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 mm (imaging optic) respectively 9 mm (excitation optics). The imaging optics allows the observation of a 30×30 mm2 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 experiments at a fired IC engine, for example OH chemiluminescence and UV-LIF measurements.
Laser-based diagnostics have been
successfully used for the observation of fuel/air mixing
and combustion process in IC engines [
Imaging measurements, however, are
typically restricted to optical engines where a significant
part of the cylinder liner and the piston surface
are replaced by large-scale quartz or sapphire
windows [
In the past, UV LIF has been used for
imaging applications with standard endoscopes with
limited success only because of weak signal
intensities in the UV combined with poor lens
speed of refractive UV endoscopes that are not
specifically designed for engine applications [
In this paper we present new hybrid endoscopic light sheet forming and wide angle imaging optics for engine diagnostics. The design concept includes diffractive and refractive optics and enables measurements at multiple spectral bands with the same front endoscope. This is required for the application of two-detectionband techniques such as simultaneous visualization of chemiluminescence and soot or fuel spray. This paper describes the optical design and the application of the miniaturized hybrid imaging endoscope in combination with a lightsheet forming optic creating a homogeneous light sheet for imaging measurements in fired IC engines.
Hybrid imaging endoscope: The design requirements for the imaging endoscope to enable the application in passenger car IC engines were: Maximum entry diameter 10 mm, avoid stiff connections between camera and engine, provide high lens speed in the UV with multiwavelength capability (different spectral bands of at least 60 nm each), allow imaging of a 30×30 mm2 field at a working distance of 35– 42 mm with a resolution better than 100×100 pixels, provide imaging on a 25 mm diameter image intensifier in at least 200 mm distance. The optical concept as shown in Figure 1 was laid out as a multi-stage system.
The front endoscope is fixed in the cylinder head. It consists of two fused silica lenses and an additional field lens that deflects an uncorrected intermediate image towards the multifunctional relay. This element is fixed to the camera at a safe distance (20 cm) from the engine.
The chromatic correction is achieved by combining the positive dispersion of refractive optics with the strong negative dispersion of a diffractive element in the hybrid relay. The optimized aspheric phase function of the DOE corrects for further aberrations of the system.
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
simultaneously with the same front endoscope. Figure 2
(top) shows two parts of the endoscopic imaging
system [
Light sheet forming optics: A micro-optical
beam-shaping endoscope was developed with
an outer diameter of 9 mm (Figure 2, bottom)
[
Figure 3 shows the design of the beamshaping endoscope with its beam pattern. The light-sheet endoscope uses three refractive cylindrical lenses (2-4). Lens 1 has an optimized aspheric shape for the conversion of the incoming Gaussian laser beam into a near-top-hat profile. For an excitation laser beam with a smooth Gaussian profile without hot spots the damage threshold of the excitation optics can be as high as 25 mJ.
Demonstration of the performance of the miniaturized 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 sheet optic are used for excitation.
With an arrangement leading to a similar paraxial object magnification at 313 nm (Hg line) the hybrid endoscope collects about 1.3 times more light than the standard UV lens (f = 105 mm, f# = 4.5, Nikon / Figure 5).
Engine application: To demonstrate the capabilities of the micro light sheet optics and the hybrid endoscope several demonstration measurements have been carried out at a SI production engine (BMW type: N46 B20). The schematic view of the setup for completely endoscopic light-sheet measurements is shown in Figure 6.
The main engine modifications are two additional ports in the combustion chamber of the fourth cylinder for mounting the micro light sheet optics and the hybrid endoscope. A frequencyquadrupled Nd:YAG laser, an articulated arm and the micro light sheet optics are used for illumination.
The front endoscope is mounted in the cylinder head. The other imaging components have a distance of at least 20 cm to the cylinder head. The space in between the optical elements can be used for mounting optical filters or beam splitters. This allows to direct different wavelength bands to separate relays and cameras.
This two wavelength capability has been used
for temperature measurements based on
toluene LIF [
Chemiluminescence: The high lens speed of the endoscope in the UV allows for single-shot OH* measurements that can be used for the detection of cycle-to-cycle variations in flame propagation. Figure 7 shows a selection of single-shot images at 20°ca before TDC (ignition at 49°ca before TDC).
The wall of the combustion chamber and the spark plug are highlighted. During these measurements the engine was operated at low load (pmi = 1.3 bar / n = 2040 min–1) to show the applicability of the hybrid endoscopes with weak UV chemiluminescence signal.
This paper presents novel endoscopes for incylinder laser-light-sheet imaging with less than 10 mm entry diameter of the optics. An imaging endoscope combining diffractive and refractive optics provides imaging from a 30×30 mm2 area at 35–42 mm working distance. The chromatic correction is provided by specifically designed diffractive elements. Both principal parts of the imaging system are separated, thus, engine and camera are not mechanically connected and engine vibrations do not affect the camera setup.
In a laboratory setup it could be shown that the hybrid element provides 1.3 times stronger light signal compared to a standard f# = 4.5 UV lens at the same image magnification. The usability of the hybrid endoscopes for multi spectral UV diagnostics has been proved in toluene LIF and OH*-chemiluminescence measurements at a production engine.
The combination of beam forming and imaging endoscopes significantly enhances the possibilities for laser-based in-cylinder imaging. It enables measurements in slightly modified engines under high load and speed conditions.
Funding by the German Federal Ministry of Education and Research (BMBF) is gratefully acknowledged.