This appears to be an exhaustive write-up of how to build a car that runs on wood gas:
http://www.gengas.nu/byggbes/contents.shtml
http://www.gengas.nu/byggbes/contents.shtml
If you use an original design, it may have soot, tar and particulates, but if you follow my link, those units wilL not; repeat NOT!!!It will work, germans used such in war 2.
It will destroy an engine after a while. As a year or so. Too much forgien particles and soot build up.
later
wayne