Combustion process in high-speed diesel engines - unina.itwpage.unina.it/anddanna/capri/capri definitivo/13b-Motori a... · Combustion process in high-speed diesel engines ... While

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Combustion process in high-speed diesel enginesConventional combustion characteristics

New combustion concept characteristics

Benefits and drawbacks

Carlo Beatrice Istituto Motori – CNR

The Requirements to the Modern Diesel Engine

V6 PSA engine

MARKET ENVIRONMENT

COST

PERFORMANCE

FUN TO DRIVE

EMISSIONS

FUEL CONSUMPTIONCustomer’s cost Low CO2 emiss.FULL

LO

AD

/SPE

ED C

ON

DIT

ION

S

NED

C P

RO

CED

UR

E

Carlo Beatrice, IM-CNR

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4-Stroke DIESEL ENGINE CYCLE

DIESEL SPRAY STRUCTURE

Break up length

θ Cone angle

Sauter Mean Diameter and air/fuel mixing are affected by different spray parameters. The air/fuel mixing process strongly affects the engine fuel consumption and the pollutant emissions.

Final SMR: 5÷÷÷÷10µµµµm

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-40 0 40Angoli di manovella dell'albero motore [°]

0

20

40

60Pr

essi

one

nel c

ilind

ro [b

ar]

0

0.5

1

1.5

2

2.5

Alz

ata

dello

spi

llo in

ietto

re [m

m]

0

100

200

300

Pres

sion

e di

man

data

del

la p

ompa

di i

niez

ione

[bar

]

P.M.S.Accensione della miscelaformatasi durante in tempodi ritardo all'accensione

Diesel engine cylinder pressure cycle

Tempo di ritardo all’accensione

Area di lavoro attivo

Ignition, flame evolution and soot formationIgnition, flame evolution and soot formationin a diesel sprayin a diesel spray

0 1 2 3 4 50

50

100

150 Injection duration

Time after injection (ms)

RO

HR

(J/m

s)

Source: Tokyo University

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Visible combustion evolution in a DI diesel engine

Conventional diesel combustion

Source: Sandia National Laboratories

Classical Diesel Combustion Concept: in principle a non-stationary heterogeneous diffusive and partially premixed turbulent combustion

NOx controlled by:• flame temperature (Zel’dovichmechanism);• local N2 and O2 concentration

PM formation controlled by:• over-rich fuel concentration;• local O2 lack;• combustion temperature

Picture of a almost steady-state burning condition


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NOx formation background


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NOx formation backgroundIM: 1985 LD SC engine with FSV Daimler Benz: 2007 HD SC engine and CFD

simulation


Source: Daimler Benz

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DI Diesel engine combustion system design

Adequate spray penetration and fuelatomization control the optimum air/fuelmixing and then the final pollutantemissions.

8

9



Not for LD engine


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Effectof EGR Rate on NOxReduction

HD Engine

Effectof OxygenConcentrationof IntakeAir on NOxFormation


For different engine load there isthe same reduction rate for NOX vs O2 concentration

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Decrease of local Gas Temperatures via EGR

In the photos the area with higher luminosity correspond to high sooting area at higher temperature

CFD simulation


Decrease of local Gas Temperatures via EGRSource: Sandia Fuel: DGE, C6H14O3

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Diesel combustion control

To assure the engine functionality an adequate control of SOC and combustion rate have to be realized.

Adequate Control of Adequate Control of AirAir--EGREGR--Fuel mixing Fuel mixing and of ignition delay and of ignition delay

timetime

BoostingBoosting

Intake Intake TemperatureTemperature

EGR Ratio (EGR Ratio (intint –– ext)ext)

Compression Compression RatioRatio

Thermal Thermal ConditionsConditions

Injection StrategyInjection Strategy

Combustion Combustion System System

ArchitectureArchitectureFuel Fuel

qualityquality

Control of:Control of:••CombComb. . NoiseNoise;;••Peak Peak pressurepressure;;••EfficiencyEfficiency;;••PollutantPollutant emissionsemissions

In every engine conditions there is an optimum Air/EGR Fuel mixing that realizes the better compromise among the output characteristics


While for SI engines, the HCCI study is oriented to reduce both FC and NOx formation, for Diesel, the HCCI/PCCI research is oriented to exploit the very low simultaneous level of both PM and NOx, preserving the low Diesel BSFC.

