Wartime automotive gasifiers used charcoal as fuel more often than wood. This permitted a cleaner fuel to be made as charcoal is about 85-98% carbon with few volatiles.
In tandem with a charcoal fuelled gasifier, a steam injector was used to produce a gas that was virtually 100% carbon monoxide and hydrogen and little else. A wood fuelled gasifier contains its own water in the wood itself.
For small applications charcoal fuel may be more preferable than wood as charcoal is lighter and packs more energy for a given mass than wood. For a wood gas driven motorbike, a small gasifier is required and for that charcoal is the better fuel.
Making Charcoal - instructions