Aircraft technology has advanced a lot since the Wright Brother's historic first flight. As time passed, airplanes evolved to become stronger and able to carry heavier loads while achieving higher speeds in the air. This required the invention of a propulsion system to generate thrust.
Today, gas turbines are the most widely used form of propulsion, but there are different types and variations of this engine.
Modern gas turbines can generate tens of thousands of pounds of thrust or mechanical horsepower, allowing aircraft to travel faster, farther, and better than ever before.
In this article, you'll learn about the four main turbine engine types: turbojet, turboprop, turbofan, and turboshaft.
SUMMARY
Basically, all aircraft engines share a common operating principle based on Newton's third law of motion:
"For every action, there is an equal and opposite reaction."
They all follow the same cycle, but they differ in how they convert energy into thrust or power. Here’s an overview:
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Turbojet: High-speed performance, noisy, fuel-intensive.
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Turboprop: Effective at moderate speeds with propeller-driven thrust.
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Turbofan: Common in airliners, reliable with quieter operations.
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Turboshaft: Powers helicopters and auxiliary units with a focus on mechanical output.
What Are Turbine Engines?
Gas turbine engines are a type of internal combustion engine that generate power by using a high-speed flow of air.
They operate by continuously cycling air using these four key processes:
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Intake
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Compression
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Combustion
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Exhaust
These engines share a fundamental design principle: separating the processes into different sections for improving performance operations.
But unlike the reciprocating engines, where these functions take place sequentially in a single chamber, gas turbines perform them simultaneously.
1. Turbojet Engine
The turbojet was the first type of gas turbine engine to be developed for aircraft propulsion, and it remains a notable design in aviation history (despite being largely replaced in modern commercial aviation).
The jet engine has superior performance in speed and altitude compared to reciprocating engines. But they have limited takeoff and climb abilities. Propellers are more capable at slower speeds and during takeoff and landing, resulting in increased fuel efficiency.
Keep in mind, it is important for all turbine engines to operate within designated temperature, rotational speed, and torque limits.
Power setting can vary depending on the installation and may include parameters such as temperature, torque, fuel flow, or RPM.
Typically, operating limits are reached before fully advancing the throttles forward, regardless of weather conditions.
Turbojets function by compressing air, mixing it with fuel, igniting the mixture, and expelling it at high speeds to produce thrust.
How it Works
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Air Intake: The intake directs air smoothly into the compressor blades. For aircraft flying at supersonic speeds, the intake system reduces airflow to subsonic speeds to protect the engine.
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Compressor: Air is compressed by multiple stages of fans, increasing its pressure significantly.
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Combustion Chamber: The compressed air is mixed with fuel and ignited. Turbojets operate with a lean air-to-fuel ratio of about 50:1, which helps manage engine cooling.
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Turbine: The turbine blades absorb energy from the high-speed exhaust gases. This energy drives the compressor.
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Exhaust: The high-pressure gases exit the engine, generating thrust.
Turbojets were widely used in early jet aircraft like the Heinkel He 178 but are now primarily found in military and high-speed applications.
2. Turboprop Engine
The turboprop engine is basically a turbojet engine connected to a propeller through a reduction gearbox. It is really great for regional flights operating at mid-range speeds and altitudes.
Now, the turboprop engine has multiple advantages over other types of engines, like its light weight, simplicity of operation, and minimum vibration.
It is most capable when flying at speeds between 250 and 400 mph and altitudes between 18,000 and 30,000 feet.
Its power output is measured in shaft horsepower and is determined by rpm and torque. The engine also produces jet thrust, but this only accounts for a small percentage of its total power.
Turboprop engines are heavier and more complex than turbojet engines, but they really excel in delivering thrust at low speeds. The trade off is that their efficiency decreases as the flight speed increases.
How it Works
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Turbojet Core: The engine’s core functions like a turbojet, generating power.
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Gearbox: The high-speed turbine shaft connects to a gearbox, which reduces the rotational speed to drive the propeller.
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Propeller: The propeller generates thrust by displacing a large volume of air at lower speeds.
