Updraft Gasifier

The updraft gasifier consists of a top fed fuel bed through which the "gasification agent" (steam, oxygen and/or air) flows in from the bottom and exits through the top as gas. Updraft gasifiers are thermally efficient because the ascending gases pyrolyze and dry the incoming biomass, transferring heat so that the exiting gases leave very cool.

The updraft gasifier has been the standard of coal gasification for 150 years and it's also popular in biomass cook stoves.
Updrafts however don't get hot enough to crack the tars that form when wood is the fuel, so they don't work well for running engines.