Why do jet engines smoke?

Question

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From the video linked above, I was wondering why the engines emit so much smoke? Has something gone wrong? Poor maintenance? Isn't it dangerous?

Yeah, they are dangerous, hard and rugged, steeped in 1950's values and aesthetics, and know about their lifetime being finite anyway ... what do you expect? — rackandboneman, Apr 27 '18 at 23:06

score 30 · Answer 1 · edited Oct 24 '16 at 21:33

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Smoke in jet engines is usually from unburnt or partially burnt fuel (or by water injection). Early jet engines used to create a lot of smoke due to these reasons (and due to lack of environmental regulations).

^{KC 135 engines creating smoke due to water injection By USAF Photographer - USAF photo, Public Domain, Link}

Water injection, while cooling the engine, quenches the flames to an extent, resulting in unburnt fuel, which comes out as smoke. However, aircraft have moved away from water injection, reducing emissions.

Another reason for the smoke is inefficient combustion- older jet engines (turbojets in first aircraft and low bypass turbofans) were not as efficient as engines today, which premix the fuel with air and pre-vaporise it before combustion, rather than using droplet combustion- basically mixing fuel more evenly with air. Requirements of fuel efficiency along with stricter regulations mean that engines today have less emissions compared to their predecessors.

^{Smoke during Boeing 707 takeoff; image from nycaviation.com}

In this case re-engining the aircraft with more efficient ones usually results in a reduction of smoke. For example, the USAF re-engined the KC-135s (from Pratt & Whitney J-57-P-59W turbojet to the CFM International CFM56 turbofan), which along with the elimination of water injection, reduced smoke.

^{On the top is KC-135s from an exercise in 1979; image from network54.com. Below that is KC-135Rs from Singapore Airforce; image from reddit.com}

edited Oct 24 '16 at 21:33

Robert Harvey

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answered Oct 23 '16 at 09:03

aeroalias

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Oh my god... The amount of pollution there is awful. I'm glad we have more stringent environmental policies now – JamEngulfer Oct 23 '16 at 21:03
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@JamEngulfer Mind you, those are all pictures from high-load scenarios - mostly take-off and landing. At cruise altitude, the emissions are much lower. That's why it took so long to get rid of the smoke - the extra efficiency loss from unburnt fuel on take-off/landing is relatively small on a typical route, while the technology to make the jet cleaner makes it more complex, and thus less reliable and more expensive. Making a clean jet engine is quite a challenge, and as always, a game of trade-offs. – Luaan Oct 24 '16 at 09:16
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@JamEngulfer There is always a chance that filters have been placed upon the photos to make the smoke more obvious that it was at the time. Monotone photos do this. That does not mean the effects were worse, simple that they are more obvious. The final image is the only marketing image. Visible emissions are potentially doctored out. – Gusdor Oct 24 '16 at 10:22
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@Gusdor I agree that contrast enhancement of the b/w images might make the smoke look darker. However, it would be a lot of work to Photoshop out smoke in the last image, and would have very little propaganda/marketing value. The modern engines do produce a lot less smoke, and the last photo looks entirely plausible to me -- compare it to any other photo you choose of a modern jet engine in level flight. – David Richerby Oct 24 '16 at 11:21
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The comparison between the last photo and the others is slightly unfair. All the smoky photos are taken at times of high load, which produces more emissions, and looking roughly along the long axis of the plane, which makes the smoke look denser. The non-smoky photo is taken at a time of low load and looking perpendicular to the long axis of the plane. Having said that, I don't dispute the conclusion, and there are plenty of photos of non-smoky take-offs of KC-135s with modern engines. – David Richerby Oct 24 '16 at 11:25

Sanchises · Answer 2 · 2016-10-24T08:17:28.187

Fundamentally, because jet engines do not burn a pre-mixed fuel/air mixture.

Fuel can burn in three ratios: lean, stoichiometrically and rich; respectively: an excess of air, exactly enough air, and an excess of fuel. Ideally, one would always burn in a stoichiometric ratio; intuitively, one can feel that this means you don't waste fuel on heating an excess on air, and you're not left with any unburnt fuel. This is how a typical spark ignition engine works: fuel is mixed in the carburetor, compressed and burnt. The throttle varies how much fuel/air mixture enters the engine, but the mixture will always be (close to) stoichiometric.

A jet engine does not have the luxury of pre-mixing the air/fuel mixture before ignition. Spraying Jet-A in the compressed, (hot!) burning air will lead to ignition as soon as it comes into contact with oxygen, whether you like it or not. The problem is that it will already ignite when the local mixture is still rich. This will inevitably lead to soot formation, which is the smoke you see. This is especially prevalant at high power settings (like during takeoff) - at a given RPM, the amount of air that is pumped through the engine is constant, and the amount of power is varied by injecting more or less fuel into the engine, thereby creating a less lean mixture at high power. ^{Note: the engine RPM will react to the power setting in a jet engine; however, the amount of air will generally lag the amount of fuel at higher power settings}

There are some methods to mitigate soot formation, which are all based on increasing the fuel-air 'mixedness' before ignition. A lot of research is being done on this by improving the combustor. The linked Wikipedia article has a very nice and thorough explanation on what exactly is being done for this, but in general (and extremely simplified), it comes down to spreading the fuel as much as possible and introducing as much air as possible (but remember that running too lean reduces efficiency), all without dousing the flames. See the image below for the complex arrangments already made in the combuster on an older cannular design.

Source: wikimedia

Pengor · Answer 3 · 2016-10-23T09:06:55.910

Collin Krum has written a fairly in-depth article on the matter for Jalopnik. It's also worth noting that not all of the engines in that video are jet engines. From the article:

Low bypass engines aren’t as efficient as high bypass engines, but water injection is the technology that is most responsible for the seemingly-eerie pictures of older airplanes riding black columns of smoke into the sky.

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Because the engine core is cooled by the injected water, the combustion chambers aren’t able to burn all of the fuel and water mixture, so some particles of the fuel and water are vented out the engine, which materializes in the form of the characteristic black smoke.

score 0 · Answer 4 · answered Oct 23 '16 at 10:14

Some of the AGE/GSE I work with uses jet engines. During startup and shutdown, surely expect the temperature not to be at its maximum. That can be one cause of unburnt fuel. Another cause could be a gradual buildup of fuel depending on the interval of the pneumatic thermometer's ability to help quench the bleed air.

Why do jet engines smoke?

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