Patent Publication Number: US-2002011069-A1

Title: Method and configuration for cleaning an exhaust-gas flow flowing in an exhaust system of a gasoline engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001] This application is a continuation of copending International Application No. PCT/EP00/00047, filed Jan. 5, 2000, which designated the United States.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a method and a configuration for cleaning an exhaust-gas flow, which is flowing in an exhaust system, from a gasoline engine that is fed with an air/fuel mixture, preferably by direct injection.  
       [0004] During the combustion of hydrocarbons which are found, for example, in gasoline, when using air in a gasoline engine, byproducts, in particular pollutants, substantially including hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NO x ), and possibly soot particles as well, are formed in addition to the principal combustion products carbon dioxide and steam. The level of pollutants and soot particles in the exhaust gas is predominantly dependent on the air/fuel ratio being supplied. Where the air/fuel ratios are low, the mixture composition is said to be “rich” (air deficit). If the air/fuel ratios are high, the mixture composition is described as “lean” (excess air).  
       [0005] Soot is formed predominantly when burning with an extreme deficit of air. Although that condition is not normally reached in a gasoline engine, it may occur locally as a result of inhomogeneity, in particular during a cold-start phase. The formation of soot is generally initiated by thermal cracking of the fuel molecules when there is a lack of oxygen and leads to the polymerization of carbon-rich macromolecules, with hydrogen being cleaved off. Those macromolecules then agglomerate to form the soot particles which are ultimately produced. The considerable increase in soot when the stoichiometric air ratio is approached results from an increasing expansion of rich mixture zones due to an injection quantity being increased. Soot which is formed in rich mixture zones in general scarcely burns without further measures.  
       [0006] In order to ensure that unburned soot particles do not pollute the environment, it is known, for example, from German Published, Non-Prosecuted Patent Application DE 41 17 676 A1, corresponding to U.S. Pat. No. 5,207,990, to provide at least one filter with a structure which promotes the deposition of soot particles in particular in the exhaust system of diesel engines. A particle filter of that type is regenerated, i.e. deposited soot particles are burnt, for example by supplying fuel and air from time to time in order to heat up the particle filter, so that even soot particles formed with a deficit of air burn again. It is also known, for example from German Published, Non-Prosecuted Patent Application DE 40 12 719 A1, to provide the filter with a catalytic converter which converts at least one pollutant component, in particular hydrocarbon (HC), carbon monoxide (CO) and/or nitrogen oxide (NO x ).  
       [0007] Moreover, when burning with an extreme deficit of air, the exhaust gas contains relatively large quantities of CO and HC, while with excess air CO and HC can be almost completely oxidized, up to a certain point. The NO x  content reaches a maximum in the range of slightly lean mixture compositions. However, the specific fuel consumption for the gasoline engine is optimized in that very range. Therefore, if gasoline engines are set to an optimally low consumption, there are high NO x  concentrations as well as moderate CO and HC concentrations in the exhaust gas. Heretofore, the fact that gasoline engines also generate particles which, however, are generally smaller and occupy a lower overall volume than in the case of diesel engines, had received scarcely any consideration. Nevertheless, particles of that type may pollute the environment.  
       SUMMARY OF THE INVENTION  
       [0008] It is accordingly an object of the invention to provide a method and a configuration for cleaning an exhaust-gas flow flowing in an exhaust system of a gasoline engine, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which provide measures and devices for improved cleaning of the exhaust-gas flow.  
       [0009] With the foregoing and other objects in view there is provided, in accordance with the invention, a method for cleaning an exhaust-gas flow, which comprises supplying an air/fuel mixture to a gasoline engine, preferably by direct injection. An exhaust-gas flow generated by the gasoline engine flows through an exhaust system. The exhaust-gas flow in the exhaust system successively flows through at least one honeycomb body having a catalytically active coating, preferably a three-way coating, and a particle filter having a coating storing at least one pollutant component, in particular hydrocarbon, carbon monoxide and/or nitrogen oxide, at least from time to time, so that it is advantageously possible to achieve improved cleaning of the exhaust gas.  
       [0010] With the objects of the invention in view, there is also provided a configuration for cleaning an exhaust-gas flow, comprising a gasoline engine receiving an air/fuel mixture, preferably by direct injection, and generating the exhaust-gas flow. An exhaust system conducts the exhaust-gas flow from the gasoline engine in a flow direction. At least one honeycomb body disposed in the exhaust system has a catalytically active coating, preferably in a three-way catalytic converter. A particle filter disposed in the exhaust system downstream of the at least one honeycomb body in the flow direction has a coating storing at least one pollutant component, in particular hydrocarbon, carbon monoxide and/or nitrogen oxide, at least from time to time.  
