Nonlinear Input Transformation for EGR and VGT Control in Diesel Engines
In diesel engines with EGR and VGT, the gas flow dynamics has
significant nonlinear effects. This is shown by analyzing DC-gains in
different operating points showing that these gains have large
variations. To handle these nonlinear effects, a nonlinear state
dependent input transformation is investigated. This input transformation is
achieved through inversion of the models for EGR-flow and turbine
flow. It is shown that the input transformation handles the nonlinear effects
and decreases the variations in DC-gains substantially. The
input transformation is combined with a new control structure that has a
pumping work minimization feature and consists of PID controllers and
min/max-selectors for coordinated control of EGR-fraction and
oxygen/fuel ratio. The EGR flow and the exhaust manifold pressure are
chosen as feedback variables in this structure. Further, the
set-points for EGR-fraction and oxygen/fuel ratio are transformed to
set-points for the feedback variables. In order to handle model errors
in this set-point transformation, an integral action on oxygen/fuel
ratio is proposed in an outer loop. An experimental validation and
comparison with a control structure without input transformation
shows that the proposed structure reduces EGR-errors at the expense of
increased pumping losses. In addition the comparison shows that the
input transformation improves the performance and achieves the same step
response for different flow conditions, thus handling the nonlinear
effects.
Johan Wahlström and Lars Eriksson
SAE Technical Paper Series,
2010
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