I refuse to go down the path of juvenile humor simply because the magic ingredient in Mercedes-Benz's new California-legal diesels, such as the ML320 CDI, is urea -- which also happens to be the magic ingredient in urine. That's rule No. 1.
Still, I think it's fascinating that urea -- an additive in Mercedes' BlueTEC emissions treatment system -- serves generally the same purpose in clean diesels as it does in human engineering. Our bodies synthesize urea as a way to carry away toxic ammonia, which is a metabolic waste product (and they say gym class never pays off).
Similarly, Mercedes' AdBlue fluid is injected into the vehicle's exhaust, where it helps neutralize and carry away toxic nitrogen oxides (NOx), which are byproducts of high-temperature diesel combustion.
Think of the BlueTEC system as a mechanical kidney.
I had plenty of time to contemplate these mysteries after topping off the ML320 CDI with $5-a-gallon diesel in Los Angeles and heading nonstop toward Monterey. Whizzing up Interstate 5 -- oh, dear -- I was getting 24 miles to the gallon, which is downright respectable for a 4,817-pound sport utility vehicle with the aerodynamics of a catapult-fired rhino. At this rate of consumption, this oil-burner has a range of about 600 miles. Alas, I don't have anything like that kind of range. Oh, look! The golden arches.
I truly sympathize with automotive engineers. For years, customers and the wags of wheel-dom have hammered them with the question: Why can't we have high-efficiency diesels in the U.S. like they have in Europe? Diesel vehicles are 25% to 40% more fuel efficient than gasoline vehicles. But no conventional Euro diesel could pass California's strict emissions tests for sulfur, particulate matter and NOx.
When a federal regulation mandating ultra-low-sulfur diesel fuel took effect in 2006, much of the problem was solved (high-sulfur fuel slowly renders diesel emissions systems ineffective). The first generation of Mercedes' BlueTEC diesel in 2007 (in the E320 sedan) used a complex system of traps and filters to deal with particulate matter. These systems actually held particulates until engine conditions were right, then released them to be vaporized in the extreme temperatures of the exhaust. Very clever.
If any of this seems to be dreary specificity, just remember: For eco-minded gearheads, it's sheer pornography.
