The fuel film thickness and temperature on the piston crown of a direct-injection spark-ignition (DISI) engine were measured using a fiber-based laser induced fluorescence (LIF) method. The engine investigated employed a wall-guided swirl-type system using a high-pressure swirl injector impinging onto the piston crown. The fuel used was isooctane with a small amount of a fluorescent liquid dopant. The measured fluorescence intensity was transformed to the fuel film thickness by way of the equations based on photophysics and the fiber optic properties. However, the fluorescence of the fuel mixture showed a strong dependency on the fuel temperature and this information was needed in the fuel film thickness calculation. The fuel film temperature, which may differ from the piston surface temperature, was measured by using a fiber-based fluorescence thermometry method.
Engine tests were performed for motored and fired conditions under the late injection stratified mode. The results of the fuel film temperature measurement showed that the mean fuel film temperature follows the piston surface temperature and the convection heat transfer from the compressed hot air directly affected the mean fuel film temperature during the compression and expansion stroke. The fuel film thickness measurement results showed that the boiling point of the liquid dopant should be much higher than that of the main fuel to meet the co-evaporation condition. The fuel film persisted during the expansion stroke and started to evaporate actively from the exhaust valve opening crank angle for both motored and fired conditions. The fuel film thickness from the fuel injection to just before the occurrence of pool fires was the same level for motored and fired engine conditions although the piston temperature was higher for the fired condition. The spark ignition and the main flame before the pool fires do not affect the fuel film thickness. The duration and thickness of the fuel film was strongly affected by the boiling point of the fuel. The fuel film evaporates quicker for a fuel having a lower boiling point.