A Complete & A Detailed Overview
Jet engines are a crucial part of planes. They serve as the primary way through which planes get their power. There is a multitude of control present in the cockpit which requires a lot of power to operate. Earlier planes used to have manual control but nowadays they are replaced with digital display and fly-by-wire technology.
Though generally, commercial planes use jet engines for their commute but there are other types of engines used for military-based and experimental purposes. These engines are ramjets and scramjets. The shape and size of these engines are very similar to that of any typical jet engine.
Jet engines have undergone many changes throughout its lifespan of around a hundred years. A major change came when the company General Electric made some significant changes to the engine which revolutionized the jet engines as a whole (this change has been talked later in the article).
WORKINGS OF DIFFERENT JET ENGINES
1. JET ENGINES
Jet engines work on the basic principle of Newton’s 3rd law of motion, for simplicity, the air going out of the engine is at a higher speed than the air coming into the engine which produces thrust.
A big fan at the front collects air for the engine to burn a mixture of fuel and oxygen(from the air) which escapes from the nozzle at the end while providing thrust. The turbine at the end is just there to drive the compressor.
Consider the mechanism in which the gases escaping from the free-floating balloon makes the balloon move forward with a lot of acceleration in the forward direction is similar to the workings of a typical jet engine.
Over the last century, many brilliant innovations have arisen in these engines which has led to the formation of new subcategories, although the basic concept behind these engines are pretty much the same.
These are subdivided as:-
a.) Turbojet Engine (used in older jetliners)
The basic idea of the turbojet engine is very simple. Air taken from an opening is compressed then mixed with fuel in a combustion chamber. The resulting hot air passes through a turbine which drives the compressor.
This excess pressure is sent to the nozzle to produce a high-velocity stream of gas which produces thrust. These engines were used by the previous generation planes. The opening air intake of these engines was very small compared to today’s jets.
b.) Turbofan Engine (used in modern jetliners)
The turbofan is simply a more advanced turbojet engine in which there is a large fan at the front which sucks tremendous amounts of air into the engine which gives more power at low speeds.
In this engine, only a portion of incoming air goes into the combustion chamber. The remainder of the air is passed through a fan and is ejected from outside of combustion chamber. This ‘cold’ gas is mixed with hot air exiting the combustion chamber. This technique reduces fuel consumption which giving the engine more power.
More power can be generated through an afterburner. In the second combustion chamber positioned after the turbine and before the end nozzle, the fuel can be mixed directly into the hot escaping gases which produce about 40% more power from the same engine. Afterburners increases the consumption of fuel dramatically.
Almost all of today’s modern jetliners are powered by turbofans. This radical modification was developed by the company General Electric.
c.) Turboprop Engine (used in short distance planes)
It is simply a jet engine attached to a propeller. The turbine at the back is turned by the hot gases, and this turns the shaft that drives the propeller. Some small airliners and transport aircraft are powered by turboprops.
Just like the turbojet, the turboprop functions the same way but it has a better propulsion efficiency at speeds below than 500 kph. To increase efficiency modern turboprops are equipped with propellers that have a smaller diameter but a larger number of blades.
You might have guessed it by now turboprop is the acronym for turbo-propeller.
d.) Turboshaft Engine (used in helicopters)
This engine operates more like a turboprop engine. It doesn’t drive a propeller instead it provides power for a helicopter rotor. This system 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.
As a matter of fact, one would imagine ramjets to very complex but you will be surprised to know it is not. It is one of the simplest engines that one can imagine. These engines doesn’t even have a moving part. Can you imagine how ridiculously simple would it be?
In jet engines, the role of the compressor is to raise the temperature of the gas. So if we are going fast enough we really don’t need the compressor because just slowing the flow down to subsonic conditions will raise the temperature and pressure to the levels for a good combustion. Therefore, for a jet engine traveling at very high speed, we don’t need a compressor. If we throw away the compressor we are left with a ramjet.
