Quick Spin: BMW three-cylinder engine

The number three has been synonymous with BMW for years, in the form of its signature 3-series model. But it could soon take on a very new meaning for the luxury brand.

BMW is working on its first three-cylinder engine, a turbocharged 1.5-liter unit with direct fuel injection and variable valve timing that was designed for improved fuel economy, but is powerful enough to replace the four-cylinder motors currently used in many of the company's cars.

By reducing friction and reciprocating mass through the elimination of one cylinder, the engine promises to be both more efficient to run -- up to 15 percent better than a four-cylinder of similar power -- while offering a smaller, lighter package that further contributes to improved fuel economy.

Essentially half of an inline-six, the compact mill has been fitted with a counter-rotating shaft placed next to the crankshaft to smooth out the inherent imbalance of the three-cylinder firing pattern.

I recently took a short test drive around the campus of BMWs U.S. headquarters in a 1-series subcompact that had been fitted with a prototype of the engine, and the results were convincing.

The engine was tuned to put out about 175 hp, near the high end of what BMW expects from the range, which will include both gasoline and diesel versions that share over 60 percent of their components.

The power delivery was strong, with virtually none of the turbo lag small displacement engines often exhibit, and its operation as smooth as advertised, the balance shaft helping to endow it with even more six-cylinder characteristics than expected. The soundtrack accompanying it was much deeper and sportier than similarly sized four-cylinder motors typically produce.

In developing its engines, BMW has hit upon 500 cc as the perfect cylinder size for providing just the right mix of acoustics and efficiency it feels is suitable for the brand. This is why its recent four, six and eight cylinder offerings have come in 2.0-liter, 3.0-liter and 4.0-liter displacements, respectively. There’s no 8.0-liter V16 in the works that I know of, but I’d be first in line for a test drive if it were.

In the meantime, the three-cylinder is set to make its debut next year in the futuristic i8 plug-in hybrid sports car, where it will combine with two electric motors to provide a mix of supercar performance and extreme fuel economy, Later, more conventional models will be added, which will likely include this 1-series subcompact and even the iconic 3-series, although BMW will not confirm any further applications at this time.

Although it should be the first luxury automaker to hit the market with a modern three-cylinder in the U.S., it won’t be entirely alone. The Smart Fortwo already employs a rather harsh and poky 1.0-liter three-cylinder that puts out 70 hp, while Ford will soon introduce a three-cylinder version of the Fiesta subcompact with a turbocharged motor good for about 123 hp.

If you’re wondering how low cylinder counts can go, Fiat sells a version of its 500 powered by an inline-two-cylinder in Europe, but has no plans to bring it here. If it catches on, it’s not a stretch that BMW could match it one day. The automaker has plenty of experience building two-pot engines…for its motorcycles.