Hybrid Passenger Car Emissions – A concept study by means of simulation
Johann Wurzenberger, AVL List GmbH
Global concerns on sustainable energy use and environmental protection are a continuous driving force on innovative power train technologies. Among all investigated power train configurations, Hybrid Electrical Vehicles (HEVs)—consisting of an internal combustion engine (ICE) and an electric motor (EM)—are considered as a viable short to midterm solution due to their similarities with conventional vehicles and due to the use of a smaller battery pack. The operating time of the internal combustion engine is substituted by electrical sources leading to reduced fuel consumption. At the same time, the engine shut-off influences the performance of the exhaust gas aftertreatment system (EAS) decisively since the EAS requires a certain temperature level for functional operation. The global optimization of both targets, low emissions and low fuel consumption, requires a system engineering view involving all hardware and software domains of the vehicle.
There is a variety of measures to hold exhaust gas aftertreatment systems at the required operating temperatures. These thermo-management measures range from passive approaches such as insulation, component positioning and arrangement up to active measures involving changed engine operating conditions and electrical heating of the exhaust line. The investigation of the different options, isolated and in combination, can become expensive if performed on real hardware. Here, simulation is an attractive alternative to master the complexity of different hardware and control strategies under arbitrary real driving conditions. To efficiently compare different powertrain and control configurations by means of simulation, models are needed featuring a high level of physical depth—thus a high level of predictability—and high computational speed. The physical depth is a key factor to describe different transient phenomena (gas exchange, heat-up, vehicle acceleration, filter loading, etc.) taking place on different time scales.
This work presents a simulation study comparing the impact of different thermo-management measures on the emissions of a gasoline powered hybrid passenger car. The simulations are done with the help of a real-time capable vehicle modelling framework that was specifically extended to handle arbitrary exhaust gas aftertreatment configurations. The system level model presents physically based sub-models of the internal combustion engine (1) including exhaust aftertreatment devices (2), electric components (3), mechanical drivetrain (4) and the corresponding controllers (5). A simplified hybrid control unit takes care of the power split between the ICE and EM where the duration of engine-off phases influences the operating temperatures of the EAS. The complete vehicle is operated in closed loop by a driver model. Real driving conditions are mimicked with the help of a random cycle generator giving velocity profiles for different shares of urban, extra-urban and highway driving. The tailpipe emissions and fuel consumption are compared for different catalyst positions and sizes (1), aftertreatment system insulation (2), in-cylinder post injection (3) and electrically heated catalysts (4). The potential gains of the passive thermo-management measures (1,2) are elaborated and the trade-off between emissions and fuel consumption is assessed for the active thermo-management measures (3,4).