Basics of Jet Engines

They're big, they're loud, and they help propel giant hunks of metal through the air at hundreds of miles per hour, the engine is one of the most important parts of any plane. The engines most people are familiar with are the ones on airliners such as the Boeing 747 or the Airbus A-320. These are turbofan engines, easy to remember because of the massive fan in their front that sucks air into its two pathways. The inner pathway works very similarly to a different type of engine, the turbojet engine used on most military aircraft, while the outer pathway simply diverts air around without doing too much else to it.

Image by Tushar Bansal, a turbojet engine at the Steven F. Udvar-Hazy Center

In order to fully understand how the turbofan engine works, we first need to examine the turbojet engine. This engine works by taking air in from the front, compressing it, and then combusting it in order to send the expanded gasses out the back and produce thrust. In order to do this, it employs two sets of compressor blades. The first set are the low pressure compressors which take the air from the atmosphere and begin to compress it down. This helps increase the difference in volume when the air is combusted and rapidly expands. The compressed air then flows through another set of compressor blades called the high pressure compressors which further compress this air. By the end of this process the air in the combustion chamber can be up to 40 times the pressure of the outside air! After the air is compressed, the fuel is injected and the air-fuel mixture is combusted. This forces the air to expand as it is expelled out the back of the engine, rotating a fan which in turn helps power the compressors. On military aircraft, there is sometimes one last step to this process: the afterburners. As the air is being expelled, the aircraft can release fuel behind the engine in order to produce last-minute expansion and increase the thrust provided by the engine in wartime scenarios. It is incredibly inefficient though, and therefore is never used on commercial or private aircraft.

Below is a diagram of both a turbojet and a turbofan engine in order to better understand how the whole process works:

Image from Aviation Stack Exchange

The turbofan engine is really similar to the one described above, except that it adds a bypass in order to cool the engine down and increase efficiency. This bypass air is reintroduced to the exhaust at the end of the process as its being expelled and rotating the turbine that drives the compressors. The only other difference is that there is a large intake fan before the compressors that helps suck air into the engine in order to feed both the combustion process and the bypass. This fan is also connected to the same shaft that turns the compressors and is powered by the exhaust leaving the engine. The interesting part of turbofan engines is that their efficiency increases as their bypass ratio increases since a lot of the thrust actually comes from the bypass. The bypass ratio is essentially the ratio of how much air bypasses the inner route vs how much air takes the inner route and is combusted.*

While the two engines mentioned above are the most popular types of jet engines there are other jet engines worth mentioning.

The first is the Ramjet. This engine essentially works like a turbojet engine except that it it relies on the ram pressure of the air hitting the engine intake to compress the air. This means that it doesn't have its own compressors, and cannot function properly at low speeds. The SR-71 Blackbird actually had a hybrid turbo-ramjet engine which allowed it take off and fly at low speeds using the

turbojet, and fly at incredibly high speeds with the ramjet (Check out Real Engineering's video on the SR-71 for more info).

Diagram of a Ramjet engine by Cryonic7 on Wikimedia

("M" values refer to the speed of the air in Mach. 1 Mach = the speed of sound)

The other is the S.C.Ramjet. This engine works almost identically to a ramjet. The only difference is that it uses supersonic air. Even on the ramjet used on the SR-71 which flew well above Mach 1, the engine spike would help slow the air down before it had a chance to combust. On a S.C.Ramjet this isn't the case. The S.C. in the S.C.Ramjet stand for Supersonic Combustion and basically tell the rest of the story. While a ramjet combusts subsonic air, the S.C.Ramjet combusts air that is at or above the speed of sound. This once again means that it's completely useless at slower speeds and needs either a turbojet engine, a rocket, or some other mechanism to get it up to supersonic speeds before the engine can kick in. Because of this it's use at subsonic and even trans-sonic speeds is minimal, but its incredibly efficiency at supersonic speeds makes it a really interesting choice when designing the next generation of supersonic aircraft.

Diagram of S.C.Rramjet engine by Emoscopes on Wikimedia

If you have any questions please feel free to leave them in the comments and I'll try my best to answer them, and If you found anything that I got wrong or if I misused any images please also feel free to leave a comment correcting me, I'm always happy to learn more!

* Fun fact: the LEAP engines get their incredible efficiency from having a huge bypass ratio, which requires a bigger engine. So big in fact, that Boeing actually had to redesign the engine placement on their 737-Max in order to retrofit the Engine onto it. Eventually I'll make a post about that whole situation too...