BMW's Innovative 4-Cyl. Blazes New Path for Smaller Engines

The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 18 years. In this installment of the 2012 Behind the 10 Best Engines series, WardsAuto looks at the development of BMW’s 2.0L DOHC direct-injected I-4.

BMW can’t leave well enough alone. A year ago, WardsAuto celebrated the auto maker’s new 3.0L N55 “TwinPower” I-6, which succeeds the N54 direct-injected, twin-turbo 6-cyl. that won three straight Ward’s 10 Best Engines awards.

The N55 delivers exactly the same output as the N54, but with lower emissions and 15% better efficiency and it won a second Ward’s 10 Best Engines award this year.

Now BMW’s all-new N20 TwinPower I-4 joins its slightly older, and 50%-bigger award-winning brother, with much of the same technology and some significant advancements. BMW’s first U.S.-market 4-cyl. since 1999, this surprising engine spins out a healthy 240 hp at 5,000 rpm and 260 lb.-ft. (353 Nm) of torque at just 1,250 rpm.

“For its breathtaking ability to propel both the Z4 roadster and the 528i sedan with thrilling efficiency, the N20 earns a 2012 Ward’s 10 Best Engines trophy by proving once and for all that engine downsizing can be a movement to embrace, rather than dread,” said WardAuto editors, who averaged an impressive 27 mpg (8.7 L/100 km) over a total 990 miles (1,593 km) in the two cars.

Now the I-4 has been introduced on the all-new ’12 BMW 3-Series, where it is rated at 23/34 mpg (10-7 L/100 km) city/highway with the 6-speed manual transmission and 24/36 mpg (9.8-6.5  L/100 km) with the available 8-speed automatic.

Because this new N20 I-4 has exactly the same 500cc per-cylinder displacement as the N55 I-6, you might expect BMW would take the easy (and most cost-effective) route by sharing many internal parts such as pistons, rods and valves. But that’s not the path BMW took.

“The N20 4-cyl.TwinPower Turbo is directly related (to the N55) in that the basic technology package was applied to a 4-cyl., and they are closely related in terms of objectives and requirements,” says Bernardo Lopez, who leads BMW Group’s Powertrain Assessment department.

“The aim was to build a 4-cyl. counterpart of that very successful 6-cyl. with similar characteristics. It was designed completely from scratch, but looking at the N55,” Lopez says. And while it shares the same cylinder displacement, its bore and stroke are slightly different.

Lopez adds that some sensors and electronics may be shared between the two, but virtually all other parts are newly designed and developed, largely because a range of new BMW “EfficientDynamics” technologies were applied, including such friction reduction measures as a new cylinder liner coating technology that is incompatible with the N55 block.

The N20’s production development history goes back three years before it was launched in Europe for 2011.

“Three years before launch is the crucial development phase,” Lopez says. “Before that, everything is concept and studies, with different prototypes of the engine in cars, convincing ourselves that this was the right decision.”

As with its 6-cyl. sibling, the TwinPower designation stands not for twin turbochargers but for the combination of BMW’s Valvetronic fully variable intake-valve lift system and (Bosch) gasoline direct injection, along with a single twin-scroll turbo supplied by Mitsubishi Heavy Industries.

And the engine was designed from the beginning to be turbocharged only. There is no naturally aspirated variant, now or planned. Among the N20’s other key components are Federal-Mogul pistons, a Faurecia exhaust manifold and a Mann & Hummel intake system.

Development priorities included basic performance requirements such as lightweight design, high specific output and strong low-end torque with throttle response comparable to the naturally aspirated I-6 it replaces.

Plus, there were tough emissions and efficiency requirements, including compliance with global emissions standards; class-leading fuel efficiency; and compatibility with other EfficientDynamics measures, including electrically powered auxiliary units that operate only when needed.

For instance, the N20’s water pump is electrically driven to reduce parasitic power losses, and the engine’s mechanically actuated oil pump can be decoupled to run in two different modes, not always at a constant speed.

Noise, vibration and harshness characteristics also were critically important and needed to be as close as possible to an I-6, which – unlike an I-4 – is inherently well-balanced.

Lopez confirms NVH was a major challenge, and it took a lot of work to come up with a package that was judged acceptable for vehicles formerly powered by one of the smoothest 6-cyl. engines on the planet.

The efforts included a number of additions to both the engine and the drivetrain, the primary one a unique over-and-under arrangement of the twin counter-rotating (Magna) balance shafts.

“With these balance shafts, we can fully compensate both second-order mass inertia and second-order moment, which means forces around an axis,” Lopez says. “The precise positioning of the balance shafts is essential, and we have them above each other below the crankshaft.

“We have done studies to optimize the arrangement of the balance shafts compared to the classical (side-by-side) arrangement, and these are really superior. They reduce the acceleration and forces that are transmitted to the rest of the powertrain and the vehicle very significantly.” This configuration is unique, and probably an industry first, Lopez says.

Also unique is the marriage of turbocharging to Valvetronic, the no-throttle variable intake valve lift system that controls both timing and lift to precisely manage intake air and reduce pumping losses. BMW has used Valvetronic on naturally aspirated engines for about a decade but has wedded it to turbocharging so far only on the new TwinPower engines.

“Valvetronic delivers additional value when combined with the turbo,” Lopez says. “You regulate the amount of air going into the engine much closer to the cylinder by controlling it via the lift of the valves, not with a throttle that is much farther away from the cylinders. The response of the engine is so much better that we have basically eliminated turbo lag.”

As good as the new N20 turbo I-4 is today, Lopez says there still is plenty of room for future improvements.

“Combustion efficiency is continuously evaluated, so we’re thinking about how to improve the combustion process. But for now, the TwinPower turbo package with Valvetronic and direct injection is something we consider very, very stable. What we need to work on further is to come up with additional measures to reduce vibrations and NVH in general.”

And he says that BMW already is designing a completely new engine family of 3-cyl., 4-cyl. and 6-cyl. engines that will boast improvements in virtually every area over these N20 and N55 TwinPower engines.

“We can never be satisfied,” Lopez says. “As much as we think that the N20 is a really great engine, we need to come up with measures that help us continuously improve its dynamics, reduce its fuel consumption and increase its comfort.”