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enter image description here

A "wet" takeoff of a KC-135 with J57 engines—By USAF Photographer (USAF photo) [Public domain], via Wikimedia Commons

Why is water-contaminated fuel bad, but water-injection is not?

The way I understand it, water-injection lowers the combustion temperature, which is good and bad, good for cool yet increased power-output / mass flow, bad for wasting [unburnt] fuel.

Why is then water dangerous when a trace amount of it is found in fuel? What's the worst that can happen apart from black exhaust and higher power?

Water-injection in jet engines and piston engines is mixed with fuel before it enters the combustion chamber, so I'm thinking fuel already mixed with water should not be any different. Of course I'm wrong. But why?

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    $\begingroup$ Have a look at accidents like the BA38 in 2008 $\endgroup$
    – PlasmaHH
    Commented Nov 29, 2016 at 9:44
  • $\begingroup$ Water injection is also used as part of anti-detonation injection (ADI) systems. $\endgroup$
    – J W
    Commented Nov 29, 2016 at 17:22
  • $\begingroup$ Water injection is intentional at a certain point of the flight, usually at take-off. It's for a short period of time and does not pollute the fuel in the tank. Water contamination on the other hand is always unintentionally. $\endgroup$
    – Mast
    Commented Nov 30, 2016 at 9:41
  • $\begingroup$ Not sure how different a turbine engine injector is from a piston (continuous vs. cycling...), but in Diesel engines if a slug of water (not just trace dissolved water) gets into a hot injector, it can flash to steam, explode, and crack the tip. $\endgroup$
    – Nick T
    Commented Nov 30, 2016 at 16:45
  • $\begingroup$ I'm pretty sure that in the photo above the water is injected into the hot exhaust gases. $\endgroup$
    – Koyovis
    Commented Apr 27, 2017 at 4:22

6 Answers 6

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Because of the quantity of water in the fuel, as opposed to a careful introduction of water into the combustion process. Typical water contamination is bad in fuel tanks as water is denser than aviation fuels and settles at the bottom of the tanks. Aviation fuels are also hydrophobic (non-polar) and so do not readily mix with water. Therefore water will settle unmixed at the bottom of the tanks, in the fuel sumps or feeder hoppers and when ingested into the engine will fuel starve it and cause it to shut down. Water in fuel lines can also freeze there during cold weather, causing fuel starvation as well.

Fuel samples should always be taken of the gas when it is both in storage as well as prior to flying the aircraft, especially after rainstorms or the aircraft has sat on the ground for long periods with low fuel in the tanks in a humid environment. Fuel samples should also be taken approximately 30 minutes after refueling and the gas allowed to settle in the tanks. A preflight sample of water contamination taken from the fuel tank sumps of a GA aircraft can be seen here.

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Water injection, however carefully controls the amount of water going into the engine along with fuel at the point of combustion, allowing for continued engine operation.

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    $\begingroup$ +1. The water and fuel will separate, leaving only water in the bottom of the tank, which is usually the part of the tank that feeds the engine. A small, controlled amount of water entering the engine can be ok, but 100% water will not burn very well (nor will it compress well in the cylinders of piston engines, which can damage the engine.) $\endgroup$
    – reirab
    Commented Nov 29, 2016 at 14:39
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    $\begingroup$ @reirab Fuel/water separation occurs in gasoline fuels, but not necessarily in jet fuels. $\endgroup$
    – J W
    Commented Nov 29, 2016 at 17:19
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    $\begingroup$ @reirab: aren't water and gasoline both nearly-equally incompressible? You'd only have an over-pressure problem on the compression stroke if some of the air was replaced with water, right? (I know physics but not a huge amount about engines, maybe I'm missing something?) $\endgroup$
    – Peter Cordes
    Commented Nov 30, 2016 at 9:09
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    $\begingroup$ Another point is, that contaminated fuel delivers unknown and uncontrollable ammount of water into the combustion chamber. Water injection, on the other hand, delivers controlled ammount of water. $\endgroup$
    – Crowley
    Commented Nov 30, 2016 at 19:17
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    $\begingroup$ This answer is only applicable to gasoline-powered engines. Jet fuel mixes with water, so layer of water would only form if there was a lot of it. The main risk with jet fuel is that the water will freeze as small flakes and clog the fuel filter (or pump or any other part of the fuel system). $\endgroup$
    – Jan Hudec
    Commented Apr 1, 2017 at 20:16
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Water-injection in jet engines and piston engines is mixed with fuel before it enters the combustion

