3D CFD Combustion Simulation of a Four-Stroke SI Opposed Piston IC Engine Maria da Conceição Rodrigues Martins Dissertação para obtenção do Grau de Mestre em Engenharia Aeronáutica (Ciclo de estudos integrado) Orientador: Prof. Doutor Francisco Miguel Ribeiro Proença Brójo Setembro de 2020 ii Dedication To my father Adelino Rodrigues Martins. ”A good head and good heart are always a formidable combination. But when you add to that a literate tongue or pen, then you have something very special.” Nelson Mandela iii Acknowledgments I am eternally grateful to my mother for the sacrifices she made for me. To my brothers and sisters, Zé, Rosária, Hirminia and Fernando, for their support and dedication that inspired me to go on. A big thank you to my supervisor, Professor Francisco Brójo for the pacience and guidence. I wish to thank all my family, for their kindness and encouragement over the years. I want to thank my friends for making me feel at home. A special thanks to Cristiano for being my special friend. iv Resumo O motor alternativo de combustão interna desempenha um papel importante no mundo dos trans- portes, existindo ainda poucas configurações alternativas com sucesso comercial. Relativamente a aplicações em aeronaves ligeiras, onde as baixas vibrações são de extrema importância, os mo- tores boxer têm predominado o mercado. O aumento do custo do combustível e o aumento da preocupação do público com as emissões de poluentes levaram a um maior interesse em novas alternativas. Nos últimos anos, com o surgimento de novas tecnologias, técnicas de pesquisa e materiais, o motor de pistões opostos surgiu como uma alternativa viável ao motor convencional de combustão interna em algumas aplicações, inclusive na área aeronáutica. Este estudo apresenta uma análise numérica do processo de combustão da mistura de octano-ar num motor de faísca a quatro tempos e de pistão oposto. O modelo utilizado nas simulações representa o volume interno do cilindro do motor UBI / UDI-OPE-BGX286. A simulação foi executada no software Fluent 16.0, dos modelos disponíveis no Fluent o modelo de transporte de espécies foi escolhido para mode- lar a combustão, e três diferentes velocidades de motor foram simuladas: 2000RP M,3200RP M e 4000RP M. Em relação aos resultados obtidos nas três simulações CFD, o comportamento geral e as propriedades do fluxo no cilindro e os gráficos obtidos foram considerados aceitáveis. Palavras-chave Combustão; CFD; Emissões; Fluent; Ignição por Faísca; Motor de Combustão Interna; Octano; Motor de Pistões Opostos; Quatro Tempos. v vi Abstract The reciprocating IC engine plays an important role in the world transport, with very few alterna- tive configurations having commercial success. In light aircraft applications where low vibrations are crucial, boxer engines have predominated. The rising cost of fuel and the growth of public concern over pollutant emissions has led to an increased interest in alternative designs. In recent years, with the uprising of new technologies, research techniques and materials, the OP engine has emerged as a viable alternative to the conventional IC engine in some applications including in the aeronautical field. This study presents a numerical analysis of the combustion process of octane-air mixture in a four-stroke SI opposed piston engine. The model used in the simulations represents the internal volume of the cylinder of UBI/UDI-OPE-BGX286 engine. The simulation was run in Fluent 16.0 software, the species transport model was chosen to model combustion from the available in Fluent, and three different engines speeds were simulated: 2000RP M, 3200RP M and 4000RP M. Regarding the results obtained from the three CFD simulations, the overall behav- ior and properties of the in-cylinder flow and the obtained graphics were considered acceptable. Keywords Combustion; CFD; Emissions; Fluent; Four-Stroke; Internal Combustion (IC); Octane; Opposed Piston (OP); Spark Ignition (SI) vii viii Contents 1 Introduction 1 1.1 Motivation ......................................... 1 1.2 Main Goals ......................................... 2 1.3 Outline ........................................... 2 2 Literature Review 5 2.1 Internal Combustion Engines ............................... 5 2.1.1 Reciprocating Engine Operating Principle ................... 5 2.1.2 Engine Characteristics/Paramenters ....................... 7 2.1.3 Operating Characteristics/Parameters ...................... 8 2.2 Classification of Reciprocating IC Engines ........................ 11 2.2.1 Work Cycle ..................................... 11 2.2.2 Ignition Method .................................. 14 2.2.3 Method of Cooling ................................. 14 2.2.4 Lubrication System ................................ 