What is a Jet Engine?
A jet engine is a type of reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term jet engine typically refers to an internal combustion airbreathing jet engine such as a turbojet, turbofan, ramjet, or pulse jet. In general, jet engines are internal combustion engines.
Airbreathing jet engines typically feature a rotating air compressor powered by a turbine, with the leftover power providing thrust through the propelling nozzle this process is known as the Brayton thermodynamic cycle.
Jet aircraft use such engines for long-distance travel. Early jet aircraft used turbojet engines that were relatively inefficient for subsonic flight. Most modern subsonic jet aircraft use more complex high-bypass turbofan engines.
They give higher speed and greater fuel efficiency than piston and propeller aeroengines over long distances. A few air-breathing engines made for high-speed applications (ramjets and scramjets) use the ram effect of the vehicle’s speed instead of a mechanical compressor.
How does a jet engine work?
Jet engines move the airplane forward with a great force that is produced by a tremendous thrust and causes the plane to fly very fast.
All jet engines, which are also called gas turbines, work on the same principle. The engine sucks air in at the front with a fan. A compressor raises the pressure of the air. The compressor is made with many blades attached to a shaft.
The blades spin at high speed and compress or squeeze the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine.
As the jets of gas shoot backward, the engine and the aircraft are thrust forward. As the hot air is going to the nozzle, it passes through another group of blades called the turbine. The turbine is attached to the same shaft as the compressor. Spinning the turbine causes the compressor to spin.
The image below shows how the air flows through the engine. The air goes through the core of the engine as well as around the core. This causes some of the air to be very hot and some to be cooler. The cooler air then mixes with the hot air at the engine exit area.
What is Thrust?
Thrust is the forward force that pushes the engine and, therefore, the airplane forward. Sir Isaac Newton discovered that for “every action there is an equal and opposite reaction.” An engine uses this principle.
The engine takes in a large volume of air. The air is heated and compressed and slowed down. The air is forced through many spinning blades. By mixing this air with jet fuel, the temperature of the air can be as high as three thousand degrees.
The power of the air is used to turn the turbine. Finally, when the air leaves, it pushes backward out of the engine. This causes the plane to move forward.
Parts of a Jet Engine
1. Fan
The fan is the first component in a turbofan. The large spinning fan sucks in large quantities of air. Most blades of the fan are made of titanium. It then speeds this air up and splits it into two parts.
One part continues through the “core” or center of the engine, where it is acted upon by the other engine components.
The second part “bypasses” the core of the engine. It goes through a duct that surrounds the core to the back of the engine where it produces much of the force that propels the airplane forward. This cooler air helps to quiet the engine as well as adding thrust to the engine.
2. Compressor
The compressor is the first component in the engine core. The compressor is made up of fans with many blades and is attached to a shaft. The compressor squeezes the air that enters it into progressively smaller areas, resulting in an increase in the air pressure.
This results in an increase in the energy potential of the air. The squashed air is forced into the combustion chamber.
3. Combustor
In the combustor, the air is mixed with fuel and then ignited. There are as many as 20 nozzles to spray fuel into the airstream. The mixture of air and fuel catches fire. This provides a high temperature, high-energy airflow.
The fuel burns with the oxygen in the compressed air, producing hot expanding gases. The inside of the combustor is often made of ceramic materials to provide a heat-resistant chamber. The heat can reach 2700°.
4. Turbine
The high-energy airflow coming out of the combustor goes into the turbine, causing the turbine blades to rotate. The turbines are linked by a shaft to turn the blades in the compressor and to spin the intake fan at the front.
This rotation takes some energy from the high-energy flow that is used to drive the fan and the compressor. The gases produced in the combustion chamber move through the turbine and spin its blades. The turbines of the jet spin around thousands of times. They are fixed on shafts that have several sets of ball-bearing in between them.
5. Nozzle
The nozzle is the exhaust duct of the engine. This is the engine part that actually produces the thrust for the plane. The energy-depleted airflow that passed the turbine, in addition to the colder air that bypassed the engine core, produces a force when exiting the nozzle that acts to propel the engine, and therefore the airplane, forward.
The combination of hot air and cold air is expelled and produces an exhaust, which causes a forward thrust.
The nozzle may be preceded by a mixer, which combines the high-temperature air coming from the engine core with the lower-temperature air that was bypassed in the fan. The mixer helps to make the engine quieter.
Types of Jet Engines
The 5 Main Types of Aircraft Jet Engines
- Turboprop Jet Engine
- Turbojet Jet Engine
- Turboshaft Jet Engine
- Turbofan Jet Engine
- Ramjet Jet Engine
1. Turbojet Jet Engine
The basic idea of the turbojet engine is simple. Air taken in from an opening in the front of the engine is compressed to 3 to 12 times its original pressure in the compressor.
Fuel is added to the air and burned in a combustion chamber to raise the temperature of the fluid mixture from about 1,100°F to 1,300° F. The resulting hot air is passed through a turbine, which drives the compressor.
If the turbine and compressor are efficient, the pressure at the turbine discharge will be near twice the atmospheric pressure, and this excess pressure is sent to the nozzle to produce a high-velocity stream of gas which produces a thrust. Substantial increases in thrust can be obtained by employing an afterburner.
It is a second combustion chamber positioned after the turbine and before the nozzle. The afterburner increases the temperature of the gas ahead of the nozzle. The result of this increase in temperature is an increase of about 40 percent in thrust at takeoff and a much larger percentage at high speeds once the plane is in the air.
