Coupled DPF regeneration and LNT desulfation

Abstract

A diesel engine exhaust aftertreatment system including a DPF and a LNT in that order is operated with simultaneous soot combustion and LNT desulfation. When a control signal to desulfate the LNT is generated, the DPF is heated to ignite combustion of trapped soot. As the trapped soot is combusting in the DPF, reductant is injected downstream of the DPF, but upstream of the LNT at a rate that leaves the exhaust rich, whereby the LNT undergoes desulfation. Soot combustion reduces the fuel penalty for desulfation by removing oxygen from the exhaust. When a reformer is configured upstream of the LNT, soot combustion helps stabilize the reformer operation. In one embodiment, there are two fuel injectors; one upstream of the DPF and one between the DPF and the fuel reformer. Methods are provided for using this type of configuration to operate the reformer when the DPF is not being regenerated.

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 SO x  from the exhaust and stores the SO x ;   generating a control signal to regenerate the LNT to remove accumulated SO x ;   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 downstream of the DPF, but upstream of the LNT at a rate that leaves the exhaust rich, whereby the LNT releases stored SO x  and is regenerated.   
   
   
       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 LNT and the DPF; and   the diesel fuel is injected upstream of the fuel reformer.   
   
   
       4 . The method of  claim 3 , wherein the aftertreatment system selectively increases an injection rate of reductant upstream of the DPF in order to reduce the temperature of the reformer. 
   
   
       5 . The method of  claim 1 , wherein the DPF is regenerated as often as the LNT is denitrated. 
   
   
       6 . The method of  claim 1 , wherein the DPF and LNT are of sizes whereby the DPF needs to be regenerated to remove accumulated soot approximately as often as the LNT needs to be regenerated to remove accumulated SO x . 
   
   
       7 . 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. 
   
   
       8 . The method of  claim 7 , wherein the reductant injected into the exhaust upstream of the DPF is injected downstream of the engine. 
   
   
       9 . A method of desulfating 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 SO x  stored in the LNT is released and the LNT regenerates   
   
   
       10 . The method of  claim 9 , wherein there is a device comprising an oxidation catalytic between the LNT and the DPF and the reductant is injected upstream of the device. 
   
   
       11 . The method of  claim 9 , 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.   
   
   
       12 . The method of  claim 9 , wherein the DPF and LNT are of sizes whereby the DPF needs to be regenerated to remove accumulated soot approximately as often as the LNT needs to be regenerated to remove accumulated SO x . 
   
   
       13 . The method of  claim 9 , wherein the aftertreatment system selectively increases the injection of reductant upstream of the DPF in order to reduce the temperature of the reformer during LNT desulfation. 
   
   
       14 . The method of  claim 9 , wherein the exhaust aftertreatment system further comprises a SCR catalyst downstream of the LNT. 
   
   
       15 . A vehicle comprising an exhaust aftertreatment system and a controller configured to operate the exhaust aftertreatment system according to the method of  claim 9 . 
   
   
       16 . The method of  claim 9 , wherein the fuel injection upstream of the DPF is continued after the DPF has finished heating. 
   
   
       17 . The method of  claim 9 , wherein:
 soot combustion in the DPF reaches a self sustaining rate;   the rate of soot combustion declines to below a self-sustaining rate due to consumption of soot; and   soot combustion is continued after the decline by maintaining the temperature of the DPF with additional fuel injection into the exhaust upstream of the DPF.   
   
   
       18 . A method of operating a diesel engine exhaust aftertreatment system, comprising:
 passing the exhaust through an exhaust line in which are installed a fuel reformer and a LNT, in that order, the LNT being adapted to adsorb and store NOx under lean conditions and to reduce the stored NOx and regenerate under rich exhaust conditions;   generating a control signal to regenerate the LNT; and   in response to the control signal, injecting fuel into the exhaust using two separate fuel injectors including a first fuel injector located upstream of the a second fuel injector;   wherein a substantial portion of the fuel injected with the first fuel injector mixes within the exhaust line with fuel injected with the second fuel injector to form a rich mixture that enters the fuel reformer, which removes oxygen from the exhaust, produces reformate, and thereby regenerates the LNT;   wherein the fuel from the first fuel injector undergoes substantially greater mixing with the exhaust and dispersion along the direction of exhaust flow than the fuel injected with the second fuel injector.   
   
   
       19 . The method of  claim 18 , wherein the first fuel injector is an engine fuel injector that injects the fuel into the exhaust prior to its leaving the engine. 
   
   
       20 . The method of  claim 18 , wherein the first fuel injector injects the fuel into the exhaust as the exhaust travels through an engine manifold upstream of a turbocharger. 
   
   
       21 . The method of  claim 18 , wherein the first fuel injector injects the fuel directly into the exhaust line. 
   
   
       22 . The method of  claim 18 , wherein the control signal is a signal to regenerate the LNT to remove stored NOx. 
   
   
       23 . The method of  claim 18 , wherein:
 the process of regenerating the LNT involves a period of heating the fuel reformer under lean exhaust conditions followed by a period of producing reformate under rich exhaust conditions; and   immediately following the transition from lean to rich, the majority of fuel supplied to the fuel reformer comes from the second fuel injector, but as the rich period progresses, the proportion of fuel supplied to the fuel reformer by the first fuel injector increases.   
   
   
       24 . The method of  claim 18 , wherein a first exhaust system device is installed in the exhaust line downstream of the first fuel injector, but upstream of the second fuel injector. 
   
   
       25 . The method of  claim 24 , wherein a portion of the fuel injected with the first fuel injector combusts in the first exhaust system device to remove a substantial portion of oxygen from the exhaust. 
   
   
       26 . The method of  claim 24 , wherein the first exhaust system device is a DPF. 
   
   
       27 . A method of operating a diesel engine exhaust aftertreatment system, comprising:
 passing the exhaust through an exhaust line in which are installed a first exhaust system device comprising an oxidation catalyst, a fuel reformer, and a LNT, in that order, the LNT being adapted to adsorb and store NOx under lean conditions and to reduce the stored NOx and regenerate under rich exhaust conditions;   generating a control signal to regenerate the LNT; and   in response to the control signal, injecting fuel into the exhaust using two separate fuel injectors including a first fuel injector located upstream of the first exhaust system device and a second fuel injector located between the first exhaust system device and the fuel reformer;   whereby a portion of the fuel injected with the first fuel injector combusts in the first exhaust system device to remove a substantial portion of oxygen contained in the exhaust from the exhaust;   the combined fuel injections result in a rich exhaust mixture that enters the fuel reformer;   the fuel reformer produces reformate that enters the LNT, which is thereby regenerated.   
   
   
       28 . The method of  claim 27 , wherein the first fuel injector injects the fuel into the exhaust as the exhaust travels through an engine manifold upstream of a turbocharger. 
   
   
       29 . The method of  claim 27 , wherein the control signal is a signal to regenerate the LNT to remove stored SO x . 
   
   
       30 . The method of  claim 27 , wherein the first exhaust system device is a DPF. 
   
   
       31 . The method of  claim 27 , wherein the exhaust aftertreatment system further comprises a SCR catalyst downstream of the LNT. 
   
   
       32 . The method of  claim 27 , wherein the amount of fuel injected into the exhaust by the first fuel injector is selectively increased in order to mitigate an increase in temperature of the fuel reformer. 
   
   
       33 . A vehicle comprising an exhaust aftertreatment system and a controller configured to operate the exhaust aftertreatment system according to the method of  claim 27 .