Abstract:
A method ( 10 ) for injecting ammonia (NH 3 ) into exhaust gas upstream of a catalyst of an aftertreatment system includes the steps of determining whether a regeneration event is imminent ( 14 ) on the basis of predetermined parameters, and determining whether dosing parameters are met ( 22 ). The method ( 10 ) further includes the steps of calculating an amount of NH 3  to fill the catalyst ( 18 ) and adjusting a quantity of NH 3  dosed ( 22 ) before the regeneration event occurs.

Description:
BACKGROUND 
       [0001]    Embodiments described herein relate to a method for injecting an emission liquid reductant into a gas stream, and more particularly, to a method for injecting ammonia into an exhaust gas stream of a diesel engine. 
         [0002]    Diesel engine combustion results in the formation of nitrogen oxides, (NO x ), in the exhaust gas. Aftertreatment systems, typically selective catalytic reduction systems (SCR systems) are used to reduce oxides of Nitrogen (NO x ) emitted from engines. Nitrogen oxides can be reduced by ammonia (NH 3 ), yielding N 2 , H 2 O and CO 2 . In the aftertreatment process, NOx reacts with the ammonia, which is injected into the exhaust gas stream upstream of an SCR Catalyst. 
         [0003]    The aftertreatment process controls the ammonia injecting rate. An insufficient injection may result in low NOx conversions, and an injection rate that is too high may result in release of ammonia to the atmosphere, known as ammonia slip. The ammonia slip increases at higher NH 3 /NOx ratios. The ammonia injection depends on the injection characteristics and the energy of the exhaust gas. If the ammonia is improperly dosed with respect to the amount, the timing, the temperature, the space velocity, the current storage level and the NOx mass flow, ammonia slippage may result. 
         [0004]    When the diesel particulate filter (DPF) of the aftertreatment system is in an active regeneration mode, the exhaust temperature and flow rate increase with respect to base operation. In this situation, an increased amount of ammonia is dosed, which depletes the ammonia stored on the DPF. 
         [0005]    Further, for an aftertreatment system that uses solid ammonia, the quantity of ammonia stored on the vehicle should be sufficient to last the service life of the vehicle, while at the same time the quantity of ammonia should be nearly depleted at the end of the service life of the vehicle. Thus, the injection of ammonia should be sufficient to reduce the NOx, and should also correspond to the storage capabilities of the SCR system. 
       SUMMARY 
       [0006]    A method for injecting ammonia (NH 3 ) into exhaust gas upstream of a catalyst of an aftertreatment system includes the steps of determining whether a regeneration event is imminent on the basis of predetermined parameters, and determining whether dosing parameters are met. The method further includes the steps of calculating an amount of NH 3  to fill the catalyst, and adjusting a quantity of NH 3  dosed before the regeneration event occurs. 
         [0007]    Another method for injecting ammonia (NH 3 ) into exhaust gas upstream of a catalyst of an aftertreatment system includes the steps of determining whether a regeneration event will occur in less than a predetermined distance to be driven, and determining whether the exhaust gas temperature is below a predetermined temperature. An amount of NH 3  to fill the catalyst to a selected storage level is calculated. The method further includes adjusting the quantity of NH 3  dosed before the regeneration event occurs under the condition that both the regeneration event will occur in less than the predetermined distance and the exhaust gas temperature is below the predetermined temperature. 
         [0008]    In another method for injecting ammonia (NH 3 ) into exhaust gas upstream of a catalyst of an aftertreatment system, the method includes determining whether a regeneration event will occur in less than about 20-miles to be driven and determining whether the exhaust gas temperature is below about 250-degrees C. The actual storage level of NH 3  of the catalyst is read, and the amount of NH 3  to fill the catalyst to a selected storage level is calculated. The method further includes the steps of increasing the quantity of NH 3  dosed before the regeneration event occurs when both the regeneration event will occur in less than about 20-miles to be driven and the exhaust gas temperature is below about 250-degrees C., and subsequently decreasing the quantity of NH 3  dosed when the exhaust gas temperature exceeds about 250-degrees C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a flow diagram showing the method of injecting ammonia into an exhaust gas stream. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Referring to  FIG. 1 , an injection method for injecting ammonia into an exhaust gas stream of a diesel engine is indicated generally at  10 . Although the following description will be directed to a method for injecting ammonia in a vehicle aftertreatment system, the system and method of  FIG. 1  can be used with any diesel engine that emits NOx. 
         [0011]    When the diesel engine combusts diesel, nitrogen oxides form and are released with the exhaust gas (EG). Nitrogen oxides, NOx, are a pollutant that are reduced in the aftertreatment system by ammonia (NH 3 ) resulting in the emission of less harmful nitrogen, N 2 , water, H 2 O, and carbon dioxide, CO 2 . 
