Engine exhaust gas cleaning apparatus

Abstract

When rich spike control is executed in a diesel engine equipped with a NOx trapping catalytic converter, there are times when the amount of NOx desorbed is too little or too much. The rich spike control start timing is determined based on the NOx accumulation amount calculated during lean combustion operation and the rich spike control end timing is determined based on the NOx accumulation amount and the NOx desorption rate. Since the actual amount of NOx remaining in the NOx trapping catalytic converter can be estimated based on the NOx desorption rate during rich spike control, the rich spike control can be ended when the amount of remaining NOx reaches a prescribed target value.

1 . An engine exhaust gas cleaning apparatus comprising: 
 a NOx trapping catalytic converter disposed in an exhaust passage of an engine;    a NOx accumulation amount computing section configured to compute a NOx accumulated amount in the NOx trapping catalytic converter during a lean combustion state;    a NOx desorption rate determining section configured to determine a NOx desorption rate at which accumulated NOx desorbs from the NOx trapping catalytic converter during a rich combustion state; and    a rich spike control section configured to perform a rich spike control to desorb the accumulated NOx in the NOx trapping catalytic converter by reducing the excess air ratio of the exhaust gas,    the rich spike control section being further configured to determine a start timing for starting the rich spike control based on the NOx accumulation amount that was computed, and to determine an end timing for ending the rich spike control based on the NOx accumulation amount and the NOx desorption rate that were computed.    
   
   
       2 . The engine exhaust gas cleaning apparatus as recited in  claim 1 , wherein 
 the NOx desorption rate determining section is configured to estimate the NOx desorption rate based on a fuel injection quantity and a rotational speed of the engine during the rich spike control.    
   
   
       3 . The engine exhaust gas cleaning apparatus as recited in  claim 2 , wherein 
 the NOx desorption rate determining section is configured to adjust the NOx desorption rate that was estimated based on the excess air ratio during the rich spike control.    
   
   
       4 . The engine exhaust gas cleaning apparatus as recited in  claim 3 , wherein 
 the NOx desorption rate determining section is configured to adjust the NOx desorption rate by applying a weighted adjustment such that as the excess air ratio becomes smaller during the rich spike control, the NOx desorption rate becomes larger.    
   
   
       5 . The engine exhaust gas cleaning apparatus as recited in  claim 1 , wherein 
 the NOx desorption rate determining section is configured to estimate the NOx desorption rate based on an exhaust gas temperature in a general vicinity of an outlet of the NOx trapping catalytic converter during the rich spike control.    
   
   
       6 . The engine exhaust gas cleaning apparatus as recited in  claim 1 , wherein 
 the NOx desorption rate determining section is configured to estimate the NOx desorption rate based on an exhaust gas flow rate during the rich spike control.    
   
   
       7 . The engine exhaust gas cleaning apparatus as recited in  claim 1 , wherein 
 the NOx desorption rate determining section is configured to estimate the NOx desorption rate based on an amount of reducing agent discharged during the rich spike control.    
   
   
       8 . The engine exhaust gas cleaning apparatus as recited in  claim 1 , wherein 
 the rich spike control section is configured to end the rich spike control when the result obtained by subtracting an amount of NOx that has desorbed according to the NOx desorption rate that was computed from the NOx accumulation amount that existed when the rich spike control was started reaches or falls below a reference value.    
   
   
       9 . The engine exhaust gas cleaning apparatus as recited in  claim 2 , wherein 
 the rich spike control section is configured to end the rich spike control when the result obtained by subtracting an amount of NOx that has desorbed according to the NOx desorption rate that was computed from the NOx accumulation amount that existed when the rich spike control was started reaches or falls below a reference value.    
   
   
       10 . An engine exhaust gas cleaning apparatus comprising: 
 NOx trapping means for trapping NOx from exhaust exiting an engine;    NOx accumulation amount computing means for computing the amount of NOx accumulated in the NOx trapping means during a lean combustion state;    a NOx desorption rate computing means for computing a NOx desorption rate at which accumulated NOx desorbs from the NOx trapping means during a rich combustion state; and    a rich spike control means for performing a rich spike control to desorb the accumulated NOx in the NOx trapping means by reducing the excess air ratio of the exhaust gas,    the rich spike control means determining a start timing for starting rich spike control based on the NOx accumulation amount that was computed, and determining an end timing for ending the rich spike control based on the NOx accumulation amount and the NOx desorption rate that were computed.    
   
   
       11 . A method for cleaning exhaust gas of an engine comprising: 
 trapping NOx from exhaust gas exiting the engine;    computing a NOx accumulated amount that was trapped during a lean combustion state;    computing a NOx desorption rate at which accumulated NOx desorbs during a rich combustion state;    performing a rich spike control to desorb the accumulated NOx by reducing the excess air ratio of the exhaust gas; and    determining a start timing for starting the rich spike control based on the NOx accumulation amount that was computed, and determining an end timing and for ending the rich spike control based on the NOx accumulation amount and the NOx desorption rate that were computed.