In ramjet, we can see a pointed inlet that slows the air to subsonic speeds which raises the pressure and temperature of the air. Then some fuel is added in the combustion chamber to product thrust. The exhaust produced is escaped from the nozzle at the end at a higher subsonic speed than the air coming into the engine.
The air passing throughout the engine is subsonic, only the exiting exhaust is supersonic.
The only big downside to this engine is that it can’t start from a standstill it needs a speed of at least Mach 3 which acts as its initial speed and can go up to a speed of Mach 6. Speeds beyond Mach 6 is not possible because the Ram effect alone raises the temperature of the subsonic air too high to encounter. This is where scramjets come into action.
SCRamjet stands for Supersonic Combustion Ramjet. The only thing happening differently from ramjets is that the combustion of the air and fuel is happening at supersonic speeds in the combustion chamber and the exhaust coming out of the end of the nozzle is also at supersonic speeds. The rest of the things discussed in ramjets are identical to scramjets.
Scramjets can have multiple openings, multiple combustion chambers and you guessed it multiple end nozzles. The same goes for ramjets.
It also has the same disadvantage as ramjets as it can’t operate from a standstill rather it needs Mach 6 as its initial speed and its final speed is unknown because we have not gone to such speeds that it is capable of. The highest ever recorded speed done by a man-made engine for non-space aircraft is Mach 9.6 or 7000mph. It was NASA’s X-43A.
Higher speeds in scramjet engines is still difficult because of 2 factors:-
a.) The aerodynamic forces on the plane becomes very large.
b.) The heat produced from the friction of air on the body becomes extremely high.
There is a crossover region at the final speed of ramjets and the initial speed of scramjets where either a ramjet or a scramjet can be used.
MACH NUMBER (M or Ma)
It is defined as the ratio of the speed of a body to the speed of sound in its surrounding medium. In other words, the speed of planes in accordance with the speed of sound. Mach 1 represents the speed of sound which is 343m/s or 1238km/h 770mph in air at 1 atm. Similarly, Mach 2 represents twice the speed of sound, & so on.
A typical civilian plane can reach speeds less than Mach 1. It is deliberately done so to avoid the complications that come with supersonic speed.
Do you want NASA to tell you more about Mach Number?, click the link: https://www.grc.nasa.gov/www/k-12/airplane/mach.html
Only one plane in aviation history was intentionally designed to break the sound barrier for civilian planes which was the Concorde. It was one of the most ambitious projects of the 20th century ranked after the Apollo program. It could do the trans-Atlantic flight in just 3 hours, half of that of conventional flights. But it never made a commercial success, it only looked good on paper that’s why after an accident in 2003 it was decided to ground all the 14 operating planes. Until then it was only reserved only for the rich as ticket prices were astronomically high.
……SONIC: THE HEDGEHOG
Subsonic is the speed that falls below Mach 0.8. Generally, all the civilian aircraft belong to this category. These speeds don’t possess any danger of flying. All kinds of planes whether it is manned and unmanned belong to this category. Going from one place to another is done in this category. At this speed, it can take about 7 hours to do the trans-Atlantic flight for any jetliner.
For more information on the physics of subsonic speeds see the article: https://www.sciencedirect.com/topics/engineering/subsonic-flight
Transonic is the speed that ranges from Mach 0.8 to Mach 1.2. Generally, the end of the rotating fan propeller of aircraft falls in this category. The spinning motion of blades makes them reach these speeds. Some civilian aircraft also belong to this category. They don’t exceed Mach 1 but they do exceed Mach 0.8.
For more information on the physics of transonic speeds see the article: https://en.wikipedia.org/wiki/Transonic#Transonic_flows_in_astronomy_and_astrophysics
Supersonic is the speed that ranges from Mach 1.2 to Mach 5. Generally, all the military fighter planes fall in this category. Whenever a plane crosses the supersonic point it produces a loud sound called as the sonic boom. Earlier, in the second world war, it was considered dangerous to cross the sound barrier to the point that they thought that this could not be done. Jet engines can easily achieve these speeds.
