Optical diagnostics for real-time combustion control, especially based on emission spectroscopy, have gained considerable importance, both with power plants and internal combustion engines.
A key element is the optical window. It must withstand the harsh and transient conditions in the combustion chamber and also show a low propensity to deterioration and fouling. Combustion chamber deposits (CCD) may rapidly weaken and eventually destroy the transparency of the window, hence inhibiting online optical diagnostics.
This work is about the experimental investigation of different window substrates and their mounting possibilities in combustion chambers. A specially prepared internal combustion engine running on liquid fuels was used. The window was mounted in the cylinder head next to the spark plug. The infrared-transparent media quartz, sapphire, ZnSe and ALON™, which are commonly used window materials, were compared experimentally. The influence of the window temperature and the effect of various fuels (gasoline, MTBE and isooctane) and of the lubricant were studied. The initial phase of deposit formation was investigated. Gravimetric analysis, transmission measurements and elementary analysis of the deposits are discussed.
It was found that the window temperature is the main parameter which influenced the deposition rate of combustion residues. Carbonaceous deposits were found to cease above a 350°C outer window temperature. CaSO4, stemming from calcium sulfonate in the lubricating oil, was found to be the most important inorganic constituent in the deposits formed on the windows. Under transient operational conditions, the kind of fuel mixture as well as the substrate can determine the transmissivity of the windows.
Strategies of how to prevent fouling of optical windows in combustion chambers are discussed.