Advanced Combustion Strategies for High Efficiency Engines of the 21 - PowerPoint PPT Presentation

Advanced Combustion Strategies for High Efficiency Engines of the 21 st Century Jason Martz Assistant Research Scientist and Adjunct Assistant Professor Department of Mechanical Engineering University of Michigan

Estimated U.S. CO 2 Emissions in 2008: ~5815 Million Metric Tons 40% 6% 4% 17% 33%

Estimated U.S. Energy Use in 2009: ~94.6 quadrillion BTUs 58% 42% 75% 94% 25% 72%

DOE Vehicle Technologies Program Technical Targets 70 Engine Brake Efficiency (%) HD TRUCK GOALS 60 CURRENT HD ENGINES 50 Veh. Eff (%) 40 PEAK ~30% ENG. EFF. 30 PASS. CAR CAFE ~ 30% GOALS mpg CURRENT AUTOS 20 35.5 VEHICLE 27.5 EFF. 10 0 2000 2005 2010 2015 2020 Year DOE Passenger Car Goals : Increase peak engine efficiency from 34% to 45% and vehicle fuel economy by ~ 30% by 2016

How Can We Improve Brake Thermal Efficiency ( η b )? η = η η η b is a product of two efficiencies: b m i n ,  W η = b : Mechanical Efficiency m W i n , W η = i n , : Net Indicated Thermal Efficiency i n , Q in Hypothetical η m for a light duty engine η i,n from fuel-air cycle simulation Reducing Displacement/ Increasing Increasing Dilution Load

Enabling High Brake Thermal Efficiency with Charge Dilution, Downsizing and Boosting To identify potential high  efficiency operating regions, GT-Power simulations were performed with a simple Wiebe function combustion model for a range of boost pressures – 25°10-90 burn duration, CA50 at 10° ATDC Family of curves represents  operation at a given boost pressure for a range of Φ Regime of high efficiency  operation (0.4 < Φ < 0.6) combines: – Dilute combustion – Boosted operation – High mechanical efficiency Lavoie, G. A., Ortiz-Soto, E., Babajimopoulos, A., Martz, J., Assanis, D.N. (2012) Thermodynamic sweet spot for high efficiency, dilute boosted gasoline engine operation, in press Int. J. Engine Res .

Lavoie, G. A., Ortiz-Soto, E., Babajimopoulos, A., Martz, J., Assanis, D.N. (2012) Thermodynamic sweet spot for high efficiency, dilute boosted gasoline engine operation, in press Int. J. Engine Res .

Combustion Regimes and Their Approximate Limits Due to high levels of charge  dilution, it is difficult to use the conventional spark ignited (SI) combustion mode in high efficiency regions (0.4 < Φ < 0.6) HCCI on the other hand lacks  flames and can run extremely HCCI SACI SI dilute, but is load limited due to Increasing excessive combustion rates Dilution SACI (Spark Assisted Compression  Ignition) combines both SI and HCCI combustion modes – Begins with spark ignited flame propagation – Completed with auto-ignition Images of SACI Combustion Zigler, B. “An experimental investigation of the ignition properties of low temperature combustion in an optical engine.” Doctoral Thesis, University of Michigan, 2008.

Drive Cycle Simulations with Different Combustion Modes GT-Drive simulations of EPA  Combustion Air City/Hwy FE CASE Size Mode Handling (mpg) GAIN UDDS (city) and HWFEET (highway) drive cycles with maps 1 Stoich-SI NA 3.3 L 25.4 BASE from GT-Power simulations 2 Advanced NA 3.3 L 31.3 23% − 1490 kg vehicle 3 HCCI TC 3.3 L 31.2 23% – Peak torque and power TC 1.4 L 34.5 4 Stoich-SI 36% maintained at 281 Nm, 161 kW 5 Lean-SI TC 1.4 L 36.7 44% The benefits of advanced  6 Advanced TC 1.4 L 40.3 58% combustion (HCCI + SACI) with Combined City/Hwy Fuel Economy boosting and downsizing appear 50 Fuel Economy (MPG) LTC (Low + 58% GAIN OVER BASE to be relatively independent Temp. Comb.) 40 + 44% + 36% Modes + 23% + 23% – Best results obtained by 30 + 0% combining both strategies 20 Fuel economy gains of up to 58%  10 1 2 3 4 5 6 are possible relative to base 0 Stoich-SI Advanced HCCI Stoich-SI Lean-SI Advanced NA NA TC TC TC TC 3.3L 3.3 L 3.3 L 1.4 L 1.4 L 1.4 L Lavoie, G. A., Ortiz-Soto, E., Babajimopoulos, A., Martz, J., Assanis, D.N. (2012) Thermodynamic sweet spot for high efficiency, dilute boosted gasoline engine operation, in press Int. J. Engine Res .

