Combustion process in high-speed diesel
engines
Conventional combustion characteristics
New combustion concept characteristics
Benefits and drawbacks
Carlo Beatrice
Istituto Motori – CNR
The Requirements to the Modern Diesel Engine
MARKET
ENVIRONMENT
COST
EMISSIONS
PERFORMANCE
FUN TO DRIVE
Customer’s cost
FUEL CONSUMPTION
NEDC PROCEDURE
FULL LOAD/SPEED CONDITIONS
V6 PSA engine
Low CO2 emiss.
Carlo Beatrice, IM-CNR
1
4-Stroke DIESEL ENGINE CYCLE
DIESEL SPRAY STRUCTURE
Break up length
θ
Cone angle
Final SMR: 5÷
÷10µ
µm
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.
2
Diesel engine cylinder pressure cycle
40
1.5
1
20
Accensione della miscela
formatasi durante in tempo
di ritardo all'accensione
P.M.S.
200
100
0.5
0
Area di lavoro attivo
0
-40
0
40
Angoli di manovella dell'albero motore [°]
Pressione di mandata della pompa di iniezione [bar]
2
Pressione nel cilindro [bar]
300
2.5
Alzata dello spillo iniettore [mm]
60
0
Tempo
di ritardo
all’accensione
ROHR (J/ms)
Ignition, flame evolution and soot formation
in a diesel spray
150
Injection duration
100
Source: Tokyo University
50
0
0
1
2
3
4
Time after injection (ms)
5
3
Visible combustion evolution in a DI diesel engine
Conventional diesel combustion
Classical Diesel Combustion Concept: in principle a non-stationary heterogeneous
diffusive and partially premixed turbulent combustion
NOx controlled by:
• flame temperature (Zel’dovich
mechanism);
• local N2 and O2 concentration
Source: Sandia National Laboratories
PM formation controlled by:
• over-rich fuel concentration;
• local O2 lack;
• combustion temperature
Picture of a almost steady-state burning condition
Carlo Beatrice, IM-CNR
4
NOx formation background
NOx formation background
5
NOx formation background
IM: 1985 LD SC engine with FSV
Daimler Benz: 2007 HD SC engine and CFD
simulation
NOx formation background
Source: Daimler Benz
6
DI Diesel engine combustion system design
Adequate spray penetration and fuel
atomization control the optimum air/fuel
mixing and then the final pollutant
emissions.
7
8
NOx formation background
Not for LD engine
Source: Daimler Benz
NOx formation background
9
Effectof EGR Rate on NOxReduction
Source: Daimler Benz
HD Engine
Effectof OxygenConcentrationof IntakeAir on NOxFormation
For different engine load there is
the same reduction rate for NOX vs
O2 concentration
Source: Daimler Benz
10
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
Source: Daimler Benz
Decrease of local Gas Temperatures via EGR
Source: Sandia
Fuel: DGE, C6H14O3
11
Diesel combustion control
To assure the engine functionality an adequate control of SOC and combustion rate have to
be realized.
Thermal
EGR Ratio (int
(int – ext)
Conditions
Injection Strategy
Control of:
•Comb.
Comb. Noise;
Noise;
•Peak pressure;
pressure;
•Efficiency;
Efficiency;
•Pollutant emissions
Adequate Control of
AirAir-EGREGR-Fuel mixing
and of ignition delay
time
Boosting
Compression
Ratio
Combustion
System
Architecture
Fuel
quality
Intake
Temperature
In every engine conditions there is an
optimum Air/EGR Fuel mixing that realizes
the better compromise among the output
characteristics
Carlo Beatrice, IM-CNR
New concepts combustion in diesel engines
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.
Low Nox
Source: SAE paper 2001-01-0655
Carlo Beatrice, IM-CNR
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HCCI/PCCI combustion process
Source: Vaglieco et al., SAE paper 2007-01-0192
Conventional Diesel Combustion
3° BTDC
TDC
4° ATDC
10° ATDC
15° ATDC
17° ATDC
21° ATDC
Nearly HCCI Combustion with diesel fuel in a LD DI Diesel engine
13° BTDC
12° BTDC
11° BTDC
10° BTDC
9° BTDC
8° BTDC
7° BTDC
Carlo Beatrice, IM-CNR
HCCI/PCCI combustion process
1
Soot Yeld under pure pyrolisys vs Temperature
Lowering flame
temperature below a
typical treshold reduces
the soot formation.
Soot Yeld [a.u.]
0.8
0.6
Low
Temperature
Combustion
regime
0.4
0.2
Tetradecane
0
1700
1800
Source: Beatrice et al., Comb. Sci. & Tech. 2001
1900
2000
2100
2200
Temperature [K]
Carlo Beatrice, IM-CNR
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HCCI/PCCI combustion process
Calculated NOx and soot formation rate vs φ-T map for the diesel combustion
Source: SAE paper 2001-01-0655
Carlo Beatrice, IM-CNR
HCCI/PCCI combustion characteristics
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 formation
Carlo Beatrice, IM-CNR
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HCCI/PCCI combustion characteristics
HCCI for Diesel fuel can be approached with PFI
or very Early injection strategies:
• PFI leads to very difficult control of global incylinder 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.
