CFD Analysis and Multi-Objective Optimization of a Diesel Engine
for Automotive Applications
Fabio Bozza
DIME - Università di Napoli “Federico II”, Napoli, ITALY
M. Costa, D. Siano
Istituto Motori CNR, Napoli, ITALY
Introduction
New combustion concepts for Diesel Engines
yHCCI (Homogeneous Charge Compression Ignition)
yPCCI (Premixed Charge Compression Ignition)
yPLTC (Premixed Low Temperature Combustion)
yCAI (Controlled Auto-Ignition)
y…
Objective: further reduction of Soot - NOx
Additional Constraints: IMEP - BSFC, HC - CO, Noise levels, reliability, driveability, costs, …
Introduction
Each combustion mode best behaves in its own (and limited) engine operating range
A switch between standard mode (Premixed- Diffusive) and advanced ones is required
Available control parameters:
yInjection Pressure, Modulation and Phase (CR)
yBOOST Level (LP/HP)
yEGR (Short/Long route, VVA systems)
ySwirl Ratio (Dedicated control valves)
yFuel properties (Dual-Fuel supplied engines)
Introduction
How to realize the control of a so large number of parameters?
Employment of advanced and integrated numerical (and experimental) tools
1D modeling (intake-exhaust system & turbo-matching)
3D modeling (spray behavior, combustion, emissions)
Additional models (prediction of radiated noise)
Definition of proper optimization techniques
Identification of optimal values of control parameters for Emission, Fuel Consumption and Noise minimization
Objective
Light-Duty Common-Rail Engine Development
1st Section: Experimental Analyses
Performance tests, Acoustic tests on a BASE engine
Characterization of the CR - Fuel Injection System
2nd Section: Models Setting & Validation
1D model, 3D model, Radiated Noise model
3rd Section: Optimization (Base Engine + CR)
Injection strategy parameterization (triple-injection)
Definition of control parameters and objectives
Results analysis: selection of optimal solutions
1
stSection: Experimental Activity
Performance tests on a BASE, Mechanical Injection Engine
4t–1 cyl. (505 cm3), Full load
Radiated Noise is measured by
a free-field microphone at 1m from the source
Spray characterization of the CR injection system on dedicated test-benches
Injector: micro-sac, 5 holes, D/L: 0.13mm/1.0mm
Rail Pressure: 28-140 MPa, Density: 12-20 kg/m3
Injection Rate Measurements and Spray Images
1
stSection: Inj. Rate Measurements
Different injection strategies are analyzed:
Low Load (3.4 mg): Triple injection, 28 MPa
Medium Load (11.87 mg): Triple injection, 71 MPa
High Load (26.35 mg): Single injection, 140 MPa
Injection Gauge Rate System (AVL) based on a Bosch tube principle
0 500 1000 1500 2000 2500 3000 3500 Time, µs
0 0.04 0.08 0.12 0.16
Injection Rate, mg/s
Low Load (Mf=3.40 mg, Pinj=28 MPa) Medium Load (Mf=11.87 mg, Pinj=71 MPa) High Load (Mf=26.35 mg, Pinj=140 MPa)
High Load
Low Load Medium Load
1
stSection: Spray characterization
Image analysis procedure (imaging technique):
Spray images are collected by a CCD camera, 1280x1024 pixels, 8 bit, 0.5 µs exposure time
Liquid spray tip penetration and the spray cone angle are reconstructed on 10 single shot images
0 50 100 150 200 250 300 350
Time, µs 0
5 10 15 20 25 30
Tip Penetration, mm
ρ=12.5 kg/m3
First Pulse Main Pulse
ρ=20.6 kg/m3
Medium Load
0 100 200 300 400 500 600 700 800 900 1000 Time, µs
0 10 20 30 40 50 60
Tip Penetration, mm
ρ=12.5 kg/m3
High Load
Medium Load (Main Pulse)
2
ndSection: 1D Analysis, Base Engine
GT-Power code:
Provides initial conditions for the 3D model
1200 1600 2000 2400 2800 3200
Engine Speed [rpm]
20 25 30 35 40 45
Air Flow Rate [kg/h]
Numerical Experimental
1D Engine layout
Numerical and experimental tip penetration:
High Load, high density case: Single shot, 140 MPa
Experimental fuel flow rate is employed
Hug-Gosman and Wave models are tested
Wave model gives good agreement with an adjusted C1 constant
2
ndSection: 3D Spray analysis
0 100 200 300 400 500 600 700 800 900 1000 Time, µs
0 10 20 30 40 50 60
Tip Penetration, mm
Experimental
Numerical (Wave C1=60)
-60 -30 0 30 60 Crank Angle, deg
0 20 40 60 80 100
Pressure, bar
Num.
