Effects of Negative Valve Overlap on the auto- ignition ... · Effects of Negative Valve Overlap on the auto-ignition process of lean ethanol/air mixture in HCCI–engines T. Joelsson,

Effects of Negative Valve Overlap on the auto-

ignition process of lean ethanol/air mixture in

HCCI–engines

T. Joelsson, R. Yu and X.S. BaiDivision of Fluid Mechanics, Department of Energy Sciences, Faculty of Engineering LTH, Lund University

Fluid Mechanical Seminar Series

2010-03-19 Fluid Mech Seminar NVO combustion study

2

Outline•

Introduction–

Background

–

Experimental Engine Setup

•

Numerical Methods and Setup–

Large Eddy Simulation

–

Multi-Zone Chemistry Model–

Computation Details

•

Results and Discussion•

Conclusion


3

Background•

To control HCCI combustion one way is to control the in-cylinder temperature

•

Using and changing Negative Valve Overlap is one way to generate ‘‘optimal’’

in-cylinder temperature and

residual gas amount

•

How does NVO influence temperature and also fuel concentration prior to auto-ignition ?

•

LES is used to resolve the flow patterns and Multi-Zone tools for the chemistry


4

GoalsUsing Numerical Simulation tools to

investigate –

the mixing process of fresh fuel/air mixture (port-fuel injected) with different amount of residual gas

–

the process of fresh/residual gas heating–

Auto-ignition process of diluted charge with residual gas under different NVO conditions

To develop a better understanding of the phenomenon


5

Negative Valve Overlap

•

Negative Valve Overlap - (NVO)

–

Trapping residual gas inside cylinder volume

–

An internal EGR system

–

With a time shift of Closing Exhaust Valves and Opening Intake Valves giving that NOT all the exhaust gases can leave the cylinder volume

–

Results of this is that a warmer burned gas amount is kept in the combustion chamber for the next burning cycle


6

Experimental Engine SetupGeometrical

Data of the Volvo D5

Displacements per cylinder 480 cm3

Valves per cylinder 4

Bore 81 mm

Stroke 93.15 mm

Connection Rod 147 mm

Combustion chamber Pancake

Compression ratio 9.1

Valve timing Fully flexible

Fuel Ethanol

Speed 1200 rpm

Valve

Openings

& Closing

Name EVO EVC IVO IVC

NVO

40 520 700 20 200

NVO

80 520 680 40 200

NVO

160 520 640 80 200


7

Experimental Results•

HCCI combustion with port-fuel injected Ethanol*

*

H. Persson, J. Sjöholm, E. Kristensson, B. Johansson, M. Richter, M. Aldén, "Study of Fuel Stratification on Spark Assisted Compression Ignition (SACI) Combustion with Ethanol Using High Speed Fuel PLIF"

SAE paper 2008-01-2401. (2008)


8

Numerical Methods•

LES was used to simulate –

the turbulent flow motion

–

Mixing processes of fresh fuel/air mixture with kept internal residual gas

–

Heat transfer with cylinder walls and compression/expansion

–

Focused on the intake and early compression stages:

0-290 CAD

•

Multi-Zone simulation–

using detailed chemistry for ethanol/air mixture

•

Constant LOAD•

Constant equivalence ratio, Φ–

Based on the LES predicted residual gas and temperature distribution fields

–

Starting from 70 BTDC (290 CAD)


9

Large Eddy Simulation Approach


10

LES models•

The governing equations–

Navier-Stokes equations

–

Low Mach number approximation –

Hydrodynamic and thermodynamic pressures

•

Spatial filtering–

Scales larger than the filter scale are resolved

–

Subfilter

scales are modeled (known as SGS)–

Small sub-filter scales are filtered out

•

Subfilter

scale models–

Transport fluxes

–

Chemical reactions


11

The Navier-Stokes Equations

( )

( )

0

j

j j

j j j j

i

i

i j ijii j i j

j i j j

phuh hD hu hu

t x t x x x

ut x

pu uuu u u u

t x x x x

ρρρ ρ ρ

ρ ρ

ρ τρρ ρ

∂∂∂ ∂ ∂ ∂+ = + + −

∂ ∂ ∂ ∂ ∂ ∂

∂ ∂+ =

∂ ∂∂∂ ∂∂ ∂

+ = − + + −∂ ∂ ∂ ∂ ∂

⎛ ⎞⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜⎝ ⎠

( ) - velocity component in direction 1,2, 3 - pressure

- density

- enthalpy

- viscous stress tensor

- thermal diffusion coefficient

i i

ij

u x i

p

h

D

ρ

τ

=


12

SGS models for transport fluxes

•

SSM-model is used to take into account the SGS stress

•

Smagorinsky-type model is used to take into account the SGS-flux for scalar transport

