Energy management strategy to reduce pollutant emissions during the catalyst light-off of parallel hybrid vehicles

The transportation sector is a major contributor to both air pollution and greenhouse gas emissions. Hybrid electric vehicles can reduce fuel consumption and CO2 emissions by optimizing the energy management of the powertrain. The purpose of this study is to examine the trade-off between regulated pollutant emissions and hybrid powertrain efficiency. The thermal dynamics of the three-way catalyst are taken into account in order to optimize the light-off. Experimental campaigns are conducted on a spark-ignition engine to introduce simplified models for emissions, exhaust gas temperature, catalyst heat transfers and efficiency. These models are used to determine the optimal distribution of a power request between the thermal engine and the electric motor with three-dimensional dynamic programming and a weighted objective function. A pollution-centered scenario is compared with a consumption-centered scenario for various driving cycles. The optimal torque distribution for the emissions-centered scenario on the world harmonized light-duty vehicles test cycle shows an 8-33% decrease in pollutant emissions while the consumption remains stable (0.1% increase). The consistency of the results is analyzed with respect to the discretization parameters, driving cycle, electric motor and battery sizing, as well as emission and catalyst models. The control strategies are promising but will have to be adapted to online engine control where the driving cycle and the catalyst efficiency are uncertain.