04 combustion

1

SAE India TOPTECH Seminaron

Engine & Powertrain SimulationJan 10-11, 2008New Delhi, India

Kevin Hoag – University of Wisconsin, MadisonJohn Wilken – Gamma Technologies Inc

Greg Hampson – ENSYSAnupam Dave – Cummins Inc

S.M. Shahed – Honeywell Turbo Technologies

Combustion Simulation

Motivation• Internal Combustion in Engines

where the “fuel meets the air”• What is it and How do we know about it?• Why is this so important?• How do they react, what are the controlling

factors, and what are the results?• Can we model it – express this behavior in general

mathematical physical terms?• Can we better understand and visualize what is

happening INSIDE the combustion chamber?• Can simulations help us improve our

understanding of combustion, improve our experimental program, improve our engine design?

How do we “know” about in-cylinder processes

Via Precision Measurements:• Cylinder Pressure Transducer• Crank Angle Shaft Encoder• High Speed Data Acquisition

System

ICP- Piezo ElectricPressure TransducerFlush mounted

CrankAngleEncoder

Combustion PrimerAtom Conservation (stoichiometry)

HmCn + EA( 0.21 O2 + 0.79 N2) => a H2O + b CO2 + e N2 : EA=1

=> a H2O + b CO2 + d O2 + e N2 : EA >1

=> a H2O + [b CO2 + c CO] + e N2 : EA < 1

Species State (properties)enthalpy or energy state of chemical species

Σ products = [a HH2O + b HCO2 + c HCO + d HO2 + e HN2 ] Σ reactants = [ 1 HmCn + EA ( 0.21 HO2 + 0.79 HN2) ]

Hydrocarbon exothermic reaction in Air

1-reactants

2-products

hfavailableenergy

Heat of Formation @ To

⎟⎠⎞⎜

⎝⎛ += ∫

T

Toiii dCphfNH ττ )(

hi

State

TTo

2

Combustion PrimerEnergy Conservation (add system to chemical species conversion)

( )∫ ⋅−= dVPWork

Hydrocarbon exothermic reaction in Air

T

H dH=0 : Hp= HR

dT=0 : HP= HR - Qhv

States

Reaction Zone

Constraints: (P,T,V)

Reactants Products

control volumeanalysis Heat, Q

HHVtotf QmH ⋅=∆ ,max

( )LHVtotf

gaswall

Qm

UQdVPNAHR

⋅

∆++⋅−= ∫

,

one system:normalized apparent heat

release

1

10

100

1000

-360 -180 0 180 360

Engine Crank Angle (deg)

Pres

sure

(bar

)

0

0.05

Cyl PressNonCombIntakeExhaustFuel InjectionBurn Rate

Intake Compress Power-Expn Exhaust

wocombIgnition >

Combustion - within a thermodynamic engine cycle

Engine Cycle - 4 stroke / Diesel

Fuel Injection

Fuel Injection

IgnitionIgnition BurningExpansionBurning

ExpansionBDCBDC

Air Induction

Air Induction Air

CompressionAir

CompressionExhaustExhaustTDCTDC Expansion

Blow DownExpansionBlow Down

Research & Testing Engine Development Process is Flow upEngine Design Goals• Measured torque/ IMEP• Measured Fuel Efficiency• Measured Emissions

Engine Geometry• Bore• Stroke• CR

Pressure Profile• Compression - rate of rise - Peak pressure• Work – IMEP

Air • Boost• MAT• EGR

Fuel • SOI• Rate of Injection• Injection Quantity

Combustion• Start of Combustion• Heat Release profile

Constraints• Reliability• Noise• Cold start

Engine Design Goals• Desired torque/ IMEP• Desired Fuel Efficiency• Desired Emissions







Design is Requirements Flow-down

3

Engine Design Goals• Desired torque/ IMEP• Desired Fuel Efficiency• Desired Emissions







Trace Through a Design Process

7

0

5

43

2

16

1

10

100

1000

0.1 1 10Log Cylinder Volume (liter)

Log

Cyl

inde

r Pre

ssur

e (b

ar)

020

406080

100

120140160

180200

-60 -30 0 30 60Crank Angle (deg)

Pres

sure

(bar

)Polytropic Trace

Wire-FramePressure

-100

0

100

200

300

400

500

600

700

800

-20 -10 0 10 20 30 40Crank Angle (deg)

AH

RR

(J/d

eg)

