KaiGywer From United States, joined Oct 2003, 11100 posts, RR: 42 Reply 1, posted (5 years 8 months 3 days 17 hours ago) and read 818 times:
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Not only Airbus. Boeings do it as well. The engines are not made by the plane manufacturer, and hence, you can have a smoky bus, or a smoky Boeing. Back to the original question, it's because the fuel isn't properly burnt (or something like that)
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L-188 From United States, joined Jul 1999, 28564 posts, RR: 72 Reply 3, posted (5 years 8 months 3 days 16 hours ago) and read 813 times:
Well probably goign to get myself in trouble for the wrong answer, but here goes my explanation.
In order to get jet fuel to light off in a jet engine, you need heat and pressure.
When it gets really cold two things in the engine thicken, the oil and the fuel. What the thick oil means is that the starter has a much harder time generating the needed pressure to light the fuel off, because it can't turn the engine as fast and that means the pressue generated is lower.
So what happens you go to start a jet motor when it is cold the fuel sprays into the combustion chamber, but because the pressure isn't there to light it off, it just smokes and is blown out of the exhaust and that is what you see.
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Air2gxs From , joined Dec 1969, posts, RR: Reply 4, posted (5 years 8 months 3 days 16 hours ago) and read 806 times:
L-188
Close, but pressure or engine speed isn't the problem. Our checklists insist on a certain minimum light-off speed before fuel and ignition are introduced. Temperature is not factored into that checklist. Usually this speed is 20% N2 or max motor, of course this is dependant on engine type. In fact, colder air is denser, so you start with a higher pressure on cold days.
The reason an engine "smokes" is incomplete combustion. A cold soaked engine and cold soaked fuel combine to make it extremely difficult to cause the entire fuel charge to ignite at once. What happens is ignition at the ignitor(s) and as the combustor heats, the ignition process propagates to the entire combustor.
Cancidas From Poland, joined Jul 2003, 4112 posts, RR: 26 Reply 7, posted (5 years 8 months 3 days 14 hours ago) and read 772 times:
Not only Airbus. Boeings do it as well.
any turbine engine will do it. it's fun standing on the ramp and watching smoke come pouring out of an ERJ engine and then hear it ignite. when it ignites you hear the *poof* and see a secondary puff of smoke. anyway, it's cool.
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Air2gxs From , joined Dec 1969, posts, RR: Reply 8, posted (5 years 8 months 3 days 13 hours ago) and read 755 times:
One addition to clear things up. The "smoke" is not really smoke as we know it, (by-product of in-efficient combustion). It is actually raw fuel coming out of the tail pipe.
To illustrate: If you can manage to get a stream of fuel (jet-a) to burn in open air, it will have a dense black smoke issuing from it. That is in-efficient combustion.
You rarely see black smoke in large quantities coming from tailpipes of modern jets because they are extremely efficient in burning their fuel.
Therefore the fuel vapor coming from the tailpipe on a cold start is not yet involved in the combustion process, but the fuel that has been lit off is burned away almost completely.
So, my use of "incomplete combustion" in my original answer should be in-efficient combustion or something along those lines.
Wjv04 From Canada, joined Jun 2001, 548 posts, RR: 5 Reply 9, posted (5 years 8 months 3 days 12 hours ago) and read 734 times:
So its fuel that has sat in the engine, for lets say over night, not being combusted during the start up?. Which would make sence because if that fuel were burining then the engine would start. Are you sure its fuel, because the smoke tends to stay in the air, just like smoke
Interesting. And yes i know that any aircraft does it, but airbuses do it alot more. Trust me, im a ramp rat here in calgary and its allways airbuses doing it. The only aircraft other then airbuses that i have seen are america west's 737's, and even then its not nearly as bad.
Air2gxs From , joined Dec 1969, posts, RR: Reply 10, posted (5 years 8 months 3 days 11 hours ago) and read 723 times:
No, its not fuel left in the combustor at shutdown, (a properly functioning engine wouldn't have much).
To understand what I'm saying you have to know how an engine lights off. I'm going to be real general and basic.
There are usually 2 ignitors in the engine. 1 or 2 may be used for light-off, depending on engine type. These ignitors may be on opposite sides or side-by-side. At the front of the combustor there are several fuel nozzles (30 on CF680C). Since there is not 1 ignitor per nozzle, only some fuel is initially ignited. The start-up process depends on the propagation of the flame from the initial light-up.
As stated above a cold engine/fuel combination inhibits the propagation of the flame front. So, you have raw fuel dumping out the back until there is sufficient heat to ignite all the fuel coming from the nozzles.
After the engine reaches a self-sustaining speed, usually 50% N2 on big fans, the iginition system stops. The fuel burn sustains itself. Ignition is then turned on, automatically or manually depending on aircraft, during phases of flight as determined by the manufacturer or operator, i.e. icing, landing, heavy precip, etc.
Thinking it through, the reason some engines "smoke" more than others can be that 2 ignitors are used in start-up rather than just 1. But, that's a guess.
As a note, I have seen engines (notably RB211's installed on L1011s) smoke eben on the hottest days. This I believe is a function of how easily the flame front moves around the combustor to light all the fuel.
JohnM From United States, joined Feb 2001, 266 posts, RR: 1 Reply 11, posted (5 years 8 months 3 days 10 hours ago) and read 711 times:
Here is another way to explain this. Jet fuel is not a flammable liquid. It is a combustible liquid. That is, it's flash point is above 100 deg. F. The stuff is not interested in burning when below it's flash point. When jet fuel is sprayed in a very cold engine, it does not burn very well at all. When things warm up, combustion improves, and it burns quite nicely. And it is smoke, not water vapor or fuel vapor. A "wet" motor is nothing like the smoke produced during cold weather starts.
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