News Room

Gasoline cars may need particulate filters; PM Emission No Longer Just a Diesel issue
Monday, 15 September 2003


Jack Peckham

Newport, R.I. -- Scientists taking real-world emissions samples from cars on the road and in laboratory test centers are finding that gasoline cars emit a lot more particulate matter (PM) than popularly believed.

New data show that PM number emissions from modern gasoline cars--including "normal emitters at high speed and load" and cars with "worn engines with high oil consumption"--may "equal or exceed diesel [PM] levels," according to real-world vehicle testing by University of Minnesota's renowned combustion particle scientists.

Why this could be a big problem: Gasoline PM and semi-volatile organic compound (SVOC) emissions can be equally (or even more) "toxic" than diesel PM, as peer-reviewed, published science studies now show (see Diesel Fuel News 9/2/02, p1).

Citing ground-breaking, real-world vehicle emissions test results--published in the December 2002 issue of Toxicological Sciences--Lovelace Respiratory Research Institute (LRRI) vice-president Joe Mauderly points out that SVOC exhaust vapors can be just as "toxic" as PM emission.

In tests, rat lung reactions to these gasoline/diesel vehicle exhaust emissions (without a diesel particulate filter--DPF) included inflammation and tissue damage. Similar reactions in humans "could aggravate asthma and may cause bronchitis" Mauderly explained to the Diesel Engine Emissions Reduction (DEER) workshop here (sponsored by U.S. Department of Energy). What's more, gasoline and diesel exhaust (without a DPF) both provoked mutagenicity in bacteria, a "crude indicator of cancer hazard," he said.

Poorly-maintained "smoking" vehicles were shown to pose the worst health threats, as these put out twice as much SVOC and PM emissions as well-maintained engines. What's more, "the toxicity of those emissions are twice as toxic as those from normal vehicles--[so] you're likely causing four times the respiratory health hazard" with "smoking" vehicles, Mauderly said.

Nor will regulatory schemes such as pushing compressed natural gas (CNG) substitution for diesel fix the PM "toxics" problems, as studies recapped by International Truck & Engine and New York City Transit once again showed here (see Diesel Fuel News 3/17/03, p10).

Adding oxidation catalysts to CNG engines merely brings CNG up to the "clean" level of diesel-particulate filter (DPF)-equipped engines, as California Air Resources Board (CARB) confirmed here. The only remaining "advantage" for CNG is lower certification levels of nitrogen oxides (NOx), yet real-world studies show that CNG engines suffer very wide variability in NOx (sometimes much higher than in-use diesels) thanks to problems with oxygen sensors and the very wide variability of "natural gas" fuel components.

So, with fuel-substitution flunking the test for "solving" vehicle emissions problems, now air regulators may have to face up to another problem: gasoline car ultra-fine PM number emissions.

Since the U.S. highway vehicle fleet burns about three times more gasoline than diesel, and since gasoline vehicles are now proven to emit substantial amounts of PM and "toxics," the current exclusive regulatory crackdown on diesel vehicle PM doesn't quite hit the clean-air bull's-eye, scientific studies indicate.

This issue is heating up in Europe right now, as the Swiss environmental authority recently proposed that Europe adopt a vehicle solid particle (PM) number emissions limit of [10.sup.11] particles per kilometer.

"This corresponds to roughly [10.sup.12] particles per kilogram of fuel," as University of Minnesota combustion particle scientist David Kittelson explained to the DEER workshop here.

"It is unlikely that our current on-road gasoline fleet meets this standard," Kittelson said, thus begging the question: "Will gasoline engines need exhaust filters?"

In a post-conference interview, Kittelson explained to us that it's possible to imagine advances in engine/combustion controls that likewise might reduce gasoline PM number emissions.

But it's also possible to imagine PM filters on gasoline vehicles, especially if regulatory authorities decide that ultra-fine PM number emissions limits (not just PM mass limits) are a key public health issue, he added.

Ironically, while diesels are sometimes "demonized" by air regulators, diesel particulate filters (DPFs) nearly eliminate PM mass emissions and drastically reduce PM number emissions as well, as numerous studies now show.

For the University of Minnesota studies, researchers used a sophisticated, truck-mounted mobile emissions lab to chase a variety of vehicles around Minneapolis-St. Paul during the summer, in part to compare weekday versus weekend air quality. Rationale: Commercial vehicle operation (mostly diesel) drastically declines on weekends, whereas gasoline traffic volume continues at roughly similar rates throughout the work-week and the week-end.

This real-world emissions sampling study showed that under steady highway cruise conditions, gasoline PM emissions weren't much different than ambient background concentrations. However, when gasoline vehicles undergo "hard accelerations," then "number-weighted [PM] size distributions for the test fleet were similar to modern heavy-duty diesel vehicles," Kittelson explained.

"Number emissions were much higher at high speed and during cold starts," he added.

Gasoline vehicles chased included a wide variety, from "new and clean" to "old and smoky," he said. "This wasn't arbitrary, but rather real-world vehicles," he said. Surprisingly, relatively clean "normal emitter" cars still matched or exceeded diesel PM number emissions at high speed/load, while older gasoline cars with high oil consumption also matched or exceeded diesel PM emissions.

While the newer vehicles had lower emissions, "gasoline engines are very sensitive to load changes--much more than diesel," Kittelson explained. What's more, "most of the tiny PM--around 10 nanometers--are from spark-ignition" rather than diesel engines. Issue: Much of the recent science/health literature indicates special concerns with these tiny particles, which are suspected of having a greater health threat than larger PM.

Gasoline vehicles at high-speed cruise produced much higher PM emissions, the vehicle-chase testing showed. Also, researchers once again exploded the "myth that gasoline doesn't put out much elemental carbon. Well, it does," Kittelson said, "especially under cold-start--there's a lot of elemental carbon. Spark-ignited engines suffer worse emissions in cold conditions, and during cold starts."

Following the road-chase tests, the U-Minn. researchers investigated emissions from a subset of light-duty gasoline vehicles tested on a chassis dyno. These lab tests confirmed that--as in the road-chase tests--PM number emissions were "much higher in cold start cycle, particularly with low-mileage vehicles."

The vehicle lab tests also confirmed that the "highest number concentrations were measured at peak vehicle speed (70 mph)" and that "larger particles are emitted early in the cycle" during cold-start tests "when the vehicle is warming up at low speeds."

The U-Minn. researchers also found a clever way to recreate a "simulated smoker" gasoline engine for chassis dyno tests, to compare with road-chase vehicle results. This scheme involve dripping lube oil into engine intake valve, imitating a leaky intake valve seal. The resulting PM emissions from this gasoline "high-emitter" were vastly higher than better-maintained vehicles, just as in the road-chase tests.

Results showed that the "nuclei mode of the [gasoline] high emitter approaches levels of older diesels," he said.

Considering all the gasoline vehicles studied, "modern in-use spark-ignition vehicles have significant mass emissions (1-7 milligrams/kilometer)" in cold unified cycle tests. "Elemental carbon emissions in these test cycles were surprisingly high, typically 30-60% of P[M.sub.2.5] mass," he said.

What's more, "emissions of extremely small particles (<10 nm) at high road speed may be very high ([10.sup.14]-[10.sup.16] particles/kilogram fuel) even for nominal low-emitters. Storage and release [of gasoline particles in exhaust systems] play an important role in the formation of these particles" - a factor that potentially could prompt new, far-reaching gasoline emissions-control schemes.