I can't understand Jet engines.

I understand the principle. Air goes in, fuel is injected, hot air goes out fast.

But I can’t work out the logic of how the above process causes the engine to move in a particular direction. When I imagine the process I just see that injecting fuel into the air and it expanding would force hot air out of both ends.

I know the fans and the intake of air stops that from happening, but if the fans are stopping it then aren’t the fans doing the… I won’t get bogged down in details. I hope you understand what I am getting at.

From HowStuffWorks:

Nice cite, Una Persson

Oh, no. Get bogged down in the details. You are this close.

Simple answer: You have it right with

It would go out both ends of the combustion can if it could. But the forward end of the can is closed, and the aft end is open. That pressure against the closed end of the can is the thrust that the engine is developing.

Complex answer: I don’t even need the combustion cans to face aft. I could redirect the exhaust gas flow with ducting. The net pressure distribution on that ducting would then give me nearly the same thrust as if the cans were directed aft. Of course, I’d have to cool the ducting and who needs that headache.

Okay, I must be misunderstanding you. How is the forward end of the combustor closed? How does the compressor air get in if it’s closed?

The burner cans already have to be cooled by secondary airflow through the combustor. Could you clarify this for me?

Lobsang: The simple answer is that it’s easier for the exhaust gases to flow out the back of the engine rather than the front. To flow forward, they would have to overcome the pressure of the compressor. The highest pressure in a typical gas turbine is at the compressor exit/combustor entrance. It’s easier just to flow out the back, past the turbine.

Or, what Una said. Er, linked.

Don’t get me started, I could talk about gas turbines for hours.

Of course, you could make your own.

No worries. I was simplifying for the case presented by the OP. If you know about gas turbines I’m probably preaching to the choir. The important difference I was trying to point out was that the forward end is closed and the aft end is open. The combustion can is perforated with holes to allow primary air flow for combustion into the can. Air flow not diverted flows past the can for secondary air flow cooling. Fuel is delivered with a spray nozzle at the front of the can. The engineering for stabilizing the flame front and combustion nucleus is way beyond me. Them guys is clever.

While I detest responding with just a link, and prefer to explain if I can, I thought in this case the question referenced matched the OP very closely, and the answer on “How Stuff Works” was straight and to the point.

Just to be straight, that was a real compliment with no sarcasm intended. I don’t visit How Stuff Works nearly enough.

      • Well… the “HowStuffWorks” link isn’t quite correct: they go into the “wall of pressure from the compressor” bit, before it even comes into play. The exhaust gasses flow mostly out the rear because that is the easiest way to flow, even when the engine’s shaft is not rotating. The exhaust gasses moving mostly out the rear does cause the front compressor to spin, but the compressor never “causes the pressure in the combustor to rise”, because it can’t. The intake compressor is driven by the outflow of exhaust gasses from the combustor.

From how stuff works -

So, the turbine fan at the exhaust runs the compressor fans in the front of the engine. Right?

I’m way over my head here, but as I understand it, there is a series of ‘fans’ that compress the incoming air to help ignite the fuel in the combustion chamber. The outgoing exhaust turns a smaller fan that turns the bigger compressor fans in front.


here we go.


For something like a M1 tank, where you need rotational energy and not exhaust gas for thrust, the final stage of the turbine is hooked up to a ‘drive shaft’ instead of the compressor blades.

Propane - Check
Couple of old box fans - Check
Copper pipe for the drive shaft - Check
Extra Wheel Bearings from my truck - Check

hmmmm. Now all I need is a 55 gal drum. :slight_smile:

You got it, Bubba!

And that idea of using rotational energy is what constitutes the difference between a turbojet and a turbofan engine. Turbofans link the turbine to a big-ass fan in the front of the engine. Engine designs vary with the number of fans present before the air bypasses the jet portion of the engine, but the concept is similar.

A turbojet passes all of its intake air through the jet’s compressor and combustion area. A turbofan diverts some, often the majority, of its intake air around the jet portion letting the fan act like nothing so much as a big ducted prop. It turns out that it’s more efficient in thrust-per-fuel to do this. I think the correct name for this unit is poop-per-pint.