       [0011] The structure and mode of operation of the invention have been provided in this way because it has been found that, with a configuration of a particle filter with a coating which, at least from time to time, stores at least one of the pollutant components downstream of at least one honeycomb body with a catalytically active coating, it is surprisingly possible to achieve improved cleaning of the exhaust gas flowing in the exhaust system. This occurs both with regard to the pollutant components and with regard to any soot particles which may occur.  
       [0012] Therefore, in accordance with another feature of the invention, if a rich air/fuel mixture is being supplied to the gasoline engine, for example during the cold-start phase, in order to improve the exhaust-gas cleaning, the coating of the particle filter stores residual HC and/or CO which has not been converted in the honeycomb body.  
       [0013] In accordance with a further feature of the invention, if a lean air/fuel mixture is being supplied to the gasoline engine, in order to improve the exhaust-gas cleaning, the coating of the particle filter stores residual NO x  which has not been converted in the honeycomb body.  
       [0014] As a result, it is advantageously possible to eliminate the elevated concentrations of pollutants in the exhaust gas in each case in a controlled and almost complete manner.  
       [0015] In accordance with an added feature of the invention, in order to further minimize the nitrogen oxide (NO x ) levels, a reducing agent, preferably from a reducing-agent reservoir through at least one reducing-agent line, is fed, for example continuously, to the particle filter as a function of a residual NO x  concentration in the exhaust gas downstream of the honeycomb body.  
       [0016] Alternatively, in accordance with an additional feature of the invention, the reducing agent is fed to the particle filter at intervals, in particular as a function of quantities of residual NO x  stored in the particle filter.  
       [0017] Supplying a reducing agent, for example ammonia, as a function of a residual NO x  concentration in the exhaust gas downstream of the honeycomb body, almost completely prevents excess metering of the reducing agent. As a result, approximately a stoichiometric ratio between residual NO x  and reducing agent is achieved and the cleaning of NO x -containing exhaust gas from the gasoline engine is advantageously improved. A further advantage is that significantly smaller quantities of reducing agents are required than if, for example, reducing agent is fed into an exhaust gas before it flows through a honeycomb body with a catalytically active coating.  
       [0018] In accordance with yet another feature of the invention, there is provided a reducing-agent line preferably ending at the particle filter. The particle filter preferably has an integrated distribution device, to which the reducing-agent line is connected.  
       [0019] In accordance with yet a further feature of the invention, in order to further minimize the hydrocarbon (HC) and carbon monoxide (CO) levels, an oxidizing agent, preferably oxygen (O 2 ), is fed to the particle filter as a function of a residual HC—CO concentration in the exhaust gas downstream of the honeycomb body.  
       [0020] Alternatively and/or in addition, in accordance with yet an added feature of the invention, the coating of the particle filter preferably, at least from time to time, stores oxygen (O 2 ).  
       [0021] Supplying an oxidizing agent as a function of residual hydrocarbons (HC) and residual carbon monoxide (CO) almost completely prevents excess metering. As a result, approximately a stoichiometric ratio between residual HC and/or CO, on one hand, and the oxidizing agent, on the other hand, is achieved and the cleaning of HC-containing and/or CO-containing exhaust gas from the gasoline engine is advantageously improved.  
       [0022] Modern gasoline engines usually have an electronic engine control unit or similar control measures. In accordance with yet an additional feature of the invention, in order to provide improved control of the exhaust-gas cleaning, for example through the use of an engine control unit, at least one measuring probe, which measures at least one pollutant component that has not been converted in the honeycomb body, is disposed between the honeycomb body and the particle filter. In this case it is preferable for at least one measuring probe to be provided in each case to measure the residual pollutant components hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NO x ). In this way, it is advantageously possible to detect residual pollutants which have not been converted in the honeycomb body and to convert them in the downstream particle filter, which has a coating with a corresponding storage capacity, with the assistance of the reducing or oxidizing agent supplied to the filter. The metering of that agent is controlled, for example, through the use of the electronic engine control unit.  
       [0023] In accordance with again another feature of the invention, the particle filter is regenerated by burning the particles. This process is triggered in particular by engine heat and/or by exothermic reactions in the honeycomb body, preferably at intervals which can be predetermined, for example through the use of the engine control unit.  
       [0024] In accordance with a concomitant feature of the invention, as an alternative to the separate configuration which has been described heretofore, the honeycomb body may, at least in partial regions, at the same time form the particle filter. It is therefore advantageously possible to achieve a space-saving structure.  
       [0025] Other features which are considered as characteristic for the invention are set forth in the appended claims.  
