Real-Time Engine Modelling of a Three Shafts Turbofan Engine: From Sub-Idle to Max Power Rate

Real-Time Engine Models are required for operation with engine electronic control systems and/or aircraft simulators for functional demonstration. The challenge for Rolls-Royce has been to establish the sub-idle speed behaviour of the engine. This paper covers the development steps by the Civil Aerospace Modelling and Simulation team to resolve this limitation in the models. The real-time engine model is now generated using two non-linear thermodynamic engine models. One of the thermodynamic engine models, normal range, covers the idle to max power range and the other is for sub-idle operation. Previously sub-idle operation was established by extrapolation from the normal range model. However, this method limited control system development by simulation for altitude starting adding time to altitude test programmes in high cost facilities. The requirement for the technique is to obtain the partial derivatives and steady-state data for the whole operating range. For the partial derivative estimation in sub-idle region, a variable perturbation size is introduced and changed according to the different shaft speed so that the sensitivity issue of using a fixed perturbation size in this operating range is resolved. Furthermore, the partial derivative of each parameter from the non-linear models is fine tuned by comparing with the steady-state values for each parameter. The summation of the integrated partial derivatives should be same as the steady-state value of each engine parameter. If an error exists then an adjustment of each integrated partial derivative is conducted according to the relative weight of each integrated partial derivatives contribution to the whole. It is highlighted that error sharing between the integrated partial derivative parameters results in less error during the validation process. The real-time engine model is constructed in state-space modular subsystems in SIMULINK, which include an engine shaft block to generate the engine shaft speeds, and fuel block to generate a signal of engine lit, etc. The database generated by the process of partial derivatives is then used in calculation of engine’s shaft speeds, temperatures and pressures. For the test of the real-time engine model obtained in this study, simulation of engine starting from stationary is conducted. Using a starter torque as the input to the engine model, starter-assisted starting can be achieved. In addition, engine relighting in flight is also conducted. The output of the real-time engine model has been compared with flight test data for engine relight and agreement has been demonstrated.