We’ve barely digested the notion of four-banger BMWs being available here again when the company invites us to try a car with even one fewer cylinder. The engine is called the B38 and it’s a turbocharged three-cylinder, with an 82.0-mm bore and a 94.6-mm stroke, displacing 1499 cc. This engine represents a new family for BMW that also will be produced in four- and six-cylinder form, all with the same half-liter per cylinder. There will also be closely related diesels in the three configurations—the 1.5-liter triple, a 2.0-liter four, and a 3.0-liter inline-six.
The new engine family employs BMW’s Valvetronic system, which controls intake valve limit to efficiently regulate engine power, as well as direct fuel injection to provide the coolest possible combustion conditions to limit knock. While direct fuel injection has been in widespread use for more than 10 years, manufacturers are still experimenting with ways to optimize the injectors. For example, BMW’s first DI turbo engine was the twin-turbo N54 introduced in the 2007 335i. That engine had its fuel injectors nestled next to the spark plugs in the apex of the combustion chamber. The current N20 four-cylinder turbo positions its injectors horizontally between the outer edges of the two intake valves.
With this new B38 engine, BMW has gone back to the centrally placed vertical injectors. Heidelinde Holzer, BMW’s powertrain integration manager, explained that the vertical injectors better mix fuel and air, leading to reduced soot formation. This in turn decreases the deposits on the backs of the intake valves, and largely eliminates the sooty exhaust-pipe residue that plagues many cars with DI engines.
The new engine uses a conventional aluminum block and head. Holzer explained that the magnesium block used on BMW’s older 3.0-liter naturally aspirated six-cylinder posed some production problems, as the scrap magnesium was difficult to recycle. To reduce the natural imbalance in three-cylinder engines, which causes the engine to rock longitudinally like a teeter-totter, there is a single balance shaft located in the oil pan.
The B38 is about 20 pounds lighter than the current four-cylinder N20 and a few inches shorter. More important, it is expected to deliver up to 15 percent better fuel efficiency, thanks to its reduced friction, more-efficient combustion, and more highly loaded operation.
Including the shared half-liter cylinders, the gasoline members of this family are expected to share up to 60 percent of their parts, although that commonality drops considerably with the diesel version. However, both types of engines will be manufactured on the same production lines, allowing BMW flexibility to quickly alter engines types and sizes to match market demand.
A prototype of the three-cylinder was installed in a European 1-series five-door hatchback for a limited test drive around the BMW USA grounds. Coupled to an eight-speed automatic, the little triple in the car we drove was turbocharged to generate 175 horsepower and 200 pound-feet of torque—about the same output as the 2.0-liter turbo four-cylinder in the recently introduced, de-powered 320i. The engine feels strong enough and is remarkably smooth. If you open the hood, you can see the engine shaking a bit on its mounts—even at its high, 900-rpm idle speed—but none of those quivers make it to the passenger cabin.
Our available roads were too short and congested for high speeds, but the engine has an appealing, six-cylinderish sound at high revs. But as you accelerate through various engine speeds and loads, there is an unrefined variability to the engine sound. It’s never terribly loud or raucous, but the tone moves from a gentle rumble, to a mild drone, to a chorus of waste-gate flutter, to bit of a snarl—more or less unpredictably. Although it has a two-mode exhaust system with a computer-controlled flapper valve, there’s never that gradual, consistent, and satisfying build-up to the high-rpm crescendo for which BMW’s inline-sixes are famous.
Of course, this engine is still at least a year away from production. Its first application will be in the i8 plug-in hybrid next year, but we can expect it to eventually appear in the next-gen Mini Cooper as well as the front-drive 1-series that will share bones with the Mini. And the four and six-cylinder versions of this engine will eventually supplant the current equivalent engines.
The engine is not all about fuel economy, though, as Holzer explained that the half-liter cylinders in the gasoline versions can be tuned to produced anywhere from 40 to 67 horsepower each. That means the family extends from a 120-hp 1.5-liter triple all the way to a 400-hp 3.0-liter inline-six. That’s more than enough power for most BMW models, but the challenge will be to make the smaller engines sound and feel like Bimmer engines should. With the three-cylinder, that goal has not yet been achieved.
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