Notar520AC
Topic Author
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Joined: Sat Jul 21, 2001 6:53 am

Fuel Vaporization/Combustion- Confused

Thu May 12, 2005 8:28 am

Hi everyone-
I'm working on a chemistry project & I'm getting myself confused. Can anyone explain to me how the whole fuel injection process works, including vaporization & combustion?

**Is 'vaporization' the same thing as lighting a puddle of gasoline on fire? Or is vaporization a result of that?

Any help greatly appreciated.
Thanks!
Justin
BMW - The Ultimate Driving Machine
 
timz
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RE: Fuel Vaporization/Combustion- Confused

Thu May 12, 2005 11:42 am

If you leave a puddle of gasoline (or water) out in the open air, eventually it will disappear. That's vaporization-- nothing to do with burning. The gasoline molecules are all intact.

When gasoline burns, the molecules are disassembled and reassembled into water and carbon dioxide.
 
air2gxs
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RE: Fuel Vaporization/Combustion- Confused

Thu May 12, 2005 9:52 pm

Vaporization (or atomization) must occur prior to combustion.
What few people appear to realize is that liquid does not burn, the vapor given off by the liquid burns. As a liquid heats up, from the local flame, it begins to give off vapor faster, which in turn burns and produces more heat. This process occurs extremely quick in flammables and combustibles. The difference in the two being the temerature in which they can light up.
 
prebennorholm
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RE: Fuel Vaporization/Combustion- Confused

Fri May 13, 2005 8:16 am

Quoting Air2gxs (Reply 2):
Vaporization (or atomization) must occur prior to combustion.

Absolutely right. Talking about turbine engines: The fuel is injected into the combustion chambers. They are placed behind the high pressure compressor stages.
The compression heats the air quite dramatically. That means that even if the fuel is in fact injected as tiny, small droplets, then it evaporates immediately after injection. Mixes with the air - and burns in a constant flame.

The fuel - kerosene - various CxHx molecules - splits into C and H, then recombines with oxygen from the atmosphere as CO2 (carbon dioxide) and H2O (water).
Always keep your number of landings equal to your number of take-offs
 
OzLAME
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RE: Fuel Vaporization/Combustion- Confused

Fri May 13, 2005 8:36 am

Quoting Air2gxs (Reply 2):
What few people appear to realize is that liquid does not burn, the vapor given off by the liquid burns.

A colleague and I did an 'experiment' that verifies just that on a winter evening at work once about ten years ago. The temperature would have been about +10C (well, it is Australia, that's cold for us); we poured some Jet A on the concrete and tried to light it with a gas torch. We did get it to light eventually, but it wouldn't self-sustain; as soon as we took the gas flame away from it it went out again.
Monty Python's Flying Circus has nothing to do with aviation, except perhaps for Management personnel.
 
oly720man
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RE: Fuel Vaporization/Combustion- Confused

Fri May 13, 2005 8:47 am

In fuel injection, the fuel is forced through a nozzle to release an aerosol that makes combustion easier and more efficient. The bigger the droplets the lower the efficiency because of the time it takes to burn within a particular combustion cycle. It's also important to get good mixing between the aerosol and the air entering the engine, otherwise the combustion efficiency is reduced and you get CO rather than CO2 being produced, and soot. Look at photos of the early jet planes on take off.

In a jet engine there is a continuous flame, or self sustaining flame as the fuel burns, so once it's lit it stays lit as long as there's a supply of fuel and oxygen. In a piston engine a spark has to be used to ignite the fuel air mixture, except in a Diesel engine where the compression itself forces combustion.

Vaporisation is just the liquid evaporating into a gas/vapor and is nothing to do with combustion.

Note that the aerosol/evaporation is used to keep the engine components cool. The latent heat of evaporation, ie. the energy transferred to the liquid as it converts to an aerosol/vapor means that the injectors do not overheat. This is another reason why aircraft fuel tanks are not insulated. Cold fuel absorbs a lot of heat energy to get up to combustion temperatures.
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aeroguy
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RE: Fuel Vaporization/Combustion- Confused

Sat May 14, 2005 12:32 pm

In line with what Air2gxs and Prebennorholm said, I'd also add a few things.

Just to make sure everyone is clear:
Atomization: process by which a liquid is broken into tiny droplets and mixed with air.
Vaporization: evaporation of a liquid to a gas state.

In a non-premixed combustion system, the fuel is atomized in air prior to being burned. That is to say, the fuel and air are not mixed on a molecular level prior to burning. The liquid droplets stay intact floating around in the air until they are burned. But evaporation of the liquid droplets is still key in order to burn them. The droplet itself doesn't actually burn, it evaporates first. Basically heat from the surrounding gas supplies the energy to evaporate the liquid fuel and the fuel vapor diffuses from the droplet surface to be burned in the air. This obviously continues until the droplet has been entirely used up.

In a premixed combustion system, the fuel is vaporized/evaporated and mixed with air prior to being burned. In this case the fuel and air are mixed on a molecular level prior to burning.

I believe we have both non-premixed and premixed combustors out there depending on the engine, with premixed being the newer technology.

I'll agree that you can evaporate fuel without burning it, but you won't burn it without evaporating it.
 
lehpron
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RE: Fuel Vaporization/Combustion- Confused

Sun May 15, 2005 4:34 am

Quoting Aeroguy (Reply 6):
Atomization: process by which a liquid is broken into tiny droplets and mixed with air.

Think of mist. The same type of cloud create by squeeing the handle of a windex kitchen cleaner or a bottle of perfume. If you poured gasoline into one of these and lighted a match in front then squeezed... DETONATION!

Except in a jet engine it is continuous with compressed air, and if you ever threw a compressed air can in a camp fire, it goes boom as well. It happens because compressed air has a lot more oxygen in it that standard ambient, this coupled with mistified fuel, explodes. Like pumping NOS in a car.

Do not try any of the above at home unless you have at least one other person to call 9-1-1. Big grin
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