From Low Tech Magazine:
Wood gas cars (also known as producer gas cars) are a not-so-elegant but surprisingly efficient and ecological alternative to their petrol (gasoline) cousins, whilst their range is comparable to that of electric cars.
Rising fuel prices and global warming have caused renewed interest in this almost-forgotten technology: worldwide, dozens of handymen drive around in their home-made woodmobiles.
Wood gasification is a proces whereby organic material is converted into a combustible gas under the influence of heat - the process reaches a temperature of 1,400 °C (2,550 °F). The first use of wood gasification dates back to 1870s, when it was used as a forerunner of natural gas for street lighting and cooking.
In the 1920s, German engineer Georges Imbert developed a wood gas generator for mobile use. The gases were cleaned and dried and then fed into the vehicle's combustion engine, which barely needs to be adapted. The Imbert generator was mass produced from 1931 on. At the end of the 1930s, about 9,000 wood gas vehicles were in use, almost exclusively in Europe.
Second World War
The technology became commonplace in many European countries during the Second World War, as a consequence of the rationing of fossil fuels. In Germany alone, around 500,000 producer gas vehicles were in operation by the end of the war.
A network of some 3,000 "petrol stations" was set up, where drivers could stock up on firewood. Not only private cars but also trucks, buses, tractors, motorcycles, ships and trains were equipped with a wood gasification unit. Some tanks were driven on wood gas, too, but for military use the Germans preferred the production of liquid synthetic fuels (made out of wood or coal).
In 1942 (when the technology had not yet reached the height of its popularity), there were about 73,000 producer gas vehicles in Sweden, 65,000 in France, 10,000 in Denmark, 9,000 in both Austria and Norway, and almost 8,000 in Switzerland. Finland had 43,000 "woodmobiles" in 1944, of which 30,000 were buses and trucks, 7,000 private vehicles, 4,000 tractors and 600 boats. (source).
Woodmobiles also appeared in the US, Asia and, particularly, Australia, which had 72,000 vehicles running on woodgas (source). Altogether, more than one million producer gas vehicles were used during World War Two.
After the war, with gasoline once again available, the technology fell into oblivion almost instantaneously. At the beginning of the 1950s, the then West-Germany only had some 20,000 woodmobiles left.
Research programme in Sweden
Rising fuel prices and global warming have resulted in renewed interest in firewood as a direct fuel. Dozens of amateur engineers around the world have converted standard production cars into producer gas vehicles, with most of these modern woodmobiles being built in Scandinavia.
In 1957, the Swedish government set up a research programme to prepare for a fast transition to wood gas cars in case of a sudden oil shortage. Sweden has no oil reserves, but it does have vast woodlands it can use for fuel. The goals of this research was to develop an improved, standardised installation that could be adapted for use in all kinds of vehicles.
This investigation, supported by car manufacturer Volvo, led to a great deal of theoretical knowledge and hands-on experience with several road vehicles (one seen above) and tractors over a total distance of more than 100,000 kilometres (62,000 miles). The results are summarized in a FAO document from 1986, which also discusses some experiments in other countries. Swedish (overview) and, particularly Finnish amateur engineers have used this data to further develop the technology (overview, below a vehicle of Juha Sipilä).