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Research Papers

Robust Separation of Signal Domain From Single Channel Mixed Signal Output of Automotive Urea Based Selective Catalytic Reduction Systems

[+] Author and Article Information
D. Upadhyay

e-mail: dupadhya@ford.com

M. Van Nieuwstadt

e-mail: mvannie1@ford.com
Ford Motor Company,
Dearborn, MI 48124

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received January 22, 2013; final manuscript received September 10, 2013; published online October 7, 2013. Assoc. Editor: Shankar Coimbatore Subramanian.

J. Dyn. Sys., Meas., Control 136(1), 011012 (Oct 07, 2013) (9 pages) Paper No: DS-13-1035; doi: 10.1115/1.4025459 History: Received January 22, 2013; Revised September 10, 2013

There is a class of sensor constrained, uncertain, chemical reactor systems that pose unique challenges with regard to the feedback signal. We refer specifically to the urea based selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the engine exhaust of diesel powertrains. These catalysts rely on adsorbed ammonia (NH3), produced from aqueous urea, for the catalytic reduction of NOx to N2. Typically, underinjection of urea will result in the slip of NOx, whereas overinjection will induce NH3 slip. The ideal control objective of such a plant is, therefore, to regulate urea injection such that the net slip over the catalyst is minimized. Meeting these control objectives is made difficult due to the presence of an output sensor that is cross sensitive to both NOx and NH3, thereby producing a mixed feedback signal. This signal confounding poses significant challenges with regard to the stability and robustness of both closed loop control as well as on board diagnostics. In the absence of a robust NH3 sensor, it becomes necessary to create alternate methods of signal disambiguation. However, so far in open literature, there has not been a detailed discussion of this problem nor has a concrete solution been proposed to robustly and continuously identify the nature of slip as NOx or NH3. In this paper, we discuss the systematic development of a new method that allows a robust and continuous determination of the slip regime from the mixed signal output of a standard NOx sensor. The full scope of the practical problem is discussed and the performance of the proposed method is shown via experimental data.

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Copyright © 2014 by ASME
Topics: Signals , Catalysts
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Figures

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Fig. 1

Illustration of single mode slip domain with slip domain transition during a HDT cycle. FTIRS signals show NOx and NH3 species individually. The cross sensitivity of the NOx sensor to NH3 is clear.

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Fig. 2

Illustration of mixed mode slip with both NH3 and NOx in the slip stream, as confirmed by the FTIRS signals, for a deteriorating SCR catalyst

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Fig. 3

Illustration of NOx slip creep behavior for a steady state operating condition

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Fig. 4

Illustration of an NH3 slip signal with high frequency modulation influenced by open-loop, FG NOx based urea injection

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Fig. 5

Prototypical signals for FG NOx, TP NOx, and TP NH3. NOx signals are always phase correlated, while the NH3 signal may become phase uncorrelated with the FG NOx signal.

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Fig. 6

Performance of SDM for slip transition from NOx to NH3 over a standard HD-FTP cycle. The transition from NOx slip only to NH3 slip only is easily confirmed by the FTIRS signals.

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Fig. 7

HD-FTP tests with a straight pipe. The straight pipe is equivalent to a missing SCR and simulates the extreme case of a fully deteriorated SCR.

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Fig. 8

SDM response over an off-cycle test drive under the influence of NOx slip creep with transition to NH3 slip

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