There are two primary functional categories of gas turbine engines: thrust-producing engines and shaft (torque)-producing engines. Thrust-producing turbine engines include the turbojet and turbofan. Shaft-producing turbine engines include the turboprop and turboshaft. While all operate on the same Brayton-cycle principle, they differ in how useful power is extracted—either as high-velocity jet thrust or as rotational shaft power.
| Engine Type | Primary Output | Propulsive Method | Typical Application |
|---|---|---|---|
| Turbojet | Thrust | High-velocity exhaust gas | Military fighters, high-speed research |
| Turbofan | Thrust | Bypass air (80-90%) & core exhaust | Commercial airliners, business jets |
| Turboprop | Torque (Shaft) | Propeller via reduction gearbox | Regional cargo, short-haul commuters |
| Turboshaft | Torque (Shaft) | Free power turbine to transmission | Helicopters, APUs, industrial power |
Turbojet Engines
The turbojet, first patented in 1930 by Frank Whittle, originally used a centrifugal (impeller-type) compressor, an annular combustor, and a single-stage turbine. Modern turbojets typically use axial-flow compressors, but the fundamental components remain the compressor, combustor, turbine, and exhaust nozzle. [Figure 1]
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| Figure 1. Turbojet |
The turbojet produces propulsive power from the reaction to high-velocity exhaust gases. The compressor increases inlet air pressure, fuel is added in the combustor, and the expanding gases rotate the turbine. The turbine drives the compressor, and the remaining exhaust energy accelerates through the nozzle to create thrust. Although turbojets are now rare in commercial service, they are still used in certain military and high-speed applications.
Turboshaft Engines
A gas turbine engine that delivers power through a shaft to operate equipment other than a propeller is called a turboshaft. Turboshaft engines are widely used in helicopters and in industrial applications such as power generation.
Early turboshaft engines mechanically coupled the power output shaft directly to the gas generator turbine. Modern designs typically incorporate a free power turbine, which rotates independently of the gas generator section. Figures 2A and B illustrate free power turbine arrangements with both front and rear output shaft configurations. Turboshaft engines are generally divided into two major sections: the gas generator and the power turbine.
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| Figure 2 |
The gas generator produces the energy required to drive the compressor and supply high-energy gases to the free power turbine. Rather than a fixed energy split, the majority of combustion energy is extracted to sustain the gas generator, while the remaining energy drives the free turbine and aircraft transmission through a high-ratio reduction gearbox. Some helicopter turboshaft engines are designed to produce a small amount of residual exhaust thrust (typically less than 10 percent), although primary propulsion is derived from the rotor system.
Turboprop Engines
The turboprop is similar in principle to the turboshaft but is specifically configured to drive a propeller through a reduction gearbox. The turboprop is essentially a gas turbine engine optimized for propeller-driven propulsion.[Figure 3]
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| Figure 3. Turboprop |
A turboprop propeller may be driven by either a fixed (direct-drive) turbine or a free power turbine. [Figure 4] In fixed-turbine designs, the turbine is mechanically connected to the compressor, gearbox, and propeller shaft. In free-turbine designs, the free power turbine drives only the gearbox and propeller shaft, allowing the gas generator and propeller to rotate at independent optimum speeds.
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| Figure 4 |
Advantages of the free-turbine design include:
- Ability to maintain very low propeller r.p.m. during taxi
- Easier engine starting, particularly in cold conditions
- Reduced transmission of propeller vibration to the gas generator
- Capability to use a propeller brake during ground operations
Unlike a basic turbojet, the turboprop requires additional turbine stages to extract sufficient energy to drive the reduction gearbox and propeller. Total turboprop thrust is the combination of propeller thrust and residual exhaust thrust, with exhaust typically contributing between 5 and 25 percent depending on design and operating conditions.
Fuel consumption of turboprops is generally lower at subsonic speeds due to their higher propulsive efficiency at lower flight velocities. This makes them especially effective for regional, short-haul, and utility operations.
