Wood gas or producer gas. Basically you heat the wood up to 1200 degrees. A gasifier needs 5 -10" vacuum so an engine is perfect for creating it.
There were around 1 million cars in the world that used it during WWII. Also at the turn of the last century there were nearly 19,000 towns that used wood for electricity in North America.
Here are some pics of old cars using it.
The bottom pic of flame is the first time I made gas from wood a few years ago.