System analysis of a Diesel Engine with VGT and EGR
A system analysis of a diesel engine with VGT and EGR is performed in
order to obtain insight into a VGT and EGR control problem where the
goal is to control the performance variables oxygen fuel ratio and
EGR-fraction using the VGT actuator and the EGR actuator. Step
responses over the entire operating region show that the channels VGT
to oxygen fuel ratio, EGR-valve to oxygen fuel ratio, and VGT to
EGR-fraction have non-minimum phase behaviors and sign reversals. The
fundamental physical explanation of these system properties is that
the system consists of two dynamic effects that interact: a fast
pressure dynamics in the manifolds and a slow turbocharger dynamics.
It is shown that the engine frequently operates in operating points
where the non-minimum phase behaviors and sign reversals occur for the
channels VGT to oxygen fuel ratio and VGT to EGR-fraction, and
consequently, it is important to consider these properties in a
control design. Further, an analysis of zeros for linearized multiple
input multiple output models of the engine shows that they are
non-minimum phase over the complete operating region. A mapping of the
performance variables oxygen fuel ratio and EGR-fraction and the
relative gain array show that the system from EGR-valve and VGT to
oxygen fuel ratio and EGR-fraction is strongly coupled in a large
operating region. It is also illustrated that the pumping losses
decrease with increasing EGR-valve and VGT opening for almost the
complete operating region.
Johan Wahlström, Lars Eriksson and Lars Nielsen
2009

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