The turbojet engine is a reaction engine. In a reaction engine, expanding gases push hard against the front of the engine. The turbojet sucks in air and compresses or squeezes it. The gases flow through the turbine and make it a spin. These gases bounce back and shoot out of the rear of the exhaust, pushing the plane forward.
2. Turboprops Jet Engine
A turboprop engine is a jet engine attached to a propeller. The turbine at the back is turned by the hot gases, and this turns into a shaft that drives the propeller. Some small airliners and transport aircraft are powered by turboprops.
Like the turbojet, the turboprop engine consists of a compressor, combustion chamber, and turbine, the air and gas pressure is used to run the turbine, which then creates the power to drive the compressor.
Compared with a turbojet engine, the turboprop has better propulsion efficiency at flight speeds below about 500 miles per hour. Modern turboprop engines are equipped with propellers that have a smaller diameter but a larger number of blades for efficient operation at much higher flight speeds.
To accommodate the higher flight speeds, the blades are scimitar-shaped with swept-back leading edges at the blade tips. Engines featuring such propellers are called propfans.
3. Turbofan Jet Engine
A turbofan engine has a large fan at the front, which sucks in air. Most of the airflow around the outside of the engine, making it quieter and giving more thrust at low speeds. Most of today’s airliners are powered by turbofans.
In a turbojet, all the air entering the intake passes through the gas generator, which is composed of the compressor, combustion chamber, and turbine. In a turbofan engine, only a portion of the incoming air goes into the combustion chamber.
The remainder passes through a fan, or low-pressure compressor, and is ejected directly as a “cold” jet or mixed with the gas-generator exhaust to produce a “hot” jet. The objective of this sort of bypass system is to increase thrust without increasing fuel consumption.
It achieves this by increasing the total air-mass flow and reducing the velocity within the same total energy supply.
4. Turboshaft Jet Engines
This is another form of the gas-turbine engine that operates much like a turboprop system. It does not drive a propeller. Instead, it provides power for a helicopter rotor.
The turboshaft engine is designed so that the speed of the helicopter rotor is independent of the rotating speed of the gas generator.
This permits the rotor speed to be kept constant even when the speed of the generator is varied to modulate the amount of power produced.
5. Ramjets Engines
The simplest jet engine has no moving parts. The speed of the jet “rams” or forces air into the engine. It is essentially a turbojet in which rotating machinery has been omitted. Its application is restricted by the fact that its compression ratio depends wholly on forwarding speed.
The ramjet develops no static thrust and very little thrust in general below the speed of sound. As a consequence, a ramjet vehicle requires some form of assisted takeoff, such as another aircraft. It has been used primarily in guided-missile systems. Space vehicles use this type of jet.
Why are jet engines so difficult to manufacture?
In engineering, some things are always difficult and expensive. The jet engine is one of them. The thing that makes jet engine manufacturing difficult are given below.
- High thermal range: jet fuel burns in the engine at a temperature of 2000° C, but the aircraft can fly below -50° C in the air.
- High Precision: the tolerance within jet engines (distance between two interacting moving components) is very tight and closer than with normal engines.
- High speed: The compressor blades rotate at astounding speeds – thousands of rpm.
- Reliability: Jet engines require high reliability (for example very high), that’s obvious. Now put all these things together, and the materials and manufacturing techniques, and equipment start to get pretty expensive.
- Accuracy: Because of the high precision of jet engine components (particularly engine blades), it takes a lot of work to design such balanced parts for the engines that produce such high power for that particular jet engine size. Compared to internal combustion engines (ICE), the end product requires more engineering construction.
- Efficiency: Jet engines are simple in terms of combustion cycles but very complex in terms of improvements in efficiency and reliability. The production efficiency is not very high, and there are many prices resulting from the price that expensive people can buy.
Advantages and Disadvantages of Jet Engines Over Reciprocating Engines:
Advantages
The following are the advantages of using a jet engine:
- The weight of a jet engine is less than a quarter that of a reciprocating engine.
- A jet engine is free from unbalanced forces because there are no reciprocating parts.
- The frontal area of a jet engine is less than a quarter of the frontal area of reciprocating engines, which greatly reduces drag. Therefore, it provides more power at higher loads.
- The speed of a jet engine is not limited by the propeller, which means that higher speeds can be achieved.
- A jet engine can be built with a much higher power output than reciprocating engines because power is not limited by detonation. Higher power can be achieved.
Disadvantages
The following are the disadvantages of using a jet engine:
- Especially at low pressure, the thermal efficiency is low. Fuel consumption is more than a reciprocating engine at low altitudes and speeds up to 150 m/s/540 kmph.
- The working place is too noisy, the materials are expensive, and life is short.
- The compression-pressure ratio is similar to the reciprocal but changes with speed.
- Some difficulties arise in the operation of the propellant unit.
Applications of Jet Engines
- Jet engines are most commonly used to power the commercial aircraft.
- Jet engines such as turbofan or turboprop engines are used in business jets and some smaller general aviation aircraft.
- They are employed in military aircraft because of their high power-to-weight ratios and the capability to run at high speeds and altitudes.
- Turboshaft engines are used to power the helicopters.
- They are utilized to power different aerospace applications, including rocket boosters and powering launch vehicles.
- These engines are sometimes employed in some high-speed marine applications like experimental naval vessels and hydrofoil boats.