High-speed PIV and LIF imaging of temperature stratification in an internal combustion engine

High-speed laser induced fluorescence (LIF) thermometry is combined with particle image velocimetry (PIV) to measure the temporal evolution of the spatial unburned gas temperature distribution in a motored spark-ignition (SI) optical engine. Single-line excitation of toluene and subsequent two-color emission detection is employed for LIF thermometry. Precision uncertainty is assessed pixel-wise and found to be ±5 K at 295 K and ±29 K at 550 K, but decreases by 34% (i.e., ±19 K at 550 K) when spatially averaging the LIF signal over a 10 × 10 pixel² region. The polytropic temperature relation is used to correlate LIF ratio with temperature to calibrate the in-cylinder gas temperature measurements. Instantaneous temperature images and temperature PDFs show homogeneous temperature distribution during compression, while significant temperature inhomogeneities from entrained colder gases are captured during early expansion. Simultaneously acquired PIV and LIF images illustrate the evolution of the cold gas distribution during expansion. The images presented reveal the capability of high-speed toluene-LIF thermometry combined with PIV in an engine to capture the 2D temperature distribution and track structures of colder temperatures.