CFD analysis of a two-stroke 70cc moped engine to reduce spillage losses Manish Garg Davinder Kumar R&D, TVS Motor Company Slide 1
Objective • To analyze the flow pattern in the engine • To understand short-circuit mechanism • Finding the various efficiencies, such as delivery ratio, charging, scavenging and trapping efficiencies at different load points • Use the model to improve the engine performance in terms of reduced spillage losses by 50% (from 20% short circuit of fresh charge in exhaust to 10%) Slide 2
Approach • As there is evidence in the literature, for 2S engines, that motoring does not replicate the exact flow conditions as in the real engine • So it was decided to model the pseudo combustion by initializing the burned gases 30 deg. ATDC of combustion. • The model is validated by ensuring the predicted and measured pressures during expansion match • Crank case was not modeled, boundary conditions were applied at the entry of the ports from measured data Slide 3
Engine Specifications Parameter Value Bore, mm 46 Stroke, mm 42 Con rod, mm 84 Comp. Ratio 9.4 EPO, ATDC 115 EPC, ATDC 244 SPO, ATDC 134 SPC, ATDC 226 Slide 4
CFD mesh and boundary conditions Measured Exhaust Pressure 4 Measured Crankcase Pressure Slide 5
Measured Pressure Data for Boundary Condition and Initialization 30 1.6 1.4 25 1.2 20 1 Pcrankavg_2500,bar 15 0.8 Pexhavg_2500,bar PCYL1avg_2500,bar 0.6 10 0.4 5 0.2 0 0 -200 -150 -100 -50 0 50 100 150 200 Slide 6
Boundary Conditions (scalar) Pressure Scalar Mass Fraction C8H18 O2 N2 Intake Intake Intake Intake (A) (A) (A) (Fresh) (Fresh1) (Fresh2) (Fresh3) (P) (P) (P) (P) Intake Port 1 0.086 0.21 0.70 1 1 0 0 Intake Port 2 0.086 0.21 0.70 1 0 1 0 Intake Port 3 0.086 0.21 0.70 1 0 0 1 NOTE: A – Active Scalar ; P – Passive Scalar 4 Fresh3 Fresh1 Fresh2 Slide 7
Boundary Conditions (wall) Wall Boundary Type Wall Temperature (K) Cylinder wall No slip 473 Dome wall No slip 473 Piston wall No slip 473 Intake Port wall No slip 423 Exhaust Port wall No slip 533 4 3 1 2 Slide 8
Initialization • Initial pressure and temperatures are taken from measurement at 30 degCA ATDC of combustion • Different species are initialized using chemical equilibrium condition for given equivalence ratio, temperature, and pressure Parameter Cylinder Intake Ports Exhaust Port Pressure (Pa) 1923405 92550 90030 Temperature (K) 1463 300 700 Slide 9
Initialization Mass Fraction Sr.No. Scalars Cylinder Intake Ports Exhaust Port 1 C8H18 (A) 0 0.085618 0 2 O2 (A) 0 0.213021 0 3 N2 (A) 0.70595 0.701361 0.70595 4 CO2 (A) 0.116693 0 0.116693 5 H2O (A) 0.093499 0 0.093499 6 Intake (Fresh) (P) 0 1 0 7 Exhaust (P) 1 0 1 8 H2 (A) 0.002088 0 0.002088 9 CO (A) 0.08177 0 0.08177 NOTE: A – Active Scalar ; P – Passive Scalar Slide 10
Models & sub-models • Solution Method [1]: Transient Solution algorithm: PISO • Turbulence Model[1]: K-Epsilon High Reynolds Number • Flow regime: Turbulent, Compressible • Solver Parameter: Under relaxation for pressure correction : 0.3 Momentum 0.7, Pressure 0.7, Temperature 0.9, Density 0.9 Turbulence 0.7 Differential Schemes: [1] MARS (Higher Order Scheme) - Momentum, Temperature, Turbulence UD - Temperature, CD - Density Slide 11
Cylinder Pressure Comparison Comparison of CFD Cylinder Pressure with experimental over a cycle Slide 12
Motion of the fresh charge in the combustion chamber Slide 13
Motion of the fresh charge in the combustion chamber Slide 14
Iso-surface of fresh charge with 50% mass fraction Slide 15
Detailed analysis of fresh mass short circuiting, showing contribution of each port Slide 16
