Wood gasification
The project
- Disclaimer
- Introduction
- Carwood
- Preparation
- Basic calculations
- Tips en tricks


DIY construction of a gasifier for mobile applications


The information on this internet site has been composed with the largest possible care. The writer rejects liability for possible loose ends, inadequacies and implications. Moreover he is not responsible for damage, loss, lesion or death, caused by DIY builders or producers, who have used the information on this internet site.


Or might I say: discouragement. I have already written it before: building a decent gas generator is more labour than building a house. Perhaps not in the absolute sense, but the required materials cannot be obtained from the local hardware store. Many parts have to be made by yourself or by a machine shop. This demands the necessary skills, but also a well filled wallet.
Building without proper preparation by means of studying the theory, is generally doomed to fail.

The largest enemy of woodgasifiers are air leaks. Therefore tidy work is a requirement. Summarized, you need much patience and time (hundreds of hours), some mathematical insight, the skill to work with metal and vehicles, and money. Do not think of hundreds, but of thousands of Euros. Also consider that building the generator itself will cost only 10% of the total time. It is the whole project, that kills the enthusiasm. Patience and perseverance are the most important qualities of a woodgasser.

Of course it is possible to build a Stratified or Imbert with oil drums and a car rim. But the life span of the unit will be restricted to a few hundreds of kilometers, and also the life span of the engine is greatly reduced. Frustration will be larger than the invested time, so don’t. I deliberately do not refer to the  FEMA plans of stratified downdraft. Oil drums are for oil and rims are to ride with.

It is very simple make woodgas. It is appallingly difficult make clean woodgas.

Well, you have not let yourself get discouraged. Built a beautiful installation. High expectations. Take into account that a woodgas operated car does not pull the rocks out of the street. A Citroën 2CV Duck will show its tail to you. Gasification is capricious: one time everything works fine, while the next moment power is less, without a direct reason. City driving and traffic-jams can lead to embarrassing situations. Be sure that you always can switch to petrol within seconds, from the drivers seat.

Still full of courage? Let’s go on then.


It seems unimportant, but a constant supply of wood is also a requirement. It is tempting to produce it manually, but the needed volumes will not allow you to have a day job. Therefore: either have it provided or produce it mechanically.

Don’t be seduced by small woodchips, made of twigs. Although a very few successful gasifiers run on chips; to start with it, you will be disappointed. The fines need to be sieved out and the compact mass is not easy to dry.

Logs, even when split are too large. On carwood you must think of the size of a matchbox  to a  cigarette box. The sort of wood does not matter, but beech contains more energy compared to pine, so a larger distance can be travelled on it. Resin is no problem for a well dimensioned gasifier.

The wood which I use has been produced by a Laimet chipper. This make of chipper cuts wood in uniform large bits, without frayed edges and fine parts.

DIY wood chopping can be done the Finnish art.
Have a look: Also possible: cutting slices of a log, which can be chopped afterwards with a small axe. Labor-intensive, but highest energy compactness. Store in a well ventilated shed or boxes. Best immediately in paper sacks.

If the generator is equipped with a so-called monorator, relatively wet wood can used. The monorator condenses the moisture from the wood and leads it to a tank.

Green wood cannot be gasified, because of the still living cell structure. Dry wood that got some rain can be filled into a monorator.

Taking wood is easiest in sacks. Papers sacks do not last long, but breath the moist. Plastic is an alternative, but does not ventilate. Netted sacks are cumbersome to fill and discharge and you do not want them on the back seat.


Studying. Particularly self-study, because there is no training or course. There are forums, but it is difficult to find specific information. Moreover the large number of inferior artists makes the overview small and the disinformation large. There are a number of downloadable books on the internet. These are an excellent basis to acquire basic knowledge. Just reading is not enough. I studied them five times before the fog cleared. These books are on top of the link section.

Which type of gasifier do you want to build? As simple as possible, without pre-heating of primary air, with simple gas cleaning and using scrap parts? Possible. Do not expect a high speed or a long-lived unit, but it is possible. Or immediately a state or the art stainless steel gasifier? Also possible, but you will need all of the before mentioned qualities.

