Just how harmful is tarry gas in an engine?

The accepted wisdom about tarry gas from a wood gasifier seems to be that you don’t want it in your engine, and it must be cleaned up at all costs. I’ve got to ask sooner or later, just how bad for the engine is it? Where does it hurt? Inlet valve? Lubrication system? I’m thinking: could I get away with a simple condenser for the tar, run the engine on shutdown on a clean fuel, and a periodic strip and clean on the cylinder head?

Just remember this. If a valve sticks open, it will get bent when the piston hits it. if it sticks shut, it could bend a pushrod (or worse). Either way you’re looking at expensive repairs.

Once they invent an engine with no valves, we won’t have to worry about tar.

in a non interference engine you’ll probably just bend intake valves… best bet to not make tar in the first place

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ouch !!! The tar will lay in the intake and migrate through with time and create repeat performances of the first disaster. I have destroyed 2 engines playing with stuff. No problems the last 5 or 6 years though … There are proper ways to deal with things if you do have a disaster though. On interference motors the piston will generally push the intake valve back up but on non-interference motors the pushrods will bend or the rocker arm studs will get raised out of their stock position. Tar is bad news … M

Hy, Brian
I bought me a store-bought gasifier about 6 montht ago.
I have run 30 or more batches of grass clipping pellets through it.
I get a big old brick of sillicon out of it; the minerals of the soil.

It runs my gen quite well. but it also gumms up the valves.
I pulled the heads and beat the valves with a hammer untill
they lusened up.

Any tar in your engine will evenualy stick a valve.

I suggest a top fired updraft gasifier to make pure charcoal.
run that in your ICE. all of the volitales goes to a boiler.

Cook your charcoal clean and feed your gen properly.
All of the tars and stuff belong in a heat thing.
The pure carbon monoxide is what runs an engine

Welcome to the Small Engine Users Corner BrianH.
You’ve asked a series of six questions.
“How harmful is tarry (wood)gas to an engine?” You will not notice it when woodgas fueling a warmed up running engine except as having lots (near gasoline levels) of engine power. After the engine cools down the whole intake/induction system will be coated with a thick sticky coating of dark asphalt like, roofing tar like, creosote. How dark colored will depend on how much microscopic black soot particles have been entrained in the microscopic sticky tar mist globules when they were hot.
If using a gasoline carburetor as your woodgas/air valve usually all of the carburetor bore internal gasoline and air bleeds, and jet nozzles will be tar coated glued shut unable to now flow normally. It will never meter a gasoline/air mix to fuel the engine again until a full teardown, chemical cleanup up and reassemble.
Once cold; the throttle plate valve will be tars glued shut; sticky on initial movement and once forced opened, be unable to then shut completly and seal off. THIS IS YOUR FIRST WARNING THE TARRING EVENT TOOK PLACE! Again this will require a chemical cleaning to restore full use.
BEING WARNED now by the cold tarred sticky throttle; BEFORE you normal crank over you must now on a hand cranker, S-L-O-W-L-Y, G-e-n-t-l-y pull over feeling for ANY turning resistance points. Electric starting - DO NOT ELECTRICALLY CRANK OVER YET. First you must now carefully, gently with a hand socket and wench “bar” turn over the engine, again feeling for any resistance points.
The whole induction system having been tar coated will include tarring of the internal exposed intake valve stem. You are trying to feel if one of the intake valves is now tarred sticky, resistant to normal movement. Force this to move, and depending on the actual engine you will end up with as a minimum a stuck open valve and NO COMPRESSION and then on a single cyclinder engine not running capable. Or, again depending on the actual engine design, the stuck opened valve will be hit by the rising piston. The piston may just push the valve mostly closed - never quite all of the way to normal seat seal - loss of compression - single cylinder - no running engine. Or; on many canted angle of contact valve edge to piston top engines BEND the valve at it’s stem and now bent it will be unable to move freely it WILL be for sure stuck off of it’s sealing seat - again loss of compression in that cylinder. Multi-cylinder engines will often start up and run with one cylinder valve stuck open with the valve then being hit by the crankshaft driven up piston. The valve head can break off loose, rattle around, scoring the machined smooth cylinder wall, damaging and imbedding in the softer piston. Piston can then hammer the broken off valve head into the underside of the cylinder head surface damaging it too. Depends. Depends. Very ugly. Very expensive.
IF you can determine a tarred sticking valve stem before any damage you can solvent chemically clean it in place with a bristle brush. You must to be able to access to do this through the intake port opening to clean the stem properly. So, OFF with the intake/carburetor and mixer! You will also have to be able to push and pull, up and down on the valve to distribute the cleaner onto the stem and get it worked into the valve guide bearing sleeve. Very unwise to crank over the engine to do this. So also OFF with the valve cover, cylinder head and/or the engine block side cover access plate also to be able to access and manually up and down push/pull on the valve. CAREFULL! Sometimes the valve spring retainer shoes/clips will pop out doing this, get lost or fall down into the opened up engine and then the valve spring retainer and spring fly off too. On overhead valve vertical cylinder engines then the valve can gravity fall down into the cylinder. Oh joy. Magnet-on-a-stick fishing time through the valve guide hole, or now for sure pull off the cylinder head.
Also realize all of your chemical tar solvent and the tars removed will drip down somewhere. On vertical or canted engines this will be down onto the top of the piston. Liquid solvent cleaner will then carry the upper removed tars down into the piston sealing rings. They must be free to move very sightly in/out, up/down and even twist to be able to gases seal for compression. Sticky piston rings and again loss of compression and no running engine on that cylinder.

