street smart

An emission-testing lab on wheels gives a close-up view of bus engines in their natural habitat.

A report from the street confirms that catalytic filtering can stop three-quarters of a diesel engine's particle discharges. A catalyzed filter will also reduce the overall volume of nitrogen oxides in exhaust, but a boost in the part that comes out as NO₂ may come back to haunt you.

When a plan took shape to measure the stuff that big engines put into the atmosphere during the stops and starts of New York City driving, there was plenty of interest. State and federal studies had already been looking at large vehicle engines in New York, especially the effectiveness of emissions-reduction technology. So Aerodyne Research Inc. of Billerica, Mass., was able to load a van with test instruments and chase big vehicles through the city's streets. Ports in the van captured samples of air laced with engine exhaust, and testing devices measured the content on the run.

According to Scott Herndon, a senior scientist at Aerodyne, diesel buses with catalyzed filters emitted 75 percent less particulate matter and slightly less NO_x than diesels without emissions controls. However, anywhere from 25 to 60 percent of the nitrogen oxides were NO₂, which is much more toxic than NO to inhale.

Less than 5 percent of the NO_x from uncontrolled diesels came out as NO₂. The fraction for compressed natural gas buses was under 10 percent.

An older 6V92 Series of two-stroke diesel engines produced NO_x at an average rate of 9.1 parts per 1,000 parts of CO₂. NO_x ran higher in newer, Series 50 four-stroke engines, at 11.5 parts for every 1,000 of CO₂, Herndon said.

Newer bus engines equipped with catalytic filters produced 8.5 parts NO_x, roughly the same rate as CNG engines, which averaged between 8 and 9 parts nitrogen oxides for every thousand parts of carbon dioxide.

Herndon pointed out that tests recorded a large variance among individual engines. While the average was 8 to 9 parts NO_x for compressed natural gas engines, for example, readings ran from a low of 5 parts to a high of 14.

Tests compared the quantities of various gases—including NO_x, carbon moNO_xide, sulfur dioxide, and methane—to the amount of CO₂ in each sample. Using CO₂ as a benchmark allowed for the dilution of exhaust in the air.

The dilution was significant and largely uncontrolled, so it varied from sample to sample. "The strongest plume was already diluted by a factor of 100," Herndon said.

The sample-gathering method introduced an uncertainty of about 2 parts target gases per thousand parts of carbon dioxide in test results.

One of the selling points for compressed natural gas engines is that the primary component of the fuel is methane, which has four atoms of hydrogen for every one of carbon. The test results, therefore, are likely to represent a smaller total mass of nitrogen oxides from natural gas engines than from diesels.

Herndon said, however, that he has been unable to track down much published data on carbon dioxide emissions of natural gas engines, although he had plenty of information about diesels.

Meanwhile, the street tests indicate that as much as
1 percent of a CNG bus's exhaust is unburned fuel. Herndon said that formaldehyde concentrations are high, "about 100 times that which we can conclusively say we see from burning diesel."

City-Tough Testing

Many of the tested buses are operasted by the New York City Transit unit of the Metropolitan Transportation
Authority, the region's state-run transportation agency. Others belong to private fleets operating under contract with the city's Department of Transportation.

In many cases, Aerodyne's researchers had specifications of the buses they were studying, especially of the diesel buses belonging to New York City Transit.

Dana Lowell, assistant chief maintenance officer in the Department of Buses for New York City Transit, said the agency had been working with the state Department of Environmental Conservation in a study of clean-diesel technology.

Instruments packed in a van identified and measured emissions content of large-vehicle engines. Exhaust samples were taken from the air in traffic and analyzed for particulates and various pollutant gases.

According to Tom Lanni, a research scientist in the Bureau of Mobile Services of the state DEC, the state-funded research project had studied almost a dozen buses on a dynamometer that simulated various street conditions and also ran each bus in steady state. To get a dynamometer large enough to handle buses, the researchers had to go to Ottawa, Ontario, and use one owned by Environment Canada, the Canadian environmental agency.

When the proposal for a street test came up, both New York agencies were eager for it. The New York City Transit buses were already operating on low-sulfur fuel, and many engines had catalyzed filters. The lab on wheels would give everybody a look at the effect of the technology on end-use vehicles.

Since the street study was conducted, the MTA has begun to phase out the 6V92 Series of two-stroke engines and replace them all with newer, four-stroke models. Dana said the changeover is due to be complete by the end of the year.

The principal test instruments were Aerodyne's Tunable Infrared Laser Differential Absorption Spectrometer and its Aerosol Mass Spectrometer. The laser system identifies pollutant gases by measuring the strength of fingerprint spectral features.

The mass spectrometer studies particulate matter, especially particles between 40 nanometers and 1.5 micrometers across. It includes a device called "an aerodynamic lens," that aligns particles and gives them identical kinetic energy. From the formula E=1/2mv², the relative velocity of particles reveals their mass, and from that information, a judgment can be formed about their size.

Later stages in the spectrometer analyze the chemical composition of most particles. It can't identify a few chemical compositions, such as black carbon. John Jayne, who was one of the developers of the system, and Manjula Canagaratna operated the mass spectrometer in the van.
Herndon and a fourth member of the team from Aerodyne, Joanne Shorter, operated the laser spectrometer.

Sampling Engine Exhaust

During five weeks in July 2001 and a couple of weeks in the previous October, Herndon and his colleagues sampled exhaust from 352 vehicles—school buses, trucks, even a few planes at the airport. Sixty percent of the tests involved passenger buses, Herndon said.

Out of more than 200 buses sampled, one had also been tested on a dynamometer. According to Herndon, the results on the street and those on the machine are comparable.

The study was part of a Supersite project overseen by the Atmospheric Sciences Research Center at the State University of New York in Albany. The project was funded by the U.S. Environmental Protection Agency with contributions from NASA's Earth Science Enterprise. Other agencies, including the New York DEC, kicked in time and money, too.

It seems to have been a cooperative effort on many fronts, beside putting up money. For instance, Lanni of the New York Department of Environmental Conservation said that he was one of the principal investigators. He got to take the wheel of the van some of the time and also operated some of the test equipment. He said his department contributed an instrument of its own to the van, an electrical low-pressure impactor, which measures the range of sizes of aerosol particles.

According to Ken Demerjian, director of the Atmospheric Sciences Research Center at SUNY Albany, although the study revealed that emissions controls have issues to address, "Overall, we were impressed with the technology."

He said that a plan for similar study in New York during the winter has not been funded yet.

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