Thickness dependent variations in surface phosphor thermometry during transient combustion in an HCCI engine

Phosphor thermometry is a semi-invasive measurement technique which is commonly used for temperature determination in combustion applications. Surface temperature measurements using thermographic phosphors rely on the assumption that the phosphor layer is thin enough in order to adopt the surface temperature non-intrusively. This work compares the temperature information, recorded from two opposing sides of phosphor films, coated on a translucent part of the combustion chamber wall inside a car engine. The film thickness was varied between 5 and 72μm and two different phosphors were studied; CdWO₄ and La₂O₂S:Eu. For both phosphors, the results showed no significant differences. Being subject to unsteady heat transfer during fired engine operation, phosphor coatings above 20μm in thickness experienced a significant temperature gradient between the front- and the backside of the coating, whereas thinner layers did not seem to be affected within the limits of measurement accuracy and precision. Cycle-to-cycle variations of the global gas temperature were not found to correlate with phosphor temperature variations. However, a strong temperature correlation between opposite sides of the phosphor coating was observed for film thicknesses below 20μm during engine cycle-to-cycle variations.