For more information on the physics of supersonic speeds see the article: https://en.wikipedia.org/wiki/Sonic_boom
Hypersonic is the speed that ranges from Mach 5 to Mach 10. Generally, only experimental planes fall into this category. Scramjets are used to power these kinds of aircraft. The fuel required to achieve hypersonic speed is so massive that the time of flight generally lasts not more than a minute.
For more information on the physics of hypersonic speeds see the article: https://en.wikipedia.org/wiki/Hypersonic_flight#Physics
High-Hypersonic is the speed that ranges from Mach 10 to Mach 25. Mind this, in all of aviation history no non-spacecraft aircraft has ever reached this milestone. The closest we have come is Mach 9.8 and that too it was unmanned. This feat was achieved recently. As mankind has not reached this category very little is known about the science of this category and continuous studies and experiments are going on to break this barrier.
For more information on the physics of high-hypersonic speeds see the article: https://en.wikipedia.org/wiki/Hypersonic_speed
6. Re-entry Speeds
Re-entry is the speed that falls above Mach 25. Space shuttle upon re-entry generally does these kinds of speeds. Doing these kinds of speeds come with a lot of challenges of its own. Tremendous amount of frictional heat is generated at a higher speed. Surfaces of the aircraft can reach upwards of 1000°C upon re-entry. At re-entry, Space Shuttle can do speeds of Mach 27 and as a result of that re-entry is considered the most dangerous part of a mission.
For more information on the physics of re-entry speeds see the article: https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/cami/library/online_libraries/aerospace_medicine/tutorial/media/iii.4.1.7_returning_from_space.pdf
SPECIALTY OF JET ENGINES ( INTERESTING FACTS & THEORIES)
- Have you ever wondered why we don’t use jet engines in rockets, in spite of both doing basically the same thing? This is because rockets work on the principle of Newton’s 3rd law of motion which is universal and doesn’t require any specific medium to work but this is not the case with airplanes as they need air as the medium.
- There is this crazy theory which some people came up with. It states that if suppose an airplane wants to go from London to New York, what if that airplane just rose vertically upwards in London and waited for the Earth to rotate just enough so that it vertically lands in New York. Crazy isn’t it?
This theory is not possible because the plane taking off from New York has the same speed as that of the rotation of the Earth and when it takes off it doesn’t have any change in its speed whatsoever. So the plane taking off from New York will land in New York.
- The shortest distance between North America & Europe in never a straight line. This is because if you want to go around the Earth horizontally while considering the curvature of the Earth, the closer you are to the poles the less distance you have traveled to complete a full circle. Hence, the more distance you will travel closer to the poles the less distance you have to travel overall. As a result of this, the flight path looks like upside-down ‘U’ shape for the northern hemisphere and ‘U’ shape for the southern hemisphere.
- You must have seen that a plane requires only 2 or 3 people to operate but a rocket needs a complete team working together on the ground as well as in the shuttle.
During the initial years of space flight around the 50s and 60s, NASA was testing the rockets to know how to operate them in future missions. Finally, they came out to this conclusion that although machines could operate the shuttle well but humans would are more reliable and would be able to do a better job. Hence, for each instrument, people are required to monitor each and every aspect of it on the ground.
- One of the great innovations in jet engines came when the company General Electric made the engine bigger without making the engine bigger.
A regular jet engine mixes air and fuel in a combustion chamber and the resulting exhaust is propelled out of the back which produces thrust but General Electric came up with a modification in which they added a big turbofan at the front that drives more air at high speed into the combustion chamber and even more through a bypass chamber around the outside thus forcing more air around as well as through the combustion chamber and ultimately the engine becomes more powerful than ever before.
Refer the video for more info about the revolutionary innovation that company General Electric made:
FOR COMPLETE GRAPHICAL INFORMATION ON JET ENGINES WATCH THIS INCREDIBLE VIDEO: