Experimental Development of a Combustion Chamber Pressure Emulator for PID Tuning of a Micro Turbojet Fuel Control System

Reliable fuel metering remains one of the main challenges in micro turbojet engines. The fuel delivery system operates under highly nonlinear conditions imposed by combustion chamber back-pressure, while its standalone characterization is rarely reported in the literature. This study presents the experimental development of a dedicated combustion chamber pressure emulator. The emulator reproduces realistic hydraulic back-pressure conditions and enables independent investigation of the fuel supply system without continuous engine operation. The proposed facility integrates an actual reverse-flow combustion chamber, controlled pneumatic pressurization, and a LabVIEW-based real-time control platform. This platform was used for fuel pump characterization and PID controller validation. A second-order dynamic model of the fuel pump was identified experimentally and employed to derive theoretical PID parameters. These parameters were subsequently used as an initial tuning basis for the experimental controller. Experimental validation was performed under atmospheric conditions and simulated combustion chamber pressure. Three bypass valve configurations were investigated, covering the complete operating envelope of the prototype micro turbojet engine. The results demonstrated accurate reference tracking over the entire investigated range. The system exhibited negligible steady-state error, rapid convergence following successive operating-point changes and stable closed-loop behavior without sustained oscillations. The pressure emulator successfully maintained representative combustion chamber back-pressure while allowing repeatable determination of fuel pump operating limits and characteristic curves. Moreover, the fuel pump exhibited an approximately linear relationship between control voltage and fuel flow for all bypass configurations, whereas the experimentally tuned PID controller provided stable and accurate dynamic responses across the investigated operating regime. The proposed methodology provides a reliable experimental framework for fuel system characterization, controller development, and validation while significantly reducing dependence on full-engine testing, thereby facilitating the implementation of advanced model-based and adaptive control strategies for future micro gas turbine applications.