=Paper=
{{Paper
|id=Vol-452/paper-4
|storemode=property
|title=In-cylinder imaging diagnostics with highly efficient UV-transparent endoscopes
|pdfUrl=https://ceur-ws.org/Vol-452/paper4.pdf
|volume=Vol-452
}}
==In-cylinder imaging diagnostics with highly efficient UV-transparent endoscopes==
https://ceur-ws.org/Vol-452/paper4.pdf
In-cylinder imaging diagnostics
with highly efficient UV-transparent endoscopes
1* 2 2 2 1
C. Gessenhardt , R. Reichle , C. Pruss , W. Osten , C. Schulz
1
IVG, University Duisburg-Essen, Germany
2
ITO, University Stuttgart, Germany
Laser-based in-cylinder diagnostics are well established in engine research. The requirement of large-scale optical
accesses, however, makes the application expensive and time consuming. It furthermore limits the engine operation
range to low loads and speeds. We introduce laser excitation and imaging optics with a minimal outer diameter of 10
2
mm (imaging optic) respectively 9 mm (excitation optics). The imaging optics allows the observation of a 30×30 mm
field with a working distance of 35–42 mm and provides a light intensity, which is higher than standard large-scale
UV optics at the same image magnification. The particular features of the endoscopes are demonstrated in experi-
ments at a fired IC engine, for example OH chemiluminescence and UV-LIF measurements.
Introduction and the application of the miniaturized hybrid
Laser-based diagnostics have been success- imaging endoscope in combination with a light-
fully used for the observation of fuel/air mixing sheet forming optic creating a homogeneous
and combustion process in IC engines [1, 2]. light sheet for imaging measurements in fired IC
Especially imaging diagnostics are of interest engines.
where in combination with pulsed UV-laser light-
sheet illumination laser-induced fluorescence Optical Design
(LIF) is used to observe the spatial distribution of Hybrid imaging endoscope: The design re-
fuel vapor, intermediate species or exhaust gas quirements for the imaging endoscope to enable
components. the application in passenger car IC engines
Imaging measurements, however, are typical- were: Maximum entry diameter 10 mm, avoid
ly restricted to optical engines where a significant stiff connections between camera and engine,
part of the cylinder liner and the piston surface provide high lens speed in the UV with multi-
are replaced by large-scale quartz or sapphire wavelength capability (different spectral bands of
windows [3]. In these facilities, standard UV- at least 60 nm each), allow imaging of a
2
transparent photographic lenses can be used for 30×30 mm field at a working distance of 35–
signal detection and conventional laser optics 42 mm with a resolution better than 100×100
are used for light-sheet formation. The construc- pixels, provide imaging on a 25 mm diameter
tion of optical engines, however, is expensive image intensifier in at least 200 mm distance.
and time consuming. Furthermore, the engines The optical concept as shown in Figure 1 was
can typically be used in a restricted speed and laid out as a multi-stage system.
load range only. Finally, IC engines are very
sensitive to modifications in the geometry and
the thermal management, which is difficult to
avoid when inserting large windows. Therefore,
an interest in micro-invasive concepts that use
endoscopic approaches has developed over the
recent years. The aim is to enable in-cylinder
laser-based imaging diagnostics at full-load and
full-speed conditions in production-line engines Figure 1: Optical design of the hybrid endos-
without significant modification. cope. Left: Front endoscope, right:
In the past, UV LIF has been used for imag- Hybrid relay optics
ing applications with standard endoscopes with
limited success only because of weak signal in- The front endoscope is fixed in the cylinder
tensities in the UV combined with poor lens head. It consists of two fused silica lenses and
speed of refractive UV endoscopes that are not an additional field lens that deflects an
specifically designed for engine applications [4]. uncorrected intermediate image towards the
In this paper we present new hybrid endos- multifunctional relay. This element is fixed to the
copic light sheet forming and wide angle imaging camera at a safe distance (20 cm) from the en-
optics for engine diagnostics. The design con- gine.
cept includes diffractive and refractive optics and The chromatic correction is achieved by
enables measurements at multiple spectral combining the positive dispersion of refractive
bands with the same front endoscope. This is optics with the strong negative dispersion of a
required for the application of two-detection- diffractive element in the hybrid relay. The opti-
band techniques such as simultaneous visualiza- mized aspheric phase function of the DOE cor-
tion of chemiluminescence and soot or fuel rects for further aberrations of the system.
spray. This paper describes the optical design
* Corresponding Author: christopher.gessenhardt@uni-due.de
Towards Clean Diesel Engines, TCDE 2009
Two different hybrid relay elements have
been developed. The first system is optimized in
the spectral range from 275–350 nm (e.g. OH*-
chemiluminescence or toluene LIF). A second
hybrid relay was designed for detection in the
380−440 nm band. Both can be used simulta-
neously with the same front endoscope. Figure 2
(top) shows two parts of the endoscopic imaging
system [5, 6].
