analysis of combustion characteristics of cai-engine with various valve strategies

Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University

ICAT 08, Istanbul, Turkey

ANALYSIS OF COMBUSTION

CHARACTERISTICS OF CAI-ENGINE WITH

VARIOUS VALVE STRATEGIES

Jin Nam KIM, Ho Young KIM, Sam S. Yoon, Woo Tae KIM and Sang Dong SA

Korea University, Hyundai Motor Company

2008. 11. 13.



ContentsContents

Introduction

Theoretical Modeling

Numerical Details

Results and Discussion

Concluding Remark



IntroductionIntroduction

Homogeneous mixture of air, fuel and residual gases is compressed until

auto-ignition occurs at sites distributed throughout the combustion chamber.

CAI Engine operation

CAI combustion in IC engine provides better performance in various aspects

compared with both SI and CI combustion.

- Operate unthrottled at part load and therefore reduce pumping losses

- Overall combustion temperature is significantly reduced by the presence of excess air or internal EGR and produces ultra-low NO emission



There is no direct method to control auto-ignition timing !!!!!!

Fuel : High volatile fuel, Dual fuel supply

Intake air : Intake charge heating auto ignition timing is advanced

Compression ratio : Low smoke and NOx emission with high compression

ratio and low intake charge temperature.

Injection time : Achieve the combustion stability by injection timing and

multi injection timing.

Recirculation or Trapping of burned gases : Exhaust Gas Recirculation,

Residual gas trapping with variable valve timing

Control the auto-ignition of CAI Engine

Uniform mixture formation

Stable ignition



Trapping of residual gas – Internal EGR in CAI

Trapping of exhaust gases by closing exhaust

valves relatively early and opening intake valve late.

Intake valves are opened and fresh charge drawn

into the cylinder.

Fresh charge and exhaust mixture is then

compressed in the next compression stroke.

The auto-ignition occurs in the final stage of

compression stroke.



Objective of this study

Poor mixing or stratification between fuel and intake gases often

poses

a great technical challenge (incomplete combustion, high emission

level)

The distribution of the internal EGR affects the mixture formation as well

as the combustion characteristics of CAI engineNumerically examine the effects of various valve strategies on the overall performance for the CAIengine

To Obtain - Flow and Mixing Characteristics

- Combustion Characteristics

- Emission Characteristics

Various Valve timing and Lift Strategies are applied



Compression Ratio 10.5 : 1

Bores X Stroke 88 mm X 97 mm

Connecting Rod Length 143.75 mm

Intake Valve Seat Diameter 32.8 mm

Exhaust Valve Seat Diameter 26 mm

ModelingModeling Engine Specification

STL format CAD data



EquationsEquationsEquationsEquations

ModelingModeling

jj j j

u St x x x

Generalized equation for continuous phase

2

, ; ;ji ik t e t t

j i j

u c ku uwhere G

x x x

Continuity 1 0

Momentum

Turbulent kinetic energy

Species

Enthalpy

Dissipation rate

SS

iu e

k

sY

h

e

k

e

e

Y

e

h

,

2

3 i

j ke e ij d u

i j i k

u upk S

x x x x

kG

1 2kc G ck

,d mS

,d jS

,d hS



mass transfer

fuel droplet

heat transfer

ls m

dmA F

dt ,

,

ln t vm g t

t v s

p pF K P

p p

1

2d

d d d d d d

dum C A u u u u V p

dt

0.687 3

3

24 1 0.15Re Re 10

0.44 Re 10

d dd

d

C

Where

Where

, "p d d dd s d fg

d C T dmm A q h

dt dt

Where "d dq h T T

By Ranz correlation

By Yuen & Chen correlation

By El Wakil formulation & Ranz-Marshall correlation

Fuel injection

Droplet Break-up

Spray injection with atomization Reitz and Diwakar model

Reitz and Diwakar model

Generalized equation for Dispersed phase

didi

dxu

dt

ModelingModeling



ReactantsReactantsReactantsReactants ProductProductProductProductEquilibrium ConstantEquilibrium ConstantEquilibrium ConstantEquilibrium Constant Reaction TypeReaction TypeReaction TypeReaction Type

Ignition and Combustion model

Shell auto ignition model – Halstead et al., 1977

Simplified multi-step reaction mechanism to predict the spontaneous auto-ignition

EBU (Eddy Break UP) Combustion model – Magnussen 1981

Initiation

Propagate

Form B

Form Q

Form B

Degenerate

Out Terminate

Out Terminate

2RH O

*R

*R

*R

*R Q

B

*R

*2R

qK

pK

1 pf K

4 pf K

2 pf K

bK

3 pf K

tK

* HeatR P

*2R

*R B*R Q

*R B

*2R

min , , pOF ebu F ebu

O P

YYR A Y B

k s s

ModelingModeling



Time step size – crank angle 0.2°

Time marching calculation – fully implicit scheme

PISO Algorithm

Differencing scheme – MARS scheme

Turbulence model - high Reynolds modelk

Numerical SchemeNumerical Scheme



3 - Dimensional Grid Generation Section of valve center

Exhaust valve

Intake valve

Negative Valve Overlap !!!

