Mercedes-Benz has advanced new engines capable of operating in a “lean-burn” mode that improves fuel efficiency and emissions, but they won’t be available in the U.S. because sulfur content in gasoline remains too high, an executive says.

Most sulfur has been scrubbed effectively from U.S. diesel fuel due to Ultra Low Sulfur Diesel federal regulations, which limit sulfur content to 15 parts per million.

But the sulfur limits for gasoline are much higher – 80 ppm at the refinery gate and 95 ppm downstream, according to the U.S. Environmental Protection Agency.

New 4- and 6-cyl. engines from Mercedes operate on lean-burn combustion cycles, but cannot function properly if sulfur levels in fuel are above 50 ppm, “That’s definitely too much,” Bernhard Heil, vice president-powertrain development for Mercedes-Benz, tells Ward’s.

Mercedes sells these “Stratified” engines in Europe that run at a much leaner fuel-to-air ratio than conventional engines, achieving better efficiency.

But until the U.S. cleans up its gasoline, those engines will not be available here because the excessive sulfur would overwhelm and “poison” the trap that captures oxides of nitrogen from the exhaust stream.

The trap is necessary because the Stratified combustion cycle inherently creates higher concentrations of NOx. A conventional 3-way catalyst will not work with a lean-burn engine, so NOx must be adsorbed and stored in a specially designed trap before it is burned off.

Integrating a lean-burn combustion cycle improves fuel efficiency up to 10%, Heil says. Sulfur in gasoline, by itself, does not impact fuel economy, he says.

Last May, the Obama administration instructed the EPA to consider the impact of lower sulfur levels in gasoline, as well as other factors, on greenhouse-gas emissions. That review is ongoing.

Meanwhile, the European Union already has reduced sulfur content in gasoline “to near zero,” Heil says.

It’s not just the U.S. that has too much sulfur in gasoline, and Heil says the problem is “comparably worse” in Africa and developing regions in Asia.

“In the rest of the world, we do have a situation that we are right now forced whenever we develop new engines to have a (lean-burn) concept for gasoline engines for Europe, but we are not able to operate them internationally,” he says.

Ideally, a global standard on sulfur levels in gasoline would correct the issue, but Heil says it does not appear such action is likely in the near future.

One of the new lean-burn engines from Mercedes is the 3.5L direct-injection V-6 in the all-new SLK roadster for Europe.

That same engine will be marketed as BlueDirect when it comes to the U.S. this summer in the ’12 Mercedes C350 sport sedan, but it will not be lean-burn.

The V-6 in the new C350 is rated at 302 hp and 273 lb.-ft. (370 Nm) of torque, up from 268 hp and 258 lb.-ft. (350 Nm) in the previous C350. Fuel economy has been improved 5% to a combined 21 mpg.

That direct-injection engine will be Mercedes’ volume V-6 and also will power U.S. versions of the E-Class, M-Class and SLK. The R-Class cross/utility vehicle will continue to use a port-injection version of the V-6.

The new C-Class also will feature a direct-injection 1.8L turbocharged 4-cyl. rated at 201 hp and 229 lb.-ft. (310 Nm) of torque. A combined fuel-economy rating of 24 mpg (9.8 L/100 km) represents a 15% improvement over the previous-generation C300.

The last 4-cyl. engine Mercedes sold in the U.S. was a supercharged 1.8L in the C-Class in 2005. The third powertrain offering in the C-Class for the U.S. is carryover, a port-injection 3.0L V-6 rated at 228 hp and 221 lb.-ft. (300 Nm) of torque.

Another new direct-injection engine for the U.S. is the 4.6L Bi-turbo V-8 under the hood of the new-for-’11 CL550 coupe, rated at 429 hp and 516 lb.-ft. (700 Nm) of torque.

With an improved fuel economy of 15/23 mpg (15.6-10.2 L/100 km), the CL550 no longer will require a gas guzzler tax. The ’10 model, with a naturally aspirated, port-injection 5.5L V-8, carried a $1,300 penalty.

The 4.6L Bi-turbo also will appear in ’12 versions of the S-Class, E-Class and CLS sedan, as well as the M-Class later on.

Reducing fuel consumption is a priority for Mercedes as it adjusts to comply with the U.S. corporate average fuel-economy mandate of 35.5 mpg (6.6 L/100 km) as a fleet average by 2016.

With cleaner gasoline, that product strategy for the U.S. could include lean-burn engines, Heil suggests.

“That’s the significant next step for fuel-economy savings, which is not possible here, but maybe for the future,” he says.

tmurphy@wardsauto.com