Institut für Photonik Technische Universität Wien

Transcription

Proceedings of the 3rd International Conference on the Frontiers of Plasma Physics and Technology (PC/5099)
S6-3
Laser Plasma-Initiated Ignition of
Engines
E. Wintner
Photonics Institute
Vienna University of
Technology
Vienna, Austria
3rd Int. Conference on the
Frontiers of Plasma Physics
and Technology
Bangkok, March 5-9, 2007
Institut für Photonik
Technische Universität Wien
Megawatt gas engine
Spark voltage vs BMEP
spark voltage [kV]
30
250 mg NOx/Nm3
25
20
15
500 mg NOx/Nm3
10
5
0
1
2
BMEP [MPa]
3
Potential future ignition systems
• Plasma Ignition
• High Frequency Ignition
• Auto Ignition
• Laser Ignition
• Pressure Wave Ignition
• Diesel Pilot Ignition
• ….
1st Application: gas engine
Production of electricity and heat
Æ overall efficiency up to 90 %
• Lean mixture
• High ignition pressures
Æ limited lifetime of the spark
plug through electrode
erosion
• High costs of the ignition
system
GE Jenbacher
NOx emission potentials
NOX [mg/Nm³]
350
330
300
250
250
240
190
200
150
100
70
50
0
direct
pre chamber laser ignition
spark ignition
direct
pre chamber
diesel pilot ignition
Source: GE Jenbacher GmbH & Co OHG
2nd Application:
spray-guided combustion
Goals of this work
Design and Construction of a Laser Ignition System
• Laser Spark Plug
• Pump Beam Multiplexing System
• Beam shaping and fiber coupling of the diode laser
Overview
•
•
•
•
•
•
Introduction
Principle of laser ignition
Realized goals of this work
Experimental
Remaining questions
Fiber transmission of
ns optical pulses
• Laser development
• In-coupling window aspects
• Conclusions
Introduction on laser ignition
• Historical
J.D. Dale, P.R. Smy, R.M. Clements: Laser-ignited
internal combustion engine – an experimental study; SAE
Congress, paper 780329, Detroit (1978) by CO2 laser!
• Objectives
Reliable and efficient ignition with clean exhaust of
gas engines (lean burn internal combustion engines)
automotive engines (in HCCI mode - homogeneous
charge compressed ignition)
Advantages of laser ignition
Free choice of position of ignition point in the cylinder
Avoids detrementous quenching effects at electrodes
and metal cylinder surfaces
Low maintenance efforts when applying diode-pumped
solid-state lasers
Possibility of ignition of very lean gas mixtures
Improvement of timing jitter in HCCI mode
substantial reduction of NOx emissions possible!
Principle of laser ignition
convex lens
laser beam
focused laser beam
plasma
I>Ithreshold
flame kernel
E>Eignition
mixture burning
Effects leading to non-resonant
breakdown
• Electron cascade growth
needs initial electrons
• multiphoton ionization
• electron tunnelling
(relevant only at intensities
higher than 1014 W/cm2)
laser energy absorption
by impurities in the gas
Plasma formation by a focused beam
through a sapphire plate
Spatial and temporal development of
a laser-induced plasma
Comparing UV emission at 2 pulse energies:
left at plasma threshold and
right far above
Experimental setup
Measurement of the plasma light emission at 310nm
laser beam
150 mm planoconvex lens
Camera
gate time = 3ns
Unseeded Nd:YAG-Laser: 1064nm, 12ns, flat top spatial profile
air, atmospheric pressure
0 ns after rising edge of laser pulse
pulse energy = 47 mJ
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
1cm
Minimum ignition energy
U. Maas/B. Lewis
10
ignition energy [mJ]
2.7 current limit
in the
combustion
vessel
propane
1
methane
0,1
H2
0,01
0
0,5
1
1,5
2
A/F-ratio
2,5
3
3,5
4
Combustion chamber
net volume = 1.2 dm3
maximum permissible
pressure = 300 bar
stabilized at
70 °C, 200 °C and
400 °C
sapphire (lens) windows
of 13 mm clear aperture
