Variable Cycle Engine Concepts and Component Technologies - An Overview

Abstract

An aircraft engine is typically engineered for increased efficiency, yet concessions must be made to meet all flight envelope requirements. Off-design operation often leads to a lower efficiency and an increased specific fuel consumption (SFC). This is somewhat tolerable for a subsonic civil platform that predominantly operates in an optimized cruise condition. This problem is amplified for supersonic civil aircraft: balancing the needs for low emissions during low-altitude flight with high efficiency during supersonic cruise introduces conflicting optimisation goals. A parallel dilemma is encountered in supersonic military aircraft, also characterized by a wider operating range. The design trade-offs required to achieve all operating requirements can lead to overall performance reductions. In both scenarios, variable cycle engines (VCEs) may offer a solution. This paper reviews the literature on existing concepts along with their technology readiness level and the main associated challenges. The discussion spans from the overall engine system down to the compression and expansion components. The foundation of a VCE lies in modifiable engine components, adjusted throughout the operating range to control their interaction with other engine components, thereby improving the overall engine efficiency. This flexibility can result in significant benefits: reduced SFC, installation drag and infrared signature as well as increased thrust. Each VCE architecture is associated with certain benefits and drawbacks when compared to a conventional engine. The ideal architecture is ultimately dictated by the mission. The corresponding variable turbo-machinery components face practicality hurdles and generally exhibit lower efficiency than their conventional counterparts. In a double bypass VCE, the compression system must deal with additional requirements, most notably the need for a secondary bypass route to modulate the airflow. This secondary flow stream can either be kept separate or mixed with the primary bypass stream. This flow control requires certain geometric alterations such as injector valves and adjustable vanes. The complexity of these alterations differs between concepts with implications to the compressor design, control system and maintenance. An additional challenge is the simultaneous adjustment of the variable components to achieve a stable transition between different engine modes, e.g. subsonic to supersonic cruise. Variable expansion systems can be directed at either core mass flow control or blade incidence angle optimisation. The later affects mostly the turbine efficiency while the former modulates the thermodynamic cycle at a broader level. Three variable turbine approaches are discussed based on the literature. The most widespread is the variable area nozzle, where stator blades rotate to adjust throat area and flow incidence angle. Secondly, variability is briefly addressed for rotor blades. For both of these approaches, the effect of the geometric variabilities on turbine pressure losses is discussed to better understand the overall profitability of variable geometry concepts. Furthermore, the actuation of both stator and rotor blades is discussed to emphasize the feasibility hurdles and the interest in alternate fixed geometry concepts. This third and last approach includes a jet-flap concept operating through cooling air jets at the blade trailing edge and a plasma actuator concept composed of an electrode pair and a voltage source. Adopting a top down approach, this paper starts by examining overall VCE architectures before focusing on the variable compressor and turbine elements. Current literature falls short of offering a comprehensive overview on VCE-technology. This paper attempts to fill in this gap while simultaneously highlighting specific technologies and design approaches that appear most promising for the future development of variable cycle engines.