Closed Loop Control of Lean NOx Traps

Abstract

There is a co ntinuing interes t in the development and application of lean burn gaso line engine sys tems, such as port fuel injec ted (PF I) and gasoline direct injected (GDI) engi nes. Th ese engin es offe r imp roved fuel econom y and lower CO 2 emissions through the installatio n of a second three-wa y catalyst, commonly referred to as a lean NOx trap (LNT). With this additional comp onent high NOx co nve rsion rates appro aching 100 % can be achieved. Sinc e the LNT operates in a batch mode , the filling of the LNT with NOx and its purgin g must be dynamically co ntrolled to achieve optimum perform ance. Since the LNT is subj ect to degradation in efficiency due to the presence of sulfur in the fuel, the LNT must be pe riodically regenerat ed by subjec ting it to a high temp eratu re (e .g. , 650 "c) and maintaining the A < 1, o r rich. Also, since the level of engine NOx can va ry with operating cond itions (e .g. , humid ity) , the filling rate of the LNT varies. Th is paper desc ribes work on closed loop adaptive control of the pu rge and fill cycles of the LNT using a feedback signal pro vided by a heated exhaust gas oxyge n (HE GO senso r located afte r the LNT. This control optimises NOx conv ersion effic iency , minimi ses loss in fuel consumption , minimises emissions, and can be used to diagnose the state of the LNT and to schedule desulfation events .