Diesels that can officially be sold in all fifty states will finally be launched by Mercedes-Benz this fall. If Mercedes gets the new ML320, GL320 and R320 BlueTec models to market before BMW launches its diesels, it will have the first light duty vehicles with urea injection.

The Mercedes SUVs will use a combination of AdBlue injection and diesel particulate filters to meet US Tier 2 Bin 5 emissions standards. AdBlue is the brand name for a solution of 1/3 urea and 2/3 water. The AdBlue injection is used to reduce emissions of nitrogen oxides.

All three models are powered by a 210 3.0L V-6 turbodiesel with 398lb-ft of torque. This updated V-6 has had the compression ratio reduced from 17.7:1 to 16.5:1 along with new pistons that improve the swirl and mixing in the combustion chamber. A seven speed 7G-TRONIC automatic transmission is used for all three SUVs.

Fuel is delivered directly to the combustion chambers through a third generation common rail injection system with piezo injectors. Pressurized intake air is delivered by a variable nozzle turbocharger.

Downstream the exhaust gases are treated three ways. A standard oxidation catalytic converter virtually eliminates carbon monoxide and unburned hydrocarbons. Since 2005 all Mercedes diesels have used a particulate filter to catch 90-95 percent of particulate matter eliminating smoke.

Finally, the selective catalytic reduction (SCR) system converts most of the nitrogen oxides (NOx) into water and nitrogen. An injector sprays the AdBlue into the exhaust upstream of the SCR catalyst. A NOx sensor downstream of the catalyst measures the NOx and combined with a temperature sensor controls the dosing of the urea solution.

In a larger, heavier vehicle like the GL, the engine has to work harder and therefore produces more exhaust and more NOx. As a result it consumes more urea. In order to accommodate the same 15,000 service interval for replenishing the urea, the GL has an 8.5 gallon storage tank, while the other two have 7 gallon tanks.

The ML and R are each tentatively rated at 18/24mpg city/hwy with the GL getting 17/23mpg. By comparison the gasoline ML350 is rated at 15/20mpg and the V-8 ML550 gets 13/18mpg. The ML550 produces substantially more power at 382hp but a similar 391lb-ft of torque.

The E320 BlueTec that's currently available doesn't include the urea injection system. Mercedes received a waiver allowing it to be leased in California for two years after which they have to be resold outside of the state. A redesigned E-Class is scheduled to be introduced in 2009 at which time it will also get the urea injection system.

MARCH 2008