DETROIT – Fans of the “Ultimate Driving Machine” had better be satisfied with the size of BMW AG’s current engines. They won’t be getting any larger, says the auto maker’s powertrain chief.

Although BMW, a brand renowned for its baked-in sportiness, intends to remain one of the industry leaders in engine performance, future power enhancements will not be achieved with larger engines, promises Klaus Borgmann, senior vice president-powertrain development.

“The time to increase horsepower by increasing displacement is over,” he says.

Future performance increases for BMW’s volume-production engines – he excludes the low-volume, ultra-high-performance engines developed for vehicles sold by its Motorsports in-house tuning division – will be achieved via advances in technology and new designs, not larger size, Borgmann says.

The twin-turbocharged 3L inline 6-cyl. BMW unveiled at the recent Geneva auto show indicates the company’s intended powertrain-development direction, he says. More than 50% of the vehicles BMW sells worldwide are powered by inline 6-cyl. engines, and turbocharging its stalwart powerplant yields about 302 hp and 295 lb.-ft. (400 Nm) of torque – figures that match or exceed high-performance V-6s from BMW’s Asian and North American competitors that typically are a half-liter larger.

At 302 hp, the twin-turbo DOHC I-6 is 47 hp stronger than the most powerful version of the same engine in normally aspirated form. But equally important, Borgmann says, is the 295 lb.-ft. of torque, a 75 lb.-ft. (102 Nm) jump that enhances engine flexibility and response, he adds, without any meaningful increase in fuel consumption because the turbochargers do not generate high boost levels.

Fuel consumption and emissions are the key reasons BMW has nixed future size increases for its volume engine families, Borgmann says.

Increasing fuel-economy and emissions-reduction pressure in all world markets makes it difficult to continue to enlarge engines, he says, adding this is particularly true in relation to carbon-dioxide emissions, the global-warming gas for which many European nations have set strict limits and tied to vehicle taxation. Output of CO2 is directly correlated to fuel consumption.

Despite recent advances in variable valve timing, cylinder-deactivation and other economy-enhancing technologies – including BMW’s own Valvetronic variable valve-lift system – Borgmann says it is impossible to escape the engineering reality that larger engines use more fuel and consequently make more emissions.

Speaking to reporters at the Society of Automotive Engineers World Congress here, Borgmann says the new turbocharged inline 6-cyl. also will employ direct (in-cylinder) fuel injection, a complementary technology seeing rapidly growing fitment for turbocharged engines to facilitate economy-enhancing downsizing.

The two technologies are so complementary, in fact, that Borgmann does not foresee direct injection seeing much future use for gasoline engines unless it is coupled with turbocharging.

“If you have turbocharging, you will only see DI (gasoline direct injection technology) with it,” Borgmann says.

A growing number of premium and performance 4- and 6-cyl. gasoline engines are combining the two systems because direct gasoline injection (DGI) has a low-rpm torque-enhancing effect that helps to mitigate the “turbo lag” that deadens the low-speed response of most turbocharged engines.

Audi AG, Mazda Motor Corp., Toyota Motor Corp. and General Motors Corp., among others, recently have launched turbocharged, direct-injected gasoline engines.

Those engines all inject fuel directly into the combustion chamber, where it combines with air in a conventional stoichiometric ratio of about 14 parts air for every one part of fuel. Borgmann says BMW is working with prototype DGI systems for naturally aspirated engines that generate stratified-charge, or “lean-burn,” air/fuel ratios that improve fuel economy 10%-15%. The downside is that stratified-charge DGI engines produce excess oxides of nitrogen emissions, mostly during the cold-start phase, that would run afoul of current or future emissions standards in the U.S.

He says the stratified-charge DGI systems will comply with Euro IV emissions regulations, however, and BMW will launch an engine later this year that uses stratified-charge injection.

Borgmann is confident BMW powertrain engineers will be able to make lean-burn DGI engines compliant with strict U.S. and California emissions standards, too, with improved technology such as cooled exhaust gas recirculation (EGR).

“It’s solvable. It just needs time,” he says.

bvisnic@wardsauto.com