Oil bath final filter

Hello I just found this site a few days ago and this is the first topic I have started myself I have replied to a couple of other threads. From what I have read so far this is a great group and when funds free up some I will upgrade but right now things are really tight.
Anyway gave just started looking into wood gas. I have an old farm with plenty of wood I currently burn about 12 cord a year in my 115 year old farm house. First on my project list is to modernize this house to lower that probably in half. Thus freeing up some wood for other things. Right now wood is just labor and updating the house is money I am spending on getting the farm going again.
Anyway enough background story for now just wanted to introduce myself a little.
As I look at different videos of wood gas genorator setups I am amazed how many people seem to pull straight air into the system without a filter. I have a few old tractors with oil bath filters and am very aware of what they call dusting a motor because you left the filter without any oil and let straight air into the motor.
I find the old simple oil bath filters work amazing well on old tractors if you just keep decent oil in them it wicks up the screens and will collect all the dust out of the air it even provides a small amount of oil to lube the valves. I have been wondering if a set of oil bath filters one for fresh air and one for the wood gas would make for a good finial filter. I would tthink an oil bath filter would do a great job of collecting any tar left but I don’t know so I wanted to ask.


Welcome Dan! Better be careful, woodgas becomes a lifetime addiction! :smile:

Oil baths have been suggested before. http://forum.driveonwood.com/search?q=oil%20bath

@wkweasel used an oil bath scrubber on his truck. I’m not sure he was satisfied with the performance of it.

They do seem to be effective. You will get some oil carryover into the engine, and the oily soot that does get through will stick to everything much more. The other issue will be oil cleaning; you’ll need to frequently change out the oil or use constant bypass filtering. You’ll also get some water condensing, which will have to be separated from the oil.

Also, tar is not something you can easily filter out. The gasifier should never produce it in the first place. The only filtering to be done is soot, ash, and moisture.


Hi Dan,

Welkome on the site! There are some fine guys here indeed.

Most people film their generator woodgas run in the “experimenting” spirit, runs usualy are short. All the later longer runs usualy run with both gas and air filtering.

As for oil bath filter, l see no problem for air filtration. The problem might occur when running woodgas trugh oil bath. The reason is water condensate wuld likely make “mayonese” in the filter and water accumulation.

This condensate it self does aid to filtration thugh.
In a way, the hay filter many guys are useing acts like a oil filter. Hay gets soaked with water and soot sticks to it.

However, l think Bruce S. uses oil filter for his charcoal gas car. I dont know what resaults he has thugh…

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Interesting. I was sort of thinking it would be in addition to the hay filter. I have plenty of hay. Guess I just wonder how efficient a hay filter would be and if a oil filter would get anything else out. Good point about it making anything that did get through more sticky I would sort of think ash and dust would behave about the same. What I do know about oil bath filters is they put less drag on a motor then an air filter and clean dusty fields out of the air better. growing up on a farm I saw several of our newer tractors not hold up as well as the older one and was told the filters had alot to do with it.
I had a tractor transmission get a mess of water in the hydraulic oil once and it was a mess. I cleaned that oil or sort of cleaned it by using it to cool metal I was heating and bending. Got the water back out and then sent the oil to a waste oil furnace. I was looking at the Gilmore video where he runs sawdust and oil mixed as fuel. I found that interesting in that it would be a great way to dispose of old oil.
Chris i think I have been bit by too many green bugs already and alternative energy is just something I have to admit I am addicted to. :grin:

thanks for the search I will spend some time later reading through the results.

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It’s definitely not 100% effective. However depending on the setup, you may not require perfect filtration to the engine. The bulk of what you might filter out is just soot, which doesn’t hurt the motor, it’s similar to graphite which is a lubricant. It’s not like dust. The main issue will be buildup over time, and we have procedures to clean that out.

I know of trucks with 20-30k miles on woodgas, with no ill effects to the engine, even with less than perfect filtration.


Hi Dan
I tryed an oil bath filter on my stationary generator and found that the soot mixed with the oil and then clogged the rest of the pipes and inlet manifold, then stuck to the valve stems and ground the valve guides to the point where the engine started using excessive amounts of oil. The compression tests on the cylinders said the engine was good but the oil consumption told a different story.
Hope this helps .


Thanks. I figured someone must have tried this and now I know why it isn’t a good idea.

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I ran across this oil bath filter arrangement a while back and it seemed to make sense. I haven’t seen any designs of what folks here have tried. Are they similar to this or different. It’s hard to believe that soot could get through this design. Maybe the real problem may be tars getting carried through in the gas and doing the damage? If so, maybe we’re back to operating temps in the hearth. Just a thought as tars can’t be filtered out as I understand the process.


That is how a oil bath filter is designed but I sort of have to believe the people saying they tried it and it caused trouble. I know it works better then a paper filter with less drag on the motor hp wise for straight air you just have to keep an eye on them with tractors because they do use oil.


