We rarely think about what's in our gasoline when we fill the tank, but carmakers have to.

Mercedes-Benz has just launched a new direct-injected 3.5-liter V-6 engine in its 2012 SLK model, with an innovative "lean burn" mode that will also be used on a smaller four-cylinder engine.

The "Stratified" engines are far more efficient under certain conditions than conventional engines, raising their efficiency and the gas-mileage ratings of the cars they're used in.

2012 Mercedes-Benz SLK

2012 Mercedes-Benz SLK

But the company can't bring the most advanced "Stratified" engine to the United States, because U.S. gasoline has far too much sulfur in it.

Europe has all but eliminated sulfur from its gasoline, while U.S. gas can still have as much as 95 ppm. (Separately, both regions have now rolled out ultra-low-sulfur diesel fuel, permitting advanced aftertreatment systems for clean diesel engines.)

The high sulfur level in U.S. gasoline poisons the trap that Mercedes-Benz fits to its new engine to capture the increased nitrous oxide emissions resulting from the lean-burn mode. They are periodically burned off in the combustion cycle.

In Europe, the aftertreatment system can handle the traces of sulfur remaining in that region's fuel. But in the States, it would be swamped by twice as much sulfur as it can handle, and quickly fail.

2007 Mercedes-Benz F700 Concept

2007 Mercedes-Benz F700 Concept

Last spring, the U.S. Environmental Protection Agency was instructed by the Obama Administration to consider the effect of sulfur levels in gasoline on greenhouse-gas emissions.

Thus far, the agency hasn't reached any conclusions or proposed any new rules for lower-sulfur gasoline.

The new engines are an offshoot of the "DiesOtto" HCCI engine program first shown in 2007 in the company's innovative Vision F700 concept car.

Drivers, incidentally, will have to start paying attention to what's in their gasoline this summer, when E15 gasoline with up to 15 percent ethanol is scheduled to roll out. The new formulation is only approved for use in cars from the 2001 model year or newer.

[Ward's Auto via MotorAuthority]


Follow GreenCarReports on Facebook and Twitter.