The Ward’s 10 Best Engines competition celebrates 13 years of recognizing outstanding powertrain development. In this second of a series, Ward’s highlights BMW’s coupling of turbocharging with direct injection for its gasoline I-6. Watch for features on the other 2007 winners throughout the year.

“Iconic” is an adjective often used when speaking of BMW AG’s inline 6-cyl. engine. Few engine types are as closely associated with their auto makers’ brand as is BMW’s legendary “straight six.”

But in an industry that lives on its ability to offer the next big thing, even legends have to advance. For 2007, in perhaps its most radical update of the I-6 layout, BMW engineers not only leveraged one of the newest gasoline-engine advances, but also revisited the past to develop a new variant that wins a Ward’s 10 Best Engines award in its first year of production.

The contemporary technology: direct injection. The more mature technology: turbocharging. BMW, rather famously, has not offered a turbocharged passenger-car engine in more than a quarter century.

But the competing mandates of generating more power while also adapting to new concerns about fuel economy and emissions led the German auto maker to employ both advances for the new N54, a direct-injected, twin-turbocharged variant (sort of) of BMW’s normally aspirated, port-injected 3L DOHC I-6, itself a 2-time Ward’s 10 Best Engines winner.

Andreas Welter, BMW’s project manager for I-6 engines, tells Ward’s adopting the complimentary advances of turbocharging and direct-injection gasoline technology helped BMW achieve its primary goal for the N54, “to be best in segment for response, low-end torque, fuel consumption and sound.”

Although the benefits of turbocharging are well known, Welter says it is newly developed direct injection for gasoline engines – BMW calls its system High Precision Injection – that really brings out the best in a turbocharged mill.

“High Precision Injection enabled us to increase the compression ratio (10.2:1) of the engine. As a result, engine performance without the influence of the charging pressure is higher than with conventional intake-manifold fuel injection.

“Based on that, we need less charging pressure to reach the defined engine performance. This leads to the virtual elimination of turbo lag, because we need only two small turbochargers that build up the relatively low charging pressure very quickly.”

Welter’s satisfaction with DIG is amplified by Michael Crane, Siemens VDO North American director-powertrain gasoline systems.

Siemens VDO supplies the sophisticated new piezohydraulic fuel injectors used by the new BMW turbocharged inline 6-cyl. – the first time such technology has been employed for a series-production gasoline engine.

“Siemens VDO believes – and OEMs are showing – that direct injection and turbocharging technologies are highly coupled and synergistic,” Crane says. “Direct-injection technology allows separate air control and fuel-injection control.”

He says the use of accurate valve timing (handled by BMW’s Double Vanos variable valve timing) and valve overlap allows fresh-air scavenging strategies that improve turbine efficiency, reduce lag and improve engine performance and fuel economy.

Since the mid 1990s, Crane says Siemens VDO has invested a stunning E5 billion ($6.6 billion) – yes, that’s billion with a “b” – in the development and production of piezohydraulic injection technology.

Auto makers now are leveraging Siemens VDO’s hard work to use DIG to its best effect: enabling downsized engines, often with forced induction and higher gear ratios, to deliver fuel-consumption improvements of 15% to 20% – with no appreciable degradation of performance.

So good is DIG that Siemens VDO expects its piezo investment to pay off to the tune of a North American DIG penetration rate (not all using piezo injection) of 7% to 10% by 2010, and 20% to 30% in Europe by 2015, Crane says.

“It was clear from the beginning that the turbocharged engine would use BMW High Precision Injection,” Welter says. “The Siemens engineers worked, together with us, on the development of the injector, ECU and high-pressure fuel pump. Our (BMW’s) core activity was the development, calibration and testing of the whole system.”

Not to be lost in all the technology talk is the magnificent performance of the N54, underscoring the company’s calling card of producing nothing but dynamically enjoyable vehicles.

Its first turbocharged mill since 1981 develops a rousing 100 hp/L, and its 300 lb.-ft. (407 Nm) of peak torque surpasses some V-8s and is available over a stupefyingly wide and satisfying range no normally aspirated V-8 could match: 1,400 rpm to 5,000 rpm.

Yet the entire engine weighs some 150 lbs. (68 kg) less than an equivalently powerful 4L V-8, BMW reckons.

Welter also is proud of BMW’s ability to retain portions of the rousing snarl for which its signature engine is deservingly famous, despite the new variant’s addition of twin turbochargers and high-pressure direct injection that has unfamiliar effects on intake sound.

“It is for sure not easy to get good sound from a turbocharged engine – with or without direct injection,” Welter asserts.

BMW enlisted Mitsubishi Heavy Industries Ltd. to supply and help develop the special turbochargers whose impellers can withstand the substantially higher exhaust temperatures of a gasoline engine in relation to a diesel. Oilburners almost universally employ turbocharging to boost performance and response.

“We carried out a lot of acoustic simulations and (based on the results) developed a turbo-specific exhaust system,” Welter says. “Target of this development was to reduce the damping of the inline 6-cyl. specific frequencies, because they are already damped by the turbocharger.”

BMW took the radical step of adopting a true dual-exhaust system for the N54 when launched in the 335i Coupe, the first time any 3-Series has employed such a layout. Engineers decided that because the turbocharger layout specified a small, low-inertia turbo driven by the exhaust from half the engine’s cylinders, it was undesirable to attempt to combine such complicated hardware into a single-manifold design. Thus each set of three cylinders exhausts through a turbocharger and a dedicated system running the length of the vehicle.

The N54’s range of technical parlor tricks is not limited to intake and exhaust advances. There are subtle, efficiency-enhancing features such as the electrically driven water pump and variable-displacement oil pump, both of which are features of its normally aspirated cousin, although the unique magnesium-aluminum engine block and Valvetronic variable valve-lift system do not translate to the new twin-turbocharged N54.

In the case of the magnesium-aluminum block, Welter says, “Right now, the crankcase-stress of the turbocharged engine is above the limit of the aluminum/magnesium crankcase.”

Valvetronic, meanwhile, which helps to generate startling steady-state fuel efficiency, could be a future upgrade for the turbocharged straight six.

Despite the already complex intake histrionics of the N54, Welter says it is technically feasible to adopt Valvetronic – which essentially eliminates the need for a conventional throttle – for the turbocharged I-6.

One thing that is unlikely for future variants of BMW’s iconic I-6 is an increase in displacement. BMW’s senior vice president for powertrain development said last year the auto maker’s long-range plans called for a “cap” in engine displacements.

Welter confirms this intrinsic strategy will carry BMW into the foreseeable future – and has helped to dictate solutions such as the auto maker’s return to turbocharging.

“Only a marginal increase of the displacement is possible (for the I-6 engine). However, an increase is not planned,” Welter says. “This is consistent with our Efficient Dynamics program and philosophy, which improves performance and at the same time reduces fuel consumption.”