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An Adaptive Proportional Integral Control of a Urea Selective Catalytic Reduction System based on System Identification Models
Abstract:
For urea Selective Catalytic Reduction (SCR) systems, adaptive
control is of interest to provide a capability of maintaining high
NOx conversion efficiency and low ammonia slip in the
presence of uncertainties in the system. In this paper, the
dynamics of the urea SCR system are represented by a
control-oriented model which is based on a linear transfer
function, with parameters dependent on engine operating conditions.
The parameters are identified from input-output data generated by a
high fidelity full chemistry model of the urea SCR system. The use
of the full chemistry model facilitated the representation of the
dynamics of stored ammonia (not a directly measurable parameter) as
well as post SCR NOx and ammonia slip.
A closed-loop Proportional-plus-Integral (PI) controller was
first designed using the estimate of stored ammonia as a feedback
signal. The performance was evaluated using the full chemistry
model, and was shown to result in a NOx conversion
efficiency of over 95%, with a maximum NH₃-slip of less than 5 ppm.
An adaptive PI controller was then designed and tested, and was
shown to lead to comparable performance. By incorporating nonlinear
components and projection maps which accounted for the nonlinear
relationship between stored ammonia and post SCR sensor
measurement, the adaptive controller was extended to a variant that
directly uses the sensor measurements (responsive to both
NOx-slip and ammonia slip). This novel adaptive
PI-controller was simulated on the full chemistry model, and shown
to be capable of delivering over 90% of NOx conversion
efficiency at a peak ammonia slip of less than 2 ppm. Experimental
results from a test vehicle demonstrated that as adaptation
proceeds, efficiency of SCR increases to levels of 90%, when driven
in city traffic, with a mean ammonia slip of about 10 ppm.