G. De Sercey, G. J. Awcock and M. Heikal University of Brighton School of Engineering UK This Work Conducted In Association With Ricardo Consulting Engineers, UK Toward A Calibrated LIF Image Acquisition Technique For In-Cylinder Investigation Of Air-to-fuel Mixing In Direct Injection Gasoline Engines OSAV2004 International Topical Meeting s

Toward A Calibrated LIF Image Acquisition Technique For In-Cylinder Investigation Of Air-to-fuel Mixing In Direct Injection Gasoline Engines Introduction The Laser Induced Fluorescence (LIF) Technique The Optical Set-up for Quantitative Measurement Calibration Strategy Tracer Optimisation Calibration Process Conclusion; - Discussion Of Results

Introduction I The Pressure for Better, Cleaner Engines GDI Engines User Demand Rocketing Fuel Cost 1970s Onwards Better Economy Is A Selling Point Manufacturers MUST Develop Cleaner Engines To Continue To Sell Cars! Fuel Injected (PFI) Engines Evolution Of European Emission Standards For Gasoline Engines (2.0 l) Imposed Pollution Limits Widespread Legislation

Introduction II Gasoline Direct Injection Engine: Injection Directly In The Engine Cylinder Better Control Over Injection Less Heat Losses Lower Consumption Reduced Emissions Achieved By Concentrating Fuel Around The Spark Plug Complex Geometry Complex Air Flow Complex Air / Fuel Mixing Stratified Mixture Intake Port Spark Plug Exhaust Air Flow Bowl-In-Piston Injector

LIF Technique Absorption Fluorescence Quenching (losses) ground electronic state excited electronic state Rotational vibrational transitions (Colour shift) Laser light Fluorescence Excited molecule (Tracer) LI Emission

Why Quantitative LIF? Qualitative LIF Shows Relative Distribution At A Particular Piston Position, Or Crank Angle (CA) Quantitative LIF Shows Absolute Distribution At Any Engine Position No Comparison Between Crank Angles No Comparison Between Experiments Gives Actual Fuel Concentration Allows Comparison Between Crank Angles Allows Comparison Between Experiments

Optical Set-up I Engine with quartz annulus Laser Nd:YAG, 266nm Shutter Sheet forming optics motor Beam dump Schott filter 532nm filter Lens-coupled gated image intensifier Cooled Camera PC Coated mirror (+ beam monitor tap)

Optical Set-up II

Calibration Strategy Must Compensate For Dependence Of Fluorescence On T & P Best Practice So Far: Measure Of T & P Dependency In A Pressure Vessel, BUT… Optical Set-up Different From The One Of The Experiment Unrealistic, As T & P Varies Spatially In The Engine! In-Cylinder Calibration Same Optical Set-up No Need To Measure P & T Provided Calibration And Experimental Images Are Acquired At The Same Crank Angle

Intake air Exhaust Intake plenum Heating tape Insulation layer 2 ID Pipe Ball valve Evaporation crucible Engine Injection hole Calibration Loop

Choice of Tracer Absorption Wavelength Achievable With A Laser Enough Fluorescence To Be Detectable With Decent SNR Low Sensitivity To Quenching Similarity To Fuel In Term Of Physical And Vaporisation Properties Non-Hazardous! Characteristics Sought For The Tracer

LIF Tracer Possibilities Fuel or TracerAbsorption (nm)Emission (nm)Boiling Point (ºC) Gasoline Various Iso-octaneNon fluorescing99 Biacetyl DMA Toluene Hexanone Acetone Pentanone

Tracer Optimisation I Test With Pure Acetone Saturation Crank Angles What Is Equivalence Ratio?

