Diesel engines, ULSD could be wiped out by potential 2014 U.S. EPA regs

Diesel Fuel News, March 31, 2003 by Jack Peckham

Charleston, S.C. -- A new study at Southwest Research Institute (SwRJ) indicates that heavy-duty gasoline engines potentially could evolve to match heavy diesels on power output and durability, while producing only a fraction of the nitrogen oxides (NOx) and particulate matter (PM) of diesels meeting 2010 U.S. EPA emissions standards.

If U.S. EPA adopts these conclusions to support even tougher heavy-duty regulations for 2014, then the many billions of dollars invested in ultra-clean diesel engines and ultra-low sulfur diesel (ULSD) fuel potentially could be wiped out.

The SwRI study, "Efficiency Convergence of Spark Ignition and Compression Ignition Engines," outlined at the recent Energy Frontiers International conference here, notes that EPA has already adopted gasoline/diesel emissions equivalence for light-duty "Tier 2" vehicles later this decade.

"The EPA considers the best available technology for lowest emissions, with some consideration of the economic and social impact," SwRI researchers Ed Owens and Tom Ryan point out in their study.

Since gasoline spark-ignition (SI) engines with three-way exhaust catalysts (for hydrocarbons, carbon monoxide and nitrogen oxides) are the "proven lowest emission commercial technology available" in light duty, then maybe they can evolve to shove aside diesels in heavy-duty, too.

"Fundamentally, there's no reason why you can't have a gasoline engine with the same power output and durability" of a heavy-duty diesel, Owens told us in a post-conference interview. "These are design issues. The current [light- and medium-duty] gasoline engines are under-cooled and operated most of the time at lighter load."

Two limiting issues for these heaviest-duty notional gasoline engines are higher power and higher durability.

For starters, this will mean addressing cylinder head thermal fatigue and cylinder liners, plus ignition system durability. "But our [SwRI engine] design guys think it can be done," Owens said.

Still uncertain are the total costs of upgrading such an engine, and the fuel economy impact. While the gasoline version might have ultra-low NOx/PM emissions, its fuel economy might not be as good as diesel.

While both gasoline and diesel technologies have become more fuel-efficient, energy penalties are expected for EPA 2007/2010 diesel emissions limits, and diesel engine/exhaust aftertreatment costs also will rise.

New Technologies

Meantime, gasoline engines are adopting sophisticated schemes such as variable valve actuation to improve efficiency. These and other potential schemes (Miller-cycle adaptation, for instance) potentially could evolve to match diesel efficiency.

But even if gasoline heavy-duty doesn't evolve by around 2010 (or by 2014) to match diesel efficiency, there's another factor: "If EPA decides air quality is the most important transportation emissions regulatory issue, then spark-ignition gasoline engines are the lowest-emission technology available," he said.

Extrapolating from recent tests on an 8-liter GM gasoline engine (not a big heavy-duty engine), SwRI calculates that a future, notional heavy-duty gasoline engine could achieve one-quarter the NOx emissions of a heavy-duty diesel, and one-tenth the PM emissions (see chart, below).

For diesel engines even to achieve the heavy-duty compression-ignition 2010 ("HDCI") standards outlined in the chart, they likely face higher costs for high exhaust gas recirculation (EGR), advanced fuel/air systems and a catalyzed PM trap with a lean-NOx trap.

The PM trap could cause a 1-4% fuel economy penalty, while the NOx trap could cause a 5-7% penalty, the SwRI study indicates.

* DPF, NOx Trap Costs

The catalyzed PM trap could cost between $500 to $5,000, while the NOx trap could cost from $500 to $9,000, the study estimates.

Advanced fuel injection could add another $500 to $3,000, and intake system modifications could add another $1,000 to $2,000, they said.

Ironically, these clean-diesel engines not only would have higher emissions than a notional 2010 gasoline heavy-duty engine, but they could cost a lot more than gasoline for notional EPA 2014 standards.

"Diesel costs at the same emissions levels are possibly a factor of two or more" versus the notional gasoline engine, the study projects.

What's more, "diesel efficiency is approaching spark-ignition efficiency at these emissions levels."

Clean heavy-duty gasoline evolution is seen possible via increased compression ratio, increased brake mean effective pressure, reduced or eliminated throttling and lower engine RPM.

A critical need for gasoline engine is knock mitigation, requiring high-energy ignition and combustion chamber design changes. "Massive EGR" seems a likely requirement, and possibly a different catalyst scheme in place of the conventional three-way catalyst.

While fuel economy of such an engine isn't known, "we're in the process of evaluating that," SwRI's Ryan told us in an interview.

Part of the efficiency improvement potentially could come from cylinder deactivation strategies, for instance.