When the first aircraft equipped with a jet engine took flight in 1939, it brought about a revolution in aviation technology. For the first time since engine-powered flight was possible, aircraft could travel long distances at speeds previously thought impossible. Today, nearly every commercial aircraft is designed with a jet engine as its primary source of power. While there are slight variations in the particular engines used in different aircraft, only three broad categories are employed for jet aircraft. In this blog, we will discuss the turbojet, turboprop, and turbofan, which will give you a better understanding of the most commonly used jet engines in civil and military aviation.
The turbojet, the oldest of the three jet engines, was first implemented in the Gloster Meteor in 1944. While the Meteor saw limited combat, it served as a model for the maneuverability and efficiency of jet engines. The turbojet would remain popular for high speed aircraft, such as the SR-71 Blackbird and Concorde, until being widely replaced with a more reliable engine design. While the turbojet is very complex in design, its function is intuitive and can be described as four distinct steps: intake, compression, combustion, and exhaust.
During the intake phase, subsonic air is directed into the engine and brought to the compressor. Modern turbojet engines only include a circular opening in which air is let in, whereas older designs implemented stationary veins to aid in directing the air. A large rotor, called the compressor, spins very quickly to add energy to the circulating air in the compression phase. The compressor squeezes the air into a smaller space, thus increasing its pressure and temperature. Once the high-energy and high-temperature air enters the combustion chamber, fuel is added and subsequently ignited, creating a high-energy exhaust. The exhaust leaves the combustion chamber and enters the turbine, where it rotates a blade that powers the compressor while the rest moves on to the nozzle. In the final step, the exhaust is slightly recompressed by the convergent shaped nozzle before releasing it into the atmosphere. The liberation of this high-energy exhaust provides thrust to the aircraft, thus propelling it forward.
While the earliest designs for the turboprop were being publicized during the same period as the turbojet, they did not receive the same widespread attention. The turboprop is only implemented in aircraft that fly at low speeds, such as small regional jets that are most efficient at speeds below 725 km/h. Turboprop operation is very similar to that of a turbojet, following the same four stages. The main difference is that the exhaust is used to spin an additional rearward turbine, which powers the propeller at the front of the engine. These engines are very reliable and have been chosen to power several popular aircraft, including the Lockheed C-130 Hercules.
The turbofan is the most modern and widely used propulsion system available. In fact, nearly every commercial aircraft has adopted this system due to its high thrust and superior fuel efficiency at high speeds. While having a similar internal construction as the turbojet engine, the turbofan is designed with a fan in the front and an additional rear turbine. Exclusive to the turbofan, an extra chamber surrounds the core engine in which air is allowed to flow. The air flowing through this external chamber gains speed and energy, and it contributes directly to thrust. The additional thrust from the engine bypass allows the engine to generate the same amount of power for a fraction of the fuel used by conventional turbojet engines.
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