How much tar is too much tar?

I’ve been running woodgas in my Onan 6.5NH for about 20 hours or so, so far. Today I removed the rubber carburetor intake tube and looked down the throat to see how much tar had gotten in there. There is a little bit of tar on the first inch or so down the throat, but the throttle plate is bright and shiny.
I have attached 4 pictures to show what I found and I would like opinions as to how bad or good the situation looks to those of you with more experience than I have.
Oh, by the way, I have a JXQ-10 gasifier, using fiberglass insulation in the final filter and I have added a GEK cyclone at the cool output that feeds the gas to the engine.

Pete Stanaitis

Is it tar or soot? i do get a similar appearance in our hose but i can easily wash the residue off my hands if i get it on them. so i don’t think it’s that harmful… hasn’t seemed to hurt the engine with well over 200 hrs on it so far. and some of those have had tar episodes… we’ve only used one engine in all of our testing.

Hi Pete; If I am not mistaken that is a fema type gasifier. I went back to rewatch some of your video’s and you said the temps. in the area above the grate were1200-1400°. Is that F or C? I obviously do not have the operating hours that alot of these folks have, but I do have an opinion. If it is a fema type I would be most worried about tar. And I believe temps. need to be near 2000°F to crack the tars. Oh well, everybody’s an expert. :wink:

The JXQ-10 is a stratified downdraft gasifier as is the fema, I guess. The highest above grate temp I ever recorded is 970 degrees (always in “F”). Below grate the best temp so far was jsut over 600 degrees F.
I know the gasifier makes tar, but this system uses a gas-to-water heat exchanger for cooling and a series of baths that the gas flows through to remove the tars, then a fiberglass filter and finally a cyclone separator. This is not a discussion of the quality of the initial gas, but a question of whether the gas, as it enters the engine, is clean enough to avoid engine problems. I am asking that question of folks who have also looked at their piping and their induction systems (carburetor horn, and throttle plate, etc.) and would care to make comparisons.
Sorry for failing to make that clear.
Later in the day today, I removed the blower from the filter and looked around as best I could with a HF camera on a cable. I also took some pix of the piping at various places. I have attached a picture of the output fitting of the cyclone separator. I see NO signs of tar there at all.

Pete Stanaitis

I just figure if you can wipe it out with your finger and it just rinses off your finger with cold water it is probably soot and “harmless” as it is actually part of the fuel for the engine. There is a waxy kind of tar that is relatively harmless and seems to happen when running pine wood. It will rinse off with warm water and doesn’t harden and stick up valves. It has the consistency of ear wax. The bad kind of tar is that gooey sticky black crap commonly called creosote and it is difficult to wash off with anything. It also hardens and sticks up valves … The outlet of that cyclone looks clean … Trouble with tar is it will migrate through a system and then splats on the first flat surface it comes in contact with which is usually the intake valve stems … ML

yup, i think Mike is bang on. also if you have ran the engine like you say you have an it was making the bad kind of tar it would be stuck already.

Pete I have been running a different batch of chips that are smaller and causing some waxy residue in the carburettor. It has been causing the butterfly shaft to tighten up after it has cooled down after use. I have found by greasing the shaft it is stopped this stuff getting in. When it had this problem the governor was not working very well at all.

Interesting. I wonder if those chips were made in the springtime, just after the leaves appear. It appears to me (I could be wrong) that the twigs and branches are a LOT “juicier” at that time of year. And I wonder if that juice contains more resins that don’t go away when they are dried.

Hi all; Pete is correct, the best time to cut trees is Jan. Feb this is when the sap is down. We always made a little more money on timber in the winter months,because of this.

Ha! Ha! Actually you are correct PeteS.
Usually though the other real practical’s of converting that “plumped” “sap rich” trunk/stem and larger limbs woods into usable fuel chips get overwheming.
Harvesting in winter sap-down also means leaf-off and even minimal conifer needles to have to deal with. Look at all of the actual Euro and North American chipping vidoes for fuel - you’ll see always been intentionally stripped of leaf/needles and twig ends. What you do not put in - you do not have to sort out later. North country frozen it even chips much easier needing less chipper power. Mid-American continent winter not frozen the fellows say MUCH less snake activity to deal with then; and much easier to handle the woods lenths without swinging air resistance and leaf weight.
Even conifers like I have do winter sap-down and go to minimal needle in the winter.
Four years ago I argued with the wife through all of Feb, March, April about having some fence line hazard trees professional faller taken down to reduce falling nighbor house liabilty, and put on the ground a few years worth of firewood. She hymned and hawed about this saying we’ll wait for income tax money settlement to be known before hiring the Pro fallers. O.K. Last half of that was wise. Then she filed for a April to June 3 months tax extention. Sigh. (matriarchal decended; these trees came from her side, and are “Hers” - I just get to use them with permission). Those big old stand-a-lone “grouse roosts” Doug Fir trees came down down the 1st of July when in full Spring growth sap-up and new needle plump. Took a LOT of chainsaw kerosene dipping and hand scrub brush washing to get that first years firewood worked up in the then hottest part of late July/Aug with the sap/pitch literally squirting out of the trunks of these wind twisted heart split and pitch filled 115 foot ol’unds with each rounds cross cut! Ended up heat crisping and ruining a chain. Pitching up sets of gloves (good), the splitting maul and all the hand tool handles (ok, sorta’ good), a set of pants (not so good). And too much that first year listening to her complaining about pitch drips off the brought in firewood onto her, her hands, her clothes, her floor. Did burn hotter though. Smoky and sootier. Then overaired to oxidize burn/control the stove sooting and that extra heat energy just went by-by up and out the chimney.
Same sooty thing happens inside a gasifer. And thats AFTER you can get all of that extra sap-up moisture with the now even more oxegen gasifer overloading dryed out and removed.
The Law of Unintended Consequences IS REAL and always demands its payments cuts.
Steve Unruh

Howdy Steve,
“The Law of Unintended Consequences IS REAL and always demands its payment cut.”
I couldn’t agree more… Bumped up against that one plenty.

The wood I had the trouble with was due to the small and splintered chips. Nothing to do with leaves appearing, it was Eucalyptus we don’t have too many trees here that loose there leaves, the climate is subtropical.

Some tars can pass through engine as fuel but will condense when engine cools and lock up engine. Those running dual fuels can burn out the tars by running with the alternate fuel a short time after shutting off wood gas. But better is not to send tars into the engine in first place. And even if you protect your engine by running on the other fuel to clean it out you still have all the condensing and clogging problems in your downstream. Again, best to keep tar production to a minimum. Soot? Only problem I see with soot is when it agglomerates with tars. The aforementioned condensing and clogging problems in the downstream.

Regarding tar-coated valve stems:
Not that its a good idea to allow tar into an engine, but I seem to remember that, somewhere in the “1939 report” ( that I reference from time to time, they talked about turning down part of the intake valve stems to minimize the effect of tar-glue seizing them. I don’t know if it actually worked or not. Obviously, if the tar is so bad that it makes the throttle plate stick, you’d still have a problem, but at least wouldn’t have to tear the engine down as often.

Pete Stanaitis

You can take out the intake valve oil seals if you are having tar issues. This would be a temporary fix of coarse and also run an acetone fuel additive and run this in your dual fuel system. Run this fuel for at least 5 min after a woodgas run if you suspect tars are getting into your engine. Also for the butterfly valve, on the 5 kW system it is EFI so no liquid fuel touches this. I did the grease trick and this has helped and also I spray in a little fuel injection cleaner during a petrol fuel run. Doing this has freed up the sticky valve plate.