Approximately 80-85% of the engine’s energy is used to drive the propeller, while the remaining energy exits through the exhaust.
Turboprop engines can be found in aircraft like the King Air and Dash 8, these typically operate on regional and cargo routes.
3. Turbofan Engine
A turbofan engine is a jet engine with a ducted fan that bypasses some intake air around the core.
Turbofan engines are all over modern commercial aviation because they are able to wonderfully balance capability, power, and their noise reduction. Turbofan engines can be low bypass or high bypass.
They are an evolution of turbojet technology, designed to generate most of their thrust through a large bypass fan.
How it Works
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Fan: A large ducted fan at the front pushes bypass air around the engine core. This bypass air generates additional thrust and cools the engine.
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Core Engine: Some air enters the core, where it undergoes compression, combustion, and exhaust, similar to a turbojet.
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Bypass Ratio: This ratio compares the amount of air bypassing the core to the amount passing through it. High-bypass engines are more efficient and quieter.
Many turbofan engines use two spools, with separate high-pressure and low-pressure compressors and turbines. This design improves performance and reliability.
Aircraft like the Boeing 747 and Airbus A320 rely on turbofan engines for reliable performance across various altitudes and speeds.
4. Turboshaft Engine
Turboshaft engines are designed mainly to transfer mechanical power over generate thrust. They are widely used in helicopters and auxiliary power units (APUs).
How it Works
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Turbojet Core: The engine core operates similarly to a turbojet.
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Power Shaft: The turbine drives a power shaft connected to a transmission.
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Transmission: This system transfers rotational energy to the helicopter’s rotor blades.
In helicopters, this allows for precise control over lift and direction. Turboshaft engines also provide onboard power for larger aircraft when grounded.
The Bell 206 and Sikorsky UH-60 Black Hawk are examples of helicopters powered by turboshaft engines.
Gas Engines vs. Reciprocating Engines
A noticeable distinction between gas turbines and reciprocating engines lies in how they handle the four main engine functions.
In a reciprocating engine, these functions occur within the same combustion chamber in sequential stages. That basically means each phase must wait for the previous one to complete.
In a gas turbine engine, each function has its own dedicated section, allowing all processes to happen simultaneously and continuously.
A typical gas turbine engine is made up of the following sections:
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Air Inlet: Directs air into the engine.
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Compressor Section: Increases air pressure through multiple stages of compression.
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Combustion Section: Mixes air with fuel and ignites the mixture.
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Turbine Section: Extracts energy from the exhaust gases to drive the compressor.
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Exhaust Section: Expels high-speed gases to produce thrust.
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Accessory Section: Provides support systems, such as power for instruments and auxiliary components.
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Supporting Systems: Includes essential systems for starting, lubrication, fuel supply, and auxiliary functions like anti-icing and pressurization.
Frequently Asked Questions
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Why are turbojets less common today?
Turbojets are not as effective and noisy. They are less suitable for commercial aviation. Turbofan engines give you a quieter, and more fuel-efficient alternative. -
What is a bypass ratio in turbofan engines?
The bypass ratio is the amount of air flowing around the engine core compared to the air passing through it. High-bypass engines are quieter and more effective at subsonic speeds. -
What are APUs, and how do they work?
Auxiliary Power Units (APUs) are turboshaft engines that provide electrical power and pressurized air on the ground. They can also serve as backup power sources during flight. -
How do turboprops achieve fuel efficiency?
Turboprops generate thrust by accelerating large volumes of air with a propeller. That make them great at moderate speeds and altitudes. -
Are turboshaft engines only used in helicopters?
No, turboshaft engines also power APUs and other mechanical systems on large aircraft.
Takeaway
Gas turbine engines have come a long way, evolving into four main types for different aviation needs.
Turbojets prioritize speed, turboprops excel at mid-range operations, turbofans balance performance and noise reduction, and turboshafts provide mechanical power for helicopters and auxiliary systems.
As you learn more about these engines you will begin to see the remarkable engineering behind modern aviation.
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