727EMflyer From United States of America, joined Mar 2005, 547 posts, RR: 0 Posted (8 years 4 months 3 weeks 6 days 6 hours ago) and read 5186 times:
The other day I saw a UA 772 take off from HNL, and was amazed at the cloud of smoke it kicked up when it began the roll. I first wrote it off to dirt on the runway being stirred up, but when it passed me at the other end, sure enough, the engines were billowing brown smoke from both engines ala 707. I've never seen a modern turbofan engine smoke like that, save the ocaisional old DC-10. So what is it that makes the engine smoky? I would guess it is burning excess fuel. Surely the airline would catch that and fix it right away!
Loggat From United States of America, joined Feb 2000, 665 posts, RR: 0 Reply 1, posted (8 years 4 months 3 weeks 6 days 6 hours ago) and read 5182 times:
maybe the catalytic converters weren't functioning properly
Seriously though, I think as the engine was spooling up, the temps of the engine increased, which allowed the sediment and unburned fuel to be burnt off and shoved out the back. I expect the browness decreased over time.
Of course, I am merely a pilot... a mechanic would be better to answer with a genuine response.
There are 3 types of people in this world, those that can count, and those that can't.
Air2gxs From , joined Dec 1969, posts, RR: Reply 3, posted (8 years 4 months 3 weeks 6 days 5 hours ago) and read 5159 times:
All engines smoke to some degree, but the right atmospheric conditions will makee the smoke more visible. I seriously doubt there is any issue with oil loss or burning simply because the engine is under ETOPS controls. This means that oil consumption is closely monitored. Any increase in oil consumption is immediately addressed and corrected.
As for excess fuel; the controls on FADEC engines pretty much make that improbable and virtually impossible on both engines, unless some common source of data is used. I don't know the B777, so I can't say.
PW100Testpilot From Netherlands, joined Mar 2005, 35 posts, RR: 0 Reply 4, posted (8 years 4 months 3 weeks 6 days 5 hours ago) and read 5154 times:
I'm only a testcell operator for PW-100 turboprops, and this engine doesn't use any of that modern stuff like ETOPS and FADEC, we monitor oil consumption by recording the amount of oil what goes in the engine, and what is left behind when the test is finished (to put it simple). For example you fill the oil tank, do a oil pressure check, shut down the engine with a MOT of 190 degrees Fahrenheit, read the oil level trough the sight glass (for example 2.1 Qts below maximum). And when you shut the engine down for the last time you read the oil level again, and with a simple calculation you can calculate the oil consumption. Btw every time when you have to shut the engine down you monitor the oil level (standard procedure is to shut down the engine when it's stabilized at 190 deg F MOT, wait for 5 minutes and then read off the oil level)
BTW I was just joking because I've seen some heavy smoking engines in our testcell, I don't really know what's causing smokey engines on aircraft. I doubt that it's excessive fuel, because that fuel would be burnt away immediately when the engine lights up
Good pilots are made.... Good technicians are born!!
DarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10 Reply 6, posted (8 years 4 months 3 weeks 6 days 2 hours ago) and read 5098 times:
Smoke (usually called soot) is formed in the fuel rich regions of the combustor, generally close to the fuel spray. Most of the soot is then consumed downstream in the dilution zone of the combustor. The smoke you see is carbon that was not consumed in the combustor and has traveled through the turbine and exhaust nozzle.
Advances in combustor technology has made engines run more fuel lean, decreasing the amount of soot produced and modern combustor designs also have better dilution zone designs that help reduce the soot.
Interestingly, although a smoky engine may look like a big polluter, (and it very well may be), but the unburned carbon is of minor concern when compared to some of the other pollutants that come out of the engine exhaust. The two biggest concerns are carbon monoxide (CO) and oxides of nitrogen (NOx) which are both regulated heavily.
Pilotpip From United States of America, joined Sep 2003, 3117 posts, RR: 11 Reply 9, posted (8 years 4 months 3 weeks 5 days 21 hours ago) and read 5015 times:
Jet fuel is not gasoline. To put it simply, it is a refined kerosene, which is a refined diesel. There is more to it than that, but Jet fuel is in that family. Many jet engines are certified to run on all three and many are even certified for 100LL for a short period of time.
