Have a look at this site, and then forget the whole idea.
Can Diesel engines be converted to run on LPG?
One way to do is to fit spark plugs to the head instead of injectors, fit ignition coil and distributor, and run only on LPG gas. Bye-Bye diesel .... . and bye-bye a lot of cash for nothing gained.
Another way is to leave the engine more or less alone, and spray LPG gas into the inlet manifold where it will mix with the air and go into the cylinders. Ignition will take place in SOME diesel engines, but may not in others, especially as the revs go up using mainly the gas.
There are so many problems, and such large costs, it simply isn't worth the bother.
Remember this, both diesel engines and LPG have been around for a long time - but not together in the same car. You have to ask yourself this: Can YOU be the first person on the planet to devise an effective LPG conversion for a diesel engined car?
You would be far better off going bio-diesel, or veggy-oil.
CharlesY