COMPUTATIONAL STUDY OF DIRECT FUEL INJECTION IN THE ROTAX 914 ENGINE A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Engineering By BRAD PAUL POLLOCK B.S.M.E, University of Toledo, 2005 2010 Wright State University WRIGHT STATE UNIVERSITY SCHOOL OF GRADUATE STUDIES December 14, 2010 I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Brad Pollock ENTITLED Computational Study of Direct Fuel Injection in the Rotax 914 Engine BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science in Engineering Committee on Final Examination Haibo Dong, Ph.D. Haibo Dong, Ph.D. Thesis Director George P.G. Huang, P.E., Ph.D. John Hoke, Ph.D. Chair, Department of Mechanical and Materials Engineering College of Engineering and Computer Science Hui Wan, Ph.D. Andrew Hsu, Ph.D. Dean, School of Graduate Studies ABSTRACT Pollock, Brad. M.S.Egr., Department of Mechanical and Materials Engineering, Wright State University, 2010. Computational Study of Direct Fuel Injection in the Rotax 914 Engine. Direct injection spark ignition (DISI) is a fuel delivery method in which the fuel is introduced directly into the combustion chamber of an internal combustion engine. Although direct fuel injection was first pioneered in the early 1920’s, it has only recently become a reliable option due to advances made in control systems and injection technology. Direct injection enables increased fuel efficiency and higher power output than a conventional Port Fuel Injection (PFI) system. By delivering pressurized fuel directly into the cylinder, the degree of fuel atomization and the fuel vaporization rate are increased. Hence, the air/fuel mixture can be more precisely maintained, benefiting both fuel economy and emissions. In addition, the cooling effect of fuel droplets changing to vapor inside the combustion chamber facilitates a higher compression ratio and lessens the likelihood of knock. DISI has witnessed a resurrected interest in the automotive industry due to its promise of better fuel economy, additional power, reduced emissions and the ability to operate on multiple fuels. The aviation industry, on the other hand, has largely forgotten about the internal combustion engine subsequent to the invention of the jet engine. However, the introduction of unmanned aerial systems (UAS) has encouraged a renewed interest in small internal combustion engines such as the Rotax 914. Although, these iii engines provide a cheap power plant, they lack the power and efficiency required for their application. Consequently, by employing DISI in UAS engines, it affords flexibility with regards to fuel choice while also providing longer flight times and more power with less weight. As with any new application of technology, DISI in these smaller engines must first be tested and refined until it can seamlessly replace PFI. Experimental testing can be costly and time consuming, but computational fluid dynamics (CFD) can help speed the design process by performing parametric analysis to determine an optimum configuration to begin testing. For this thesis, a model of the Rotax 914 engine was developed to computationally model the effects of direct injection on the engine. Gambit was adopted for geometry generation and meshing, while Fluent was used for fluid motion and combustion simulation. A PFI version of the computational model was validated against experimental results of a Rotax 914 engine in order to add fidelity to the model. DISI was then applied to the model and a study was performed to determine operation capabilities under different operating conditions. iv Contents CHAPTER 1: INTRODUCTION .................................................................................................... 1 1.1 STATEMENT OF PROBLEM ............................................................................................................1 1.2 BACKGROUND AND RELEVANCE TO PREVIOUS WORK..........................................................4 1.3 GENERAL METHODOLOGY AND PROCEDURE TO BE FOLLOWED ..........................................5 1.4 LITERATURE REVIEW ...................................................................................................................6 1.5 THESIS OUTLINE.......................................................................................................................15 CHAPTER 2: COMPUTATIONAL MODELING ............................................................................ 18 2.1 MODELING AND MESHING.........................................................................................................18 2.1.1 Modeling.......................................................................................................................18 2.1.2 Meshing ........................................................................................................................22 2.1.3 Boundary Conditions.....................................................................................................26 2.2 COMPUTATIONAL MODELS ........................................................................................................32 2.2.1 Turbulence Models .......................................................................................................32 2.2.2 Combustion Models ......................................................................................................34 2.2.3 Spark Ignition................................................................................................................36 2.2.4 Autoignition ..................................................................................................................37 2.2.5 Injections.......................................................................................................................39 2.2.6 Solver Settings ..............................................................................................................40 CHAPTER 3: EXPERIMENTAL SETUP, RESULTS AND MODEL VALIDATIONS .............................. 46 3.1 EXPERIMENTAL SETUP...............................................................................................................46 3.2 EXPERIMENTAL RESULTS............................................................................................................48 3.3 COMPUTATIONAL RESULTS ........................................................................................................53 3.3.1 Computational Cycle.....................................................................................................53 3.3.2 Cold Flow ......................................................................................................................58 3.3.3 Computational Results..................................................................................................62 3.4 VALIDATION STUDY ..................................................................................................................64 3.4.1 Physical Comparison.....................................................................................................64 3.4.2 Boundary Conditions & Initial Conditions .....................................................................66 v 3.4.3 Output Comparison.......................................................................................................69 CHAPTER 4: DIRECTION INJECTION STUDY ............................................................................. 73 4.1 PFI SETTINGS ..........................................................................................................................73 4.2 SPARK TIMING STUDY ...............................................................................................................76 4.3 INJECTION ANGLE.....................................................................................................................78 4.4 INJECTION TIMING....................................................................................................................82 4.5 SPARK ENERGY STUDY...............................................................................................................83 4.6 CONCLUSIONS .........................................................................................................................85 CHAPTER 5: BOWL PISTON DIRECTION INJECTION STUDY ...................................................... 87 5.1 COLD FLOW / CHARGE MOTION STUDY .......................................................................................87 5.2 INJECTION ANGLE.....................................................................................................................92 5.3 INJECTION TIMING....................................................................................................................95 5.4 CONCLUSIONS .........................................................................................................................99 CHAPTER 6: FUTURE WORK AND RECOMMENDATIONS ....................................................... 102 CHAPTER 7: CONCLUSIONS .................................................................................................. 107 REFERENCES................................................................................................................... 109 vi Nomenclature PFI Port Fuel Injection DISI direct injection spark ignition SI Spark ignition Φ Equivalence ratio GDI Gasoline Direct Injection CFD Computational fluid dynamics HCSI Homogeneous Charge Spark Ignition