Abstract:
The invention is a method for diagnosing operation of a nonthermal plasma discharge device and a lean NOx trap disposed in the exhaust of an internal combustion engine. The method further includes reducing power to the nonthermal plasma discharge device and determining that the nonthermal plasma discharge device is operating properly when a concentration of NOx of exhaust gases exiting the lean NOx trap increases in response to reducing power to the nonthermal plasma discharge device. Additionally, the nonthermal plasma discharge device is found not to be operating when the NOx concentration remains substantially constant in response to a decrease in power supplied to the nonthermal plasma discharge device.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates to an aftertreatment system for treating exhaust gases from a lean burn engine and, more particularly, to an aftertreatment system comprising a nonthermal plasma discharge device.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is well known by those skilled in the art that internal combustion engines burning a lean mixture of fuel and air consume significantly less fuel than when operating at a stoichiometric mixture of air and fuel. Presently, there are very few lean burn gasoline engines in production due to difficulties in meeting emission requirements. The difficulty is that conventional precious metal catalyst oxidizes CO and hydrocarbons and reduces NOx at high efficiency when the air-fuel mixture is very close to stoichiometric; but, NOx conversion efficiency drops off substantially when the exhaust gases are lean.  
         [0003]     It is known in the art to use a lean NOx trap (LNT) aftertreatment system for processing the products of lean combustion. During lean combustion, NOx is trapped in the LNT. When the LNT is full, the engine is operated rich for a short period of time. The rich exhaust gases cause the absorbed NOx to desorb from catalyst surfaces. Furthermore, the rich exhaust gases contain CO and unburned hydrocarbons that reduce NOx to N2. Although commonly called a lean NOx trap, the LNT actually stores only NO2 to a high degree. Because NOx coming from the engine is predominantly comprised of NO and very little NO2, an oxidation catalyst is provided upstream to cause NO to oxidize to NO2.  
         [0004]     The inventors of the present invention have recognized a difficulty in relying on an oxidation catalyst to perform the oxidation of NO to NO2. Specifically, the oxidation catalyst is only partially active below a temperature of about 200° C. Thus, during warm up or at very low power conditions, the reaction from NO to NO2 is marginal. Consequently, NO proceeds through the LNT and out the vehicle tailpipe unprocessed.  
         [0005]     A known problem with lean NOx traps is their susceptibility to SOx contamination. Most hydrocarbon fuels contain some sulfur. The sulfur oxidizes mostly to SO2 during the combustion process in the combustion chamber. If an oxidation catalyst is placed upstream of the LNT, the SO2 is further oxidized to SO3. SO2 can pass through the exhaust system with no harmful effect. However, SO3, in the presence of water vapor in the exhaust, forms particulates containing sulfuric acid. These become absorbed in the LNT and reduce its conversion efficiency. To overcome sulfur degradation of LNT performance, it is known to periodically regenerate the trap, commonly called deSOx. The SOx can be desorbed and made to exit the LNT when its temperature is raised to a high temperature, in the range of 700-800° C., for a period of time, typically greater than a minute. The inventors of the present invention have recognized several problems with sulfur contamination: first, the LNT operates at less than its optimal efficiency for much of the time due to the sulfur contamination and secondly, the deSOx operation is cumbersome, penalizes fuel economy, and the deSOx temperature is near the temperature at which permanent damage to the LNT occurs making control of deSOx regeneration a challenge. Furthermore, deSOx regeneration is not completely reversible. The propensity of an oxidation catalyst to oxidize SO2 to SO3 is harmful to the LNT. Some LNTs contain precious metals, such as platinum, in their formulation. In such LNTs, the oxidation of SO2 to SO3 happens regardless of whether there is an oxidation catalyst upstream or not.  
         [0006]     The inventors of the present invention have further recognized that it is desirable to provide any aftertreatment system with an onboard diagnostic procedure to detect system deficiencies.  
       SUMMARY OF THE INVENTION  
       [0007]     To overcome disadvantages in the prior systems, the inventors of the present invention have recognized that a nonthermal plasma discharge device can be used to convert NO to NO2 in place of an oxidation catalyst.  
         [0008]     Disclosed is a method for diagnosing operation of a nonthermal plasma discharge device and a lean NOx trap disposed in the exhaust of an internal combustion engine. The method includes reducing power to the nonthermal plasma discharge device and determining that the nonthermal plasma discharge device is operating properly when a concentration of NOx of exhaust gases exiting the lean NOx trap increases in response to reducing power to the nonthermal plasma discharge device. Additionally, the nonthermal plasma discharge device is determined to not be operating when the NOx concentration remains substantially constant in response to a decrease in power supplied to the nonthermal plasma discharge device.  
         [0009]     Also disclosed is method for diagnosing operation of a nonthermal plasma device similar to that discussed above, except that instead of power to the nonthermal discharge device being reduced, it is fuel supply to the nonthermal discharge device which is reduced for diagnostic purposes.  
         [0010]     An advantage of the present invention is in providing information about deterioration of the exhaust aftertreatment system.  
         [0011]     Other advantages, as well as objects and features of the present invention, will become apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a schematic of a typical gasoline engine;  
         [0013]      FIG. 2  shows a schematic of the exhaust aftertreatment system of the engine shown in  FIG. 1 ;  
         [0014]      FIG. 3  is a flowchart showing a method of operating an engine and aftertreatment system according to an aspect of the present invention; and  
