2. BUILDING YOUR OWN WOOD GAS GENERATOR
Ihe following fabrication instructions, parts lists, and illustrations
describe the prototype gasifier unit shown schematically in
Fig. 1-3. These instructions are simple and easy
to follow. The dimensions in the following plans are given in inches
rather than in millimeters to make construction easier for those who might
be unfamiliar with the metric system and to allow the builder to take
advantage of available, alternate construction materials. It will be
obvious to the experienced engineeer, mechanic, or builder that most of
the dimensions (for example, plate thicknesses and cleanout diameters) are
not critical to the acceptable performance of the finished gasifier
The prototype gasifier unit described in the following text was actually
constructed and field tested on a gasoline engine farm tractor
(a 35-hp, John Deere 1010 Special); see Fig. 2-1.
The unit operated very well, and on par with the European, World War II
designs, but it has not had the test of time nor the millions of
operating hours like the older Imbert design. This new stratified design
was developed for the construction of simple, inexpensive emergency
wood gas generator units. The prototype design below should be considered
to be the absolute minimum in regard to materials, piping and filter
arrangement, and carburetor system connections.
The gasifier unit, as described below, is designed to maintain proper
cooling, even at moderate vehicle speeds. If this unit is to be used
on stationary engines or on slow-moving vehicles, a gas cooler and a
secondary filter must be placed in the piping system between the generator
unit and the carburetor. The ideal temperature for the wood gas at the
inlet to the carburetor manifold would be 70oF, with
acceptable peaks of 140 to 160oF. For every 10 degrees above
70oF, an estimated 1% horsepower is lost. Cooler gas has
higher density and, therefore, contains more combustible components per
The millions of wood gasifiers built during World War II proved that
shape, form, and construction material bad little or no effect on the
performance of the unit. Judicious substitution or the use of
scavenged parts is, therefore, quite acceptable. What is important is
- the fire tube dimensions (inside diameter and length) must be
correctly selected to match the rated horsepower of particular engine
which is to be fueled,
- airtightness of the gas generator unit and all connecting piping
must be maintained at all times, and
- unnecessary friction should be eliminated in all of the air
and gas passages by avoiding sharp bends in the piping and by using
piping sizes which are not too small.
2.1. BUILDING THE GAS GENERATOR UNIT AND THE FUEL HOPPER
Figure 2-2 shows an exploded view of the gas
generator unit and the fuel hopper; the list of materials is given in
Table 2-1(all figures and tables mentioned in
Sect. 2 are presented at the end of Sect. 2). Only the dimensions of the
fire tube (Item lA) must be reasonably close; all other dimensions and
materials can be substituted as long as complete airtightness is
maintained. In the following instructions, all item numbers refer both to
Fig. 2-2 and to Table 2-1.
The prototype unit described in this report was construeted for use with
a 35-hp gasoline engine; the unit has a fire tube diameter of 6 in.
(as determined from Table 2-2). A gas generator
unit containing a fire tube up to 9-in. diameter (i.e., a gasifier unit
for fueling engines up to about 65 hp) can be constructed from the
following instructions. If your engine requires a fire tube diameter of
10 in. or more, use a 55-gal drum for the gas unit and another 55-gal
drum for the fuel hopper.
The following fabrication procedure is very general and can be applied
to the construction of gas generator units of any size; however, the
specific dimensions which are given in the parts list and in the
instructions below are for this particular prototype unit. All accompanying
photographs were taken during the actual assembly of the prototype unit.
The fabrication proecdure is as follows:
- Using the displacement or horsepower rating of the engine to be fueled
by the gasifier unit, determine the dimensions (inside diameter and length)
of the fire tube (Item lA) from Table 2-2.
Fabricate a cylindrical tube or cut a length of correctly sized pipe
to match the dimensions from Table 2-2.
(For the prototype gasifier unit illustrated in this report, a 6-in.-diam
firetube was used; its length was 19 in.)
- The circular top plate (Item 2A) should be cut to a diameter equal to
the outside diameter of the gasifier housing drum (Item 3A) at its top.
