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Experimental and Numerical Study of a Two-Stroke Poppet Valve Engine Fuelled with Gasoline and Ethanol

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Experimental and Numerical Study of a Two-Stroke Poppet Valve Engine Fuelled with Gasoline and Ethanol EXPERIMENTAL AND NUMERICAL STUDY OF A TWO-STROKE POPPET VALVE ENGINE FUELLED WITH GASOLINE AND ETHANOL A thesis submitted for the degree of Doctor of Philosophy by Macklini Dalla Nora Department of Mechanical, Aerospace and Civil Engineering College of Engineering, Design and Physical Sciences Brunel University London November 2015 II Abstract The restrictions imposed by CO2 emission standards in Europe and many countries have promoted the development of more efficient spark ignition engines. The reduced swept volume and number of cylinders of four-stroke engines has significantly improved fuel economy by means of lower pumping and friction losses. This approach, known as engine downsizing, has demonstrated its potential of reducing fuel consumption on its own as well as applied to hybrid vehicles where a low weight engine is desired. However, aggressive engine downsizing is currently constrained by thermal and mechanical stresses and knocking combustion. In order to overcome these limitations, the present work evaluates the application of a conventional poppet valve direct injection engine into the two-stroke cycle. Two-stroke engines have the ability to produce higher power with reduced swept volume and less weight than four-stroke engines thanks to the doubled firing frequency. These advantages, although, are sometimes offset by poorer emissions resulted from fuel short-circuiting; lower thermal efficiency resulted from short expansion process; and reduced engine durability due to lubrication issues. Therefore, in this research the four-stroke engine architecture was employed so these shortcomings could be addressed by the use of direct fuel injection, variable valve actuation and a wet crankcase, respectively. The burnt gases were scavenged during a long valve overlap by means of boosted air supplied by an external compressor. An electrohydraulic fully-variable valve train enabled the optimisation of the gas exchange process in a variety of engine operating conditions. The air-fuel mixture formation was evaluated through computational fluid dynamic simulations and correlated to experimental tests. In addition, the engine operation with ethanol was assessed in a wide range of engine loads and speeds. Finally, the engine performance, combustion process, air-fuel mixing and gas exchange results were presented, discussed and contextualised with current four-stroke engines. Keywords: Two-stroke poppet valve engine; gasoline and ethanol direct injection; engine downsizing; supercharged two-stroke cycle. III List of contents List of illustrations .............................................................................................. VII List of tables ....................................................................................................... XV Acknowledgements ........................................................................................... XVI Notation ........................................................................................................... XVII Chapter One Introduction .................................................................................1 1.1 Preface .......................................................................................................1 1.2 Research objectives ...................................................................................3 1.3 Thesis outline ..............................................................................................3 Chapter Two Literature review .........................................................................6 2.1 Introduction .................................................................................................6 2.2 Overview of current gasoline engines ....................................................... 11 2.3 Two-stroke cycle engines ......................................................................... 14 2.3.1 History and background ..................................................................... 14 2.3.2 Engine operation fundamentals ......................................................... 16 2.3.3 The scavenging process .................................................................... 18 2.3.4 The charging process ......................................................................... 22 2.3.5 Mixture formation and combustion ..................................................... 24 2.4 Two-stroke engines in the contemporary automotive sector ..................... 26 2.4.1 Engine downsizing ............................................................................. 29 2.4.2 Stratified charge combustion .............................................................. 33 2.4.3 Controlled auto-ignition combustion ................................................... 36 2.4.4 Vehicle hybridisation .......................................................................... 39 2.5 Biofuels ..................................................................................................... 41 2.6 Summary .................................................................................................. 47 Chapter Three Experimental methodology ....................................................... 48 3.1 Introduction ............................................................................................... 48 3.2 Experimental setup ................................................................................... 48 3.2.1 Engine specifications ......................................................................... 49 3.2.2 Emissions measurement .................................................................... 51 3.2.3 Fuel supply system ............................................................................ 53 3.2.4 Data acquisition and control ............................................................... 56 3.2.5 Dynamometer, intake air supply and hydraulic systems .................... 59 IV 3.3 Data analysis ............................................................................................ 60 3.3.1 Heat release analysis ......................................................................... 61 3.3.2 Overall engine parameters ................................................................. 63 3.3.3 Engine-out emission analysis ............................................................. 65 3.3.4 Gas exchange calculations ................................................................ 70 3.4 Summary .................................................................................................. 74 Chapter Four Numerical methodology ............................................................ 75 4.2 Introduction ............................................................................................... 75 4.3 Mathematical model .................................................................................. 75 4.4 Cold flow simulation .................................................................................. 80 4.4.1 Mesh independency study ................................................................. 81 4.4.2 Time-step independency study .......................................................... 86 4.4.3 Cold flow model validation ................................................................. 87 4.5 Fuel spray simulation ................................................................................ 88 4.5.1 Spray calculation ................................................................................ 89 4.5.2 Spray model validation ....................................................................... 90 4.6 Summary .................................................................................................. 94 Chapter Five Experimental assessment of the two-stroke poppet valve GDI engine…………… ............................................................................................... 95 5.1 Introduction ............................................................................................... 95 5.2 Test procedures ........................................................................................ 95 5.3 Results and discussion ............................................................................. 98 5.3.1 Performance and gas exchange analysis .......................................... 98 5.3.2 Combustion and heat release analysis ............................................ 107 5.3.3 Emission analysis............................................................................. 118 5.4 Summary ................................................................................................ 124 Chapter Six Investigation of the gas exchange process in the two-stroke poppet valve engine .......................................................................................... 125 6.1 Introduction ............................................................................................. 125 6.2 Test procedures ...................................................................................... 125 6.2.1 Valve opening duration tests ............................................................ 126 6.2.2 Valve lift and exhaust backpressure tests ........................................ 127 6.2.3 Procedures for estimating the in-cylinder lambda at lean conditions128 6.3 Results and discussion ........................................................................... 129 V 6.3.1 Effects of valve opening duration ..................................................... 129 6.3.2 Effects of
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