         [0015]      FIGS. 4 and 5  are flowcharts showing diagnostic procedures to determine operational activity of the nonthermal plasma discharge device according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     A 4-cylinder internal combustion engine  10  is shown, by way of example, in  FIG. 1 . Engine  10  is supplied air through intake manifold  12  and discharges exhaust gases through exhaust manifold  14 . An intake duct upstream of the intake manifold  12  contains a throttle valve  32  which, when actuated, controls the amount of airflow to engine  10 . Sensors  34  and  36  installed in intake manifold  12  measure air temperature and mass airflow (MAF), respectively. Sensor  24 , located in intake manifold  14  downstream of throttle valve  32 , is a manifold absolute pressure (MAP) sensor. A partially closed throttle valve  32  causes a pressure depression in intake manifold  12 . When a pressure depression exists in intake manifold  12 , exhaust gases are caused to flow through exhaust gas recirculation (EGR) duct  30 , which connects exhaust manifold  14  to intake manifold  12 . Within EGR duct  30  is EGR valve  18 , which is actuated to control EGR flow. Fuel is supplied to engine  10  by fuel injectors  26 . Each cylinder  16  of engine  10  contains a spark plug  26 . The crankshaft (not shown) of engine  10  is coupled to a toothed wheel  20 . Sensor  22 , placed proximately to toothed wheel  20 , detects engine  10  rotation.  
         [0017]     Engine  10  is described as a spark-ignition engine. However, the present invention applies also to a compression-ignition type engine, which could be a homogeneous-charge, compression-ignition or diesel engine  
         [0018]     Continuing to refer to  FIG. 1 , electronic control unit (ECU)  40  is provided to control engine  10 . ECU  40  has a microprocessor  46 , called a central processing unit (CPU), in communication with memory management unit (MMU)  48 . MMU  48  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  46 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  50 , random-access memory (RAM)  54 , and keep-alive memory (KAM)  52 , for example. KAM  52  may be used to store various operating variables while CPU  46  is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU  46  in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  46  communicates with various sensors and actuators via an input/output (I/O) interface  44 . Examples of items that are actuated under control by CPU  46 , through I/O interface  44 , are fuel injection timing, fuel injection rate, fuel injection duration, throttle valve  32  position, spark plug  26  timing, and EGR valve  18 . Various other sensors  42  and specific sensors (engine speed sensor  22 , engine coolant sensor  38 , manifold absolute pressure sensor  24 , air temperature sensor  34 , and mass airflow sensor  36 ) communicate input through I/O interface  44  and may indicate engine rotational speed, vehicle speed, coolant temperature, manifold pressure, pedal position, cylinder pressure, throttle valve position, air temperature, exhaust temperature, exhaust stoichiometry, exhaust component concentration, and air flow. Some ECU  40  architectures do not contain MMU  48 . If no MMU  48  is employed, CPU  46  manages data and connects directly to ROM  50 , RAM  54 , and KAM  52 . Of course, the present invention could utilize more than one CPU  46  to provide engine control and ECU  40  may contain multiple ROM  50 , RAM  54 , and KAM  52  coupled to MMU  48  or CPU  46  depending upon the particular application.  
         [0019]     The exhaust aftertreatment system  80  coupled to engine  10  is shown in  FIG. 2 . A nonthermal plasma discharge device (NPDD)  56  is located downstream of the exhaust manifold  14 . Downstream of NPDD  56  is a lean NOx trap (LNT)  58 . Downstream of LNT  58  is a catalyst containing precious metals on its internal surfaces. NPDD  56  converts NO to NO2, but does so at higher efficiency in the presence of hydrocarbon materials. Thus, preferably, an injector  66  is placed upstream of NPDD  56  supplying fuel or another hydrocarbon. Alternatively, engine  10  is a direct injection engine in which fuel injectors  26  provide fuel into cylinders  16 . In a DI engine, fuel can be supplied to NPDD  56  by injecting after the combustion has occurred in the cylinder and before the exhaust valve closes. Thus, excess fuel is supplied at a time in the cycle in which significant oxidation of the fuel does not occur. In another alternative, the combustion process in the cylinder can be specifically tailored to provide some excess hydrocarbons into the exhaust gases, eg., by fuel stratification. In yet another alternative, a fuel injection pulse from a port fuel injector  26 , as shown in  FIG. 1 , can be caused to occur during valve overlap, i.e., when both intake and exhaust valves are open, allowing fuel to travel through cylinders  16  without being combusted.  
         [0020]     Continuing to refer to  FIG. 2 , exhaust aftertreatment system  80  comprises exhaust gas sensors. Sensors  64 ,  68 ,  70 , and  72  are exhaust gas oxygen (EGO) sensors. Alternatively, sensors  64 ,  68 ,  70 , and  72  are NOx sensors. In a further alternative, there are EGO and NOx sensors located at each of  64 ,  68 ,  70 , and  72 . Sensors  64 ,  68 ,  70 , and  72  provide signals to I/O  44  of ECU  40 . A signal from ECU  40  controls fuel injector  66 .  
         [0021]     It is well known to those skilled in the art that LNT  58  traps primarily NO2. A typical exhaust gas composition has a NO2/(NO+NO2) ratio significantly less than 10%. Thus, if raw exhaust gases were fed to LNT  58 , only a small fraction of the NOx, i.e., the NO2 portion, would be stored within LNT  58 . In the exhaust aftertreatment system  80  shown in  FIG. 2 , NPDD  56  is placed upstream of LNT  58  to convert NO to NO2. Within NPDD  56 , an electrical discharge, in the presence of a small concentration of hydrocarbons causes NO to oxidize to NO2. The exhaust stream, in which the NO has been converted to NO2, is conducted to LNT  58 , in which the NO2 is trapped. This continues until LNT  58  no longer can store more NO2, at which point, LNT  58  is purged by causing the air-fuel ratio in the exhaust to become rich. Rich exhaust gases cause the NO2 to desorb from LNT  58 . Thus, a purge is initiated by operating engine  10  at a rich air-fuel ratio. Alternatively, fuel can be added to exhaust gases to cause the overall stoichiometry to be rich.  
         [0022]     Precious metal catalyst  60 , located downstream of LNT  58 , has two functions. It oxidizes hydrocarbons, aldehydes, and CO during lean and rich operation. During rich operation, stored oxygen in NO2 serves as the oxidant, decomposing or reacting with reductants into N2 and O2. During stoichiometric operation, catalyst  60  also reduces NOx.  
         [0023]     The efficiency at which NPDD  56  converts NO to NO2 is affected primarily by two variables: the amount of electrical energy supplied to the NPDD, P elec , and the amount of hydrocarbons supplied, m f, inj : 
 
