oil bath

[DutchEdwin]

I believe the actual fluid dynamics is when the air make a 180 degree turn from the inlet lip back up to the filter material, the dirt due to the higher density than air, can't make the turn. As a result it collects in the oil at the bottom of the filter. The horse hair filter at the top of the filter is a secondary and final filter.

GTK, you are almost on track. For the large parts of dust you are right. They can't make the turn and will collect in the oil.

But...the actual filter would be the steel mesh inside. The trick is that through this mesh air can move very quickly. The air speed in the filter is, as you know, very high. This makes it difficult to collect the dirt. It is the same when you want to catch a fly. Travels fast too. The air will travel around the mesh. The dirt will try to follow the airstream, but because of it's mass it cannot follow, thus colliding into the steel mesh. The more mesh there is, the more chance it has in colliding. But on steel dirt will not collect. This is where the oil comes in. Having the mesh lubricated with oil, the dirt colliding with the oil will slow down the velocity of the dirt, and act like a spring and shock absorber. Eventually when the speed of the dirt is low enough the stickiness of the oil will trap the dirt. Result, clean air.

The oil has a second function. Because it is constant being swept into the mesh it will drop out and so trying to take out the dirt.

This method of cleaning air in a high speed air environment is still in use for various materials of dirt you want to filter and various methods (and materials) of collecting (like sticking it with static electricity, or tribology if you want).

[grey beard]

Caterpillar Diesel Engine Company has video tapes taken inside oil bath air filters. As was discussed here, the oil is in great turbulence inside the filter chamber during engine operation.

What engineers have learned from these studies is that, while oil bath air cleaners function very well at most engine speeds, they don't work at all at idle speed, where isufficient air is being aspirated to maintain this turbulence. When the turbulence produced by air movement is not present at idle, air is pulled through the oil in large bubbles. These bubbles still contain unfiltered - read dirty - air.

Cummins Diesel says that one tablespoon of dirt aspirated into a new engine can ruin it completely. The dirt turns into carburundum-like cinders in the presence of combustion chamber temperatures, and wrecks havoc on piston rings and cylinder walls.

My understanding from the diesel schools I have attended is that this shortcoming was the reason for the industry abandoning the oil bath filtration system. I'm making the perhaps dangerous assumption that gasoline engine air filtration events are pretty similar to those of diesel engines, although diesels aspirate nearly twice the amount of air as do similar displacement gasoline engines, because they do not have throttle plates in their intake air circuits.

Did you know that a whole industry has been built around dry-cleaning larger paper air filters, in the trucking and heavy equipment industry? Paper filters can be cleaned several times before they must be disgarded. This saves money for fleet owners. Merle Coggins is an instructor for a Volvo equipment dealership, and could speak to this issue.

[Joe Flanagan]

Wow. Really interesting. Another reason this is one of the best forums out there. Thanks grey beard.

[grey beard]

We surely have sawn some serious lumber out of Mr. Warsaw's original thread, what?

[Merle Coggins]

Interesting info on Caterpillar's study. I will add that from my training I've been told that oil bath air filtration is very good in sertain applications. In fact it's still used today in some applications. Our Volvo Construction Equipment can be ordered with optional oil bath air filters. These are mounted in series with the standard dual element air filter. It is uses in extremely dusty applications as a pre-filter to extend the life of the paper type elements. In those applications the recommended service interval is 250 hours, whereas the recommended engine oil change interval is 500 hours.

I also know that nothing makes more of a mess than when an engine snuffs out and begins running backwards with an oil bath air filter. I saw it happen on a Cat engine in an American crane. It had pushed all of the oil out of the filter and blew it all over the engine compartment. What a mess.

Merle

[Joe Flanagan]

We surely have sawn some serious lumber out of Mr. Warsaw's original thread, what?

Yes, and not for the usual reasons, which is a nice change. His posts tend to turn into events.

[John Mulders]

What engineers have learned from these studies is that, while oil bath air cleaners function very well at most engine speeds, they don't work at all at idle speed, where isufficient air is being aspirated to maintain this turbulence. When the turbulence produced by air movement is not present at idle, air is pulled through the oil in large bubbles. These bubbles still contain unfiltered - read dirty - air.

As we all stand still more and more in traffic jams it is a good thing it was abandonned !

John