Optical measurement techniques are valuable tools for the experimental investigation of combustion engine related processes like fuel injection, spray propagation, mixing and evaporation, ignition and combustion. This techniques are non-intrusive and therefore don’t disturb the investigated process, but they need an optical access to the combustion chamber. For production engines this is usually not possible (except sometimes with endoscopic techniques), therefore specialised experimental facilities are required that can both operate at engine relevant conditions (pressures, temperatures, flows) and provide good optical access at the same time.
At LAV two experimental test rigs are available for such investigations: A high pressure and high temperature constant volume cell and a single stroke machine (rapid compression and expansion machine). The cell can operate at gas pressures > 100bar and gas temperatures up to 750K (with electrical heating only, > 1000K with H2 precombustion). Excellent optical access is provided by up to five quartz or sapphire windows. It is suitable for all kinds of experiments involving liquid and gaseous fuel injection and diffusion combustion processes at very well controlled conditions, but the flow conditions in the cell (i.e. the quiescent environment) are quite different from the flow conditions present in real engines. The second available optically accessible test rig at LAV, the single stroke or rapid compression and expansion machine bridges the gap between the idealized, almost time invariant conditions found in the constant volume cell and the continuous, cyclic operation of real engines by isolating a single compression and expansion stroke while at the same time still offering an excellent optical access to the combustion chamber. Compression ratio and stroke length can be adjusted easily to achieve compression pressures > 100bar and temperatures > 950K, therefore it is suitable for experiments with all possible types of combustion including HCCI, and premixed, externally ignited as well as dual-fuel combustion processes.
Optical measurement techniques available at LAV and applied in those test rigs encompass high speed imaging (with plain, laser diode or laser light sheet illumination for the observation of liquid fuel spray structures), Schlieren techniques (for the observation of evaporated fuel or gaseous fuel injections), chemiluminescence imaging (qualitative observation of ignition and combustion), spectrographs (to analyse the light spectra emitted by the combustion), soot pyrometry (including 2D-pyrometry, to measure resp. visualize soot production / reduction processes), Phase Doppler Anemometry (to measure liquid droplet sizes and velocities in fuel sprays) and Laser Doppler Anemometry resp. high speed PIV to measure flow fields and turbulence levels.
Projects with participation of the Group “Optical Diagnostics”