Foil Air Bearings for High-Temperature Turbocharger Applications

Within the realm of turbocharging, there are a number of different design challenges that influence the design process on both large-scale marine applications and smaller-scale commercial automobile applications.  From aerodynamic loads to dynamic control systems to rotor dynamics and bearing challenges, turbochargers represent a special subset of turbomachinery that requires complex and integrated solutions.  Turbocharger rotors specifically, have unique characteristics due to the dynamics of having a heavy turbine and compressor wheel located at the overhang ends of the rotor. The majority of turbocharger rotors are supported within a couple floating-ring oil film bearings.  In general, these bearings provide the damping necessary to support the high gyroscopic moments of the impeller wheels.  However, there are several disadvantages of working with these oil systems that have allowed different technologies to start to surface for these turbomachines.  With the floating-ring oil models, varying ring speed ratios and oil viscosity changes significantly influence the performance of the rotor dynamic model.

Dan blog bearing for turbochargers
Figure 1 – Floating-Ring Bearing Model for a Turbocharger

The application of oil-free bearings have started to emanate due to the overall consistency of their performance and the minimized heat loss associated with air as the damping fluid. Studies on these bearing types for turbomachinery applications are neither trivial nor unique, as they have seen plenty of exposure within the commercial and military aircraft industries within turbo compressors and turboexpanders. However, the success of these specific applications are due to the fact that these turbomachines operate with light loads and relatively low temperatures. The main design challenges with foil air bearings are a result of poor rotor dynamic performance, material capabilities, and inadequate load capacities at high temperature/high load applications.

Foil Air Bearing
Figure 2 – Foil Air Bearing

Foil air bearings operate based on a self-acting hydrodynamic air film layer during normal operation, but they exhibit serious wear on start up and shut down if not properly attended to. Prior to developing a gas film on start up, these bearings must handle the sliding that occurs between the rotor and the inner surface of the bearings. For this reason, solid lubricants like polymer foil coatings were considered for these bearings. Polymer coatings have a serious temperature restriction which do not allow them to be considered for high-temperature applications above 300 °C. Different chrome oxide based coatings have shown greater performance at higher temperatures. Initial testing of these coatings showed significantly poor performance at lower temperatures of 25 °C and difficulties with adhesion through repeated thermal cycles. However, NASA has developed a new high temperature PS400 formulation of this coating that performs well under different load conditions and between the temperature range of 25 °C and 650 °C. Essentially, the viability of these bearings within the automotive market has become a reality with individualized bearing designs. The question now becomes whether the foil gas bearing manufacturers can penetrate the market from a larger-scale and create a standard for these turbocharger setups to run free of oil altogether. To learn more about the simulation of both floating-ring oil film bearings and foil air bearings using the SoftInWay platform, please visit: http://www.softinway.com/software-applications/bearing-design/

References:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000004303.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090033769.pdf

 

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