For an engine equipped with dual switch- type oxygen sensors upstream and downstream of the three way catalyst（TWC）

an innovative adaptive dual-loop λ control strategy was proposed. The front oxygen sensor upstream of the TWC was used to perform PI feedback control of the closed-loop control factor and the rear sensor downstream of the TWC was used to monitor and correct the λ in real- time. If the front oxygen sensor voltage output characteristic shifts remarkably due to aging or oil-poisoned

the traditional closed-loop PI method can not prevent the λ from deviating from 1

resulting in a deteriorated efficiency of the TWC. Therefore

in the proposed control strategy

every time the front oxygen sensor voltage steps

a freezing mode and a freezing period are calculated depending on the offset of the rear oxygen sensor voltage from its reference value and applied to freeze PI adjustment and delay the closed-loop control factor to invert jump for a specific period. The closed-loop control factor is maintained at the maximum/minimum state during the freezing period. In this way

an asymmetric controller with unilateral time delay is formed to compensate the λ offset so that the λ can be controlled precisely in the ideal operation range to achieve an optimum conversion efficiency of the TWC. The advanced software ASCET is used to graphically model the controller and automatically generate C codes. After being integrated with the baseline driver code

C codes are downloaded to MPC5634 hardware platform and installed on an engine bench to experimentally demonstrate the effectiveness of the proposed control strategy. The results show that the λ can still be precisely controlled within the ideal range of 0.99 to 1.01 even if the front oxygen sensor malfunctions.