The maintenance of aero engines is a costly and essential undertaking required to maintain their functionality. Precise condition monitoring and predictive maintenance techniques could reduce these costs significantly. One of the primary challenges in aero engine maintenance is understanding the nonlinear system performance over the engine's lifecycle. To address this challenge, this dissertation investigates the impact of combined module degradation and its aerothermodynamic interactions on the performance-specific and safety-critical parameters of aero engines. A dynamic performance model of the research turbofan engine is developed using the Pseudo-Bond-Graph theory. This model combines unsteady equations of motion for compressible gases and solids in one system of differential equation, accounting for aerothermodynamic interdependencies of unsteady effects in the entire thermodynamic cycle. Hence, the dynamic engine model is calibrated based on performance-specific data of the real research aero engine, which is obtained from measurement campaigns at MTU Hannover GmbH. Then, the model is compared with iterative transient simulation methods from the literature to investigate the effects of dynamics on predicted parameters. The results indicate that aerothermodynamic interdependencies of unsteady effects have a significant impact on the time-dependent parameters and overall performance of aero engines. After verifying the plausibility of the model, the impact of combined module degradation on the performance is investigated. A sensitivity analysis is conducted, demonstrating that wear-related changes in the efficiency and capacity of the high-pressure system, which result in significant changes in performance. Furthermore, the evidence of aerothermodynamic interactions due to combined modul degradation is presented and the influence on the nonlinear performance of the engine is quantified. The developed methods and results can improve aero engine maintenance practices by being integrated into the condition monitoring of aero engines during operational use, allowing for a more precise prediction of the nonlinear perfromance of aero engines.