Source SAE Paper 2000-01-0331
Carlo Beatrice, IM-CNR
HCCI Combustion approach for LD DI Diesel engines
With conventional LD DI combustion systems the
homogeneous approach is more and more
stringent with heavy problems of knocking
conditions
Syngle-Cylinder LD DI Diesel engine
1500 rpm @ 4.5 bar IMEP
100
50
Cylinder pressure [bar]
120
N-Heptane
40
80
60
30
40
20
20
10
0
320
0
340
360
380
400
Crank Angle [°]
420
Rate of Heat Release [%/°]
60
High
knocking
conditions
-20
440
Source MTZ
Carlo Beatrice, IM-CNR
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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.
PCCI combustion is a “stratified highly diluted quasi-total premixed combustion”
70
40
60
Diesel fuel
SOC after EOI
Controlled SOC
30
1400 rpm @ 3 bar IMEP
50
20
40
30
10
ROHR [%/°]
CYLINDER PRESSURE [bar]
.
4-Cylinder LD DI diesel eninge
20
0
10
SOImain = 11 BTDC
0
-10
-30
-20
-10
0
10
20
30
40
50
60
C.A.[°]
Source: Neely et al., SAE Paper 2005-01-1091
Carlo Beatrice, IM-CNR
Advanced combustion management in modern
diesel engines
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PCCI vs Conventional Diesel Combustion
Low Load with Early Injection Strategy
200
4-Cylinder LD DI Diesel engine
0.05
Emission Indexes [%]
160
Conventional Diesel
PCCI Combustion
Emission Indexes [g/kWh]
1500 rpm @ 2 bar of BMEP
0.04
120
0.03
80
0.02
40
0.01
0
0
HC raw
CO raw
NOx raw
BSFC
Smoke
Carlo Beatrice, IM-CNR
PCCI vs Conventional Diesel Combustion
Medium Load with Late Injection Strategy
4-Cylinder LD DI Diesel engine
0.12
2000 rpm @ 5 bar of BMEP
Emission Indexes [%]
160
Conventional Diesel
PCCI Combustion
0.09
120
0.06
80
0.03
40
Emission Indexes [g/kWh]
200
0
0
HC raw
CO raw
NOx raw
BSFC
Smoke
Carlo Beatrice, IM-CNR
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Problems of PCCI application to DI Diesel engine
Few problems at low load. Heavy problems at medium high load:
adequate EGR distrb.
• SOC control;
NOx;
HCs;
CO;
PM.
• pollutants;
• FC;
η thermod.
η comb.
impingment
overleaning
low comb. temp.
quenching
low O2 local conc.
Fouling components
mech. stress (knocking)
oil dilution
cylind. to cylind. EGR distrib.
• cylinder to cylinder balancement;
cylind. to cylind. thermal cond.
• engine durability;
• transient engine control;
response of combustion to all above factors durnig
transient conditions
Initial Conditions
Conventional
combustion
application
reaction scheme:
44 species and
112 reactions
for n-heptane
Detailed
kinetics
CHEMKIN 3.1
Multimethod
solvers
DVODE /SDIRK
solver and
time step
locally
chosen
Fuel Injection
Breakup models
Fuel evaporation
KIVA3V_Rel2
Carlo Beatrice, IM-CNR
Combustion Model
Ignition delay
Numerical +Solvers
Combustion Model
KIVA
CFD Computations
18
Particles formation modeling – sectional approach
• The continuous dimensional distribution function is
discretized into classes of fixed molecular weight
• In the earlier stages of the engine combustion Æ 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
low smoke;
Reduced
≈ FC;
CR
≈NOx;
High CO, HCs;
extension of PCCI
application area
NADI TM system (IFP)
Carlo Beatrice, IM-CNR
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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
Carlo Beatrice, IM-CNR
Advanced technologies to realize practical application of
HCCI combustion: Advanced Injection Systems
Improved solenoid injector
or
Piezo injector
Injection flow rate
To improve the premixed Air/EGR/Fuel charge inside the cylinder, to employ injection system
Source: Hammer (BOSCH), ATA congress, Bari 2004
with injection rate shaping will be useful.
final high flow rate
first flat slope
low impingment;
low oil dilution;
low overleaning.
time
fuel distrib. control;
comb. rate control.
Source: Imarisio et al., ATA congress, Siracusa 2006
Source: Gastaldi et al. (RENAULT), ATA congress, Siracusa 2006
Carlo Beatrice, IM-CNR
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Advanced technologies to realize practical application of
HCCI combustion: Advanced Air Layout Systems 2
re-opening
exhaust valve
during intake
stroke
VVA system can be a very useful tool to
control the in-cylinder temperature during the
warm up in the NEDC.
valve lift
Source: Lisbona et al., TDCE congress, Ischia 2007
exhaust stroke
cam angle
Carlo Beatrice, IM-CNR
Advanced technologies to realize practical application of
HCCI combustion: Closed Loop Combustion Control
Closed Loop Comb. Control is the prerequisite for SOC control and EGR effects
Source: Lisbona et al., TDCE congress, Ischia 2007
PCCI application
in a veichle with
closed loop
comb. control
Source: Hűlser et al., SAE paper 2006-01-1146
Carlo Beatrice, IM-CNR
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THE FUTURE HCCI DI CI engines
The improvement ok knowledge in the HCCI combustion characteristics is fundamental to
define the line-guides for HCCI combustion control technologies.
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 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.
Carlo Beatrice, IM-CNR
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