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Fuel Consumption and Emission Reduction for Hybrid Electric Vehicles with Electrically Heated Catalyst
Abstract:
Hybridization is a promising way to further reduce the CO2 emissions of passenger vehicles. However, high engine efficiencies and the reduction of engine load, due to torque assists by an electric motor, cause a decrease of exhaust gas temperature levels. This leads to an increased time to catalyst light-off, resulting in an overall lower efficiency of the exhaust aftertreatment system (ATS). Especially in low load driving conditions, at cold ambient temperatures and on short distance drives, the tailpipe pollutant emissions are severely impacted by these low ATS efficiency levels.
To ensure lowest emissions under all driving conditions, catalyst heating methods must be used. In conventional vehicles, internal combustion engine measures (e.g. usage of a dedicated combustion mode for late combustion) can be applied. A hybrid system with an electrically heated catalyst (EHC) enables further methods such as the increase of engine load by the electric motor or electric catalyst heating. Since these methods result either directly or indirectly in additional fuel consumption there is a conflict of objectives between a fast catalyst heat-up and the reduction of fuel consumption.
This trade-off is addressed by an energy and emission management, which controls the split of the propulsion power between the combustion engine and electric motor, the electric catalyst heating and the selection of the combustion mode. Such a management system, based on offline-optimized rules, is presented in this paper. The results for different driving cycles are compared to those from a conventional non-hybrid vehicle in a detailed simulation framework, for a 48V P0 Diesel hybrid including an exhaust aftertreatment system with EHC. With the help of the EHC the NOx level was reduced significantly especially during city and short distance drives. On the WLTC a CO2 saving of 7 % was achieved, maintaining the same NOx emissions level.