Rotax 912 oil scavenge system
 

Coming from a motorsport background, I assumed the 912 had a mechanical scavenge pump, so I always turned the thing round quite enthusiastically before it ever gurgled.. Then my instructor explained it actually works on compression... :ugh:

OK, after feeling a bit daft I now get a gurgle after a couple of slow turns, and realise it must use some of the expanding gases to pressurise the sump, thus forcing any pooling oil out to the sump tank.

How is this pressurisation achieved? Is there a fixed port from the bottom of the piston strokes to the crankcase, or is there some other devious contrivance/valve?

All you want to know about Rotax 912s and more at:

Rotax-owner.com Information Training Support

Nothing so elaborate, The oil collects in a conical shape at the bottom of the sump, it is blown back to the header tank by compression from the top of the piston on compression stroke being forced by the piston rings under hand cranking. The secret in getting it to "burp" quickly is to keep the pressure in the crankcase constant by adopting a constant rhythm throughout the hand cranking process. Different installations result in a different time frame for the oil to return to the header tank. This system is very simple, nothing to go wrong and when the engine is running the oil returns to the header tank in a mist that is returned to a liquid state in the special chamber in the tank.
The 912 series of engines are brilliant when maintained and treated properly, astoundingly good fuel consumption and lightweight.
As the previous poster stated the R.O.A.N site is excellent for all things rotax.

Jon

Or speak to Conrad Beale at Skydrive. Top bloke.

Interesting... watching the video explaining the oil level check procedure, the narrator says the pressurisation occurs due to gasses passing by the piston rings. I always understood this was a bad thing in most engines, but obviously this is how it works for Rotax. Doesn't the constant blow-by cause errosion of the ring material? Just curious... I guess next time I see a rotax being serviced at the hangars, I'll ask :)

ALL piston engines have blowpast of the piston rings. It will be more pronounced when being hand cranked as the speed of the piston travelling up the cylinder is slow thus allowing time for the compression pressure to escape past the rings, in the case of the rotax 912 it permits said pressure to force the oil back to the tank, hence the "burp" when all the oil is back, (ie) the burp is the gases escaping from the crankcase via the return oil hose. When the engine is at operating temp, thus expanded rings and pistons, coupled to the fact that the piston is travelling many many times faster up and down the cylinder and the compression gasses have less time to get by the rings.

Jon

Gotchya. Obviously why crankcases have breathers on cars... well, when I was a lad they did anyway... Currently all fingers crossed that the weather is flyable tomorrow, so I can practice my new slower gurgle cranking.