Fuel Economy Evaluation of Three-Way Catalyst Control Strategies in a Multimode Combustion Context
Sandro Nuesch, University of Michigan
Highly diluted, low temperature homogeneous charge compression ignition (HCCI) combustion leads to ultra-low levels of engine-out NOx emissions. A standard drive cycle, however, would require switching between HCCI and spark-ignited (SI) combustion. During stoichiometric SI operation the three-way catalyst (TWC) achieves close to perfect conversion of all species. Theoretically, in lean HCCI mode the TWC is still able to reduce unburnt hydrocarbons and CO while engine-out NOx is very low.
It Is shown that in practice the HCCI operation would require NOx aftertreatment for two reasons. First the increasingly strict emissions standards in the scope of a whole drive cycle would require conversion of the HCCI engine-out NOx even if it is considerably lower that the SI NOx emissions. Second during combustion mode switches the transient engine behavior might cause high spikes in engine-out NOx which need to be absorbed by the aftertreatment system other than TWC.
A TWC is capable of reducing emissions as long as its oxygen storage is not full. But the lean exhaust gas during HCCI operation fills the oxygen storage and eventually leads to a drop in emission conversion efficiency of the catalyst. If the levels of NOx become unacceptable a mode switch to a rich combustion mode is necessary in order to deplete the oxygen storage. The resulting lean-rich cycling leads to three forms of fuel penalization: Firstly during a mode switch each combustion mode has to be operated far outside its optimized range leading to increased fuel consumption during the switch. Secondly the fuel-beneficial HCCI mode cannot be used up to the value its load-speed range suggests and its potential advantages are reduced. Thirdly the succeeding rich operation in order to remove oxygen from the catalyst leads to another significant reduction in fuel economy.
To quantify these penalties, TWC aftertreatment system is modeled and calibrated to combustion mode switch experiments from lean HCCI to rich spark-assisted compression ignition (SACI) mode and back. Steady state fuel and emissions maps acquired in experiments were used. The engine model was integrated with a longitudinal vehicle simulation and a finite state machine simulating the combustion mode switches. Different lean-rich cycling strategies were tested in order to predict and compare their influence on drive cycle fuel economy.
Authors: Sandro Nuesch, Jeff Sterniak, Li Jiang, Anna G. Stefanopoulou