Compressor Modeling for Control of Automotive Two Stage Turbochargers
There is a demand for increasing efficiency of automotive engines, and
one way to achieve this is through downsizing and turbocharging. In
the design compromises are made, for example the maximum power of the
engine determines the size of the compressor, but since the compressor
mass flow range is limited, this affects the torque for low engine
speeds. A two stage system, with two different sized turbochargers,
reduces this compromise, but the system complexity increases. To
handle the complexity, models have come to play a central role where
they aid engineers in the design. Models are used in simulation, for
design optimization and also in the control synthesis. In all
applications it is vital that the models have good descriptive
capabilities for the entire operating range studied.
A novel control oriented compressor model is developed, with good
performance in the operating regions relevant for compressors in a two
stage system. In addition to the nominal operating regime, also surge,
choke and operation at pressure ratios less than unity, are
modeled. The model structure can be automatically parametrized using a
compressor map, and is based on static functions for low computational
cost. A sensitivity analysis, isolating the important characteristics
that influence surge transients in an engine is performed, and the
gains of a novel surge controller are quantified.
A compressor map is usually measured in a gas stand, that has
different surrounding systems, compared to the application where the
compressor is used. A method to automatically determine a turbo map,
when the turbo is installed on an engine in an engine test stand is
developed. The map can then be used to parametrize the developed
compressor model, and effectively create a model parametrized for its
intended application.
An experimental analysis of the applicability of the commonly used
correction factors, used for estimating compressor performance when
the inlet conditions deviate from nominal, is presented. Correction
factors are vital, to e.g. estimate turbocharger performance for
driving at high altitude or to analyze second stage compressor
performance, where the variations in inlet conditions are large. The
experimental campaign uses measurements from an engine test cell and
from a gas stand, and shows 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 is developed, enabling measurements to
be analyzed with modified corrections. An adjusted shaft speed
correction quantity is proposed, incorporating also the inlet pressure
in the shaft speed correction. A high altitude example is used to
quantify the influence of the modified correction.
Oskar Leufven
2010
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Senast uppdaterad: 2021-11-10
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