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The Rotax 912 engine has dry sump forced lubrication system. They say rotating the prop in the correct way (counter-clockwise when looking from the prop side) will get the oil from the bottom of the crankcase to the oil tank.

Questions:

  1. How does this "force back by blow-by gases" work? Is it air which gets through the carburetors and cylinders to do the crankcase what pushes the oil from the engine? If oil gets from the bottom of the crankcase to the top part of the oil tank, where the "burping" sound emerges then? In the oil line, or at the bottom of the crankcase?

  2. If the oil gets pushed by air, why is it bad to rotate the prop in the opposite direction? Air would get to the bottom of the crankcase and air would get to the oil input line and "burp" at the bottom of the oil tank. But if engine then runs in correct direction again, won't the oil be sucked from the bottom of the oil tank again? My only guess is it won't due to the air in the oil pump which would just spin without sucking any oil...

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The Rotax 912 engine uses a dry sump, and does not have a scavenger pump to return oil to the sump. Instead it uses pressure escaping past the piston rings to force the oil back into the sump.
The "burp" you hear coming from the (open) oil tank happens when most of the oil in the crankcase has been returned to the tank: the air escaping past the rings bubbles up through the oil tank.

The propeller is pulled through "forward" (in the normal direction of rotation) for a few reasons, mainly that it's the direction that the gearbox & accessories are all designed to turn. Turning in the normal direction of rotation also ensures that oil is pumped through the rest of the system, particularly the valve train.
Reverse rotation can pump air into the system which may cause problems if you try to start/run the engine immediately after. I believe the recommended procedure is to "burp" the engine a few times to re-prime the oil system if it's pulled through backwards.

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  • $\begingroup$ Thanks for the answer! "the air escaping past the rings bubbles up through the oil tank" - is that really correct? I thought the oil pipe from the crankcase goes to the TOP of the oil tank and that oil "falls" from the top in the oil tank. Oil from the tank to the engine is "sucked" from the bottom of the oil tank. $\endgroup$ – k3a Sep 22 '14 at 18:08
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This "burping" is probably happening while the engine is running too, it just can't be heard due to the engine noise. The burping noise probably emerges at the bottom of the crankcase (not in the tank) when almost all oil has been returned. This burping probably occurs because air pressure from the cylinders creates more oil flow out of the engine than the oil pump for the oil coming in, that's why there is always some oil in the tank (which is good!). That also explains why burping shouldn't be done just after stopping the engine - as we want to let the oil drip to the bottom of the crankcase to get as much oil as possible out of the engine.

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