η conv =function ( P   elec   , m   f, inj )   (1) 
 
 Both P elec  and m f, inj  penalize system fuel economy. The desired conversion efficiency can be achieved while minimizing fuel economy penalty. The amount of fuel energy consumed in providing the electrical power to drive the NPDD  56  can be determined from: 
 
 P   elec =( m   f, eq   *ΔH   R )/η overall  
 
 where m f, eq  is the equivalent fuel consumed in providing electrical energy to the NPDD  56 , ΔH R  is the enthalpy of reaction of fuel, and η overall  is the overall efficiency of the engine in converting the fuel&#39;s chemical energy into electrical energy and providing that to the NPDD  56 . The value of η overall  is a function of engine operating conditions and is so computed. Alternatively, a constant value of η overall  is used if the magnitude of the range in η overall  over the engine operating map is inconsequential. The total effective fuel consumed in the NPDD  56  is: 
 
 m   f, tot =η overall   *P   elec   *ΔH   R   +m   f, inj .   (2) 
 
 Equation 1 above is solved with the additional constraint that m f, tot , according to equation 2, is minimized. In the above discussion, the hydrocarbon supply is defined as fuel. If the hydrocarbon supply is other than fuel, the above equations apply, except that ΔH R  is the enthalpy of reaction of the fluid being supplied. 
 
