Running LPG as an additive rather than an alternative fuel in a high pressure boosted over fueled diesel turbo can work in a similar way to nitrous oxide in a petrol car.
Turbo diesels reach a point where even with the boost up they can't suck in enough air to get enough oxygen to burn cleanly hence big soot on drag racing tractors and diesel dragsters and circuit racing trucks.
Injecting propane not only gives an additional fuel burst, it also releases extra oxygen that can help with the excess diesel and remove some of the subsequent black smoke.