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Why is there residual oil in a preserved or new engine?

Posted: Thu May 21, 2020 10:43 pm
by TB500
I saw some videos on YouTube where a new (/preserved) engine starts and a fire ball and much smoke come out of the tailpipe.

https://youtu.be/sNwG9vC5qnM
https://youtu.be/ZEPhgZeZARI
https://youtu.be/EDc8o_h-dn0

Below these videos some people explain that the engines are just burning residual oil out of the engines hot section (combustion chamber and turbine I guess) as the engine is started for the first time. This was generally known as 'wet start'.

My question is: Why is there any residual oil in the new/preserved engine in the first place?

I don't work in the aviation industry so I don't know what happens during the construction or preservation process of a jet engine but I think the parts are put together so that it must be possible to remove any oil from the components/sections. Therefore you could conduct the first start of the engine without a fire ball or much, much smoke.

This may also reduce the wear on the components of the engine as well as the wings if the engines are mounted below them.

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Re: Why is there residual oil in a preserved or new engine?

Posted: Thu May 21, 2020 11:14 pm
by Stitch
My guess is that they pre-lubricate the parts so that when the engine starts for the first time you don't have bare metal against bare metal. It could also be used as an anti-corrosive agent while the engine is being shipped.

Re: Why is there residual oil in a preserved or new engine?

Posted: Sat May 23, 2020 5:33 pm
by 7673mech
The oil is a preservative to prevent corrosion.

Re: Why is there residual oil in a preserved or new engine?

Posted: Sun May 24, 2020 1:22 pm
by Tristarsteve
The fuel system is filled with a thin preserving oil when the engine leaves the factory.
After installation, you drain this oil out via the fuel filter drain, but there is always some left. On the initial engine start this oil has to leave the engine via the burners before the fuel can get there and ignite.
The oil can then ignite in the jet pipe.