October 08 2009   Story: Bryan Joslin

If last year's $4-a-gallon gas prices didn't provide enough motivation for carmakers to get serious about offering more efficient models to American consumers, then surely President Obama's signing into law new Corporate Average Fuel Economy (CAFE) standards did. The new mandate means every full-line carmaker will have to produce a line of products that can achieve a company-wide average of 35 mpg or better to avoid paying penalties by 2016. While this means certain manufacturers will have to develop entirely new product lines and shelve some of their most popular models, companies like BMW and the other Europeans may be able to quickly tip the scales in their favor by bringing in some of the super-efficient diesel models they already sell in the rest of the world. One such car that could easily make the trans-Atlantic journey is BMW's 320d.

Diesels have long played an important role in BMW's success in its home market, with the four-cylinder 3-series leading the charge. Several options exist, including a 1.6-liter, 116-horsepower 316d and the 2.0-liter, 143-horsepower 318d, but the 2.0-liter, 177-horsepower 320d is the one that could actually appeal to American buyers. We picked up our 320d Coupe in Munich and spent several days driving it throughout southern Germany, in traffic that ranged from stop-and-go city driving through der Altstadt in the Bavarian capital to flat-out Autobahn cruising, with a little bit of everything else mixed between.

The heart of the 320d is an engine that's fairly new to the BMW lineup. It uses an aluminum block and cylinder head and hollow camshafts to keep weight at a minimum. Sixteen valves give it a more free-revving nature than most diesels, and the four-valve-per-cylinder arrangement conveniently leaves room for the centrally located direct injection nozzle. Separating the 320d from the 318d is not 200 cc of displacement, but rather the use of piezo-electric injectors instead of conventional magnetic-pulse injectors. It may not sound like a big deal, but the piezo units spray so much more quickly and precisely that multiple injections are possible within a single combustion cycle, allowing for a longer, leaner, more powerful burn.

A single, variable-geometry turbocharger is used to boost output across a wide rpm range. Peak output of 177 horsepower comes at 4000 rpm, and torque maxes out at 221 at just 1750 rpm. By modern standards, those numbers might not sound terribly heady, but they're enough to get the 3311-pound coupe to 60 mph in about 7.7 seconds and push it to a top speed of 144 mph, putting it roughly on par with a last-generation 325i. All this in a car that is rated at 49 mpg (58.9 miles per imperial gallon) on the European combined cycle.

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As impressive as this sounds on paper, it doesn't mean much if the real-life experience is a letdown. Having spent the majority of this year alternating between a Jetta TDI (the most popular diesel car in America) and a 335d sedan (a car that totally eradicates all preconceptions of what a diesel power is all about), we had a pretty good idea what to expect. Namely, we expected the kind of refinement we've come to trust from our 335d (if not the Jetta), but with a much more typical diesel power delivery (not terribly exciting, as with the Jetta). We know that most Europeans are willing to suffer with dismal athleticism if it means paying less at the pump, and while we were hoping the 320d wouldn't be a total performance dud, we did set our sights intentionally low.

There's something to be said for setting low expectations; by doing so we were actually blown away by how "normal" the 320d felt. It possesses virtually all the requisite refinement of any modern BMW. The one notable exception — as an American, anyway — is the typical, unmuffled clatter of a small-displacement diesel at idle (its engine note is entirely indistinguishable from that of any other oil-burning four-banger). Aside from that, there's nothing about the way this car drives that sets it apart from most other 3-series coupes.

With all that torque from such a low engine speed, the 320d pulls away from stops with the kind of fervor BMWs are generally known for. The lightweight engine loves to rev — at least in diesel terms — to its 5000-rpm redline. In urban driving, the sweet spot is between 2000 and 3000 rpm, where torque progressively yields to horsepower. Keep it in that zone and you'll have no problem squirting in and out of narrow traffic opportunities. Gearing is perfect for the 30- to 50-mph grind.

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On the highway, the 320d is even more in its element. It has long Autobahn legs, preferring to ride the torque curve in fifth and sixth gears. The tach barely seems to move when we venture out into the overtaking lane to pass hoards of German families packed to the gills with a weekend's worth of gear and luggage crammed in their Audi and Volvo and Mercedes wagons. We positively fly to an indicated speed of 130 mph when the road offers the opportunity. In the rearview mirror, our colleague — in a diesel-V6-powered, full-size SUV from another German maker — gets smaller and smaller, unable to keep pace with our BMW coupe. All that diesel noise you hear at idle is eliminated once the revs are above 2000, and once you're cruising down the highway the only noise from under the hood is the occasional faint whine of the turbocharger boosting up under load.

After five days of bouncing between Munich, Stuttgart and Ingolstadt, we refueled the 320d and checked our actual fuel economy. After doing the necessary mathematical conversions, we netted a real-world average of 49.5 mpg. That figure represents more than a 10 percent advantage over our Jetta TDI, and a 25 percent advantage over our six-cylinder 335d. BMW's EfficientDynamics program, which applies minor efficiency increases to numerous components to achieve a compound effect, is certainly effective in achieving this figure. Little things like the Auto Start-Stop feature that shuts the engine down when the car is stopped, electrically-boosted power steering that eliminates the constant drag on the engine, and brake regeneration that only engages the alternator when the car is decelerating, all contribute to a better bottom line.

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As much as we love driving our 335d, we can't help but think an "entry-level" 3-series like the 320d would be a better way of spreading the clean-diesel gospel. In Europe the 320d is slotted in just below the 325i, a position that could easily be justified in America where the 328i anchors our current lineup. A six-speed automatic transmission — crucial to any vehicle's success today — is already available as an option in Europe, with minimal loss of performance (0-to-60 mph takes only a tenth of a second longer, and top speed drops by only a single mph). If the European test-cycle figures translated directly into EPA ratings American buyers could drive an automatic-equipped 320d capable of getting 33 mpg city and 52 mpg highway (43 mpg combined). Imagine, Honda Insight-like overall economy, but with all the dynamic (and social, if we're being honest) advantages of a 3-series.

We're not the glassy-eyed optimists we used to be; we know there won't be a 320d for America, especially in this model generation. But at least BMW is now admitting that four-cylinder engines in some form will be needed to meet the upcoming CAFE standards, and we have to believe a strong, smooth diesel-four would be a great fit. The 3 may be too upmarket already for the marketing folks to consider the step back to four-cylinders in this market, but therein lies the beauty of the 1-series and its future offshoots. Don't be surprised if the upcoming X1 Sport Activity Vehicle represents the first-ever BMW with a small diesel engine in this country. If it happens to drive a lot like our 320d, we won't be disappointed at all.