[Stubby79]

I'd assume you need to use propane or CNG, due to the compression igniting said fuel...as far as I understand it, diesels inject at/close to TDC, so the power stroke starts then...if you have already introduced a fuel that will ignite before TDC, it will combust as soon as there is enough compression, before TDC, causing knock/detonation. Not good for your engine. Propane has a notably higher octane rating, so it is less likely to happen with it.

Also, from what I understand, "they" claim you get better mileage because you get better/more thorough combustion with propane mixed in, which is part of the reason you don't need to run a whole lot to get a benefit. While gasoline might vaporize well, I doubt it's anywhere near as thoroughly and evenly vaporized compared to a gaseous fuel, so it would not give as much of a benefit.

My thoughts (with no real proof) are that having a bit of propane mixed in effectively increases the compression the cylinder when it ignites, which in turn allows you to get more work(actual mechanical work/power out) out of the same volume of diesel injected. Less pumping losses effectively.