We like Mercedes-Benz's diesels, but to further reduce emissions, they require a special ingredient: urea. Turns out the fluid is mandatory not only for the exhaust system, but to start a Bluetec vehicle in the first place.
Mercedes is designing its vehicles so they cannot start if the supply of urea, which can be checked in a manner similar to oil levels during regularly scheduled maintenance, is depleted. The system even features sensors making sure the exhaust is being properly neutralized, so owners don’t fill the urea tank with water. This is so diesels can be successfully used as an alternative to hybrid and electric vehicles. With the use of the urea compound, diesels can meet emissions standards that would otherwise be too high. When the urea solution is combined with the pollutant NOx in exhaust, nitrogen and water is produced.
Despite the name, urea isn’t derived from urine, but from natural gas. The fluid could get relatively expensive to replace, but many diesel drivers say it is worth it for the high torque and good mileage they get by using a diesel. Mercedes is putting the urea system on its ML320 and R320 crossovers as well as the GL320 SUV. The automaker will introduce the system on the E320 sedan for the 2010 model year. Source: Automotive News