Of all airplane components, one of the most fascinating and complex aircraft parts are jet engines. Early jet engines included turbojets, while more modern aircraft use turbofans. Regardless of the variations in design, jet engines are giant pieces of machinery that generate incredible thrust to power aircraft. They draw in large amounts of air, mix parts of this air with fuel, and expel the resulting gasses out of their backside to move both the engine and the aircraft forwards. This blog will explore the way jet engines have performed this function over time and the way jet engine designs inform their functions.
In modern aircraft, nearly 90% of the air taken in by the jet engine passes directly through the engine to generate thrust without mixing it with fuel or igniting it. The simple force of this air through the fan at the front of turbofan engines does the majority of the lifting work, which is why you no longer see flames behind a jet engine. Since turbofans push air mainly around the core of the engine, they are much quieter than previous turbojet style jet engines, producing less vibrations and performing with greater efficiency.
Today, jetliners rely on large, high-bypass engines, those of which have high ratios of air bypassing the core as compared to air entering the core of the engine. Only the air that directly enters the core following the fan is mixed with fuel. Early versions of turbojet engines relied on greater amounts of fuel, especially those that depended on afterburners. The last civilian turbojet to fly was the Concorde, which used afterburners to inject additional fuel into the exhaust from the engine in order to generate forward thrust. This helped aircraft accelerate during takeoff, and it also worked to break the sound barrier. A major downside of turbojet engines, especially those paired with afterburners, is that they have no bypass and consume great amounts of fuel, making them expensive to operate.
On the other hand, turbofans save money by taking advantage of propulsive efficiency. By using bypass to generate 70-80% of thrust during takeoff and 95-100% of thrust after the aircraft is at cruising altitude, turbofan engines move larger volumes of air at lower speeds to increase efficiency. When understanding jet engines, it is often helpful for pilots to remember the path of airflow from the propeller to the exhaust in order to observe how the engine generates its power. For this, you can remember the phrase, “suck, squeeze, bang, and blow.”
The terms in this phrase each pertain to a different step in order of the air inside a jet engine, beginning with the fan. Jet engines begin by “sucking” in air through the fan and pushing most of this air around the core, while the remaining 10% of this air is directed into the core. This air then enters the “squeeze” phase. Here, it moves through a series of small spinning blades in the core, those of which are attached to a shaft that compresses the air. The spinning increases pressure by creating torque. Next, the compressed air is ignited with a “bang!” In this phase, the air passes through a combustor where it is mixed with fuel, forming a rapidly expanding hot gas mixture. Finally, the mixture enters the “blow” phase when it passes through both the high pressure and low pressure turbines. These turbines are spun by the passing gasses, which in turn moves a shaft that spins both the compressors and the fan at the very front of the engine. This entire “suck, squeeze, bang, and blow” system exists not to generate thrust directly, but mainly to power the massive fan at the front of the engine. At around 20,000 RPM, the spinning of the turbine does the work of converting thermal energy back into mechanical energy for flight.
Each blade of a jet engine’s fan does an immense amount of heavy lifting and could theoretically lift nine double-decker buses on their own. They are often made of metals like aluminum or titanium, and they spin at around 3,000 RPM. In order to maintain this speed which is lower than the speed of the turbines, special stages are needed in the engine. As such, engines rely on gear boxes to reduce the rotation. Today, technology allows for lightweight gearboxes, high bypass ratios, and increased fuel efficiency, all of which make jet engines powerful and fascinating pieces of modern technology.
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