INJECTION PRESSURE AS A MEANS TO GUIDE AIR
UTILIZATION IN DIESEL ENGINE COMBUSTION
H. Dembinski, Scania AB Sweden
H.-E. Angstrom, KTH Stockholm, Sweden
E. Winklhofer, AVL List GmbH, Austria
London, March 10 and 11, 2015
Goteborg, November 26, 2015
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1
MOTIVATION
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2
THE QUESTION
How can higher injection pressure end
up with lower engine out soot ?
injection pressure
/
soot
t = 0.2 ms aSOI
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3
THE QUESTION
How can higher injection pressure end
up with lower engine out soot ?
 Can we understand the mechanisms ?
 If so – how to exploit them ?
 Which kind of analysis would we require ?
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4
CONTENT
1.Injection pressure – spray at nozzle exit
2.Spray interaction with in-cylinder gas
 Momentum transfer
 Heat transfer
3.Ignition
4.Premixed and diffusion flames
 Soot formation
 Soot oxidation
5.Enhancing soot oxidation
6.How things come together
 pressure temperature flow
7.Summary
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
1. Injection pressure - spray
Fuel injection pressure
injector internal flow
Such flow is visualized in 2D model nozzle tests.
E. Winklhofer et al.: „Basic flow processes in high
pressure fuel injection equipment“, ICLASS 2003
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
1. Injection pressure - spray
Internal flow is visualized in 2D model nozzle tests.
Liquid into liquid injection (Diesel)
Pin= 400 bar
liquid
Fuel flow is subject to cavitation in
local shear and boundary layers.
needle
needle
body
liquid
Shear layer
cavitation
boundary layer
cavitation
1 bar
1 bar
30 bar
High cross flow velocity gradients
force static pressure to drop below
vapor pressure.
Driving parameters are:
• Geometry
• Velocity gradients
• Pressure
• Vapor pressure of fuel
boundary layer
cavitation
White: liquid phase (Diesel)
Black: gas phase (cavitation bubbles) at Diesel vapor pressure << 1 bar
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
1. Injection pressure - spray
Internal flow is visualized in 2D model nozzle tests.
Liquid into liquid injection (Diesel)
Pressure field at inflow into nozzle hole
Fuel pressure is discharged within
fractions of a millimeter at the
entrance to the nozzle hole.
Geometry influence on local static
pressure – and hence on cavitation - is
highest in areas of high pressure drop.
A
Measurements were done as Diesel fluid
was just below cavitation limit
B
Sharp (A) and
round (B) inlet
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
1. Injection pressure - spray
Liquid into air: 2D model nozzle to see
internal flow together with spray.
Average of 30 events
Pin= 60
120
825 bar
800 µm 800 µm
fuel
cavitation
spray
air
A nozzle flow – spray experiment:
At high injection pressure we see
• Well developed cavitation down to nozzle hole exit
• Atomizing spray with highly stable spray cone
angle
Note the dimensions: 0,8 mm nozzle hole + 0,8mm
free spray
Conclusion: high injection pressure stabilises spray
cone angle near nozzle exit.
laminar
turbulent
turbulent and
cavitating
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
2. Spray interaction with in-cylinder gas
Momentum transfer
Heat transfer
Fuel injection pressure
spray dia. - mm
exit velocity:
Vspray/VBernoulli ~ 0.9
Spray observation in optical Diesel research engine:
Spray diameter fluctuation measurement to
document spray targeting and spray fluctuation in
far field
1500 bar
2
0
Z = 15 mm
-2
0
1 ms
2
spray diameter fluctuation
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
2. Spray interaction with in-cylinder gas
Momentum transfer
Heat transfer
Length - time and Diameter - time shadow traces of Diesel sprays in
an optical engine
Start of Injection
2
End of Injection
20 mm
Spray observation in optical
Diesel research engine:
500 bar
0
-2
2
800 bar
Spray diameter fluctuation
measurement to document spray
targeting and spray fluctuation in
far field
0
10
-2
1500 bar
2
Spray
shadow
800 bar
Strahlschatten
0
0
0
1 ms
2
Spray tip propagation and spray core length.
Z = 15 mm
-2
0
1 ms
2
Spray targeting and spray diameter
(spray angle) fluctuations.
Conclusion:
• high injection pressure
stabilises spray targeting.
