They filter it and use gasifiers that make less creosote, but the best and simplest is to use charcoal like they did in ww2. They also need ample warm up time to get hot enough before they run the engine off it, and if it's a diesel they still need enough fuel to provide the ignition. If people are interested i have literature on it, it's old technology that laid the foundations for the fuels we use today. It can also be used in heating applications (far easier than running it in an internal combustion engine), which makes automating the process much easier too, and can utilize almost any feedstock provided the right preparation. This is also the same way that modern coal power plant works, by gasifying the coal they remove the solids from the process with much greater efficiency and control.