for longitudinal and
transversal transmission
Experimental setup
Selected experimental laser systems
Q-switched
Nd:YAG Laser
pulse duration = 6ns
wavelengths:
1064 nm, 532 nm, 355 nm
beam quality parameter M2=1.8
and later M2=1.1
Plasma formation in different
gaseous media
air
N2
methane
NO PLASMA N2
1,0
Intensität[1012 W/cm²]
0,8
0,6
10 bar
0,4
0,2
0,0
0
5
10
Zeit [ns]
15
20
Transmitted beam intensity at
T = 293 K, prel = 10 bar
Realized goals by
combustion chamber experiments
Determination of plasma threshold intensity and
minimum laser pulse energy for ignition (MPE)
of several fuel gas –air mixtures and mixtures
depending on
–
–
–
–
–
methane-air
hydrogen-air
hydrogen-methane-air
biogas
isooctane
Minimum ignition energy (MIE) not important for practical
considerations, however for comparison with theory
Realized goals by
combustion chamber experiments
Determination and optimization of
– laser wavelength (1064 nm, 532 nm, 355 nm)
– focusing optics
– MPE dependance on fill pressure (≤4 MPa)
– MPE dependance on gas-air equivalence ratio L
(especially lean side limit)
– MPE dependance on temperature
Focus intensity for plasma formation
vs. pressure
2,0
12
threshold intensity [10 W/cm²]
2,5
1,5
1,0
0,5
0,0
0
5
10
15
20
25
relative pressure [bar]
30
Pplasma > 99%, T = 293 K, air
Influence of pressure and temperature
on plasma formation
Plasma Durchbruchsenergie [mJ]
2,5
350 °C
50°C
100°C
150°C
200°C
250°C
50 °C
300°C
350°C
2,0
1,5
1,0
0,5
0,0
0
10
20
Zünddruck
[bar] start
pressure
at ignition
30
40
Mininimum pulse energy
vs. excess air ratio Λ
MPE [mJ]
Methane-air, T=473 K, p=30 bar
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
1,0
1,2
+/- 0,05
λ
1,4
1,6
1,8
λ +/- 0,1
2,0
2,2
2,4
+/- 0,12
Combustion pressure versus time at
different Λ
excess pressure [bar] (+30 bar)
100
Methane-air,
T=473 K, p=30 bar
λ=1.73 +/- 0,1
λ=1,88
λ=2,0
λ=2,11
λ=2,3
λ=2,4 +/- 0,12
80
60
40
20
0
0
1000
2000
time [ms]
3000
4000
5000
40
40
35
35
30
30
25
20
pinit (bar)
pinit (bar)
Direct comparison
laser ignition – spark plug ignition
25
20
15
15
10
1,40 1,50 1,60 1,70 1,80 1,90 2,00 2,10 2,20 2,30
A/Frel ( )
0,00
ignition reliability of laser ignition
0,25
0,50
0,75
1,00
10
1,40 1,50 1,60 1,70 1,80 1,90 2,00 2,10 2,20 2,30
A/Frel ( )
0,00
ignition reliability of spark plug ignition
0,25
0,50
0,75
1,00
Combustion chamber of constant volume; methane-air, Tgas = 200°C;
A/Frel =1.77 on the engine for reliable run, maximum BMEP = 19 bar, typical
spark duration = 400 – 500 µs; laser M2 < 1.2; laser pulse energy constant
25 mJ well above the plasma breakdown threshold for all conditions, overall
ignition attempts: 1201 for spark plug, 642 for laser;
Plasma and flame kernel diagnostics
Schlieren
Schlieren
PLIF
Laser ignition – first engine test
first engine test 08.2000
Laser ignition of lean fuel-air mixtures
Problem:
• slow flame front propagation
Countermeasures:
• higher laser energy: undesirable
• turbulence already realized in engines
• multipoint ignition
• addition of hydrogen
• “self-ignition“ of the whole volume,
improved by laser-initiation
Approach for multipoint ignition
laser
fiber
sapphire
window
diffraction grating
coupling optics
ignition optics
piston
Laser ignition
Heat release - single point / multipoint
7
6
dQB [%/°CA]
ROHR laser
multi point ignition
5
4
ROHR spark ignition
3
2
1
0
-40
-20
0
20
crank angle [CA°]
40
60
80
Increase
of flame
velocity by
hydrogen
addition
% hydrogen replacing methane indicated
First demo of laser-triggered HCCI
Influence of laser plasma on HCCI combustion; weak color: without plasma, intense
color: laser plasma at 40° BTDC; A/Frel = 2.3, 90% methane, 10% isooctane.
