DEF is a solution that is about 32.5% urea and 67.5% deionized water. Basically, it is injected into the exhaust flow and reduces nitrogen oxides which, in turn, contribute to lower exhaust emissions.
The DEF injection is integrated into the emissions system of the vehicle. It is fed from a separate DEF tank that must be refilled regularly. The consumption rate of the DEF solution can range from 2% to 6% of the amount of diesel fuel the engine uses, depending on the system set up and operation parameters.
In the motor coach application, a separate gauge displays the quantity of DEF in the urea tank. If the tank gets extremely low, a warning will be displayed followed by an engine de-rating. If the tank becomes empty, the de-rated engine will continue to run until it is shut off, at which time it cannot be re-started until DEF is present in the tank.
DEF is available at most service stations in a 2.5 gallon container. In large highway truck stops, island pump dispensers can be found. Generally, at these fuel island pumps, the price per gallon is substantially lower that the 2.5 gallon carry out containers.
So, is the DEF dependence a pain? Not really. I have travelled thousands of miles with a DEF system. Once you get used to it, the extra chore becomes second nature. It requires very little time to top up the DEF tank every once in a while. It really is not that big of a deal.
Does it really make a difference? Wipe your figure on the inside of your exhaust outlet. No black soot! So it appears that the DEF system does help your diesel run cleaner. The effort is worth it. Just keeping the world a little greener.