Claim that gasoline cars could beat clean-diesels on 'global warming' stirs more public debate

Controversial claims by Stanford University researcher Mark Jacobson that cars with diesel particulate filters (DPFs) can still cause more net "global warming" emissions than gasoline cars continue to stir debate.

Jacobson's initial claims (based on certain assumptions about diesel car emissions, measurement methods, and regulatory limits) were first contradicted last year by University of Minnesota's renowned engine combustion particle scientist David Kittelson (see Diesel Fuel News 9/16/02, Ps,. 1/7/02, P1 3), who pointed out the excellent carbon-trapping efficiency of diesel particulate filters (DPFs).

Reducing "black carbon" from combustion could have a greater short-term impact on reducing "global warming" than simply reducing carbon dioxide ([CO.sub.2]), which takes many decades to have much impact on climate change, recent scientific studies indicate.

But in another presentation this month to a California Air Resources Board (CARB) "international vehicle technology symposium" on cutting climate-change emissions from vehicles, Jacobson now cites other recent studies about vehicle particulate matter (PM) emissions. By his calculations, these data supposedly that show gasoline cars can have an advantage over diesel cars on to Jacobson compares the average miles per gallon of the latest gasoline/electric hybrid cars (Honda Insight, Toyota Prius) versus the Volkswagen Golf, Jetta and Beetle diesel cars. However, the "Insight" is a tiny, two-seater vehicle, and the Prius is smaller than some diesel cars cited in the comparison.

Nor do these hybrids deliver the torque, highway mileage, and highway acceleration performance of the larger, new-generation diesels -- and larger gasoline hybrids have yet to emerge, or show they can out-perform diesels in larger vehicle categories.

What's more, the possible emergence of diesel-electric hybrids (much better on fuel economy/[CO.sub.2] than gasoline hybrids) is left out of the "gasoline beats diesel" claims.

Still, adding a diesel particulate filter (DPF) and nitrogen oxides (NOx) trap for upcoming Tier 2/LEV-2 emissions limits would further penalize diesel's average fuel economy (due to hydrocarbon reductant for NOx/PM trap regeneration), thus making the gasoline-electric hybrid even better on both [CO.sub.2] and particulate matter, Jacobson claims.

However, while NOx/PM trap regeneration might represent a few percent fuel penalty, diesels still have an overwhelming fuel-economy (and hence [CO.sub.2]) advantage over gasoline.

What's more, in a 2001 study for Swedish National Road Administration comparing 45 diesel and gasoline cars, the DPF-equipped diesels regularly beat gasoline on PM emissions. "If diesel manufacturers choose to equip their models with particle filters, the diesel cars will definitely emit fewer ultrafine particles than cars with spark-ignition direct-injection engines," that study found. What's more, the PSA/Peugeot DPF-equipped diesel often beats the gasoline cars on PM number emissions, not just mass emissions, that report found.

A separate study in 2002 by Europe's Ecotraffic found that the DPF-equipped PSA car produced only a small fraction of the PM emissions of its gasoline car counterpart.

Another presentation to the same CARB vehicles/climate change symposium likewise undercuts sweeping claims that gasoline or gasoline-hybrid cars would beat diesel on total global-warming impact.

New generation diesel cars can have an average 3 6-42% fuel economy advantage over gasoline cars, implying about a 20-25% [CO.sub.2] reduction (minus some DPF regeneration fuel penalty), Ford Motor researcher Matti Maricq showed at the CARB symposium. DPFs and ultra-low sulfur diesel (ULSD) fuel reduce both soot and sulfate PM emissions, while improved catalysts can also reduce hydrocarbons that could nucleate to form PM, he said. Hence the "black carbon" (BC) impact of clean-diesels on "global warming" would be greatly diminished.tal "global warming" impact