2009 Mercedes-Benz ML320 BlueTEC, GL320 BlueTEC, and R320 BlueTEC

More so than most auto manufacturers, Mercedes-Benz has been committed to diesel engines, which are inherently more fuel-efficient and often deliver more torque than their gasoline counterparts. Diesels can have downsides, however, including low power output, dirty exhaust, and a characteristic clatter.

Modern diesels have dramatically curtailed these negatives while preserving their efficiency advantage. But although the engines are very popular in Europe, many automakers are unable to sell them in the United States because of U.S. emissions regulations, which are not only very strict but differ from one state to another. This year, for instance, seven northeastern states (Connecticut, Maine, Massachusetts, New York, Pennsylvania, Rhode Island, and Vermont) have adopted California's more restrictive emissions standards, which very few diesels can meet.

With the arrival of ultra-low-sulfur diesel fuel in the fall of 2006, Mercedes-Benz was able to introduce the first diesel clean enough to be meet emissions regulations in all fifty states, the E320 BlueTEC sedan. (Nothing being easy with regard to diesels and regulations, the E320 BlueTEC in fact is available for lease, but not for purchase, in California and the Northeast. Go figure.)

The company's other current diesel offerings, the ML320 CDI, the GL320 CDI, and the R320 CDI are offered only in 42 states, but come this fall, 2009 Mercedes diesel SUVs will be available from coast to coast (to purchase as well as to lease). The 2009 ML320 BlueTEC, GL320 BlueTEC, and R320 BlueTEC use the same clean, smooth, and potent 3.0-liter turbocharged, direct-injection diesel V-6 as the E320 BlueTEC sedan.

The output and fuel economy of the cleaner BlueTEC engine is virtually the same as the 3.0-liter V-6 in this year's CDI diesels. Both the five-passenger ML320 SUV and the six-passenger (three-row) R320 minivan/SUV crossover are rated at 18 mpg city/24 highway. We actually saw an indicated 26 mpg on our test drive along two-lane back roads in upstate New York and southern Vermont, a route with few traffic lights and locals who were in no hurry.

Whereas the E320 BlueTEC sedan is genuinely fast - pull out quickly into traffic and the traction control has to intervene to prevent the rear tires from chirping - in the much heavier SUVs, the BlueTEC V-6 isn't nearly as quick. The ML320, at least, is no slouch, and its advertised 0-to-60-mph time of 8.0 seconds essentially mirrors that of the gasoline V-6-powered ML350 (while easily exceeding the gasoline model's 15/20 mpg EPA ratings). The ML320 BlueTEC is rated to tow only 5000 pounds, though, which is less than the gasoline V-6's 7200-pound rating and also, surprisingly, less than the outgoing diesel's 7200-pound tow rating.

The big GL320 BlueTEC isn't as quick to gather speed, as indicated by its 9.0-second 0-to-60-mph time, which is notably slower than the gasoline V-8-powered GL450. (A gasoline V-6 is not offered in the GL.) But of all the BlueTEC diesel Mercedes SUVs, the GL makes the biggest fuel economy gains. Its 17/23 mpg EPA ratings crush the 13/18 mpg figures of the gasoline-engine version, even as it matches the V-8's 7200-pound towing capacity. We saw even better mileage, an indicated 26 mpg, under the same conditions as the ML, which is really impressive for an SUV as big as the GL320.

Unfortunately, we ran out of time before we could sample the BlueTEC engine in the R-class. That vehicle sits between the ML and the GL in seating capacity (with room for six), weight, and factory-estimated performance. Although the R-class isn't a bad choice for families needing more than five seats, it's been a very slow seller, owing mostly to its untraditional appearance; it looks more like a stretched, high-roofed wagon or a minivan than an SUV.

The new Mercedes diesels are cleaner than ever before, with no discernable smell and particulate filters that trap soot. What's not so great is their timing. Stuttgart's new oil burners are hitting the market at a time when diesel fuel prices have crested $5 a gallon, with diesel running about fifty cents per gallon more than unleaded premium. In this environment, do diesels still make sense?

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