First demo of laser-triggered HCCI
Laser-stabilized HCCI engine operation at reduced inlet temperature fades away after
plasma is turned off (zero value for CA50: no combustion at all; CA50: crank angle at
50% burn point; A/Frel = 2.04, 90% methane, 10% isooctane, plasma at 25° BTDC)
Remaining questions
• Propagation of pulsed radiation via optical
fibers
• Choice of a compact, robust and economic
laser source
• Durability of windows
Various fibers tested
• Step index fibers with diameter 100 to
1000 um
• Hollow core dielectric capillaries
• Hollow glass fibers with cyclic olefin
polymer coated silver [1]
• Hollow-core photonic crystal fibers,
preliminary experiments [2]
[1] Cooperation with M. Miyagi, Tohoku University, Sendai, Japan
[2] Cooperation with A.M. Zheltikov, Lomonosov Moscow State University,
Moscow, Russia;
S.O. Konorov et al., J. Phys. D: Appl. Phys. 36 (2003) 1-7
Problems encountered with optical fibers
The high power of Q-switched laser pulses easily
can destroy optical media
Countermeasures:
•Thick step index fibers: only diameters > 300 µm
allowed reliable propagation of the pulses
•Hollow capillaries or fibers do not contain
solid-state matter within the core
Beam quality of fiber output
Great emphasis was put on high laser beam quality
when aiming towards lowest pulse energies for ignition:
M2 = 1.1
In all cases of fibers except photonic crystal fiber
the output beam quality was
M2 ca. 30-50
Overview of photonic crystal fibres
(PCFs)
• are optical fibers that employ a
microstructured arrangement of low
index material in a background
material of higher refractive index.
• The backgrond material is undoped
silica and the low index region is
typically provided by air voids running
along the length of the fiber.
Types of PCF
• High index guiding fibres based on modified
total internal reflection principle.
• Low index guiding fibres based on the
Photonic Band Gap (PBG) effect.
„The bandgap effect can be found in nature
where bright colours that are seen in the
butterfly wings are the results of naturally
occuring periodic microstructures“.
Photonic band gap fiber used
2-dimensionally periodic cladding, 5 µm
periodicity, 14 µm core
diameter, air-filled or
evacuated, several
passbands, e.g.
around 1 µm
(Cooperation with
Blaze Photonics, Bath,
UK)
Experimental setup
Q-Switched Laser
Focusing lens
f=75mm
CCD Camera
and/or Energy
Detector
Collimating lens
f=75mm
OSC
Vacuum Chamber
Relation between pressure and
laser energy for breakdown
Output from the fiber
Experimental results on
evacuated PCFs
Evacuation below 10 mbar allows to propagate up to
600 µJ of 1064 nm ns pulses in single mode shape. Peak
intensity >1012 W/cm2 (i.e. ca. 1000 x higher than
destruction limit of silica! Care has to be taken when
coupling in to avoid destruction of fiber walls.
Problem: high coupling loss (80%), propagation loss
<0.1 dB/m, recently solved: only 16 % loss;
Maximum throughput: 0.6 mJ, still not enough
Mininimum pulse energy vs.
excess air ratio λ for 3 temperatures
Allow to define the requirements on the ignition laser
MPE [mJ]
• < 5ns pulse duration
• > 10mJ Pulse energy
• Stable against mechanical
& thermal stress
20
30 bar
16
12
8
150°C
275°C
400°C
4
0
1,2
1,6
2,0
2,4
λ [−]
2,8
3,2
Prototype: specially designed laser
Developed scheme of laser ignition
Laser setup
Experimental setup for
self-developed laser
Fiber-coupled, longitudinally diode-pumped,
passively Q-switched solid-state laser
A schematic view
1: Laser Diode Fiber
4: Passive Q-Switch Cr4+:YAG
2: Collimating Lens
5: Output Coupler
3: Nd:YAG + Input Coupler
Experimental setup
In the laboratory
Laser spark plug : preliminary design
