Sweeping changes are afoot at Bavarian Motor Works, as the automaker confirms plans to bring three-cylinder diesel and gasoline engines to market as part of a new, modular engine family. The base engine in the family—its heart, if you will—is a four-cylinder, from which an inline-six will be derived; that engine in turn forms the basis for the three. This three-cylinder, BMW engineer Theodor Melcher hastens to point out, “has nothing to do with the spirit of renunciation that many drivers associate with those engines.” (In layman’s speak, he’s claiming that there won’t be much sacrifice when you downsize to the three. We’ll see.) He also says the three-cylinder engine is very similar to an inline-six in its characteristics. Another BMW official adds, “You can do things with a three now that you needed a six for in the past.”
All engines will be fed by high-pressure direct-fuel-injection systems and turbocharging. The gasoline engines will be equipped with BMW’s Valvetronic variable valve timing and lift, while the diesel engines will have variable-geometry turbochargers. To ensure smooth operation, the three-cylinders will get one balance shaft, which rotates at the speed of the crankshaft; the four-cylinders get two balance shafts that counter-rotate at double engine speed. The inline-six won’t need such shafts, because it is already inherently balanced. (For more on engine layout and balance, check out our piece on the physics of engine cylinder-bank angles.)
The engines will displace 400 to 500 cc per cylinder. This means the three-cylinder engines will range in size from 1.2 to 1.5 liters, the four-cylinders from 1.6 to 2.0, and the inline-six from 2.4 to 3.0. BMW is rather precise about its output targets: 50 to 70 hp and 45 to 75 lb-ft of torque per cylinder for the gasoline burners, and 30 to 65 hp and 55 to 75 lb-ft for the diesels. That translates to over 400 hp for a top-spec six. Both single- and twin-turbo setups will be employed.
Commonality is Important
Commonality is a key focus for BMW. Valves and pistons are slated to be shared among gasoline and diesel variants, and the injector will always always mounted in the center above the combustion chamber. The location of the throttle control will be standardized across all the powerplants, and the chain drive will be located at the rear of the engine, partially due to pedestrian protection requirements.
Overall, BMW is aiming for the engines to share 40 percent of their components—which will help the assembly line adjust its build mix rapidly according to demand—but there will be high-performance derivatives that will employ different pistons and connecting rods. There also will be hybrid components that will work among various engines.
A wide array of vehicles will get the new engines. The three- and four-cylinder engines can be mounted transversely in front-wheel-drive models (the next 1-series, for example) as well as longitudinally for rear-drive-based cars (3-, 5-, 7-series, etc.); the I-6 can only be mounted longitudinally. BMW’s i brand vehicles will see these engines, and so will the next-gen Mini. Will a rear-wheel drive BMW actually get a three-cylinder engine? Why not?, says an insider. While that’s something less than a confirmation, it’s possible they could even motivate an extremely efficient (and incredibly, epically slow) 5- or 7-series. But the company concedes it’s going to take considerable effort to convince customers of the layout even at the lower end of the spectrum with Mini, much less BMW and its executive flagships.
A two-cylinder spin-off isn’t likely. If it happened, it would be strictly used as a range extender for electric cars, says a BMW engineer. But, more important, what about the V-8 and V-12? “They will always be part of BMW,” we are told. But volumes are low and basically negligible for the V-12, which was characterized to us as “virtually hand-built.” But at least BMW sees the value of holding onto some of its heritage as it (turbo)charges into the future.
by blog.caranddriver.com
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