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We use a technology that is called producer gas. The technology was primely used during World War II when there were shortage of oil in Sweden.

Simply explained
There is a uncompleted burn of the wood in the producer. This means that there is too little air added to the process so the wood smoulders at high temperature. The wood then come apart to the gas carbon monoxide, witch you then purify and run the engine on. The engine is a usual otto engine that has a gas mixer instead of a carburetor.

...
Wood contains water and when you make gas of the wood the water becomes steam. In the cooler (the red unit with lots of small pipes) the gas is cooled so that the steam becomes water again. The water then pours down to a square canister underneath the cooler.

When you start a gas producer you need to get a high temperature before you can run the engine on the carbon monoxide. That´s why there is a start fan (the black round thing to the right). Usually we use the start fan 2-10 minutes before we close the valve and start the engine. The gas coming out of the start fan should be flammable and burn with a blue colour.