An interesting post , there also seems to be a lot of urban myth and hearsay floating around. Some of the buses around here (deepest Somerset) run on LPG and have done for years, but these are a one way conversion, and LPG is then the only fuel they can use.
Where does the DVLA state that adding LPG to a diesel engine is illegal in the UK, chapter and verse please?
This aside, adding Propane to a Diesel engine adds extra power only because your adding extra fuel, it burns cleaner only because LPG is a cleaner burning fuel, there is no more myth or magic to it further than just that. Diesels do not have a throttle butterfly unlike a petrol engine and so run in effect full throttle all the time and the fuel is the only thing varied to change power output, more fuel = more power and more RPM's. You may as well just turn the diesel pump up and add more fuel that way.
What I'd be more concerned about is fuel burn times as I would expect that Diesel burns a lot faster than the relatively slow burning LPG, this may lead to overheating issues with prolonged use. The slow burn of LPG is why when converting a petrol engine to LPG you have to advance the timing by quite a few degrees to get full power and to avoid overheating.
The much more clever approach for more power and a cleaner burn on Diesel is water methanol injection, this works in two stages:
1. Methanol improves the burn efficiency of Diesel, plus your adding extra fuel to the combustion
2. Water cools the incoming air making it more dense, therefore upping the compression ratio, it also cools the resulting explosion resulting in less NOx and CO production, these gases are only produced when burning a fuel quickly (part of the reason that slow burning LPG is so clean as a car fuel).
Water methanol is easy to get ready mixed, it's called concentrated screen wash, look for the screen wash with a flammable sticker.
This is proven technology and is even used in the aviation industry on jet and turboprop engines.
