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The axial-flow compressor’s advantages are: 

• High peak efficiencies; 

• Small frontal area for given airflow; 

• Straight-through flow, allowing high ram efficiency; and 

• Increased pressure rise by increasing number of stages, with negligible losses. 

The axial-flow compressor’s disadvantages are: 

• Good efficiencies over only narrow rotational speed range, 

• Difficulty of manufacture and high cost, 

• Relatively high weight, and 

• High starting power requirements (partially overcome by split compressors). 

## Diffuser 

The diffuser is the divergent section of the engine after the compressor and before the combustion section. It has the all-important function of reducing high-velocity compressor discharge air to increased pressure at a slower velocity. This prepares the air for entry into the flame burning area of the combustion section at a lower velocity so that the flame of combustion can burn continuously. If the air passed through the flame area at a high velocity, it could extinguish the flame. 

## Combustion Section 

The combustion section houses the combustion process, which raises the temperature of the air passing through the engine. This process releases energy contained in the air/ fuel mixture. The major part of this energy is required at the turbine or turbine stages to drive the compressor. About ²⁄ ³  of the energy is used to drive the gas generator compressor. The remaining energy passes through the remaining turbine stages that absorb more of the energy to drive the fan, output shaft, or propeller. Only the pure turbojet allows the air to create all the thrust or propulsion by exiting the rear of the engine in the form of a high-velocity jet. These other engine types have some jet velocity out the rear of the engine but most of the thrust or power is generated by the additional turbine stages driving a large fan, propeller, or helicopter rotor blades. 

The primary function of the combustion section is, of course, to burn the fuel/air mixture, thereby adding heat energy to the air. To do this efficiently, the combustion chamber must: 

• Provide the means for proper mixing of the fuel and air to assure good combustion, 

• Burn this mixture efficiently, 

• Cool the hot combustion products to a temperature that the turbine inlet guide vanes/blades can withstand under operating conditions, and 

• Deliver the hot gases to the turbine section. 

The location of the combustion section is directly between the compressor and the turbine sections. The combustion chambers are always arranged coaxially with the compressor and turbine regardless of type, since the chambers must be in a through-flow position to function efficiently. All combustion chambers contain the same basic elements: 

1. Casing 

2. Perforated inner liner 

3. Fuel injection system 

4. Some means for initial ignition 

5. Fuel drainage system to drain off unburned fuel after engine shutdown 

There are currently three basic types of combustion chambers, variations within type being in detail only. These types are: 

1. Can type 

2. Can-annular type 

3. Annular type 

The can-type combustion chamber is typical of the type used on turboshaft and APUs. [Figure 1-52]  Each of the 

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