US20070089406A1

US20070089406A1 - Method of avoiding undesired NO2 emissions of internal combustion engines

Info

Publication number: US20070089406A1
Application number: US11/582,913
Authority: US
Inventors: Andreas Doring
Original assignee: MAN Nutzfahrzeuge AG
Current assignee: MAN Truck and Bus SE
Priority date: 2005-10-18
Filing date: 2006-10-18
Publication date: 2007-04-26
Also published as: DE502006000393D1; EP1777384A1; DE102005049655A1; EP1777384B1

Abstract

A method of reducing undesired NO₂emissions of an internal combustion engine that operates with excess air and has an exhaust gas section, including providing the exhaust gas section with at least one catalytic converter that operates with NO oxidation activity, wherein the catalytic converter is adapted to increase the NO₂content in the exhaust gas of the internal combustion engine for exhaust gas post treatment processes. During operation of the internal combustion engine, and adapted to a respective operating level of the internal combustion engine and a state of the catalytic converter, a content of materials that compete with the NO oxidation is varied in such a way that downstream of the catalytic converter only that theoretical NO₂content is present in the exhaust gas that is required for a subsequent exhaust gas post treatment.

Description

The instant application should be granted the priority date of Oct. 18, 2005 the filing date of the corresponding German patent application 10 2005 049 655.5.

BACKGROUND OF THE INVENTION

The present invention relates to a method of reducing undesired NO₂emissions of internal combustion engines that operate with excess air and have an exhaust gas section that is provided with at least one catalyzer or catalytic converter that operates with NO oxidation activity, whereby the catalytic converter having NO oxidation activity is adapted to increase the NO₂content in the exhaust gas of the internal combustion engine for exhaust gas treatment processes that follow.
For a number of exhaust gas post treatment measures in oxygen-rich exhaust gas of internal combustion engines, nitrogen dioxide is a key molecule. For example, as described in EP 0341832 A1, nitrogen dioxide is used, for example, to oxidize soot or carbon in particle filters. It also serves for the acceleration of the selected catalytic reduction of nitrogen oxides with the aid of urea, ammonia and hydrocarbons, as discussed in EP 1054722 A1, JP200000225323 and US 2002 006 4956. For the NO_x, retention catalytic converter technology, it is necessary to be able to store NO_x, in lean phases as nitrate; such methods are described in DE 196 36 041 A1 and JP 19990220418.
In this connection, the NO₂necessary for these technologies is generated on platinum-containing catalytic converters from the nitrogen monoxide emitted by the engine according to the following equation:
2NO+O₂⇄2NO₂ Equation 1
Depending upon the design of the catalytic converters, the platinum content, and the application, the catalytic converters have reaction start temperatures between 180 and 330° C. Reaction start temperature is here meant to designate that temperature at which 50% of the nitrogen monoxide is oxidized to nitrogen dioxide. However, the NO₂proportion of the overall nitrogen oxides again drops as the temperature increases, since at high temperatures the thermodynamic NO/NO₂equilibrium is on the side of NO. For a better understanding, this interrelationship is illustrated in FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph in which the NO₂proportion is plotted against temperature.
As the velocity increases, i.e. as the retention time over the catalytic converter decreases, a decline of the NO₂concentrations can also be observed.
Since the statutorily prescribed reduction cycles of vehicles and engines generally provide low temperature regions in which the threshold values are to be maintained, the catalytic converters are designed such that they can already display high NO₂contents at low temperatures. Otherwise, one could hardly operate one of the previously addressed technologies for exhaust gas post treatment at these temperatures. Thus, for example, at 200° C. it is possible only with the aid of nitrogen dioxide to produce acceptable conversions during the reduction of nitrogen oxides with the aid of ammonia according to the equation:
NO+NO₂+2NH₃→2N₂+3H₂O Equation 2
Similarly, the oxidation of carbon-containing particles at 350° C. at acceptable material quantity change velocities is possible only with the aid of NO₂according to the equation:
NO₂+C→NO+CO Equation 3
The high NO₂concentrations at low temperatures means, however, that at higher temperatures generally more NO₂is produced than is required. This leads to the main problem of the NO₂based technologies, namely that unused NO₂can be found in the emissions. NO₂is a toxic molecule that can lead to chemical attack of the breathing passages. In addition, it reacts in the atmosphere with oxygen to form ozone, and hence increases the ozone concentration close to the ground. This is particularly problematic in cities and urban areas since here the ozone concentrations can increase significantly and thus also contribute to damage to breathing passages and circulatory systems. A further problem is that the emission thresholds are to be maintained not only in the new state of the internal combustion engine or vehicle, but also after a certain running time. Since the effectiveness of catalytic converters customarily decreases over time due to chemical deactivation, thermal loads, etc., the catalytic converters are designed such that they still maintain the threshold values even after a prescribed running time. However, this means that they have to be over designed, which in the case of catalytic converters that operate with NO₂oxidation activity, in the new state they generate significantly more NO₂than would be required. There is thus a conflict in goals between good performance of the post treatment system at low temperatures and over a long running time, and the undesired NO₂emissions at high temperatures and a new catalytic converter.
It is therefore an object of the present invention to provide a method that reliably prevents the NO₂emissions without having a negative impact upon the effectiveness and efficiency of the exhaust gas post treatment system that is respectively used.

