Bodobodo From Canada, joined May 2000, 553 posts, RR: 15 Posted (11 years 4 months 3 days 15 hours ago) and read 2062 times:
Hi,
I was at the airport taking photos this past weekend in cold Montreal weather and saw a few 737-200's start their engines in unspectacular fashion. However when a 767-300 did the same thing (in -15 celsius weather) both engines created quite a bit of smoke on startup.
What causes this and is the high bypass ratio in newer engines the reason why this is more apparent in newer planes (I've seen it on an Airbus 320 also) than on jets with older engines?
A320FO From Austria, joined Oct 2000, 211 posts, RR: 1 Reply 1, posted (11 years 4 months 3 days 14 hours ago) and read 1797 times:
Its due to condensation.
Basically a runnig engine is fairly hot. After shutdown it will cool down, with air trapped inside slowly releasing moisture (warm air will absorb more water). On a cold day this moisture will freeze inside the engine, also trapping small amounts of oil and fuel residues.
Now during start up, the engine temperature increases very fast, releasing the frozen water / oil particles. These exhaust gases now come in contact with the very cold air from the fan section, again condensing (thats the way clouds form),providing the casual viewer with the pictures above.......
The 767 in the picture probably was sitting in the cold during the night. This phenomena will not occur during short turn arounds, as the engine doesn't cool down enough.
L-1011-500 From United States of America, joined Nov 2000, 368 posts, RR: 2 Reply 3, posted (11 years 3 months 2 weeks 3 days 18 hours ago) and read 1601 times:
The L-1011 was FAMOUS for this...even in warm weather!!! Quite a treat for spotters.
Cfm&b From , joined Dec 1969, posts, RR: Reply 4, posted (11 years 3 months 2 weeks 3 days 9 hours ago) and read 1561 times:
On the Next Gen. 737 aircraft, the aft sump oil drain can be seen in the exhaust plug. Oil will drain and accumulate in this area at shutdown. For this aircraft, smoke will blow out at start-up as this oil clears the plug. Other than that, A320FO's answer pretty much hit it right.
Tn283 From , joined Dec 1969, posts, RR: Reply 5, posted (11 years 3 months 2 weeks 2 days 20 hours ago) and read 1523 times:
That L-1011 picture almost looks like a "pickled" motor being started for the first time after installation. I see this on F-15's about once a week after a rebuilt engine is installed. Smells terrible!
CX flyboy From Hong Kong, joined Dec 1999, 6129 posts, RR: 57 Reply 7, posted (11 years 3 months 1 week 6 days 23 hours ago) and read 1475 times:
Yeah, but with this reasoning (condensation) surely all engines on the aircraft should have the same thing when starting....this doesn't always happen. Sometimes only one or two engines do it. Explanations?
Mriihi From Finland, joined Aug 2000, 54 posts, RR: 0 Reply 9, posted (11 years 3 months 1 week 4 days ago) and read 1422 times:
The reason for L-1011 smoke is obvious. The pilot is warming his tires before take-off. I think he has been watcing NHRA drag Racing the night before... NOT !!!
F-WWAI From Andorra, joined Dec 1999, 131 posts, RR: 0 Reply 10, posted (11 years 3 months 1 week 3 days 19 hours ago) and read 1408 times:
hmm, there's another reason why cold engines can smoke at starts.
cold engines, like your car engine, need rich fuel to light up. if the fuel flow is not high enough, the fuel does not light in the combustion chamber and evaporates through the exhaust. when it eventually ignites, you may hear a loud "boom" and the vapors disappear.
have fun,
FW
VC-10 From United Kingdom, joined Oct 1999, 3656 posts, RR: 42 Reply 11, posted (11 years 3 months 1 week 3 days 13 hours ago) and read 1415 times:
A320FO
Where does this moisture come from ? The air in the engine has been drawnfrom the ambient air outside the engine, therefore the air can't be holding anymore water than the air outside the engine as that is where it came from.
I think the more likly answer is that due to the temperature cold day fuel enrichment was used.
The smoke on an RB211 is due to start enrichment that supplies extra fuel during the start until 2 or 3% N3 below idle.
PopFiction From , joined Dec 1969, posts, RR: Reply 12, posted (11 years 3 months 1 week 3 days 7 hours ago) and read 1388 times:
When I worked at Millionair a few years back I saw a king air 350 do a hot start - I was standing 20 feet from it and watched flames shoot out of the left exhaust stack - I saw it go way above the fuselage . Scared shitless and running around trying to find a fire extinguisher like 5 other guys . The king air pilot didn't even see it - fired up and blasted , we told him about it later
Wilcharl From United States of America, joined Jun 2000, 1158 posts, RR: 3 Reply 13, posted (11 years 3 months 1 week 3 days 4 hours ago) and read 1385 times:
Any L-1011 built before this day in 1983 is over 18 and can legally smoke in the united states...... Its a nasty habbit, but as most of the L-10 fleet is legal, what can we do? they are all dying off anyways...