Exp.
2
ndSection: 3D Analysis, Base Engine
Fire code (AVL)
Spray model employs the measured pinj
Break-up: Wave model
Combustion: ECFM-3Z
NO: Zeldovich
Soot: Nagle et al.
Validation with
experimental data
3D domain:
40000 cells
2200 rpm Full Load
2
ndSection: Noise Emission Model
Torregrosa’s Approach:
Decomposition of the
in-cylinder pressure cycle
High-pass filter (4.5kHz) on the total pressure FFT
res comb
mot
tot p p p
p = + +
mot tot
excess res
comb p p p p
p + = = −
101 102 103 104
Frequency [Hz]
80 120 160 200 240
Pressure Amplitude [dB]
4.5 kHz
-90 -60 -30 0 30 60 90
Crank Angle, [deg]
0 25 50 75 100
Pressure [bar]
Total Pressure Motored Combustion Resonance
-20 -10 0 10 20
Crank Angle [deg]
-0.4 -0.2 0 0.2 0.4 0.6
Pressure [bar]
Resonance Pressure
2
ndSection: Noise Emission Model
Proper indices are defined:
In (accounts for mechanical noise):
I1 (accounts for pressure gradients):
or
I2 (accounts for resonance phenomena):
=
mot comb idle
dt dp
dt dp n
I n
max max 1
=
idle
n n
I log10 n
+
=
mot
comb comb
idle
dt dp
dt dp dt
dp n
I n
max
2 max 1
max
1
= ∫
∫
dt p
dt p I
mot res 2
2 6
10
2 log 10
2
ndSection: Noise Model Validation
Overall Noise Correlation [dB]:
Ci: adjustable tuning constants
Validation on acoustic measurements on
the BASE engine:
Good agreement !
1200 1600 2000 2400 2800 3200
Engine Speed [rpm]
90 95 100 105 110 115
Overall Noise [dB]
Exp Calc
2 2 1
1
0 C I C I C I
C
ON = + n ⋅ n + ⋅ + ⋅
2
ndSection: Remarks
Proper models are available for Base Engine and spray evolution realized by the CR system
A prototype Engine is “Numerically” analyzed, equipped with the previously tested CR system
Medium Load operating point: 2200rpm @ 11.87mg/cyc
A optimization procedure detects the best
injection profile to minimize FC-NO-Soot-Noise
3
rdSection: Optimization
Parameterization of the injection profile:
5 degrees of freedom:
SOIP, pilot%1, pilot%2, dwell_1 and dwell_2
3D Run window [-123°,110°]: pdV
IMEP V
HP ∫−+
= −
− 123110
123
1
0 500 1000 1500 2000 2500
Time, µs 0
0.02 0.04 0.06 0.08
Injection Rate, mg/s
Experimental Profile Parameterized Profile
1 2
3 4
8
10 9
pilot%_1
soip
dwell_1 pilot%_2
5
6 7
dwell_2
-60 -40 -20 0 20 40 60
Crank Angle, deg 0
20 40 60 80 100
Pressure, bar
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
NO Mass Fraction
Experimental Profile Parameterized Profile
10-5 10-4 10-3
Soot
NO
Soot Pressure
Optimization Layout (ModeFRONTIERTM code)
Pilot%_2 Dwell_2
3
rdSection: Optimization
SOIP
Dwell_1
Pilot%_1
Injection Profile coordinates
3D run
Noise Model
Noise
IMEP Soot
NO
Objectives Parameters
3D input file
3
rdSection: Optimization Results
Optimization Method (Scheduler): MOGA II
Multi-Objective Problem:
Infinite solutions located on Pareto Frontiers
How to select a single Solution? MCDM tool
Set of preferences are defined to identify compromise solutions:
1st set: High IMEP (same as Low Fuel Consumption)
2nd set: Low NO
3rd set: Low Noise
NO-Soot trade-off (Pareto-Frontier) ∼500 points
Bubble-size:
HP-IMEP
3
rdSection: Optimization Results
1st sol: #297
High IMEP
1st sol.
2nd sol.
3rd sol.
3rd sol.