Heya MonkeyMensh,

And of course there is the TurboProp, which hooks up the jet to the prop to pull the plane.


Hmmmm. SDMB airlines. Still need a couple of 55 gallon drums, LOTS of aluminum, and hinges, I’m sure we will need hinges. :slight_smile:

I’ll work on the logo.

Great stuff. $15 a year, fly anywhere.

Right on.

I am such an aviation geek. I got to tour the Boeing plant in Long Beach on Thursday and I’m still jacked up.

Hook that pesky turbine up to anything. Helicopter rotors, power generators, and (if I get my way) … TABLE SAWS!

I design jet engine components for a living, so perhaps I can help. First, read the HowStuffWorks link - it’s explanation is correct, though brief. For some other details:

It is not necessarily true that the lowest-resistance flowpath is out the back, not the front. At 0 RPM, they’re about the same in area and drag. When the engine is running, the flow from the combustor is out the back because the compressor discharge is forcing it that way. The compressor does indeed cause the pressure in the combustor to rise, to the highest pressure in the entire engine in fact, because making pressure rise is what compressors do, in short.

The “wall of pressure” explanation is right but may not be detailed enough, as it doesn’t explain that static pressure and velocity are convertible into each other. The compressor discharges through a diffuser which slows the flow down and increases its static pressure (mainly because combustion gets harder to control at higher velocities), but that static pressure pushes on the forward wall of the combustion chamber and produces some of the engine’s net thrust, just like a rocket engine pushes on the front of its own combustion chamber. That’s much of the “wall of pressure” right there.

Think about this: Why is it possible for a steam engine to move a piston which provides enough useful work to move a pump (piston) which pumps water into the boiler? Very simple: the output piston yields more energy than is required to move the input piston. That’s what an engine is about.

So, in a jet engine you have a turbine which yields X HP and you only need a fraction of that to pump the air into the combustion chamber. If you used a piston compressor to do it nobody would have a problem understanding it, just like the steam engine. So just imagine a piston compressor and then realise a turbine compressor does the same thing, only better.

And then we can get into “Ram Jets”. Bawahahahaha

Just to add some more to “the gas goes where there is the least pressure”.

When starting a gas-turbine engine some external energy is used to start rotation of the engine BEFORE fuel is introduced. It can be an electrical motor that cranks the compressor, or bleed air from an APU (Auxiliary Power Unit). Engines vary, but most of the ones I’ve used have the inner (high-speed) compressor spinning at about 10% minimum before fuel and ingition are introduced. By doing this you avoid the gases not “knowing” where to go - the pressure from the spinning compressor means the back is the most efficient exit.

Most of the power goes to turning the compressor section, roughly 70-75%.

      • Yes, it must be true: elsewise, the combustor exhaust gasses would flow out the engine the wrong direction, and the jet engine would eventually stop spinning. There is no other reason that it would maintain flow out the rear, unless that was the path of least resistance. One cannot argue that “the forward motion…” or “the spinning compressor…” maintains flow, because the forward motion and spinning compressor are caused by the combustor exhaust stream.
  • Or to put it a simpler way: a ramjet has a little opening at the front, and a bigger opening at the rear. A turbine engine is a ramjet with a set of fans attached, but the fans do not repeal the laws of the basic physics involved.

Well, not really.

Turbine engines run very nicely at zero velocity, at least once you get them started. Ramjets run poorly at zero velocity. Turbofans and turbojets are extracting energy from the exhaust gas stream to compress inlet air for combustion. Once you get 'em burning they keep on turning.

Ramjets, on the other hand, use the dynamic pressure of the vehicle’s motion to compress inlet air. You scoop in a bunch of air and slow it down. The static pressure rises, you fling some fuel into the compressed air and stand back. Without that motion and dynamic air pressure a ramjet won’t run.

And how about that scramjet demo by NASA! Interesting times…