Applications of the LIF method for the diagnostics of the combustion process of gas-IC-engines

摘要

Within the underlying project, the task was to develop methods for optical measurements in a hydrogen-fuelled engine with direct-injection, with the goal of measuring the jet patterns during injection, the stratification of the charge at ignition point and the propagation of the flame during combustion. Therefore, the method of planar laser-induced-fluorescence (PLIF) was chosen. In order to apply this technique for the named tasks, particular methods the visualisation of fuel distribution and the flame front were developed. The measurements were carried out on a single cylinder research engine installed at the Institute for Internal Combustion Engines at Graz University of Technology. This engine features optical access through a quartz-glass liner and a window in the piston while providing a layout equivalent to modern passenger car engines and the possibility to operate in fired mode. As it is hardly feasible to directly excite molecular hydrogen by means of laser light, it is necessary to add a tracer substance to the fuel that provides high fluorescence intensity while not changing the properties of the fuel. Consequently, Triethylamine was chosen as a tracer to be mixed with hydrogen at 200 ppm, which allows it to be used up to a maximum pressure of 200 bar while still providing a strong LIF signal. Due to the excellent linearity of the signal to the local air/fuel-ratio it was possible to develop a method for the calibration of the images in order to compensate for inhomogeneities of the laser beam and staining of the optical access and to ultimately allow a quantification of the fuel distribution. The results are images scaled on air/fuel-ratio which can be used for a direct optimisation of mixture formation processes and the validation of CFD-models. For the analysis of the combustion process the method was adapted with two different approaches. For homogeneous charges a new method was applied by marking the flame front using the tracer within the fuel, so that both are burned together. However, as this method is limited to measurements with a homogeneous distribution of tracer within the measured volume, an alternative technique had to be applied for the measurement of stratified charges. In this case, a direct visualisation of the flame front was achieved by exciting the OH-radicals formed during combustion. As this method has significantly increased demands on measuring equipment and is more time consuming, both methods are used in parallel on specific measuring tasks.