Stoichiometry & Equivalence Ratio A Stoichiometric Air-fuel Mix Is 14.7:1 It Represents The Air-fuel Mixture At Which Complete Combustion Of All Elements Of The Fuel Occur Equivalence Ratio, Φ, Is Used To Express The Ratio Of The Investigated Air-fuel Mixture To The Stoichiometric Ratio At The Stoichiometric Ratio, Φ = 1 At Φ > 1 The Mixture Is Rich, Leading To Unburned Fuel At Φ < 1 The Mixture Is Lean, Which Can Bring Reduced Pollutant Emissions

Tracer Optimisation II Test With Various Acetone Concentrations In Iso-Octane Optimum Between 2 And 10%

Calibration Process Overview Engine Motored In Closed-Loop Mode Calibration Images Acquired (For Each CA And Equivalence Ratio) And Processed To Extract Average IntensityAverage Intensity Average Intensities Plotted And Piece-Wise Linear FittedPiece-Wise Linear Fitted Calibration Look-Up-Table (LUT) GeneratedLook-Up-Table (LUT) Generated Engine Motored In Normal Mode Fuel Mixing Experiments Performed & Images Acquired (Error Images Derived, At Each CA, Mid-Term, BUT In Closed Loop Mode )Error Images Derived Error Image Corresponding To The Same CA Subtracted Calibration Map Applied Quantitative Air-to-Fuel Ratio Maps

Calibration Process Detail I Image Average At 210º CA, Equivalence Ratio Images Acquired At, E.G. 210º CA, Equivalence Ratio Pixel Average At 210º CA, Equivalence Ratio 1.2

Calibration Process Detail II Fluorescence Of An Homogeneous Mixture Depending On Piston Position

Calibration Process Detail III Calibration Look-up Table

Calibration Process Detail IV Homogeoneous Mixture Φ = 1.28 Pixel Average Point Operation = (Homogeneous Image Pixel Value – ) Error Image Derivation of Error Image (Performed In Mid-Term Of Experiments To Be Calibrated)

Calibration Process Summary Error Subtraction Calibration Quantitative Data Raw Experiment Image - Error Image Corrected Image

Review; - Why Quantitative LIF? Qualitative LIF Shows Relative Distribution At A Particular Crank Angle Quantitative LIF Shows Absolute Distribution At Any Engine Position No Comparison Between Crank Angles No Comparison Between Experiments Gives Actual Fuel Concentration Allows Comparison Between Crank Angles Allows Comparison Between Experiments

Quantitative Results I Equivalence Ratio Scale:

Quantitative Results II Uncalibrated Fluorescence Calibrated Fluorescence; - Equivalence Ratios Crank-Angle Compensation Allows Valid Fuel Mixing Studies To Be Conducted Over All Relevant Crank Angles A Range of Injection Strategies (At 1500 RPM) Start of Injection (SoI) At 0.5º, 30º, 60º ATDC A Range of Engine Speeds (At SoI 60º ATDC) 1500, 1000, 500 RPM

Review; - Why Quantitative LIF? Qualitative LIF Shows Relative Distribution At A Particular Crank Angle Quantitative LIF Shows Absolute Distribution At Any Engine Position No Comparison Between Crank Angles No Comparison Between Experiments Gives Actual Fuel Concentration Allows Comparison Between Crank Angles Allows Comparison Between Experiments

Quantitative Results III Mixture Distribution at 90º CA For A SoI At TDC, With Superimposed DFVR Air-Flow Predictions Comparison With Dynamic Flow Visualisation Rig (DFVR) DFVR Is A PIV Technique Using Water Seeded With Particles To Visualise Flow LIF And DFVR Results Are Compared At The SAME Crank Angle –Good Correspondence Rich Mixture (1.2<Φ<1.8) On Exhaust Side –Carried With Flow Out Of Bowl Lean (Φ<0.5) On Intake Side Dilution By High Velocity Air From Open Intake Valve

Quantitative Results VI Coefficient of Variation (CoV) Can Be Determined To Study Stability Of The Mixing Process CoV Is The Image RMS Difference Values Divided By Image Mean CoV Mixture Stability for Various Start of Injection Timings (White = >25%) Injection at TDC Injection at 30CA Injection at 60CA 25% 10% 0% These Results Suggest That 30CA Is The Most Stable Scenario Tests Performed On A Firing Engine Support This Evidence Injection At 30CA Gives Best Emissions Performance And Minimum Knock (Pre-ignition)

Conclusions A New Strategy Has Been Developed For Calibration of LIF Measurements Critical To Understanding Air-Fuel Mixing In The Cylinder It Is Efficient And Realistic Thanks To Calibration At Full Range Of Equivalence Ratios, Crank Angles And Engine Speeds It Is Effective Predictions From Motored Test Engine Give Good Agreement With Independent Investigations