Laser diagnostics in Combustion: Microoptical Sensors
Microinvasive engine diagnostics
Optical diagnostics are successfully used for time-resolved, non-intrusive in-situ measurements in many fields of research. Optical methods form an essential part of modern IC engine research . For the development and optimisation of novel laser-optical diagnostics methods special transparent single-cylinder engines with a good optical access are required. The cylinder wall and piston of such a "glass engine" contain large integrated windows made of quartz glass which provide an almost unobstructed view into the combustion chamber.
For measurements under more realistic engine operating conditions in industrial research production engines are modified to enable a glimpse into the combustion chamber. These modifications (i.e. large windows for the optical access), however, are always time consuming and expensive and in some cases even influence the physical properties (e.g., heat transfer, in-cylinder flow field, compression ratio) of the examined engine. Hence, for the further development of promising concepts like direct injection with stratified load or HCCI (homogeneous charge compression ignition) and for the investigation of knocking in novel highly compressing IC engines optical measurements on engines that are not substantially modified or not changed at all (i.e. production engines) are inevitable.
In order to realize these measurements on production engines, in current research projects at the IVG together with other universities and industry partners innovative microoptical systems are developed. With these systems the necessary diagnostics can be applied on production engines with minimal invasive optical access for the first time.
The endoscopic optics have an outer diameter of less than one centimeter or are directly integrated into a fully functional spark plug. In cooperation with the ITO (Institut für Technische Optik, Universität Stuttgart) a novel, very sensitive UV endoscope  and a fiber-optic spark plug  were developed. These optics are now tested and improved in engine experiments at the IVG. For the optimization of combustion processes in DISI engines it is essential to have time resolved information about the fuel distribution close to the spark gap. Laser-induced fluorescence (LIF) based on the use of well characterized fuel tracers like toluene and 3-pentanone can deliver information about temperature, pressure and mixture composition (i.e. fuel/air-equivalence ratios) . The fiber-optic spark plug is designed to perform LIF measurements in a defined small probe volume (~2 mm³) close to the spark gap (cf. figure 1). Its ignition function is fully maintained. When constructing such a sensor, besides its optical properties, high demands on pressure- and temperature resistance have to be met. In recent experiments with the spark-plug sensor fuel LIF was measured in a commercial IC engine and temperature and fuel/air ratios were measured in a heatable flow setup in the lab.
Figure 1: Draft of fiber-optic spark plug.
Figure 2: Hybrid UV endoscope and its application on a four-cylinder production engine.
The novel hybrid microoptical system, consisting of a UV endoscope and two microoptics for the generation of lightsheets or illumination patterns, was realized for the two-dimensional observation of processes in cavities with limited optical access. It is denoted "hybrid" because of the combination of refractive and diffractive optical elements. The diffractive elements are employed to define the specific excitation beam shaping and they help to reduce aberrations in the imaging system. The design of the endoscope comprises a small light weight front endoscope that is chromatically uncorrected and hybrid relay elements for the correction of spatial and chromatic aberrations. By means of two relay elements, two spectral detection ranges are defined and optimized for the imaging of toluene-LIF (275 - 350 nm) and 3-pentanone-LIF (380 - 440 nm) signals (cf. Figure 4). Different spectral detection regions can be selected by changing the respective relay element. Its modular design makes the endoscope flexible for use with different filter combinations and enables multiple camera experiments with only one "keyhole" access into the examined object. The separation of front endoscope (in the engine) and relay optics and camera (on fixed positions in the lab) is especially important for engine experiments. That way the engine movement can not harm the sensitive optics and cameras.
Under application-relevant conditions, the new hybrid UV endoscope proved to be about an order of magnitude more light efficient than a commercial UV endoscope and showed a better spatial resolution over a broadband spectral detection range. Even in comparison to a regular 105 mm UV lens with the same paraxial object magnification the hybrid UV endoscope gathers more light (i.e. a factor of 1.3 at 300 nm and about twice the amount of light over a broader spectral detection range up to 440 nm).
Figure 3: Photos of hybrid endoscope. Top and left: Endoscope with field lens, bottom: Relay optics with diffractive optical element (DOE).
Figure 4: Draft of an experimental setup for the simultaneous imaging of two separate spectral regions through one "key hole".
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