Contradictory diesel cetane requirements might confound advanced diesel technologies

Diesel Fuel News, Sept 29, 2003 by Jack Peckham

Washington, DC -- Ultra-low-emissions homogenous charge compression ignition (HCCI)--at least at part-load--could emerge as a major compliance strategy for heavy-duty diesel engine makers for 2010.

But there's a fuel issue: HCCI engines suffer if cetane is boosted to levels resembling those of California Air Resources Board (CARB) diesel, usually well over 50-cetane number, as Caterpillar explained at last month's Diesel Engine Emissions Reduction (DEER) workshop.

Rather, HCCI seems to work better with the more ordinary mid-40s cetane diesels; performance suffered when a typical U.S. 45-cetane diesel fuel was boosted to 50.

Still, HCCI research & development is at early stages, so it's conceivable that engineers might yet figure out some way for HCCI to tolerate somewhat higher cetane, Cat engineers tell us.

--Contradicts World-Wide Fuels Charter

On the other hand, if HCCI is proven to be utterly intolerant to high cetane, then this is just the opposite of what the world's diesel automakers would want. They're pushing the "World-Wide Fuels Charter" (WWFC), which favors not only near-zero sulfur levels, but also high (around 55) cetane number in order to help reduce cold-start emissions, cut noise and aid fuel economy.

Heavy-duty engine makers are signatories to the WWFC, too. But if HCCI indeed emerges as the favored ultra-clean diesel technology for heavy-duty, then they might change their minds on high cetane.

HCCI's tremendous advantage is ultra-low emissions of nitrogen oxides (NOx) and particulate matter (PM), the two toughest (and costliest) pollutants to slash from diesel engines. While nobody has figured out how to make HCCI work at full-load yet, Cat has achieved stable multi-cylinder operation at >75% load--with ordinary, 45-cetane diesel fuel. Cat's test results to date are steady-state; the enormous complexity of controlling HCCI through transients still hasn't been figured out.

Nevertheless, Cat thinks HCCI is a possible contender for U.S. EPA's ultra-low NOx/PM limits for heavy-duty 2010 engines.

Now, it turns out that Cat's not alone seeing HCCI's strong 2010 potential.

At the Hart World Fuels Conference here, Volvo/Mack powertrain senior vice-president Denis Leblond said that "partial HCCI at light load is likely for 2010 using fuels similar to today's" diesel fuel. However, "full HCCI requires either a major breakthrough in engine design or a carefully controlled fuel" that may require higher volatility than today's diesel, he added.

On the other hand, while HCCI technology may seem promising, diesel cetane boost would help most diesel engines, as International Truck & Engine fuels & lubricants manager Rodica Baranescu said here.

"We don't know when HCCI will happen," Baranescu said. "It's much too early to say we don't need cetane improvement."

Meantime, "the World-Wide Fuels Charter is an excellent road map and a stretch target for North American diesel," she said. That's because typical U.S. diesel doesn't even achieve the lowest WWFC "category 1" minimum of 48, she said.

--Toyota Wonders About Market Demand

A separate National Conference of Weights & Measures (NCWM) "premium diesel" specification (adopted by many U.S. states) instead requires a minimum 47 cetane, as well as mandatory coldflow, lubricity and stability limits, she noted.

Meantime, automakers likewise favor higher cetane, lower aromatics and a more uniform ultra-low sulfur diesel (ULSD) quality, as Toyota regulatory affairs manager Tom Stricker pointed out here.

While a recent test of Toyota's "DPNR" clean-diesel car system came close to U.S. EPA Tier 2/Bin 5 limits, it's still not been proven to meet EPA's very long "useful life" limits (120,000/150,000 miles). "The jury's still out whether it's robust enough for the U.S.--if there is a U.S. [light-duty diesel] market," he said.