Materials selection for dynamic flow control application in turbocharger turbines

This paper investigates material candidates for use in a turbocharger turbine technology known as the active control turbocharger (ACT) which is a distinct technology to the Variable Geometry Turbine (VGT) for turbochargers but broadly based on this technology. This concept involves the use of an actuated nozzle mechanism that is oscillated to provide a more active change of the turbine inlet area to the turbine resulting in response to incoming instantaneous exhaust gas flow pulsating characteristics to provide greater extraction of exhaust gas pulse energy. Careful materials selection is required for this highly dynamic application to overcome the creep, fatigue, oxidation and high temperature challenges associated with the diesel engine exhaust conditions to which this technology is exposed to. The investigation of materials suitability for this application was conducted for steady and transient flow conditions. It was found that the vane undergoes cyclical loading at a maximum stress of 58 MPa for 109 cycles of operation at an inlet temperature of 800oC and pressure of 240 kPa. The vane experiences maximum stresses in the closed position which occurs at a vane angle of 70o. It has been found that the implementation of ACT technology is possible using currently available materials. Using the information obtained from the transient analysis, a material selection process was developed to incorporate the specific application requirements of the ACT application. A two tiered weighting decision process was applied; first to analyse the relative importance of various material properties to each application requirement and then to the properties of individual materials. Materials Nimonic 90 and IN X750/751 obtained the highest overall scores from the selection process and were shown to be capable of withstanding the creep requirements to a minimum safety factor of 2, a failure mechanism of primary concern to the high temperature application. Nimonic 80A, although receiving a final rating 8% lower than Nimonic 90, also showed promising potential to offer a solution, with superior corrosion properties to both Nimonic 90 and IN X750/751. In addition to the specific results, a significant contribution of this work has been in providing a foundation for future numerical and material selection analyses for ACT development