RayChuang From United States of America, joined Jun 2000, 7560 posts, RR: 6 Reply 14, posted (11 years 3 months 1 week 2 days 20 hours ago) and read 1377 times:
I do know that due to a quirk with the RB.211 engines on the L1011, when the engine starts up from cold you get a lot of engine lubricating oil burned, hence the big cloud of smoke from the initial engine run up. Mind you, this is definitely not "politically correct" by EPA standards of today.
What's interesting is that you don't see this with RB.211 installations on the Boeing 747 and 757 when the engine starts up from cold; they must have changed the combustor design to avoid this problem. I believe you don't see this on L1011-500's with their more modern RB.211-524 engines, either.
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 15, posted (11 years 3 months 1 week 2 days 12 hours ago) and read 1365 times:
A320FO,
Therefore, white/gray engine smoke occurs only if the ambient temp is below freezing?
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 16, posted (11 years 3 months 1 week 2 days 12 hours ago) and read 1360 times:
Moreover, A320FO, if we need an ambient temperature less than freezing to condense the moisture, why doesn't it make a continuous vapor cloud?
A320FO From Austria, joined Oct 2000, 211 posts, RR: 1 Reply 17, posted (11 years 3 months 1 week 1 day 23 hours ago) and read 1374 times:
VC-10,
fuel enrichnment sure does contribute to the smoke. And the moisture captured in the engine comes mostly from the abient air. Maybe you have observed the effect of water building on the warm side of a single-sheet window, despite a only small difference in humidity. This is a similar process. As long as air is moving through the engine, there is no chance for the water to settle in the engine, it is blown out. On a standing engine, the air more or less remains trapped inside, with the moisture inside being just a little higher (due to just a very small quantity of remaing, unburnt, liquid fuel. Now, the materials used in the engine are perfect heat conductors, cooling down fairly quickly on a cold day. This energy exchange and transfer may even cool the inner surfaces to a temperature just slightly lower than the ambient temperature, especially with air movement on the outside surface, forming frost / condensation on the inner surface, despite the ambient humidity below just below saturation. Trapping fuel and oil residues, this will form a white-blueish cloud during start-up, the unburnt and condensing fuel from start-up enrichment will ad to this cloud. In both pictured cases, the aircraft spent a longer time sitting in the cold.
Just as an add-on to the pictured TWA L-1011, it was used for troop transports into the Arabian area during the guf-war (european winter). I happened to see several widebodies sitting in NUE (my original home-town) during that time.
EssentialPowr,
the ambient temperatur being below freezing is favorable, but not essential. If the condensed water doesn't freeze, it will be blown out during the engine start, mostly before fuel and ignition, thus heat, kick in. Obviously, frost can't be blown out, it needs the engine's heat to disappear, mostly all at once, in the mentioned cloud.
Im not a scientist, so if anybody has more knowledge, I stand to be corrected, as I am based on actual observations and basics....
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 19, posted (11 years 3 months 1 week 1 day 20 hours ago) and read 1340 times:
A320FO,
You provided a guestimate in good faith, but your scenario violates a basic law of thermodynamics, and your assumption that a shut down engine is a sealed volume is not valid. If your scenario were correct, every cold day start would be smoky...
Water vapor is invisible in jet exhaust except during extreme alt/temp conditions (contrails). The white/gray smoke in these pictures is due to large amounts of unburned hydrocarbons at low combustion temperatures. At the temp increases, and the residual oil/fuel is removed, the exhaust will turn to black when viewed from a distance on older generation engines, and be totally invisible on newer engines.
Turbofans have extra oil sumps when compared to turbojets. The A3007 on the ERJs and Citation X is a classic "cold start" smoker; it is also notorious for oil seal leaks. That engine can smoke on startup on a 70F day...The older RB211 has 3 spools, therefore an extra sump or two over a 2 spool design. More sumps, more leak potential. As an engine ages, things loosen up, and leaks and drips occur. Ever walk under a JT8D w/ a clean white shirt on? Ever watch a cold JT8d start, when the seals have shrunk and the oil has had time to pool?
A320FO From Austria, joined Oct 2000, 211 posts, RR: 1 Reply 20, posted (11 years 3 months 1 week 1 day 19 hours ago) and read 1347 times:
As I said, I always stand to be corrected.
Not every cold day gives the scenario I described, it is just a possibility. If the air is not calm or the spread is great, it will not happen. You are completely right on the fact that the smokes color is due to oil/fuel residues, thats what I mentioned. The billowing and extremely large cloud is based on steam also, produced from the sudden heating, I doubt that the amount of residual oil is that high to justify that extend of smoke. Otherwise, the huge cloud would have to appear on every start-up, even in warm air, as a certain amount of unburnt fuel and oil are always present.
Concerning air movement in a standing engine, well, the N1 / fan stage can not be considered as enclosed, but the N2 / HP section actually can. I have often observed a windmilling N1 stage and a standing N2 stage, both on the cockpit instruments and during occasional observations on the engine itself. This is also where I have noticed the frost build-up mentioned. The frost can also build on the fan and compressor blades, as they can be considered as airfoils, with air expanding, thus cooling and releasing moisture.
