Exhaust gas recirculation technologies offer significant benefits in reducing vehicle emissions. Long route EGR also decreases vehicle fuel consumption when compared with short route EGR. The new turbocharger, designed by Cummins, was developed for commercial vehicle applications where the durability requirements are particularly high.
“Although this type of technology is widely used in passenger cars, in a commercial vehicle it’s very difficult to introduce due to the durability requirements and the long-term effects of the corrosion and erosion environment that is observed in the turbocharger compressor stage,” explains Dr Michael Burkinshaw, group leader of tribology and advanced processing at Cummins Turbo Technologies.
Development, which took around four years, adopted a materials science-based approach. “We believe this is the best method because it enabled us to understand certain results in a laboratory environment and learn why some solutions wouldn’t work on the turbocharger, which is more efficient than developing the turbocharger system, identifying the failures and then investigating why those are occurring.”
Potential materials and surface treatments were selected including untreated aluminum alloy impeller and compressor cover material concepts as well as surface treatments such as anodizing, plating and polymeric-based coatings and characterized in terms of their durability and consistency of performance in long route EGR environments.
“The next stage was to try and understand how we would test these materials and surface treatments in a range of environments. Once we had established a myriad of test conditions and conducted such experiments, we performed analysis using different techniques and subsequently employed a cause and effect matrix, which enabled us to establish the best solutions and do more specific tests.”
Test coupons replicating the microstructure, mechanical properties and topography of the deck of the impeller and compressor cover volute were created. Evaluation in the lab was conducted using a combination of rig and prototype testing and focused on thermal shock, corrosion, erosion, aerodynamic performance and fatigue. The test samples were analyzed using high resolution microscopy, chemical analysis and interferometry techniques.
“In some tests we knew what we were aiming for but in others that were new to us, such as the corrosion tests, we had to do a comparison in order to understand the benefits of the treatment,” Burkinshaw says. “We learned after initial corrosion tests and discussions with our customers, that we needed to understand corrosion behavior of concepts when subject to a much broader range of condensate pHs and chemistry. As a result of the comprehensive testing, we’re now able to provide our customers with a wide condensate pH and chemistry operating window in which our technology can durably work.”
An anodise surface treatment was identified as the best performing concept for the impeller (above). Two respective surface treatments, namely a spray and a plating, were identified as the best performing concepts for the compressor cover (top). Having already received positive feedback from customers, the technology is now in the final prototype stages for adoption in future commercial engine applications.