I was expirementing around a while back and got a 2.5 hp briggs to run off of smoke. The way it is possible is using green wood to create as much smoke as possible you need to use a damper and a valve to controle the arflow into the fire and into the engine. the engines do not put out very much power at all but it is possible end of wwII the germans were running a few of their engines off of smoke when they were on flat ground and would convert back over whent they were fighting or pulling hills they pretty much had a manual valve on teh fuel line to shut he fuel flow off.