Increased premixed level and EGR reduce local over-rich air/fuel ratio.

Source: SAE paper 2001-01-0655

New concepts combustion in diesel engines

Low Nox


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HCCI/PCCI combustion process

13° BTDC 12° BTDC 10° BTDC 8° BTDC9° BTDC 7° BTDC11° BTDC

3° BTDC TDC 4° ATDC 10° ATDC 15° ATDC 21° ATDC17° ATDC

Conventional Diesel Combustion

Nearly HCCI Combustion with diesel fuel in a LD DI Diesel engine

Source: Vaglieco et al., SAE paper 2007-01-0192


Lowering flame temperature below a typical treshold reduces the soot formation.

Soot Yeld under pure pyrolisys vs Temperature

0

0.2

0.4

0.6

0.8

1

1700 1800 1900 2000 2100 2200

Temperature [K]

Soo

t Yel

d [a

.u.]

Tetradecane

Source: Beatrice et al., Comb. Sci. & Tech. 2001

Low Temperature Combustion regime



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Calculated NOx and soot formation rate vs φ-T map for the diesel combustion

Source: SAE paper 2001-01-0655



When fuel burns under diesel combustion, fuel molecules are oxidated under different φ and T conditions

Source: SAE paper 2001-01-0655 Reduced NOx and Soot formationCarlo Beatrice, IM-CNR

HCCI/PCCI combustion characteristics

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Source SAE Paper 2000-01-0331

HCCI for Diesel fuel can be approached with PFI or very Early injection strategies:

• PFI leads to very difficult control of global in-cylinder A/F, oil dilution by fuel, unburned HCs, SOC control and noise limitation;

•Early injection leads to same problems but less critical, depending on the combustion system and injection strategy.

•In both cases, due to the high boiling point of heavy fractions of the fuel, the homogenization is never reached.


HCCI/PCCI combustion characteristics

HCCI Combustion approach for LD DI Diesel engines

Source MTZ

With conventional LD DI combustion systems the homogeneous approach is more and more stringent with heavy problems of knocking conditions

0

10

20

30

40

50

60

320 340 360 380 400 420 440Crank Angle [°]

Cyl

inde

r pre

ssur

e [b

ar]

-20

0

20

40

60

80

100

120

Rat

e of

Hea

t Rel

ease

[%/°

]

N-Heptane

1500 rpm @ 4.5 bar IMEP

Syngle-Cylinder LD DI Diesel engine

High knockingconditions


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From HCCI to Premixed Low Temperature Combustion (PCCI/LTC)PCCI with diluted air/fuel charge by high EGR rate can be defined as the middle between HCCI and diesel combustion.They are characterized by almost premixed stratified Air/EGR/Fuel charge with a better link between injection event and SOC as in the diesel combustion.

Source: Neely et al., SAE Paper 2005-01-1091

1400 rpm @ 3 bar IMEP

0

10

20

30

40

50

60

70

-30 -20 -10 0 10 20 30 40 50 60C.A.[°]

CY

LIN

DE

R P

RE

SS

UR

E [b

ar]

.

-10

0

10

20

30

40

RO

HR

[%/°

]

Diesel fuel

SOImain = 11 BTDC

4-Cylinder LD DI diesel eninge

SOC after EOIControlled SOC

PCCI combustion is a “stratified highly diluted quasi-total premixed combustion”


Advanced combustion management in modern diesel engines

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PCCI vs Conventional Diesel Combustion

0

40

80

120

160

200

HC raw CO raw NOx raw BSFC

Emis

sion

Inde

xes

[%] Conventional Diesel

PCCI Combustion

1500 rpm @ 2 bar of BMEP

4-Cylinder LD DI Diesel engine

0

0.01

0.02

0.03

0.04

0.05

Smoke

Emis

sion

Inde

xes

[g/k

Wh]

Low Load with Early Injection Strategy


PCCI vs Conventional Diesel Combustion

Medium Load with Late Injection Strategy

0

40

80

120

160

200

HC raw CO raw NOx raw BSFC

Em

issi

on In

dexe

s [%

] Conventional DieselPCCI Combustion

2000 rpm @ 5 bar of BMEP

4-Cylinder LD DI Diesel engine

0

0.03

0.06

0.09

0.12

Smoke

Emis

sion

Inde

xes

[g/k

Wh]


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Problems of PCCI application to DI Diesel engine

Few problems at low load. Heavy problems at medium high load:

• SOC control;

• pollutants;

• FC;

• engine durability;

• cylinder to cylinder balancement;

• transient engine control;

NOx;HCs;CO;PM.

adequate EGR distrb.impingmentoverleaning

low comb. temp.quenching

low O2 local conc.

