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Engine system reliability can be improved by advanced electric architectures, while the reduction of hydraulic components, fuel tubes, and fittings can enhance the maintainability of the engine and minimize pilot workload.

Global warming and environmental friendliness considerations have recently been key for multiple global industries. In the commercial aviation industry, reduction of aircraft emissions, including CO2 and noise, is an urgent priority. Both aircraft and aircraft engine manufacturers are striving to improve design to accommodate the needs of commercial airlines. Engine manufacturers in particular have worked long and hard to improve each engine component, e.g., compressors, turbines, combustors, etc.

Schematic of an MEE (more electric engine) electric fuel pump system vs. a conventional fuel pump system.
However, the need for another approach to further improve engine efficiency motivated researchers from IHI Aerospace to focus on the system approach, including the control system, fuel system, or other engine systems, ultimately resulting in an MEE (more electric engine).

The MEE is a new engine system concept that seeks engine efficiency improvements, which results in a reduction of engine fuel burn and CO2 emissions. The key concept of the MEE system involves the architecture for the electrical power generation by the engine and changing the power source for accessories from mechanical/hydraulic to an electric motor.

IHI focused on the electrification of the engine fuel pump system because of its contribution to fuel burn reduction. The researchers conducted a feasibility study of the MEE fuel system for an assumed small-size turbofan engine, and the result indicated an improvement in specific fuel consumption (SFC) by about 1% during cruising. The SFC improvement would be accomplished by removing the fuel bypass circuit and eliminating the ACOC (aircooled oil cooler), which worsens engine efficiency.

There are several technical challenges for the practical design of the MEE motor-driven fuel pump system. Failure of the engine fuel pump may induce IFSD (in flight shut down) of the engine and result in catastrophic failure of the aircraft. To avoid such critical situations and ensure better reliability than that of the conventional system, a fault-tolerant design of the electric drive is mandatory.

Schematic depicting the MEE fuel pump system.
For the MEE fuel pump system currently being developed, IHI has proposed the use of a permanent magnet blushless ac servo motor. In the servo motor control system, the electrical current is adequately controlled corresponding to the required torque from the fuel pump. The control system contains a limiter function to avoid overload of the motor, and a motor power-off function for emergency shutoff of the fuel supply.

In addition, IHI proposed the introduction of advanced fault-tolerant technologies such as a unique active-active redundant motor control system to the MEE. The active-active control enables the supply of the same amount of fuel to the engine combustor in case a single open failure occurs in one of the redundant motor systems.

Conventional vs. MEE Fuel Systems

An example of a conventional aircraft and engine fuel feed system.
IHI researchers propose integrating the fuel feed system between the aircraft and engine, as shown in this concept.
In current commercial aircraft, the aircraft system consists of various subsystems, and each subsystem is independently designed to accommodate the specific requirements or operational conditions designated for each subsystem. The independency sometimes causes duplicated functions and complicated system design.

One typical example of a segregated system is the engine fuel system. It is independent from the aircraft fuel system and both systems are designed to accommodate various conditions, which are designated at the interface point between aircraft system and engine inlet. Integrating the aircraft and engine fuel system would be helpful to construct a more efficient and simplified system, but it seems not to be practical in the conventional aircraft system.

The aircraft system contains electric motor-driven fuel boost pumps, shutoff valves, and cross feed valves as a minimum. There may be fuel transfer pumps, such as electric motor-driven pumps or ejector pumps, for transferring fuel between the tanks. In the case of a twin engine aircraft, the left- and right-hand engines are supplied with fuel from the left and right wing tank, respectively, during normal operation. If IFSD occurs in one of the engines, the cross feed valves would be activated to supply fuel from the opposite side tank to avoid an imbalance of fuel mass.