DETROIT – For all of the positive press diesel engines are receiving in the U.S. of late, the market leader here still has reason to believe the diesel’s American revival is up for debate.

In North America, General Motors Corp.’s work on cleaning up diesel’s oxides of nitrogen (NOx) and particulate emissions to comply with Tier II Bin 5 standards that take effect in 2007 could compromise the fuel-economy advantage diesels currently hold over their gasoline counterparts, Gary Smyth, GM’s director-powertrain systems research laboratories, says during the Convergence 2004 conference here.

Smyth says technologies needed to improve both diesels and gasoline engines “are controls-enabled.” He says the ubiquity of those controls between the two propulsion options is causing “the roadmaps for diesel and gasoline (to) exhibit significant convergence if you look at” the strategies being employed – such as turbocharging, lean combustion, tweaking compression ratios and direct injection.

Although gas and diesel development paths may be converging, the complexity being introduced to already-sophisticated diesels may drain some of their fuel-efficiency advantage.

GM committed to OHV gasoline engines.

“If we look at diesel, our focus is not to improve the efficiency of diesel from a fuel economy standpoint, but rather to meet noxious-emission requirements,” he says. “In doing that, as we add significant complexity to the aftertreatment system, we will be adding cost, and we could quite possibly lose fuel economy as we strive to get to (the U.S.’) Bin 5 emissions.”

North American Tier II standards clearly are “the most stringent levels globally” when it comes to emissions, forcing auto makers to produce cars that are 50%-60% cleaner than European vehicles under the Euro IV and even 2010’s Euro V mandates, Smyth says.

While GM is not jumping off the diesel bandwagon, it is investing significantly in technologies that will further the progress of gasoline engines to the point where they will equal diesel performance, while maintaining the cost and cleanliness advantage. (See related story: GM Cautious About U.S. Diesels)

“I believe that as you look at the ultimate potential, you will find the benefit of the diesel engine vs. the gasoline engine will diminish from where it is today, and the issue we’ll have to deal with is the significant cost disadvantage that the diesel will have,” Smyth says.

GM’s progress with gas engines is being assisted by various improved controls – such as Displacement on Demand, which provides a 6%-8% efficiency improvement in some cases. DoD will be implemented in several million GM units in coming years, including significant penetration in “value” engines, such as overhead-valve units that power light trucks and SUVs, Smyth says.

More and more control-centric complexity can be expected in GM-built OHV engines in years to come, because OHV configurations offer the most power in an ideal packaging and cost solution, Smyth says.

Another key enabler to lengthening the gasoline engine’s lifeline is advancement in combustion technologies, which can lower emissions levels significantly.

When it comes to refining combustion, the ultimate breakthrough may come on the wings of HCCI (homogenous charge compression ignition), which is an up-and-coming combustion technology beneficial to lowering both diesel and gasoline emissions.

“There has been significant development done by GM and outside of General Motors on HCCI, and I would have said one year ago HCCI was closer to a dream,” Smyth says. “Today, HCCI has become much closer to a reality.” (See related story: California CO2 Law Puts Heat on Engine Developers)

He says HCCI represents “the next-generation combustion system” because it is a chemically controlled flameless combustion process that minimizes NOx and particulate emissions.

HCCI’s emergence still may not become reality until about 2009, according to Environmental Protection Agency estimates, but so-called pre-mix combustion is making HCCI’s case far more palatable than it once was, Smyth says.

“There’s really a continuum driving from the more classical combustion to HCCI,” Smyth says. “And that’s what we call pre-mix combustion – where we either inject very early or we inject late and we have some premixing,” which reduces the temperature of the combustion event and lowers NOx output.