3D CFD Modeling of Gas Exchange Processes in a Small HCCI Free Piston Engine

Transcription

1 3D CFD Modeling of Gas Exchange Processes in a Small HCCI Free Piston Engine Aimilios Sofianopoulos, Benjamin Lawler, Sotirios Mamalis Department of Mechanical Engineering Stony Brook University aimilios.sofianopoulos@stonybrook.edu Phone: September 27 th, 2017

2 Outline Introduction Model development and research methods Gas exchange simulation and analysis Conclusions and future work 2

3 Free Piston Engines No crank slider mechanism Piston motion controlled by force balance Piston coupled with linear alternator to produce electric power Advantages: Linear piston motion - reduced friction losses Variable compression ratio Drawbacks: Necessity for high speed piston motion controls Limited to 2-stroke operation Combustion chamber Linear Alternator Gas Spring Source: Haag et al. [1] 3

4 Gas Exchange Methods Gas exchange process: Critical for delivering fresh charge to the combustion chamber Major source of UHC emissions from short-circuiting Scavenging efficiency: Loop Crossflow Uniflow n sc = m fresh,trapped m total,trapped = 1 RGF Trapping efficiency n tr = m fresh,trapped m freshscavenge cylinder Effective Equivalence Ratio: φ = φ 1 RGF = φ n sc The following slides will discuss the design of a loop-scavenged HCCI engine Source: Goldsborough et al. [3] 4

5 Central Idea Combine free piston engine architecture with HCCI combustion of dilute natural gasair mixtures. Targets: η sc = % for high residual gas trapping (no preheating) Minimum short-circuiting losses Investigate the effect of engine design on the gas exchange process through 3D CFD modeling φ = φ n sc Source: Goldsborough et al. [3] 5

6 Modeled Engine Head Stroke (mm) 106 Combustion Chamber Bore (mm) 53 S/B (-) 2:1 Scavenge Ports CR geometric 23:1 CR effective 20:1 h c (mm) 4.8 S/V at TDC (m -1 ) 492 V d (cm 3 ) 233 Piston V c (cm 3 ) Frequency (Hz) 20 Fuel Natural gas [15] Exhaust Port Opening (EPO) (CAD ATDC) -217 Exhaust Port Closing (EPC) (CAD ATDC) -143 Air Box Scavenge Port Opening (SPO) (CAD ATDC) -212 Scavenge Port Closing (SPC) (CAD ATDC)

7 Model Setup Turbulence modeling: RANS with RNG k-ε [8] Amsden [9] law of the wall model for velocity and temperature SAGE combustion model Chemical kinetics mechanism developed by Hockett et al. [11]: Verified in HCCI relevant conditions,141 species 709 reactions Multi-zone combustion modeling approach of Babajimopoulos et al. [12] Grid: 0.7 mm base grid for the entire computational domain Refinement to 0.35 mm around TDC Velocity-based AMR (2 levels maximum) during gas exchange Temperature and velocity-based AMR around TDC Piston motion calculated from a system level model and used as input to the CFD model 7

8 Port Design Process 2 scavenge ports and 1 exhaust port in the cylinder Used the specific time-area analysis proposed by Blair [4]: Approximation of port dimensions based on target BMEP Developed for SI engines Large data scatter Adequate initial approximation for port design Modified accordingly for HCCI mixture preparation needs A sv (s/m) Scavenge Port 1 Scavenge Port 2 Intake Port Exhaust Port Blowdown A sv,p = PC Aθ dθ PO 360 V d f A sv,sp1 = A sv,sp2 = A sv,int = A sv,ep = A sv,bd = BMEP BMEP BMEP BMEP BMEP

9 Piston Burned gas backflow Scavenge Flow Regimes SPC Combustion Chamber Fresh charge flow Dilute mixture backflow Scavenge Port Closing Scavenge Burned gas Port backflow Intake Jet Opening SPO Dilute Scavenge Mixture Port Backflow Φ (-) Scavenging Efficiency (%) 28.5 Airbox 9

10 Piston Loop losses Analysis of Trapping Losses (1) Direct entrainment losses Backflow from exhaust EPC Combustion Chamber Scavenge Exhaust Port Direct Entrainment Opening Losses Exhaust EPO SPO ITL (CAD) = m fresh exhaust (CAD) m freshscavenge cylinder Trapping Efficiency (%) 87.7 Percentage of intake charge lost to the exhaust (%) 12.3 Airbox Intake 10

