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.