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Fuel Vaporization/Combustion- Confused  
User currently offlineNotar520AC From United States of America, joined Jul 2001, 1606 posts, RR: 3
Posted (11 years 3 weeks ago) and read 4768 times:

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.

BMW - The Ultimate Driving Machine
7 replies: All unread, jump to last
User currently offlineTimz From United States of America, joined Sep 1999, 7228 posts, RR: 7
Reply 1, posted (11 years 2 weeks 6 days 21 hours ago) and read 4742 times:

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.

User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 2, posted (11 years 2 weeks 6 days 11 hours ago) and read 4715 times:

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.

User currently offlinePrebennorholm From Denmark, joined Mar 2000, 7142 posts, RR: 53
Reply 3, posted (11 years 2 weeks 6 days ago) and read 4661 times:

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
User currently offlineOzLAME From Australia, joined Feb 2005, 338 posts, RR: 0
Reply 4, posted (11 years 2 weeks 6 days ago) and read 4657 times:

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.
User currently offlineOly720man From United Kingdom, joined May 2004, 7154 posts, RR: 11
Reply 5, posted (11 years 2 weeks 6 days ago) and read 4659 times:

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.

wheat and dairy can screw up your brain
User currently offlineAeroguy From United States of America, joined Aug 2001, 69 posts, RR: 0
Reply 6, posted (11 years 2 weeks 4 days 20 hours ago) and read 4608 times:

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.

User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 20
Reply 7, posted (11 years 2 weeks 4 days 4 hours ago) and read 4580 times:

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

The meaning of life is curiosity; we were put on this planet to explore opportunities.
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