         [0024]     Referring now to  FIG. 3 , a routine for operating engine  10  starts in step  100 . In step  102 , the engine is operated at a lean air-fuel ratio. In step  104  it is determined whether [NOx] at the sensor is greater than a [NOx] threshold . Preferably, [NOx] threshold  is determined as a function of engine operating condition. If the threshold is exceeded, it is determined whether the NOx trap is likely to be full. If full, control passes to step  108  in which a purge cycle is accomplished by causing the engine air-fuel ratio to be rich. At the same time, the amount of electrical energy, P elec , and the amount of hydrocarbons, m f, inj  supplied to NPDD  56  are altered. Preferably, these are curtailed to save energy during the purge. Alternatively, P elec  and m f, inj  are operated at a different level than during trapping. The NOx exiting LNT  58  is conducted into PM catalyst  60 , in which NOx is reacted to N2 and O2, step  110 . If in step  106  it is determined that LNT  58  is not full, one or both of steps  112  and  114  occur: increasing P and increasing m f, inj  to NPDD  56 . Both steps  112  and  114  are shown as consequences of a negative result from step  106 . Alternatively, one of steps  112  and  114  can be accomplished. Then, in step  104 , it is determined whether the action taken in step  112  or  114  caused a decrease in [NOx] below the threshold level. If not, the other of steps  112  and  114  is caused to occur.  
         [0025]     According to another embodiment of the present invention, the loop in  FIG. 3 , comprising steps  102 ,  104 ,  106 ,  112 , and  114 , can be used to update the constants in the operating model of NPDD  56 , according to equation  1 . In yet a further embodiment of the present invention, accessing steps  112  and  114  in  FIG. 3  indicates that the system is not providing the expected conversion of NO to NO2 in NPDD  56 . As mentioned above, one alternative is to adjust the model. Another alternative is to access a system diagnostic routine when steps  112  and  114  are repeatedly accessed with limited improvement in NO conversion efficiency in NPDD  56 .  
         [0026]     A diagnostic routine for NPDD  56  is shown in  FIG. 4 . Beginning in step  198 , the diagnostic routine is initiated. The diagnostic routine is begun only when the LNT has been recently purged. That is, operation of the diagnostic routine is undertaken at a time when a high level of NOx at NOx sensor  70  would signify a problem with NPDD  56  converting NO to NO2, not a problem with LNT being unable to trap NO2. If the starting condition is met, power supply to NPDD  56  is that determined from an engine model of NPDD  56  performance. Based on the present engine operating condition, the model provides an expectation for electrical power and fuel to be supplied to NPDD  56 . Alternatively, a lookup table based on engine operating conditions is used to determine electrical power and fuel to supply to NPDD  56 . Power to NPDD  56  is reduced in step  202 . In step  204 , it is determined whether the NOx concentration at sensor  70  has increased. If there has been an increase in NOx concentration, it is determined that NPDD  56  is working, step  206 . The diagnostic is ended in step  212 . If the result from step  204  is negative, that is NOx concentration has not changed in response to a change in power to NPDD  56 , the model is adjusted in step  214 . Next, it is determined in step  208  whether NPDD  56  power is zero. If so, control passes to step  210  in which it is registered that NPDD  56  is not working. If a negative result from step  208 , control passes to step  202  to further reduce NPDD  56  power. The power is progressively reduced until NPDD  56  power is turned off  
         [0027]     The purpose of progressively lowering power to NPDD  56 , as shown in the loop of steps  202 ,  204 , and  208 , is to ensure that NPDD  210  is truly not working. In one scenario, if the power to NPDD  56  is higher than need be, then dropping the power does not result in a measurable difference in NOx concentration at the exit of LNT  58 . Thus, to obtain an accurate determination of NOx concentration this possible scenario is ruled out by steps  202 ,  204 , and  208 .  
         [0028]     An alternative to the diagnostic method of  FIG. 4  is to instead turn off power to NPDD  56  in step  202 , i.e., turn it off completely rather than progressively reduce NPDD  56  power. Steps  208  and  214  are unnecessary in the alternative. If a negative result is returned from step  204 , control passes directly to step  210 . The difference between  FIG. 4  and the alternative to  FIG. 4  is that by turning off NPDD  56  completely, the NO to NO2 conversion is completely turned off. During even a short interval in which NO is not converted to NO2, NO breaks through exhaust aftertreatment system  80  leading to a momentary increase in exhaust emissions. By reducing the power to NPDD  56  according to the diagnostic method shown in  FIG. 4 , NO to NO2 conversion is lessened but not completely stopped. The emission impact of the  FIG. 4  diagnostic is less than the alternative method.  
         [0029]     An alternative diagnostic strategy in which fuel supply is reduced is shown in  FIG. 5 . The steps are analogous to those in  FIG. 4 , expect that it refers to fuel supply to NPDD  56 .  
         [0030]     While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. Thus, the above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.