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The greatest problem in a wood gas system is tar. It causes difficulties of many kinds. Presence of tar can be first noticed from the knocking sound on the ICE. The tar makes the inlet valves stuck but when the operating temperature of the ICE is normal the valves are not completely stuck. When the ICE cools down the valves are completely stuck and when the ICE is started again the stuck valves can cause the valve push rods to bend. The inlet valve opens but it does not close until the piston pushes it back. This can damage the piston. Normally the valve can be loosened without removing the cylinder head. This can be done in the following way:

  1. Remove the valve cover.
  2. Rotate the crankshaft by hand so that the piston goes down.
  3. Tap the valve down with a plastic hammer.
  4. Rotate the crankshaft by hand so that the piston comes up and pushes the valve up.
  5. Repeat steps 2 - 4 until the valve spring can move the valve easily.
  6. Check the condition of the valve push rod and replace or straighten if bent.

Tar cannot be easily removed with easily available chemical solvents. The tar gathered in the pipes and on the flaps can be removed by burning it with a gas flame. Tar can sometimes dissolve and then it can be wiped away with a cloth soaked with denatured alcohol.

Tar is produced for many reasons. The main reason is too low temperature in the combustion chamber. If the temperature in the combustion chamber drops below 800°C tar can be formed. This can occur when:

  1. The ICE is let idle for a long time. The gas flows slower in the combustion chamber and this reduces the size of the glowing charcoal layer and the temperature drops. The ICE must not let idle for long periods. The system works best when the load on the ICE is steady all the time.
  2. The fuel is too wet. The vaporizing process needs heat and the temperature in the combustion chamber drops. The moisture content of the fuel must always be below 20%.
  3. An arch is formed in the combustion chamber. When removing the arch with a steel bar the charcoal in the combustion chamber is often stirred too much and fresh wood enters the combustion chamber and tar is produced. If an arch is formed it must be removed carefully with one vertical stroke with the steel bar.
  4. The critical dimensions of the gas generator are not correct. The critical dimensions are:
the inside diameter and length of the air nozzles
Too big inside diameter cools down the combustion chamber and reduces the amount of carbon monoxide in the gas thus reducing its heating power. Too much oxygen can even destroy the whole generator or the gas filter. Too small inside diameter reduces the amount of gas generated thus reducing the power or speed of the ICE. Too short air nozzles let the fuel go too low in the generator before charring. So the raw fuel has not time enough to char and burn and tar is produced. Too long air nozzles cause arch formation in the combustion chamber.
dimensions of the combustion chamber ring
This is the most stressed part. It should be made of the best steel available and constructed so that is can easily be removed and replaced. Too big combustion chamber ring lets the fuel flow easily downwards and the fuel has no time to char thoroughly before passing through the combustion chamber ring. This reduces the heating power and temperature of the gas and tar is formed. Too small combustion chamber ring causes high suction resistance and enough gas can not go through the combustion chamber ring. The heating power and temperature of the gas are high but the amount of gas is low.
dimensions of the hearth
A gas generator which uses wood chips as the fuel needs a movable hearth. Wood chips turns to fine charcoal and ash which soon clog the charcoal layer on the hearth. The hearth must not move continuously but now and then. The movement must be vertical. A horizontal movement would "eat" the charcoal layer from beneath thus reducing its mass and generate tar in the gas. The length of the movement must be adjustable to match the size of the fuel particles. Chopped wood does not need movement at all and the smaller the fuel particles are the longer movement is needed. If the movement is too long fuel consumption is high and the ash collection chamber is quickly filled with ash and unburned charcoal. Too short movement makes the charcoal layer to clog quickly or the hearth must be moved continuously. The length of the movement can be adjusted to the proper value for each type of fuel and ICE based on experience.
the distances between the parts listed above
The distance between the air nozzles and the combustion chamber ring must also correctly match the fuel used. Too short distance does not allow the charring and burning occur properly and tar is produced. Too long distance causes high suction resistance and the mass of the charcoal layer is reduced and tar can be produced. For the generator built during this project the proper distance was 90 mm (4.4 liter engine, wood chips as fuel).

The distance between the combustion chamber ring and the hearth depends on the distance mentioned above. In the test equipment it was 1.5 times the distance between the air nozzles and the combustion chamber ring that means 135 mm. Shorter distance causes the charcoal layer to reduce in mass thus lowering the temperature and producing tar. Longer distance causes high suction resistance, slower gas flow in the generator, lower temperature, tar production and poor reaction on the ICE for example when increase in speed is required.

Translated by Olli.