I feel a bit stupid for asking this, but I simply cannot wrap my head around how a jet engine directs its thrust backwards.
A piston engine opens and closes valves, such that air is alternately sucked in and compressed to create force. That makes sense and I understand how it works.
But with a jet engine there are no valves opening and closing, so why doesn’t the air that is sucked in just get pushed right back out from the thrust generated? How can an engine simultaneously suck in air from the front and force it out the back?
It makes no sense to me :-/
In: Engineering
Think of it like a desk fan or a ceiling fan. Air is cycled through a LOT of REALLY fast fans. Fuel gets shot in, which accelerates the air that gets pushed out the back. It’s the exploding jet fuel that creates the extra force!
The way forward is obstructed – there is the high pressure compressor with dozens of layers of fan blades mashing air backwards. The way out back is much less obstructed, just the turbine blades (much easier to turn than driving the compressor backwards).
The same way the flame is directed when you hold a lighter in front of a spray can. You are igniting already moving gas.
The turbofan (or turbojet) pulls air in mixes it with fuel and then ignites it.
The fan and turbine in the engine works to try to push everything in the right direction. Furthermore, the actual design of the engine itself makes it easier for flow to occur in one direction and not the other, encouraging everything to go in the right direction.
However things to go wrong, and without physical valves to prevent it, the flow goes backwards, this is known as a compressor stall. Notably this can be caused if the airflow into the engine is suddenly disrupted for some reason, which if you think about it makes sense. If for example, the engine’s airflow is suddenly and drastically reduced, there is then a lot less air for the fans in the engines to be pushing in “right direction”, which may result in the combustion overpowering the air being pushed in it’s way, allowing it to go “backwards”
They used to be more common (though not sure how common is “more common”) because older jet engines were obviously produced with older and less advanced understanding of making jet engines work. Luckily for us, they are much less common nowadays, and would likely mean there was already some damage with the engine.
dont feel stupid man the jet engine is pretty wild. it works by sucking air in and compressing it with fans and then burning fuel to create hot gases. those gases expand and get forced out the back really fast creating thrust. think of it like when you blow up a balloon and let it go. the air rushes out one way and it’s like whoosh you go the other way. pretty neat right
The combustion in a jet engine does not increase the pressure of the gas, only the volume. The combustion chamber expands towards the rear to allow the gasses to expand without increasing in pressure. The pressure on the inlet side of the combustion chamber is actually slightly higher then on the exhaust side, but the area is much smaller. So there is a lot more gas exiting the combustion chamber then entered it, even though it is at roughly the same pressure.
The pressure is generated from the turbine in front of the combustion chamber. You know how a turbo in a piston engine will compress the gas before it enters the cylinder so the compression ratio in the cylinder can be lower. A jet engine basically have such a big turbo that their compression ratio is 1. This big turbine in the front to compress the air that much is driven by the exhaust fan at the back, just like in a piston engine turbo. And because the exhaust have so much more volume then the intake air it will generate a lot more force on the exhaust fan then it takes to run the intake turbine.
The shape of the jet engine pinches inwards at the end of the compressor blades. This essentially crams the air into a funnel so fast the air has to compress.
If you look at a cross section of a jet engine, it’s usually really wide at the front, there’s a cone/cylinder-like thing running down the middle from front to back that goes from narrow to wide back to narrow, and the back end is again wide. The part in the middle creates a venturi effect (fancy name for taking air in a wide tube and shoving it quickly through a sudden narrowing of the tube) which squeezes the incoming air and makes it move much faster.
Into this faster moving air you add jet fuel and ignite it. What happens is the jet fuel and compressed air ignites into something very hot, and the fuel and air mix creates a lot of extra stuff – mostly gas – from burning (carbon monoxide, carbon dioxide, water vapor, etc.). Jet fuel takes up little space. Compressed air (venturi effect) takes up little space. But burning jet fuel with compressed air creates hot gases which takes up a LOT of space which has to move out the back end of the engine – enough space to help push the aircraft forward.
So, what’s coming out of a jet engine isn’t just fast moving air, it’s fast moving air mixed with jet fuel and combusted. And what comes out is a LOT of hot gases that’s a lot more than just the air that went in.
Modern jets have vectored thrust:
https://en.m.wikipedia.org/wiki/Thrust_vectoring
Before that, You just had to aim the front of the plane, and the ass end always made you go in a straight line.
The direction of rotation and the angle of the blades keeps the air moving in one direction. Also, the air intake is massive compared to the exhaust. This is necessary because you have to take in a huge volume of air to compress it, mix it with fuel, and shoot it out the exhaust.
This all depends on the engine being able to suck in air. If it’s not sucking in enough air, the flow can in fact reverse. This is called compressor surge (or stall). This can happen in planes when the airflow is disrupted by the plane, say, trying to fly at too steep of an angle of attack.
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