Crank-Angle Resolved Oilfilm Thickness Measurement Between Piston Ring and Cylinder Liner in a Spark Ignition Engine

A new measuring technique has been developed in order to determine the oilfilm thickness between piston ring and cylinder liner in a combustion engine by use of laser-induced fluorescence (LIF) technique. In clear contrast to further investigations, where optical probes have been mounted at a fix position in the cylinder liner allowing single measurements at defined times of the piston stroke, only, the optical probes used in the presented work are mounted in the piston and the piston ring. Fiber optics, led via a grasshopper-link, carry the exciting laser radiation (λ = 488 nm) to the probes and backwards the fluorescence radiation (λ ∼ 540 nm) to a photomultiplier. Thus, the developed measuring system allows a continous, crank-angle resolved measuring of the oilfilm thickness. As the LIF-technique is a relative measuring technique, calibration work, especially in the combustion engine, had to be done. Using eddy current sensors at a fix position in the cylinder liner the oilfilm in a single point could be determined. Grooves of defined depth in the cylinder liner provided the fluorescence signal amplification in each stroke so that adverse influences of oil pollution or temperature could be eliminated. A method has been developed to eliminate transmission influences contributed to the grasshopper link system. The measuring technique adapted to a single-cylinder SI engine has been tested under motored and fired conditions. Parameters under motored conditions were engine speed, oil- and coolant-temperature and throttle position. Experiments under fired conditions (part load) have been done varying oil and coolant temperature. Correlations between oilfilm thickness and HC-emissions under motored conditions have been found. Incylinder pressure, oil-temperature and engine speed are the most influencing parameters.