Jet engine

[Panther91]

-I know all about how the turbine engine works, but how does it mix fuel? It doesn't have any mixture control like piston engines and I saw in the Learning Center that that is automatic. When I'm flying the F-16 in Falcon 4, as I'm getting to higher altitudes the fuel flow decreases all the time. Does it mean that the turbine engine is automaticaly leaning the mixture or what? Is the aircraft computer responsible for that or just the engine? What if some jet doesn't have a computer, or some older jets from 1950s which didn't have computers for sure?

-Why doesn't the jet engine have magnetos? Or maybe they do have it for starting the engine but they don't need it all the time like piston engines. Is this because the combustion in the jet engine never stops because the air and fuel are mixing all the time and the flame is there from start to shut down so there's no need to ignite it all the time except when starting the engine? I mean, in piston engine the combustion happens in the closed cylinder so when the mixture ignites the mixture burns very fast and then the mixture is gone, because the air can't enter the cylinder until intake stroke.

[pepper_airborne]

Yet engines dont need a mixture, you control a jet engine by adding extra fuel to it. it sucks in the air, compresses it, throws in the fuel and ignites that.

i know the jet engine on a Bell 206B is started by a gas turbine, as a helper, which throws in the hot air, then the pilots introduces fuel into the main engine which then ignites and can sustain itself. Most planes have an APU to help them startup there main engines.

[DaveSims]

Jet engines are a completely different beast than piston engines.  To answer your questions, the fuel is automatically controlled on a turbine engine to match the airflow and power settings it needs.  The magnetoes in a piston engine are the equivalent of a distributor in a car, the produce the spark for the spark plugs to ignite the fuel in each piston at the right time.  In a jet the combustion is continous and doesn't need an ignition as such.  However, most turbines do have an ignition control which is part of the starting process.

[Panther91]

To answer your questions, the fuel is automatically controlled on a turbine engine to match the airflow and power settings it needs.

So how does the fuel get matched with the airflow? Does the aircraft computer do that or the engine by itself somehow?

[DaveSims]

I'm not sure exactly what mechanism there is for the fuel control, but I do know turbines have a fuel control unit.  How it works I'm not sure, but the pilot does not have any control over it other than off or on.

[Brett_Henderson]

I'm no jet mechanic..

[Hagar]

All you wanted to know about fuel metering for gas turbine engines. http://www.globalsecurity.org/military/library/policy/army/fm/1-506/Ch4.htm

[Panther91]

But I always understood that turbine fuel control is the throttle only. It's pretty much a variable valve. If you want more power, you open the valve more.

So the valve only controls how much fuel are you introducing into the combustion chamber, right? The valve does not also control how much air is going in the chamber like in a piston engine because the air comes in the chamber directly from the intake and compressor and it depends only on how fast the turbine and the compressor are turning,right? More power, faster turbine and compressor and more air coming in the chamber, but the valve doesn't have to do anything with the air, only with the fuel. Is this correct?

[Brett_Henderson]

But I always understood that turbine fuel control is the throttle only. It's pretty much a variable valve. If you want more power, you open the valve more.

So the valve only controls how much fuel are you introducing into the combustion chamber, right? The valve does not also control how much air is going in the chamber like in a piston engine because the air comes in the chamber directly from the intake and compressor and it depends only on how fast the turbine and the compressor are turning,right? More power, faster turbine and compressor and more air coming in the chamber, but the valve doesn't have to do anything with the air, only with the fuel. Is this correct?

That's an over-simplification, I'm sure.. but yeah, that's how I understand it.

Mixture ratios for piston engines are different in that they happen at a throttle.. which by definition is what controls the amount of air allowed into the combustion chambers. You limit an engine's RPMs by "throttling" the amount of air let through.

With a turbine, the mixture is a non-issue. You aren't trying to throttle the inlet air. How much airflow there is, is a function of the spinning turbines..whose speed s a direct function of the energy being generated, which is of course how much fuel is being introduced.

To speed up a turbine, you give it more fuel. To speed up a piston engine, you open the throttle.

To slow down a turbine, you reduce the amount of fuel. To slow down a piston engine, you close the throttle  (reducing the available air flow).

[DaveSims]

Brett

I figured you could explain it a little better than me.  I had an aircraft powerplant class in college, but we didn't even get into the details of how the engine works, other than suck, squeeze, bang, blow.  

[Splinter562]

Brett is correct as usual. In it's most basic form, you are simply increasing or decreasing the fuel flow with the throttle.

In reality, a simple throttle to fuel valve connection is difficult to control in flight. For example, you could get the mixture too lean or too rich to support the flame when moving the throttle quickly because the compressor (which determines the air part of the fuel/air mixture) takes time to spool up and down. To solve this problem the pilot is not directly connected to the fuel valve, but to another unit that helps regulate the fuel flow in response to the throttle leaver angle and the present conditions.

In the olden days, before computers, this was a Hydromechanical Unit (HMU). I don't know much about them, but it looks like there is a fairly detailed bit about the HMU on the globalsecuricy link from above if you're interested in all the gory details of how it works.

In modern times, jet engines are controlled by what is called a Full-Authority Digital Engine Control (FADEC). The throttle is simply an electronic input to the computer (like a very, very expensive version of your joystick's throttle). The computer also receives information electronically from a sensor suite which monitors conditions both inside the engine and outside the aircraft. It crunches this huge volume of information and decides how best to regulate the fuel flow (and other parts of the engine)

[Brett_Henderson]

[quote]Brett

I figured you could explain it a little better than me.

[DaveSims]

[quote][quote]Brett

I figured you could explain it a little better than me.

[Panther91]

1. N1 and N2? Do turbojet engines which have only one turbine have also this N1 and N2 or only other jet engines which have 2 turbines instead of one? Like turbofan, turboprop and turboshaft?

2. I know that N1 is the rotational speed of the low pressure compressor and N2 is the rotational speed of the high pressure compressor and N2 always has a higher RPM percentage. How come they are not rotating at the same speed and how is this possible?
Does the letter 'N' up here mean something or not?

3. Do I have to monitor N2 and is it spinning faster than N1?

4. Some aircraft like Beechcraft King Air 350 with a turboprop engine has a torque gauge in percentage (some in ft-lbs, what is that) and a gas generator RPM gauge. What does each of these gauges show?

5. On a piston engine the propeller spins faster and faster as airspeed increases. Does the RPM of the compressor and the turbine also increase with airspeed?

6. When power is set to reverse in aircraft with turboprop engines does the turbine continue to spin in the same direction or what? How is this reverse achieved? I don't see any changes with the propeller rotation.

[C]

6. When power is set to reverse in aircraft with turboprop engines does the turbine continue to spin in the same direction or what? How is this reverse achieved? I don't see any changes with the propeller rotation.

The blade is "fined" until it is in negative pitch - so produces reverse thrust without changing the direction of its rotation.