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The Ultimate Combustion Machine

The Ultimate Combustion Machine

The industry has moved to V-6s but BMW still believes its iconic I-6 design is the engine of the future.     

The Ward’s 10 Best Engines competition has recognized outstanding powertrain achievement for 18 years. In this installment of the 2012 Behind the 10 Best Engines series, WardsAuto looks at the future of BMW’s iconic I-6 layout.

What more can be said about BMW’s brilliant N55 "TwinPower" turbo? The I-6 is celebrating its second straight Ward’s 10 Best Engines award and its much-loved N54 predecessor won three straight trophies before the N55 arrived.

Besides being sexy and powerful, the N55 also is practical and efficient. The I-6 soon will be the only engine in the German auto maker’s lineup that is available in every one of its models, from the entry-level 1-Series to the top-of-the-line 7-Series sedan and even the X-model cross/utility vehicles.

WardsAuto editors and auto enthusiasts gush over the engine. Even BMW’s corporate beancounters like it. Is there anything left to say? Yes, plenty.

For one, sticking with an inline architecture while the rest of the world’s auto makers have moved to “V” configurations increasingly is a controversial strategy for a big, high-volume producer such as BMW.

Few can argue against an inline design for balancing combustion events and torque pulses from six reciprocating pistons, but I-6s do suffer from being long and tall. Their dimensions have put them at odds with designers trying to improve aerodynamics and safety advocates mandating crush space underneath hoods for pedestrian safety.

Packaging issues, safety rules and other concerns forced most manufacturers to take the V-6 route years ago. BMW and Volvo are among the few remaining mainstream auto makers still using I-6s in vehicles where everyone else offers V-6s.

But then again, those other auto makers did not make I-6s that turn driving into a blissful, out-of-body experience.

"There comes a point while driving the 335i when the mind pauses to take note of the propulsive power and the aural beauty of the N55 TwinPower inline 6-cyl.," muses WardsAuto editor Tom Murphy. "It is a moment that makes the day a little sweeter, when all distractions and heavy thoughts melt away and the driver achieves oneness with an internal-combustion machine that is so much more than an assemblage of metal, plastic and rubber parts.”

"Let there be no doubt this is the best 6-cyl. on the market," addsWardsAuto editor James Amend.

No auto maker would throw away that kind of driving experience just to make a designer happy, especially a company that still considers the engine the most important part of the car.

Fritz Steinparzer, BMW's head of Gas Engine Development, makes it clear the auto maker is sticking with the I-6 architecture and argues compellingly that it is the optimum configuration for future engines.

"I think it is very important that to come to a very good 6-cyl. engine, you have to choose the right base configuration," he says. "For six cylinders, this means an inline 6-cyl. And when you come to a turbocharged engine, that is much more important than it is to naturally aspirated engines."

It is not just the inherent balance and smoothness of an I-6 that makes it BMW's preferred choice but also, perhaps surprisingly, a packaging advantage when it comes to turbocharging, Steinparzer says.

"The best configuration for a turbocharged engine is to bring in the air on one side and have the exhaust manifold and the turbocharger on the other side," Steinparzer says. "This is a very clear design concept.

"On a ‘V’ engine, you have to bring the air to both rows of cylinders, which is much more complicated and needs more packaging space. So maybe there is a little bit of a disadvantage in length (with the I-6), but for all other dimensions, it has only advantages."

European pedestrian-protection rules that require substantial clearance between the top of the engine and the inside of the vehicle's hood also are not a problem, Steinparzer says. The I-6 may be taller, but its height is concentrated in the center of the car. A turbocharged DOHC V-6, by contrast, needs a lot of plumbing within its “V” and has large, tall cylinder heads on both sides.

What’s more, the I-6 has less mass and lower friction compared with a V-6 of the same displacement. "You have only one cylinder head instead of two, an easier chain drive and two camshafts instead of four, which need additional bearings," Steinparzer says.

Beyond power and torque, fuel efficiency and customer-pleasing noise, vibration and harshness characteristics, the N55's design priorities include excellent throttle response with near-zero turbo lag and high-rpm capability.

"It's not so easy for a turbocharged engine to have the same response as a naturally aspirated engine," Steinparzer says, "But that was our target. And I think that for a sporting brand such as BMW, it is very important for a gasoline engine to be able to deal with high rpms."

The smile-inducing N55 DOHC turbo was installed in nearly 200,000 new BMW vehicles globally in 2011, and volumes are continuing to rise as it is used as the standard engine for BMW's larger, more upscale entries and the up-power choice (over the N20 turbo I-4) in smaller vehicles.

While its N54 predecessor generated identical output from a twin-turbo forced-induction system, the N55 does it more efficiently and cost-effectively with a single twin-scroll turbo. It also benefits from BMW's unique "Valvetronic" induction technology, which continuously varies intake valve lift to micro-manage fuel/air input, and therefore power, without a conventional throttle, while "Double Vanos" dual-cam phasing controls both intake and exhaust valve timing.

While Valvetronic has been used on other BMW engines over the last decade, the N55 I-6 boasts the industry's first high-tech ménage a trois of third-generation Valvetronic, turbocharging and direct gas injection.

The biggest challenge BMW engineers faced during the N55’s development was getting the combination of Valvetronic, turbocharging and direct injection exactly right, Steinparzer says. "We did it for the first time with this engine," he says. "We have now introduced it also in the new N20 4-cyl. engine and this year in the V-8."

As customer and government demands for ever-better fuel efficiency relentlessly accelerate, Steinparzer says BMW is working on a variety of new strategies that will continue to improve efficiency without compromising performance.

In the near future, all BMW engines will be turbocharged, Steinparzer says. Further down the road are such things as homogeneous charge compression ignition. “That is not ready for series production today, and it is too early to say if it will be ready in four or five years, but we do see a potential for that technology to bring us benefits in fuel consumption," he says.

Further reductions in engine mass and internal friction also are on the table, as well as myriad other opportunities. “The gasoline engine is not at the end of its development," Steinparzer says.

To BMW fans especially, this is good news indeed.

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