The debate over using diesel engines for U.S. light vehicles continues to rage.

At the eye of the storm are the Environmental Protection Agency's ultra-tight Tier II emissions standards — slated for phase-in beginning next year — that auto industry interests say threaten to throttle the diesel's promise.

And with momentum gathering to adopt the diesel as an easy answer for the auto industry's steadily declining fuel economy, proponents have hoped regulatory interests might promote a cutback on the diesel-restricting aspects of Tier II in order to allow compression-ignition engines a chance to get started.

But Jeffrey R. Holmstead, EPA assistant administrator for air and radiation, says the EPA is not inclined to cut diesel a break in order to compete with other emerging powertrain technologies such as hybrid-electric vehicles or fuel cells.

“We want to be able to stand up and say we're (the EPA) technology neutral,” Holmstead tells attendees at the recent Society of Automotive Engineers conference in Troy, MI, on “Diesels and Their Future in the U.S.”

“I think it would be a mistake to relax the standard for diesel engines,” he says.

Tier II drastically cuts allowable levels of carbon monoxide, particulates and oxides of nitrogen (NOx). Diesels, which inherently produce higher levels of NOx, face the prospect of meeting Tier II NOx levels that are, on average, less than one-tenth of current permissible output.

California also recently passed a law — currently being challenged by the auto industry — that clamps down on transportation-related output of carbon dioxide (CO2), the well-known “greenhouse gas” linked to global warming. And because CO2 output is directly proportional to fuel efficiency, diesels emit as much as 40% less CO2 than a comparable gasoline engine. That would make diesels handy in California, if it weren't for the NOx issue.

Holmstead concedes, “The future of diesel power for (U.S.) passenger vehicles is at a critical juncture.” But he says the diesel's “environmental negatives” will be overcome.

Just don't look for a helping hand from the regulators: “Everything we see today points to a successful implementation of the Tier II program,” insists Holmstead, who adds the EPA has tested four diesel-powered cars that comply with Tier II, bin 5 standards, the strictest of Tier II's “bin”-based emissions categories.