Experimental results
Dependence
(70 W diode)
of:
-- Pulse duration (left) and
-- Pulse energy (right)
on: -- Reflectivity of output mirror and
-- Initial transmission of Q-Switch
Dependence
(300 W Diode)
of:
-- Pulse jitter (left) and
-- Pulse energy and wasted energy (right)
on: -- Pump duration
Dependence
of:
(300 W diode)
-- Pulse duration
-- Plasma transmission (f = 7.5 mm, E = 1.7 mJ)
on: -- Resonator length
Typical beam profiles (300 W diode)
Energy
E = 6 mJ
Energy
E = 10 mJ
Pulse duration
t = 1 ns
Pulse duration
t = 1.5 ns
Simulation results
Energy distribution
- Reflectivity output mirror
R=50%
- Reflectivity output mirror
R=50%
- Initial transmission Q-switch I0=40% - Initial transmission Q-switch I0=90%
Depositions – window temperature
Laser-induced depositions
beam profile
Laserbeschichtungseffekt:
Suprasil
Suprasilfenster
Suprasil window
6
5
Abbrennen der
Ablagerungen
4
3
Bereich des Laserbeschichtungseffektes
Laserenergiedichte ED [mJ/mm²]
Suprasil
2
1
0
0
5
10
15
Einsatzzeit [h]
20
kein Laserbeschichtungseffekt
25
Laserbeschichtungseffekt:
SaphirfensterSaphir
5
4
Abbrennen der
Ablagerungen
3
Bereich des Laserbeschichtungseffektes
Laserenergiedichte ED [mJ/mm²]
Saphir
Sapphire window
2
vom heißen Brenngas
umspültes Linsenfenster
(LF blieb sauber)
1
0
0
5
10
Einsatzzeit [h]
kein Laserbeschichtungseffekt
15
Focusing optics
Lens window
Ignit ion
Plasma
Focal lenght
Cylinderhead
Pist on
combined optics (lens-window)
separate optics
Conclusions I
Using optimized (aspheric) focusing optics and laser
beams of near Gaussian characteristics can
Æ minimize the minimum laser pulse energy needed
for ignition (MPE) of stoichiometric mixtures down to
sub-mJ levels having also been realized in engine
operation.
Æ
The lean side limit for methane-air mixtures for
reliable laser ignition was determined as λ = 2.1
(as opposed to spark ignition up to λ = 1.7); for
hydrogen-air mixtures even up to λ = 7.
Conclusions II
Fiber delivery represents one of the most critical
aspects of laser ignition. As seen from now, only the
Æ
hollow-core photonic band gap fiber represents an
option for transportation of Q-switched laser pulses.
It seems more realistic to equip every cylinder with its
Æ own laser source which has to be much cheaper than
the ones available today.
A compact diode-pumped, passively Q-switched
Æ Nd:YAG laser has been developed, delivering
Ep >10 mJ in τp = 1.5 ns which can potentially be
implemented in a laser spark plug.
Financed by
GE Jenbacher GmbH & Co OHG
and
FFF Austrian Industrial Development Fund
Coworkers and Collaborators
Gabor Ast
Soren Charareh
Alexander Dozenko
Reinhard Gilber
Herbert Kopecek
Harald Maier
Georg Reider
Anfisa Stachiv
Bernhard Schwecherl
Martin Tesch
Martin Weinrotter
Photonics Institute
Christian Forsich
Maximilian Lackner
Gerhard Totschnig
Franz Winter
Institute of
Chemical
Engineering
Kurt Iskra
Theo Neger
Harald Rüdisser
Institute of Experimental
Physics, Graz University
of Technology
Cooperations
Martin Frenz
Heinz Weber
Jaroslav Bobitski
Alexander Manenkov
Alexei Zheltikov
M. Miyagi
Blaze Photonics
Bernhard Geringer
Institute for Applied
Physics, Bern University
Lvivska Politechnika
A. Prokhorov General Physics
Institute, Moscow
Moscow State University
Tohoku University, Sendai, Japan
Bath, UK
Institute of Internal Combustion
Engines, Vienna University of
Technology
Thank you for your
attention!

Institut für Photonik Technische Universität Wien

Transcription

Similar documents

stadionkurier 05FC Schwarzenbach

LENZINGER BERICHTE Elektrisch leitfähige Folien

Prüfungsfragen und Antworten für das Kfz-Techniker

WFNS - aasns

ESP Catalog - Guitars Collector