• Spray diameter fluctuations
appear at ever higher
frequency
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
2. Spray interaction with in-cylinder gas
Momentum transfer
Heat transfer
Spray in hot compressed in-cylinder gas
Heat transfer from gas into spray with resultant
• fast expansion of spray vapor plume
• and self ignition
T = 930 K
p = 53 bar
Schlieren imaging in optical research engine, 500 rpm
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
2. Spray interaction with in-cylinder gas
Momentum transfer
Heat transfer
Prail = 300 bar
Prail = 800 bar
Prail = 1100 bar
Spray in hot compressed in-cylinder gas
Heat transfer from gas into spray with
resultant
• fast expansion of spray vapor plume
• and self ignition
Spray
core
• Injection pressure enhances fuel vapor
transport
Spray
vapor
cloud
Average spray contours at 0.40 ms after SOI. Nozzle: 1x 0.115 mm
Schlieren imaging in optical research engine
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PRESSURE – GEOMETRY - FUEL FLOW – SPRAY
– EVAPORATION - IGNITION
3. Ignition
0 µs
50 µs
100 µs
150 µs
µs
Ignition and spray – flame interaction in
optical engine, 85 mm bore, p = 60 bar
Combustion chamber is externally
illuminated, high speed camera records of
one cycle.
Time sequence shows
• sprays before ignition,
• combustion of premixed fuel vapor in „blue flame“
pockets,
• and start of diffusion combustion
Full glass optical piston
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3. Ignition
PRESSURE – GEOMETRY - FUEL FLOW – SPRAY
– EVAPORATION - IGNITION
Ignition and spray – flame interaction in optical
heavy duty engine, 127 mm bore, p = 153 bar
80 mm piston
window dia.
A
Piston bottom window
Time sequence shows
• combustion of premixed fuel vapor in „blue flame“ pockets,
• and start of diffusion combustion
• Note that blue premixed flame is only visible at ignition for
up to 100 µs (0,6 deg CA at 1000 rpm)
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INJECTION PRESSURE – COMBUSTION
4. Premixed and diffusion flames
Soot formation
Soot oxidation
Diesel combustion movie
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HD DIESEL OSCE WITH
PISTON BOTTOM WINDOW
Fired operation for
15 cycles
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HD DIESEL OSCE WITH
PISTON BOTTOM WINDOW
Topic: 20 bar IMEP
1400 rpm
Fired operation for
15 cycles
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INJECTION PRESSURE – COMBUSTION
– AIR UTILIZATION
4. Premixed and diffusion flames
Soot formation
Soot oxidation
14
x 10
4
„metal engine“
500 bar
1000 bar
1500 bar
2000 bar
soot
total
- KL*area
total soot
(KL*area)
12
„optical engine“ flame evaluation shows
• Faster soot oxidation at higher injection
pressure
8
6
Soot oxidation needs flame (soot) – air mixing
In optical engine
1500
2000
500bar
1000bar
• Lower FSN at higher injection pressure
10
4
Injection at
„Metal engine“ soot measurement:
1000
Data show that injection pressure has significant
influence on air utilization = soot oxidation.
500 bar
2
0
How can this happen ?
0
10
KL is evaluated from high speed
flame movies with 2-color method
20
30
40
CAD
50
60
70
80
deg CA
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INJECTION PRESSURE – COMBUSTION
4. Premixed and diffusion flames
Soot formation
Soot oxidation
How can it be that injection pressure
improves air utilization ?
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HOW CAN INJECTION PRESSURE IMPROVE
2000 bar
AIR UTILIZATION?
4. Premixed and diffusion flames
Soot formation
Soot oxidation
near end of injection
2000 bar
10 deg CA after end of injection
2000 bar
At ongoing injection
• Backflow of flames into combustion
chamber center following spray – vapor flame reflection on piston bowl wall
After end of injection
• Speed up of swirl motion in center of piston
bowl
0 m/s
55 m/s
21
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HOW CAN INJECTION PRESSURE IMPROVE
2000 bar
AIR UTILIZATION?
4. Premixed and diffusion flames
Soot formation
Soot oxidation
10 deg CA after end of injection
near end of injection
200 bar
2000 bar
200 bar
2000 bar
A comparison with very low injection
pressure
Spray momentum is too small for
effective interaction with reflecting piston
bowl wall
Conclusion 1
Injection pressure drives the flame back
into areas of un-used air
0 m/s
55 m/s
22
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INJECTION PRESSURE – COMBUSTION
4. Premixed and diffusion flames
Soot formation
Soot oxidation
How can it be that injection pressure
improves air utilization ?
1. It introduces flame transport into areas with un-used air
2. And further:
flame transport enhances turbulent motion
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5. Enhancing soot oxidation
FLAME MOTION AND
TURBULENT KINETIC ENERGY
Flame (soot) transport
Turbulence: data on local turbulent kinetic energy
At 8,5 EOI
Flow field
12,5
16,5
21,5 deg CA
500 bar,
FSN = 1,2
1000 bar
FSN = 0,5
Significant rise of local turbulence with high
injection pressure…
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5. Enhancing soot oxidation
KINETIC ENERGY RELAXATION
AFTER END OF INJECTION
𝑢𝑥2 + 𝑢𝑦2
𝐾𝐸~
.