15 2.2.5 Application ..................................... 15 2.2.6 Fuel ......................................... 15 2.2.7 Method of Mixture Generation .......................... 16 2.2.8 Valve or Port Arrangement ............................ 17 2.2.9 Combustion Chamber Design ........................... 18 2.2.10 Cylinder Arrangement ............................... 19 2.3 Opposed Piston Engine .................................. 21 2.3.1 Historical Overview of OP Engine ........................ 22 2.3.2 Challenges Facing OP Engines .......................... 27 2.3.3 Aeronautical Applications ............................. 27 2.4 Thermodynamic Cycle ................................... 29 2.4.1 Constant Volume Cycle .............................. 30 2.4.2 Thermodynamic Analysis of Air-Standard Otto Cycle ............. 31 2.5 Combustion / Combustion Considerations ....................... 34 2.5.1 Classifications of Fundamental Combustion Phenomena ........... 35 2.5.2 Combustion in Four-Stroke SI Engines ..................... 36 2.5.3 Ignition Timing .................................. 37 2.6 Combustion Analysis-Thermodynamic and Kinetic ................... 38 2.6.1 Mixture Composition ............................... 39 2.6.2 Combustion Stoichiometry ............................ 39 2.6.3 Mixture Ratios ................................... 40 2.6.4 Reaction Rates/Kinetics .............................. 41 2.7 Emissions .......................................... 42 2.7.1 Carbon Monoxide (CO) .............................. 43 2.7.2 Nitric Oxides (NOx) ................................ 43 2.7.3 Unburned Hydrocarbon (HC) .......................... 44 2.8 Computational Fluid Dynamics (CFD) .......................... 45 2.8.1 Governing Transport Equations ......................... 45 ix 2.8.2 Solution of Governing Equations ......................... 48 2.8.3 Turbulent Racting Flows/Combustion-Numerical Approach ......... 49 2.8.4 Combustion Modeling ............................... 49 3 Case Study-Combustion Simulation 51 3.1 Model Generation and Meshing Process ......................... 52 3.1.1 Geometry ...................................... 52 3.1.2 Mesh ........................................ 53 3.2 Physical Model Setup ................................... 55 3.2.1 Solver ........................................ 55 3.2.2 Models ....................................... 55 3.2.3 Boundary Conditions ............................... 58 3.2.4 Mesh Interfaces-Dynamic Mesh ......................... 59 3.3 Solution ........................................... 60 3.3.1 Solution Methods ................................. 60 3.3.2 Solution Controls ................................. 61 3.3.3 Monitors ...................................... 62 3.3.4 Solution Initialization ............................... 62 3.3.5 Calculation Activities ............................... 62 4 Results 63 4.0.1 In Cylinder Variables ............................... 63 4.0.2 Flame Propagation ................................ 66 4.0.3 Inlet Flow-Volumetric Efficiency ......................... 68 4.0.4 Outlet Flow-Emissions .............................. 69 5 Conclusion 73 5.1 Future Studies/Work ................................... 73 Bibliography 75 A Model Decomposition 79 B Mesh Details 81 C Problem Setup Details 85 C.1 Boundary Conditions ................................... 85 C.2 Dynamic Mesh ....................................... 86 D Results 91 D.1 Flame Propagation ..................................... 91 D.2 Inlet Flow .......................................... 92 D.3 Matlab Code ........................................ 92 D.4 Outlet Flow ......................................... 93 x List of Figures 2.1 Reciprocating IC Engine. Adapted from [1]. ....................... 6 2.2 The four-stroke engine cycle. Edited from [2]. ..................... 12 2.3 The two-stroke engine cycle[2]. .............................. 13 2.4 Types of fuel Injection in SI engines. Adapted from [3]. (a) Single point injection; (b) Multipoint injection; (c) Direct injection; 1-Fuel supply; 2-Air intake; 3-Throttle; 4-Intake manifold; 5-Fuel injector (or injectors); 6-Engine .............. 16 2.5 Types of fuel Injection of CI engines. Adapted from [4]. ................ 16 2.6 Engine Classification by Valve Location. Adapted from [5]. (a) L-head; (b) I-head; (c) F-head; (d) Valves in block on opposite sides of cylinder, T-head. ......... 17 2.7 Overhead valve and camshaft arrangement. ....................... 17 2.8 Combustion chamber for SI engines. Adapted from[6]. E- Exhaust valve; I- Inlet valve; S- Spatk plug. .................................... 18 2.9 Combustion chamber for SI engines. Adapted from [7]. (a) wedge chamber; (b) hemispherical head; (c) bowl in piston chamber; (d) bath-tub