The Oomph Behind BMW's Diesel


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Editor's Rating: star rating

The 330d, only available in Europe, is nonetheless a prime example of the mileage-friendly diesels that are coming to the U.S.

Why review a 2006 European car that's not sold in the U.S.? Because this car signals the flotilla of new diesels that will wash up on American shores starting this year. In fact, it's a close cousin of the 335d diesel sedan BMW (BMWG) plans to bring to the U.S. this fall.

Indeed, until this year, unless you visited Europe, it had been difficult to understand what driving a modern diesel is like. Instead of making the trek, I recently had the opportunity to try out the 330d courtesy of Honeywell Turbo Technologies.

It didn't take long for me to get it. After a quick pit stop, things clicked while merging onto I-76, Pennsylvania's main east-west artery and one of the oldest, hilliest interstates around. Shifting into second gear and hitting the throttle, I was pinned back into my seat as the BMW 330d hurtled up the hill. I was surprised by the oomph.

In seconds, I was cruising at 70 mph in third gear, with the engine revving at a barely audible and surprisingly low 3,500 rpm. With a redline of 4,800 I had plenty of revs left—and three more gears—to slalom me through the trucks and minivans lumbering up the hill ahead. This car is a far cry from the rumbling diesels I used to coax to life as a valet 25 years ago.

Bang for the Buck

And the best part? After 220 miles, I didn't have to fill up at the rest stop. Starting in New York with a full tank, the fuel needle had drifted about one-fourth of the way down. On every other drive I've done on the 360-mile trek from New York to Pittsburgh—including in a Toyota (TM) Prius—a fill-up was mandatory somewhere in the middle. Yet with a range in excess of 600 miles, the European-spec'ed BMW 330d still had 400 miles to go.

And the mileage, you ask? The trip computer was drifting between 40 mpg and 45 mpg as I rolled up and down the Appalachian ridge that bisects Pennsylvania.

In an era of $4 gas, not having to fill up at the rest stop was the sort of "road high" I associate with learning to drive. It's the feeling that I could afford to drive fast and far, in a fun car, without worrying too much. Financially stretched drivers wrestling with the pain of letting go of their long romance with size and horsepower may regard hybrids as poky concessions, a sort of medicine to swallow hard. Diesels are set to offer similar mileage, but with a surprisingly visceral appeal.

Surprise, Surprise

It all comes from what's under the hood. The BMW I tested is nothing exotic. A regular on Europe's roads, this 2006 model is simpler than the advanced diesels that have started appearing in the U.S. this year (BusinessWeek.com, 3/24/08). Diesels deliver more spinning force into the tires—torque, in car speak—so they feel faster at low speeds and deliver more power at higher speeds than gas engines.

It's not that the diesel has more absolute power. In tests conducted by Car and Driver magazine, the very same 3-liter turbo-diesel I test-drove generated 228 hp, or less than the gas-fired 255 hp, 3-liter engine in a BMW 330i. In absolute acceleration, the gas-fired 330i nipped the diesel, taking 5.6 seconds to get to 60 mph vs. a still respectable 6 seconds for the diesel.

But the turbo-diesel engine spits out a massive 369 lb-ft of torque, 70% more than the 3-liter gas engine. This is why the diesel still felt faster, both to Car and Driver's reviewers and me. Torque creates acceleration—it's that feeling of being pushed back into your seat. Put another way, if a car accelerates smoothly to 120 mph over 2 miles you may not even sense it. But accelerate to merely 40 mph in 100 feet and you'll feel a stronger head-snapping sense of speed. This sort of pickup is why diesels feel faster. Conversely, the absence of high torque is what can make smaller, lighter cars feel underpowered, leaving some drivers dissatisfied.

Diesels offer some surprises beyond the starting line, too. Another wrinkle in the way diesels work: They deliver torque more evenly, from low gears through high. That makes them smoother to drive, with less frequent gear shifts. And diesels can accelerate from passing speeds with gusto. In Car and Driver's tests, the diesel accelerated from 50 mph to 70 mph nearly 2 seconds faster than its gas-powered cousin.

Aston is Energy Environment editor for BusinessWeek in New York .

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