•

Spatial and density-weighted filtering is as follow:

( , ; ) ( ; ) ( , )

1( , ; ) ( ; ) ( , ) ( , )

i i i i i

i i i i i i

x t F x x x t dx

h x t F x x x t h x t dx

ρ ρ

ρρ

∞ ∞ ∞

−∞ −∞ −∞∞ ∞ ∞

−∞ −∞ −∞

′ ′ ′Δ = − Δ

′ ′ ′ ′Δ = − Δ

∫ ∫ ∫

∫ ∫ ∫


13

SGS models for chemical reactions

Reaction progress variable

Tabulation of the chemical source term

( , , )c f c h p=

( )

* **

* *

( ) (0)( ) , ( ) ( ( ), )

( ) (0) i u

jj j

j j j j

h hc h h Y T

h h

u cc cu c u c c

t x x Sc x x

ττ τ τ

τ

ρρ μρ ρ ρ ρ

∞

−= =

−⎛ ⎞∂∂ ∂ ∂ ∂⎟⎜ ⎟⎜+ = + − +⎟⎜ ⎟⎟⎜∂ ∂ ∂ ∂ ∂⎝ ⎠


14

Mean and Fluctuations from LESStatistical analysis of velocity and temperature

( ) ( ) ( )

1 1 2 2 3 3 1 2 3 1 2 3

12

2 2 2

1 1 2 2 3 3

1 2 3 1 2 3

2

1 2 3 1 2

ˆ ( , , ; ) ( , , )

1ˆ ˆ ˆ

3

1ˆ ( , , ,CAD)

1 ˆ( , , ,CAD)

i i

V

V

V

u G x x x x x x u x x x dx dx dx

u u u u u u u dVV

T T x x x dx dx dxV

T T x x x T dx dx dxV

′ ′ ′ ′ ′ ′ ′ ′ ′= − − − Δ

⎛ ⎞⎟⎜ ⎡ ⎤ ⎟′ ⎜= − + − + − ⎟⎜ ⎢ ⎥ ⎟⎣ ⎦⎜ ⎟⎝ ⎠

′ ′ ′ ′ ′ ′=

⎡ ⎤′ ′ ′ ′ ′ ′ ′= −⎢ ⎥⎣ ⎦

∫∫∫

∫

∫∫∫

∫∫∫12

3

⎛ ⎞⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠


15

LES numerical solver•

Staggered deforming grids

•

Predictor/Corrector time integration–

Predictor 2nd

order Adam-Bashforth–

Corrector 2nd

order Crank-Nicolson

•

Spatial discretization–

5th

order WENO scheme for the convective terms

–

4th

order central-difference-scheme for the diffusive terms


16

Multi-Zone Simulation•

Cantera

solver tool

•

Marinov

Ethanol reaction model•

Two part divide solver–

Chemical Reaction model Constant Volume

–

Non-reaction part Compression of Volume

•

From 290 –

400 CAD


17

Computational ConditionsInitial conditions of pressure and temperature for residual gas BEFORE EVC and when LES cycle starts and the fuel/air ratio for the multi-zone simulation

Name PINITIAL TINITIAL PRESIDUAL TRESIDUAL PhiCONST PhiLOADNVO

40 1 bar 500 K 1.45 bar 551 K 0.3 0.55

NVO

80 1 bar 500 K 2.9 bar 662 K 0.3 0.61

NVO

160 1 bar 500 K 8.5 bar 863 K 0.3 1.0

11

fuel

airres air res fuel

u b

st

m

mpVMm m m m

FRT xA

φ

⎛ ⎞⎟⎜ ⎟⎜ ⎟⎜⎛ ⎞ ⎟⎜⎝ ⎠⎟⎜ ⎟= = − − =⎜ ⎟⎜ ⎟ ⎛ ⎞⎜⎝ ⎠ ⎟⎜ ⎟⎜ ⎟⎜⎝ ⎠


18

Residual Gas Distribution

•

Pure residual gas xb

= 1•

Pure fresh fuel/air intake gas xb

= 0

Large Eddy Simulation

Results and Discussionon Flow and Concentration Spatial

and Temporal Distribution


20

Stratification of mixtures

2 ( )c b bD x xχ = ∇ ∇i


21

In-cylinder Temperature


22

Stratification in mixtures and in-cylinder temperature @ 270 CAD

NVO 40 NVO 80


23

Temperature stratification: Summary•

Temperature stratification develops due to wall heat transfer and the mixing of the fresh charge with the residual gas–

