Even as diesel prices rise faster than gasoline, Mercedes-Benz remains committed to diesel in the U.S. market (the automaker notes that diesel vehicle sales have risen 40% in the U.S. in the past five years), predicting consumers will appreciate the improved engine performance—torque output is 30% to 50% higher on a V6 diesel compared to a V8 gasoline engine—and a fuel tank range of up to 600 miles (depending on application).
Mercedes is expanding its diesel offerings in the U.S. with the introduction of its next-generation BlueTEC diesel engines technology. First introduced in the E-Class in 2007, BlueTEC uses oxidation catalysts, particulate filters, a Nox storage catalyst and Selective Catalytic Reduction catalyst to reduce harmful Nox emissions. The latest generation—debuting in the ‘09 GL 320 BlueTEC, ML 320 BlueTEC and R 320 BlueTEC—adds AdBlue (urea-based solution) injections into the exhaust, which releases ammonia to covert up to 88% of Nox emissions into harmless nitrogen gas and water. This allows Mercedes to meet the stringent U.S. Tier II BIN 5 emission requirements, which means the diesels can be sold in all 50 states.
Utilizing the existing architecture of the Mercedes V6 diesel engine family, the AdBlue compatible 3.2-liter has been tweaked to improve emission performance. Engineers had to redesign the piston cavity to improve the air/fuel mixture, reducing the compression ratio from 17.7 to 16.5, while the new variable-geometry turbocharger has been tuned for improved low rpm operation. New sensors—located at the front and the rear of the SCR catalyst—and a reconfigured engine control unit, control the amount of AdBlue injected into the exhaust stream.
Mercedes first used AdBlue in 2005 in its commercial vehicle fleet in Europe to comply with Euro5 emission standards. It took Mercedes engineers two more years to develop an AdBlue injection system that could be packaged in a light-duty SUV: “The most difficult task we faced was packaging this technology to fit inside the SUV; then the challenge was optimizing the system for light-duty operating cycles,” says Thomas Ruhl, director of development of R-Class, SUV hybrid and BlueTEC for Mercedes-Benz. Engineers also had to make sure the AdBlue solution would not freeze in cold climate areas (the solution can freeze at -11°C) through the use of a small area in the fuel pump that is used to re-circulate the AdBlue, thereby reducing the freezing temperature to approximately -30°C—KMK