2
Flame (soot) transport
𝑢𝑥2 + 𝑢𝑦2
𝐾𝐸~
.
2
Kinetic energy vs CAD
Flow field
500 bar,
FSN = 1,2
CAD
Local Turbulent Kinetic Energy
…and fast decay of turbulence at
end of injection
1000 bar
FSN = 0,5
Turbulence
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5. Enhancing soot oxidation
KINETIC ENERGY RELAXATION
AFTER END OF INJECTION
𝑢𝑥2 + 𝑢𝑦2
𝐾𝐸~
.
2
2000 bar
2
2
Soot 𝑢𝑥 + 𝑢𝑦 Soot oxidation
𝐾𝐸~
.
formation
2
Turbulence is driver for
soot – oxygen mixing…
Soot transport
…and results in enhanced
soot oxidation
Note: high turbulence for effective soot oxidation is only available right at the end of injection.
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Flame - summary
6. How things come together
pressure temperature flow
„Metal engine“ soot measurement:
• Lower FSN at higher injection pressure
„optical engine“ flame evaluation shows
• Faster soot oxidation at higher injection pressure
Soot oxidation needs flame (soot) – air mixing
Data show that injection pressure has singificant influence on air
utilization = soot oxidation.
How can this happen ?
1. Transport soot to meet with air
2. Use turbulence for fast soot – air mixing
Examples have shown
• that and how both, transport and turbulence, are controlled by fuel injection and spray momentum reflection on piston bowl walls.
• Effective soot oxidation must happen close to the end of injection to benefit from high temperature oxidation rates.
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7. Summary
INJECTION PRESSURE AS A MEANS TO
GUIDE AIR UTILIZATION IN DIESEL ENGINE
COMBUSTION
Spray:
Spray turbulence and atomizaton are driven by cavitation
Cavitation is controlled by shear and boundary layer flow under influence of local spray hole geometry
At usual temperatures ( 100 °C) and injection pressures ( 500 – 2500 bar)
„normal“ behaviour of Diesel sprays: liquid spray core of droplets and ligaments, fuel vapor and
diffusion flame
Flame:
Temperature and pilot injection control ignition, soot formation in diffusion flame,
Soot oxidation:
Injection pressure is most effective to control flame transport and turbulent mixing for fast oxidation
Analysis:
in optical Diesel engines with realistic temperature, pressure and geometry parameters
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INJECTION PRESSURE AS A MEANS TO
GUIDE AIR UTILIZATION IN DIESEL ENGINE
COMBUSTION
Thank you
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29
6 MINUTES
TO STOP THE ENGINE, CLEAN THE
COMBUSTION CHAMBER AND START AGAIN
Diesel engine movie
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30
HD DIESEL OSCE WITH
PISTON BOTTOM WINDOW
Fired operation for
15 cycles
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31
HD DIESEL OSCE WITH
PISTON BOTTOM WINDOW
Topic: 20 bar
IMEP
1400 rpm
Fired operation for
15 cycles
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THE RISK OF GLASS DAMAGE
MOUNTING
PRESSURE
TEMPERATURE
INERTIA FORCES
LOCAL CONTACT
UNKNOWN
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PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
1. Injection pressure - spray
Internal flow is visualized in 2D model nozzle tests.
Liquid into liquid injection (Diesel)
liquid
Pin= 400 bar
needle
body
liquid
1 bar
Fuel flow is subject to cavitation in
local shear and boundary layers.
High cross flow velocity gradients
force static pressure to drop below
vapor pressure.
Driving parameters are:
• Geometry
• Velocity gradients
• Pressure
• Vapor pressure of fuel
White: liquid phase (Diesel)
Black: gas phase (cavitation bubbles) at Diesel vapor pressure << 1 bar
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34
PRESSURE – GEOMETRY - FUEL FLOW - SPRAY
1. Injection pressure - spray
Internal flow is visualized in 2D model nozzle tests.
Liquid into liquid injection (Diesel)
Pin= 400 bar
liquid
needle
body
liquid
Fuel flow is subject to cavitation in
local shear and boundary layers.
High cross flow velocity gradients
force static pressure to drop below
vapor pressure.
Shear layer
cavitation
1 bar
30 bar
Driving parameters are:
• Geometry
• Velocity gradients
• Pressure
• Vapor pressure of fuel
boundary layer
cavitation
White: liquid phase (Diesel)
Black: gas phase (cavitation bubbles) at Diesel vapor pressure << 1 bar
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35