Contribution of Low Voltage Diesel Mild Hybridization and Novel Exhaust After- Treatment Systems in the Context of Future Post EU6 RDE and CO2 Requirements

Autoren

Dr.-Ing. Frank Bunar, Robert Scholz, MSc, Miles Brammer, MEng, Dipl.-Ing. Olaf Friedrichs, Dr.-Ing. Maximilian Brauer, IAV GmbH, Berlin

Zusammenfassung

Future CO₂ reduction and post EU6 emission requirements will drive the electrification of all combustion engine powertrains for passenger cars and light commercial vehicles forward significantly. For diesel engines, this general trend, which is especially prevalent
in Europe, will lead to greater stress on the overall system complexity and cost. Alongside the well-established, high voltage hybridizations of gasoline engines, a cost optimized, low voltage technology for diesel engines is currently a very promising path to combine powertrain efficiency, driving performance and emission control at an acceptable cost level. One promising technology strategy is a mild hybrid P0 application coupled with an advanced exhaust aftertreatment (EA T) system including lean NOx trap (LNT) technology that can form the standard for next generation powertrain technology in 2021 at EU6d and beyond. The technical potential of such a strategy is summarized in the first part of the paper, shown exemplarily on an existing C-segment demonstrator vehicle targeting ambitious real driving emissions (RDE) requirements.
In the second part of this paper, a predictive investigation explores the functional performance benefits of an alternative technology strategy for pre-selected advanced hybrid architectures, P2, P4 and a combined P1+P4 layout, using a computer aided engineering (CAE) approach. It looks at different mildly hybridized diesel powertrain architectures in interaction with an electrically heated catalyst (EHC) in a tailored but high-performance exhaust aftertreatment system. Different concepts of 48V mild hybridization are applied to a representative, generic D-segment passenger car in a modular CAE environment. Combined innovative operation strategies of both powertrain and exhaust aftertreatment are set up in order to meet future, ambitious real driving conditions legislation, focusing on nitrous oxides (NOx) and carbon dioxide (CO₂) including cold start and short urban driving conditions. The post EU6 RDE scenario considers more challenging driving conditions to demonstrate robust NOx-control with priority towards inner city, low speed and low load driving conditions on a level of <=40 mg/km. A key challenge is an exemplary low load driving cycle (LLC) close to the borderline of current RDE boundary conditions characterized by v*a_pos, relative positive acceleration (RPA) and minimum average vehicle speed (v_mean). The CAE results indicate that advanced, mild hybridization technology, characterized by the position of the electric motor(s) (EM) downstream of the internal combustion engine (ICE) of diesel powertrains, shows high optimization potential for both costumer-relevant fuel efficiency and driving performance improvement with even further improved robustness for meeting low-level NOx-emission targets. A key element of P2- and P4-layouts is the functionality of limited, pure electric driving in ultra-low speed conditions.
This enhanced functionality is not offered with the P0 architecture due to a very limited torque assist functionality for the ICE. Considering a combined EM strategy (e.g. P1+P4) in combination with an automatic transmission, an additional functionality of combined series and parallel hybrid operation strategy is enabled. This powertrain operation mode shows the overall highest potential for NOx-emission robustness down to extremely low speed and low load driving conditions. Considering fuel efficiency and NOx-control requirements a novel, physical model based, powertrain control logic with closed loop control was developed. An additional synergetic effect in combination with mild hybrid electric vehicle (MHEV) powertrain technology is the electrified aftertreatment system, which can increase NOx-robustness for ultra-low load city cycle scenarios as well. In particular, the application of a 48V electrically heated oxidation catalyst (eDOC) is considered. Finally, the predictive results of combined CO₂ and NOx improvement investigations indicate a highly beneficial contribution of tailored, low voltage MHEV applications towards mid-term 95 g/km CO₂ and post EU6 RDE NOx requirements. The model based CAE approach enabled the wide screening, optimization and control of the complex powertrains in an effective and robust manner.

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