Now that you have sucked up a huge amount of knowledge and know what the goals are, you must buy yourself a vehicle first. A strong car, which can carry the weight. Sufficient space under the hood for filters, mixer and tubing. A mechanically adjustable distributor, unless you can hack the ECU’s and rewrite the software. A big lung engine with low rpm and sufficient power reserve at 3000 rpm. A small car with a small engine cannot be used. The whole gasifier unit for 1200 cc  four cylinder is not five times smaller compared to a 5.7  liter V-8. More like half the size. The little car cannot carry that. And a small car with a small engine needs the most power to keep up with other traffic. It has no reserves.

It sounds tempting to use a turbo charged car. The turbo, however, is between the gas/air mixer and the intake manifold. A strong vacuum appears in the turbo when the gas pedal is in idling position. The seals of the turbo will be damaged and leak oil into the engine.
A supercharged engine is very well possible. Although the maximum engine speed is limited to about 3,000 rpm, there will be less or no power decrease.

The modern vehicles will meet only partial the requirements. Cars from the seventies and eighties are more suitable. An American pickup truck is ideal. A large European car is also possible. Trailers are good to keep the extra weight away from the car, but are more cumbersome to drive. Choices, choices .......

Perhaps you work in or have access to a machine shop. That is ideal to start the project. But ones own workshop is always necessary. Lathe, column drill, angle grinder and welding machine are the minimum equipment.

Basic calculations

This is a tough and dull paragraph. But also the basis of an Imbert gas generator. Therefore an  important paragraph. I will not explain how the formulas have come about. For that you need to study the literature. This is only a summary.

We start with the most important formula, which is nevertheless difficult to find in literature: the calculation of the quantity of required gas. The dimensions of all important components of the whole gasifier unit are based on this. This is it:

G = V x n x 0.5 x 0.48 x 0.72 [l/s]

G is the needed quantity of gas in liters per second
V is the engine displacement in liters
n is the rpm
0.5 is the four stroke factor
0.48 is the mixture composition (1: 1.1)
0.72 is the filling degree of the engine (assumption)
60 for the conversion to seconds

Example: G = 2.32 x 2,750 x 0.5 x 0.48 x 0.72 = 18.4 [liter cold gas per second]

This quantity of gas is sucked by the engine every second. These are the numbers which I have used for my Volvo: 2,320 cc and 2,750 rpm, a little under 3,000. It is better to dimension the generator too small than too large. A generator which generally is used under its nominal capacity, can produce tar. Certainly when slow driving in town or on long idling.

Next calculation is the determination of the diameter of the restriction. We assume thereby a superficial gas speed through the restriction of 2.5 [m/s]. I foresee now glaring looks, but you will encounter this number in the literature also, study therefore!

d = square root from (4/pi x G/Vi)

d is the restriction diameter
pi is 3.142
G is quantity of cold gas per second
Vi is the superficial velocity being: 2.5 [m/s] = 25 [dm/s] for an Imbert

Example: d = square root from ((4/3,142) x (18,4/25)) = 0.97 [dm] = 97 [mm]

This is an important dimension, because it determines all other dimensions of the hearth. These measures can be calculated, but I will not bother you with that. They depend entirely on which type of Imbert you want to build. For this reason I refer to the tables in the literature. With the restriction diameter you can read the remaining dimensions in the tables.
For a simple Imbert with V-hearth use the tables in the “Handbook or biomass downdraft gasifier systems”. Tables for gasifiers with effective primary air pre-heating can be found in FAO 72. And as the attentive student notices, the literature is not always uniform. That is not a big problem, certainly not if you ensure that nozzles and restriction are interchangeable. Nozzles that are adjustable in length and a height adjustable restriction by means of shims.

Tubing diameters depend on the gas quantity, but also on the temperature. In most tubing, we want a laminar flow (< 5 m/s). In some tubing we need a turbulent flow(> 6 m/s). Unfortunately turbulence raises the pressure drop in the system and reduces the filling degree and with that, engine power output.

To determine the tubing diameter, we first calculate the gas flow in liters per second. “We had that number already?!” you will notice; indeed, the quantity of cold gas. But since the gas is hot, an increase of volume occurs. We recalculate this flow using a conversion in Kelvin. 0 degrees Celsius are 273 Kelvin. 350 degrees Celsius are 273 + 350 = 623 Kelvin. 18.4 [l/s] at 350 degrees becomes:

(623/273) x 18.4 = 42.0 [l/s]

See, that asks for a wider tube! Those 350 degrees is the temperature of the gas which comes out of the generator of the Volvo. Because the gas has internally exchanged energy with primary air by effective double heat exchangers, this temperature is rather low. Without heat exchangers, the temperature would be 600 to 700 degrees. So pay attention, which type of Imbert you want to use.

