Surface Engineering of Turbomachinery Components to Meet Future Industry Trends
Novel Solutions for Materials Engineering
Increased demands are being placed upon turbochargers as a result of ever more stringent emissions regulations, a necessity for improved product performance and reliability and a requirement for a wider product operating range. Consequently, a greater level of stress is placed upon the materials from which turbochargers are manufactured. In order to achieve all of the necessary legislation and customer requirements, novel solutions to materials engineering challenges are being developed.
Two such challenges faced by the turbocharger industry are high temperature tribology and the corrosion and erosion environment generated by long route exhaust gas recirculation (LR EGR). Cummins Turbo Technologies has developed innovative materials engineering solutions using surface treatments applied to cost effective substrates, rather than utilising much more expensive, exotic alloys.
High Temperature Tribology
As a consequence of experiencing excessive vane and slot wear whilst in service and experiencing temperatures > 500 °C, a thermochemical diffusion surface engineering solution was developed for variable geometry systems.
Figure 1 - Variable Geometry System
Such technology was generated using a systematic approach to tribological testing and analysis, which was pioneered by Cummins Turbo Technologies as shown in Figure 2. After comprehensive evaluation of the problem using the expertise at Cummins Turbo Technologies, it was determined that during particular operating conditions the conventional uncoated nozzle and shroud plate experienced high levels of abrasive wear and elevated friction forces.
Figure 2 - Cummins Turbo Technologies’ Approach to Tribological Testing and Analysis
In order to solve the problem, a multi-disciplinary cross functional team at Cummins Turbo Technologies used the relevant customer and regulatory requirements to identify and develop the surface treatment concept. Using cutting-edge techniques, the fundamental mechanical and physical properties of the thermochemical diffusion treatment were determined. Subsequently, the surface treatment was characterised in terms of friction and wear performance using a tribometer, which simulated the variable geometry nozzle to shroud plate interface and associated operating conditions.
The surface treatment was tested on a turbocharger, on both gas stand and engine, with the results correlated to tribometer-based testing to ensure consistency of performance. This approach to the problem ensured that a robust engineering solution was identified and implemented into production; the resultant positive impact on nozzle vane wear performance can be observed in Figure 3.
Figure 3 - Comparison of Wear Performance between Untreated and Surface Treated Nozzle Vanes
Large Wear Volume
Long Route Exhaust Gas Recirculation
The turbocharger compressor stage, which includes the compressor wheel and cover (Figure 4), is subjected to a corrosive and erosive environment when an engine is operating LR EGR. The chemistry and pH of the condensate present in the compressor stage varies depending on engine operating conditions and the chemistry of the fuel. Furthermore, the architecture of the LR EGR system dictates the density and size of erosive particles to which the compressor stage is subjected.
Figure 4 - Compressor Wheel and Compressor Cover
In order to minimise the wear and corrosion observed on the compressor wheel and cover as a result of interaction with condensate and erosive particles, surface treatments were developed for the two components. Numerous concepts for both components were identified using a multi-disciplinary cross functional team at Cummins Turbo Technologies, using regulatory and customer requirements as a guideline.
Subsequently, the concepts were subjected to numerous in-house tests which were developed using the expertise of Cummins Turbo Technologies:
Such experiments utilised surface treated test bars and also components in order to correlate the data from fundamental laboratory to turbocharger-based testing. The acquired data was analysed using the expertise of Cummins Turbo Technologies and various advanced techniques, with the results input into a comprehensive scoring matrix in order to select the most appropriate solution for the two respective components.
The process developed for the compressor wheel was of an anodising type, whereas a polymeric coating was developed for the compressor cover. As a result of the research and development work conducted, these technologies have been proven to significantly reduce the wear and corrosion observed on compressor wheels and covers subjected to LR EGR environments. A comparison of untreated and surface treated compressor wheels and covers can be observed in Figure 5.
Figure 5 - Untreated and Treated Compressor Wheels and Covers
Untreated at Top
Treat at Bottom
Experts at Cummins Turbo Technologies are continually developing novel solutions to materials engineering challenges in order to meet customer requirements for more robust and durable products.