Back to the hot microsopic tars mists ability to entrain microscopic soot particles - they WILL also out of the lower gasifier hearth core trap entrain and carry into the engine much larger abrasive fuel ash particles. This carried in, and solvent washed down abrasive ash WILL wear the upper cylinder bore, piston sides (called skirts) and the sealing pistion rings resulting sooner or later in again loss of compression and now poor starting and once started loss of power. This time however the abrasive worn away metals will NOT be a cleanable restorable condition. Ash entrained in the tars sucked into a running engine WILL be released as these tars are combustion burn up and and this now released ash wash down past the piston rings into the crankcase lubricating oil. Now abrasive engine oil WILL prematurely wear the crankshaft and bearings AND the lower cylinder bore, piston skirts and sealing piston rings. These abraded away metals will circulate into the oil accelerating all internal engine wearing dramatically. Abraded away, precision machined metal surfaces are NOT just cleanable restorable!
So Answer #1.

The other answers are here:
Remember back when I said a tar fueled engine will have near gasoline levels of power?
This is because these tars are made up of long chained complex hydrogen and carbon molecular links. Tars ARE powerful HC fuels. These should have been thermally broken down, and apart back in the gasifier hot heart and made into the molecular simpler CLEANABLE fuel gases of hydrogen, carbon monoxide and methane gases.
Volatile light tars should not be wasted as rich energy valuable gasifer internal fuels to then be condensed or filter out becoming an avoidable environmental hazard waste contaminated tarry liquid wash and tar goo soiled filter disposal “problem”. A properly operated and fueled gasifier IS the actual waste clean up, convert to energy machine itself for 90% of these! The 10% gasifer removed, heavy, heavy non-volitive tars are the actual stabilized asphalt that can be used as a paving material or woodstove/incinerator heavy fuels.
Mineral ash goes back into the soil where it originally came from.
The engine exhaust atmopheric released simple H, C and O molocules as H2O and CO2 go back into the air where they came from to be again plant captured, sun energy converted and recycled back into new fuels. It IS the sun energy that drives this all.
Ma’ Nature hates you squandering and wasting her bounty. And SHE always gets the last word. Eventually in sterile dead soils unable to feed anything until the dis-balancer in her cycles all dies off from starvation and self-pollution and she can then balanced regenerate her cycles.

I suggest everyone should intentionally tar up a simple to recover from single cylinder engine for the tarry hands-on experience.
Two old Tecumseh lawnmower engines for me. Honest; the trashed Honda tiller later wasn’t actually woodgas/tar but a wooden garden stake broken off plastic air cleaner cover. Grrr. Plastic that should have been metal! 45 minutes in the late summer dusty garden and the oil was like $$$ liquid silver from the piston, piston rings and cylinder bore metal wears. Abrasives are quick, quick. Another dead, dead killed engine by me.


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Thanks for all the replies- I certainly wouldn’t try to run a valuable engine on tarry gas, but it would be interesting to experiment with a sidevalve engine that’s on it’s last legs anyway. The question would be: would ten minutes running on petrol or LPG be enough to clean the tar residues out of the intake before shutting down?

Chris mentioned engines without valves- would a fan scavenged two-stroke fit the bill?

you don’t have to build a charcoal gasifier to run your ice… a well designed wood gasifier will produce tar free gas. i can say this from experience.