SUMMARY OF THE INVENTION

The inventive solution, for reducing undesired NO₂emissions with the arrangements of the aforementioned type, provides a method wherein during operation of the internal combustion engine, and adapted to the perspective operating level of the internal combustion engine and the state of the catalytic converter that operates with NO oxidation activity, the content is varied of materials that compete with the NO oxidation in such a way that downstream of the catalytic converter only that theoretical NO₂content is present in the exhaust gas that is required for the subsequent exhaust gas post treatment. As a result, greater than required NO₂contents advantageously do not even result.
The catalytic converter that operates with NO oxidation activity that is used in this connection advantageously contains platinum and/or platinum oxide as an active component.
An expedient possibility for introducing into the system the materials that compete with the NO oxidation is realized via the internal combustion engine, as a result of which a homogeneous distribution is achieved in the exhaust gas without any additional expenditure.
A further possibility is to effect the variation of the content of the material that competes with the NO oxidation by adding such material into the exhaust gas of the internal combustion engine downstream of the engine and upstream of the catalytic converter that operates with NO oxidation activity. The advantage of proceeding in this manner is that the engine procedures are not impacted.
The inventively added materials that compete with the NO oxidation can be hydrocarbons. In this connection, it is particularly advantageous to use the fuel that is used to operate the internal combustion engine and is comprised of hydrocarbons. This can be accomplished in that the hydrocarbon concentration in the exhaust gas of the internal combustion engine is varied by varying engine parameters, such as start of injection, and/or injection pressure and/or AGR rate, and/or position of a butterfly valve on an intake side, and/or reduced injections.
For the technical transformation of the inventive method, it is advantageous to provide an electronic control unit that with the aid of sensors connected thereto, and/or with the aid of determination models stored in the control unit in the form of computational steps and/or with the aid of characteristics stored therein, determines the required theoretical NO₂content, which is a function of the respective operating level of the internal combustion engine. In conformity with the determined theoretical NO₂contents, the electronic control unit controls the adjustment element or adjustment elements that vary the content of the material that competes with the NO oxidation in the exhaust gas upstream of the catalytic converter that operates with NO oxidation activity. In this connection, the control is effected in such a way that the required theoretical NO₂content is adjusted in the exhaust gas downstream of the catalytic converter that operates with NO oxidation activity. Such electronic control units, which are in a position to work out complex control programs, are readily used for the control of internal combustion engines and can be used to carry out the inventive method.
For the determination of the theoretical NO₂contents, and for the control of the adjustment element or elements, a plurality of parameters can be used that are already included by the electronic control unit for controlling the internal combustion engine, or can be included without great expense. Furthermore, reference can be made to determination models that are already implemented in the control unit in the form of programs or which can be easily implemented therein. The aforementioned parameters and determination models, ofwhich at least one is used for determining the theoretical NO₂content and for controlling the adjustment element, can include, for example, the total quantity of exhaust gas, hydrocarbon concentration in the exhaust gas upstream of the catyalytic converter having NO oxidation activity, the raw emission of NO and/or NO_x, the raw emission of the exhaust gas substituents that are to be reduced, the maximum permissible residual emission of the exhaust gas substituents that are to be reduced downstream of the exhaust gas post treatment, the maximum permissible residual emission of NO₂downstream of the exhaust gas post treatment, the maximum permissible residual concentration of the added materials downstream of the catalytic converter having