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Piston Bowl Optimization for a Diesel Engine with Variable Compression Ratio
STAR Global Conference 2016
Prague, March 2016
Christian Schramm, Carolus Gruenig, Maximilian Brauer, Matthias Diezemann
Why VCR?
© IAV · 03/2016 · Star Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
95 g/km CO2 in 2020
Thermal efficiency
CR
Specific power / CR
downsizing
Low friction CR
powertrain
Real Driving Emissions
• Soot / NOx at high CR
high
• EGR at full load CR
with constant PFP
• HC / CO at low load CR
Cold Test -7°C for
Diesel
• Cold start ability CR
• HC, CO emission CR
• Fuel conversion CR
efficiency
Why Variable Compression Ratio (VCR)?
2
Content
© IAV · 03/2016 · Star Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
1. Introduction
2. Simulation Approach
3. Simulation Results
4. Testing Results & Validation
5. Summary
3
Content
© IAV · 03/2016 · Star Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
1. Introduction
2. Simulation Approach
3. Simulation Results
4. Testing Results & Validation
5. Summary
4
Articulated
con-rod
Swinging
crank
Linking
gear rack
Linking
con-rod
Crankpin
eccentric
Piston pin
eccentric
Piston
height
adjustment
Con-rod
length
adjustment
Additional
volume
Tilting
cylinder-
block
Lifting
cylinder-
block
Shifting
crankshaft
axis
Introduction / Technical Solutions
© IAV · 03/2016 · Star Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Multiple link mechanisms
Alteration of kinematic lengths in the crank train
Repositioning of unmoving parts
All suitable concepts have a variable squish gap height
5
Reduced piston bowl volume for CR = 20 reduces free spray penetration length
Changed top dead center position for CR = 11 leads to different spray targeting
Piston bowl shape needs to be developed for VCR requirements
Introduction / Challenges with VCR
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Variation Range for VCR
Determined based on GT-Power studies
Selected variation range: CR = 11 … 20
Impact on Combustion Chamber Geometry CR = 20.0
CR = 11.0
Ds Quench = 4.7 mm
Ds Quench = 0.0 mm
CR = 20.0
CR = 11.0
Ds Quench = 4.7 mm
Ds Quench = 0.0 mm
CR = 20 (Part Load) CR = 11 (Full Load)
6
Content
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
1. Introduction
2. Simulation Approach
3. Simulation Results
4. Testing Results & Validation
5. Summary
7
Automated CFD meshing
and model generation of
segment models
Define geometry based on
a parametric piston bowl
shape
Piston bowl shape or
selected parameters are
scaled to fit a predefined
compression ratio
Call pro-STAR / STAR-CD®
Automated Post-
processing: Collecting
simulation results in one txt-
/pdf-file
Simulation Approach
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Results File
txt/pdf
Input File
txt
STAR-CD Sector Model
Parametric Bowl
Analyze Results and
Decide new Variants
Manually
(by Engineer)
Automatically (by
Optimization Tool)
CR = const.
8
IAV Tool
Simulation Approach
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Automated vs. Manual Approach
Automated Approach Manual Approach
+ Requires less man power
+ Very suitable to find a global optimum
+ Effective usage of computational
resources
- Too much parameters if all geometric
parameters have to be considered
- Useful only for ’fine tuning’ of a
geometry (global optimum)
- Need of more simulation runs to find an
optimum
+ Very suitable to investigate different
bowl layouts
+ Separation of secondary effects
possible / plausibility checks
- Normally the optimum cannot be
detected
- More time-consuming
For the given task the manual approach has been selected
