Fundamental Differences Between Conventional and Geared Turbofans

Abstract

The potential for improving the thermodynamic efficiency of aircraft engines is limited because the aerodynamic quality of the turbomachines has already achieved a very high level. While in the past increasing burner exit temperature did contribute to better cycle efficiency, this is no longer the case with today’s temperatures in the range of 1900...2000K. Increasing the cycle pressure ratio above 40 will yield only a small fuel consumption benefit. Therefore the only way to improve the fuel efficiency of aircraft engines significantly is to increase bypass ratio — which yields higher propulsive efficiency. A purely thermodynamic cycle study shows that specific fuel consumption decreases continuously with increasing bypass ratio. However, thermodynamics alone is a too simplistic view of the problem. A conventional direct drive turbofan of bypass ratio 6 looks very different to an engine with bypass ratio 10. Increasing bypass ratio above 10 makes it attractive to design an engine with a gearbox to separate the fan speed from the other low pressure components. Different rules apply for optimizing turbofans of conventional designs and those with a gearbox. This paper describes various criteria to be considered for optimizing the respective engines and their components. For illustrating the main differences between conventional and geared turbofans it is assumed that an existing core of medium pressure ratio with a two stage high pressure turbine is to be used. The design of the engines is done for takeoff rating because this is the mechanically most challenging condition. For each engine the flow annulus is examined and stress calculations for the disks are performed. The result of the integrated aero-thermodynamic and mechanical study allows a comparison of the fundamental differences between conventional and geared turbofans. At the same bypass ratio there will be no significant difference in specific fuel consumption between the alternative designs. The main difference is in the parts count which is much lower for the geared turbofan than for the conventional engine. However, these parts will be mechanically much more challenging than those of a conventional turbofan. If the bypass ratio is increased significantly above 10, then the geared turbofan becomes more and more attractive and the conventional turbofan design is no longer a real option. The maximum practical bypass ratio for ducted fans depends on the nacelle drag and how the installation problems can be solved.

On the Optimization of a Geared Fan Intercooled Core Engine Design

“…The interested reader is referred to the very valuable study presented in Ref. [43] for more details on gearbox ratio optimization and the general advantages and disadvantages associated with geared fan configurations.…”

Assessment of Vehicle Performance Using Integrated NPSS Hybrid Electric Propulsion Models

“…These two parameters are determined using Equation 5 and Equation 6. The diameter equation was generated by iterating through the sizing process to estimate power as a function of diameter.…”

“…The conventional turbofan architecture benefits greatly from increases in higher bypass ratios and smaller cores; however, the achievable efficiencies are limited by size, drag, and weight constraints [1,2]. There have been many recent efforts to reduce the effects of these mitigating factors such as lighter weight materials [3,4], decoupling of the low pressure turbine from the propulsor to increase efficiency and decrease weight [5], more tightly coupled airframe and engine integration [6], and advanced concepts such as intercooled and recuperated engines [7]. The common theme among all of these proposed and developed concepts is that, regardless of the technology, all of the power needed for propulsion is generated within the gas generator core.…”

Feasible Concept of an Air-Driven Fan with a Tip Turbine for a High-Bypass Propulsion System

“…However, when the BPR rises to ultra-high, the effects of the techniques in those engines are limited. The geared turbofan engine (GTF) is a successful technology for high/ultra-high BPR engines in commercial airplanes [9]. By using a gearbox, the GTF can enable the fan and LPT to rotate at different rotational speeds.…”