23rd International Colloquium Tribology - January 2022 199 Radioactive Tracer Engine Wear Test Development Peter M. Lee, Gregory A.T. Hanson Southwest Research Institute, San Antonio, Texas, USA. 1. Introduction Engine wear has been a concern since the internal combustion engine was first developed. As internal combustion engines became main stream and vehicle manufacturers started selling them in their millions each year, an industry was established around testing lubricants for, amongst other things, wear protection. Southwest Research Institute (SwRI ® ) is part of this Industry, testing lubricants through engine Sequence tests to qualify them for lubricant standards. In parallel with this, SwRI ® has developed research capabilities to assist lubricant, additive and OEM clients in understanding wear phenomena in engines. SwRI ® developed Radioactive Tracer Technology (RATT ® ) for real time engine wear evaluations. This involves irradiating engine components using either bulk or surface layer activation using radioactive sources. The decaying activated parts release isotopes, which can be detected by radioactive detectors. Once the engine is assembled, the engine components wear during engine operation, becoming wear debris in the oil and releasing the isotopes. The level of isotopes present in the oil is then detected, and the strength of the signal for each isotope correlated with the mass of wear material in the oil. 2. Testing A 2.0L Ecoboost direct injection gasoline engine with activated valvetrain, top piston rings, liner and turbo thrust plate was used, and conditions investigated to further understand the technology and guide future RATT ® engine wear testing programs. The project team had three questions: 1. What is the correct data collection time for the RATT ® signal? 30, 120 or 300 seconds. 2. What effect does engine build mid test have on test repeatability? 3. Is there a ‘settle’ time for the engine to return to ‘normal’ wear conditions after a severe engine cycle that has created high wear events? In addition, the team had interest in measuring valvetrain wear during both hot and cold oil temperature engine cycles in preparation for future engine test development work on modern engine architectures. Also, with continued drive towards reduced viscosities, SAE 0W-16 and SAE 0W-08 oil was used to investigate the effects on wear using viscosities lower than the recommended engine lubricant viscosity of 5W-30. Table 1 shows the test lubricants used in this project. Well established operating conditions for this engine were used, as shown in Table 2. Table 1: Test Lubricants, Viscosity and Additive package Test Lubricant Viscosity Add. Pack 0W-08 0W-08 A 0W-16 0W-16 A 5W-30 Medium Wear (MW) 5W-30 A 5W-30 Low Wear (LW) 5W-30 B 5W-30 High Wear (HW) 5W-30 C 200 23rd International Colloquium Tribology - January 2022 Radioactive Tracer Engine Wear Test Development Table 2: Engine operating conditions; name and description Operation Condition Description Stop-Start, Very Cold (SS_VC) Start, immediate hard acceleration at high load to moderate speed for 10 sec, drop to low load/ moderate speed for 20 sec, stop engine, soak off 1 min, repeat for extended period of time. Very cold engine temperatures. Boundary Lubrication (Boundary) Start at moderate load and moderate engine speed. Hold throttle while slowly ramping engine speed to idle. Warm engine temperatures. Transient Speed: Low Load, Cold (TS_LL_Cold) Ramp engine speed from medium to high RPM at low engine torque and cold engine temperatures. Transient Speed: Low Load, Hot (TS_LL_Hot) Ramp engine speed from medium to high RPM at low engine torque and hot engine temperatures. Shortened Sequence IVA (IVA) Ramp engine speed from idle to 1500 RPM at constant torque. Warm engine temperatures. 3. Results During each sample time (30s, 120s and 300s) the oil is constantly flowing through the detector. As such, the wear measurements taken are an average of that time. Significant differences were observed comparing results with the standard deviation obtained over the sample times. Most reliable results are obtained when the standard deviation is the lowest and the wear rates are significantly higher than the standard deviation. 120s was the best for the camshaft, top ring face, liner and turbo thrust plate wear. For the top ring side wear, 300 seconds gave the best results. Only one sample rate can be used, therefore, 120 second sampling time was selected for all future work. Mid matrix the engine was removed from the test stand, disassembled and reassembled twice with the purpose of observing any change in wear rate due to the rebuild. It was observed that immediately following the rebuild, a spike in wear occured for the camshaft and top ring face and that it took only 4 hours of running to re-establish its pre teardown wear rates. Tests were undertaken on the High Wear oil to investiage the effects of severe high wear engine cycles on subsequent cycles. The majority of the components showed no significant change in wear when comparing the before and after severe wear test. The few that did, showed a temporary reduction in wear. A 1h run-in stabilized this. Camshafts primarily operate in the mixed to boundary lubrication regime. In these regimes, there is little lubricant and therefore, bulk viscosity has little impact on wear. In these regimes, it is the wear resistance of the additive package that is of greatest importance. In Figure 1, it is important to remember that we are seeing the effects of two things on the results: The same additive package with three different viscosity grades and the same viscosity grade with three different levels of wear protection. Figure 1: Camshaft wear for each test cycle and lubricant The effect of additive package can clearly be seen in the IVA, SS_VC and TS_LL_Hot cycles with the expected low wear oil being low, medium wear oil being in the middle and the higher wear oil being noticeably higher. This shows that it is possible to develop a camshaft wear test that will differentiate between test lubricants for modern architecture engine. The current IVA and IVB engine tests were developed as low temperature wear tests in the belief that the additives are not activated at the lower temperatures. However, it is interesting to note that in this work significantly more camshaft wear was observed during the hot engine cycle. Both the Boundary and TS_LL_Cold cycles did not show the anticipated relationship between wear and additive package. For the boundary cycle, this may have been due to the fairly low levels of wear being measured vs. the average standard errors. For the TS_LL_Cold cycle, it may have been due to the increased viscosity (caused by cold oil temperatures) of the lubricant forcing the contact more into mixed regime, where additive package will have less effect. The 0W-08 lubricant was run through the engine as the last five engine runs. This engine was designed to operate on a 5W-30 lubricant and therefore, this was not expected to go well, hence it being the final part of the text matrix. Tests were run in order of anticipated engine wear severity from the least to the most. The turbo failed during the final test. The turbo failure may have been due to the low viscosity of this lubricant or the fact that the engine had 23rd International Colloquium Tribology - January 2022 201 Radioactive Tracer Engine Wear Test Development already run the 97-hour run-in and 48 test cycles or, more likely, a combination of both. In general, comparing the 0W-08 wear results with other wear results, the higher wear values anticipated were not observed. 4. Conclusion Five distinct conclusions resulted from this work: 1. 120s was found to be the most statistically accurate sample time for RATT ® wear measurements. 2. The effect of engine tear down and rebuild on differences in preand post-test wear did not have a significant effect. Results showed a 4h post build run-in sufficient to stabilize wear rates. 3. Results showed little evidence to support concerns that severe engine cycles could have an effect on wear during the next engine test cycle. 4. Hot and cold test cycles showed camshaft wear could be produced and measured on this engine. It was observed that significantly more wear occurred during the hot engine cycle. 5. The engine operated successfully on SAE 0W-08 lubricant. Lower wear was measured than anticipated. This work has maintained confidence in the RATT ® engine wear technology and will guide future RATT ® engine wear testing programs undertaken at SwRI ®