Control of EGR and VGT for emission control and pumping work minimization in diesel engines
Legislators steadily increase the demands on lowered emissions from
heavy duty vehicles. To meet these demands it is necessary to
integrate technologies like Exhaust Gas Recirculation (EGR) and
Variable Geometry Turbochargers (VGT) together with advanced control
systems. A control structure with PID controllers and selectors is
proposed and investigated for coordinated control of EGR valve and VGT
position in heavy duty diesel engines. Main control goals are to
fulfill the legislated emission levels, to reduce the fuel
consumption, and to fulfill safe operation of the turbocharger. These
goals are achieved through regulation of normalized oxygen/fuel ratio
and intake manifold EGR-fraction. These are chosen as main
performance variables since they are strongly coupled to the
emissions, compared to manifold pressure or air mass flow, which makes
it easy to adjust set-points depending on e.g. measured emissions
during an emission calibration process. In addition a mechanism for
fuel efficient operation is incorporated in the structure, this is
achieved by minimizing the pumping work.
To design a successful control structure, a mean value model of a
diesel engine is developed and validated. The intended applications of
the model are system analysis, simulation, and development of
model-based control systems. Model equations and tuning methods for
the model parameters are described for each subsystem in the model.
Static and dynamic validations of the entire model show mean relative
errors that are less than 12%.
Based on a system analysis of the model, a key characteristic behind
the control structure is that oxygen/fuel ratio is controlled by the
EGR-valve and EGR-fraction by the VGT-position, in order to handle a
sign reversal in the system from VGT to oxygen/fuel ratio. For efficient
calibration an automatic controller tuning method is developed. The
controller objectives are captured in a cost function, that is
evaluated utilizing a method choosing representative transients. The
performance is evaluated on the European Transient Cycle. It is
demonstrated how the weights in the cost function influence behavior,
and that the tuning method is important in order to improve the
control performance compared to if only a standard method is used. It
is also demonstrated that the controller structure performs well
regarding all control objectives. In combination with its efficient
tuning, the controller structure thus fulfills all requirements for
successful application.
Johan Wahlström
2006
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Last updated: 2021-11-10
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