Introduction: Air-Breathing Aircraft Engine Types

This section describes various types of gas turbines and introduces piston engines. Aircraft propulsion depends on the extent of thrust produced by the engine. Sec­tion 10.11 presents the thrust and power available from various types of engines. Statistics for various types of aircraft engines are previous at the end of this chapter. Gas turbine sizes are progressing in making engines both larger and smaller than current sizes – that is, expanding the application envelope.

10.4.1 Simple Straight-Through Turbojet

The most elementary form of a gas turbine engine is a simple straight-through turbojet, shown schematically in Figure 10.4. In this case, the intake airflow goes straight through the entire length of the engine and exits at a higher velocity and temperature after the processes of compression, combustion, and expansion. This type of engine burns like a stove in a pressurized environment. Readers may note the “waisting” of the airflow passage as a result of the compressor reducing the volume as the turbine expands. Typically, at the LRC condition, the free-stream tube located far upstream is narrower in diameter than at the compressor face. As a result, airflow ahead of the intake plane slows down during the precompression phase.

Components associated with the thermodynamic processes within the engine have assigned station numbers, as listed in Table 10.3. (A bare engine does not have an intake and exhaust nozzle.)

Table 10.3. Gas turbine station number

Number

Station

Description

Free Stream

Far upstream (if precompression is ignored, then it is the same as 0)

0-1

Intake

A short divergent duct as a diffuser to compress inhaled airmass

1-2

Compressor

Active compression to increase pressure; temperature rises

2-3

Burner*

Fuel is burned to release the heat energy

3-4

Turbine

Extracts the power from heat energy to drive the compressor

4-5

e

Nozzle*

Generally convergent to increase flow velocity (Station 5 is also known as e, representing the exit plane)

Note:

* The burner is also known as the combustion chamber (CC) and the nozzle as the exhaust duct.

Overall engine efficiency improves if the higher energy of the exhaust gas of a straight-through turbojet is extracted through an additional turbine, which can drive a fan in front of a compressor (i. e., for a turbofan engine) or a propeller (i. e., for a turboprop engine).

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