2007 Mercedes-Benz E320 BlueTec - Road Tests
Do perceptions match reality in this efficient, smooth, and strong diesel?
BY MICHAEL AUSTIN, PHOTOGRAPHY BY AARON KILEY
May 2007
Here’s a number for you: With 400 pound-feet on tap, the Mercedes-Benz E320 BlueTec diesel has 17 more pound-feet of torque than a BMW M5. That’s something of a wild-ass comparison, though, because the diesel E320 produces only 208 horsepower—292 fewer than the BMW and 60 fewer than a gasoline E350. The more relevant numbers for a diesel are its fuel mileage, where the E320 BlueTec is rated higher than its gas counterpart by 7 mpg in the city and 11 mpg on the highway, according to the EPA.
Even though the fuel savings are significant, it’s hard to ignore the E320’s torque, especially since it peaks at 1600 rpm. The source of this power is an all-new DOHC 3.0-liter aluminum V-6 that’s fed by an intercooled Garrett turbocharger nestled between the cylinder banks that produces a maximum boost of 29 psi. The seven-speed automatic in the E320 BlueTec is the same as the one found in all other two-wheel-drive E-class models, but the diesel gets the low 2.65:1 final-drive ratio found on the E550 and E63 AMG.
Before we go any further, we should mention that the 2007 E320 BlueTec is on sale in only 45 states. Residents of California, Maine, Massachusetts, New York, and Vermont will have to wait until next year, when the BlueTec receives a 50-state-compliant upgrade to its emissions system. Furthermore, new passenger diesels from any manufacturer must run on ultra-low-sulfur diesel (ULSD), which has at most 15 parts per million of sulfur content and is identified by a sticker at the pump. Unfortunately, EPA regulations only require refiners to produce ULSD as 80 percent of their total diesel output; in December 2010, retailers will be required to sell only ULSD. California has already made the switch. In the rest of the country, finding the right kind of diesel can be a guessing game.
To see how well the E320—which costs $1000 more than a comparable E350—matches up against its gasoline counterpart, we drove one and an E350 Sport from New York City to our offices in Ann Arbor. Apart from the engine, the only differences on the diesel car are smaller front brakes, 16-inch wheels, and a softer suspension; 17-inch wheels are standard on the E350, and Sport models ride on 18s.
The E320 BlueTec is perfectly at home on the highway. It wafts over bumps, and gentle pressure on the pedal easily adds 10 mph to the cruising speed. The E350, in contrast, is livelier and feels lighter than the 136-pound difference between the two cars suggests. But even though throttle response on the E350 is sharp, it takes more pedal pressure for passing acceleration. The E320 lacks the soundtrack of the gas V-6 but makes up for it with torque you can feel every time you hit the throttle. In short, the diesel feels faster.
The bad news is that the E320 BlueTec is slower than the E350 in every respect. Our E320’s 0-to-60 time of 6.8 seconds trails the E350 Sport’s by 0.9 second, and the E320 is also slower than the 6.6 seconds claimed by Mercedes-Benz. The diesel might feel faster, but the numbers don’t lie.
When it comes to fuel-economy numbers, however, the diesel delivers. Our lead feet averaged 34 mpg on our trip, which included some city driving, and we were able to make the entire 650-mile journey on one tank. The E350 averaged 26 mpg, which is hardly shameful but nothing to brag about. Yet its cruising range will outlast even the most steadfast bladder—plus, it’s the more satisfying car to drive. The BlueTec isn’t lacking; it’s just that the E350 is terrific. And even with the fuel-economy figures from our experience, you would have to drive 49,000 miles to recoup the cost of the diesel over the gasoline, considering average fuel costs at the time of this writing of $2.43 a gallon for diesel and $2.39 for premium gasoline.
The E320 BlueTec doesn’t deliver as many smiles and doesn’t save much money over the premium-sipping E350. If you log an awful lot of highway miles, however, or think that gas is going to get a lot more expensive, or feel that better gas mileage might help save the planet, the E320 BlueTec may very well be worth the price.
COUNTERPOINT
DAVE VANDERWERP
For $1000—same as for the huge sunroof option—Mercedes offers a 43-percent EPA combined fuel-economy bump for the E-class sedan, the E320 diesel. That’s not only a cheaper premium than any hybrid’s, but it also provides a larger fuel-economy boost than most. The quiet and smooth Benz diesel got 31 mpg on my commute, an impressive number I’ve never achieved in a hybrid. Too bad it would still take at least 30,000 miles to break even in fuel savings alone.
LARRY WEBSTER
The kick in the fanny one gets when the boost finally builds in the Benz diesel seemed at first to be entertaining enough to warrant the extra $1000 over the cost of the gas-engined E350. But as the miles wore on, the diesel engine—which is only slightly noisier than the gas V-6—lost its luster. The turbo lag got old, especially after stepping out of the quicker E350. When gas prices climb to four bucks a gallon, I’ll gladly accept a diesel Benz, but until then, I’m a gas man.
What’s ‘BlueTec’?
For 2007 and beyond, diesel-engined cars have to meet the same emissions standards as gasoline-engined cars. The problem for automakers is that diesel compliance is more difficult.
Diesel engines offer better fuel economy in part because they use a high compression ratio and lean air-fuel ratios. Those characteristics, however, also alter the proportions of the exhaust gases — carbon monoxide, oxygen, hydrocarbons, and oxides of nitrogen (NOx)—in such a way that they make conventional catalytic converters ineffective at reducing NOx and particulates.
Complicating the issue in the past has been the high sulfur content of the diesel fuel sold in the U.S., which contaminated diesel-specific catalytic converters and particulate traps. Starting in October 2006, however, the EPA mandated that 80 percent of diesel production be of the ultra-low-sulfur variety, which allowed Mercedes to introduce its BlueTec diesel engine.
“BlueTec” is the Mercedes name for its clean-diesel system. There is not one singular whiz-bang part here but a combination of exhaust components and engine controls. Combustion is controlled by high-pressure fuel injectors (maximum fuel pressure is about 23,000 psi) that squirt fuel in bursts to provide the amount of power demanded by the driver and also keep the typical diesel knock at bay.
Once the exhaust leaves the cylinder, an oxidizing catalytic converter, a particulate trap, a “denox” storage converter, and SCR (“selective catalytic reduction,” which specifically treats NOx emissions) go to work. This system is effective enough to meet present federal requirements but not the more-stringent California regulations. So for 2007, the E320 BlueTec is only available in the 45 states that have not adopted California’s regs.
Mercedes says there’s a second version of BlueTec that would allow the E320 to meet California standards. It involves injecting a fluid called AdBlue—an aqueous urea solution—into the exhaust-gas flow. This process releases ammonia, which converts up to 80 percent of the oxides of nitrogen into harmless nitrogen and water in the downstream SCR unit. AdBlue injection has not been approved by the EPA or CARB as of this writing, but Mercedes expects to have the system on at least one 2008 model.
Audi and VW have also announced that a clean diesel with a urea-injection system will be featured on its 2008 diesel cars and marketed under the BlueTec name.
—Larry Webster
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