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Short-circuit mechanism due to gas exchange The negative pressure of exhaust pressure pulse has a major impact on the short-circuit process Slide 20
Mass flow rate through inlets • Total fresh mass flow Inlet 5 through scavenge ports: Inlet 4 10.7 kg/hr Inlet 1 • Inlet 1: 28.35%, Inlet 2: Inlet 3 Inlet 2 18.60%, Inlet 3: 8.39%, Inlet 4: 20.50% Inlet 5: 24.16% • Fresh mass escaping: 2.8 kg/hr (26%) • Residual gas content: 16% Slide 21
Mass flow rate of passive scalar through intake port (attached boundaries) Slide 22
Mass flow rate of passive scalar through intake port (attached boundaries) Inlet 5 Inlet 4 Inlet 1 Inlet 3 Inlet 2 Slide 23
Mass flow rate of passive scalar through exhaust port (attached boundaries) Slide 24
Mass flow rate of passive scalar through exhaust port (attached boundaries) Inlet 5 Inlet 4 Inlet 1 Inlet 3 Inlet 2 Slide 25
Mass flow rate of active scalar through intake port (attached boundaries) Slide 26
Mass flow rate of active scalar through exhaust port (attached boundaries) Slide 27
Standard efficiencies (2500@WOT) • Delivery Ratio (Fresh mass delivered/Ref. mass[swept vol.*density]): 96.86% • Charging Efficiency (Fresh mass retained/Ref. mass): 73.69% • Trapping Efficiency (Fresh mass trapped/fresh mass intake): 76.08% • Scavenging Efficiency (Fresh mass in cylinder/cylinder mass): 82.10% Slide 28
Results at different load points 25% 50% 50% 50% wot 3500 2500 3500 5000 3500 Trapping Eff 80.03 78.24 77.04 81.49 74.69 Scvanging Eff 82.79 79.84 84.88 81.02 85.58 Delivery ratio 89.66 88.17 97.58 84.52 99.98 Charging Eff 71.75 68.98 76.72 68.88 79.55 fresh in exhaust 20.07 22.09 23.03 18.48 25.36 Slide 29
Experimental validation CFD vs. PIV measurement at 180 degCA ATDC PIV window Slide 30
Experimental validation CFD vs. PIV measurement at 226 degCA ATDC PIV window Slide 31
Experimental validation Watson method Slide 32
Mass flow rate of passive scalar through exhaust port (attached boundaries) Inlet 5 Inlet 4 Inlet 1 Inlet 3 Inlet 2 Slide 33
Design iterations • Design 1 : Inlet 1 and Inlet 5 area is reduced by 15 % each and added to Inlet 3 , however port entry area is not changed Inlet 5 Inlet 4 Inlet 1 Inlet 3 Inlet 2 Slide 34
Design 1 Base Design 1 Slide 35
Design Iterations • Design 2 : Inlet 1 and Inlet 5 area is reduced by 15 % each and added to Inlet 3 , port area is changed throughout Inlet 5 Inlet 4 Inlet 1 Inlet 3 Inlet 2 Slide 36
Design Iterations • Design 3 : Inlet 1 , Inlet 2 , Inlet 4 and Inlet 5 angle with horizontal is increased from 10 deg to 15 deg. Inlet 5 Inlet 4 Inlet 1 Inlet 3 Inlet 2 Slide 37
Short Circuit Analysis Fresh-3 Fresh-1 Fresh-2 Slide 38
Fresh-1 short-circuit through exhaust outlet boundary Slide 39
Fresh-2 short-circuit through exhaust outlet boundary Slide 40
Fresh-3 short-circuit through exhaust outlet boundary Slide 41
Intake short-circuit through exhaust outlet boundary Slide 42
Cumulative intake short-circuit through exhaust outlet boundary 12% drop in short circuit losses Slide 43
Conclusions • CFD model is established for a two-stroke 70cc moped engine to predict and improve the short- circuit (spillage losses) of fresh charge. • Two key reasons identified for the short-circuit losses are port design and gas exchange process. • Three different port designs are attempted to reduce the spillage losses. The best design resulted in 12% reduction of same. • A combine 3d-1d approach will be tried out to improve the gas exchange process. Slide 44
Thank you ! Slide 45
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