oil and water engines! not at pet-d'troll!
anyway the diesel engine must be under load to have a super combustion, the oil "naturally" fouls the diesel engines, especially at idle, the cylinders freeze, the segments get damaged etc ...
all that is oil produces soot. however, the combustion of a vegetable oil will always be cleaner than a mineral oil (petroleum).
wikipedia wrote:Vegetable oil contains no nitrogen, no sulfur, no heavy metals and no harmful pollutants.