This invention relates to lean-burn gasoline engine control and, more specifically, to lean NOX trap (LNT) purge cycle management.
A LNT is an additional three-way catalyst exhaust after-treatment for lean burn port fuel injected and direct injected gasoline engines. The LNT is purged periodically to release and convert the oxides of nitrogen (NOX) stored in the trap during the preceding lean operation. To accomplish the purge, the engine has to be operated at an air-to-fuel ratio that is rich of stoichiometry. As a result of the rich operation, substantial amounts of feedgas carbon monoxide (CO) and hydrocarbons (HC) are generated to convert the stored NOX. Typically, the purge mode is activated on the basis of estimated trap loading. That is, when the estimated mass of NOX stored in the trap exceeds a predetermined threshold, a transition to the purge mode is initiated. The rich operation continues for several seconds until the trap is emptied of the stored NOX, whereupon the purge mode is terminated and the normal lean operation is resumed. The end of the purge is usually initiated by a transition in the reading of the HEGO sensor located downstream of the trap, or based on the model prediction of the LNT states. Since the engine is operated rich of stoichiometry during the purge operation, the fuel economy advantage of the lean operation is lost.
In accordance with the present invention, the transition to the purge mode is delayed if it is expected that in the near term the engine will be subjected to high load and high speed conditions. Since the lean operation is limited (or is primarily beneficial) to low loads and low engine speeds, the transition to the purge mode may be advantageously delayed if it is expected that during the next few seconds of the ensuing operation, the engine will be subjected to high load and high speed conditions. Thus, by delaying the purge, fuel economy improvements can be attained without a detectable loss in emission performance.