A circular hole should then be cut in the center of the top plate; the
diameter of this hole must be equal to the outside diameter of the fire
tube. The fire tube (Item 1A) should then be welded at a right angle to
the top plate (Item 2A) as shown in Fig. 2-3.
- The grate (Item 4A) should be made from a stainless steel mixing bowl
or colander. Approximately 125 holes with diameters of 1/2 in. should
be drilled in the bottom and up the sides of the mixing bowl; see
Fig. 2-4. A U-bolt (Item 5A) should be welded
horizontally to the side of the grate, 2 in. from its bottom.
This U-bolt will be interlocked with the shaker mechanism
(Item 12A) in a later step.
- The support chains (Item 6A) are to be attached to the grate in three
evenly spaced holes drilled under the lip of the mixing bowl or
colander; see Fig. 2-5. These chains are to
be connected to the top plate (Item 2A) with eyebolts (Item 7A),
as shown in Fig. 2-6. Each eyebolt should have two
nuts, one on each side of the top plate, so that the eyeholts can be
adjusted to the proper length. When assembled, the bottom of the
firetube should be 1.25 in. above the hottom of the mixing bowl.
- A hole equal to the outside diameter of the ash cleanout port
(Item SA) should be cut into the side of the gasifier housing drum
(Item 3A); the bottom edge of this hole should be about 1/2 in.
from the bottom of the drum. Because of the thin wall thickness of
oil drums and garbage cans, welding is not recommended; brazing such
parts to the drums or cans will ensure both strength and airtightness
(see Fig. 2-7).
- Two holes, equal to the outside diameters of the ignition ports
(Item 10A), are to be cut with their centers at a distance from the
top of the housing drum (Item 3A) equal to the firetube length less
7 in. (19 in. less 7 in. equals 12 in. for this prototype unit); the
holes should be placed opposite each other as shown in
Fig. 2-2. The ignition ports should be attached
to the wall of the housing drum by brazing.
- When the ash cleanout port (Item 8A) and the ignition ports (Item 10A)
have been attached to the wall of the gasifier housing drum
(Item 3A), they should then be closed with pipe caps, Items 9A and 11A
respectively. The threads of the pipe caps should be first coated with
high temperature silicone (Item 27A) to ensure airtightness.
An optional steel crossbar welded to the pipe cap will reduce the
effort required to open these caps later.
- The shaker assembly (Item 12A) is shown in
Fig. 2-8. The 1/2-in. pipe (Item 1AA) should be
brazed into the side of the housing drum (Item 3A), 1.5 inches from
the bottom of the drum; the length of this pipe which protrudes into
the drum must be chosen so that the upright bar (Item 2AA) is in line
with the U-bolt (Item 5A) on the grate. Likewise, the length of the
upright bar must be selected so as to connect into the U-bolt.
- Weld the upright har (Item 2AA) to the head of the bolt
(Item 3AA). The threaded end of the bolt should be ground down or
flattened on one side, as shown in Fig. 2-9,
to positively interlock with a slot to be drilled and filed in the handle
(Item 4AA). The handle can be formed or bent into any desired or
- A hole should be drilled in the pipe cap (Item 7AA) so that there
is a close fit between this hole and the bolt (Item 3AA). The close
fit will help to ensure airtightness.
- Before assembling the shaker, as shown in
Fig. 2-8, coat the bolt (Item 3AA) with a small
amount of grease. Before inserting the bolt, fill the pipe (Item lAA)
with high temperature silicone (Item 27A) to ensure airtightness.
Tighten the nuts (Item 6AA) so that the position of the handie
(Item 4AA) is maintained by friction, yet is capable of being turned
and agitated during cleanout or stationary operation.
- Fabricate the supports (Item 13A) for the gasifier unit housing drum
(Item 3A) out of rectangular, iron bar stock. The shape and height of the
support flanges must be determined by the frame of the vehicle to
which the gasifier is to be mounted. The supports can either be bolted
to the hottom and side with the 114-in. bolts (Item 14A) or can be
brazed directly to the drum; see Fig. 2-10.