Ha! Ha! Quick reloop: " sucked in tars entrained with abrasive fuel ash . . . wearing away precision machined metal surfaces . . " Cylinder bores, sealing ring surfaces, precision machined piston surfaces. 2-strokes have these too.
Gasoline/petrol was originally a tars cleaning solvent before it was a motor fuel. It WILL liquify ALL the induction system tar coatings and carry these plus all the entrained soots (ok - just more delivered carbons fuel) and abrasive ash particles (NOT OK) into the cylinder.
Yes a side valve engine would be great to try on. Easy clean up recovery from. Pop the cylinder head, finger nail feel the cylinder wear ridge before and after a bit to see for yourself, eh? Easy on a side valve. And as Mr Dausie says nessary anyhow to tap the valves back down closed.

Hey ArvidO since tree trunk, limb and stem woods ARE so easy to gasify power with and eveything else like leaf, needle, and such are so damndable hard does kinnda seem like we are being sent a message ain’t it?

Steve Unruh

Steve, That was by far the best tar rant and explanation I have ever read !! Very well put !! You covered all the bases. I am saving the text for later posting. Best regards and smooth sailing … Mike
PS, I always have to remind folks when they do a conversion to make sure that the air inlet for the PCV system is not in the woodgas path. I have had to modify several throttle bodies now to prevent this as they like to take this air from points downline to avoid adding a different filter … Woodgas in crankcase and one good backfire or blowby equals blown up oil pan and valve covers …

Oh, I forgot to mention that one of the best ways to clean off the valve stems if you know you have screwed up is to leave the engine running on gasoline if you have that option and then boil up a jar of water and pour it into the intake while revving the engine a bit. When that boiling water hits the manifold vacuum it will turn to steam and hopefully steam clean the stems. If you shut the engine down without doing this and let it even begin to cool the tar will solidify and it’s too late charlie … Obviously if you pour the water in too fast you can liquid lock the engine and crack a piston or the such so do it carefully. Gasoline does a poor job on real tar but rinses soot perfectly. M

Steve, Thanks for taking the time to put that information on here. It will be printed off and put with the book.

I have always said when Steve talks everyone should listen. Maybe I should also add get there pencil and paper out.

Hey BrianH.
You actually live I think in the Land of Lister.
Internet look up the work there of a fellow Briton named Ken Boak.
He has a site called powercubes? and sponsors a Yahoo Group about IC engine home made CHP. I do not recall the name of his Y.Group.
He works with the old 6.5 hp stationary Lister CS engines for vegioil fueling, has been working to woodgas these either dual fueled and has now even set up and spark converted these now and straight woodgas fueled one of these now.
For awhile I had an Indian produced clone engine of one of these in a 12 hp version. Too hard to get parts for this now in the USA with US EPA import ban restictions.
Very simple engine. Easy, easy to pull down for cleaning and maintences with straight on THICK valve stems, literallty three bolts for full valve access; 10 more bolts and nuts to piston out remove. Then no more bolts to pull and replace the wet cyinder liner. As eat anything combustable and recover engine as I’ve ever seen. Sigh. Wife hated it. Too scary for her with it’s open flywheels.

ChrisKy and BrianH google search up sleeve valve and rotory valve engines for an interesting look see at “valveless” designs that were in production and used in addition to the Wankle. I’ve work ed on and owned twi of the three if these. Very interesting the things been tryed that Did work, and why they are no longer generally used.

Steve Unruh

Daucie you need to tighten up the restricter on your machine. You can do a simple mod to it by welding in a 2" nipple end into the reduction cone. Then you can screw on bell reducers with different openings.

A few years ago I worked for a startup company doing gasification and Fischer Tropsch. They ruined a 20kw generator with less than 50 hours by not heeding my warnings about temperature and tar. Seems they couldn’t quite grasp the concept that a gasifier needs to run on a bit of vacuum. But, then again… I didn’t have the right initials behind my name. What would I know?

With my veggie fueled diesel engines, I’m in the process of adding water/methanol injection. Coking of injectors is a real issue and meth injection cleans the insides of the engine well. I’ve wondered if that might be a good pro-active maintenance measure for woodgas applications, too.

Thats funny Mike!! I was thinking the same thing when I read Steves post.Good explaination, I have not tarred up any engine yey but it can be very messy and ruin what couild be an enjoyable time. :slight_smile:

A Wankel Rotary Engine such as made by Mazda have no valves.

Problem solved?

Not sure how it would go if the side and apex seals get gummed up though.

Yeah, even with “no valves” there are lots of other places you don’t want tar. Best to not make any in the first place.

Hey, Chris! So far as I know Wankel engines do not have valves. Mazda used them in some car models back in the 1970’s. Have you ever heard of anyone running a Wankel rotary engine on wood gas?