Negative Valve Overlap !!!

Mesh GenerationMesh GenerationAt TDC : 320,000 cells

At BDC : 700,000 cells

By Pro-am, Es-ice



Exhaust Valve TimingA1 A2 A3 A4 A5 A6

EVO 140 130 120 140 140 140

EVC 290 290 290 300 310 320

IVO 440

IVC 590

NVO 150 150 150 140 130 120

Intake Valve TimingA1 B1 B2 B3 B4 B5 B6 B7

EVO 140

EVC 290

IVO 440 430 420 440 440 440 420 460

IVC 590 590 590 600 610 620 570 610

NVO 150 140 130 150 150 150 130 170

Parametric Studies The maximum valve lift of intake and exhaust valve is 2 mm.

Case A1 is the benchmarking case

Case A1,A2,A3 are EVO advanced

Case A4,A5,A6 are EVC retarded

Case B1 and B2 are IVO advanced

Case B3,B4 and B5 are IVC retarded

Case B6 andB7 are intake stroke

shifted of 20 CAD

(advanced and retarded, respectively)



Intake valve lift (mm)A1 C1 C2 C3

Intake valve 1&2

2 3 4 5

Intake valve lift strategies (mm)A1 D1 D2 D3 D4

Intake valve 1

2 0

Intake valve 2

2 4 6 4 6

Intake Valve 1

Intake Valve 2

Exhaust Valve 2

Exhaust Valve 1

Parametric Studies

Case C1, C2 and C3 are increasing both intake valve lift.

Case D1,D2,D3 and D4 are various intake valve lift scenario considering swirling motion

in cylinder.



Results and DiscussionResults and Discussion



0 200 400 600 800 10000

5

10

15

20

25

30

35

40

Cy

lind

er

Pre

ss

ure

(b

ar)

CAD (Crank Angle Degree)

1D Pressure 3D Pressure

The results of 3D simulation is analogous to those of 1D simulation. The maximum pressure difference is about 5%

1D Gas Dynamic Engine System Simulation (Ricardo WAVE)

Comparison of in-cylinder pressure in 1D and 3D simulation result



Additional pressure phase of residual gas re-

compression

compared with a standard 4-stroke engine cycle due to

the negative valve overlap for all cases.

When EVC is retarded (Case A4,A5 and A6),

under-lap period is decreased and less amount of the

residual

gases remains.

Pressure inside the cylinder

The pressure at recompression stroke is decreased

12

r

1

n

ii

n

2i

i

C C

C

1

1

n

i iin

ii

CVC

V

The Case A6 has the largest uniformity index because of

30 CAD retarded EVC, resulting in a larger amount of

intake air and vigorous mixing

Uniformity Index

Various Exhaust Valve Timing

Uniformity Index ( Weltens, 1993)

-considering EGR mass fraction and volume at each cell



Case A1(EVO : 140 °CA)



Mass fraction of internal EGR at the valve center

- EVO advanced (Case A1, A2 and A3)

The mass fraction of internal EGR is analogous to the result of case A1.

The improvement of the mixing characteristics has not occurred even though the EVO is

advanced.



Case A1 (EVC : 290 °CA)




Mass fraction of internal EGR at the valve center

- EVC retarded (Case A1, A4, A5 and A6)

The highly accelerated intake flow penetrates the piston head and causes a vigorous mixing in the case A6, unlike the case in A2 and A3.

The case A6 has the best mixing characteristics between the internal EGR and thefresh air at the end of compression stroke.



Various Intake Valve Timing

Intake valve timing had practically no effect on the

pressure during negative valve overlap

All other mixing characteristics such as cylinder temperature and flow fields also remained unchanged.

Two dominant temporal increases in the intake port

gas concentration, commonly referred to as the

“early backflow”, “late backflow”.

The most advanced IVO timing, Case B2 and B6 were

shown to the highest values for early backflow.

The most retarded IVC timing, Case B5 was the

largest value for late backflow.

