The propane assists in the burn of diesel for a more effective burn. For it to work properly on a Turbo engine, by design blessed with a lean air-to-fuel ratio, and can be fed concentrations of LPG up to about 6-8% of the intake air volume.
Introducing LPG gas into the combustion air intake of a diesel engine acts as an accelerant, promoting the even burning of the diesel fuel, and more complete combustion, resulting in more power being produced. Propane by itself will not self-ignite inside a diesel-fuel compression-ignition engine. During the compression stroke, the air/LPG mixture is compressed and the temperature is raised to about 400?C, not enough to ignite the LPG, which has an ignition temperature of about 500?C. When the diesel fuel is atomized into the cylinder under high pressure, it immediately self-ignites (diesel ignites at about 385?C. ), and causes the LPG to burn as well. Since the LPG is in mixture with the air, the flame front from the diesel spreads more quickly, and more completely, including igniting the air/fuel mixture which is in contact with the cylinder walls, which are cool in comparison to the super-heated air inside the combustion chamber. Much of the cleaner burning of the fuel is attributed to this ignition against the "cooler" components of the engine, and accounts for raising the percentage of combustion from a typical 75% for a well-tuned diesel engine running on pure diesel fuel alone, to 85-90% with the addition of LPG. Obviously, this more complete combustion also gives a nice boost in power, with an accompanying increase in fuel economy and reduction of pollutants.
Understanding that it is a very fine line between lots of extra power and a dose of LPG that will render an engine scrap metal in a hurry, so consider carefully before you decide to "turn it up"( it must be tuned properlly to work safely).