A Second Law Perspective on Critical IC Research for High Efficiency Low Emissions Gasoline Engines

Transcription

1 A Second Law Perspective on Critical IC Research for High Efficiency Low Emissions Gasoline Engines University of Wisconsin Symposium on Low Emission Technologies for IC Engines June J.T. Farrell, J.H. Farenback-Brateman, C.H. Schleyer, J.G. Stevens, and W. Weissman (presenter) ExxonMobil Research and Engineering

2 Outline Availability Analysis Methodology and Simulation Tools Engine / Vehicle Simulations PFI Lean high CR stratified (SIDI) Ultra-Lean High CR Boosted (ULBDI) Mid-size vehicle fuel economies in US city and highway cycles Outline of Other Areas for Efficiency Gains Hurdles and Research Challenges State of the Art Fuel Chemistry and Gaps Autoignition Burn rate Summary

3 Availability Analysis Methodology Availability = Work Available vs. Standard Conditions; Allows Determining Maximum Achievable Potential and Where Losses Occur Reversible Chemistry Reversible Cooling Reversible Isothermal Expansion T, P, μ* T, P, μ T, P, μ T, P, μ Carnot Cycle W Sensible + W Expansion + W Chemical W S = T T 1 T Tˆ * chemical potential C p dtˆ W E = -RT ln(p / P) W C =- G (T )+RT { n i,p ln (P /P i,p ) - n i,r ln(p /P i,r )}

4 Simulation Tools Engine specs: Geometry Valve timings, burn parameters, manifold press., lambda, etc. Modified MIT engine simulator incorporating availability algorithms; CHEMKIN for thermo. GT- Power engine simulator for air flow and turbocharging Efficiency, fuel consumptions, availabilities over range of engine speeds and torques ADVISOR (DOE/NREL) Max. torque over range of engine speeds Driving cycle, vehicle characteristics, transmission specs Engine work outputs and availability balances Drive cycle fuel economies

5 Engine Cases Engine Power kw Comp. Ratio Liters # of Cylinders Lambda Min/Max PFI / 1. SIDI * / 1.7 ULBDI / 4. * stochiometric only at WOT Stochiometric PFI Operation Allows Use of TWC to Meet Emissions Specs SIDI and ULBDI Engines Cases Unconstrained by NOx Limits During Lean Operation Shows efficiency possible if NOx controlled by combustion innovations (e.g., HCCI) and/or aftertreatment advancements

6 Availability Breakdown for Base PFI Engine Available Energy, % fuel Cylinder Heat Losses Combustion Irreversibilities Exhaust to Ambient Fluid Flow Losses Mechanical Losses Brake Work Power, % Peak 8 1 Large Availability Losses in Low Load Region Are Due to Throttling and Low Power Output per Stroke Relative to Throttling and Mechanical Losses About 2% Lost Throughout by Conversion of Chemical Energy to Heat W lost = mt dq T

7 SIDI vs PFI Available Energy, % fuel PFI Engine Cylinder Heat Losses Combustion Irreversibilities Exhaust to Ambient Fluid Flow Losses Mechanical Losses Brake Work Available Energy, % fuel SIDI Engine Cylinder Heat Losses Combustion Irreversibilities Exhaust to Ambient Fluid Flow Losses Mechanical Losses Brake Work Power, % Peak Power, % Peak 8 1 Operating Lean vs. Throttling Increases Efficiency Reduces cylinder heat, exhaust availability and fluid flow losses Higher CR Reduces Exhaust Losses Combustion Losses Increase at Low Load/Lean Conditions due to Lower T and Higher m W lost = mt dq T

8 Fuel Consumption Impacts: Through to ULBDI Fuel Consumption (g/s) Cylinder Heat Losses Combustion Losses Exhaust Losses Fluid Flow Friction 3% peak power Fuel Consumption (g/s) Cylinder Heat Losses Combustion Losses Exhaust Losses Fluid Flow Friction 15% peak power Brake Work Brake Work. PFI SIDI ULBDI. PFI SIDI ULBDI Comparison of PFI, SIDI, and ULBDI Fuel Consumption at Representative Speed/Load Conditions Shows: Reduction in cylinder heat and exhaust losses due to higher CR & leaner operation for SIDI and ULBDI vs PFI + Added benefits in these areas and in fluid flow losses for ULBDI due downsizing and increased enleanment at low load Combustion losses increase slightly in ULBDI due leaner operation

