In LNG carriers where duel fuel engines are employed, the primary fuel is a natural gas (Methane) and in LPG carriers the fuel used is (propane) where LPG has high concentration of propane. These cargoes which are been carried in these vessels can be used to operate on diesel engines. And one more advantage of these DFDE is it can operate on 100% diesel fuel also.
Initially when first LPG carriers were introduced the boil off gases from the tanks (i.e. Propane) were used as fuel for power generation used in boiler combustion. This boil off gas rates will vary according to the quantity of LPG carried on board and the quality of the liquefied petrolium gas. It is approximately 0 .12 to 0.16 percentage per day of ship cargo capacity.
This boiled off propane gas obtained from LPG tanks has a composition like methane, butane, and propane. Among this, propane has greater proportion when compared to the other gases. Propane alone can contribute around 70% of LPG. But nowadays the same boiled off gases are used in main propulsion diesel engines and for this purpose the diesel engines are changed over to duel fuel diesel engines thereby reducing the diesel consumed by the engine, hence reducing the overall operating cost of the ship as the daily fuel cost alone contribute to half of the daily running cost of the engines.
Normally during maneuverings inside congested waters the propane injection is not efficient because it require a steady service speed. So during the rough weather and in maneuverings periods the marine diesel or heavy fuel oil is used because it produces more power than propane injection. When the ship reaches sea speed or service speed, it is changed over to the propane that will provide all the require sailing power during the calm weather. These engines will operate completely on diesel and as the ship begins to sail at its full ahead speed the propane gas are admitted in the pilot injection valve to replace the diesel fuel to 80% or even more.
Normal diesel engines can be converted in to duel fuel engines because they relatively have the same compression ratio, structural design, etc. Duel fuel diesel engines have the advantage that the power of the engine can be increased by adding a few more cylinders and hence there is no limit for the size of the ship.