Closed loop control of lean NO(x) traps

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Closed loop control of lean NO(x) traps

Asik, J.R.

There is a continuing interest in the development and application of lean burn gasoline engine systems, such as port fuel injected (PFI) and gasoline direct injected (GDI) engines. These engines offer improved fuel economy and lower CO2 emissions through the installation of a second three-way catalyst, commonly referred to as a lean NO(x) trap (LNT). With this additional component high NO(x) conversion rates approaching 100% can be achieved. Since the LNT operates in a batch mode, the filling of the LNT with NO(x) and its purging must be dynamically controlled to achieve optimum performance. Since the LNT is subject to degradation in efficiency due to the presence of sulfur in the fuel, the LNT must be periodically regenerated by subjecting it to a high temperature (e.g., 650 deg C) and maintaining the lamda less then 1, or rich. Also, since the level of engine NO(x) can vary with operating conditions (e.g., humidity), the filling rate of the LNT varies. This paper describes work on closed loop adaptive control of the purge and fill cycles of the LNT using a feedback signal provided by a heated exhaust gas oxygen (HEGO sensor located after the LNT). This control optimises NO(x) conversion efficiency, minimises loss in fuel consumption, minimises emissions, and can be used to diagnose the state of the LNT and to schedule desulfation events.