Output Selection and Its Implications for MPC of EGR and VGT in Diesel Engines
Control of exhaust gas recirculation (EGR) and
variable geometry turbine in diesel engines is a challenging
problem and model predictive control (MPC) seems to be a
promising method. In MPC the choice of output variables, and
thereby the criterion, has a direct impact on the optimization
problem to solve and the resulting control performance. Different
selections of outputs are investigated and discussed, proposing
that it is beneficial to include EGR-fraction and pumping losses
in the criterion while having the oxygen/fuel ratio as a constraint.
The rational for this constraint is that, in diesel engines, it is
allowed to have the oxygen/fuel ratio larger than a set-point. The
proposed design also includes integral action of the EGR-fraction
to handle model errors and prediction of engine load and
speed. A comparison is made between the proposed MPC, a
proportional integral derivative (PID) controller, and an MPC
with intake manifold pressure and compressor flow as outputs,
which is the common choice in the literature. Comparisons
are performed in simulation on the European transient cycle
showing the following two points. First, the proposed design
gives 9% lower oxygen/fuel ratio error, 80% lower EGR-error,
and 12% lower pumping losses compared to an MPC design
with intake manifold pressure and compressor flow as outputs.
Second, the proposed design gives 9% lower EGR-error and 6%
lower pumping losses compared to a control structure with PID
controllers with oxygen/fuel ratio and EGR-fraction as the main
outputs.
Johan Wahlström and Lars Eriksson
IEEE Transactions on Control Systems Technology,
2013
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