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Residual Gas Effects on Combustion in an Air-Cooled Utility Engine

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Residual Gas Effects on Combustion in an Air-Cooled Utility Engine RESIDUAL GAS EFFECTS ON COMBUSTION IN AN AIR-COOLED UTILITY ENGINE by Brian Philip Albert A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science (Mechanical Engineering) at the UNIVERSITY OF WISCONSIN-MADISON 2004 i ABSTRACT Air-cooled utility engines are often equipped with cams designed to ensure maximum charging efficiency, but the resulting valve lift and timing often correlates to poor engine performance at low speeds and light loads, which manifests itself in poor combustion stability and elevated pollutant emissions. The poor performance at low-speeds and light- loads can be attributed to the presence of high levels of exhaust gas residuals (EGR) trapped in the cylinder. In this project the combustion stability and pollutant emissions of an air-cooled utility engine were investigated at low speeds and light loads with high residual fractions. An in- cylinder sampling valve was used to measure the amount of residual fraction in the charge. Varying the cam phasing, opening and closing times of both cams, effected a change in the amount of residual gas retained. A Fast Flame Ionization Detector (FFID) was used to measure the cycle-resolved hydrocarbon emissions from the combustion process, and a standard five-gas analytical emissions bench measured steady-state emissions. In addition, spark timing and different fuel mixture preparation techniques were tested to investigate their influence on combustion progress. The different fuel preparation systems included a stock carburetor, a carburetor-mounted liquid fuel injection (CMLFI) system, and a homogeneous mixing system (HMS). The residual fraction was found to be relatively insensitive to the fuel mixture preparation system, but was to a moderate degree sensitive to the ignition timing. The ii residual fraction was found to be strongly affected by the amount of valve overlap and engine speed. The measured residual fraction was compared to three models used for EGR prediction in order to quantify their applicability to utility engines. The three models used for comparative analysis include: Yun and Mirsky [16], Fox, Cheng and Heywood [12], and an ideal gas model. None of the three EGR models investigated was able to accurately predict the residual fraction over the entire range of conditions tested. Cylinder pressure-derived parameters were used to evaluate combustion performance parameters for cycle-to-cycle analysis of the investigated operating conditions. The CMLFI showed a slight improvement in performance relative to the other fuel preparation systems. Advancing ignition improved the cyclic variability of the tested conditions, with an exception at very light load operation at low speeds. The combustion stability was found to be sensitive to increasing amounts of overlap. The carburetor produced higher steady-state emissions, unburned hydrocarbons (uHC) and oxides of nitrogen (NOx), than the other fuel preparation systems. Advancing ignition and decreasing engine speed increased the measured uHC concentrations. The resulting uHC increased with increasing valve overlap. A strong prior cycle effect was found in the cycle- resolved hydrocarbon concentrations at high residual fraction. iii Dedicated to my daughter, Allison Eileen Albert iv ACKNOWLEDGEMENTS I would like to take this opportunity to thank the many people and organizations that helped contribute to this project. Specifically, I would like to thank Professor Jaal Ghandhi for all his support and encouragement during my undergraduate and graduate career. The last few weeks never would have happened if it weren’t for his cooperation. Thank you Jaal for all the knowledge you have instilled in me. I would like to thank all the members of the Wisconsin Small Engine Consortium: Briggs and Stratton Corporation, Harley-Davidson Motor Company, Kohler Corporation, Mercury Marine Corporation, MotoTron, Fleetguard Nelson Inc., and the Wisconsin Department of Commerce. In particular, I would like to pay a special thanks to Art Poehlman for his assistance with the Intek engine parts, Phil Pierce for his technical recommendations regarding the various systems pertinent to the project, and Matt Manthey for implementing the “skip-fire” program into MotoTune. Thanks to all the faculty, staff, and students at the UW-Engine Research Center, they truly made my time spent at the ERC more enjoyable. I would also like to thank my parents, Phil and Pauli Albert, for all their love and support. They have truly influenced who I have become. Finally, I would like to thank my beautiful wife, Meghann, for having the patience to allow me to complete this endeavor. I love you! v TABLE OF CONTENTS ABSTRACT............................................................................................................................... i ACKNOWLEDGEMENTS..................................................................................................... iv TABLE OF CONTENTS.......................................................................................................... v LIST OF FIGURES ................................................................................................................ vii LIST OF TABLES................................................................................................................... xi NOMENCLATURE ...............................................................................................................xii 1.0 INTRODUCTION .............................................................................................................. 1 1.1 Motivation....................................................................................................................... 1 1.1.1 Small Engine Motivation ......................................................................................... 1 1.1.2 Emissions Regulations ............................................................................................. 2 1.2 Objective......................................................................................................................... 4 2.0 BACKGROUND AND LITERATURE REVIEW ............................................................ 5 2.1 Introduction..................................................................................................................... 5 2.2 Fuel Mixture Preparation and Emissions........................................................................ 5 2.3 Exhaust Gas Residuals.................................................................................................. 10 2.3.1 Effect of EGR on Engine Performance and Emissions ......................................... 10 2.3.2 Predicting EGR Fraction........................................................................................ 14 2.3.3 EGR Fraction Measurement .................................................................................. 15 2.4 Techniques to Evaluate Cyclic Combustion Variability............................................... 17 2.4.1 Peak Pressure vs. the Location of Peak Pressure................................................... 17 2.4.2 Cycle-to-Cycle IMEP............................................................................................. 19 2.4.3 Fast-FID ................................................................................................................. 19 3.0 EXPERIMENTAL SETUP............................................................................................... 22 3.1 Engine Description........................................................................................................ 22 3.2 Engine Dynamometer, Torque Sensor and Shaft Encoder ........................................... 24 3.3 Engine Control.............................................................................................................. 25 3.4 Air Delivery System ..................................................................................................... 25 3.5 Fuel Systems ................................................................................................................. 28 3.5.1 Carburetor .............................................................................................................. 29 3.5.2 Carburetor-Mounted Liquid Fuel Injector ............................................................. 30 3.5.3 Homogeneous Mixture System.............................................................................. 32 3.6 Air-Fuel Ratio Analyzer ............................................................................................... 33 3.7 Ignition System ............................................................................................................. 33 3.9 Exhaust.......................................................................................................................... 36 3.9 Pressure Transducers .................................................................................................... 37 3.10 Data Acquisition System............................................................................................. 38 3.11 Exhaust Emission Measurement................................................................................. 38 3.12 Fast-FID ...................................................................................................................... 40 3.13 In-cylinder Sampling Valve........................................................................................ 40 vi 4.0 RESIDUAL GAS FRACTION MEASUREMENTS
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