1.Trap and Burn
First, let’s consider the Volkswagen Jetta TDI, which uses a device to trap and store NOx. The system captures the pollutant and periodically changes the engine's air-to-fuel mixture to burn off the NOx.
This reduces NOx emissions by up to 90 percent compared to an untreated engine. And it requires nothing at all from the owner.
A particulate filter further reduces emissions. In the Jetta application, the 16-valve engine produces 140 horsepower and 235 foot pounds of torque. It features a common rail fuel system with high-pressure injectors. The engine delivers “real world” fuel economy improvements of 30 percent compared to a similar gasoline model.
The Volkswagen Jetta TDI Sportwagen, which captures and burns NOx, does not require special fluids or maintenance.
2.Spray with Urea
Meanwhile, the Mercedes clean diesel vehicles requires customers to return to a dealership every 10,000 miles—this could be done during regular maintenance visits—to refill a special tank holding urea. The urea solution is a chemical synthesized from natural gas—not urine.
An instrument panel light will warn the driver if the tank falls below one gallon of fluid. If the urea level gets too low, a counter will appear on the dashboard informing the driver that 20 restarts remain. If the warnings continue to be ignored, the vehicle will not start until the urea fluid is added.
The Mercedes-Benz Bluetec vehicles—including the ML320, GL320, and RL320 models—feature improved combustion from a third-generation common rail system that uses piezo-electric injectors. A tiny squirt of urea solution—which Mercedes calls “AdBlue”—is injected into the exhaust mix before reaching the tailpipe. The average consumption of AdBlue is around 0.025 gallons per 60 miles. The solution is held in a seven-gallon tank in the space usually occupied by a spare tire—so the vehicles will have run-flat tires instead of a spare.
The Mercedes R320 Bluetec, and other Mercedes clean diesel vehicles, must maintain a sufficient level of urea in order to effectively treat NOx emissions.
Urea for Larger Engines, NOx Traps for Smaller
The NOx trap approach is more expensive to manufacture than the urea-based system. Both engines meet the most stringent diesel emissions requirements in the world—California’s Tier II/Bin 5 (or Ultra Low Emission Vehicle) standards. Those standards had effectively outlawed diesel engines in California (and other states following California’s standards) for a generation.
The different approaches demonstrated by Mercedes and VW are likely to be used in future engines, with larger engines using the urea approach, and smaller 4-cylinders going without urea.