BMW's Three-Cylinder Engine: Same Power, Better Gas Mileage Than Four

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How do you get better gas mileage from engines that create the same power as what you've built your brand on?

If you're BMW, the answer is that you launch an entirely new family of small engines derived from the world-famous inline six-cylinders you're famous for.

But if you're fitting them with variable valve timing, direct injection, and turbocharging, you can downsize them to a displacement rarely seen in the U.S.: 1.5 liters.

And because you have built your legacy around engines with displacement of roughly 0.5 liters per cylinder, that means your engine will have only three cylinders--the first time BMW has ever offered such a configuration for sale in a production car.

With regulations on emission (or gas mileage) tightening all over the globe--in the U.S., Europe, and China--BMW has to launch smaller, more efficient engines.

And its new three-cylinder, which will appear roughly one year from now in the BMW i8 plug-in hybrid sports coupe, will play a big role in meeting those regulations.

To be used in a variety of "BMW Group" products--which means not only BMWs, but also MINIs--the three can be mounted lengthwise or transversely.

That means it can be used in the company's first-ever front-wheel-drive platform for the next generation of MINI vehicles, as well as the lowest end of the BMW range.

At a briefing on Monday at the New Jersey headquarters of BMW's U.S. brand, Dr. Heidelinde Holzer from the engine team in Munich explained the concept of the new three.

BMW 1-Series (European model) fitted with prototype 1.5-liter 3-cylinder engine

BMW 1-Series (European model) fitted with prototype 1.5-liter 3-cylinder engine

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BMW's engine designers have always considered half a liter to be the ideal size for a combustion chamber, meaning that fours would be roughly 2.0 liters and sixes would be roughly 3.0 liters.

Look at BMW's lineup and you'll see lots of engines and model that fall into that pattern.

But with higher output from smaller displacements, could a 1.5-liter engine that was direct-injected, turbocharged, and fitted with variable valve timing actually produce the power of a low-end four--but with fuel efficiency up to 15 percent higher?

BMW believes the answer is yes, and it offered up a pair of bright red test cars fitted with developmental test versions of the new 1.5-liter three-cylinder engine.

Holzer said the new engine would have outputs of 120 to 200 horsepower, depending on how it was tuned.

And she noted that its first appearance on the market--roughly one year from now--would be in the BMW i8 plug-in hybrid sports coupe, which uses the new three mounted transversely to power the rear wheels, while an electric motor powers the fronts.

BMW i8 Concept live photos, 2011 Frankfurt Auto Show

BMW i8 Concept live photos, 2011 Frankfurt Auto Show

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That layout is known as a "through-the-road hybrid," and is used right now by only a few European makes not sold in the States.

Another plus for the three-cylinder layout: Because its crankshaft throws are offset 270 degrees  to accommodate firing pulses, and it has a balance shaft to counteract vertical vibrations, the new engine family comes closer to the smoothness of its inline six--Holzer said--than the character of a four-cylinder engine.

The result, Holzer said, is that the BMW i8 will have acceleration comparable to that of today's BMW M3.

From behind the wheel, the engine in the test cars produced a smooth surge of power, which reached the rear wheels viaBMW's seven-speed Dual-Clutch Transmission.

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