NO oxidation activity, the maximum permissible residual emission of the added materials downstream of the post treatment system, the catalytic converter temperatures, the exhaust gas temperatures, the catalytic converter model for determining the NO₂conversions in the catalytic converter having NO oxidation activity, the catalytic converter model for determining the conversion rates of the exhaust gas substituents that are to be reduced in the exhaust gas post treatment, the speed of the engine, the intake or supercharge pressure, the quantity of fuel, the catalytic converter aging over the running time, the actual NO₂concentration and/or the actual NO₂content downstream of the catalytic converter having NO oxidation activity, the NO₂residual concentration and/or the NO₂residual content of the exhaust gas downstream of the exhaust gas post treatment system.
The actual NO₂concentration and/or the actual NO₂content and/or the residual NO₂concentration and/or the residual NO₂content that are advantageously usable for a direct regulation of the theoretical NO₂content can be determined by means of one or more sensors. In this connection, the determination of the actual NO₂concentration and/or the NO₂actual content in the exhaust gas stream is effected between the catalytic converter having the NO oxidation activity and the following arrangement for exhaust gas post treatment, while the residual NO₂concentration and/or the residual NO₂content in the exhaust gas stream is determined downstream of the arrangement for the exhaust gas post treatment.
With the actual NO₂concentration and/or the actual NO₂content as an actual value, or the residual NO₂concentration and/or the residual NO₂content as an actual value, a closed control loop can be realized in a straightforward and hence advantageous manner that, by continuous actual value/theoretical value comparison, and continuous setting of the adjustment element or elements, which vary the quantity of the added material, adjust the theoretical NO₂content.
A further advantageous possibility for realizing a direct regulation of the theoretical NO₂contents is to use the hydrocarbon concentration determined by one or more sensors in the exhaust gas as an actual value in a closed control loop, and by means of the control loop, by continuous actual value/theoretical value comparison and continuous setting of the adjustment element or elements, to convey the hydrocarbon concentration determined by the sensor to the theoretical value thereof, and hence to adjust the correct theoretical NO₂value. In this connection, the hydrocarbon concentration can be detected not only between the catalytic converter having NO oxidation activity in the arrangement for the exhaust gas post treatment, but also downstream of the arrangement for the exhaust gas post treatment. It is to be understood that different theoretical values are prescribed in the control loop as a function of the site of determination.
As described above, it is particularly advantageous, for vehicles having the aforementioned type of internal combustion engines, to use the hydrocarbons contained in the fuel for the inventive method. In this connection, reference is made to the known effect that non-combusted hydrocarbons in the exhaust gas, significantly reduce the NO₂yields via catalytic converters that operate with NO oxidation activity (chemical engineering technology (72) 2000 pages 441-449, W. Weisweiler: “Removal of Nitrogen Oxides from Automobile Exhaust Gases that contain Oxygen”). Up to now, this effect has been viewed as being undesirable, because as a result larger catalytic converters or higher precious metal contents were necessary. The inventive method intentionally utilizes this effect in order to control or regulate the NO₂yields in such a way that always only that quantity of NO₂results that is actually required for the subsequent exhaust gas post treatment system. The NO₂emission can thus be held to a minimum in all operating states of the internal combustion engine, taking into consideration the state of the catalytic converter that operates with NO oxidation activity.