9
Simulation Approach
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine 10
1. Step:
Definition of different
piston bowl base
concepts (3)
CR = 20.0
CR = 11.0
Ds Quench = 4.7 mm
Ds Quench = 0.0 mm
e.g. open w-bowl omega
flat dish
open w
r in (mm)
z in
(m
m)
0 10 20 30 40 50 -20
-15
-10
-5
0
5
2. Step:
Optimization of
Piston Bowl Main
Dimensions
3. Step:
Optimization of
Piston bowl features
(collar, cone, …)
Simulation Approach
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Operating Conditions
Consideration of 2 operating conditions
12
BM
EP
in b
ar
0
5
10
15
20
25
30
Engine Speed in mm-1
1000 2000 3000 4000 5000
Part Load:
n = 1200 min-1
IMEP ~8.5 bar, CR=20
Part Load:
n = 1200 min-1
IMEP ~8.5 bar, CR=20
Full Load:
n = 4000 min-1
Full Load, CO2-Reduction
Power Increase or
Reduction of Friction
CR=11
Model Setup
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Turbulence / Heat Transfer
Turbulence Model k-ε-model, high
Reynolds
Wall Function Kader
Spray
Nozzle model MPI2
Atomization model Huh
Break-up model Reitz +
Submodels
Droplet-Wall-Interaction Bai
Liquid Fuel C12H26 (DF2)
Combustion/Ignition
Ignition model Double Delay
Ignition Model
Combustion model ECFM-3Z
Evaporated Fuel C12H26 (DF2)
Emissions
NO Zeldovich
Soot ERC
Cyclic Symmetry
Boundaries
Injector
BDC TDC
13
CFD, Liquid
Measurement
Maximum (95%)
Average (50%)
Minimum (5%)
Rail Pressure: 800 barInjected Mass: 25 mg
Chamber Pressure: 50 barChb. Temperature: 320 °C
CFD, Vapour
CFD, Liquid Fuel
CFD, Evaporated Fuel
CFD Spray ChamberConti PCR NG Q355, 8 Holes
Ma
ss F
ractio
n [
1]
0.00
0.25
0.50
0.75
1.00
Dro
ple
t D
iam
ete
r [µ
m]
0
50
100
150
200
Time [ms]
0.00 0.25 0.50 0.75 1.00 1.25 1.50
(all CFD)
Inje
ctio
n R
ate
[kg
/s]
0.00
0.01
0.02
0.03
0.04
Pe
ne
tra
tio
n [
mm
]
0
10
20
30
40
50
Co
ne
An
gle
[d
eg
]
0
10
20
30
40
Time [ms]
0.00 0.25 0.50 0.75 1.00 1.25 1.50
CFD, Liquid
Measurement
Maximum (95%)
Average (50%)
Minimum (5%)
Rail Pressure: 800 barInjected Mass: 25 mg
Chamber Pressure: 50 barChb. Temperature: 320 °C
CFD, Vapour
CFD, Liquid Fuel
CFD, Evaporated Fuel
CFD Spray ChamberConti PCR NG Q355, 8 Holes
Ma
ss F
ractio
n [
1]
0.00
0.25
0.50
0.75
1.00
Dro
ple
t D
iam
ete
r [µ
m]
0
50
100
150
200
Time [ms]
0.00 0.25 0.50 0.75 1.00 1.25 1.50
(all CFD)
Inje
ctio
n R
ate
[kg
/s]
0.00
0.01
0.02
0.03
0.04
Pe
ne
tra
tio
n [
mm
]
0
10
20
30
40
50
Co
ne
An
gle
[d
eg
]
0
10
20
30
40
Time [ms]
0.00 0.25 0.50 0.75 1.00 1.25 1.50
Test-rig
CFD
CFD
Measurement / Reference
Engine Speed: 3700 1/minBMEP: max.
CFD CombustionMB OM651 Q355
EKAS: 5%Rail Pressure: 1600 bar, 8 Holes
Crank Angle [°CA]
225.0 270.0 315.0 360.0 405.0 450.0 495.0
Cyl. P
ressu
re [
ba
r]
0
30
60
90
120
150
180
Bu
rn
Ra
te [
J/°
CA
]
0
15
30
45
60
75
90
CFD
Measurement / Reference
Engine Speed: 1200 1/minBMEP: 7.0 bar
CFD CombustionMB OM651 Q355
EKAS: 95% , 8 Holes
Crank Angle [°CA]
225.0 270.0 315.0 360.0 405.0 450.0 495.0
Cyl. P
ressu
re [
ba
r]
0
20
40
60
80
100
120
Bu
rn
Ra
te [
J/°
CA
]
0
20
40
60
80
100
120
Model Calibration
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Spray Model Calibration
Combustion Model Calibration
n = 1200 min-1, IMEP ~8.5 bar n = 4000 min-1, Full Load
Test-rig
CFD
Test-rig
CFD
14
Content
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
1. Introduction
2. Simulation Approach
3. Simulation Results
4. Testing Results & Validation
5. Summary
15
Boost Pressure, EGR-Rate,Swirl at IVC, Inj. Fuel Mass,Injection Timing (PI, MI)
base (CR = 16.2)
open wflat dish final w variant
n = 1200 min-1, IMEP ~8.5 barCR = 20
Constant: Variable:
CA50 %IMEP,pCyl,max
omega
n = 4000 min-1, Full LoadCR = 11
Bowl Concepts: Single Variants:
Wall
Heat
Lo
ss i
n (
%)
85
90
95
100
105
110
115
120
125
Wall
Heat
Lo
ss i
n (
%)
65
70
75
80
85
90
95
100
105
110
115
Simulation Results
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Piston geometry selected for max. ISFC benefit
Increase of Soot and NO unavoidable with CR=20
CR = 20.0
CR = 11.0
Ds Quench = 4.7 mm
Ds Quench = 0.0 mm
Soot – NO – ISFC – Trade-off
base (CR = 16.2) final w variant
omegaflat dishopen w
Variable:CA50 %,IMEP, pCyl,max
Constant:Boost Pressure, EGR-Rate, Swirl at IVC, Inj. Fuel Mass, Injection Timing (PI, MI)
3D Simulation
Pa
rtic
ula
tes
in %
10
40
70
100
130
160n = 4000 min-1, Full Load, CR = 11
ISFC
in %
90
95
100
105
110
NOx in Comparison to CR = 16.2 in %
90 100 110 120 130 140 150 160
ISFC - 5.8 %
ISFC
in %
90
95
100
105
110
NOx in Comparison to CR = 16.2 in %
75 80 85 90 95 100 105 110
Pa
rtic
ula
tes
in %
0
100
200
300
400
500
NOx + 37 %; PM + 34 %
PM - 65 %
n = 1200 min-1, IMEP ~8.5 bar, CR = 20
omega - reduced bowl depth
omega - scaled
flat dish
open w
16
Part Load, CR = 20 Full Load, CR = 11
base (CR = 16.2) final w variant
omegaflat dishopen w
Variable:CA50 %,IMEP, pCyl,max
Constant:Boost Pressure, EGR-Rate, Swirl at IVC, Inj. Fuel Mass, Injection Timing (PI, MI)
3D Simulation
Par
ticu
late
s in
%
10
40
70
100
130
160n = 4000 min-1, Full Load, CR = 11
ISFC
in %
90
95
100
105
110
NOx in Comparison to CR = 16.2 in %
90 100 110 120 130 140 150 160
ISFC - 5.8 %
ISFC
in %
90
95
100
105
110
NOx in Comparison to CR = 16.2 in %
75 80 85 90 95 100 105 110
Par
ticu
late
s in
%
0
100
200
300
400
500
NOx + 37 %; PM + 34 %
PM - 65 %
n = 1200 min-1, IMEP ~8.5 bar, CR = 20
PM -65%
NOx +37%; PM +34%
ISFC -5.8% ISFC -1.1%
base (CR = 16.2) final w variant
omegaflat dishopen w
Variable:CA50 %,IMEP, pCyl,max
Constant:Boost Pressure, EGR-Rate, Swirl at IVC, Inj. Fuel Mass, Injection Timing (PI, MI)
3D Simulation
Par
ticu
late
s in
%
10
40
70
100
130
160n = 4000 min-1, Full Load, CR = 11
ISFC
in %
90
95
100
105
110
NOx in Comparison to CR = 16.2 in %
90 100 110 120 130 140 150 160
ISFC - 5.8 %
ISFC
in %
90
95
100
105
110
NOx in Comparison to CR = 16.2 in %
75 80 85 90 95 100 105 110
Par
ticu
late
s in
%
0
100
200
300
400
500
NOx + 37 %; PM + 34 %
PM - 65 %
n = 1200 min-1, IMEP ~8.5 bar, CR = 20
Simulation Results
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
O2-Iso-Surface (kO2 = 5%) min max
Temperature [K]
Part Load (n = 1200 min-1, IMEP ~8.5 bar, 365 deg CA)
Omega-Bowl CR = 20, sSQUISH = 0.9 mm
w-Bowl CR = 20, sSQUISH = 0.9 mm
Flat Bowl CR = 20, sSQUISH = 0.9 mm
Basis CR = 16, sSQUISH = 0.9 mm
17
Omega-Bowl CR = 11, sSQUISH = 5.6 mm
w-Bowl CR = 11, sSQUISH = 5.6 mm
Flat Bowl CR = 11, sSQUISH = 5.6 mm
Basis CR = 16, sSQUISH = 0.9 mm
Full Load (n = 4000 min-1, Full Load, 375 deg CA)
Boost Pressure, EGR-Rate,Swirl at IVC, Inj. Fuel Mass,Injection Timing (PI, MI)
base (CR = 16.2)
open wflat dish final w variant
n = 1200 min-1, IMEP ~8.5 barCR = 20
Constant: Variable:
CA50 %IMEP,pCyl,max
omega
n = 4000 min-1, Full LoadCR = 11
Bowl Concepts: Single Variants:
Wall
Heat
Lo
ss i
n (
%)
85
90
95
100
105
110
115
120
125
Wall
Heat
Lo
ss i
n (
%)
65
70
75
80
85
90
95
100
105
110
115
Simulation Results
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Smaller Swirl Number @ SOI due to smaller piston bowl volume
Smaller piston bowl surface potential for decreased wall heat losses
CR = 20.0
CR = 11.0
Ds Quench = 4.7 mm
Ds Quench = 0.0 mm
Swirl and Wall Heat Losses
omega - reduced bowl depth
omega - scaled
flat dish
open w
18
