It is theorized that systems and equipment for an All Electric Aircraft (AEA) will be developed 20 years hence as a More-Electric Aircraft (MEA) with no bleed system, which is a concept typified by the Boeing 787, and electric powered propulsion (including electric distributed thrust or electric hybrids by gas turbine power generation), which is expected to be realized after the 2040s. In this trend, a More-Electric Engine (MEE) plays the following roles:
- Role: electric engine systems; Description: to eliminate system waste and improve the management of the fuel burn and heat
- Role: increased electric power demand and engine control; Description: to optimize the extracting power of the generated power and engine control due to increased electric power demand
- Role: integrated thrust control; Description: to integrate the fuel, the electric power, the control and thrust systems toward system optimization
When it comes to electric power demand and engine control, no-bleed systems of the MEA generate more electric power because they transfer the secondary power by compressed air from the conventional compressor to power generation by the rotational energy of a high-pressure spool shaft. The AEA, with a single aisle and 150 seats, may need 1MW of electric power, or for an eight-hour flight, 7.8MWh. In order to realize these electric power demands, the engine system plays a role in power generation in the total energy management system that consists of power generation, distribution, storage and consumption.
The power generation system is necessary for the engine system considering the improvement of generating efficiency for future increased power demand, stability of engine control, and limits of mounting structures of an extraction mechanism and generators. General aero-engines have a low-pressure (LP) spool and high-pressure (HP) spool in the compressor and the turbine, respectively, and fans that acquire thrust are driven by a low-pressure shaft and the generator and the pumps are driven by the torque of a high-pressure shaft. The generator drive by the torque of a low-pressure shaft should be examined in consideration of weighing up the merits and demerits of the high-/low-pressure shafts:
High-pressure Shaft Merits:
•Able to start the engine by the starter or the generator
•Range of rotation speed is narrow (about twice)
High-pressure Shaft Challenge:
•A surge margin of the low-pressure compressor decreases when extracting power increases
Low-pressure Shaft Merits:
•Able to save fuel compared to high-pressure shaft extraction (idling on the ground)
•Able to generate power by a windmill while the engines are off
Low-pressure Shaft Challenges:
•Range of rotation speed is large (about fivefold)
• As opposite to the high-pressure shaft, the operation margin of the low-pressure compressor decreases when extracting power decreases
Regarding integrated propulsion control, the electric power management needs to be linked with every status and situation of the aircraft and requires a stable power supply and quick response in an emergency. A conventional fuel pump driven by mechanical power needs to be mounted on the engine AGB, but for the electric power system it needs to provide flexibility upon being mounted on the fuel pump, and fuel system integration should be assumed from the point of a holistic and optimized system.
Tightening the information and communication between the control system of MEE and the aircraft system may enable integrated control such as removing limiting conditions temporarily by analysis of the information and the status in coordination with engine control and aircraft control in an emergency. Coexistence of aircraft control, electric power management and stability of engine operation must function as total energy management at normal times and provide safety and stability in an emergency.
Research targets of the integrated propulsion control include:
- Integrated System: Integration with the fuel system; main research target is to optimize function and configuration and configure redundancy of the engine fuel pump by integration with the aircraft fuel system
- Integrated System: Integration with the electric power system; main research target is to integrate the extracting power control of a multiplexed power generation system corresponding to the increase of generation capacity, power supply/demand management information with a load and engine control
- Integrated System: Integration with the control system; main research target is to improve the engine responses in operability (e.g. engine control responding quickly in an emergency [speed maintenance during turbulence, at a go-around and while ground proximity warning system is operating])
Read the full article in Aerospace Engineering/Aerospace & Defense Magazine entitled “Designing a Power Generation System for a More-Electric Aircraft” that focuses on the expectation that MEE will be provided as a future key technology for the total energy management of MEA. Specifically, the engine will be an important factor not only for the conventional role of high efficiency and low emission, but also for optimization between the propulsion system and electric power system because an MEA integrates power management into the electric power generated by engines. In addition, the MEE will also have to consider the importance for optimization of the aircraft in integrated management by advancing management of fuel burns and the thermal control system, optimization of engine control and information sharing with the entire aircraft, and the integration of flight control.
This article has been summarized. It was originally published as “Designing a Power Generation System for a More-Electric Aircraft” in SAE International/Tech Briefs Media Group’s Aerospace Engineering/Aerospace & Defense Magazine, April 2016. It is based on SAE Technical paper 2015-01-2408 by Hitoshi Oyori, IHI Aerospace Co. Ltd., and Noriko Morioka and Tsuyoshi Fukuda, IHI Corporation, doi:10.4271/2015-01-2408.