Piston BowlPiston Bowl

Cylinder HeadCylinder HeadFuel SprayFuel Spray

Wire-Frame

PressureTrace

Heat ReleaseRate

Geom, Air & Fuel

Define “Wire-Frame”Cylinder Pressures to achieve design goals within constraints a’ priori

Idealized Cycle - CI• Polytropic compression• Polytropic expansion• Limited Pressure Cycle

-Constant V@TDC-Constant P=Pmax

• IMEP adjusted by τburn

7

0

5

43

2

16

1

10

100

1000

0.1 1 10Log Cylinder Volume (liter)

Log

Cyl

inde

r Pre

ssur

e (b

ar)

Starting Point:Engine torque (IMEP) requirement

120/NVdIMEPncylPower ⋅⋅⋅=

Wire-Frame / Cycle Diagram

Pmax

CR

Boost

∆P

IMEP

τburn

+

+ / -

VddVP

IMEP ∫ ⋅=

Comparison of SI-gasoline and CI-dieselRealistic Cycle Diagrams

160

20

40

60

80

100

120

140

0 5 10 15 20

Volume Ratio (V/Vtdc)

Pres

sure

(bar

)

SI Cycle

SI Polytropic 1.30

Diesel Cycle

CI Polytropic 1.37

0.1

1

10

100

1000

0.1 1 10 100

Volume Ratio (V/Vtdc)

Pres

sure

(bar

)

SI Cycle

SI Polytropic 1.30

Diesel Cycle

CI Polytropic 1.37

CI-Diesel

CI-DieselSI-Gas

SI-Gas

Linear Pressure vs. Volume Ratio Log Pressure vs. Log Volume Ratio

16

NA-SI engines – no boost, low CR & low slope, large constant volume combustionTDI-CI engines – high boost, high CR ratio & steep slope, small constant volume comb

Max Efficiency = fn(Charge Comp, Geometric CR,…)

0%

10%

20%

30%

40%

50%

60%

70%

80%

0 10 20 30Compression Ratio

Indi

cate

d Ef

ficie

ncy

γ =1.4

γ =1.3

γ =1.25

• Idealized Cycle Efficiency

• Losses & Limitations• Mechanical limits 2-10%• Fuel + Air charge is not

Air• Heat Transfer - 20%• Pumping losses 5-10%• Friction – 10%• Slow Burning – 5%• Fuel pressurization – 2%• NOx reduction 0-10% or

more• Actuation - non-ideal

valve or fuel timing

air

si

ci

η 1 1

CRγ 1

4

0

100

200

300

400

500

600

700

800

-60 -30 0 30 60Crank Angle (deg)

AH

RR

(J/d

eg)

Heat Release

020406080

100120140160180200

-60 -30 0 30 60

Crank Angle (deg)

Pres

sure

(bar

)

Wire-FrameMotored

Design Process: Flow-Down to Burn Rate“Fulcrum” of the design

Pressure vs. Crank AnglePmax

Peoc

Requirements (IMEP, Pmax…)

“Wire-Frame”Cycle Diagram

Pressure Trace

Heat Release

Rate

AHRR vs. Crank Angle

Fuel/Air/Ignition/EGR

Geometry, …

7

0

5

43

2

16

1

10

100

1000

0.1 1 10

Log Cylinder Volume (liter)

Log

Cyl

inde

r Pre

ssur

e (b

ar)

Engine Thermodynamics and Combustion Simulations

Performance Results

(IMEP, P, T…)

Cycle Diagram

Pressure Trace

BurnRate

Flow-down proceeds from engine performance Requirements to Necessary Causes – i.e. reverse causality

Simulation and Engine testing proceeds from Causes to

performance Results– i.e. forward causality, direction of

time


Geometry, Event, …

Subject of Combustion Simulations start here



Pressure Trace

Heat Release

Rate


Geometry, …

time

Simulation Options for Burn RatesPerformance

Results(IMEP, P, T…)

Cycle Diagram

Pressure Trace

BurnRate



Burn Rate Profile Options1) Experimentally Derived Heat Release Profile2) Semi-Empirical Burn Rate Profile3) Parametric Mathematical Burn Rate Profile

Starting with the burn rate profileallows us to essentially ignore what causesthe heat release in favor of focusing on theeffects of a given burn rate has on the engine system

Burn Rate Definition for SimulationsNon-predictive combustion / imposed burn rate profile

