Closed-loop measurement of NO_x storage capacity and oxygen storage capacity of a lean NO_x trap

摘要

A novel method to measure the oxygen storage capacity (OSC) and the NO_x storage capacity of lean NO_x traps (LNT) has been developed. The method relies on closed-loop control of the response of an HEGO sensor located after the LNT. The HEGO sensor reacts to the residual CO passing through the exhaust system during a purge of the LNT. This response can be used to infer the NO_x capacity and oxygen capacity of the LNT, as well as the oxygen capacity of a TWC (three-way catalyst) located before the LNT. A lean NO_x trap must be periodically purged through momentary rich engine operation to liberate the chemically trapped NO_x. A substantial portion of the purge fuel is also required to react with the oxygen stored in the LNT and in the TWC. During the purge of an LNT, the engine is operated rich for one or two seconds. Rich A/F operation generates a rich pulse of predominately CO that travels down the exhaust system and reacts with the TWC and LNT. The CO reacts with the stored oxygen, forming CO_2. The CO also reacts with NO_x stored as NO_2 in the LNT, forming CO_2 and N_2. Unreacted CO passes out of the LNT and interacts with an HEGO sensor located after the LNT. The HEGO responds by creating a momentary voltage pulse greater than zero, corresponding to the rich A/F pulse. Capture and analysis of this pulse by the PCM (Powertrain Control Module) results in a parameter that can be used in a closed-loop feedback control system. This control system can be used to measure the OSC (oxygen storage capacity) and the stored NO_x. The experimental method used is described and data are presented showing the temperature dependence and magnitude of the OSC and the stored NO_x. This method is currently used for off-line measurement of these two important LNT parameters but in the future can be used for on-line measurements and even for real-time feedback control.