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Better Way Found to Test Emissions of Heavy Duty Trucks

University of Denver scientists have developed a new highway emissions test for trucks that is far more reliable and less costly than the present test. If used, it could save the trucking industry millions of dollars and ensure that truck emissions are measured accurately.

“We have invented a drive-through Streamlined Heavy-duty Emissions Test (SHED) which measures all the pollutants of importance in mass emissions units—the current government standard,” says Donald Stedman, John Evans Professor in the Department of Chemistry and Biochemistry at Denver. “The test allows the truck to simply accelerate in its normal way with its normal load.”

University of Denver

The new SHED test for truck emissions is more accurate and less costly.

The SHED technique measures realistic truck exhaust emissions in 20 seconds or less and the truck does not even have to stop. The system basically places a 50-foot long tent roof over the roadway which captures some of the exhaust. “Under the roof,” says Stedman, “is a perforated sampling tube with suction provided by an in-line blower. The perforations are designed to accelerate the air sample down the tube at about the same speed as the truck accelerates under the roof.”

The result is a measurement that is closely comparable to federal emissions standards.

That is an upgrade to existing testing practices in two ways. First, the current method tests only for how much or little light (opacity) a truck’s exhaust lets in. This actually has little correlation with the government standard for pollution which is measured in units of smoke mass. Oxides of nitrogen, an important part of diesel exhaust, are not measured at all.

Second, the test now in place is time-consuming which drives up transportation costs. The truck must be pulled over and stopped with the engine idling. The tester mounts an opacity monitor on the hot exhaust pipe, then has the driver floor the accelerator to allow the motor to go to its maximum RPM. “This is something that never occurs under normal circumstances,” Stedman says.

The SHED testing technique has undergone two successful tests. The first was done in collaboration with researchers at Texas A&M University. The second took place in Vancouver, British Columbia. In the latter test, more than 1,000 heavy-duty truck emissions were measured. The results were presented in April at the Coordinating Research Council On-Road Mobile Source Emissions Conference. See

Stedman says that practical applications of the SHED test include “improved estimates of the impact of truck emissions on air quality and fast emission screening at selected drive-through locations such as weigh stations, transit terminals and border crossings.”

The SHED test could also help The United States fulfill an unmet part of the North American Free Trade Agreement (NAFTA), he notes. NAFTA allows trucks from Mexico to deliver directly to destinations in the U.S. But in practice that doesn’t happen, in part because of fears that the Mexican trucks might be more polluting than the better-regulated U.S. fleet.

“Border crossing loads now are emptied from one truck and refilled into another,” says Stedman, “significantly increasing the cost and time of transportation.”

Earlier work by Stedman and University of Denver colleagues resulted in a technique that enabled an emissions test for cars that, by using remote sensing, could be carried out in about one second as the car drove by. That resulted in on-road emission monitoring being made a part of the 1990 U.S. Clean Air Act Amendments, although that mandate has been more or less ignored, except in Colorado. In Colorado, almost 300,000 drivers each year get a postcard from Air Care Colorado stating that they have passed their emission test by means of remote sensing during the course of their normal driving and thus do not have to report for their biennial emission test.

Stedman is hopeful that his new truck test will take off in the USA as successfully as his car test has been accepted in Colorado.

Donald Stedman | Newswise
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