η thermod.η comb.

Fouling componentsmech. stress (knocking)oil dilution

cylind. to cylind. EGR distrib.cylind. to cylind. thermal cond.

response of combustion to all above factors durnigtransient conditions


KIVACFD Computations

Fuel Injection

Initial Conditions

Fuel evaporationBreakup models

Ignition delay+

Combustion Model

Detailedkinetics

solver and time step

locallychosen

KIVA

3V_R

el2

MultimethodMultimethodsolverssolvers

DVODEDVODE /SDIRK/SDIRK

Combustion ModelCHEMKIN 3.1CHEMKIN 3.1

Numerical Solvers

Conventionalcombustionapplication

reaction scheme:

44 species and 112 reactions

for n-heptane

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• The continuous dimensional distribution function isdiscretized into classes of fixed molecular weight

ParticlesParticles formationformation modelingmodeling –– sectional approach

• In the earlier stages of the engine combustion number weighted particle distribution is mono-modal• With the progress of combustion the size distributions change from monomodal into bi-modal: after 12 CAD, first mode corresponds to particles smaller than 3nm and second mode to a peak between 30 and 60 nm.

D’Anna, A., Detailed kinetic modeling of Particulate Formation in Rich Premixed Flames of Ethylene, Energy Fuels 22, 2008

Advanced technologies to realize practical application of HCCI combustion: Combustion System Architecture

To increase premixing level, reducing unburned compounds, reducing cylinder wall wetting (oil dilution) and extend the HCCI application, an accurate combustion system design is needed

ReducedCR

low smoke;≈ FC;≈NOx;High CO, HCs;

extension of PCCI application area

NADI TM system (IFP)


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Advanced technologies to realize practical application of HCCI combustion: EGR

EGR is the main driver for NOx control

Source: Imarisio et al., ATA congress, Siracusa 2006

The use of an advanced EGR layout with LP+HP EGR can extend the engine tolerability to the high EGR rate increasing the HCCI application area at medium load


Advanced technologies to realize practical application of HCCI combustion: Advanced Injection Systems

To improve the premixed Air/EGR/Fuel charge inside the cylinder, to employ injection system with injection rate shaping will be useful.

Source: Imarisio et al., ATA congress, Siracusa 2006

Improved solenoid injectoror

Piezo injector

time

Inje

ctio

nflo

wra

te

first flat slope

final high flow rate

Source: Hammer (BOSCH), ATA congress, Bari 2004

low impingment;low oil dilution;low overleaning.

fuel distrib. control;comb. rate control.

Source: Gastaldi et al. (RENAULT), ATA congress, Siracusa 2006


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Advanced technologies to realize practical application of HCCI combustion: Advanced Air Layout Systems 2

exhaust stroke

re-opening exhaust valve during intake stroke

cam angle

valv

e lif

t

VVA system can be a very useful tool to control the in-cylinder temperature during the warm up in the NEDC.

Source: Lisbona et al., TDCE congress, Ischia 2007


Advanced technologies to realize practical application of HCCI combustion: Closed Loop Combustion Control

PCCI applicationin a veichle withclosed loopcomb. control

Closed Loop Comb. Control is the prerequisite for SOC control and EGR effectsSource: Lisbona et al., TDCE congress, Ischia 2007

Source: Hűlser et al., SAE paper 2006-01-1146


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THE FUTURE HCCI DI CI engines

The correct development and application of all technologies will help to bring the LD DI engines to match the future stringent emission regulation preserving the fuel consumption, fun to drive and performance at full load.

The improvement ok knowledge in the HCCI combustion characteristics is fundamental to define the line-guides for HCCI combustion control technologies.

The practical application of HCCI to the real CI engines will depend on the acquisition of the necessary knowledge to control the desiderate characteristics of the air/EGR/fuel charge inside the cylinder before the start of the combustion.


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Combustion process in high-speed diesel engines - unina.itwpage.unina.it/anddanna/capri/capri definitivo/13b-Motori a... · Combustion process in high-speed diesel engines ... While