In the tube after the generator, we want a turbulent gas stream, to avoid settling of dust particles in the tube. Take 10 [m/s] =100 [dm/s]

Diameter pipe D = gas flow/gas speed = 42.0/100 = 0.42 [dm2] = 4,200 [mm2]

 Pipe diameter d = square root ((4 x 4,200) /pi) = 73 [mm]

76.1 is x 1.5 [mm] or 3” is existing tube and fits very well.

After the filtering we want a laminar flow to avoid much resistance and power loss. So, up to 5 [m/s]. In practice you use the same size tubing in the whole system; in my situation, 76.1 x 1.5 mm.  Downstream the gas decreases in temperature, shrinks and automatically a lower speed is obtained. Better a too wide than a too tight tube.

In the “Handbook or biomass downdraft gasifier systems” a chapter has been dedicated to the cyclone. Also the website of Bill Pentz is very instructive. Take into account that a slim cyclone removes also a large part of the fine dust. A too generous sized cyclone has less resistance, but only removes the coarse particles. Take an entrance speed in the cyclone of 25  to 30 meters per second, taking the temperature of the gas into account. Calculation of the entrance diameter is the same as before on tubing diameters. The remaining dimensions can be calculated or derived from the above mentioned documents.

For the glass-fibre filter surface area is a formula:

Af = 1.5 x V [m2]

Af is filter surface area in [m2]
V engine displacement in [litre]

For the Volvo:
Af = 1.5 x 2.32 = 3.5 [m2]

I must admit that it has become less: 2.8 [m2].

Also for the total cooling area there is a directive. With “total cooling area” I mean all surfaces which are in contact with the open air, therefore also the tubing. The filter barrel has been insulated, therefore that does not count. Of course, the cooler itself has the most surface area. The directive is:

Ak = V x n x 1.25 [m2]

Ak is the cooling area in [m2]
V engine displacement in [litres]
n is the rpm divided by thousand

For the Volvo:
Ak = 2.32 x 2.75 x 1.25 = 8.0 [m2]

In practice it is difficult to realise. For this reason I have chosen for a cyclone, an apparatus which not only filters, but because of the very high gas speeds and the high gas temperatures, cools extraordinarily well with a relatively small surface. The gas tube, coming from the filter barrel, is finned, so that the out coming gases of approximately 100 degrees Centigrade are cooled down to 40 degrees over a length of only 70 cm, so it is under the dew point. Water condenses in the tube before it heads under the trunk towards the engine. An additional advantage is that possible mineral deposits are rinsed to the cooler.

The cooler itself can best be made of thin walled stainless steel tubes with a diameter of 15 to 25 mm. Gas always goes up in the cooler. Or up by two third of the tubes and down by a third. Up, because of the earlier-mentioned flush effect. The condensate rinses down along the tube wall, also cleaning the dry part. The reason for two third up and one third down is the warmer, expanded gas going in. While being cooled, the volume shrinks and less tubes are needed for the same gas speed. A slight turbulent flow is best for heat exchanging.

Practical tips and tricks

First think well where to put the gasifier. The whole unit behind the trunk is not possible because it is too heavy. Mount the cooler in front of the car if possible. The delivery tube can pre-cool the gas, thus making the cooler more efficient. A generator weighs 60-100 kilograms. Empty. Hot filter barrel up to 50 kilograms when sufficiently large and insulated. Frame: 20-30 kilograms. Before purchasing the car, pay attention to the chassis. You need to attach the frame somewhere sturdy.
The trunk can also be used when it is large enough. Accessibility to the lower parts must be possible, without cutting big parts out of the trunk bottom. Pay attention to ventilation! Take care that no gases ventilate into the interior. Carbon monoxide is poisonous! Be sure that the back seat has an airtight seal to the trunk when a gasifier is put in.  A trailer simplifies the construction, but has disadvantages in the city. Moreover it reduces the top speed.