As already mentioned, it is expedient in vehicles to recover the hydrocarbons needed for lowering the NO₂contents from the carry along fuel that contains hydrocarbons. This can, for example, be produced by an adjustment of the fuel injection via an electronic control unit (ECU). Thus, it is possible, by an adjustment of the time point of injection and/or the injection pressure to influence the emissions of non-combusted hydrocarbons. A further possibility is to reduce the air/fuel ratio, for example by throttling the intake air, which leads to an increase of the hydrocarbon emissions. If an external exhaust gas return is provided, the emission of hydrocarbons can similarly be varied by varying the rate of return of the exhaust gas. It is also possible to use reduced injections; in this connection, a second injection is undertaken relatively late following the actual main injection. Since due to the expansion of the combustion chamber the latter is already greatly cooled off, no complete combustion of the fuel any longer takes place, resulting in considerable emissions of hydrocarbons.
The known methods that achieve the necessary content of non-combusted hydrocarbon in the exhaust gas upstream of the catalytic converter that operates with NO oxidation activity by influencing the combustion process in the cylinder all have the drawback that this can lead to fuel striking the cylinder walls. This leads to dilution of oil and/or slaking of the cylinder liner. It can therefore be very expedient not to produce the hydrocarbons via the internal combustion engine, but rather to supply the hydrocarbons by means of a separate metering device upstream of the catalytic converter that operates with NO oxidation activity, it after discharge from the cylinder chamber.
At this point it should be noted that of course technologies for exhaust gas purification are known there based upon the supply of hydrocarbons. For example, with NO_xstorage technology, hydrocarbons and carbon monoxide are needed in the rich phases for the regeneration of the NO_xretention. The inventive method differs from this technology on the one hand in that the catalytic converter that operates with NO oxidation activity in contrast to an NO_xretention catalytic converter, operates with excess air even where hydrocarbons are added, and on the other hand the content of hydrocarbons that are not combusted are not relied upon for the minimization of the NO₂emission.
A further exhaust gas purification technology that is used with high hydrocarbon emissions, is the so-called HC-SCR process. In this connection, nitrogen oxides are catalytically reduced with the aid of hydrocarbon. The inventive method also fundamentally differs from this process. On the one hand, with the HC-SCR reaction on platinum-containing catalytic converters the NO_xconversion maximum is at low temperatures (<250° C.) (MTZ (57) 1996, pages 506-514, “NO_xReduction for Diesel Engines, Part 1: Model Gas Test with Nitrogen-Free Reduction Means” T. Wahl, U. Jacob, W. Weisweiler), so that the method can be expediently utilized only at temperatures between 200 and 300° C. In contrast, the inventive method is typically used to reduce the NO₂contents at higher temperatures of 280-400° C. A further difference is in the concentration of the hydrocarbons, whereas for the HC-SCR method great excesses of hydrocarbons in relation to the nitrogen oxides that are to be reduced are necessary (typically 5-15 times the quantity), with the inventive method the hydrocarbon concentrations are less than those for NO_x. In addition, for the HC-SCR method generally short-chained hydrocarbons are used that are either separately carried along or must be recovered from the fuel by cracking (Chemical Engineering Technology (70) 1999 (10) pages 749-753 “Catalytic Cracking of m-dodecane and Diesel Fuel to Improve the Selective Catalytic Reduction of NO_x, in Automotive Exhaust Containing Excess Oxygen” S. Kurze, W. Weisweiler).
Finally, in connection with particle filters, for the active regeneration, to supply hydrocarbons in order to temporarily greatly increase the exhaust gas temperature and thereby to realize the oxidation of the deposited soot particles. Also with this technology, the content of non-combusted hydrocarbons is not relied upon for the minimization of the NO₂emission.
To add the correct quantity of hydrocarbons for the adjustment of the theoretical NO₂contents, a determination or calculation of the actual and theoretical NO₂contents are needed for the catalytic converter that operates with NO oxidation activity and/or for the complete post treatment system. In this connection, the theoretical NO₂contents are to be selected such that the desired conversion of the undesired exhaust gas components, such as NO_x, or soot, can be produced downstream of the overall system with minimal NO₂emissions. For this purpose, appropriate models are first produced at test stands and are transferred into an electronic control unit (ECU). These models then determine the optimum NO₂contents during the time that the internal combustion engine is running. In this connection, incorporated into the theoretical NO₂model is at least one of the following parameters or determination models:
the total quantity of gas,
hydrocarbon concentration in the exhaust gas upstream of the catalytic converter that operates with NO oxidation activity,
the raw emission of NO and/or NO_x,
the raw emission of the exhaust gas substituents that are to be reduced,
the maximum permissible residual emission of the exhaust gas substituents that are to be reduced downstream of the exhaust gas post treatment,
the maximum permissible residual emission of NO₂downstream of the exhaust gas post treatment,
the maximum permissible residual concentration of the supplied materials downstream of the catalytic converter that operates with NO oxidation activity,
the maximum permissible residual emission of the supplied materials downstream of the post treatment system,
the catalytic converter temperatures,
the exhaust gas temperatures,
the catalytic converter model for the determination of the NO₂conversions in the catalytic converter that operates with NO oxidation activity,
the catalytic converter model for the determination of the conversion rates of the exhaust gas contents that are to be reduced in the exhaust gas post treatment (described in EP 0498598 A1, DE 4315278 A1, and EP 98114698 A1),
the speed of the engine,
the intake or supercharge pressure,
the quantity of fuel,
the catalytic converter aging over the running time,
the actual NO₂concentration and/or the actual NO₂content downstream of the catalytic converter that operates with NO oxidation activity,
the residual NO₂concentration and/or the residual NO₂content of the exhaust gas downstream of the exhaust gas post treatment system.
Some of these parameters can be determined directly with the aid of sensors, such as, for example, the quantity of exhaust gas with the aid of a flow measuring device for the NO_xraw emissions of the aid of the NO_xsensor. However, an indirect determination via suitable models and easily accessible measurement data is also possible, as is described in DE 4315278 A1. An example for this would be the determination of the fuel mass from the speed of the internal combustion engine, the number of cylinders and the fuel injection quantity per stroke. However, a determination with the aid of characteristics is also conceivable:
No_xraw emissions from engine speed and engine load,
NO raw emissions from engine speed and engine load,
NO₂conversion from exhaust gas quantity and catalytic converter temperature, possibly additionally NO_xraw emission,
theoretical quantity of the exhaust gas components that are to be reduced from engine speed and engine load, possibly exhaust gas quantity.
With the aid of the thereby determined theoretical NO₂quantity, the quantity of hydrocarbons that is to be supplied can be determined, and by means of the ECU, by varying engine parameters such as beginning of injection, AGR rate, injection pressure or butterfly valve position, can be adjusted or hydrocarbons can be metered in upstream of the catalytic converter that operates with NO oxidation activity, as a result of which the NO₂contents can be varied.
If a suitable NO₂sensor mechanism, such as described, for example, in EP 1384069 A1, JP 19980118026, JP 19970296435, and US 2002/0064956, is available, then instead of a controlled operation a closed control loop can be provided. In this connection, the actual NO₂contents continuously readjusts the theoretical NO₂contents with the aid of the variable hydrocarbon concentrations. The NO₂sensor mechanism used for this purpose can not only be disposed directly downstream of the catalytic converter that operates with NO oxidation activity and yet upstream of components for the exhaust gas post treatment, such as SCR catalytic converter or particle filter, but also downstream of the entire post treatment devices, yet still upstream of the discharge into the atmosphere. Instead of NO₂sensors, it is also possible to build in a sensor mechanism for the determination of the hydrocarbon concentration, as described in DE 19991012102 A1.
It is to be understood that the previous aspects represent examples only; one of skill in the art can embody the inventive method in many ways.
The specification incorporates by reference the disclosure of German priority document 10 2005 049 655.5 filed Oct. 18, 2005.
The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.

