Thermal stratification in an internal combustion engine due to wall heat transfer measured by laser-induced fluorescence

Laser-induced fluorescence of toluene was used to image spatial fluctuations of gas temperature in an optically accessible engine. These thermal inhomogeneities develop due to wall heat-transfer and convection during the compression stroke. They are known to be important for slowing heat release when operating the engine in compression auto-ignition (CAI) mode. The engine had a pent-roof four-valve head typical for automotive spark-ignited engines and a flat-top piston with a window. Measurements were performed in the central vertical symmetry plane of the cylinder with the engine motored and fed with nitrogen. Toluene was seeded homogeneously into the intake gas. Fluorescence was excited at 248 nm and detected spectrally integrated. Toluene fluorescence decreases strongly with increasing temperature. Estimating the absolute in-cylinder temperature from isentropic compression along the measured pressure trace, we found the magnitude of this decrease in the engine to be consistent with literature data from heated flow-cell measurements. This calibration allowed for determination of the spatial fluctuations in temperature against the multi-cycle average temperature. Precision error, mostly from laser mode fluctuations, was between 1.6 and 4.0 K depending on crank angle. The results show that the temperature-fluctuation field transitions during the compression stroke from small, evenly distributed inhomogeneities to much greater fluctuations, mostly localized near walls, but convecting into the cylinder center very late in the stroke. There are subtle differences in the spatial structure of the near-wall fluctuation field between cylinder head and piston top. Compiled from the entire imaged area, at top dead-center the standard deviation of the fluctuations was 1.9% of the temperature differential between gas phase and wall, consistent with corresponding literature data. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.