Cold-Start Modeling and Real-time Optimal Control of the Three-Way-Catalyst
Paper i proceeding, 2021
The three-way-catalyst (TWC) is an essential part of the exhaust aftertreatment system in spark-ignited powertrains, converting nearly all toxic emissions to harmless gasses. The TWC’s conversion efficiency is significantly temperature-dependent, and cold-starts can be the dominating source of emissions for vehicles with frequent start/stops (e.g. hybrid vehicles). In this paper we develop a thermal TWC model and calibrate it with experimental data. Due to the few number of state variables the model is well suited for fast offline simulation as well as subsequent on-line control, for instance using non-linear state-feedback or explicit MPC. Using the model could allow an on-line controller to more optimally adjust the engine ignition timing, the power in an electric catalyst pre-heater, and/or the power split ratio in a hybrid vehicle when the catalyst is not completely hot. The model uses a physics-based approach and resolves both axial and radial temperature gradients, allowing for the thermal transients seen during heat-up to be represented far more accurately than conventional scalar (i.e. lumped-temperature) real-time models. Furthermore, we also use a physics-based chemical kinetics reaction model for computing the exothermic heat of reaction and emission conversion rate which is temperature and residence-time-dependent. We have performed an experimental campaign with a standard spark-ignited engine and a commercial TWC, where we measured steady-state operation and cold-start transient behavior. This experimental data allowed us to tune the model, where we found excellent matching between the measured and modeled tailpipe emissions. Modeling the radial temperature gradient improved the relative accuracy of the conversion efficiency by 15%, and simulations indicate the potential for an absolute improvement by 15 percentage points for some cases. Furthermore, the modeled TWC temperature evolution for a cold-start was typically within ±10 ° C of the measured temperature (with a maximal deviation of 20 °C). The proposed model thus bridges a gap between heuristic models suited for on-line control and accurate models for slower off-line simulation.