Development of a Measuring System for the Visualization of the Oil Film between the Piston and Cylinder Liner of a Gasoline Engine

The design of cylinder liners, pistons, and piston rings is subject to different conflicting goals. In addition to a loss-free seal of the combustion chamber, sufficient oil must be present between the friction partners. Both the reduction of piston assembly friction and the minimization of oil consumption are crucial to achieve the strictly defined CO2 and emission standards. To master this challenge and find the best compromise requires a lot of system-specific know-how. The automobile and engine manufacturers focus mainly on friction-reducing measures, which are analyzed with different measuring methods such as the floating-liner method, the strip-down method, or the instantaneous indicated mean effective pressure (IMEP) method. However, the interpretation of the results and the development of realistic simulation models lacks information about the oil film behavior and the film thickness. In order to gain this missing knowledge, instruments for oil film visualization and oil film thickness measurement have to be developed. In the FVV-project "Piston ring oil transport - Glassliner", the two-dimensional laser-induced fluorescence method (2D-LIF) is used to visualize the lubricating oil film between the piston and the cylinder liner. Green light emitted by a double-pulsed Nd:YAG laser is expanded by sheet and collimator optics. A dichroic mirror next to the optically accessible cylinder housing with integrated sapphire window reflects the expanded laser beam onto the oil film. The laser beam causes a fluorescent dye mixed in the lubricant to emit light at a longer wavelength, i.e. red. Due to this wavelength shift (Stokes shift), the fluorescent light can be separated with an optical narrowband filter and detected by a camera. During stationary operation of a single-cylinder gasoline engine, oil accumulation is observed throughout the piston assembly. The results reveal the oil flow to the combustion chamber changes significantly with engine speed and load. When the throttle closes during high engine speed, a large amount of oil enters the combustion chamber. Thus, the transition from fired-to-motored engine operation is associated with increased oil consumption.