Ultra High-Bypass Propfan / Open Rotor Engines
Advances in propeller aerodynamics, materials, and computational design have renewed interest in ultra high-bypass open-rotor (propfan) engines. Modern composite materials and swept, scimitar-shaped blades allow higher propulsive efficiency with reduced weight and improved structural integrity.
Conventional propellers typically achieve a pressure ratio near 1.05:1, while advanced propfan concepts can approach approximately 1.2:1. Contra-rotating propellers reduce swirl losses by recovering rotational energy from the first rotor with a second rotor, improving overall efficiency. [Figure 4E]
Propfan designs—sometimes referred to as open-rotor engines—use highly swept, multi-blade configurations that differ substantially from traditional turboprops. [Figure 4D] These engines aim to combine turbofan cruise speeds (around Mach 0.75–0.8) with significantly improved fuel efficiency.
Although not yet in widespread commercial service as of 2026, modern open-rotor development programs such as CFM’s RISE demonstrator reflect renewed industry interest in achieving fuel savings of 20 percent or more compared to current high-bypass turbofans. Noise reduction and integration challenges remain active areas of development.
Turbofan Engines
The turbofan can be described as a ducted fan driven by a gas turbine core. Unlike a turboprop, the fan is enclosed within a nacelle and contributes both bypass airflow and additional compression.[Figure 5] Modern fan pressure ratios typically range from approximately 1.4:1 to 1.8:1, depending on design.
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| Figure 5. Turbofan |
Turbofan design combines the high-speed capability of the turbojet with the improved propulsive efficiency of a turboprop. Fan installation configurations include:
- Fan mechanically connected to the low-pressure compressor (Figure 6A)
- Fan driven by a separate turbine spool (Figure 6B)
- Aft-fan configurations integrated with turbine stages (Figure 6C), though this arrangement is uncommon in modern engines
Figure 6
The aft-fan configuration is rarely used today due to efficiency and foreign object damage considerations.
Turbofan engines are classified by bypass ratio: ultra low-bypass, low-bypass, medium-bypass, high-bypass, and ultra high-bypass. Bypass ratio is defined as the mass of air flowing around the core divided by the mass flowing through the core.
Ultra low-bypass engines (less than 1:1) are common in military aircraft such as the F/A-18 Hornet and F-22 Raptor, where compact diameter and high thrust-to-weight ratio are critical.
Low-bypass engines (approximately 1:1) were used on earlier jetliners such as the Boeing 727 and Douglas DC-9.
Medium-bypass engines typically range from 2:1 to 3:1. High-bypass turbofans range from approximately 4:1 to 9:1, while ultra high-bypass designs exceed 10:1.
Modern high-bypass engines, including geared turbofan designs such as the Pratt & Whitney PW1100G-JM used on the Airbus A320neo, incorporate a reduction gearbox between the fan and low-pressure turbine. This allows the fan and turbine to rotate at optimal speeds, improving fuel efficiency and reducing noise.
The Boeing 787 uses ultra high-bypass engines such as the GE GEnx, with bypass ratios exceeding 10:1. In these engines, 80–90 percent of total thrust is produced by the fan stream rather than the core.
High- and ultra high-bypass turbofans achieve superior fuel efficiency by increasing mass airflow and reducing exhaust velocity, thereby improving propulsive efficiency and reducing noise. For medium to large commercial aircraft, the high-bypass turbofan remains the dominant propulsion system.
Ultra High-Bypass Turbofan Engines (Ducted, Variable Pitch)
Variable-pitch or variable-area turbofan concepts are under active research and development. These engines aim to combine the flexibility of turboprops with the cruise performance of turbofans. Features may include variable fan pitch, adaptive bypass ratios, and variable exhaust nozzles. [Figure 7]
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| Figure 7 |
Although not yet widely certified for commercial airline service as of 2026, such technologies represent potential future evolution of the turbofan, offering improved efficiency across a broader operating envelope.