Remember to seal all bolt holes for airtightness.
- Completely cover the bottom of the housing drum (Item 3A)
with 1/2 in. of hydraulik cement (Item 28A). The cement should also
be applied to the inside of the drum for about 5 in. up the inside
walls near the bottom. All edges should be rounded for easy ash
- The fuel hopper (Item 15A) is to be made from a second container
with its bottom up as shown in Fig. 2-11.
Remove the bottom, leaving a 1/4-in. lip around the circumference.
- A garden hose (Item 17A) should be cut to a length equal to the
circumference of the fuel hopper (Item 15A) and should then be, slit along
its entire length. It should be placed over the edge of the fuel
hopper from which the bottom was removed. This will prevent injury to
the operator when adding wood fuel to the unit. To insure close fit of
the garbage can lid (Item 16A), a piece of weather stripping
(Item 18A) should be attached under the lid where it makes contact with
the fuel hopper.
- Cut four support bars (Item 19A) to lengths 2.5 in. longer than
the height of the fuel hopper (Item 15A). Drill a 3/8-in. hole in each
end of all four support bars; these holes should be centered 3/4 in.
from the ends. Bend 2 in. of each end of these support bars over at
a right angle; then, mount them evenly spaced around the fuel hopper
(Item 15A) with 1/4-in. bolts (Item 20A). One of the bends on each
support bar should be as close to the lower edge of the fuel hopper
- Cut four metal triangular standoffs (Item 21A) and braze, weld, or
rivet them flat against the edge of the garbage can lid (Item 16A)
as shown in Fig. 2-12; they must be aligned with
the four support bars (Item 19A) attached to the fuel hopper.
During operation, the garbage can lid must have a minimum 3/4-in.
opening for air passage; the standoffs should provide this clearance,
where they are engaged into the holes in the top edges of the support
bars (Item 19A); see Fig. 2-13,.
- Two eye hooks (Item 22A) should be attached to opposite sides of
the garbage can lid (Item 16A). Two screen door springs (Item 23A)
should be attached to the garbage can handle-s and used under tension to
keep the top lid (Item 16A) either open or closed.
- Cut the oil drum lock ring (Item 24A) to the exact circumference of
the top plate (Item 2A) so that it will fit snugly around the gasifier
unit housing drum (Item 3A).
- Cut four 2 by 2 by 1/4-in. tabs (Item 25A); then, braze these tabs
to the lock ring (Item 24A), evenly spaced and in alignment with the
support bars (Item 19A) on the fuel hopper. Drill a 3/8-in. hole in
each tab to align with the holes in the fuel hopper support bars
(Item 19A). The lock ring is shown in Fig. 2-14.
- The connecting pipe (Item 29A) between the gasifier unit and the
filter unit should be attached to the gasifier housing drum
(Item 3A) at a point 6 in. below the top of the drum. Ihis pipe must
be a minimum of 2-in. in diameter and should be at least 6 ft long for
cooling purposes. At least one of the ends of this pipe must be
removable for cleaning and maintenance. On this prototype unit,
an airtight electrical conduit connector was used; this connection
is visible in Fig. 2-1. Many similar plumbing
devices are available and can be used if they are suitable for operation
at 400oF and above. The pipe can also be welded or brazed
directly to the housing drum.
- When assembling the gasifier unit, the upright bar (Item 2AA)
on the shaker assembly must be placed inside the U-bolt (Item 5A)
on the grate.
- The lock ring will then clamp the gasifier unit housing drum
(Item 3A) and the top plate (Item 2A) together. The fuel hopper support
bars (Item 19A) must be attached to the tabs (Item 25A) on the lock ring
with bolts (Item 26A). High temperature silicone (Item 27A) should
be applied to all edges to make an airtight connection. The lock ring
connections are shown in the lower portion of
2.2 BUILDING THE PRIMARY FILTER UNIT
Figures 2-15 and 2-16 show
exploded views of the primary filter unit; the list of materials is
given in Table 2-3 (all figures and tables mentioned
in Sect. 2 are presented at the end of Sect. 2). In the following
instructions, all item numbers refer to either Fig. 2-15
or 2-16 and to Table 2-3.