• Instantaneous Zero-Dimensional Burn Rate is directly imposed as a simulation input

• Total amount of energy released from the fuel depends on the total mass of fuel and the properties of the gas in the cylinder

• “Burn Rate” tells the simulation the rate which the available fuel + air are converted to products

• “AHRR” or the “Apparent” Heat Release Rate is generally the result of pressure trace analysis

• AHRR rarely matches the Burn Rate magnitude, but often is proportional to the Burn Rate “shape profile”

5

Cylinder Pressure “measurement”Cylinder Voltage to Pressure• Measure a relative dynamic voltage • Calibration• Pinning & Drift• Crank Angle Offset• Cleaning & Checking• Volume Estimates for PV linearity plots

pressure change

drift

graphics: Formulating an Engine Simulation Procedure, Tyler Feralio, RPI University, Troy NY

refrefcal

PovotvdVdPtP +−⋅⎥⎦

⎤= ))(~()(

1. Pressure Data Pre-Processing Checks

Syed Wahiduzzaman Loic BarbierGamma Technologies Inc. Ecole Nationale Superieure D’Electriciteet de Mecanique

Source:

• Classical Heat Release Analysis (method used in most other cylinder pressure analysis tools):

• Result is “apparent” heat release (NOT burn rate)• Employs simplifying assumptions, can enable

quick computations in a test lab while the engine is running

Apparent Heat Release Rate

( )LHVtotf

gaswall

QmUQdVP

NAHRR⋅

∆++⋅−= ∫

,

• Apparent Heat Release Rate Profile– Derived from Experimental Cylinder Pressure Trace -> Apparent– Zero-dimensional– Assumptions implicit in Heat Release Rate calculations are generally

not consistent with simulation method– However, if the goal is to get correct “profile” wrt ignition, 10/90 burn

duration, centroid of HRR, integral of HRR, then• Normalized AHRR can be used as a simulation input• Simulations will generally scale the NAHRR based upon fuel inputs

1. Experimental Burn Rate Profile

-100

0

100

200

300

400

500

600

700

800

-20 -10 0 10 20 30 40

Crank Angle (deg)

AH

RR

(J/d

eg)

0

1

2

3

4

-20 -10 0 10 20 30 40Crank Angle (deg)

Cum

ulat

ive

AH

R (K

J)

6

• SI Wiebe Model– Non-predictive burn rate imposed according to the SI Wiebe

function (50% burned, 10-90% duration, exponent)– Enables transition from NAHRR to a modeled/fit Burn rate– Can be parameterized– Very fast execution

2a. Burn Rate Profile - SI 2b. Burn Rate Profile - DI Diesel• DI Wiebe Model (No default entries)

– Non-predictive burn rate imposed according to the three-term DI Wiebe function

– Enables transition from NAHRR to a modeled/fit Burn rate– Can be parameterized– Very fast execution time

Simulation Options for Predictive Combustion

Performance Results

(IMEP, P, T…)

Cycle Diagram

Pressure Trace

BurnRate



Predictive Burn Rate/Combustion Options

1) Two-Zone Rate 2) Multi-Zone Flame model3) Multi-Zone Spray model4) Full 3D Computational Fluid

Dynamics

Once we are comfortable we have the proper burn rate for our engine and operating conditions, then we can go to the next level – predictive combustion

Why Use Predictive Combustion?• Non-predictive combustion is useful when the subject

of the study does not directly affect combustion rate in a significant way– Exhaust acoustics– Intake manifold geometry– Turbocharger performance– Engine model build up

• Predictive combustion is required when subject of study directly affects combustion rate– Injection timing and profiles– EGR rates– Swirl

• Predictive combustion also useful for running a variety of loads/speeds where it may not be practical to impose burn rates from cylinder pressure

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Predictive Combustion for SimulationsPredictive combustion / computed burn rate

• Burn rate is computed directly from: fuel injection rate, spray/gas interactions, species concentrations, time and spacial evolutions, geometry, …

• Global behavior is integral of Local & Multi-dimensional behavior (non-uniform temperature & species concentrations, but pressure equalizes quickly)

• Non-uniform spatial and temporal distributions enable realistic emissions predictions

• Physics based & More universal – physics connects the dots between known data

• Inputs are more physical and suitable for engineering design exploration studies

