Introduction to Jet Engines
Jet engines have been the driving force behind aircraft for nearly a century. This article explains the technology behind jet engines in a logical, step-by-step manner. A jet engine propels an aircraft forward using a simple principle—the same principle that makes an air-filled balloon move: Newton’s third law of motion. Just as the reaction force from the escaping air moves a balloon, the reaction force produced by the high-speed jet exiting the tail of a jet engine propels it forward.
How Jet Engines Work
The primary function of a jet engine is to generate a high-speed jet at the exhaust. The faster the jet speed, the greater the thrust force, which propels the aircraft forward. This high-speed exhaust is achieved through a combination of techniques. Heating incoming air to a high temperature causes it to expand dramatically, creating the required high-velocity jet. This is accomplished in the combustion chamber, where atomized fuel is burned. Effective combustion requires air at moderately high pressure and temperature.
Compressors and Turbines
To achieve these conditions, the air passes through a series of compressor stages. The rotating blades of the compressor add energy to the air, raising its pressure and temperature to levels suitable for combustion. The energy needed for the compressor’s rotation comes from a turbine, located immediately after the combustion chamber. The turbine and compressor are connected by the same shaft. High-energy gases exiting the combustion chamber drive the turbine blades, which have a specialized airfoil shape to generate lift and cause rotation.
Energy Extraction and Acceleration
As the turbine absorbs energy from the gases, the pressure of the gases decreases. These steps result in a high-temperature, high-speed airflow exiting the engine. The narrowing shape of the engine casing near the outlet further accelerates the jet, increasing velocity. The synchronized operation of the compressor, combustion chamber, and turbine enables the aircraft to move forward efficiently.
Modern Advancements in Jet Engines
Modern jet engines use improved turbine-compressor arrangements, such as a two-spool system. In this setup, two independent turbine-compressor stages operate on concentric shafts. The outer turbine operates at lower speed and pressure, reducing stress on its longer blades and increasing their lifespan. Some advanced engines even feature three spools for greater efficiency.
The Evolution to Turbofan Engines
The traditional engine described above is specifically known as a turbojet engine, which produces significant noise. A revolutionary advancement was the addition of a large fan to the low-pressure spool, creating the turbofan engine. Turbofan engines, used in most commercial aircraft, bypass a large volume of air around the core. The narrowing bypass duct increases the velocity of this bypassed air.
Advantages of Turbofan Engines
In a turbofan engine, most of the thrust is generated by the fan’s reaction force. The fan also enhances airflow through the system, improving overall thrust while slightly increasing fuel consumption. This makes turbofan engines highly fuel-efficient. Additionally, the mixing of bypassed cold air with hot exhaust reduces exit jet velocity, significantly lowering noise levels. With their quieter operation and superior fuel economy, turbofan engines continue to dominate modern aircraft propulsion systems.
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