Simultaneous LNT and DPF regeneration

Abstract

A method is provided for operating a diesel engine exhaust aftertreatment system including a DPF and a LNT. The LNT is regenerated as soot is combusting in the DPF. To accomplish this, reductant is injected between the DPF and the LNT during soot combustion, whereby the LNT experiences rich conditions as the DPF experiences lean conditions. Preferably, the DPF is of small size whereby the DPF is heated easily and needs to be regenerated almost as often as the LNT needs to be denitrated. In an exemplary process, DPF regeneration begins in response to a control signal to denitrate the LNT. Preferably, the reductant is diesel fuel and there is a fuel reformer between the DPF and the LNT. Oxygen consumed by soot combustion reduces the fuel penalty for regenerating the LNT and promotes stable reformer operation. Heat generated in the DPF helps warm the fuel reformer.

1 . A method of operating a diesel engine exhaust aftertreatment system, comprising:
 passing the exhaust through a DPF and a LNT in that order, whereby under lean conditions the LNT adsorbs NOx from the exhaust and stores the NOx;   generating a control signal to regenerate the LNT to remove stored NOx;   in response to the control signal, heating the DPF to ignite combustion of trapped soot; and   as the trapped soot is combusting in the DPF, injecting a reductant upstream of the LNT, but downstream of the DPF, at a rate that leaves the exhaust rich whereby NOx stored in the LNT is reduced and the LNT regenerates.   
   
   
       2 . The method of  claim 1 , wherein there is a device comprising an oxidation catalytic between the LNT and the DPF and the reductant is injected upstream of the device. 
   
   
       3 . The method of  claim 1 , wherein:
 the reductant is diesel fuel;   there is a fuel reformer between the DPF and the LNT; and   the diesel fuel is injected upstream of the fuel reformer.   
   
   
       4 . The method of  claim 1 , wherein heating the DPF comprises injecting reductant into the exhaust upstream of the DPF, whereby combustion of the reductant heats the DPF. 
   
   
       5 . The method of  claim 4 , wherein the reductant injected into the exhaust upstream of the DPF is injected downstream of the engine. 
   
   
       6 . The method of  claim 1 , wherein the DPF is of a small thermal mass, whereby over the course of the LNT regeneration the DPF generates more heat through soot combustion than the heat required to heat the DPF to ignite combustion of trapped soot. 
   
   
       7 . The method of  claim 1 , wherein the DPF is regenerated as often as the LNT is denitrated. 
   
   
       8 . A method of denitrating a LNT in a diesel engine exhaust aftertreatment system, comprising:
 passing the exhaust through a DPF and a LNT in that order;   injecting fuel into the exhaust upstream of the DPF, whereby at least a portion of the fuel combusts to heat the DPF to a temperature at which soot trapped in the DPF combusts to regenerate the DPF; and   as the trapped soot is combusting in the DPF, injecting a reductant upstream of the LNT, but downstream of the DPF, at a rate that leaves the exhaust rich, whereby NOx stored in the LNT is reduced and the LNT regenerates.   
   
   
       9 . The method of  claim 8 , wherein there is a device comprising an oxidation catalytic between the LNT and the DPF and the reductant is injected upstream of the device. 
   
   
       10 . The method of  claim 8 , wherein:
 the reductant is diesel fuel;   there is a device comprising a fuel reformer between the LNT and the DPF; and   the diesel fuel is injected upstream of the fuel reformer.   
   
   
       11 . The method of  claim 8 , wherein the DPF is of a small thermal mass, whereby over the course of the LNT regeneration the DPF generates more heat through soot combustion than the heat required to heat the DPF. 
   
   
       12 . The method of  claim 8 , wherein the DPF is of a small size, whereby the DPF would generally not be effective if not regenerated at least about 20% as often as the LNT is denitrated. 
   
   
       13 . The method of  claim 8 , wherein the DPF is of a small size, whereby the DPF would generally not be effective if not regenerated at least about once every 10 minutes of engine operation. 
   
   
       14 . The method of  claim 8 , wherein injecting reductant into the exhaust upstream of the DPF comprises injecting reductant into an exhaust manifold upstream of a turbocharger. 
   
   
       15 . The method of  claim 8 , wherein injecting reductant into the exhaust upstream of the DPF comprises injecting fuel into the exhaust in an engine cylinder using a late fuel injection. 
   
   
       16 . The method of  claim 8 , wherein the exhaust aftertreatment system further comprises a SCR catalyst downstream of the LNT. 
   
   
       17 . A power generation system comprising a diesel engine, an exhaust aftertreatment system, and a controller configured to operate the exhaust aftertreatment system according to the method of  claim 8 . 
   
   
       18 . A vehicle comprising an exhaust aftertreatment system and a controller configured to operate the exhaust aftertreatment system according to the method of  claim 8 . 
   
   
       19 . A method of operating a diesel engine exhaust aftertreatment system, comprising:
 passing the exhaust through a DPF and a LNT in that order;   using a first fuel injector, injecting fuel into the exhaust upstream of the DPF, whereby at least a portion of the fuel combusts to heat the DPF to a temperature at which soot trapped in the DPF combusts to regenerate the DPF; and   as the first fuel injector is injecting fuel into the exhaust, injecting fuel into the exhaust between the DPF and the LNT using a second fuel injector, whereby the exhaust downstream of the second fuel injector becomes rich and the LNT undergoes regeneration.   
   
   
       20 . The method of  claim 19 , wherein the first fuel injector continues injecting fuel into the exhaust for a significant period after soot combustion in the DPF  10  has achieved a self-sustaining rate. 
   
   
       21 . The method of  claim 19 , wherein there is a device comprising an oxidation catalytic between the LNT and the DPF and the second fuel injector injects fuel upstream of the device. 
   
   
       22 . The method of  claim 19 , wherein:
 the reductant is diesel fuel;   there is a fuel reformer between the LNT and the DPF; and   the second fuel injector injects fuel upstream of the fuel reformer.