cfd analysis of a two-stroke 70cc moped engine to reduce...

CFD analysis of a two-stroke 70cc moped engine to reduce spillage

losses

Manish Garg

Davinder Kumar

R&D, TVS Motor Company

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%)

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

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

CFD mesh and boundary conditions

4

Measured Exhaust Pressure

Measured Crankcase Pressure

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

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

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

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

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

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

Comparison of CFD Cylinder Pressure with experimental over a cycle

Cylinder Pressure Comparison

Motion of the fresh charge in the combustion chamber

Iso-surface of fresh charge with 50% mass fraction

Detailed analysis of fresh mass short circuiting, showing contribution of each port

Short-circuit mechanism due to gas exchange

The negative pressure of exhaust pressure pulse has a major impact on the short-circuit process

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

Mass flow rate of passive scalar through

intake port (attached boundaries)



Inlet 1

Inlet 2 Inlet 3

Inlet 4

Inlet 5


exhaust port (attached boundaries)



Inlet 1

Inlet 2 Inlet 3

Inlet 4

Inlet 5

Mass flow rate of active scalar through


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%

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

PIV window

Experimental validation CFD vs. PIV measurement at 180 degCA ATDC

Experimental validation CFD vs. PIV measurement at 226 degCA ATDC

PIV window

Experimental validation Watson method



Inlet 1

Inlet 2 Inlet 3

Inlet 4

Inlet 5

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

Design 1

Base Design 1

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

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

Short Circuit Analysis

Fresh-1 Fresh-2

Fresh-3

Fresh-1 short-circuit through exhaust outlet boundary

Intake short-circuit through exhaust outlet boundary

12% drop in short circuit losses

Cumulative intake short-circuit through exhaust outlet boundary

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.

Thank you !

cfd analysis of a two-stroke 70cc moped engine to reduce...

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