Control-Oriented Model of Molar Scavenge Oxygen Fraction for Exhaust Recirculation in Large Diesel Engines
Exhaust gas recirculation (EGR) systems have been introduced to large
marine engines in order to reduce NOx formation. Adequate modelling for
control design is one of the bottlenecks to design EGR control that also
meets emission requirements during transient loading conditions. This paper therefore focus on deriving and validating a mean-value model of a large
two-stroke crosshead diesel engines with EGR. The model introduces a number
of amendments and extensions to previous, complex models and shows in theory
and practice that a simplified nonlinear model captures all essential
dynamics that is needed for EGR control. Our approach is to isolate and
reduce the gas composition part of the more complex models using nonlinear
model reduction techniques. The result is a control-oriented model (COM) of
the oxygen fraction in the scavenge manifold with three molar flows being
inputs to the COM, and it is shown how these flows are estimated from
signals that are commonly available. The COM is validated by first
comparing the output to a simulation of the full model, then by comparing
with measurement series from two engines. The control oriented nonlinear
model is shown to be able to replicate the behavior of the scavenge oxygen
fraction well over the entire envelope of load and blower speed range that
are relevant for EGR. The simplicity of the new model makes it suitable for
observer and control design, which are essential steps to meet the
emission requirements for marine diesel engines that take effect from 2016.
Kraen Nielsen, Mogens Blanke, Lars Eriksson and Morten Vejlgaard-Laursen
ASME Journal of Dynamic Systems, Measurement, and Control,
2017
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