Small engine Gasifier for the post apocalypse
Gasification was used in World war 2 to provide a petrol alternative to the taxi’s of London and Europe when petrol was unavailable.
Gasification is in its simplest form, a heating of carbon based materials ( wood, coal, charcoal) to produce gases, that will burn in an engine.
This can be accomplished by heating wood in a confined space and siphoning off the gases and piping them into an engine –
I am not sure this will work on a Diesel engine or a 2 stroke engine as they require different things to function effectively, than a 4 stroke Otto engine
With this in mind you should be able to create one with a little research, and i would suggest looking on-line for plans from FEMA
you then should have no issue generating electricity for some time after the fall of civilization.