Types of Turbine Engines

Turbojet Engine

The turbojet engine consists of four sections—compressor, combustion chamber, turbine section, and exhaust.

The compressor section passes inlet air at a high rate of speed to the combustion chamber. The combustion chamber contains the fuel inlet and igniter for combustion. The expanding air drives a turbine, which is connected by a shaft to the compressor, sustaining engine operation. The accelerated exhaust gases from the engine provide thrust. This is a basic application of compressing air, igniting the fuel-air mixture, producing power to self-sustain the engine operation, and exhaust for propulsion.

Schematic

Inlet air → Drives the Turbine → Drives the Compressor/Gear box → Then turn the propeller → Combustion Chamber contains fuel for combustion (Ignition) → Expanding air drives a turbine (Power Extraction) → → Turbine connected by shaft to the compressor → Exhaust from the engine (Thrust)

Inlet Air ──▶ [ Compressor ] ──▶ [ Combustion ] ──▶ [ Turbine ] ──▶ Exhaust
                        ▲                                  │
                        └─────────── ( Shaft ) ────────────┘

Advantages of turbojet engine

• Relatively simple design

• Capable of very high speeds

• Takes up little space

Disadvantages of turbojet engine

• High fuel consumption

• Loud

• Poor performance at slow speeds

• Limited in range and endurance

Turboprop

A turboprop engine is a turbine engine that drives a propeller through a reduction gear, which is a combination of a gas turbine engine, reduction gearbox, and propeller.

However, the difference is that the turbine in the turboprop engine usually has extra stages to extract energy to drive the propeller. In addition to operating the compressor and accessories, the turboprop turbine transmits increased power forward through a shaft and a gear train to drive the propeller. (Approximately 85–90 percent of the engine’s energy is converted into shaft power to drive the propeller)

Schematic

Inlet air

→ Drives the Turbine

→ Drives the Compressor/Gear box

→ Drives the Propeller

Inlet Air ──▶ [ Compressor ] ──▶ [ Combustion ] ──▶ [ Turbine ] ──▶ Exhaust
                         ▲                                  │
                         │          (Main Shaft)            │
                         └──────────────────────────────────┤
                                                            ▼
    [ Propeller ] ◀────── [ Reduction Gearbox ] ◀───────────┘
     (85-90% Power)        (Speed Conversion)

Advantages of turboprop engine

• Very fuel efficient

• Most efficient between 200-350 knots

• Most efficient at mid-range altitudes of 18,000-30,000 feet

Disadvantages of turboprop engine

• Limited forward airspeed

• Gearing systems are heavy and can break down

Turbofan Engine

Turbofans were developed to combine some of the best features of the turbojet and the turboprop. Turbofan engines are designed to create additional thrust by diverting a secondary airflow around the combustion chamber.

Almost all airliner-type aircraft use a turbofan engine. In high-bypass turbofan engines, as much as 80–90 percent of total thrust is produced by the bypass airflow. This engine is quieter and has better fuel efficiency in this speed range.

Advantages of turbofan engine

• Fuel efficient

• Quieter than turbojets

• High thrust capability

• Improved fuel economy at subsonic speeds

Disadvantages of turbofan engine

• Heavier than turbojets

• Larger frontal area than turbojets

• Less efficient at supersonic speeds compared to turbojets

Turboshaft Engine

The fourth common type of jet engine is the turboshaft. It delivers power to a shaft that drives something other than a propeller. The biggest difference between a turbojet and turboshaft engine is that on a turboshaft engine, most of the energy produced by the expanding gases is used to drive a turbine rather than produce thrust. Many helicopters use a turboshaft gas turbine engine.

Advantages of turboshaft engine

• Much higher power-to-weight ratio than piston engines

• Typically smaller than piston engines

Disadvantages of turboshaft engine

• Rotor and gearbox noise typically dominate overall helicopter sound levels.

• Gear systems connected to the shaft can be complex and break down

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