23rd International Colloquium Tribology - January 2022 381 Investigation into the effect of lubricant viscosity in engine bearing-film-thickness-using-embedded-ultrasonic-transducers Henry-Brunskill Corresponding author: henry@pktopk.co.uk Peak to Peak Measurement Solutions ltd., Sheffield, UK Andy-Hunter Peak to Peak Measurement Solutions ltd., Sheffield, UK Am-Ho-Sung- Hyundai Motor Company, Seoul, S. Korea Junsik-Park- Hyundai Motor Company, Seoul, S. Korea Rob Dwyer-Joyce Leonardo Centre for Tribology, University of Sheffield, Sheffield, UK 1. Introduction With the introduction of the new Euro 7 emmission standard, the automotive industry is under increased pressure to reduce CO2 emissions to lessen the impact on global warming and climate change. This has led to a trend in reducing viscosity of the engine oil to improve fuel economy and emissions. In doing so, there is a risk that the engine bearing lubricant films become too thin and scuffing/ film breakdown can occur, leading to decreased service intervals and in some cases, engine failure. In this work, non-invasive ultrasonic sensors were installed in a fired engine dyno test to measure the film thicknesses in the main and big-end bearings. The results were compared to models. 2. Ultrasonic-Measurement-of-Lubricant-Film- Thickness The micro-ultrasonic sensors are mounted on the rear of a component and a high-frequency pressure wave is generated that reflects of the lubricated interface. The reflected wave amplitude is proportional to the stiffness of the interface, K, which can be related to the lubricant film thickness, h, via and a series of know parameters: the transducer frequency, ω, the lubricant bulk modulus, B, and the acoustic impedance of the materials either side of the contact, z1, z2. More detail on the method is given in [1, 2]. A small form factor ultrasonic DAQ hardware system was developed and implemented to capture ultrasonic data 14kHz per channel. 3. Engine,-Instrumentation,-and-Test-Details 3.1 Engine Details A diesel engine with the following specification was used for testing: • Displacement (cc): 1598 • Engine speed (rpm): 750~5200 • Rated power (ps): 136@4000rpm • Maximum torque (kgfm): 32@1750rpm 3.2 Main Bearing Film Thickness Instrumentation A linear array of 4 off 0.7 × 2.5mm ultrasonic transducers was mounted on the rear surface of the 2nd and 3rd unmodified main bearing shells for the measurement of lubricant film thickness. At each transducer location, a K-type thermocouple was also bonded to the bearing. Small holes were machined in the bed-plate to eject the 0.4mm micro-coaxial and 0.6mm thermocouple cables. Images describing the instrumentation can be seen in Figure 1. 382 23rd International Colloquium Tribology - January 2022 Investigation into the effect of lubricant viscosity in engine bearing film thickness using embedded ultrasonic transducers Figure 1: Photographs of the instrumented bearing shell and modified bed plate and a sketch showing the measurement concept. 3.3 Big End Bearing Instrumentation A linear array made up of 5 identical transducers and 2 in-built thermocouples were embedded in a slot machined in the 1st big end journal pin in the crankshaft. The slot was positioned at the estimated angular position so that the transducers where measuring at the point of the minimum film thickness considering the complex engine cycle. A 3mm hole was machined in the centre of the shaft to eject the transducer and thermocouple cables. A slip-ring was mounted on the end of the crankshaft to eject the rotating signal cables. Images describing the crankshaft instrumentation can be seen in Figure 2. Figure 2: Photograph of the instrumented big-end pin on the crankshaft shell and a sketch showing the measurement concept. 3.4 Fired Engine Test Conditions Tests were conducted to establish the oil film thickness between different lubricants and engine operating parameters. During testing, the engine parameters were modified and held until steady state thermal conditions were reached before sensor measurements were logged. Three different lubricants were used for testing. Table 3 shows the different test conditions to be presented in this paper. Speeds (RPM): 1000, 1500, 2000, 2500, 3000, 3500, 4000 Loads (Nm): Unfired, 50, 100, 150 Nm, Full load (exact Nm varies by test condition) Lubricants: 0W20, 0W30, 0W16 4. Results This section shows some example results from the different sensor configurations. 4.1 Main Bearing Measurements An example main bearing lubricant film thickness measurement is shown in Figure 3. Also shown are the simulated results modelled using Gamma Technologies GT-Suite v.2016. Figure 3: Example measured and simulated main bearing lubricant film thicknesses with 0W20 lubricant, 4000RPM, and full load. Figure 4: Measured and simulated main bearing film temperatures for 3 lubricants at full load, and various speeds. 4.2 Big End Bearing Measurements An example big end minimum lubricant film thickness measurement is shown in Figure 5. The calculated total power loss for two different lubricants can be seen in Figure 6. 23rd International Colloquium Tribology - January 2022 383 Investigation into the effect of lubricant viscosity in engine bearing film thickness using embedded ultrasonic transducers Figure 5: Example measured big-end lubricant film thicknesses with 0W20 lubricant, 100 Nm, and various speeds. Figure 6: Calculated power loss for two lubricants at 100 Nm and various speed conditions. 5. Discussion-and-Conclusion In this project, the ultrasonic lubricant film thickness measurement method has been successfully demonstrated in automotive engine main and big end bearings. The results consistently showed that lower viscosity oils reported thinner films and reduced total power loss, proving that viscosity reduction is one viable approach to work towards meeting the new regulatory standards. None of the results reported reached critically thin levels below the combined component surface roughness (approx. Ra = 0.35 µm). The measured main bearing results consistently reported good correlation with the simulations, although in all cases, slightly thicker films. This is most likely because the sensors were in fixed positions and the minimum film location is dynamic. Additional work could be performed to extrrapolate minimum films To increase confidence in new lubricants, on-road duration testing is planned. References [1] Dwyer-Joyce R. S.; Drinkwater B. W.; Donohoe C. J.: The measurement of lubricant-film thickness using ultrasound. In: Proc. R. Soc. Lond. 459957- 976. 2003 [2] Beamish, S. L.; X. Brunskill, H.; Hunter, A.; Dwyer-Joyce, R.: Circumferential film thickness measurement in journal bearings via the ultrasonic technique. In: Tribology International, 148/ 2020