24th International Colloquium Tribology - January 2024 191 LIF Signal Calibration for Bench Simulating Experiments and Engine Oil Film Thickness Investigations Polychronis S. Dellis 1* 1 National Technical University of Athens, School of Mechanical Engineering, Mechanical Design and Automatic Control Section, Athens, Greece * Corresponding author: pdellis@mail.ntua.gr 1. Introduction The Laser Induced Fluorescence (LIF) measurement technique when carefully applied to engine cylinder liners or flat reciprocating simulation mechanisms is able to produce oil film measurements data that can, either be compared with direct simultaneous capacitance oil film thickness (OFT) measurements or when calibrated appropriately, provide a picture of the oil film in between and under the piston-rings. For the purpose of calibration, insitu dynamic measurements or static calibration techniques have been applied in the past [1, 2]. Bench calibration being less prone to thickness errors as its concept relies on a micrometer, meticulously modified to be used as the measurement reference, whereas dynamic calibration offers significant advantages, as it takes into account local temperatures at the measurement point, lubricant degradation, i.e. issues that affect fluorescence [3]. This study is focused on presenting the two calibration methods, propose a new dynamic method and how is temperature affecting the calibration coefficient. Eventually, light is shed on the inconsistencies found between the proposed methods. 2. Background Engine oil consumption reduction is an ever-increasing trend that requires controlling of the OFT in between and under the piston rings. The combination of fibre optics and LIF for measuring OFT is a non-intrusive method based on the emission of photons during the relaxation process following the initial excitation of the substance molecules with a light source,that is the blue laser light. Spectral discrimination, achieved by appropriate optical filtering, is giving the intensity of the fluorescence that is related to the OFT [3]. 2.1 Experimental Techniques - Previous Applications of Static and Dynamic Calibration Ring profile fitting with known film thickness taken from the capacitance measurements was used to calibrate the LIF signal. Temperature can bring about significant changes in the viscosity of engine oils and their fluorescence spectrum. Previous attempts were performed to achieve static calibration with a high-resolution micrometer [2, 4]. The optical fibre was installed to the modified micrometer anvil that accommodates the fibre and supplies a continuous oil flow between the anvil and the micrometer spindle. Thus, the OFT flowing through the interrogation zone could be read from the micrometer scale respectively. The area of interest was fully flooded with oil as it is pumped from a small reservoir, a condition which is important for very thin oil films ranging from 0 to 5 μm, where the photobleaching effects might have a more pronounced effect in the quantum yield [2]. Brown et al [5], proposed an in-situ calibration method with grooves etched on the piston-ring surface. The static calibration method showed very low repeatability for the data points corresponding to an OFT of 10 μm. Obtaining calibration coefficients of high accuracy in the region 0 to 10 μm is vital for improving the accuracy of the LIF system [2]. Experiments with an immersed silica block incorporating grooves of known depth overcame this deficit [2]. Nakayama et al [6], measured OFT on engine bearings using LIF and presented a static calibration apparatus. The researchers also presented a dynamic calibration technique. Figure 1 shows the device that used a shaft with two thickness gauges pasted on its surface having a known thickness of 48 μm and Figure 2 shows the OFT tests. Figure 1: Dynamic calibration using thickness gauges [6] Figure 2: OFT by dynamic calibration method [6] 2.2 Dynamic Calibration with Grooves on Piston-Ring For the purpose of dynamic calibration, a piston-ring specimen was modified to be used for the experimental set-up of the simulating single-ring test rig according to the specifications of these type of measurements (microns range, flat reciprocating liner and steady piston-ring). The dynamic calibration is based on etching a groove or series of grooves of known depth on the surface where the optical fibre travels above. The best surface finish possible was desired to be achieved with the spark eroding machine (Electro-Discharge Machining, EDM) with the groove depth ideally being between 15 and 25 μm and the surface roughness R a <1 μm [7]. In Figure 3, the first attempt shows the groove depth from 0 to 65 μm. 192 24th International Colloquium Tribology - January 2024 LIF Signal Calibration for Bench Simulating Experiments and Engine Oil Film Thickness Investigations Figure 3: First unsuccessful attempt to etch groove on the surface of the piston-ring [7] The electrode used for spark eroding was specially manufactured Cu-Cr-Zr and to enhance the accuracy of the manufacturing process the technique was applied for longer periods of time to minimize the risk of failed attempts. Figure 4 shows the measured 3-D surface profile of the groove. Figure 4: 3-D meshed axonometric of groove [7] Results were derived from tests at an average oil temperature of 40-°C. Test oil coded 3B was supplied by CASTROL, the properties of which are shown in Table 1. Laser power was set at 50 mW, 0.39 kV photomultiplier (PMT) voltage. Table 1: Tested lubricant properties [7] CASTROL Code 003B SAE Grade 0W-30 Viscosity Index 182 V 100 (cSt) 12.16 V 40 (cSt) 68.93 HTHS (mPa s) 3.30 Polymer Castrol Code A Base Fluid Poly alpha olefin V100, V40: Kinematic Viscosity at 100- °C, 40- °C HTHS: High Temp High Shear viscosity The dynamic coefficient is derived when LIF and groove data are directly superimposed. Figure 5 shows a schematic of the groove area over which the optical fibre travels. Five surface roughness profiles were averaged so that a mean derived value would correspond to the areas over which the fibre passes. In Figure 6 the LIF points vs the averaged groove data are superimposed. After this representation, the dynamic calibration coefficients were derived and a temperature parametric study followed. Figure 5: The groove with the blue - highlighted line shows the area over which the optical fibre travels [7] Figure 6: Matching of groove and averaged LIF data [7] The shortest stroke available was set (5 mm) for the experiments so that as many data as possible could be acquired. When the optical fibre travels on top of the specially machined groove, the highest number of data points will be acquired from the data acquisition system over the groove width measured at 1,525 mm. The averaged data will represent the highest accuracy possible. 3. Conclusions - Matching LIF and surface roughness data is the key point of the dynamic calibration technique. - A higher calibration coefficient is derived for high temperature parametric testing. - Considerations such as background noise, groove flooding, PMT voltage, absolute OFT (less than 5 μm) and OFT to voltage ratio affect the measurements’ accuracy. References [1] Arcoumanis C., Duszynski M., Lindenkamp H. and Preston H., “Measurements of oil film thickness in the cylinder of a firing diesel engine using LIF”, SAE 982435, 1998. [2] Duszynski M., “Measurement of Lubricant Film Thickness in Reciprocating Engines”, PhD thesis, Imperial College of Science, Technology and Medicine, 1999. [3] Dellis, P. “Aspects of lubrication in piston cylinder assemblies”, PhD Thesis, Mechanical Engineering Department, Imperial College London, April 2005. [4] Pyke E. A., “Investigation of Piston Ring Lubrication Using Laser Induced Fluorescence”, PhD thesis, Imperial College of Science, Technology and Medicine, March 2000. [5] Brown M. A., McCann H. and Thompson D. M., “Characterisation of the Oil Film Behaviour Between the Liner and Piston of a Heavy-Duty Diesel Engine”, SAE Paper 932784, 1993. [6] Nakayama K., Morio I., Katagiri T. and Okamoto Y., “A Study for Measurement of Oil Film Thickness on Engine Bearing by using Laser Induced Fluorescence (LIF) Method”, Central R&D, Daido Metal Co., Ltd, SAE 2003-01-0243, 2003. [7] Dellis P., “An Attempt to Calibrate the Laser Induced Fluorescence Signal used for Oil Film Thickness Measurements in Simulating Test Rigs”, Tribology in Industry, 37 No. 4, 525-538, 2015.