| Energy [PJ/year] | ||||
|---|---|---|---|---|
| Fuel | Sector | Device | EndUse | |
| Liquid fuel | Aviation | Gas Turbine | Mechanical | 493.3 |
5 Jet engine
Adapted from (Paoli and Cullen 2020)
Jet engines are devices using a gas turbine for converting chemical energy into work in the form of kinetic energy of a stream of air. The change of momentum of the air jet is used to propel the aircraft forwards.
They use gas turbines to convert fuel to mechanical energy, which is then converted to thrust.
5.1 Characterisation
There are three main designs of jet engine: turboprop, turbofan and turbojet. They differ in the design by-pass ratio, which is the share of air which is not passed through the combustor.
Turboprops are used for small aircraft that fly at low Mach numbers, turbofans are used for most commercial flight applications while turbojets are only used for military applications. The turbofan design has the highest bypass ratio and efficiency of all.
5.2 Key issues that affect efficiency
The thermal efficiency of the core gas turbine influences the overall efficiency of the jet engine, which is affected by the parameters discussed in Chapter 4.
The transfer efficiency represents the efficiency with which the work generated by the core can be turned in kinetic energy.
The propulsive efficiency represents how well the kinetic energy of the jet is transferred to the vehicle. A lower jet velocity increases the propulsive efficiency. This can be achieved by increasing the “bypass ratio” of the engine.
5.3 Efficiency limits
According to Paoli and Cullen (2020), the current efficiency and efficiency limits for gas turbines used in industry are:
| Current \(\eta_D\) | 30–40% |
| BAT (Best Available Technology) | 41% |
| TEL (Technological Efficiency Limit) | 54–58% |
5.4 Final Energy used
This table shows the quantity of final energy \(F\) used in the UK in one year:
(A small amount of energy is used in spark ignition engines for smaller planes, but we are neglecting this here).