1. Combustion - Two-Zone• Thermodynamic Two-Zone model

– Models Unburned and Burned zones – Burn rate controlled by input profile or predictive model– Converts Unburned reactant mixture to burned products– Compression of unburned and burned products– Good for NOx emissions predictions, especially SI– Burn Rate can be predictive and coupled to flame

advancement in simulations & can include geometry dependency

piston

unburnedburned

TuTb

“flame”

2. Combustion Flame Model - SI• SI Turbulent Flame Model

– Predictive burn rate taking into account:• cylinder temperature and pressure• composition in the cylinder including fuel, fresh air, and

egr/residuals• Spark timing, position, and gap• Fuel properties• Flame area/Wall wetted area• In-cylinder flow

– Slow execution– Combustion chamber geometry (head and piston) may

be read from an .STL file or generated automatically from dimensions entered by the user

SI NOx and Knock

• Predictive NOx Model (extended Zeldovich)– Temperature tracked in many zones– Sensitive to heat release rate and composition

(including EGR)– Sensitive to pressure, temperature, equivalence

ratio, and dilution ratio

• Predictive Knock Model– Correlate predicted knock index to measurements

of knock initiation and strength– Knock index predicts knock trends– Cylinder wall temperatures should be specified by

either the detailed or solution reference objects

8

Diesel Engine Emissions – Causes• Diffusion flame structure - Fuel Rich spray

– Oxygen poor fuel heating (pyrolysis) >> PM, Smoke– Stoichiometric combustion >> NOx

• Incomplete combustion – CO, UHC, PM

Piston BowlPiston Bowl

Cylinder HeadCylinder Head

Fuel SprayFuel Spray

CFD simulations of direct injectiondiesel engine combustion chamber

Mechanisms of emissions productiondirect injection diesel engines-Heywood

TemperatureFuel

Lean flame-out Region, HC

Initial rapidcombustionnoise

Fuel jetmixing with air

rich mixture

Burned gas NO

“Pre-mixed” combustion mode

Flame quenchon walls, HC

Burned gas NO

White/Yellow flameSoot oxidation

Rich zonesin fuel jetSoot formation

“mixing controlled” combustion mode

Fuel vaporfrom nozzleSac volume

NOx Emissions

NOx Production = f ( Time, Air, Temperature)

In-cylinder NOx Reduction Mechanisms (simplified)NOx Reducton

Mechanism Retard EGR Water HCCI

• Less time late combustion fast comb

• Displace O2 dilution dilution dilution poss.

• Lower Temp. expansion burn heat capacity heat capacitylean

homogeneous

dd t

NO ....6 1016

Texp ,69 090

TO2

12 N2

Emissions Control – Fuel Timing Retard

Combustion Timing : -10, -5, 0, +5 atdc• Retard Timing

=> Lower Temperature = > Lower NOx

• Challenge: timing retard => lower pressure

=> lower efficiency

Cylinder Pressure

NOx -10

-50+5

Emissions Control – EFI

Fuel injection timing precisionFuel injection duration

Importance of tight controls on fuel injection on emissions

Fuel

Con

sum

ptio

n

Oxi

des

of N

itro

gen

, NO

x

Soot

Par

ticu

late

Hyd

roca

rbon

s, H

C

9

Emissions Control - EGR Emissions Control - EGR

Sandia NL, 9th Int’l Engine Conference, 6/05

Local Equivalence Ratio and Temperature dictate NOx and Soot formation propensity

3. Combustion - DI Diesel• DI Jet Model / Hiroyasu Model

– Predictive burn rate model, Takes into account:• Cylinder pressure and temperature• Injection timing, rate, velocity, and plume shape• Composition in the cylinder including fuel, fresh air,

and EGR/residuals• In-cylinder flow (Swirl and Tumble)

– Slow execution– Predictive NOx Model– Predictive SOOT Model (trends only)

Calibration• Match measured pressure using imposed non-

predictive HRR • Base engine model with imposed burn rates should be

well correlated BEFORE attempting detailed combustion model calibration

• Use single cylinder model with imposed BC to save time

• Calibrate at several points in operating range– Speed– Load– EGR rates

• All calibrated multipliers should be constant for all speeds, loads and EGR rates

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Calibration

On single-cylinder model:• Impose fuel injection rate • Impose pressure/temperature

boundary conditions• Adjust Ignition delay parameters• Adjust Evaporation parameters• Adjust Entrainment parameters

