Good Morning MattR.
I have done both horizontal and vertical gasifing of “stick form” wood to run small IC engines.
Stick form being what I would call fine splitting of my normal nominal 18" / 46cm length woodstove firewood. This was split into irregular 4"x 4" /100mm x 100 mm cross sections.
As you said the pyrolisis and oxidization sections worked OK/fine. The REDUCTION conversion sections were always the problem area. Just two stickwood hang-ups/cross-lays, or a couple of burnt around remaining limb knot chunks will cause a gasses bypass channels to form. Then the upper sections CO2 and vaporized H2O goes past unreduced, unconverted to CO, H2 and CH4 fuel gases. Engine power then stumbles to 1/2 of normal expected woodgas power. Or; if under a load, often coughs chokes and dies. Shaking does not help. In fact seems to then just overstettle and flow packs other areas of the Reduction bed increasing the gasses bypassing channel flow problem.
Angled down into the lower oxidization/upper reduction Char breaking/poker ports work well. But that then that requires much involed Operator knowlege/skill/and winningness to use.
I know a couple of old codgers doing and willing to do this w-a-y up in the mountains off grid.
Works much, much better with easily burning down softwoods like fir/spruce and pines trunk sections without any limb/knot wood sections in the sticks. Chokes out on them with any attemp to use slow buring hardwoods like oak or even maple. Red alder processes good but much less power with a slower energy release and higher ash content causing more gasses flow clogging and needed cleaning out.
I once on another Site/Forum got a Unkranian memeber there to search out and put up links to 50-70’s Soviet/Russian forestry equipment wheeled, tracked and light rail that had raw fueled stick wood gasifiers. Thier wood forms were more European at 12"-16" / 30-40cm X 4"-6" 100-150mm. These did have active human Operators using top and side poker ports. Even larger less power expected IC engines ~200-400 big slow CID for 20-30 hp. More interesting for me was thier reversal blower top hopper venting to allow in hopper drying down of 40-60% wet, raw as cut fuel wood. VERY steamy and smoky emmitting. We could never do that now. Be many neighbor complaints for visible “smoke”.
All in all, in my experiences much better to spend the human time in fuel wood prepping down to at least chunk sizing to be able to then unattened 2-4 hour system run with less system metals killing waste heat produced and a average overall better quality fuel gas. Then you will get a 2X better wood volumn use per horsepower/kWme hour used.