STUTTGART, Germany — The soon-to-be-launched and much-hyped Mercedes-Benz E320 Bluetec has reportedly failed to meet emissions criteria in the important states of California, Maine, Massachusetts, New York and Vermont.

Mercedes said its E320 CDI was "the cleanest diesel vehicle in the world," but apparently not clean enough for U.S. regulators.

Despite their popularity in Europe, diesels have never been commonplace in the United States and have a reputation as a technology for big rigs only. But there's a more ecological explanation for the lukewarm reception. Diesel is actually more harmful to the environment than gasoline because it generates more nitrogen oxides (NOx), gases that play a major role in the formation of acid rain and haze.

It is no wonder, then, that many U.S. states impose strict emissions levels for diesel-powered vehicles. European regulators in turn have begun to demand "NOx traps" for diesel vehicles.

Mercedes-Benz tried to tackle the problem by utilizing a catalytic device that converts NOx to nitrogen. Its Bluetec system, introduced in the Vision GL320 concept shown at the North American International Auto Show in January, injects an aqueous urea fluid called AdBlue into the exhaust system. AdBlue is intended to separate NOx into hydrogen and water.

Despite the setback in the United States, Mercedes has already been using Bluetec-equipped commercial vehicles for some time, and it plans to offer Bluetec-equipped cars in Europe by 2008.

What this means to you: Diesel is more frugal, but presents unique emissions problems for engineers. Sooner or later, it's got to work — Mercedes isn't the only one desperate to get modern diesels, so popular in Europe, into the U.S.