On-line adaptation of wall-wetting model parameters

Spark-ignited engines equipped by a three-way catalyst require a precise control of the air fuel ratio (A/F) fed to the combustion chamber. A stoichiometric mixture is necessary for the proper working of the catalyst in order to meet the legislation requirement. A critical part of the A/F control is the feed-forward compensation of the fuel dynamics. Conventional strategies are based on a simplified model of the wall-wetting phenomena whose parameter values (the well known X and \tau_f in the Aquino's model) are stored in off-line computed look-up tables. Unfortunately, different factors such as aging of the engine components, wide range of possible fuels, errors in the parameters calibration over the whole engine map deteriorate the control performances in terms of emissions. In this paper, we present a strategy for the slow on-line adaptation of the wall-wetting dynamics. In particular the algorithm detects when parameter mismatch occurs and then uses a least squares method to perform the adaptation. The aim is to reduce temporary lean (rich) excursions during fastaccelerations (decelerations), and obtain a good compensation during rapid throttle transients. The whole procedure was designed and tested by numerical simulations based on experimental data.