Claims

1. A method of reducing undesired NO₂emissions of an internal combustion engine that operates with excess air and has an exhaust gas section, the method including the steps of:

providing said exhaust gas section with at least one catalytic converter that operates with NO oxidation activity, wherein said catalytic converter is adapted to increase the NO₂content in the exhaust gas of the internal combustion engine for exhaust gas post treatment processes that are adapted to follow; and

during operation of the internal combustion engine, and adapted to a respective operating level of the internal combustion engine and a state of said catalytic converter, varying a content of materials that compete with the NO oxidation in such a way that downstream of said catalytic converter only that theoretical NO₂content is present in the exhaust gas that is required for a subsequent exhaust gas post treatment.

2. A method according to claim 1, wherein said catalytic converter that operates with NO oxidation activity contains platinum and/or platinum oxide as an active component.

3. A method according to claim 1, wherein said step of varying a content of materials is effected via the internal combustion engine.

4. A method according to claim 1, wherein said step of varying a content of materials is effected by adding the materials into the exhaust gas of the internal combustion engine downstream of the engine and upstream of said catalytic converter that operates with NO oxidation activity.

5. A method according to claim 1, wherein the materials are hydrocarbons.

6. A method according to claim 5, wherein said hydrocarbons are non-combusted fuel.

7. A method according to claim 5, wherein the hydrocarbon concentration in the exhaust gas of the internal combustion engine is varied by means of a variation of engine parameters.

8. A method according to claim 7, wherein said engine parameters are at least one of beginning of injection, injection pressure, AGR rate, position of a butterfly valve on a suction side, and reduced injections.

9. A method according to claim 1, which includes the further steps of determining the theoretical NO₂content in an electronic control unit with the aid of sensors and/or calculations and/or characteristics, in conformity therewith controlling at least one adjustment element of said electronic control unit, and by means of said at least one adjustment element varying the proportion of the material in the exhaust gas upstream of said catalytic converter that operates with NO oxidation activity in such a way that the theoretical NO₂value is adjusted downstream of said catalytic converter.

10. A method according to claim 9, which includes, for a determination of a theoretical NO₂content and for the control of said at least one adjustment element, using at least one of the following parameters and determination models: total quantity of exhaust gas; hydrocarbon concentration in the exhaust gas upstream of said catalytic converter that operates with NO oxidation activity; the raw emission of NO and/or NO_x; the raw emission of the exhaust gas substituents that are to be reduced; the maximum permissible residual emission of the exhaust gas substituents that are to be reduced downstream of the exhaust gas post treatment; the maximum permissible residual emission of NO₂downstream of the exhaust gas post treatment; the maximum permissible residual concentration of the added materials downstream of the catalytic converter that operates with NO oxidation activity; the maximum permissible residual emission of the added materials downstream of the post treatment; the catalytic converter temperatures; the exhaust gas temperatures; a catalytic converter model for determination of NO₂conversions in the catalytic converter that operates with NO oxidation activity; a catalytic converter model for determination of conversion rates of the exhaust gas substituents that are to be reduced in the exhaust gas post treatment; the speed of the engine; the intake or supercharge pressure; the quantity of fuel; the catalytic converter aging over running time; the actual NO₂concentration and/or the actual NO₂content downstream of the catalytic converter that operates with NO oxidation activity; and the residual NO₂concentration and/or the residual NO₂content of the exhaust gas downstream of the exhaust gas post treatment.

11. A method according to claim 10, which includes determining the actual NO₂concentration and/or the actual NO₂content and/or the residual NO₂concentration and/or the residual NO₂content by means of one or more sensors.

12. A method according to claim 10, wherein determination of the actual NO₂concentration and/or the actual NO₂content in the exhaust gas is effected between said catalytic converter that operates with NO oxidation activity and the following arrangement for the exhaust gas post treatment.

13. A method according to claim 10, wherein determination of the residual NO₂concentration and/or the residual NO₂content in the exhaust gas is effected downstream of the arrangement for the exhaust gas post treatment.

14. A method according to claim 10, wherein the actual NO₂concentration and/or the actual NO₂content serves as an actual value in a closed control loop, and wherein by continuously comparing actual value and theoretical value and continuous setting of the at least one adjustment element for variation of the quantity of the materials to be added, said control loop adjusts the theoretical NO₂value.

15. A method according to claim 10, wherein the residual NO₂concentration and/or the residual NO₂content serves as an actual value in a closed control loop, and wherein by continuously comparing actual value and theoretical value and continuous setting of the at least one adjustment element for variation of the quantity of the materials to be added, said control loop adjusts the theoretical NO₂value.

16. A method according to claim 5, wherein a sensor is used to determine the hydrocarbon concentration in the exhaust gas.

17. A method according to claim 10, wherein the hydrocarbon concentration serves as an actual value in a closed control loop, and wherein by continuously comparing actual value and theoretical value and continuous setting of the at least one adjustment element for variation of the quantity of the materials to be added, said control loop adjusts the theoretical NO₂value.

18. A method according to claim 16, wherein for determination of the hydrocarbon concentration, the sensor is disposed between said catalytic converter that operates with NO oxidation activity and the arrangement for the exhaust gas post treatment.

19. A method according to claim 16, wherein for determination of the hydrocarbon concentration, the sensor is disposed downstream of the arrangement for the exhaust gas post treatment.