Again you fellows are overlooking the number one reason oil bath filtering is by far not very good for woodgas filtering: ALL WOODGAS IS WET GAS!
One of these tractor/truck/industrial AIR oil bath filter let get rain water in it turns it into a slimy, grey-gooy clogging mess.
I know. I’m old. Cut my early twenties teeth servicing these. Without the down turned rain cap. Maure-pit or pond sink that piece of equipment and it was hours disassembly, solvent washing, cleaning-up time.

And . . . what you gonna do with all of the soots/carbons contaminated oil, eh? Just dump it? Invest then in a whole 'nother centrifuge oil-cleaning operation?
There are reasons why all have evolved to either water washable filters; or a hot-dry shake out the soot-cake filtering.

Read, look widely why folks DO as they do. Shun idealism “perfect”, “better”, filtration for practical. Shun lay-about-cheap for purpose-made performance.
J-I-C Steve Unruh


Steve thanks for the info on the wet gas. I do know full well what you mean about the old oil bath filets getting water in them and the mess it makes. The entire reason I asked this question in the first place was to find out from those of you who had tried it why it didn’t work for wood gas. I do fully believe it when you guys tell me it didn’t work in this application.

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Oil bath filters, or at least the design i am using… ( the black plastic tank besides the gasifier )
The space above the oil i have filled that with a spread of chopsticks and bamboo fiber
In the blue tube the actual steel wool can / is higher then on the drawing
The whole set is also acting as a backflow prevention when i would use the flare


Thank you Koen for the filter plan … very inspiring!

How often should you change the oil?

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Hi Thierry,

It depends on the dirt ofcourse, but i check and clean the stainless almost daily ( standard routine i want to teach )

The plastic drum has a drain plug and also a top level plug, to check the oil level, so i can top up the oil to the correct level easy. ( similar as the plug on a gearbox or axle )


Koen I see some of why you are willing to put the effort into making charcoal now. You have me considering building both charcoal and wood to experiment with around the farm.


I believe Koen is on the optimal track for reliability, ease of operation, weight, and system complexity. Others have pointed out quite correctly that the energy losses from burning off the hydrocarbons are arguably unacceptable, as was the conclusion of government regulators in northern Europe in WWII.

Koen is pursuing secondary chemical processes to recover some of those losses. In temperate zones though, I envision the perfect marriage - a stationary simplified gasifier designed to slip the char and burn any tar in a close coupled gas stream for heating and supplying hot water. If 5 cords of firewood are needed to heat a home here, then WAG, closer to 8 or 9 would be used if burning primarily the hydrocarbons.

That would result in a pile of charcoal sufficient for a lot of driving, or tractor work, hopefully even excess for biochar.



Hi Garry,
Besides being it a less complex system to teach / maintain, all the filtering and cleansing setups are reduced to…
All the products from gasification can be used in one way or the other, depending the local circumstances.

Once you start to like the catalystic functioning of glowing charcoal, using it in a well balanced way, then you’l start wondering why they all forgot about the technology…

Some people are giving me the advice to use / build a gasifier to do raw bamboo…
Yes i can, yes i could… but could it ever been that easy as a Gary Gilmore Simple fire with charcoal ?

Your idea for the perfect marriage comes closest to my vision… Home heat stationary CHP, driving on electric or clean liquid fuel from the system…

The slipped char does a wonderful trick with air-pollution, water Treatment, adiabatic cooling / house humidity…
and at the end you can used it as fuel… feeding the ashes to the garden…

If i would live in a cold climate… i would know what to do, but would not have so much Bamboo :grin:


l was allso thinking about the heating stove/charcoal maker combo to produce motorfuel. l love the black gold. But l found now l can refuel my woodcar about anywhere on earth, if l run out of fuel. With charcoal, its a bit tougher…


I love the oil filter design. Any suggestions on dimensions or cross section area based on displacement? For cleaning do you just clean the steel wool daily and change the oil less frequently to keep down cost for fresh oil? Or do you change all of the oil every day and filter it for reuse? Are your slots made with a hacksaw blade?


Hi Bruce,

I did not use a pre defined sizing yet, just used what was on hand with in the back of my mind not to create an obstruction in the gasflow.

The slots are made with a 3mm cutting disc
The steel wool is rinsed daily with diesel, the oil only topped up when needed.

One of the “misleading facts” or “misconceptions”, if someone comes here for a show or presentation, is, that it looks so easy and always starts flawless, always have good power…
This is because i always clean the filters, always take care of good quality fuel and so on…
Most people are spoiled in long time maintenance interval’s and getting less aware of the needs for the engine.

Why would i build a huge filter if there is not a reason beyond having longer maintenance interval’s ?
My systems can run whole day full load and maintenance is done in the blink of an eye.
If the operator does not clean his filter before start, it will prevent easy start or good performance at the spot and he will know immediately.
Its all about habits…