Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 20 years. This installment of the 2014 Behind the 10 Best Engines series highlights development of BMW’s 3.0L turbodiesel I-6 used in the 535d.

Diesel engines still suffer from a negative image among many U.S. consumers, who – from past bad experience or hearsay – incorrectly view them as poky, smoky and noisy.

But modern compression-ignition engines own more than half of the European market and are fast gaining popularity elsewhere, including here. This is largely due to European tax policies that make diesel fuel cheaper than gasoline, and also due to their general excellence these days.

As we know, diesels enjoy the advantages of higher torque for stronger acceleration and significantly better fuel efficiency compared with similar-size gasoline engines. But their drawbacks are higher engine cost and (in the U.S.) higher fuel prices and spotty availability. There's also the oily fuel itself, which is much cleaner today but still better to keep off your hands when refueling.

That said, today's advanced diesels are simply too good to ignore, so WardsAuto editors tested six of them (more than ever before) and awarded half of those 2014 10 Best Engines trophies. And this smooth, quiet, powerful BMW turbodiesel inline-6 may be the best yet.

After testing it in a ’14 BMW 535d sedan, they called it “remarkable,” “stealthy fast,” “responsive,” “a blast to drive” and “arguably...the all-around quietest diesel we've even driven.” This despite the new N57 I-6 losing 10 hp and 12 lb.-ft. (16 Nm) of torque compared with the previous-generation Ward's 10 Best Engines award-winning (2009 and 2010) I-6 diesel it replaces.

It also uses a single variable-geometry turbocharger instead of the previous twin turbos and less expensive solenoid-type fuel injectors in place of piezo-electrics.

But the most important difference is substantially better EPA fuel-economy ratings at 26/38 mpg (9.0-6.1 L/100 km) city/highway in the ’14 535d vs. the earlier engine's 23/36 mpg (10.2-6.5 L/100 km) in the slightly smaller previous-generation 535d.

In real-world Detroit-area driving, WardsAuto editors topped 35 mpg (6.7 L/100km), better than some 4-cyl. diesels and gas-electric luxury hybrids tested. They also found its improved stop-start system smoother and faster to respond than before.

Through an exchange of emails, BMW diesel engine development chief Peter Nefischer highlights the N57's top design priorities.

“Efficiency, low emissions and the best possible power-to-weight ratio were the key objectives,” he says. “Also, a compact package was very important in terms of total vehicle architecture.”

New Turbocharger Key to Performance

Nefischer says the team’s toughest challenges were delivering a lightweight design and achieving high output without producing excessive vibration. And emissions were kept in check by combining “efficient turbocharging” with high-pressure direct fuel injection and reduced friction.

Of those technologies, the new turbocharger perhaps is the most interesting. Instead of the previous engine's two separate turbos or the single twin-scroll units appearing on many of today's turbocharged gasoline engines, BMW opted for a single, relatively large Garrett turbocharger with innovative Variable Turbine Geometry (VTG), a U.S.-market first.

Its electronically controlled vanes around the turbine continuously vary boost by adjusting their angle depending on driving conditions. At low engine speeds, they are open at a flatter (more perpendicular) angle to aim incoming exhaust gas more directly at the turbine blades, which increases the turbine's speed (and therefore boost) for strong acceleration response.

Then, as engine and vehicle speed increase, they tuck inward for less direct flow against the turbine blades, which moderates boost.

BMW touts the advantages of VTG as high torque at both low and high engine speeds; very precise boost-pressure control; lower emissions and fuel consumption over the engine's entire operating range; and no need for a turbo wastegate to relieve overpressure.

The Bosch high-pressure common-rail fuel-injection system, which delivers diesel to the top centers of individual cylinders at pressures of up to 26,107 psi (1,800 bar), is capable of multiple injections through a single combustion cycle. Its major advantages include both lower emissions and reduced noise.

BMW says the N57's all-aluminum block (with cast-iron cylinder sleeves) and hollow camshafts reduce weight, as well as noise, vibration and harshness. NVH also is improved with twin internal balance shafts for smooth idle and a higher rev range; extra insulation at the oil sump and engine front cover; and relocation of the camshaft timing chain from the front to the rear (flywheel end) of the engine to squelch external engine noise.

Other engineering updates relative to the previous diesel I-6 include a weight-optimized crankshaft; a low-friction belt drive for the alternator and other ancillaries; new sensors; and a new electronic control module.

However, the basic internal dimensions remain the same as in many current BMW engines: 84-mm (3.31-in.) bore and 90-mm (3.54-in.) stroke. That gives the 4-cyl. engines an even 2.0L displacement and the I-6s a common 3.0L while enabling component sharing among them.

Because U.S. emissions standards require diesels to be as squeaky clean as gasoline engines, the N57’s “BluPerformance” emissions system (like most others in the U.S. market) is a costly, complex “chemistry factory within the car,” in the words of one BMW executive, which is managed and controlled by nine separate sensors.

To comply with the most stringent emissions limits in both North America and Europe, it includes a diesel particulate filter, an oxidation/NOx storage catalyst and a selective catalytic reduction system, the latter requiring periodic replenishment of urea-based diesel exhaust fluid.

Beyond the 535d midsize sedan, the engine is available in BMW's X5 xDrive 35d all-wheel-drive midsize CUV and in the (all-new for ’15) 740Ld xDrive long-wheelbase AWD luxury sedan.

In all of those applications, it's rated at 255 hp at 4,000 rpm and a stout 413 lb.-ft. (560 Nm) of torque between 1,500 and 3,000 rpm. It's good for 5.7-second 0-60 mph (97 km/h) launches in the 535d, 6.7 seconds in the heavier X5 and 6.1 seconds in the much larger 740Ld. Yet it's as much as 30% more fuel efficient than gas-powered alternatives in those vehicles.

When asked whether there is opportunity for improvement in fuel economy, emissions and output as U.S. CAFE and European CO2 regulations continue accelerating into the future, Nefischer says, “Yes, but we will inform you in due time.”

So he won't drop a clue, but we know engineers by nature never are satisfied, and there always is room for improvement in such “BMW EfficientDynamics” areas as combustion system efficiency, friction reduction and electrification of accessory drives.