However, various backflow patterns were observed !!!

Case B6 presents the most optimal operating condition in terms of improved thermal efficiency

The heat loss due to late backflow is unfavorable because of the longer residence period of the hot EGR in the intake port;

B2,B6

B5

B6



Distribution of Scalars - Increasing Intake Valve Lift (Valve 1 & 2)

Internal EGRInternal EGRInternal EGRInternal EGR FuelFuelFuelFuel O2 at 710 CADO2 at 710 CADO2 at 710 CADO2 at 710 CADTemperatureTemperatureTemperatureTemperature O2 at 715 CADO2 at 715 CADO2 at 715 CADO2 at 715 CAD

1000 1200 0.5 0.8 0 0.07 0.04 0.090.04 0.09

Case C1Case C1

Case C2Case C2

Case C3Case C3

•The areas with high internal EGR mass fraction correspond to relatively high temperature areas.

•Auto-ignition areas correspond to the fuel concentration from fuel and oxygen mass concentration



As the intake valve lift increase, more cool fresh air could be induced

Combustion and Emission - Increasing Intake Valve Lift (Valve 1 & 2)

Lower pressure and temperature

NO formation is modeled by Zeldovich’s reaction mechanism

Around1800 K

Case C1, yielding the highest temperature (2083 K)

- The largest NO emission

When temperature greater than1800 K, - NO formation increased.



Intake valve strategy (different valve lift 1 & 2)

Intake Valve 1

Intake Valve 2Case

Intake

Valve 1

Intake

Valve 2

Case BM 2 mm 2 mm

Case D1 2 mm 4 mm

Case D2 2 mm 6 mm

Case D3 0 4 mm

Case D4 0 6 mm

To relate the intake valve lift profile to the flow structure



Intake valve strategy (different valve lift 1 & 2)

0 01

2 20 0

1

( ) ( )

( ) ( )

n

i i i i ii

s n

i i ii

m y y u x x v

m x x y y

0 01

2 20 0

1

( ) ( )

( ) ( )

n

i i i i ii

tx n

i i ii

m y y w z z v

m z z y y

0 01

2 20 0

1

( ) ( )

( ) ( )

n

i i i i ii

ty n

i i ii

m z z u x x w

m x x z z

Equation by mattarelli et al. X Y

Z

440 460 480 500 520 540 560 580

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

NT

x

Crank Angle [deg]

Case BM Case D1 Case D2 Case D3 Case D4

440 460 480 500 520 540 560 580

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

NT

y

Crank Angle [deg]

Case BM Case D1 Case D2 Case D3 Case D4

With Intake Vale Lift 1=0 as in Case D3 and Case D4, the swirl intensity substantially increases at

the

end of the intake stroke.

The x-axis tumble intensity increases as the valve lift increases but gradually loses

The flow direction (from positive, negative value) of y-axis tumble gradually changes the intake stroke.

Intake valve 1 lift =0Case D3 and D4



Internal EGRInternal EGRInternal EGRInternal EGR FuelFuelFuelFuel O2 at 710 CADO2 at 710 CADO2 at 710 CADO2 at 710 CADTemperatureTemperatureTemperatureTemperature O2 at 715 CADO2 at 715 CADO2 at 715 CADO2 at 715 CAD

1000

1200

0.5 0.8 0 0.07 0.04

0.09

0.04 0.09

Case BMCase BM

•The areas with high internal EGR mass fraction correspond to relatively high temperature areas.

•Auto-ignition areas correspond to the fuel concentration from fuel and oxygen mass concentration.

•A larger auto-ignition spot in the reaction zones appeared for the most homogeneous mixture as shown in Case D4.

Distribution of Scalars - Intake valve strategy (different valve lift 1 & 2)

Case D1Case D1

Case D2Case D2

Case D3Case D3

Case D4Case D4



Concluding RemarkConcluding Remark

To investigate the effects of various valve strategy and its subsequent

combustion for a CAI engine, parametric studies were conducted

The results represent that

When the EVC motion was retarded, the less residual gas was remained and more

intake air was supplied

The mixing characteristics were improved with the high momentum of the intake air

The advancing EVO motion virtually had no effect on the mixing characteristics

Some areas with high internal EGR mass fraction inside the cylinder correspond to

relatively high temperature areas.

High fuel concentration and the auto-ignition spots were affecting each other and

Case D4 with the most homogeneous mixture yielded the best combustion efficiency



Thank you

For your attention

analysis of combustion characteristics of cai-engine with various valve strategies

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