cfd analysis of a two-stroke 70cc moped engine to reduce...
TRANSCRIPT
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Slide 1
CFD analysis of a two-stroke 70cc moped engine to reduce spillage
losses
Manish Garg
Davinder Kumar
R&D, TVS Motor Company
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Slide 2
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%)
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Slide 3
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
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Slide 4
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
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Slide 5
CFD mesh and boundary conditions
4
Measured Exhaust Pressure
Measured Crankcase Pressure
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Slide 6
Measured Pressure Data for Boundary Condition and Initialization
0
5
10
15
20
25
30
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-200 -150 -100 -50 0 50 100 150 200
Pcrankavg_2500,bar
Pexhavg_2500,bar
PCYL1avg_2500,bar
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Slide 7
Boundary Conditions (scalar)
Pressure Scalar Mass Fraction
C8H18 (A)
O2 (A)
N2 (A)
Intake (Fresh)
(P)
Intake (Fresh1)
(P)
Intake (Fresh2)
(P)
Intake (Fresh3)
(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
Fresh1 Fresh2
Fresh3
4
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Slide 8
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
1 2
3
4
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Slide 9
Initialization
Parameter Cylinder Intake Ports Exhaust Port
Pressure (Pa) 1923405 92550 90030
Temperature (K) 1463 300 700
• 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
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Slide 10
Initialization
NOTE: A – Active Scalar ; P – Passive Scalar
Sr.No. Scalars Mass Fraction
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
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Slide 11
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
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Slide 12
Comparison of CFD Cylinder Pressure with experimental over a cycle
Cylinder Pressure Comparison
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Slide 13
Motion of the fresh charge in the combustion chamber
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Slide 14
Motion of the fresh charge in the combustion chamber
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Slide 15
Iso-surface of fresh charge with 50% mass fraction
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Slide 16
Detailed analysis of fresh mass short circuiting, showing contribution of each port
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Slide 17
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Slide 18
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Slide 19
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Slide 20
Short-circuit mechanism due to gas exchange
The negative pressure of exhaust pressure pulse has a major impact on the short-circuit process
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Slide 21
Mass flow rate through inlets
• Total fresh mass flow through scavenge ports: 10.7 kg/hr
• Inlet 1: 28.35%, 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%
Inlet 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
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Slide 22
Mass flow rate of passive scalar through
intake port (attached boundaries)
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Slide 23
Mass flow rate of passive scalar through
intake port (attached boundaries)
Inlet 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
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Slide 24
Mass flow rate of passive scalar through
exhaust port (attached boundaries)
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Slide 25
Mass flow rate of passive scalar through
exhaust port (attached boundaries)
Inlet 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
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Slide 26
Mass flow rate of active scalar through
intake port (attached boundaries)
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Slide 27
Mass flow rate of active scalar through
exhaust port (attached boundaries)
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Slide 28
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%
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Slide 29
Results at different load points
25% 3500
50% 2500
50% 3500
50% 5000
wot 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
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Slide 30
PIV window
Experimental validation CFD vs. PIV measurement at 180 degCA ATDC
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Slide 31
Experimental validation CFD vs. PIV measurement at 226 degCA ATDC
PIV window
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Slide 32
Experimental validation Watson method
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Slide 33
Mass flow rate of passive scalar through
exhaust port (attached boundaries)
Inlet 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
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Slide 34
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 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
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Slide 35
Design 1
Base Design 1
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Slide 36
Design Iterations
Inlet 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
• Design 2: Inlet 1 and Inlet 5 area is reduced by 15 % each and added to Inlet 3, port area is changed throughout
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Slide 37
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 1
Inlet 2 Inlet 3
Inlet 4
Inlet 5
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Slide 38
Short Circuit Analysis
Fresh-1 Fresh-2
Fresh-3
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Slide 39
Fresh-1 short-circuit through exhaust outlet boundary
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Slide 40
Fresh-2 short-circuit through exhaust outlet boundary
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Slide 41
Fresh-3 short-circuit through exhaust outlet boundary
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Slide 42
Intake short-circuit through exhaust outlet boundary
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Slide 43
12% drop in short circuit losses
Cumulative intake short-circuit through exhaust outlet boundary
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Slide 44
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.
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Slide 45
Thank you !