The A320s CFMs are exactly mounted at my eye-level, making it easy to look inside with a flashlight...
I have no experience with older turbojets or the A3007, all I know is that the CFM56 oil seals are fairly reliable, and have been improved recently, still lowering oil leakage and consumption.
A320FO
PS Fly8, yes, I earn my money under the red-white-red tail........
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 21, posted (11 years 3 months 1 week 1 day 17 hours ago) and read 1345 times:
A320FO,
Are you talking about vater vapor (steam?) or smoke? The difference is readily apparent, whether it comes out of a CFM56 or a smoke stack at a power generation plant.
N1 and N2 on a CFM56 are independent spools. N1 can always "free wheel"; N2 is geared to the accessory drive and starter and will NEVer "free wheel." Just b/c you observed N1 windmilling and N2 stationary doesn not mean that N2 "holds" air at a higher pressure or warmer temp after engine rotation stops. No way.
One could theoretically pass a string entirely through the N2 section immediately after the engine stops. If the string will fit, will air pass through it? Definitely...
So, again, if steam (visable water vapor) made up the smoke, then "smoke" would be visable during every startup...
Finally, if you understood the thermodynamic diff b/t steam and water vapor, you'd understand that "sudden heating" doesn't make sense. What's the approx turbine inlet temp on a typical gas turbine before ignition and fuel are introduced? 150C? Is that enough heat to keep water in the vapor phase, and therefore invisable? Quite (In fact, it's MOre than enough - hint @ steam/vapor question). When does the engine start to smoke? AT LIght OFF!
COMBustion burns off the old hydrocarbons, and until the EGT is stable, produces smoke for a second or 2 during the start sequence, esp on older, "leakier" engines. That's it.
I appreciate your attempts to explain this thread, but had to correct it b/c it was way off base.
Tom775257 From United Kingdom, joined Dec 2000, 153 posts, RR: 0 Reply 22, posted (11 years 3 months 1 week 16 hours ago) and read 1325 times:
Just a fact which I haven't seen mentioned, forgive me if it has, but as with any hydrocarbon burning engine, back to very simple chemistry, fuel+ Oxygen -> Co2 + WATER!! So when you first start the engine, the temperature of the metal is cold enough to start condensing the water in the gaseous phase caused by the combustion, hence you get exactly what you get in clouds, small water droplets in air suspension i.e. a coloidal system, hence white (in the same way as, for example milk...a fat in water coloid)
This, in my opinion, is the main thing causing the difference between so called smoke output with the same engine as when cold than hot. Admittedly there will be slightly more fuel introduced, but this would give black smoke, not, the white ploom shown in the original 767 piccy. Of course once started, the metal will warm up, and will therefore not condense out water. Perhaps a similar analogy would be when you turn on your car airconditioner on a humid day, you will initially get a ploom stuff looking like steam...same physics with the evaporator alalogous to the rest of the engine, and the air analogous to the Carbon dioxide, water vapour mix...
Sorry if this has already been said. If wrong please correct.
cheers.
VC-10 From United Kingdom, joined Oct 1999, 3656 posts, RR: 42 Reply 24, posted (11 years 3 months 1 week 12 hours ago) and read 1318 times:
Tom775257
You get white smoke from jet engine when too much fuel is added during start, not black
25 EssentialPowr: Tom775, "Fuel + O2 = Co2 and H2O" is...a tremendous oversimplification. Without going into organic chemistry, let it suffice to say there are MAny mor
26 AirT85: Hello, To add onto this question, When i wasin Philly a weekend ago, a Fokker 100 started up its engines and fire came out the right one, why is this?
27 Tom775257: EssentialPowr: Sorry, any over simplification was because I didn't want to confuse issues. My point is this: Yes, the metal is at ambient temperature,
28 JohnM: Just to throw a monkey wrench in this thing, this is my reasoning. I believe that Jet A, having a relatively high flash point ( above 100 deg F) does
29 Tom775257: JohnM, Thanks for the interesting reply. It would be very interesting to know from actual exhaust gas analysis the components of cold start smoke i.e.
30 EssentialPowr: The basic question was of smoke vs water vapor on startup. There is no question that water vapor condenses to a saturated, and therefore visable state
31 EssentialPowr: John M, I agree - ref my post "until EGT is stable..." Lower flashpoint fuels produce smoke at lower than optimum combustion temps. Higher flashpoint
32 ATA L1011: Well their was a design flaw in the design of the Rb211-22b which caused a little oil to leak on the manifold and cause smoke upon start up. It was wo
33 Fr8tdog: hmmmm water wont burn? It indeed does, when exposed to temps in excess of 2000 degrees F. the water separates back O2 and H and will actually aid in t
34 PW4084: I don't even know if anyone is still interested in this thread and I am sure my uneducated input won't cause any revelations but here goes: I was taug
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