Part Load, CR = 20 Full Load, CR = 11
Content
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
1. Introduction
2. Simulation Approach
3. Simulation Results
4. Testing Results & Validation
5. Summary
19
Testing Results & Validation
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Test-bench Environment
Single Cylinder Engine (SCE)
Replacement of piston
Variation of CR by inserting washers between
crank train and cylinder block
20
Base piston Optimized Piston
n = 1200 min-1, IMEP ~8.5 bar, CR = 20 n = 4000 min-1, Full Load, CR = 11
Injection
Pilot injection controlw.r.t. heat release
const. Injection Rate
SCE
CFD
EGR
Ext. EGR controled w.r.t.Iso-NOx / Iso-Soot
constant
Fuel Mass
Fuel mass controled w.r.t. IMEP
constant
Change w
.r.t
. B
ase in (
%)
-80
-60
-40
-20
0
20
ISFC Soot NOx
Change w
.r.t
. B
ase in (
%)
-20
0
20
40
60
80
100
ISFC
Soot NOx
Testing Results & Validation
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
Comparison Test-rig / CFD Results
Testing results confirm CFD predictions
Slight differences in predicted fuel consumption due to different conditions
(testing)
21
-5.8% -4.2%
+34%
+77%
+37% +32%
-1.3% -1.1%
-64% -65%
±0% -5%
Part Load, CR = 20 Full Load, CR = 11
Content
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
1. Introduction
2. Simulation Approach
3. Simulation Results
4. Testing Results & Validation
5. Summary
22
n = 1200 min-1, IMEP ~8.5 bar, CR = 20 n = 4000 min-1, Full Load, CR = 11
Injection
Pilot injection controlw.r.t. heat release
const. Injection Rate
SCE
CFD
EGR
Ext. EGR controled w.r.t.Iso-NOx / Iso-Soot
constant
Fuel Mass
Fuel mass controled w.r.t. IMEP
constant
Change w
.r.t
. B
ase in (
%)
-80
-60
-40
-20
0
20
ISFC Soot NOx
Change w
.r.t
. B
ase in (
%)
-20
0
20
40
60
80
100
ISFC
Soot NOx
(testing)
-5.8%-4.2%
+34%
+77%
+37%+32%
-1.3%-1.1%
-64% -65%
±0%-5%
Part Load, CR = 20 Full Load, CR = 11
n = 1200 min-1, IMEP ~8.5 bar, CR = 20 n = 4000 min-1, Full Load, CR = 11
Injection
Pilot injection controlw.r.t. heat release
const. Injection Rate
SCE
CFD
EGR
Ext. EGR controled w.r.t.Iso-NOx / Iso-Soot
constant
Fuel Mass
Fuel mass controled w.r.t. IMEP
constant
Change w
.r.t
. B
ase in (
%)
-80
-60
-40
-20
0
20
ISFC Soot NOx
Change w
.r.t
. B
ase in (
%)
-20
0
20
40
60
80
100
ISFC
Soot NOx
(testing)
-5.8%-4.2%
+34%
+77%
+37%+32%
-1.3%-1.1%
-64% -65%
±0%-5%
Part Load, CR = 20 Full Load, CR = 11
Summary
© IAV · 03/2016 · STAR Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine
• Almost all technical solutions for a variable
compression ratio (CR) have a variable squish gap
height
• For a CR variation range from 11 to 20 the squish
gap height must be changed by 4.7 mm
Big differences in spray-wall-interaction
• Manual, step-wise optimization of piston bowl design
at 2 operating points
• Piston bowl optimization results:
~5% reduction of ISFC/CO2 at part load
The increase of NO/Soot emissions at part
load caused by the increase of CR could not
be prevented
At rated power slight reduction of ISFC/CO2
(~1.2%) and strong decrease of soot emissions
• In general the simulation results were confirmed by
single cylinder engine
23
n = 1200 min-1, IMEP ~8.5 bar, CR = 20.0 n = 4000 min-1, Full Load, CR = 11.0
SCE (Testing) CFD
Change w
.r.t
. B
ase in (
%)
-70
-60
-50
-40
-30
-20
-10
0ISFC
Soot
NOx
Change w
.r.t
. B
ase in (
%)
-10
0
10
20
30
40
50
60
70
80
ISFC
Soot NOx
Thank You
Christian Schramm
IAV GmbH
Auer Straße 54, 09366 Stollberg (GERMANY)
Phone +49 371 237-34570
www.iav.com
© IAV · 03/2016 · Star Global Conference in Prague · C. Schramm · Piston Bowl Optimization for a VCR Diesel Engine 24