related to wall and exhaust gas temperature

–

related to the ratio between intake and residual gases

•

Decrease of T’

happens due to turbulence mixing and piston expansion–

the slopes are related to the intake -

residual

gas ratio–

Starting point of injection of fresh charge


24

Charge stratification: Summary•

The value of xb

-mean relates to the amount of intake gas injected into the cylinder–

High NVO gives low amount of intake gas which gives

–

xb

-mean valve starts to decrease when intake valve opens

•

xb

-fluctuations depends on the openings of the valves–

related to the intake/residual gas mass ratio

•

The decrease rate of xb

-fluctuations is related to the amount of intake gas that is in the cylinder and the mixing time–

Scalar dissipation rate

–

Intake valve opening time

Multi-Zone Simulation

Results and Discussionon Detail Chemistry Analysis


26

Initialization for Multi-Zone Simulation

NVO 40 NVO 80 NVO 160


27

Initial Conditions & Pressure

Name PhiCONST PhiLOADNVO

40 0.3 0.55

NVO

80 0.3 0.61

NVO

160 0.3 1.0


28

Constant LOAD Constant Φ

CO2

emission Distribution over Combustion phase


29

Temperature Distribution over Combustion phase



30


CO emission Distribution over Combustion phase


31

Detailed Chemistry: Summary•

NVO affect the pressure peak and pressure rise-rate–

Large NVO has lower peak and slower pressure-

rise-rate

•

The richest zones are not ignited first–

The rich zones have lower temperature

•

The ignition process is more sensitive to the temperature field than to the charge stratification field, at least for the current cases


32

Conclusions•

Large eddy simulations coupled with detailed ignition chemistry

•

Different valve timing generates different temperature and mixture compositions

•

Several mechanisms control this mixing and heat transfer process–

Initial temperature difference between colder walls and hot residual gas

•

Influences strongest at early intake stroke–

Inhale of even colder intake gas

–

In-cylinder turbulence enhances diffusion in both temperature and concentration fields

•

Strongest influence at BDC and beyond


33

Conclusions•

Temperature and concentration fields have strong correlation

•

The detailed chemistry simulations show that the ignition process is mainly correlated to the temperature stratification–

Hot regions ignite first

•

contain higher quantities of residual gas, higher temperature in these zones

•

Overrides at stoichiometric condition

–

NVO 80 give an optimal conditions when its weighted fuel concentration spreads along a longer temperature field than the other cases

–

No radicals were in the residual gas part

Thank you for you attention!

Effects of Negative Valve Overlap on the auto-ignition process of lean ethanol/air mixture in HCCI–enginesOutlineBackgroundGoalsNegative Valve OverlapExperimental Engine SetupExperimental ResultsNumerical MethodsLarge Eddy Simulation ApproachLES modelsThe Navier-Stokes EquationsSGS models for transport fluxesSGS models for chemical reactionsMean and Fluctuations from LESLES numerical solverMulti-Zone SimulationComputational ConditionsResidual Gas DistributionLarge Eddy SimulationStratification of mixturesIn-cylinder TemperatureStratification in mixtures and �in-cylinder temperature @ 270 CADTemperature stratification: SummaryCharge stratification: SummaryMulti-Zone SimulationInitialization for Multi-Zone SimulationInitial Conditions & PressureCO2 emission Distribution �over Combustion phaseTemperature Distribution �over Combustion phaseCO emission Distribution �over Combustion phaseDetailed Chemistry: SummaryConclusionsConclusionsSlide Number 34

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Effects of Negative Valve Overlap on the auto- ignition ... · Effects of Negative Valve Overlap on the auto-ignition process of lean ethanol/air mixture in HCCI–engines T. Joelsson,