Paradigm Shift in Emission Testing

FEV’s emission-based EGR control conditions diesel engines for the upcoming “Real Driving Emissions procedures”

12. May 2015 | Engineering Service

Emission testing shall reflect real driving conditions more precisely: This is the European commission’s aim in di­scussing the RDE (Real Driving Emissions) test procedure included in the upcoming Euro 6c emissions legi­slation. The new test procedures will cover a wider range of engine speed, engine load and ambient conditions. As a result, limiting the design and calibration of emission reduction systems to a reduced emission-relevant range is no longer sufficient to fulfill the legislative requirements. “More complex emission control systems, including EGR systems with several EGR circuits and coolers will become increasingly common,” explains Dr. Thorsten Schnorbus, Deputy Department Manager for Diesel Engine Predevelopment at FEV GmbH. “A fast and robust control system that is easy to calibrate will become increasingly important in this context.”

Advantages of FEV’s model-based approach

For this purpose, FEV has developed an innovative, model-based approach for direct emission-based EGR control in diesel engines. This approach shows that, given the more stringent requirements, a high potential exists for improvements with regard to engine-out emission as well as tail pipe emission behavior. The concept involves less calibration effort compared to current EGR approaches. In parallel, the full fuel consumption reduction potential of the engine can be utilized as a result of the flexible definition of the EGR fraction.

>>A fast and robust EGR control system that is easy to calibrate will become
increasingly
important

Neutralize ambient influences effectively

Changes in environmental or operating conditions are compensated by physical-based system reactions, such as EGR redistribution and/or EGR cooler bypass control. As a result, NOx emissions as well as fuel consumption stay very constant when there are major changes in ambient temperature. Also the intake manifold temperature can be kept at a very similar level, which is expected to have a positive effect on engine-out HC and CO emissions. The similar intake manifold temperature level leads to a very stable exhaust gas temperature keeping the aftertreatment conversion efficiency consistent.

NOx-based EGR control with multi-loop EGR

NOx-based EGR control with multi-loop EGR

Functional principle of the EGR control

The air path control concept is based on an integrated NOx model from which a set point is derived for the target oxygen concentration in the cylinder (at “intake valve closure”). To compensate for the influence of modified boundary conditions on the NOx emissions or to actively use the degrees of freedom of the EGR control, the specified value of the NOx engine-out set point can be adjusted independently and adapted by algorithms taking into account the current conversion rate of the NOx aftertreatment.

To distribute the amount of EGR required to achieve the desired oxygen concentration in the cylinder, dedicated set points are defined for the oxygen concentration at each mixing point. Each set point can be controlled on the basis of different parameters. Since the entire air system is represented by dedicated physical models, it is also possible to determine the current oxygen concentration at every mixing point. Under highly transient operating conditions, faster EGR paths can be used to compensate slower EGR paths by means of dynamic control intervention, so that the desired specified oxygen concentration in the cylinder is achieved even during highly dynamic cycles.

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