9 Mid-Size Vehicle MPG Estimates US CITY US HIGHWAY AVERAGE PFI (Base) SIDI (1.2x) ULBDI (1.5x) 5% FE improvement = 33% FC benefit

10 Other Areas for Efficiency Gains Adiabatic Engine Design Turbocompounding, Turbogenerator, Bottoming Cycles Low Friction Designs/Lubes ULBDI Medium Power Example Fluid Flow Cylinder Heat Losses Combustion Losses Exhaust Heat Losses Friction Output Work New Concepts That Minimize Combustion Loss Integral: mt T dq Hybridization to Narrow Operating Range Burn Width Reduction Has Small Effect HCCI burn width (~ 5 CA) vs SIDI (~ 4 CA) yields 1-2 % benefit Main HCCI advantage is lower NOx HCCI Approaches That Control Burn Rate Through Wall Cooling Will Suffer Efficiency Debits Due to Increased Heat Losses

11 Research Needs for Creating Future Fuel / Engine / Aftertreatment Systems Current Areas of Research Focus High CR/Lean Operation Turbocharging HCCI Longer Term Adiabatic Engine Major Hurdles NOx Control Ignition Timing, Knock & Noise Control Research Needs for Overcoming Hurdles Chemistry and Fluid Dynamics End gas & HCCI auto-ignition Burn rate in various T/P/ composition profiles Lean De-NOx Catalysis Advanced Exhaust Recovery Concepts Low Cost /High Energy Recovery Concepts Innovations From Various Fields Outside Engine Area; e.g., High Efficiency Thermopiles Reduced Combustion Losses New System Address Through Effects in Work-loss Integral

12 Constant Volume Combustion Losses Similar for Gasoline and Diesel Range Fuels Fraction Availability Destroye H 2 cetane iso-octane butane CH Equivalence Ratio 1. T = 7K P i = 16 bar Entropy Produced During Constant Volume Combustion Similar for ~ C 4 and Larger Molecules Initial mixture properties approach pure air (fuel mole fraction <.2) Final mixture composition (CO 2, H 2 O, N 2 ) and temperature effectively same Largest Fuel Effects on Reducing Availability Losses Will Be Indirect Enabling strategies (HCCI, high CR SI) that minimize other availability losses

13 Temperature (K) Engine Advancements Move Autoignition Chemistry into New Regions Gasoline HCCI 1 2 Pressure (bar) 3 MON RON Diesel HCCI T/P History During Compression Stroke 4 Cylinder Pressure (bar) Fuels with same RON,MON paraffin + naphthene paraffin + aromatic paraffin + olefin Crank Angles (degrees) Fuel Ignitability Key Property for Future Engines SI Engines High CR and turbocharging demand increased knock resistance at peak load Turbocharging, internal EGR move autoignition requirements outside RON/MON region HCCI Fuel structure effects (ignition kinetics) more prominent Wide variability in HCCI approaches precludes definition of a single metric Increased Focus on Understanding Kinetics Constitutes Key Need dq/dt (J/deg)

14 Fuel Burn Rates in Advanced Engines Depend on Interactive Effects of Chemistry and Fluid Dynamics Promoting Lean Burn at High Pressure Will Help to Maximize Fuel Economy SI Engines Effect of high pressure on lean burn limit is incompletely understood Flame speed effects may be dramatically different at very high pressure HCCI May Not Be Flame-less Inhomogeneity / staged ignition can give rise to combination of autoignition + flame propagation late ignition Temperature (K) flame early ignition Cylinder Radius (cm) time Cylinder Radius (cm)

15 Summary SI Concepts Currently Being Developed Have Potential to Increase Fuel Economy by Factor of ~ 1.5 Main Challenges are NOx Control, Knock, HCCI Operability Significant Potential for Further Efficiency Improvements Heat transfer, exhaust losses and combustion irreversibilities are key areas to address New Fuel / Engine / Aftertreatment Combinations Open Areas For Innovation Improved understanding of the engine chemistry and fluid dynamics related to auto-ignition and burn rate Aftertreatment catalysis New system concepts (low irreversibility combustion, adiabatic engine, etc.)