This paper describes the development and testing of a measuring system which can visualize lubricating conditions in the tribological system outlet valve guide - valve stem in a real fired engine with petrol as well as with compressed natural gas (CNG) combustion using a laser induced fluorescence spectroscopy (LIF) based method. In order to make optical measurements possible, a patented prototype cylinder head based on a bifuel engine from serial production was developed and built. This special head has two bore holes and a transparent valve guide made of sapphire. To increase the strength of the fluorescence signal the two tracers Pyrromethene 567 and Rhodamine 640 based on research at the MIT were added to the oil. As it was needed to quantify the thick­ness of the oil film in the tribological system an in-situ calibration method using valves with grinded furrows was developed. The complete measuring system was thoroughly tested on a cylinder-head test rig. After this the tested and adjusted system was transferred to a fired engine test bench where the lubrication behaviour as well as the oil film thickness was investigated qualitatively and quantitatively with engines running on petrol and on compressed natural gas. This was carried out under the variation of three different valve seal lips. By means of this set up, the differences in wear behaviour between petrol and CNG combustion could be explained.weiterlesen