Experimental and Numerical Analysis of a Turbocharging System for High Performance Automotive Engines

The global transition towards sustainable energy systems is essential to mitigate climate change and reduce environmental pollution. In line with the European Union's Fit for 55 initiatives under the European Green Deal, the automotive industry faces stringent targets to reduce greenhouse gas emissions, including ambitious CO₂ reduction goals for passenger cars, heavy-duty vehicles, and zero-emission urban buses by 2030 and beyond. Despite the rapid progress in electrification, internal combustion engines (ICEs) remain vital in transportation, necessitating innovations to align with decarbonization goals. This doctoral research focuses on the role of advanced turbocharging systems in reducing emissions and enhancing ICE performance. Turbocharging systems are critical in optimizing air handling, reducing NOx emissions, and addressing unique challenges posed by hydrogen combustion. Innovations in turbocharger design, such as lean or ultra-lean combustion regimes and precise control of waste-gate valves, can significantly mitigate environmental impact while maintaining performance. The PhD thesis investigates key aspects of turbocharger performance, emphasizing unsteady turbine and compressor behaviour, heat transfer effects, and control-oriented modelling to improve predictive capabilities for advanced engines design. The findings underscore the necessity of advanced modelling techniques to accurately predict turbocharger performance under real-world conditions, particularly for future hydrogen internal combustion engines requiring ultra-lean combustion strategies. A comprehensive understanding of unsteady flow dynamics and heat transfer phenomena is essential for optimizing turbocharging systems, enabling more efficient and sustainable engine designs aligned with European emission reduction targets. Experimental investigations conducted at the Test Bench for Components of Propulsion Systems of the Mechanical, Energy, Management, and Transportation Engineering Department (DIME) of the University of Genoa focused on multiple aspects of turbocharger dynamics. For centrifugal compressors, steady and unsteady flow conditions were analysed, particularly under pulsating flow generated by a motor-driven cylinder head. This analysis provided insights into the energy content and hysteresis effects in the compressor map, crucial for understanding interactions between the intake system and turbocharger. The thesis further explored heat transfer effects on turbocharger efficiency maps, proposing a new model to correct measured data and isolate the adiabatic performance of compressors. Tests conducted under quasi-adiabatic conditions demonstrated the accuracy and practical applicability of this method, which requires minimal geometrical inputs and can be implemented on standard test benches. For turbine systems, investigations focused on unsteady flow phenomena in the exhaust manifold and on the waste-gate valve impact on the turbine steady state performance. The pulsating wave shapes and their influence on turbine performance are analysed in detail. An experimental campaign was conducted to analyse the turbine’s performance across different waste-gate valve openings using the test bench at the University of Genoa. Based on experimental data, a specific 1D numerical model was developed to assess the interaction between the flow through the impeller and the waste-gate valve. Initial tuning of the model utilized turbine characteristic maps measured with the waste-gate valve fully closed. The study then examined waste-gate valve behaviour for varying degrees of opening. Results indicated the necessity of a more refined model to accurately represent the swallowing capacity of the waste-gate valve. After independently tuning the 1D models of the turbine and the waste-gate valve, their behaviour under parallel flow conditions was analysed. The results revealed significant interactions between the two components, which led to reduced turbine performance due to a decrease in the impeller’s swallowing capacity. This reduction directly impacts the power delivered to the compressor and consequently increases the engine back pressure. The findings suggest that traditional methods may be insufficient to accurately assess turbocharger behaviour at high waste-gate valve openings. Enhanced modelling techniques are therefore essential to improve the understanding of turbine performance and, finally, the overall performance of turbocharged engines. In addition, a one-dimensional model of the engine exhaust manifold-turbine inlet system was developed. This model was based on the actual geometries and experimental data collected during the investigations. The results obtained from the model were compared with the experimental findings, demonstrating its capability to accurately replicate the observed phenomena and providing further validation for the modelling approach. In conclusion, this study aims to refine 0D/1D simulation models for turbocharged ICEs through an integrated approach combining experimental and computational methodologies. Enhanced predictive accuracy of turbine thermomechanical efficiency, waste-gate valve flow dynamics, and cylinder backpressure modeling ensures optimized engine design and after-treatment integration. These advancements support the development of efficient, and environmentally compliant automotive solutions, contributing to Europe’s net-zero carbon ambitions.