BMW 4-Cyl. Techy, Torquey and Efficient

The Ward’s 10 Best Engines competition has recognized outstanding powertrain achievement for 19 years. In this installment of the 2013 Behind the 10 Best Engines series, WardsAuto looks at the development of BMW’s turbocharged 2.0L I-4.

More and more auto makers are substituting boosted smaller engines for naturally aspirated larger ones, and that has 4-cyl. turbos proliferating like dandelions in spring lawns.

WardsAuto editors tested eight small turbocharged  I-4 engines for 2013 and awarded three that stood out from the crowd – the same three they picked last year. Those top 2.0L turbos come from BMW, Ford and General Motors. Comparing their specs shows some interesting differences.

The BMW N20 I-4, as tested in a 328i sedan, puts out a healthy 240 hp and 255 lb.-ft. (346 Nm) of torque beginning at an astonishingly low 1,250 rpm. That's 12 and 32 fewer ponies and 15 lb.-ft. (20 Nm) and 5 lb.-ft. (7 Nm) less twist than the Ford and General Motors engines, respectively.

Yet the creamy-smooth BMW turbo 4-cyl. feels at least as powerful as the other two off the mark thanks to strong low-rpm torque and no discernible turbo lag. It also tops the GM and Ford powerplants in highway fuel-economy ratings.

“It's uncommon to experience so much thrust so soon after step-off, particularly with turbocharged engines needing time to spool up,” WardsAuto editors say. “But BMW has managed to bid Auf Wiedersehen to turbo lag.”

On the fuel-economy front, judges collectively logged 355 miles (571 km) in the 328i and averaged close to 30 mpg (7.8 L/100 km), topping the 328i's rivals in this year's competition by several miles per gallon.

Two key elements behind that rare balance of performance and efficiency are the car's smooth 8-speed automatic transmission and less-than-smooth stop/start system, which kills the engine (with a shudder) at rest and restarts it (with another) when the brake is released.

But a big part of the engine’s efficiency also is due to the N20's unique “TwinPower” technology trifecta: direct fuel injection, twin-scroll turbocharger and BMW’s Valvetronic variable intake-valve-lift system.

BMW's first U.S.-market 4-cyl. since 1999, the N20’s development history goes back to three years before it was launched in Europe for 2011. And it was designed from the start to be turbocharged only. There is no naturally aspirated variant, now or planned.

According to BMW, its design targets were: High specific engine output (up to 120 hp/L); best torque delivery at very low rpm; response behavior similar to naturally aspirated engines; low fuel consumption in test cycles and customer drive cycles; capability to meet all legal emissions limits including SULEV; best noise vibration and harshness behavior; lightweight design; and the ability to be integrated with all-wheel-drive.

Another key objective was compatibility with the growing suite of fuel-sipping technologies that BMW calls “EfficientDynamics,” including electrically driven auxiliary units that operate only when needed. The N20’s water pump is electric, and its mechanically driven oil pump can be decoupled to run in two different modes instead of at a constant speed.

BMW engine general manager Klaus Hirschfelder says developing the turbocharger components was his team's most difficult challenge, “because of the very high temperatures, nearly 1,000° C (1,832° F). This turbocharger application is fairly new, and the materials undergo high stress due to those very high temperatures.”

That challenge was overcome with the use of high-heat-resistant materials, including austenitic stainless steel, which can withstand up to 980° C (1,796° F).

“That is the gas temperature,” Hirschfelder says. “And a lot of areas inside the turbine, especially in the twin-scroll tongue, can have this temperature.”

Another challenge was the combination of turbocharging and the VVT system, as well as direct gas injection. “With a fuel pressure of 200 bar (2,900 psi), which is relatively high for gasoline, you must make sure that all of the components can handle that.”

Still another hurdle was the engine’s balance shafts. “We have two balancing shafts to (offset) the second moment forces, and this system was a challenge because you have a chain drive and you must make sure that it can withstand the maximum engine speed of 7,000 revs,” he says.

BMW's analysis shows the unique over-and-under positioning of those twin counter-rotating balance shafts is the optimum arrangement to answer second-order moment and mass inertia.

While its cylinder displacement is the same as that of its N55 3.0L turbocharged 6-cyl. big brother, the N20 shares few components with the I-6. “The N20 TwinPower Turbo is directly related (to the N55) in that the basic technology package was applied to a 4-cylinder,” explains Bernardo Lopez, who leads BMW Group’s powertrain assessment department.

“They are closely related in terms of objectives and requirements. 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.”

The I-4 and I-6 TwinPower engines do share the same 91-mm pistons. But while that size is common to all BMW in-line engines except the Mini I-4, the design is different for higher-power engines compared with less potent variants.

As good as the N20 currently is, there always is room for improvement, Hirschfelder says. The N20 will get a successor, and performance and fuel consumption will be improved even more.

Will those stop/start issues be dealt with? “I am sure the next generation will be much better. (The problem) is the mounts and the starter itself, and we have to respond to those customers who don't like it.”

Hirschfelder points out that BMW was a pioneer of auto stop/start in Europe, initially in manual-transmission cars. The difference is that when drivers engaged the clutch pedal of a manual-shift car, they felt little of the vibration that is experienced in an automatic-transmission car.

“As engineers, we haven't seen it as bad as some customers do in the field, and our American customers tend to be more sensitive to (stop/start shudders) than Europeans.”

Of course, for those who can’t wait for a cure down the road, WardsAuto editors found the perfect solution for the stop/start blues: The automatic system can be turned off with the push of a button.