The prototype primary filter unit was made from a 5-gal paint can.
That size seems to be sufficient for gasifiers with fire tubes up
to 10 in. in diameter. If a fire tube diameter of more than 10 in. is
required, then a 20-gal garbage can or a 30-gal oil drum should be
used. The Filter unit could be fabricated in any shape or form as long
as airtightness and unobstructed flow of gas are provided. If a 5-gal
container is used, it must be clean and free of any chemical residue.
The top edge must be straight and without any indentations. If an
alternate container can be found or fabricated, a larger diameter will
permit longer operation between cleanings.
The piping (Item 29A in Figs. 2-2 and
2-15) which connects the gas generator unit to
the primary filter should be considered to be a necessary part of the
cooling system and should never have an inside diameter less than 2 in.
A flexible automotive exhaust pipe was used on the prototype filter unit
deseribed below; it was shaped into a semicircular are so that increased
length would achieve a greater cooling effect. The fabrication procedure
for the filter unit follows:
- A hole equal to the outside diameter of the drain tube
(Item 13B in Fig. 2-15) should be cut into
the side of the filter container (Item 1B); the bottom edge of this hole
should be about 1/2 in. from the inside bottom of the container.
- The drain tube (Item 13B) should be inserted into the previously
cut hole in the filter container and should be positioned so that its
nonthreaded end is near the center of the container and is
about 1/2 in. off the hottom. Once this position has been ensured,
braze (do not weld) the drain pipe into the side of the filter
container. Close the threaded, exterior end of the drain pipe with
the pipe cap (Item 14B).
- Coat the bottom of the filter container (Item IB) with a 1/2-in.
layer of hydraulic cement (Item 28A), taking care not to plug or
obstruct the end of the drain tube (Item 13B) with cement
(i.e., fill the drain tube with a paper, styrofoam, or other easily
removable, but rigid material). The cement should also be applied
for about 1.5 in. up the inside walls of the container near its bottom.
Round the edges slightly; the cement is to provide a pathway for any
liquid condensate to drain out through the drain tube. The cement must
be allowed to harden before proceeding with the fabrication steps below.
Remove the filler material from the drain tube when the cement has hardened.
- A circular bottom plate (Item 2B) should be cut to a diameter 1/2 in.
smaller than the inside diameter of the filter container (Item 1B). This
will allow for heat expansion and easy removal for cleaning. This
bottom plate should be drilled with as many 3/4-in. holes as are
practical for the size of the plate. Three evenly spaced 3/8-in.
holes should also be drilled around the edge of the bottom plate for the
spacer bolts (Item 3B).
- Fig. 2-16 shows the detail of using three
bolts (Item 3B) as spacers for the hottom plate (Item 2B). The length
of the bolts should be adjusted to provide a clearance of about
2-in. between the layer of cement in the bottom of the container
(Item 1B) and the bottom plate (Item 2B).
- A rectangular divider plate (Item 4B) should be cut to a width
1/4 in. less than the inside diameter of the filter container
(Item IB) and to a height 2.5 in. less than the inside height of the
container. This divider plate should then be welded at a right angle to
the centerline of the bottom plate (Item 2B) as shown in
- Cut a piece of high-temperature hydraulic hose (Item 5B) to
a length equal to the circumference of the filter container. It
should be slit along its entire length and then placed over the top
edge of the filter container (Item 1B) to ensure airtightness.
- A circular lid (Item 6B) should be cut equat to the outside
diameter of the filter container (Item IB). Three holes should be
cut into this lid for the exhaust pipe (Item 29A) from the gasifier
unit, the blower (Item 7B), and the filter exhaust pipe (Item 1OB)
to the engine manifold. Note the arrangement of these holes: the
pipe (Item 29A) from the gasifier unit must enter the lid on
one side of the divider plate (Item 4B); the blower (Item 7B) and
the filter exhaust pipe (Item 1OB) to the engine manifold must be
located on the other side of the divider plate. This arrangement
can be seen in Fig. 2-18.
