Lnt control method for vehicle

Abstract

An LNT control method for a vehicle may minimize the loss ratio of fuel efficiency while satisfying limits of NOx by changing a rich mode control plan in accordance with the purification rate of NOx in LNT which changes with the degree of deterioration of LNT catalyst.

What is claimed is: 
     
         1 . An LNT control method for a vehicle, comprising:
 a deterioration determining step that determines whether LNT is deteriorated; and   a rich mode necessity determining step that performs determination so that a rich mode may be performed more times in comparison with the case when LNT is not deteriorated, when determining that the LNT is deteriorated.   
     
     
         2 . The method of  claim 1 , wherein the rich mode necessity determining step determines whether a rich mode of LNT is necessary, by multiplying NOx factors set based on values obtained by modeling the amount of NOx adsorbed to LNT by vehicle speed & gear factors set based on the vehicle speed and the number of gears and comparing that with a predetermined rich safe factors. 
     
     
         3 . The method of  claim 2 , wherein the NOx factors are composed of normal NOx factors determined for a normal state without LNT deteriorated and aged NOx factors specifically determined for the deteriorated state of LNT, and the vehicle speed & gear factors are composed of normal vehicle & gear factors determined for a normal state without LNT deteriorated and aged vehicle speed & gear factors specifically determined for the deteriorated state of LNT, and in the rich mode necessity step, one of the normal NOx factors or the aged NOx factor and one of the normal vehicle speed & gear factors or the aged vehicle speed & gear factors are selected, multiplied, and then compared with the rich safe factors. 
     
     
         4 . The method of  claim 3 , wherein, in the rich mode necessity step, the result of multiplying the NOx factor with the vehicle speed & gear factors is additionally multiplied by an LNT temperature factors determined in accordance with the temperature of LNT and then the resultant is compared with the rich safe factors, and from the result, when the result of multiplying the NOx factors, the vehicle speed & gear factors, and LNT temperature factors is the rich safe factors or more, it is determined that the rich mode of LNT is necessary. 
     
     
         5 . The method of  claim 3 , wherein the NOx factors have a tendency to increase in magnitude with the increase in the amount of NOx and the aged NOx factors are set to have a tendency to be relatively greater than the normal NOx factors, and the vehicle speed & gear factors have a tendency to increase with the increase in vehicle speed and the number of gears and the aged vehicle speed & gear factors are set to have a tendency to be relatively greater than the normal vehicle speed & gear factors. 
     
     
         6 . The method of  claim 1 , further includes a final determining step that determines whether to perform the rich mode of LNT by further determining one or more conditions of the atmospheric pressure, coolant temperature, fuel temperature, acceleration pedal signal, and engine operation range, when it is determined that it is necessary to perform the rich mode of LNT, from the result of performing the rich mode necessity determining step. 
     
     
         7 . The method of  claim 1 , wherein, in the deterioration determining step, a slip ration is calculated by detecting the amount of a reducer consumed in NOx reduction and the amount of a reducer slipping, using signals from lambda sensors at the front and rear ends of LNT, and it is determined that LNT has been deteriorated, when the calculated slip ratio exceeds a predetermined critical value.