words: Stu Fowle

In Europe, diesel engines are offered in everything from Ford Fiestas to large SUVs. Unconfirmed speculation around the Motive office has even European lawnmowers running spark plug–free. But here in the U.S., the developing diesel revival clings to volume segments in which the risk of offering an alternate-fuel option may be justified. For Volkswagen, that means Jettas. For BMW, it means the 3-series and X5. For Audi, the Q7. Similarly, Mercedes-Benz is betting on its crossovers — the ML-, GL-, and R-classes — to heighten stateside diesel awareness. We recently took a quick spin in what's sure to move the most units of these three, the 2009 ML320 Bluetec.

Of course, the ML320 diesel isn't exactly new, serving as an update of the 2008 ML320 CDI. The two use the same engine, the same transmission, and they even boast the same EPA figures of 18/24 mpg. The big difference that earns the Bluetec name comes post-combustion, courtesy of AdBlue. That's the name Mercedes-Benz uses for a system that injects a urea-based solution into the exhaust flow, which in turn releases ammonia and converts harmful nitrogen oxides into nitrogen and water. Computers measure the necessary injection rate based on engine activity, and the average usage rate measures out to a figure that conveniently requires a refill every 10,000 miles — right when oil changes are due.

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The EPA has a 10-point rating scale to measure the cleanliness of vehicle emissions based on a number of factors, and the new system scores a 6/10, matching the score of the gas-powered ML350. Last year's CDI model, without the help of blue pee-pee juice, scored just a 1/10. The EPA says its Air Pollution test "represents the amount of health-damaging and smog-forming airborne pollutants the vehicle emits." This doesn't include CO2 measurements, which are even more in the ML's favor since diesels, thanks to their higher efficiency, produce less CO2. According to fueleconomy.gov, the ML320 Bluetec's carbon footprint, or annual tons of CO2 emitted driving 15,000 miles with a 55/45 percent split between city and highway, is 10.6 tons. An ML350, by comparison, emits 10.8.

So yes, the new Bluetec name means that you can feel better about yourself. But for Mercedes-Benz, it means quite a lot more. Because the U.S. (and in particular the five states operating under California's more stringent laws) are harder on Oxides of Nitrous emissions and more lax on CO2, the old ML diesel wasn't 50-state legal and would've had a hard time keeping up with ever-tightening limits. With the ML Bluetec, Mercedes has met the ULEV II emissions standard and can now sell its diesel vehicles in those liberal areas that are probably most likely to buy one. And that's good news for anyone who wants diesels to succeed here.

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But enough about hot air — how does it drive? Quite well, actually. The engine's 398 lb-ft of torque seem to be available right at idle, while horsepower, as in all diesels, comes with a relatively unimpressive peak number (210 in this case). That doesn't matter, because the ML320 never feels sluggish on our 25-mile route through rural northwestern Chicagoland. It pulls fluidly from stops and out of corners, working in harmony with M-B's impressive seven-speed automatic. Just for kicks, we scaled a small grassy hill in the corner of a parking lot. With feather-light throttle input, the ML idled its way up the hill without drawing the attention of any irritable farmers living nearby.

Fuel economy for our short drive was equally impressive. The ML returned an indicated 24.9 mpg over 45-mph roads with the occasional stop sign or slow corner. That number exceeds the ML's highway figure of 24 mpg and is well above the city number of 18 mpg. Having been accused in the past of being too generous with fuel economy rankings, we suspect the EPA has overcorrected its numbers just a bit. Plus, when there's an average-fuel-economy gauge staring you in the face, it is hard to resist the urge to drive the score up.

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More important is the fact that the Bluetec's economy makes the ML350's 15/20 numbers downright depressing. However, there is a catch. Here in the Chicago area, diesel is just 20 to 30 cents higher than a gallon of premium fuel, so the economics work in diesel's favor. However, in neighboring Michigan, the disparity between premium and diesel gallons is closer to a dollar, throwing any cost benefit out the window. And that doesn't include the $1500 premium an ML320 Bluetec carries over an ML350 right from the beginning.

As with owners of hybrids, the diesel buyers paying a little extra to earn it back later will probably never realize their goal. Instead, a diesel like the ML320 Bluetec should be considered on the basis of image, preference, or need. The ML diesel will sell to trailer towers, torque lovers, and people buying into the clean diesel movement. How convenient that we fit into all three categories?