, a NOx mass flow and / or an additive mass flow. 10. The method according to any one of claims 8 and 9, wherein the average droplet size of the additive (4) is increased with an increase in exhaust gas temperature.