LiU startsida
Abstract |
Modeling for control of centrifugal compressors
Downsizing and turbocharging of engines provide a way to meet
increasing demands for efficiency and performance in the automotive
industry. An engine design is a result of compromises, e.g. the
selection of charging system, and the trend is to reduce these
compromises by increasing system complexity. Models have come to play
a central role to handle this rise in complexity, and are used for
simulation, system optimization and control synthesis. The models
should describe the entire operating range, be capable of
extrapolation, be easily parameterizable, and wide cover a range of
applications.
A novel compressor model is developed which, in addition to the
nominal op- eration, also covers surge, choke and operation at
pressure ratios less than one. The model is based on data from more
than 300 compressor maps, measure- ments from engine test stands, and
a surge test stand. The general knowledge gained from the in-depth
analysis is condensed in the model equations. The model can be
automatically parametrized using a compressor map, is based on static
functions for low computational cost, and is shown to extrapolate low
speed compressor operation well. Furthermore, it is shown to be
applicable to compressors of different size, ranging from small car
applications to large heavy duty vehicles. Compressor restriction
operation is modeled down to a standstill compressor, and shown to
agree well with gas stand measurements. Further, the analysis
contributes with new knowledge and models for choking pressure ratio
and flow.
A method to automatically determine a turbo map, when the turbo is in-
stalled on an engine in an engine test stand is developed. The method
can be used to validate manufacturer maps or expand the region covered
in a map. An analysis of the limits that an engine installation
imposes on the reachable points in the compressor map is
performed. The addition of a throttle before the compressor is
suggested to increase the reachable map region, and an engine and test
cell control structure that can be used to automate the measurements
is proposed. Two methods that compensate for the deviation between
mea- sured and desired speeds, are proposed and investigated. A gas
stand map is compared to the map generated in the engine test stand,
and a generally good agreement results.
An experimental analysis of the applicability of the commonly used
correc- tion factors, used for estimating compressor performance when
the inlet condi- tions deviate from nominal, is performed. Correction
factors are vital, to e.g. estimate turbocharger performance for
driving at high altitude or to characterize second stage compressor
performance, where the variations in inlet conditions are
large. Measurements from an engine test stand and a gas stand show a
small but clearly measurable trend, with decreasing compressor
pressure ratio for decreasing compressor inlet pressure, for points
with equal corrected shaft speed and corrected mass flow. A method
that enables measurements to be analyzed with modified corrections is
developed. As a result, an adjusted shaft speed correction quantity is
proposed, incorporating also the inlet pressure in the shaft speed
correction.
Oskar Leufven
2013
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Last updated: 2021-11-10