Figure 4: Experimental setup for demonstration
measurements under lab conditions
With an arrangement leading to a similar pa-
raxial object magnification at 313 nm (Hg line)
Figure 2: Endoscopic imaging system with front
the hybrid endoscope collects about 1.3 times
endoscope and relay element (top)
more light than the standard UV lens
and light sheet endoscope (bottom)
(f = 105 mm, f# = 4.5, Nikon / Figure 5).
Light sheet forming optics: A micro-optical
beam-shaping endoscope was developed with
an outer diameter of 9 mm (Figure 2, bottom)
[5]. It converts an incoming UV-laser beam into a
divergent homogeneous excitation light sheet
with a divergence angle of 22° and a width of
<0.5 mm. The steel housing mounts cylindrical
lenses inside a square, spark-eroded carrier
channel.
Figure 3 shows the design of the beam-
shaping endoscope with its beam pattern. The Figure 5: Comparison of the hybrid endoscope
light-sheet endoscope uses three refractive cy- and a commercial UV lens by simul-
lindrical lenses (2-4). Lens 1 has an optimized taneous LIF measurements of a tolu-
aspheric shape for the conversion of the incom- ene doped nitrogen flow
ing Gaussian laser beam into a near-top-hat pro-
file. For an excitation laser beam with a smooth Engine application: To demonstrate the ca-
Gaussian profile without hot spots the damage pabilities of the micro light sheet optics and the
threshold of the excitation optics can be as high hybrid endoscope several demonstration mea-
as 25 mJ. surements have been carried out at a SI produc-
tion engine (BMW type: N46 B20). The schemat-
ic view of the setup for completely endoscopic
light-sheet measurements is shown in Figure 6.
Figure 3: Design of the micro light sheet optics
Measurement Results
Demonstration of the performance of the mi-
niaturized optics: In an atmospheric-pressure lab
experiment (Figure 4) LIF measurements at a
toluene-doped nitrogen flow have been carried
out to contrast the hybrid endoscope with a
commercial UV lens. A frequency-quadrupled
beam of a Nd:YAG laser and the micro light Figure 6: Measurement setup for detection of
sheet optic are used for excitation. two spectral regions and laser illumi-
nation
The main engine modifications are two addi-
tional ports in the combustion chamber of the
fourth cylinder for mounting the micro light sheet
optics and the hybrid endoscope. A frequency-
quadrupled Nd:YAG laser, an articulated arm engine vibrations do not affect the camera set-
and the micro light sheet optics are used for il- up.
lumination. In a laboratory setup it could be shown that
The front endoscope is mounted in the cy- the hybrid element provides 1.3 times stronger
linder head. The other imaging components light signal compared to a standard f# = 4.5 UV
have a distance of at least 20 cm to the cylinder lens at the same image magnification. The usa-
head. The space in between the optical ele- bility of the hybrid endoscopes for multi spectral
ments can be used for mounting optical filters or UV diagnostics has been proved in toluene LIF
beam splitters. This allows to direct different wa- and OH*-chemiluminescence measurements at
velength bands to separate relays and cameras. a production engine.
This two wavelength capability has been used The combination of beam forming and imag-
for temperature measurements based on tolu- ing endoscopes significantly enhances the pos-
ene LIF [7]. A technique, called two-color toluene sibilities for laser-based in-cylinder imaging. It
thermometry, takes advantage of the red-shift of enables measurements in slightly modified en-
the toluene fluorescence spectra with increasing gines under high load and speed conditions.
temperature [8]. By detecting two spectral re-
gions of the toluene LIF signal simultaneously, Acknowledgement
local variations in temperature could be deter- Funding by the German Federal Ministry of
mined. Education and Research (BMBF) is gratefully
acknowledged.
Chemiluminescence: The high lens speed of
the endoscope in the UV allows for single-shot References
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cylinder laser-light-sheet imaging with less than Pruss, W. Osten, and C. Schulz, Hybrid endosco-
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