- The connecting pipe (Item 29A) between the gasifier unit and the
filter unit should be attached to the lid (Item 5B) of the filter
container. At least one of the ends of the connecting pipe
(Item 29A) must be removable for eleaning and maintenance.
On this prototype unit, an airtight electrical conduit connector was
used. Many similar plumbing devices are available and can be used if
they are suitable for operation at 400oF and above. The
pipe can also be welded or brazed directly to the lid.
- Attach the blower (Item 7B) to the filter container lid
(Item 6B). On the prototype gasifier illustrated in this report,
a heater blower from a Volkswagen automobile was used.
Connections for a vertical extension tube (Item SB) will have to be
fabricated as shown in Fig. 2-19. A closing
cap (Item 9B) is required for the blower exhaust tube.
A plumbing cap of steel or plastic with a close fit can be used or
fabricated to fit. The vertical extension and the closing cap are
visible in Fig. 2-1.
- The gas outlet (Item 10B) to the carbureting unit on the engine
should be 1.25 in. minimum diameter. In fabricating this connection,
all abrupt bends should be avoided to ensure free flow of gas. Using
plumbing elbows is one solution. The gas outlet (Item 10B) can either
be welded or brazed to the lid (Item 6B) of the filter container
or an airtight, electrical conduit connector can be used.
- Latching devices (Item 11B) should be welded or brazed to the
lid (see Fig. 2-20) and to the sides
(see Fig. 2-21) of the filter container.
An air tight connection between the lid and the filter container must
- Cut two lengths of high-temperature hydraulic hose (Item 12B)
equal to the height of the divider plate (Item 4B); cut a third length
of hose equal to the width of the divider plate. Slit cach hose along
its entire length. Place the first two hoses on each side of the
divider plate, and place the third hose along the top edge of the
divider plate as shown in Fig. 2-17.
- Insert the divider plate (Item 4B) into the filter container
(Item 1B), making sure that the hoses (Item 12B) create an airtight seal
along all sides. By changing the length of the spacer bolts (Item 3B),
adjust the height of the divider plate so that it is exactly
flush with the top of the filter container. Make sure that the lid
(Item 5B) will seat flatly and tightly against the top edge of the
- Fill the filter container (Item 1B) on-both sides of the divider
plate with wood chips, the same kind as would be used for fuel in
the gasifier unit. After carefully packing and leveling these wood
chips, place the lid (Item 6B) on the filter container, and close
the latches tightly.
2.3. BUILDING THE CARBURETING UNIT WITH THE AIR AND
Figures 2-22 and 2-23
show exploded views of the carbureting unit; the list of materials is
given in Table 2-4 (all figures and tables
mentioned in Sect. 2 are presented at the end of Sect. 2).
In the following instruction's, all item numbers refer both to
Figs. 2-22 and 2-23
and to Table 2-4. The following is'a simple and
easy way to assemble a carburetor to achieve both air mixture and
throttle control. It can be mounted to either updraft or downdraft
manifolds by simply turning the unit over. Most of the fabrication
procedure below is devoted to the assembly of two buttertly
valves: one for the throttle valve and one for the air mixture valve.
The remainder of the carburetor unit can be assembled from ordinary,
threaded plumbing parts.
The inside diameter of the piping used in the carburetor unit must be
related to the size of the engine and should never be smaller than
the intake opening on the engine manifold. If in doubt on the inside
diameter for the pipe and/or hose sizes, always go with a larger
diameter. This will reduce friction losses and will give longer operating
hours between cleanings.
When the wood gas leaves the filter unit it should normally be below
180oF. About 2 ft from the filter container, an automotive
water hose can be connected to the pipe on the carbureting unit.
