Li, L., Yang, Y., Zhao, F., Zhang, Z. et al., "Application of Near-field Acoustic Holography to Low Temperature Engine Start-up Noise Issue Resolution," SAE Technical Paper 2015-01-1666, 2015, doi:10.4271/2015-01-1666.
An issue of engine squealing in low temperature range (around −25°C) right after start-up emerged for a significant number of vehicles in a vehicle program. The earlier effort had focused on typical common culprits such as the pulley-belt systems etc. However, much effort, by subjective listening and guestimating hardware replacement, yielded no fruitful results for more than a year. The application of near-field acoustic holography on top of the engine in a climate chamber quickly identified several noise source locations. Further noise source identification effort on top, front and left side of the engine pinpointed the top three noise source locations: the idler pulley and water pump on front, the air compressor on right side, and the air intake throttle on top. Then a series of experiments were subsequently conducted, leading to a conclusion that the idler pulley was the source of the issue. Finally, investigation of the pulley system confirmed that the insufficient lubricating capacity at low temperature was the root cause. By switching to a type of lower base oil viscosity lubricating grease characterized by lower rotation torque in cold temperature conditions, the issue was successfully resolved.