A Biogas Engine is an engine that works similar to other buildcraft engines but uses biofuels instead. The engine must be heated up with lava in an internal tank and fuel must be added. These fuels include Water (not the best choice for burning), Liquid Honey, Milk (lasts for a very long time), Seed Oil (good power output, 3MJ/T), and Biomass (5MJ/T !). These engines do NOT take the refined version, biofuel.