This is actually somewhat wrong (or at least imprecise) and I think it is adding to the misunderstanding. The water and fuel are never "mixed" in the traditional sense as liquids. An aerosol of fuel is injected... Also, an aerosol of water is injected. The intention is to have a typical fuel combustion which is also in the presence of water to cool the combustion. This is a rather important distinction from a physics perspective.

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  • $\begingroup$ Actually, the cooling is best done at the intake to increase the density of the air which enters the compressor. The lower temperature helps to burn more fuel, so thrust is increased due to a higher air mass flow and a higher fuel flow. $\endgroup$
    – Peter Kämpf
    Commented Nov 29, 2016 at 21:40
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    $\begingroup$ I see that you're experienced here, but FWIW, water injection is used so that a leaner (approaching stoichiometric) fuel mixture can be used while keeping the combustion cool enough to avoid damaging the engine and/or causing premature combustion. So, you're actually burning less fuel per mass of air flow since a stoichiometric burn is most efficient. Though yes, overall air throughput is increased, so maybe we're singing the same song. $\endgroup$
    – justinm410
    Commented Nov 29, 2016 at 22:02
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Without some help, water doesn't usually mix around in the remaining fuel, but can combine to form larger droplets or even pools at the bottom of the tank.

  • If large quantities exist, it can be ingested by the engine and lead to loss of power or stalling.
  • Water not removed from the tank supports microbial growth.
  • Water in the system can freeze and cause problems for filters and other components.
  • I've heard of water in fuel causing erratic fuel quantity readings due to short circuiting the sensor (not sure how common that is).
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Water in fuel will freeze, causing clogging of filters and nozzles, leading to the engine flaming out. That's why an antiicing additive is added to jet fuel - so that the water does not become ice.

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Water in the fuel tanks can freeze, possibly causing blocked fuel lines or filters. This can cause the engines to produce insufficient thrust, or even to flame out. Even if the ice adheres to the inner surfaces of the fuel system in a thin layer during cruise and doesn't restrict fuel flow appreciably, it can be dislodged by a sudden increase in fuel flow (for instance, during the landing sequence), and create a blockage further downstream.

Water used for water injection does not cause this problem, as the only place where it and the fuel both are is in the engines themselves, which tend to be a bit on the warm side for ice formation. Also, water has an extremely high heat capacity (meaning that it takes the addition or subtraction of a lot more heat to change the temperature of a given quantity of water by a given amount than it takes to change the temperature of the same quantity of pretty much any other liquid by the same amount), which causes it to cool down during flight much more slowly than the fuel does; the proportion of water in water-contaminated fuel isn't enough to change its heat capacity by much compared to pure fuel, but pure water is another matter altogether.

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    $\begingroup$ "in the engines themselves, which tend to be a bit on the warm side for ice formation" That must be the understatement of the day :) $\endgroup$
    – user
    Commented Apr 28, 2018 at 19:07
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Water injection is introduced into the combustion chamber to cool the temperature which allows for a higher rpm of the engine which gives you more thrust. This occurs after the fuel has ignited. Water contamination means poor ignition where water and fuel are trying to combust resulting in possible flameout.

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    $\begingroup$ Welcome to aviation.SE! $\endgroup$
    – Pondlife
    Commented Apr 20, 2019 at 19:33
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    $\begingroup$ I fail to see what your answer adds to existing answer. $\endgroup$
    – Manu H
    Commented Apr 20, 2019 at 20:01

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