Advanced Fuel Metering for Aero Engines

Traditionally the fuel system of most modern day aero engines consist of two basic functional blocks; - pumping system - fuel metering system. The pumping system basically consists of a first stage centrifugal pump followed by a second stage gear pump. The function of the pumping system is to provide sufficient fuel at the required pressure to ensure engine operation (steady state and transient) over the complete operating envelope and under the consideration of worst case fuel inlet properties. The fuel metering system basically consists of a fuel metering valve, an emergency valve and a shut off valve. The fuel metering valve itself is usually realised as controllable main metering valve, the pressure drop across which is hydro-mechanically controlled via a pressure drop regulator. The pressure drop regulator spills the excess fuel delivery back to the pump inlet in order to maintain the constant pressure drop. Whilst this approach to fuel system design has gained world wide industry acceptance, there still remains potential for further simplification and improved system reliability. This paper examines two alternative concepts for future fuel system designs namely, - Electrically controlled variable speed fuel pump - Reduced complexity fuel metering unit. For both systems this paper examines and discusses, the technologies involved and the relative merits of each system in terms of: • system performance • fault detection / accommodation • failure reaction and • production costs.