Considerations of the All Electric (Accessory) Engine Concept

The concept of the all electric (accessory) engine (AEE) involves the use of an electrical machine integrated on an engine main shaft acting as a starter generator, providing power to both engine and airframe systems. Through removal of the various constraints caused by the present mechanical accessory drives, this concept apparently provides a number of advantages. However, the concept of an AEE has been around for some years and yet has not been adopted to-date. A brief review of electrical machine types previously considered as internally mounted starter/generators highlights basic problems of the concept. The reasons for favouring the Switched Reluctance Machine for this application are then explained and some present and planned developments on this basis outlined. An approximate sizing of starter/generators to replace shaft off-takes alone and bleed in additions in a civil turbofan application is presented, showing more detailed design considerations. Other system component aspects and concerns are also covered before addressing the effects on the ease of adopting the AEE concept of likely future changes to civil engines. Major differences relative to the civil case are indicated for other engine type applications. Conclusions are then drawn on the likely future of the AEE concept.