MrChips From Canada, joined Mar 2005, 920 posts, RR: 0 Reply 10, posted (8 years 4 months 3 weeks 5 days 20 hours ago) and read 5004 times:
Quoting EMBQA (Reply 5): Why do 18 wheelers leave a smoke cloud when their running hard..?? It's the same reason. They both burn the same fuel...only aviation jet fuel is more refined and pure
That's close...diesel fuel has lubricating additives in it to keep the fuel pumps and injectors lubricated...kind of like two-stroke gasoline, with oil added.
Almost all piston engines will reduce the air:fuel mixture at high power settings by introducing more fuel into the air charge. By running rich, combustion temperatures are reduced substantially. This is especially important in a diesel engine, where temperatures and loads are much higher than in a gas engine. I am assuming that a jet engine may operate on a similar principle.
Quoting AR1300 (Reply 8): I don't think so.If I'm not mistaking, cars use 90+ octane gas while JET 1A is 70ish.Avgas is 100/130 octane, but is not used for jet mainliners.
Octane rating is NOT a measure of purity. It is a measure of the percentage of octane (8 carbons, burns slowly) in gasoline. The unreported number is the percentage of heptane (7 carbons, burns quickly) present in the gasoline. In a high-performance engine, the pressures are very high, and heptane will tend to auto-ignite in a BIG way (explode). This explosion, detonation, not only robs you of power, but can destroy your engine. The solution? reduce the heptane content of the gasoline, and increase the octane content.
Therefore, octane rating is a measure of the gasoline's resistance to autoignition. Interestingly, even though kerosene (Jet-A) has a lower vapor pressure than gasoline (it doesn't evaporate as fast), it has a much lower resistance to autoignition.
Avioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11 Reply 14, posted (8 years 4 months 3 weeks 5 days 11 hours ago) and read 4920 times:
MrChips, Great answer.
In simplist terms when the engines are being told to produce max power, even a FADEC will dump a slightly excessive amount of fuel into the combustor section which will produce the "smoke" you see.
As engine turbine sections have been engineered to run hotter and hotter over the year you've seen less and less smoke.
When we raised the EGT of the J-79 15deg in the 80's the smoke was very significantly reduced.
The -9 series JT-8's and -2 series CFM-56 vs the JT-8-200series and -7B series CFM-56 are a great example of improved engineering in the cumbustion controls and turbine section engineering.
The RB211's are of the generaation that includes the 200 series JT-8's and do smoke a bit more than the V2500 or TAY or Trent for any given set of circumstances.
If the FMS is allowed to direct the ATS on the 737NG, smoke is an extremely rare thing.
There are engines in Tulahoma that don't make any smoke and almost no IR signature. The next few years will be very interesting indeed.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
DarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10 Reply 15, posted (8 years 4 months 3 weeks 5 days 9 hours ago) and read 4880 times:
Quoting Avioniker (Reply 14): As engine turbine sections have been engineered to run hotter and hotter over the year you've seen less and less smoke.
Yep, advances in turbine design have allowed higher combustor temperatures which consumes more soot in the combustor. However, NOx increases exponentially with combustor temperature, and NOx pollutants are a much bigger deal to the environment than soot particles (smoke).
727EMflyer From United States of America, joined Mar 2005, 547 posts, RR: 0 Reply 16, posted (8 years 4 months 3 weeks 5 days 6 hours ago) and read 4845 times:
Thanks for the great responses guys! So it seems for a modern engine like the PW4000's I saw to smoke like they did the pilots needed more power than normal, resulting in higher fuel consumption, resulting in lower exhaust gas temperatures, resulting in less soot burn off. Now I know
Phollingsworth From United Kingdom, joined Mar 2004, 825 posts, RR: 6 Reply 17, posted (8 years 4 months 3 weeks 5 days 6 hours ago) and read 4841 times:
Quoting DarkBlue (Reply 15): Yep, advances in turbine design have allowed higher combustor temperatures which consumes more soot in the combustor. However, NOx increases exponentially with combustor temperature, and NOx pollutants are a much bigger deal to the environment than soot particles (smoke).
