The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 16 years. This month, Ward’s examines the long-term prospects for BMW’s 3.0L I-6 turbodiesel and other diesels in the U.S. market.

Diesels are beloved in Europe, but many homeland critics say the light-duty diesel engine never will find a place in the heart of the U.S. consumer.

They say diesel technology is too expensive, diesel fuel prices are too unpredictable and diesel’s reputation in America is too soiled to ever catch on. And they say California LEV III emissions regulations soon will chase diesels from U.S. shores for good.

All we can say is the critics obviously have never driven BMW AG’s 3.0L I-6 turbodiesel.

Whether it is snapping your head back in the 335d as it hits 60 mph (97 km/h) in just six seconds or allowing the X5 cross/utility vehicle to effortlessly cruise for almost 600 miles (966 km) on one tank of fuel, the engine is a masterpiece of efficiency and power that U.S. luxury buyers suddenly are beginning to appreciate.

“Had you asked me about diesel prospects a year ago, I would have been a bit ho-hum, but right now we are fighting for production allocation with other markets in the world to satisfy a growing demand in the U.S.,” says Martin Birkmann, manager-product planning and strategy, BMW of North America, LLC.

The turbodiesel currently enjoys a 30% take rate on the X5 CUV, and diesel X5s command the highest pre-sold order bank of any BMW product in the U.S., Birkmann says. Demand for the diesel-powered 3-Series, BMW’s only other U.S. diesel offering is not as high, but still double that of a year ago.

“From everything we hear from consumer researchers and retailers, people are just wide-eyed when they come out from behind the wheel of a diesel after a test drive,” Birkmann says.

“Wide-eyed” is a good description of Ward’s 10 Best Engines judges when they first sampled the outpouring of torque the engine starts to deliver at just over idle.

In the 335d, old misconceptions about compression-ignition engines, known in the industry as “the triple prejudice of noisy, smelly and slow,” evaporate when you first tip in the throttle.

The engine makes an angry grunt, and suddenly a whopping 425 lb.-ft. (576 Nm) of torque starts twisting the rear wheels and pushing the car forward as if a semi truck just hit you from behind.

This kind of performance, combined with the engine’s inherent efficiency, is what attracts many European BMW buyers to the diesel option. In fact, about two-thirds of all BMWs sold in Europe are powered by diesels. It’s also a big reason why the engine has made the Ward’s 10 Best Engines list two years in a row.

But it isn’t just the torque, or 88 hp/L specific output, that impressed us. The engine’s inherent efficiency also scored lots of points. The 335d is rated at 23/36 mpg (10.2-6.5 L/100 km) in city/highway driving.

The diesel X5 gets 19/26 mpg (12.3-9 L/100 km), a little less awe-inspiring until you consider the CUV weighs well over 5,000 lbs. (2268 kg). It also trounces the I-6 gasoline engine in fuel-economy and performance.

It is this more practical aspect that seems to be attracting U.S. buyers, Birkmann says.

“What diesel buyers in Europe and the U.S. have in common is the idea of efficiency and economy. They are intrigued by the idea,” he says. Growing resale values for the diesel option also are giving oil burners a boost.

But Birkmann says the real key to selling diesels in the U.S. is putting potential buyers behind the wheel and getting dealers fully informed and engaged in selling the technology.

“Once the retail organization has enough belief in the product to put their own effort behind it, and do a convincing job explaining and selling that product, for me, that’s the answer.”

Diesel sales really started to gain momentum last summer during the U.S. government’s “Cash-for- Clunkers” stimulus, Birkmann says.

While the program had little direct relevance for BMW because of the brand’s premium prices and the fact the BMW ownership base did not have a lot of $2,000 clunkers to trade in, the auto maker started offering a $4,500 diesel incentive it called an “Eco credit.”

That strategy got people into showrooms and generated a dialog between dealership sales people and customers about the positive aspects of compression-ignition engines.

Once sales people started understanding the benefits of diesel technology and what features would resonate with consumers, momentum started building, Birkmann says.

Unfortunately, he is less forthcoming about future plans BMW has for the technological heart of the 3.0L turbodiesel, its 2-stage sequential turbo.

Virtually eliminating turbo lag, the unique layout features a small turbocharger that spools up quickly for immediate low-rpm throttle response, while a larger, more powerful turbo gradually takes over to provide even more induction air as engine speed increases.

The strategy provides flawless linear response throughout the engine’s entire rpm range, but Birkmann says there are many ways to enable turbocharged engines to have maximum breathing and linear power delivery.

“This can be accomplished with a single turbo in combination with Valvetronic, as we’ve done on the gasoline side. It could be done with twin-scroll turbos, variable-vanes or staging two turbos sequentially,” he says.

“But first we look at the level of performance and output and what is needed. Then we go to the drawing board, without being hung up on whether it is sequential, variable-vane, etc.”

Dean Tomazic, vice president-Engine Performance and Emissions Div. at FEV Inc., says many more engines equipped with 2-stage sequential turbochargers are coming, both from BMW as well as other auto makers.

BMW and Mercedes-Benz already have 2.0L and 2.2L 4-cyl. diesels equipped with sequential turbochargers in Europe.

The engines are astonishingly powerful, with the slightly larger Mercedes engine producing 369 lb.-ft. (500 Nm) of torque. Both are expected to be available in the U.S., although exactly when is unclear.

Because of the sequential turbochargers, the engines are pleasant to drive and do not have the lag or power surges normally found on small, powerful turbocharged engines.

“They give you such a good feeling of having high torque available at low engine speeds that it makes the car very, very nice to drive,” Tomazic says.

Many other auto makers will be applying 2-stage sequential turbochargers on smaller displacement engines, Tomazic says.

“Nothing is confirmed for a 4-cyl. diesel yet in the U.S.,” BMW’s Birkmann says, but he clearly does see a future for diesels in North America. They are a natural fit for U.S. driving patterns, which emphasize highway mileage, and for U.S. drivers, who like lots of low-end torque.

And he believes the technology is making a fresh start with consumers, whose old ideas about diesels can be dispelled.

“If you look at diesels today, with precision, high-pressure injection systems and compare them with our first approach, it’s almost like a completely different idea,” Birkmann says.

And Tomazic dismisses the critics’ view that diesels will not be able to meet future emissions regulations.

“How often have people said: ‘Well, if we do this, then the diesel is dead.’ It happened under Tier II (emissions regulations) and the same thing will happen under Lev III,” he says. “It won’t be any different.

“People will find a way with additional refinement and innovative ideas to make it happen.”

dwinter@wardsauto.com