       [0026] Although the invention is illustrated and described herein as embodied in a method and a configuration for cleaning an exhaust-gas flow flowing in an exhaust system of a gasoline engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     
    
    
     [0027] The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0028] The FIGURE of the drawing is a diagrammatic and schematic illustration of an exemplary embodiment of the invention, on the basis of which further features, advantages and configurations thereof the will be explained. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0029] Referring now in detail to the single FIGURE of the drawing, there is seen a configuration for cleaning an exhaust-gas flow which flows in an exhaust system or tract  2  of a gasoline or spark-ignition engine  1  that is fed with an air/fuel mixture, preferably through the use of direct injection. At least one honeycomb body  3  with a catalytically active coating, preferably a three-way catalytic converter, and a particle filter  4 , are disposed in succession in the exhaust system, as seen in a flow direction S. The particle filter  4  has a coating which, at least from time to time or temporarily, stores or accumulates at least one pollutant component, in particular hydrocarbon (HC), carbon monoxide (CO) and/or nitrogen oxide (NO x ).  
     [0030] The coating of the particle filter  4  is preferably provided in such a way that, at least partially, when a rich air/fuel mixture is fed to the gasoline engine  1 , residual HC and CO which has not been converted in the honeycomb body  3  is stored. When a lean air/fuel mixture is fed to the gasoline engine  1 , residual NO x  which has not been converted in the honeycomb body  3  is stored.  
     [0031] In each case at least one measuring probe  5 ,  6 ,  7  is provided in order to measure the residual pollutant components hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NO x ). These probes are disposed between the honeycomb body  3  and the particle filter  4  and are in communication, for example, with a non-illustrated electronic engine control unit which advantageously also includes programs for controlling exhaust-gas cleaning. In particular, the electronic engine control unit can be used to determine metered quantities of reducing and/or oxidizing agents which may need to be added.  
     [0032] Therefore, an oxidizing agent is fed to the particle filter  4  or a reducing agent is fed to the particle filter  4  from a reducing-agent reservoir  8  through a reducing-agent line  9  by a pump  11 , as a function of a measured residual HC—CO concentration or a measured residual NO x  concentration in the exhaust gas downstream of the honeycomb body  3 . Preferably, the particle filter  4  has an integrated distribution device  10  which is connected to the reducing-agent line  9 , in particular for supplying the reducing agent.  
     [0033] The reducing agent which is preferably used is fluid ammonia that is carried in the reducing-agent reservoir  8  and can be supplied when required. Alternatively, the reducing agent may also be carried as a stored precursor, for example urea, in the reducing-agent reservoir  8  and can be produced on demand, in particular by pyrolysis. The reducing agent is then fed as a fluid to the particle filter  4 , in particular through the distribution device  10 .  
     [0034] The structure of the particle filter  4  which promotes deposition of soot particles is preferably a porous structure or a channel structure. In the case of a channel structure, channels are preferably at least partially disposed offset and/or transversely. In order to regenerate the particle filter  4 , i.e. to burn the soot particles which have been deposited therein, the particle filter  4  is at least disposed sufficiently close enough behind the honeycomb body  3  for the particles to burn, in particular as a result of exothermic reactions in the honeycomb body  3 , preferably at predeterminable intervals.  
     [0035] Preferably, according to the invention, the honeycomb body  3  may also at the same time form the particle filter  4 , at least in partial regions.  
     [0036] It should also be pointed out that further components, as well as the configuration of the honeycomb body  3  and the particle filter  4  according to the invention, may be provided in the exhaust system or line  2  of a gasoline engine  1 . In particular, it is possible for at least one water trap to be disposed upstream of the honeycomb body  3 . This water trap keeps the honeycomb body  3  and its catalytic coating as dry as possible, in order to be able to effect the desired oxidation or reduction processes in the honeycomb body  3  even at exhaust-gas temperatures of only a few hundred degrees centigrade. Water traps therefore contain materials which are able to collect and store large quantities of water below a defined temperature.  
     [0037] In addition, it is possible for an electrically heatable catalytic converter to be disposed in the exhaust system  2  upstream of the honeycomb body  3 , in order to provide an exhaust-gas temperature which is elevated at least from time to time. In that way, the pollutants are catalytically converted even immediately after the engine has been started. Finally, the honeycomb body  3  itself may be electrically heatable.  
     [0038] The present invention is particularly suitable for exhausts of gasoline engines. In this case, the particle filter  4 , in addition to its task of trapping any soot particles, is also advantageously responsible, during the cold-start phase, in particular, for providing the supplementary oxidation of residual hydrocarbons (HC) and carbon monoxide (CO). When the gasoline engine  1  is running with load, the particle filter  4  is responsible, in particular, for providing the supplementary reduction of residual nitrogen oxides (NO x ).