SACI Combustion Experiments: UM FFVA Engine Engine Displacement 550 cc Bore 86 mm Stroke 94.6 mm Connecting Rod Length 152.2 mm Piston Pin Offset 0.8 mm Compression Ratio 12.5:1 Number of Valves 4 Piston Shape Shallow Bowl Gasoline 87 Fuel Type (RON+MON)/2 Sturman Hydraulic Valve System – Fully-flexible valve actuation (FFVA) – Four valves actuated electro- hydraulically – Variable lift, timing, duration – Independently controlled

Internal EGR for Charge Dilution Negative Valve Overlap (NVO)  – The exhaust valve is closed early during the exhaust displacement stroke and the intake valve is opened late during the intake stroke • This retains products from the previous cycle (internal residual) – Can control internal residual quantity from cycle to cycle – Internal residual fraction increases with more NVO – Varying internal residual changes compression temperature, which affects auto- ignition combustion phasing 8 0.28 510 Internal Residual Gas Fraction (-) NVO RGF Exhaust Intake Temp at IVC Event Event Temperature at IVC (K) 6 0.26 500 Pressure (bar) 0.24 490 4 0.22 480 2 IVO IVC EVO EVC 0.2 470 0 115 115 120 120 125 125 130 130 135 135 90 180 270 360 450 540 Crank Angle (deg) Negative Valve Overlap (deg)

Load Extension of LTC with SACI SAE 2011-01-1179 (Manofsky et al.)  – Demonstrated control over burn rate and combustion phasing at various loads – Extended high load limit to ~7.5 bar IMEP n 60 4.93 bar Increasing 5.96 bar 50 Load and 6.59 bar Spark 6.89 bar 40 AHRR (J/CA) Advance 7.31 bar 30 20 10 0 -40 -20 0 20 40 60 Crank Angle (deg)

Current Study  Goals – Examine methods for modifying heat release behavior at constant load and CA50 – Control burn rate (CA 10-90) and combustion phasing (CA50) independently  Approach – Change both variables (spark timing AND compression temperature) simultaneously – Temperature will affect flame propagation rate and timing of auto- ignition – Spark timing should compensate for the change in temperature, allowing constant CA50

Vary Spark and Compression Temperature at Constant CA50 (~8 dATDC) and Load (~6.5 bar IMEP n ) 0.4 12 Increasing iEGR 0.35 NVO eEGR 10 EGR Fraction (-) Total 0.3 Pressure (bar) Exhaust Intake 8 0.25 Event Event 6 0.2 0.15 4 0.1 -30 -25 -20 -15 2 Spark Advance (dATDC) 0 820 90 180 270 360 450 540 Temperature at 40 dBTDC (K) Crank Angle (deg) 810 Strategy  – Constant fueling rate of 19 mg/cycle 800 Constant Φ = 1.0, Φ′ = Φ (1 – EGR) ~ 0.62 – – Constant intake temperature (45° C) 790 – Vary temperature by trading off NVO and external EGR – Compensate for changes in combustion phasing with 780 -30 -25 -20 -15 spark timing Spark Advance (dATDC)

Control Over Burn Rate and Duration with SACI 100 1 32 dBTDC 32 dBTDC Rate of Heat Release (J/deg) Hottest 22 dBTDC 22 dBTDC Mass Fraction Burned (-) 80 0.8 Case 13 dBTDC 13 dBDTC Coldest Case 60 Coldest 0.6 Case Auto-Ignition 40 0.4 Hottest 20 0.2 Case 0 0 -30 -20 -10 0 10 20 30 40 -30 -20 -10 0 10 20 30 40 50 Crank Angle (deg) Crank Angle (deg) Results  – Time of auto-ignition = maximum change in slope of rate of heat release – Burn rate can be controlled at constant CA50 – addresses a major shortcoming of HCCI

Peak Heat Release Decreases with Higher Fraction of Flame Heat Release 100 14% Mass Maximum Heat Release Rate (J/deg) 100 32 dBTDC Burned by Rate of Heat Release (J/deg) 22 dBTDC 80 Flame 90 13 dBTDC 80 60 32% Mass Auto-Ignition Burned by 70 Flame 40 60 20 50 0 40 -30 -20 -10 0 10 20 30 40 0.15 0.2 0.25 0.3 Crank Angle (deg) Fraction of Flame Heat Release (-) Possible explanations  – As more mass is burned by the flame, less mass is available for auto-ignition – For a higher portion of flame based heat release, the mass consumed by auto-ignition is closer to the wall and has a higher temperature gradient

Operational Constraints 8 3.5 Ringing Intensity (MW/m 2 ) 7 Ringing COV of 3 COV of IMEPn (%) 6 Intensity IMEP n (%) 2.5 (MW/m 2 ) 5 4 2 3 1.5 2 1 1 -30 -25 -20 -15 -30 -25 -20 -15 Spark Advance (dATDC) Spark Advance (dATDC) 2.5 As spark is advanced:  – More mass is consumed by the flame 2.25 EI-NOx (g/kg fuel) – Less mass auto-ignites simultaneously – Trends are opposite of what advancing 2 spark alone gives 1.75 EI-NO x – Ringing intensity and NO x decreases (g/kg fuel) – COV of IMEP n increases 1.5 -30 -25 -20 -15 – Caused by flame or auto-ignition? Spark Advance (dATDC)

Effect on Thermal Efficiency 0.5 Net Gross Thermal Efficiency (-) 0.45 0.4 0.35 0.3 -30 -25 -20 -15 Spark Advance (dBTDC) Thermal efficiency remains relatively constant despite changes in compression  temperature and burn rate At constant load, we can manipulate the combustion behavior (to reduce NO x  and ringing) without negatively affecting thermal efficiency