0 5 10 15 20 25 30 35 40 45 50
NO, g/kgFUEL 0
5 10 15 20 25 30
SOOT, g/kgFUEL
0 1 2 3 4 5
NO g/kgFUEL 0
5 10 15 20 25 30
SOOT, g/kgFUEL
Zoom
Pareto-Frontier
HP-IMEP 3.5 bar 5.5 bar
2nd sol: #428 Low NO
3rd sol: #115
Low Noise
0 5 10 15 20 25 30 35 40 45 50 NO, g/kgFUEL
0 5 10 15 20 25 30
SOOT, g/kgFUEL
0 1 2 3 4 5
NO g/kgFUEL 0
5 10 15 20 25 30
SOOT, g/kgFUEL
Zoom
Noise Level
Pareto-Frontier
98 dB 110 dB
3
rdSection: Optimization Results
NO-Soot trade-off (Pareto-Frontier)
Bubble-size:
NOISE
1st sol: #297
High IMEP
2nd sol: #428 Low NO
3rd sol: #115
Low Noise
1st sol.
2nd sol.
3rd sol.
3rd sol.
3
rdSection: Optimization Results
0 1 2 3 4 5
HP-IMEP, bar 96
100 104 108 112
Overall Noise, dB
428
115 297
0 1 2 3 4 5
HP-IMEP, bar 0.0x100
2.0x10-4 4.0x10-4 6.0x10-4 8.0x10-4 1.0x10-3 1.2x10-3
NO Mass Fraction
428 297
115
1st sol.
2nd sol.
3rd sol.
1st sol.
2nd sol.
3rd sol.
NO-IMEP Noise-IMEP
Alternative Representations:
IMEP Penalty ∼ 1.2 bar
Noise Reduction∼8 dB
3
rdSection: Optimal Modulation
1st Sol: High IMEP (#297):
Quasi-Homogeneous combustion mode (HCCI)
2nd Sol: Low NO (#428):
Near-TDC injection
PCCI comb. mode
3rd Sol: Low Noise
(#115):
Delayed Main &
greater dwell
Standard premixed-
diffusive mode -120 -100 -80 -60 -40 -20 0 20
Crank Angle, deg 0
0.02 0.04 0.06 0.08
Injection Rate, mg/s
# 297 (High IMEP & Low Soot)
# 428 (Intermediate Case)
# 115 (Reduced NO & Noise)
High IMEP (HCCI)
Low NO (PCCI)
Low Noise (Standard)
#297
#428
#115 Heat Release Starts
3
rdSection: Final Results
Injection Strategy effects on pressure cycle and Rate of Heat Release (ROHR)
#297 High IMEP
#428 Low NO
#115 Low Noise
-60 -40 -20 0 20 40 60
Crank Angle, deg 0
20 40 60 80 100
Pressure, bar
0 40 80 120
Rate of Heat Release, J/deg
# 297
# 428
# 115
Noise, HP-IMEP = 106.4 dB, 5.16 bar 101.1 dB, 4.89 bar
98.7 dB, 4.0 bar
Pressure
ROHR
Injection Strategy effects on SOOT and NO
#297 High IMEP
#428 Low NO
#115 Low Noise
3
rdSection: Final Results
-20 0 20 40 60
Crank Angle, deg 10-7
10-6 10-5 10-4 10-3
NO Mass Fraction
10-7 10-6 10-5 10-4 10-3 10-2 10-1
Soot Mass Fraction
# 297
# 428
# 115 NO
Soot
BEST COMPROMISE
Conclusions
Optimal inj. strategies are identified through:
Coupled 1D, 3D and Noise emission tools, plus a multi-objective optimization procedure
Code validation is realized with reference to BASE Engine and CR Fuel Injection System
Results:
High IMEP: Advanced start of both Pilot & Main
Low NO: Delayed Pilot & reduced dwells
Low Noise: Delayed Main & greater dwells
Besides the practical results, main interest consists in the developed overall procedure
Different objective importance, depending on the load level
High IMEP & Low Noise at high load
Low NO & Low Soot at low load
Great Potential of extension & validation
Inclusion of Boost, EGR, Swirl, Pinj, and so on
Different combustion modes established
Conclusions
Work in progress
-360 -270 -180 -90 0 90 180 270 360
Crank Angle, deg 0
10 20 30 40 50 60 70
Pressure, bar
Exp.
1D Results
1500 rpm, BMEP=1.5 bar
C1 C3 C5 C2 C4 C6
6 Cyl. Turbocharged Engine
VGT – EGR – Common Rail
1D Scheme
1D Model Validation
Work in progress
0 1000 2000 3000 4000
Time, microsec 0
0.01 0.02 0.03 0.04
Total Injection Rate, mg/microsec
0 5 10 15 20 25
Total Injected Fuel, mg
Caso 2 700
3D Model Validation
Injection Parameterization
Work in progress
Optimization !!
…to be continued
THANKS FOR THE ATTENTION