The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 17 years. In this installment of the 2011 Behind the 10 Best Engines series, Ward’s looks at the development of’s N55 I-6.
If an auto maker has a delightfully smooth and powerful gasoline direct-injection, twin-turbocharged I-6 that has won three straight Ward’s 10 Best Engines awards, why would you redesign a masterpiece? And if you did, wouldn’t a bump in displacement and output be in order?
Yet,’s new 3.0L N55 I-6 is the same size and pumps out exactly the same power as its N54 predecessor: 300 hp and 300 lb.-ft (407 Nm) of torque. The trick is the new design is 15% more fuel efficient, with lower emissions and better low-end torque.
Ward’s editors tested the N55 last year in an ’11 335i sedan, but it now has replaced the N54 on the 135i, 335i, 535i, X3 X5, X6 35i and Z4 35i. More applications for the new 6-cyl. are expected, including a 640i model that will be introduced this fall featuring a new, higher rated version of the N55 engine called the N55HP.
The N54 currently only is available as the N54HP in the Z4 sDrive35is, 335is Coupe and Convertible and 740i/Li.
“Why did we make a new engine?” BMW Drivetrain Development Product Manager Hans Hohenner asks rhetorically. “It's always the constant improvement in emissions.”
“We have to meet ULEV-2 (California’s Ultra-Low Emissions Vehicle-2 rule), and we are introducing new cars, such as the 5-Series Gran Turismo, where we have the need for such an engine. We also had to replace a larger, naturally aspirated engine in a very heavy car, the X6.”
Two years ago, Ward’s editors described the all-aluminum N54 as “all-around awesome,” “absolutely phenomenal,” and “one of the best volume internal-combustion engines ever produced.”
Ward’s editors predicted it would turn up in the next-generation 7-Series “and probably most everywhere else there’s a BMW engine bay needing fuel-efficient performance.” But now BMW has trumped the N54 with the new N55.
How does the new I-6 differ from its heralded predecessor? For starters, the N55 features a single, more compact, faster-responding twin-scroll turbocharger that replaces the N54’s twin turbos.
The N55 also boasts BMW’s third-generation Valvetronic throttle-less variable intake technology.
Valvetronic uses an electric motor in the cylinder head to continuously vary the amount of valve lift, which eliminates the need for a throttle, while BMW’s Double Vanos VVT controls both intake and exhaust valve timing to help optimize output, efficiency and emissions.
The combination of turbocharging, Valvetronic and gasoline direct injection is a first for BMW.
The twin-scroll turbo is a smaller, lighter, less-expensive package with lower friction and virtually zero turbo lag (not that the N54 twin-turbo has much), and the new Valvetronic system offers advantages over previous versions, including improved idle stability.
“It's a faster system, and we can compensate the amount of air that is provided to certain cylinders,” Hohenner says.
The better low-end torque, a peak 300 lb.-ft (407 Nm) from 1,200 rpm all the way to 5,000 rpm, also comes from the Valvetronic.
A third major improvement over the N54 is integration of all powertrain controls into a single Digital Motor Electronics unit mounted directly to the engine. This shortens the wiring harness and allows the electronics box to be more easily air cooled.
Hohenner, who was part of the original N54 development team, foreshadowed this development when Ward’s interviewed him two years ago.
“Another thing that has to improve,” he said at the time, “is the way the electronics are packaged. Maybe mount it directly to the engine with different cooling. We would no longer need a specific cooling fan, and we could use the space for other things.” And that’s exactly what BMW engineers did.
“We got rid of these (separate) E boxes in the car,” Hohenner says. “We had nice and cozy places for all the electronics in the engine compartment, but now the DME is attached directly to the engine, and it includes the Valvetronic controls.”
Also significant: The single turbocharger’s lone exhaust stream feeds one catalytic converter, which heats up faster than the previous twin-turbo, dual-catalyst configuration. That results in lower start-up emissions.
“We have an air-gap-insulated exhaust manifold that gets heat faster to the catalyst,” Hohenner says. “The catalyst reaches its operating temperature in just five to six seconds (compared with 14 seconds previously) because it's using heat from the engine with little loss because of that air gap.”
Because 95% of test-cycle emissions are produced in the first five seconds after engine start, bringing the catalyst up to temperature faster reduces total emissions.
Have any of BMW’s ongoing “Efficient Dynamics” engineering efforts found their way into the N55?
“Absolutely,” Hohenner says. “We started early with electrifying our engines and drivetrains. Ancillaries that are traditionally mechanical can be electrified and integrated on an as-needed basis. When you need it, it runs. When you don’t, it doesn’t.
“You don’t need to spend some power to drag it along; (with) a mechanical pump, I always have to drag even when I don’t need it. An electric one, I only use when I need it.”
So the N55 has a new electric oil pump and an electric water pump. Both deliver pressure only as needed to optimize efficiency.
The alternator also is electronically controlled. It takes advantage of the engine’s kinetic energy by increasing its voltage output during engine deceleration. BMW calls this Brake Energy Regeneration. But unlike hybrid-electric-vehicle regenerative braking, BER does not actually recover vehicle kinetic energy directly from the brakes.
“The DME increases the voltage output from the alternator, and this over-voltage charges the battery faster. It is generating energy all the time, but it generates more energy (during engine overrun) because we can then increase the voltage,” Hohenner says.
He adds that this process requires a deep-cycle battery, which is allowed to get as low as 60% state of charge.
“So on acceleration and during steady cruise, we run (ancillaries) off the battery as much as we can, then on decel we very quickly put as much energy back into the battery as possible. It also can be taken out of the system so it does not drag.”
Another difference between the two engines is fuel delivery. The N54’s GDI system uses high-priced piezo injectors. The N55 employs less-costlysolenoid-type injectors.
Why? “(Piezo) is very expensive technology and we don't use its full potential,” Hohenner says. “We use much more of its potential in Europe, where we have lean-burn engines. With lean-burn engines, the piezo injectors really pay off.”
Is there still room for improvement? “One thing would be to get the catalyst to work even sooner. We need to improve it even more to get to lower emissions.
“There is always a way, but it's getting more expensive. And the integration of our components could be further optimized,” Hohenner says.