         [0012]    In the injection method  10 , the rate of NH 3  injection is based on a soot loading at a diesel particulate filter (DPF). Particulate matter or “soot” from the exhaust gas accumulates on the DPF, and if left unchecked, can create a back pressure in the aftertreatment system. A regeneration event is the periodic oxidation of the collected particulate matter in the aftertreatment system during routine diesel engine operation. When the diesel particulate filter of the exhaust system experiences a build-up of soot, the soot is oxidized to “regenerate” the filter. An active regeneration is initiated periodically depending on the soot loading, the distance driven since the last regeneration, or the amount of time since the last regeneration, among other factors. 
         [0013]    The injection method  10  may be implemented by software on the vehicle, such as at an engine control unit (ECU), however other controllers are possible. At monitoring step  12  and determination step  14 , the ECU monitors and determines whether active regeneration of the DPF is going to occur in less than a predetermined parameter, for example a distance remaining/distance to be driven X. The distance remaining X is an easily measured parameter that relates to the soot loading on the DPF when active regeneration is impending. An exemplary distance remaining X is about 20-miles, however other distances are possible. 
         [0014]    Alternatively, the determination of whether regeneration is imminent may be made by reading a soot level loading at the DPF (load-pf), an amount of time since the last regeneration of the DPF (time-pf), a system back pressure (bp), among other parameters. It is possible that these parameters can be read by sensors, can be estimated values, or can be calculated values that are compared with pre-programmed values. 
         [0015]    At determination step  14 , if active regeneration is not going to occur in less than the predetermined parameter, for example distance remaining X, then the method loops to monitoring step  12 . If active regeneration is going to occur in less than the predetermined parameter, for example distance remaining X, then the storage level of NH 3  at the catalyst is read. 
         [0016]    The NH 3  dosing method  10  is configured to maintain NOx conversion efficiency during all operating conditions. An NOx reduction catalyst is located downstream of the location of NH 3  injection, and the catalyst stores NH 3  on the catalyst surface. At a low temperature, for example about less than 250° C., the NH 3  is stored on the catalyst. As the temperature increases, the storage capability of the catalyst diminishes and the NH 3  on the surface is released to either react with the incoming exhaust gas stream or escapes through to the tailpipe. 
         [0017]    At catalyst read calculation step  16 , the storage level SL of the catalyst is read. At fill catalyst step  18 , the storage level SL is used to calculate the amount of NH 3  to fill the catalyst. The current storage level (SL) is compared to the actual storage level of the catalyst to determine how much NH 3  is to be injected to maintain a selected storage level, which is typically less than the true physical capacity of the catalyst. Other factors may also determine how much NH 3  is injected, such as the temperature of the exhaust gas, the flow rate of the exhaust gas, and the area of the engine map that the engine is currently operating in, which will dictate, temperature, flow rate, soot accumulation rate, and the regeneration state, among other factors. 
         [0018]    A replaceable cartridge is used as a source of NH 3 . The cartridge stores a quantity of NH 3  that should be sufficient to last the service life of the vehicle, while at the same time, the quantity of NH 3  should be nearly depleted at the end of the service life of the vehicle. At cartridge usage calculation step  20 , the amount of NH 3  usage to empty the cartridge by the service time is calculated. The ECU tracks the NH 3  that is dosed from the cartridge, for example with an accumulator. 
         [0019]    The catalyst will store the dosed NH 3  during certain conditions of low temperature and low flow rate of exhaust gas. As the exhaust gas temperature increases, the NH 3  storage capacity of the catalyst decreases. At parameter adjustment step  22 , the quantity of NH 3  injected is adjusted when dosing parameters are met. The dosing parameters may be a low temperature of exhaust gas, for example less than about 250-degrees C. Alternatively, the dosing parameters for increasing the amount of NH 3  dosed may be a low temperature and a low flow rate of exhaust gas, for example when the engine is idling at about 700 rpm. It is possible that the dosing parameters are other temperatures and flow rates, or may include other parameters. When the dosing parameters are met, the quantity of NH 3  dosed is adjusted to fill the catalyst with NH 3 . 
         [0020]    Using the method  10 , if the ECU determines that regeneration is going to occur within the predetermined parameters, for example in less than about 20 miles of vehicle usage, and if the ECU determines that the dosing parameters are met, for example the exhaust gas temperature is below 250-degrees C., then the quantity of NH 3  dosed is increased to rapidly fill the catalyst with NH 3  before regeneration occurs. 
         [0021]    Subsequently, when the regeneration event occurs, as indicated by an increase of temperature or some other factor, the dosing of NH 3  will either be decreased or stopped. The NH 3  dosing will be either decreased or completely eliminated during the regeneration event because there is typically a high exhaust gas temperature, high exhaust gas flow rates, and a high likelihood of NH 3  slippage during the regeneration event. The catalyst will use the stored NH 3  on the catalyst for reducing NOx until the NH 3  is depleted. 
         [0022]    At tailpipe NOx calculation step  24 , the quantity of NOx to be reduced to meet tailpipe emission regulations is calculated. The dosing parameters  22  may be adjusted based on the NOx levels at the tailpipe  24 . If insufficient NH 3  is being injected to reduce the NOx to meet the emission regulation, the quantity of NH 3  injected can be increased.