In WWII, Europe was in a real fuel shortage. Oil imports were largely blocked, and what oil was available was used for aviation and tank fuel. Germany had a lot of coal, but no oil deposits. To make up for their huge lack of oil, German ingenuity turned to gassification of coal to make synthetic fuel. To run cars without using their precious oil, the German government and many European civilians tried a method of running a car by burning wood, and it apparently worked! Ford, GM, and other car manufacturers made kits, and many cars were converted to run on wood, either using a mass produced conversion kit, or a homemade version. A car using a factory built kit would run for 100,000 miles fairly problem free.
The way this worked was that wood was heated until it began to break down chemically. When wood burns in a normal fire, the wood decomposes chemically due to the heat, and some of the gasses produced by the wood are flamable, and they burn as they are released. That is the flame that you see.
With the WWII era wood burning cars, wood was heated to a temperature hot enough to decompose the wood, but the gas was not allowed to burn. It was stored in a chamber, and injected into the cylinders of a regular internal combustion car. Some of the government made cars were similar to a WV Beetle, and a small military runabout, shown in the photos above. FEMA, the Federal Emergency Management Agency, prepared a very complete manual on how to build these units, written by Oak Ridge National Laboratory, foreword by Thomas B. Reed. The site is called Gengassidan.