Imaging of flame propagation and temperature distribution in an all-metal gasoline engine with endoscopic access via anisole fluorescence

Abstract

Laser-based optical diagnostic tools are widely used to investigate in-cylinder processes in internal combustion engines. For laser-induced fluorescence (LIF), many tracers have been used in the past. Recently, anisole has been characterized spectroscopically for engine-relevant pressures and temperatures and emerged as a potentially advantageous alternative to more commonly used tracers in the past due to its photo-physical properties. Its high fluorescence quantum yield and large absorption cross section result in high signal intensity. This is particularly beneficial for endoscopic imaging systems, which typically have worse light collection efficiency than traditional imaging systems in fully optically accessible engines with transparent liners. In this work, we exploited the strong anisole LIF signal in two single-shot experiments: to image flame propagation, and the instantaneous gas-phase temperature during compression stroke and gas exchange process. Measurements were performed in a production gasoline engine modified for endoscopic optical access via an advanced UV endoscope system. First, LIF of anisole was compared to that of toluene during compression stroke. Anisole LIF yields a much higher signal-to-noise ratio and better image quality with lower tracer concentrations. Due to the higher signal of anisole LIF, small structures of the turbulent flame burning into the anisole/isooctane mixture were well visible after ignition. Second, the red-shift of the anisole fluorescence spectrum with increasing temperature and oxygen partial pressure was exploited for ratiometric temperature measurements based on single-shot images. The available spectroscopic data were used to develop several signal ratio models, which were calibrated in situ using a heated tracer/bath gas mixture introduced inside the combustion chamber. The calibrated signal ratio models were then extrapolated to the engine-relevant ranges. Models with two-step exponential interpolation show better agreement with the adiabatic temperature than linear or 3D surface exponential interpolation. The gas-phase temperature images based on single shots were obtained using one selected model, showing a near uniform and a stratified temperature distribution during the compression stroke and gas exchange process, respectively.