This rubber hose will keep engine vibration from creating air
leaks in the filter unit or in the connecting piping. The hose must be a
fairly new item; such hoses have a steel spring inside to keep them
from collapsing when negative pressure is applied. The spring will soon
rust if it has first been subjected to water and then to the hot wood
gas enriched with hydrogen. The fabrication procedure for the assembly
of two butterfly valves follows:
- The manifold adapter (Item 1C in Fig. 2-22)
must be fitted with bolts and/or holes for mounting onto the engine's
existing intake manifold. Because gasoline engines are produced with
so many different types of intake manifolds, ingenuity and common sense
must be used to modify the manifold adapter (Item 1C) for each
different engine to be operated on wood gas. A gasket (Item 7C)
should be cut to match the shape of the engine intake fitting.
- The butterfly valve (Item 3C) is shown in Figs.
2-24 and 2-25; two such
valves are required. A 3/8-in. hole should be drilled through the
diameter of each valve body (Item 1CC) at the midpoint of its length.
- The valve plate (Item 2CC) must be oval in shape with the dimensions
given in Table 2-4. An oval valve plate must be
used so that, in the closed position, the valve will be about 10o
off center. This will ensure that the valve will come to a complete stop
in the closed position.
- The edges of the valve plate (Item 2CC), around the longer diameter
of the oval, should be beveled to provide a positive, airtight closure.
Two evenly spaced, 1/4-in. holes should be drilled along the shorter
diameter of the oval plate.
- The valve support rod (Item 3CC) should be filed or ground flat
on one side as shown in Fig. 2-24; the flat area
must begin 1/4 in. from one end and must continue for a distance equal
to the inside diameter of the valve body (Item 1CC).
- Two 3/16-in. holes should be drilled into the flat area of the
valve support rod (Item 3CC); these holes must align with the holes in
the valve plate (Item 2CC). They must also be tapped (with threads)
to accept the valve plate screws (Item 4CC).
- The butterfly valve (Item 3C) should be assembled by first placing
the valve support rod (Item 3CC) through the hole in the valve body
(Item 1CC). The valve plate (Item 2CC) should be dropped into one
end of the valve body and then inserted into the flat area of the
valve support rod. The two screws (Item 4CC) should be used to attach
the valve plate to the support rod. Check to see that the assembled
valve plate rotates freely and seats completely in the closed
- A nut (Item 6CC) should be welded flat against one side of the
throttle arm (Item 5CC) near its end. A 1/8-in. hole should be drilled
into the side of the nut and must be threaded to accept the set screw
(Item 7CC). At least one hole should be drilled into the throttle arm
for attachment of the engine throttle control or air control linkages.
- Place the nut (Item 6CC) on the throttle arm over the end of the
valve support rod (Item 3CC) and use the set screw (Item 7CC) to
secure the assembly. The throttle arm can be placed in any convenient
orientation. Assembled butterfly valves are shown in
- The remaining parts of the carburetor assembly should be screwed
together as shown in Fig. 2-27. Pipe thread
compound should be used to make airtight connections. The assembled
carburetor unit should be attached to the engine's intake manifold
as shown in Fig. 2-28
- This prototype gasifier was designed to operate if gasoline were
unavailable; but, if dual operation on wood and gasoline is desired,
the elbow (Item 2C) could be replaced with a tee, allowing a gasoline
carburetor to also be mounted.
- The arm on the butterfly valve (Item 3C) which is closest to
the elbow (Item 2C) is to be connected to the foot- (or, on tractors,
hand-) operated accelerator. The other butterfly valve is to be
used as the air mixture control valve and can be operated with a
manual choke cable. If the engine has an automatic choking device,
then a hand operated choke cable should be installed. Both butterfly
valves and their connecting control linkages must operate smoothly with
the ability to adjust the valve yet keep it stationary in the selected
position during operation. The linkages must close the valves
airtight when the engine is off.
- The air inlet (Item 6C) should be connected by an extension
hose or pipe, either iron or plastic, to the existing engine's
air filter in order to prevent road dust or agricultural residue
from entering the engine.
- The wood gas inlet (Item 5C) is to be conneeted to the outlet
piping (Item 10B as shown in Fig. 2-15)
from the wood gas filter unit. Part of this connection should be
a high-temperature rubber or neoprene hose to absorb engine