11 Analysis of Trapping Losses (2) Direct Entrainment Losses Looping Losses SPA = 50 o SPA = 90 o Flow deviating from prescribed direction Direct entrainment losses Fresh Charge Flow Looping motion Looping losses Fresh Charge Flow 11

12 Effect of Target Point TP 1 2 n trapping (%) Trapping Losses (% m in ) n scavenging (%) Moving target point from the center to the liner wall: Reduced direct entrainment losses Increased scavenging efficiency TP 1 TP 2 12

13 Effect of Scavenge Port Angle SPA ( ο ) n trapping (%) Trapping Losses (% m in ) n scavenging (%) SPA = 50 o SPA = 70 o SPA = 90 o Increasing SPA: Reduced direct entrainment losses Increased looping losses Reduced overall trapping losses 13

14 Effect of Unsweep Angle z target UPM( ο ) UPM 60 o 45 o 30 o n trapping (%) Trapping Losses (% m in ) n scavenging (%) o Increasing UPM: Reduced short-circuiting losses Increased looping losses Reduced overall trapping losses 14

15 Multicycle Operation Ongoing work: LES 10 consecutive cycles were simulated: Repeatable gas exchange process was predicted 38.1% gross indicated efficiency was calculated, with PRR = 5.8 bar/cad, at φ = 0.32 Ongoing work: Large Eddy Simulations Focus on combustion process Understand mixing between fresh charge and residual gas, thermal stratification, and effects on ignition timing and burn rates 15

16 Conclusions and Future Work Developed 3D CFD model of a small free piston engine Analyzed flow patterns contributing to scavenge flow and trapping losses Identified effects of port design parameters on scavenging and trapping efficiencies Simulated gas exchange for achieving trapping efficiency >98% for scavenging efficiency of 35% Simulated multi-cycle operation HCCI combustion of dilute natural gas-air mixtures was achieved no air pre-heating 38% gross indicated efficiency Future work: Use LES to understand how mixing and thermal stratification can be used to control ignition and burn rates 16

17 Thank you! Publications Sofianopoulos, A., et al., Multi-dimensional modeling of a 1 kwe Free Piston Linear Alternator. TFEC- IWHT, Sofianopoulos, A., et al. "Gas Exchange Processes of a Small HCCI Free Piston Engine A Computational Study." Appl. Therm. Eng. (2017), 17

18 Appendix 18

19 References 1. Haag, J., et al., Numerical and experimental investigation of in-cylinder flow in a loop-scavenged two-stroke free piston engine. 2012, SAE Technical Paper. 2. Goto, S., et al., Development of free piston engine linear generator system part 2-investigation of control system for generator. 2014, SAE Technical Paper. 3. Goldsborough, S.S. and P. Van Blarigan, Optimizing the scavenging system for a two-stroke cycle, free piston engine for high efficiency and low emissions: a computational approach. 2003, SAE Technical Paper. 4. Blair, G.P., Design and simulation of two-stroke engines. 1996: Society of Automotive Engineers Warrendale, PA 5. Mikalsen, R. and A. Roskilly, A review of free-piston engine history and applications. Applied Thermal Engineering, (14): p Han, Z. and R.D. Reitz, Turbulence modeling of internal combustion engines using RNG κ-ε models. Combustion science and technology, (4-6): p Amsden, A., KIVA3V. A Block-Structured KIVA Program for Engines with Vertical or Canted Valves. 1997, Los Alamos National Lab., NM (United States). 8. Senecal, P., et al., Multi-dimensional modeling of direct-injection diesel spray liquid length and flame lift-off length using CFD and parallel detailed chemistry. 2003, SAE Technical Paper 9. Hockett, A., G. Hampson, and A.J. Marchese, Development and Validation of a Reduced Chemical Kinetic Mechanism for Computational Fluid Dynamics Simulations of Natural Gas/Diesel Dual-Fuel Engines. Energy & Fuels, (3): p Babajimopoulos, A., et al., A fully coupled computational fluid dynamics and multi-zone model with detailed chemical kinetics for the simulation of premixed charge compression ignition engines. International journal of engine research, (5): p