Design of fuzzy logic controller for fighter aircraft fuel tank pressurization

Military fighter aircraft are designed to operate in difficult conditions, such as suddenly changing pressure and temperature environments. This brings complex requirements and interfaces between many systems on board. Fuel system is one of the critical systems with the function of pressurizing the fuel tanks it uses. The source of the air that pressurizes the fuel tanks is important for reducing fuel evaporation, especially at high altitudes where ambient air pressure is much lower than the sea level. This air comes from the environmental control system and is converted into nitrogen-enriched air thanks to the modules that separate oxygen and nitrogen molecules in the air in inerting system. The bleed air taken from the engine compressor causes a significant reduction in the power or thrust produced by the engine. Traditional mechanical valves, called climb and dive valves, are generally developed to maintain a certain pressure limit. In aircraft using a closed pressurization system, the climb valve allows air to come out as the atmospheric pressure decreases, while the dive valve allows air flow from the source during descending. On the other hand, while the transfer of fuel from the tank takes place, the empty space (ullage) in the tank increases, resulting in a pressure drop. Mechanical valves operate according to the binary logic. Predefined pressure limits are set and up to this level, the valve is closed and when it reaches the limit, it becomes open. This may be sufficient to maintain fuel tank pressurization but does not take into account engine compressor air consumption. In this thesis, the design and implementation of a fuzzy logic controller that can be applied to the electro-mechanical valve to regulate the flow of air coming into the fuel tanks for pressurization is discussed. Mamdani's method is selected for the inference system. Fuzzy logic was preferred to design a controller due to the modeling complexity caused by the sudden change of air pressure in the fuel tanks. At the same time, fuzzy logic was found appropriate, as it reflects the experience and perspective of experts with linguistic terms, rather than with a high level of mathematical infrastructure and analytical models for designing a controller. The main purpose in this study is to reduce the waste of use of supply air while meeting the pressurization requirement. Since modern fighter aircraft often perform different types of missions, two flight profiles have been created to test the controller. Four experimental and four control groups/cases were analyzed by simulation to compare the effectiveness of the new type of electro-mechanical valve with traditional mechanical ones. The designed controller has kept the fuel tank pressure within predetermined limits, while providing significant savings in the amount of supply air. In the case of fuel transfer, it was observed that bleed air consumption was decreased by 90% while in the absence of fuel transfer, bleed air was consumed almost negligibly. With this study, a new perspective was obtained for tank pressurization of the aircraft fuel system. The designed controller can be applied for a type of electro-mechanical valve and bleed air consumption can be saved. Therefore, saved bleed air can be distributed to other systems or used to generate more power.