DI jet model – Results Example

Combustion Prediction in a Truck DI Diesel Engine

Heat Release with and without EGR

NOx Prediction: Effect of SOI & Fuel Rate

50% load

75% load

100% load

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NOx Prediction: EGR Effect

50% load

75% load

100% load

NOx Prediction: Speed Effect

50% load

75% load

100% load

NOx and Soot Tradeoff 3. Combustion – 3D CFD• 3D CFD (e.g. DOE/Los Alamos KIVA-3V code, StarCd,

Fluent….) • Full 3D CFD code simulates process which control combustion

& emissions:– gas flow, compression, turbulence & mixing (finite volume)– fuel jet breakup (e.g. Kelvin-Helmholtz model) – fuel droplet breakup (e.g. Rayleigh-Taylor model )– ignition (e.g. Shell model )– combustion (e.g. Lam/Turb characteristic timescale models ) – NOx (e.g. Extended Zel’dovich mechanism )– soot formation (e.g. Hiroyasu model )– soot oxidation (e.g. Nagle-Strickland model )

12

FLOW SOLVER: KIVA

Initialization

Phase A

Phase B

Phase C

Read Input DataCalculate gas viscosity

Initialize time step, piston velocity

Spray Modeling (injection, drop breakup, collision, evaporation)Combustion chemistry

Emission modeling

Fluid phase calculationMass, momentum, velocity, temperature, pressure, turbulence

properties (implicit solver, iterations)

Snapping/Rezoning gridsRemapping fluid properties to new grids

Update cell properties

IN-CYLINDER MODELS• Fluid Phase

– Continuity equations– Momentum equations– Internal energy equations– K-epsilon equations

• Boundary Conditions– Physical boundaries:

• inflow/outflow, rigid walls, periodic boundaries.– Numerical boundary conditions:

• Temperature: adiabatic, fixed T.• Velocity: free slip, no slip, turbulent law-of-the-wall• Turbulent parameters • Droplets: handled by drop-wall impingement model

DIESEL SCHEMATIC (ERC)

Fuel injectionnozzle cavitation

spray breakup

Drop/wall impingement

Drop distortionvaporizationturbulent dispersion

Autoignition/combustionsoot/NOx formation

Wall heat transfer

3D models in Practice• 3D Combustion & Spray CFD

– Very Slow Run times– Open cylinder simulations – need to include valve events

in meshing and BC at inlet to manifolds– Closed cylinder simulations – need BC: P, T, TKE at IVC

from other source (e.g. 1D solver)– Impose or compute fuel injection rates

• Calibration – this is significant, do not under-estimate– Need comprehensive data set over wide parameter space– Each submodel carries calibration constants (breakup,

evaporation, turbulence, mixing, wall impingement, ignition, heat transfer, NOx, Soot, meshing )

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OUTPUT – Temperature at 30o Plane

Cra

nk A

ngle

(deg

)

OUTPUT - Fuel and CO mass fractions

OUTPUT

simulation time evolution animation

Summary - Engine Thermodynamics and Combustion Simulations

Performance Results

(IMEP, P, T…)

Cycle Diagram

Pressure Trace

BurnRate



Step 1 impose burn rate



Pressure Trace

Heat Release

Rate


Geometry, …

Step 2 predictive combustion

Step 0 For engine design studies, determine “target” heat release rate to meet engine requirements

Sim

ulat

ion

& Ti

me

14

Summary - Engine Thermodynamics and Combustion Simulations (1)

• Burn Rate is “fulcrum” of engine system thermodynamics• Burn Rate integrates up to cylinder pressure and to

engine performance and can be defined by requirements• Take a “process approach” to Combustion Simulations

i. start simple => experimental pressure trace & NAHRR• use this to get the breathing & turbo model working• does not depend upon fuel injection rate profile

ii. go to semi-empirical, mathematical fit – Wiebe• more universal, flexible, enable “what if” and design

requirement studiesiii. parameterize the fit – parametric Wiebe coefficientsiv. increase generality with predictive combustion

• SI flame • DI jet models (requires fuel injection rate profile)

v. CFD simulations• maximum degree of predictive capabiliyt• but requires tuning and calibration• longer run times

Summary - Engine Thermodynamics and Combustion Simulations (2)

• Combustion Rate & its impact on engine operating condition

• “Process approach” to Combustion Simulations

• When to add complexity

04 combustion

Documents

h2o b co2 e n2

understanding of combustion

combustion chamber

h2o b co2 d o2 e n2

h2o b co2 c

upengine design goals

chemical species conversion

requirements flow