Well, it needs a bit more research but purely from the empiricallly proven auto ignition temp standpoint it would seem that adding RUG to a diesels fuel oil would make for more difficult starting and that its lowering of the viscosity of the fuel oil would be the main factor in helping cold starts. The high compresion ratio is employed in a diesel is to provide the heat to ignite the fuel, petrol engines don`t need this as they have a high temperature spark to ignite the charge and without water injection or higher octane rated fuel, suffer serious detonation problems at higher compression ratios. Never used RUG blend and have only ever used 1 pint of veg oil in a diesel engine so I confess to being a theorist with regards to veg oil, on the other hand I have a little bit of practical experience of using just about any type of mineral oil in quite a number of diesel engines from 25 cu inches to 1000 cu inches capacity and have come to the conclusion that above a certain viscosity, around that of ATF, it becomes difficult to start most diesels due to poor atomisation, this is borne out by the professionally designed vehicle veg oil systems having the facility to start on diesel then when the system is heated sufficiently switch over to veg oil. The better starters on the more viscous mineral based oils seem to be smaller DI engines with glow plugs fitted or 250 cu inch or larger DI engines without, IDI engines with glow plugs start OK, older IDI engines without glow plugs don`t seem to.
Mark.
