losses Manish Garg Davinder Kumar R&D, TVS Motor Company - - PowerPoint PPT Presentation

Slide 1

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

Manish Garg Davinder Kumar R&D, TVS Motor Company

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%)

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

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

Slide 5

CFD mesh and boundary conditions

4

Measured Exhaust Pressure Measured Crankcase Pressure

Slide 6

Measured Pressure Data for Boundary Condition and Initialization

5 10 15 20 25 30

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

• 200
• 150
• 100
• 50

50 100 150 200 Pcrankavg_2500,bar Pexhavg_2500,bar PCYL1avg_2500,bar

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 Intake Port 2 0.086 0.21 0.70 1 1 Intake Port 3 0.086 0.21 0.70 1 1

NOTE: A – Active Scalar ; P – Passive Scalar

Fresh1 Fresh2 Fresh3 4

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

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

Slide 10

Initialization

NOTE: A – Active Scalar ; P – Passive Scalar

Sr.No. Scalars Mass Fraction Cylinder Intake Ports Exhaust Port 1 C8H18 (A) 0.085618 2 O2 (A) 0.213021 3 N2 (A) 0.70595 0.701361 0.70595 4 CO2 (A) 0.116693 0.116693 5 H2O (A) 0.093499 0.093499 6 Intake (Fresh) (P) 1 7 Exhaust (P) 1 1 8 H2 (A) 0.002088 0.002088 9 CO (A) 0.08177 0.08177

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

Slide 12

Comparison of CFD Cylinder Pressure with experimental over a cycle

Cylinder Pressure Comparison

Slide 13

Motion of the fresh charge in the combustion chamber

Slide 14

Motion of the fresh charge in the combustion chamber

Slide 15

Iso-surface of fresh charge with 50% mass fraction

Slide 16

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

Slide 17

Slide 18

Slide 19

Slide 20

Short-circuit mechanism due to gas exchange

The negative pressure of exhaust pressure pulse has a major impact

• n the short-circuit process

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

Slide 22

Mass flow rate of passive scalar through intake port (attached boundaries)

Slide 23

Mass flow rate of passive scalar through intake port (attached boundaries)

Inlet 1 Inlet 2 Inlet 3 Inlet 4 Inlet 5

Slide 24

Mass flow rate of passive scalar through exhaust port (attached boundaries)

Slide 25

Mass flow rate of passive scalar through exhaust port (attached boundaries)

Inlet 1 Inlet 2 Inlet 3 Inlet 4 Inlet 5

Slide 26

Mass flow rate of active scalar through intake port (attached boundaries)

Slide 27

Mass flow rate of active scalar through exhaust port (attached boundaries)

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%

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

Slide 30

PIV window

Experimental validation

CFD vs. PIV measurement at 180 degCA ATDC

Slide 31

Experimental validation

CFD vs. PIV measurement at 226 degCA ATDC

PIV window

Slide 32

Experimental validation

Watson method

Slide 33

Mass flow rate of passive scalar through exhaust port (attached boundaries)

Inlet 1 Inlet 2 Inlet 3 Inlet 4 Inlet 5

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

Slide 35

Design 1

Base Design 1

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

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

Slide 38

Short Circuit Analysis

Fresh-1 Fresh-2 Fresh-3

Slide 39

Fresh-1 short-circuit through exhaust

• utlet boundary

Slide 40

Fresh-2 short-circuit through exhaust

• utlet boundary

Slide 41

Fresh-3 short-circuit through exhaust

• utlet boundary

Slide 42

Intake short-circuit through exhaust

• utlet boundary

Slide 43

12% drop in short circuit losses

Cumulative intake short-circuit through exhaust outlet boundary

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.

Slide 45

Thank you !