Effect of Incorporating the Thermal Management of the Three-Way Catalyst on Energy Efficiency and Tailpipe Emissions for a P2 Parallel Hybrid Vehicle

The energy management of hybrid electric vehicles (HEVs) is a complex subject that can be addressed with the tools provided by optimal control theory. Optimization algorithms explored so far in the literature, like dynamic programming (DP) or equivalent consumption minimization strategy (ECMS), have systematically analyzed the potential CO2 reduction for different topologies and degree of hybridization. However, the management of engine and electric machine (EM) neglects that the catalyst material in the aftertreatment system needs to reach a certain temperature to properly convert pollutant emissions. In this study, the thermal management of the catalyst in a gasoline HEV has been investigated, and two algorithms have been proposed. Two strategies based on the ECMS are presented: the first one explicitly considers the catalyst temperature; the second one keeps the underlying structure of ECMS, but it adds a high-level rule to indirectly encompass catalyst management. To have a reliable catalyst temperature, a monodimensional model for the three-way catalyst (TWC), incorporating chemical kinetics, has been implemented. Finally, both strategies have been assessed via numerical simulations on two different driving cycles: the Worldwide harmonized Light vehicles Test Cycle (WLTC) and the Transport for London cycle (TfL), an urban driving cycle that is selected as a worst-case scenario for the thermal management of the aftertreatment system. On the WLTC both strategies show a 2% increase in fuel consumption with a potential 60% NOx reduction. On the urban cycle, only the second strategy is able to ensure the catalyst heating in a reasonable timespan. However general trends are still confirmed: when the catalyst thermal management is incorporated into the energy management strategy, since the first ignition, the engine produces extra power and charges the battery so that the TWC reaches the light-off temperature over a time-lapse comparable with a conventional vehicle. The stored energy is exploited at higher power demands to reduce the fuel consumption. The average engine load is hence shifted upwards in comparison to a fuel economy-oriented strategy.