Jet Propulsion A Simple Guide to the Aerodynamics and Thermodynamic Design and Performance of Jet Engines

Introduction

This chapter looks at the layout of some jet engines, using cross-sectional drawings. This begins with relatively simple engines and leads to engines for a recent large aircraft, the Boeing 787 and an engine for the smaller Bombardier C-series. Two concepts are introduced in the chapter. One is the multi-shaft engine with separate low-pressure and high-pressure spools. The other is the bypass engine in which some, very often most, of the air compressed by the fan bypasses the combustor and turbines.

Any consideration of practical engines must address the temperature limitations on the turbine. The chapter ends with some discussion of cooling technology and of the concept of cooling effectiveness.

The turbojet and the turbofan

Figure 5.1 shows a cut-away drawing of a Rolls-Royce Viper engine. This is typical of the simplest form of turbojet engine, which was the norm in the 1950s when it entered service, with an axial compressor coupled to an axial turbine, all on the same shaft. (The shaft, the compressor on one end and turbine on the other are sometimes referred to together as a spool.) Even for this very simple engine, which was originally designed to be expendable as a power source for target drones, the drawing is complicated. For more advanced engines such drawings become unhelpful at this small scale and simplified cross-sections are therefore more satisfactory and will be shown. A simplified cross-section is also shown for the Viper in Figure 5.1, as well as a cartoon showing the major components.

More recent turbojet engines had two spools so that the compression and expansion were split into parts. For flight at sustained speeds well in excess of the speed of sound a turbojet engine remains an attractive option and a two-shaft example, the Rolls-Royce Olympus 593, is shown in Figure 5.2. Four of these engines were used to propel the Concorde at around twice the speed of sound. The low-pressure (LP) compressor and LP turbine are mounted on one shaft to form the LP spool. The LP shaft passes through the high-pressure (HP) shaft on which are mounted the HP compressor and the HP turbine. The compression process is split between two spools for reasons to do with operation at speeds below the design speed, including starting; this is discussed in some detail in Chapter 12.