Unlike test cycles, such as NEDC or WLTC, measurements based on real driving emissions (RDE) prescribe no set speed-load profiles. The vehicle still has to meet the average emission limits of the WLTC. IAV is addressing this new challenge with additional software control layers: The emission coordinator sets targets for engine control and exhaust gas aftertreatment control, making sure that the demanded real driving emissions are met.
The new RDE requirements, for example, include the conformity factors (CF) for nitrogen oxide emissions. From September 2017, a CF of 2.1 will be in place which will be tightened to 1.5 from January 2020. Even under RDE conditions, newly registered vehicles will then on average only be allowed to emit a maximum of 2.1 or, later on, 1.5-times the WLTC limit – and under widely differing ambient conditions, too. RDE covers a temperature range of minus 2 to plus 30 degrees Celsius as well as altitudes of up to 1,300 meters. Besides NOx emissions, of course, the increasingly tighter CO2 limit values must also be met: 95 grams per kilometer from 2020, with an anticipated 78 grams from 2025.
Overarching approach needed for RDE
“In future, exhaust gas aftertreatment components will need to work at maximum efficiency under these widely differing conditions”, explains Enrico Neumann, team manager of Diesel Engine Algorithm Development at IAV. “Needless to say, the engine must only be allowed to generate a minimum of emissions.” This will demand some calibration rethinking: Whereas individual components, such as engine and exhaust gas aftertreatment (EAT), have so far been separately optimized for maximum efficiency, an overarching approach is now called for. The control units for the engine, diesel particulate filter (DPF), lean NOx trap (LNT) and SCR system will now be joined by control layers that keep an eye on the overarching RDE targets and coordinate the interaction of engine control unit (ECU) and EAT control. IAV refers to this new layer as the emission coordinator.
As part of the emission coordinator, the NOx target estimator constantly watches the vehicle’s NOx and CO2 emissions and keeps a type of “account” for nitrogen oxide emissions. “It does this by calculating the integral of emissions over the journey which lets it determine whether the values still remain within the statutory requirement under RDE conditions as well”, Neumann says. “Our aim is to achieve a CF of 0.8 while at the same time keeping CO2 emissions as low as possible.” To solve this, the NOx target estimator sets targets for NOx and CO2 emissions which the emission coordinator then converts into targets for engine-out emissions and EAT. Doing so, it considers for current boundary conditions, such as ambient temperature or traveling speed, and on this basis decides which strategy currently provides the best compromise between NOx and CO2 emissions. It would be conceivable for EAT to request higher exhaust gas temperatures and the emission coordinator to decide whether and how it passes this request on to the engine control system.
Initial results confirm expectations
IAV’s Diesel section has already used MATLAB / Simulink to set up an initial version of an emission coordinator for an EAT system made up of LNT and SCR, and found out that coordinating the two domains of engine and EAT does work in the way intended. “During the course of 2016, we are planning to examine the new approach in detail and validate the new software concept under RDE conditions”, Neumann says. “This will also involve us testing it in a virtual vehicle on journeys that are actually driven.” automotion will be reporting on this project in forthcoming issues.