The welding must be carried out tidy and neat. Not particularly for a fancy appearance, but to prevent leaks and cracks. Notice that heat expansion of metal sheets can be eliminated, especially when preheat mantles are used.

Leaks are the woodgasser’s biggest enemy. In the generator itself, leaking air burns up the gas partly, the result being poor gas and an overheated generator. For this reason always have a temperature gauge on the out coming gas tube. Preferably readable from the drivers seat.

A monorator is recommended, because wood is never entirely dry. See the link section. The outside mantle is exposed to acid condensation from the wood. Use corrosion-durable material.

The grate is shaken from the drivers seat. A vacuum gauge on the out coming gas tube indicates when to shake. Furthermore a vacuum meter after the filter barrel and after the final filter. Of course mounted on the dashboard.

Seal flanges with high temperature silicone sealant. Lids and hatches with silicone baking mats. Axles with graphite cord. Duct tape can lie in the car as emergency. Not as final solution. Start your project out well immediately. Starting with making a mess with the idea of making it tidy later, always fails.

There are several ways to filter. The best one is the most extensive: cyclone and glass-fibre screen filter. Without a cyclone it is possible too, e.g. if the cyclone cools the gas too much. This way of dry filtering can only be done when tar free gas is produced. Otherwise the filter fabric plugs and is ruined. Always put in a metal sheet to have the gas bounce against. Thus you prevent a glowing particle to burn a hole in the fabric. If your are not convinced of the gas quality and suspect tars, use wet filtering. See literature.

The blower can be on different spots. When you choose for a sucking fan, then have it as close as possible to the engine. The disadvantage is that the first, cold, wet gas is sucked through the filter train. Glass-fibre filtering and sucking fans are not compatible. For this reason a pushing blower is better, also because these do not get polluted fins. Only vane pumps and narrow centrifugal fans (vacuum cleaner) qualify. Squirrel cage fans produce insufficient pressure or vacuum. Never use a sucking fan when the gas also has to pass the motor. You can image what happens when a small leak mixes air with the gas and passes the sparking motor…
I use a vacuum cleaner from the seventies that can also blow. However, the thing needs a 4000 Watt inverter. But yeah, you never know where 230 Volt aboard is good for…

For cooling use piping as much as possible, fixed and better  too large of a diameter than too small. If possible fit the tube under the rear axle, instead of going over. Otherwise you get a water lock where the tube goes up. At the engine a water lock in inevitable. Put in a drain to a condensate tank. Piping over the roof and sides is ugly, cumbersome to make and has sharp edges. The only advantage is that no water lock before the engine is expected.

The cooler is most efficient far away from the generator. Then the delivery tube towards the cooler works efficient despite its small surface. Always ask the national road association if a cooler in the front is allowed. Some countries don’t, because it can wound pedestrians severely. I mounted the cooler between front axle and bumper, sloped upward.

A reheater after the cooler dries the moist gas. The gas from the cooler is still 100% humid. We do not want to have it pass the final filter and enter the engine this way. That is why hot coolant heats the gas about 10 degrees Centigrade.

The final filter is meant to catch the soot in case the glass-fibre filter fails. Do not use a paper filter, because it absorbs moist on standstill. Cold starts supply even more water and a huge pressure drop over the filter will decrease engine power. Household fibre material is excellent material for the final filter. An oil bath filter is also possible.

Well, the gas/air mixer…..
Enough examples are showed in the literature. I walked a whole different route. Unfortunately for you, I keep that information secret for the time being. Most other woodgassers need one hand to control the mixture manually. I have both hands free for starting and driving. The mixer is automatic and self-regulating.

For building a stainless steel gasifier, I refer to the book of Vesa Mikkonen in the link section. It has 300 pages with detailed information. There is nothing for me to add. For someone who wants build a decent gasifier, it is worth its weight in gold. It contains drawings and both practical and experienced information. Also recommendable if a less sophisticated gasifier is to be built.

For building a functional gasifier, using second hand materials, I refer to the site of Stig-Erik Werner. However, for the nozzles use drilled stud bolts for length adjustment. A stainless steel nut, drilled to the right diameter, welded to the bolt as a tip. A flange between the hopper and generator makes maintenance easier.

Jim Mason has developed a simple, but very effective gasifier and sells them. Buy it ready welded or as loose parts. Drawings for free to download.

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