Yup, soot, HC, and CO trend the opposite direction from NOx, technology levels remaining fixed. ICAO actually tracks all of these emissions (soot is referred to in the smoke number) for operational large gas turbines. The databank is actually publicly available. What you will find is that the engines for longer range aircraft are typically much closer to the NOx limits than the engines for the shorter range aircraft. That is because they have much higher OPRs and TITs (T4).
Oh, overall almost all jet engines run very lean, i.e., overall fuel to air ratio is very low. This keeps the turbines from melting. However, the actual combustion region may be relatively rich and therefore more smoke.
Avioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11 Reply 18, posted (8 years 4 months 3 weeks 5 days 5 hours ago) and read 4825 times:
Roger on the NOX. Problem is they're finding other, worse stuff coming out of the tailpipes as a result of higher levels of recombination from the higher temperatures.
Time will tell I'm afraid...
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
NORTHSEATIGER From United Kingdom, joined Sep 2003, 432 posts, RR: 5 Reply 21, posted (8 years 4 months 3 weeks 13 hours ago) and read 4607 times:
We had an Antanov visiting a while back. The engineer said "Can I have some oil for my Engines please" we said "Sure what kind you need" and he replied "Doesn't matter any will do". Now that was a smokey old girl !!.
Lightsaber From United States of America, joined Jan 2005, 11878 posts, RR: 100 Reply 22, posted (8 years 4 months 3 weeks 3 hours ago) and read 4542 times:
Quoting DarkBlue (Reply 6): Smoke (usually called soot) is formed in the fuel rich regions of the combustor, generally close to the fuel spray. Most of the soot is then consumed downstream in the dilution zone of the combustor. The smoke you see is carbon that was not consumed in the combustor and has traveled through the turbine and exhaust nozzle.
Quoting DarkBlue (Reply 6): Advances in combustor technology has made engines run more fuel lean,
Darkblue, excellent answers, but if I may expand. Anywhere the Stoichiometric ratio is > 3 (fuel/air ratio >0.2) in a jet engine will produce the carbon char that is the seed of smoke. Smoke is nothing more than an agglomeration of Carbon from the liquid fuel in solid form.
While GE is going for a pure lean burn technology to reduce NOx and smoke, this results in slightly higher CO emissions and a lower peak relight altitude.
Pratt and RR are going for RQL combustors or "Rich mix, quick mix, lean burn" combustors. In those combustors, which have a very rich area near the injector (the "dome" of a combustor) one works HARD to keep the localized phi low. However, the surplus of fuel produces a surplus of chemical "radicals" (CH, OH, etc.) that help stabilize the reaction.
Pratt has gone to F119 style fuel injectors that have a greater amount of air participate in the breaking up of the fuel (atomization). This produces far less smoke for a given dome stoichiometry (as the localized fuel concentrations are less fuel rich). Or it lets the dome be run richer (for a given smoke limit) allowing greater power without the smoke trail (very important in military applications where you don't want the opponent to be able to say "shoot at the thing at the end of the smoke trail").
The UA 772 has the old style Pratt airblast injectors (internally referred to as "Talon injectors"). These old style injectors have very little of the total combustor air participating in the breakup of the fuel; thus the higher smoke. As others have alluded, the higher the thrust setting the greater the fuel/air ratio in the engine thrust the increased production of smoke (its about cubic with f/a ratio). The combustor in the 772 (pw4090) is an annular derivation of a conventional combustor. Only the pw4098, pw4162 (option), pw6122/24, and pw4168 (option) have low emission ("Talon") combustors in Pratt engines.
Factoid1: Pratt offers a lean burn, dual stage combustor for the IAE V2500. Guess how many have been sold? Zero!
Factoid2: Pratt has the best high altitude relight in the industry (one can restart a Pratt at altitudes GE could only dream of) but has the worst smoke of the three engine makers.