Mercedes-Benz can boast of having a new engine lineup that’s so advanced that it can operate in a “lean-burn” mode that is very fuel-efficient and also has reduced emissions.
But since sulfur content in gasoline remains too high, it won’t be offered in the U.S. Ultra Low Sulfur Diesel federal regulations limit sulfur content to 15 parts per million and so most of the sulfur has been removed from U.S. diesel fuel.
The U.S. Environmental Protection Agency sets the sulfur limits for gasoline much higher – 80 ppm at the refinery gate and 95 ppm downstream.
Bernhard Heil, vice president-powertrain development for Mercedes-Benz, told Ward’s that new 4- and 6-cyl. engines from Mercedes run on lean-burn combustion cycles but they don’t work properly if sulfur levels in fuel are above 50 ppm.
He said that those are “definitely too much.” Mercedes sells these “Stratified” engines in Europe that run at a much leaner fuel-to-air ratio than conventional engines, getting higher fuel efficiency.
These engines will not be available in the U.S. until its gasoline is cleaned up as the excessive sulfur will overwhelm and “poison” the trap that captures oxides of nitrogen from the exhaust stream. This trap is required since the Stratified combustion cycle inherently produces higher concentrations of NOx.
Since a conventional 3-way catalyst will not work with a lean-burn engine, the NOx must be adsorbed and stored in a specially designed trap before it is burned off. Heil said that integrating a lean-burn combustion cycle boosts fuel efficiency up to 10%. He said that sulfur in gasoline, by itself, does not affect fuel economy.
In May 2010, the Obama administration told the EPA to consider the effect of lower sulfur levels in gasoline, as well as other factors, on greenhouse-gas emissions.
The EPA has yet to complete its review. Heil said that the European Union has already cut sulfur content in gasoline “to near zero.” Actually, this problem isn’t just in the U.S., as the gasoline in Africa and developing countries in Asia is “comparably worse.” [via Ward's Auto]