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. Control structures are 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.
To design successful control structures, 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. Dynamic validations show that the
proposed model captures the essential system properties, i.e.
non-minimum phase behaviors and sign reversals.
A first control structure consisting of PID controllers and
min/max-selectors is developed based on a system analysis of the
model. A key characteristic behind this 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. This structure also minimizes the pumping
work by opening the EGR-valve and the VGT as much as possible while
achieving the control objectives for oxygen/fuel ratio and
EGR-fraction. For efficient calibration an automatic controller tuning
method is developed. The controller objectives are captured by a cost
function, that is evaluated utilizing a method choosing representative
transients. Experiments in an engine test cell show that the
controller achieves all the control objectives and that the current
production controller has at least 26\% higher pumping losses compared
to the proposed controller.
In a second control structure, a non-linear compensator is used in
an inner loop for handling non-linear effects. This compensator is a
non-linear state dependent input transformation. PID controllers and
selectors are used in an outer loop similar to the first control
structure. Experimental validations of the second control structure
show that it handles nonlinear effects, and that it reduces EGR-errors
but increases the pumping losses compared to the first control
structure.
Substantial experimental evaluations in engine test cells show that
both these structures are good controller candidates. In conclusion,
validated modeling, system analysis, tuning methodology, experimental
evaluation of transient response, and complete ETC-cycles give a firm
foundation for deployment of these controllers in the important area
of coordinated EGR and VGT control.
Johan Wahlström
2009
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Last updated: 2021-11-10
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