Modeling and Control of an Electrically-Heated Catalyst
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Modeling and Control of an Electrically-Heated Catalyst Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Beth Ann Bezaire, B.S. Graduate Program in Mechanical Engineering The Ohio State University 2011 Thesis Committee: Dr. Shawn Midlam-Mohler, Co-Advisor Dr. Giorgio Rizzoni, Co-Advisor Dr. Yann Guezennec Copyright by Beth Ann Bezaire 2011 ABSTRACT Current model-based design research on automotive catalytic converters mainly fall into three basic categories: either modeling the catalyst as a continuous system based on physics, discretizing the system to reduce modeling complexity, or developing a highly- simplified, mean-value model for control. Continuous models are computationally intensive and therefore not well-suited for implementation into a vehicle model for Hardware in the Loop or control design. Highly-simplified models are calibrated for a particular system without incorporating the governing physical laws into the model, and mean-value models are only able to predict the response for a single lumped element. Although a simplified, mean-value model can be developed to accurately predict system response, it does not lend itself to being extended to broader applications without significant re-calibration efforts. Therefore, a model is needed that can account for the physics of the system so it can be extended to further applications while decreasing computation time to allow the model to be implemented for Hardware in the Loop and vehicle control design. This research investigates the development of such a model to predict automotive catalytic converter thermal response during warm-up. A one-dimensional, lumped- parameter model of a three-way catalyst was developed in Matlab/Simulink. The catalyst ii length was divided into discrete elements. Each discrete element contained states for the temperatures of the gas, substrate, and can wall. Heat transfer mechanisms were modeled from physics-based equations. For each discrete element, these equations modeled the enthalpy of the gas flow axially through the catalyst, convective heat transfer between the gas and substrate, conduction between discrete elements axially along the catalyst for the substrate and for the can, conduction between the substrate and can wall, and convection from the can wall to ambient. Model predictions were validated against experimental results for thermal transients. The application of this model was analysis for an extended-range electric vehicle application with electrically-heated catalyst (EHC). The model was used to compare the catalyst thermal response with and without the EHC. These results facilitated the development of a control strategy for the EHC, as well as recommendations for improving the overall vehicle control strategy. For further development, this model can also be extended to a two- or three-dimensional application. A two-dimensional catalyst model would be of interest to account for temperature gradients in the radial direction through the catalyst. iii DEDICATION To the one who gave His life for mine. iv ACKNOWLEDGEMENTS I would like to thank Dr. Shawn Midlam-Mohler for all of his guidance, his continued support, and his enthusiasm for advanced vehicle technologies and teaching. Thanks to Dr. Giorgio Rizzoni for the unparalleled opportunity to attend Ohio State University and be so involved in the EcoCAR program. Thanks to everyone at the OSU Center for Automotive Research for providing such excellent support of the students and motorsports teams. Thanks to the Advanced Vehicle Technology Competition EcoCAR: The Next Challenge program for providing the foundation for such an outstanding hands- on experience and for investing so effectively in the education of students. Thank you to the OSU-Honda Partnership program for the financial support to allow me to complete my graduate program while investing in the OSU EcoCAR program. And finally, I would like to thank the entire Ohio State University EcoCAR team for continually challenging, teaching, and inspiring me. v VITA January 6, 1986 ………………………………………Born – Detroit, Michigan May 2008 …………………………………………… B.S. Mechanical Engineering, University of Michigan-Ann Arbor July 2008 to Present ………………………….………Graduate Research Fellow, The Ohio State University Center for Automotive Research FIELDS OF STUDY Major Field: Mechanical Engineering vi TABLE OF CONTENTS Page Abstract………………………………………………………………..............…..……..ii Dedication…………………………………………………………….……….………....iv Acknowledgments…………………………………………………………….………….v Vita…………………………………………………………………….……….………..vi List of Tables……………………………………………………………………….........x List of Figures………………………………………………………………..……..…...xi Chapters: 1 Introduction ...................................................................................................................... 1 1.1 Motivation............................................................................................................... 1 1.2 EcoCAR: The Next Challenge................................................................................ 2 1.2.1 EcoCAR Emissions and Energy Consumption .................................................. 3 1.3 Ohio State EcoCAR Plug-In Hybrid Electric Vehicle ............................................ 4 1.4 Thesis Overview ..................................................................................................... 6 2 Literature Review............................................................................................................. 8 2.1 Introduction............................................................................................................. 8 2.2 Vehicle Tailpipe Emissions .................................................................................... 8 2.3 Three-Way Catalytic Converter .............................................................................. 9 2.3.1 Three-Way Catalytic Converter Design Considerations .................................. 13 2.3.1.1 Geometric Surface Area ............................................................................ 13 2.3.1.2 Heat Transfer Characteristics .................................................................... 14 2.3.1.3 Mounting Location.................................................................................... 14 2.4 Cold Start .............................................................................................................. 14 2.4.1 Startability and Emissions Prevention ............................................................. 15 2.4.2 Aftertreatment .................................................................................................. 20 2.4.2.1 Secondary Air Injection ............................................................................ 20 vii 2.4.2.2 Catalytic Converter Technologies ............................................................. 21 2.5 Electrically-Heated Catalyst (EHC) ..................................................................... 22 2.5.1 EHC Hardware Design .................................................................................... 22 2.5.2 EHC Emissions System Design ....................................................................... 26 2.5.2.1 Converter Packaging Location .................................................................. 27 2.5.2.2 Air Injection .............................................................................................. 27 2.5.2.3 Electrical Supply ....................................................................................... 27 2.5.2.4 Control Strategy ........................................................................................ 28 2.5.3 EHC Vehicle Applications ............................................................................... 29 2.6 Catalyst Modeling ................................................................................................. 30 2.6.1 Detailed Physical Models ................................................................................ 30 2.6.2 Simplified Kinetic Models ............................................................................... 31 2.6.3 Highly-Simplified Models ............................................................................... 32 2.7 Additional Emissions Considerations for Hybridized Vehicles ........................... 33 3 Electrically-Heated Catalyst Implementation ................................................................ 36 3.1 Introduction........................................................................................................... 36 3.2 Catalytic Converter Selection ............................................................................... 37 3.3 System Configuration ........................................................................................... 38 3.4 Control Strategy Development ............................................................................. 41 3.5 EHC Integration and Testing In-Vehicle .............................................................. 43 3.6 Engine Dynamometer Experimental Setup .......................................................... 46 4 Physics-Based Catalytic Converter Thermal Model ...................................................... 48 4.1 Introduction........................................................................................................... 48 4.2 System Description ..............................................................................................