What causes oil in an air filter

Ricci: "So oil in the air filter box is normal. It comes from the ventilation (I think)"

Faith has helped you
Right.

If the crankshaft slips in the oil, producing oil mist, which happily lubricates the cylinder liner, and then the pistons compress the elastic air, e.g. during the work cycle in the crankcase, some air (and oil mist) must inevitably be able to escape somewhere, otherwise it would come in the crankcase ( Exception engine with exactly counter-rotating pistons) with the considerable pressure fluctuations in the V2 to an "air jam" (with loading of the oil seals of the engine housing), if the pistons would have to "pump up" in the work cycle (against the air in the crankcase) like against an air spring - > Loss of performance.
In the case of the two-stroke engine, it is well known that the gasoline-air mixture is very effectively pressed into the combustion chamber via the overflow ducts.
In the four-stroke cycle, this "pumping power" from the underside of the piston evaporates unused.

Logo, at least a small equalization of fluctuations in the pressure peaks for the air pressure must be possible via the vent.
Oil vapors inevitably escape into the airbox.

Perhaps that is why the old English ladies (Norton, BSA, Triumph) with parallel piston two-cylinder engines liked to oil out all kinds of engine cover gaskets?
The overpressure created by the 2 large pistons in the engine housing was particularly high here.

If you plug the vent even further with foam, you may not be doing yourself a big favor and increasing the pumping work of the pistons.

Depending on the design, the air pressure fluctuations that the engine ventilation has to cope with should be greater in a BMW boxer engine and an asymmetrically running V2 with a certain piston size than with an in-line four-cylinder with comparatively smaller pistons.
There, 2 smaller pistons always work in opposite directions, i.e. 2 pistons compress the air in the crankcase and 2 pistons decompress.
This results in an internal pressure equalization, quasi an internal air pressure short circuit.
But not with the V2. The SV runs with a 90 ° cylinder angle offset.

I could imagine that now at lower speeds the oil vapors can swing in and out of the airbox sooner and further through the ventilation hose and thus get into the airbox housing sooner than at high speeds.
At the high frequency, the oil will probably not even make it up to the airbox through the hose.

I suspect:
Anyone who, like me, likes to brake a lot by downshifting the engine (especially when driving down passes) could possibly have longer negative pressure in the airbox and thus suck in more oil mist (and relatively less gasoline-air mixture) for longer than a driver who prefers to go with it the disc brakes work alone and burn the small oil mist more easily at full load.
But that's speculation.

After a 2,000 km tour of the Alps over many passes in South Tyrol and Switzerland, I noticed that the new air filter was soon oily again.
In contrast, the oil consumption was very low.


In very old engines with air filters made of sheet metal (NSU Max, etc.) you could use the escaping oil to oil the wire mesh, on whose oily surface the dust should stick when the intake air flows through.
These very coarse filters clean much more poorly than paper filters commonly used today.
This is likely to have a noticeable effect on the life of the piston / cylinder.
This was particularly noticeable on the pistons of the open two-stroke mopeds. Road dust acts like sandpaper.

For environmental reasons, the weak oil vapors are now sucked in in the air filter box and burned at the same time.

Unfortunately, today's paper air filters "clog" over time, as the much finer and more effective pores of the paper are closed by the oil.
Also a good deal for the dealer.


If you fill in too much oil, you will have more splash losses and therefore minimally less performance.
In addition, more oil warms up more slowly. Cold oil is therefore more viscous - the engine runs a little heavier, at least initially.

Half the height at the oil sight glass at the SV is completely sufficient. So far I have never had oil temperature problems, even with a very "brisk" pass trip with luggage at over 2,000 m - but with primitive temperature measurement at the oil filler neck.
However, I don't stand and chug around with my SV much in the lower speed range, but prefer a "wind-oriented" driving style
That too cools

But I had with good intentions topped up oil to max. After the service and before a long journey.
Is not recommended!
The performance continues to decrease when the air filter is too oiled, meaning that less air and relatively more gasoline are sucked in. Fatter mixture.

The paper air filter partially "clogs" (look at the paper filter when changing the air filter, especially at the bottom near the rear cylinder) and thus brings logo less air flow, more gasoline in the mixture (with carburettor engines) and less power.
Injection engines should readjust the mixture.

Presumably because of this, some SV-Motor logo appears to be more revving at the top when changing the filter (allegedly increased performance, with new paper filters and of course with new K&N filters).
The K&N filter should even like the "oiling" via the vent, in contrast to the paper filter which then becomes clogged, _must_ be oiled.

However, it should clean a little less, right?

The fine dust is m.W. less filtered through the very fine pores in the paper, but rather sticks to the oil in the larger holes in the fabric - or not ...
One would have to examine the pore opening in K&N more closely.
The lower flow resistance of the fabric filter with the same filter size must come from somewhere.


If someone never uses oil (very strange!), Frequent cold starts at low temperatures and brakes with the engine may be due to the fact that some gasoline is sometimes sucked past the piston rings into the crankcase during the warm-up phase with an over-rich mixture and larger piston clearance, especially when the engine is braking a lot.

This can be dangerous if, after the winter, when the weather is nice, you take the motorway and at full load and high speed and the corresponding heat development in the engine housing, the gasoline content then evaporates rapidly.
The oil level may then drop dramatically in a short time.

I also know that from car engines that are driven a lot on short trips in winter.
This can be extreme for people who drive down a long hill every morning in winter with a cold engine and engine brake. The "oil" level (including petrol) can even rise!


When motorcycles smell of special oil, it was mostly two-stroke engines with "racing oil" or four-stroke engines with the "top oil" that was used in the past, which was also added to the tank.
Is not used today and completely unnecessary.


The oil separator at the bottom of the SV-Lufi-box can be dimensioned very small for this system, since it should only catch a few drops of the oil mist and possibly lead it back into the engine.


A special cross-country oil (from racing / aviation or similar) brings m.W. nothing at all, possibly just a clutch slip because of the additives.

The largest share in the clogging of the LuFis has m.W. the oil mist, possibly in connection with dust.

TelexF