This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
“I have to take my car in every year for an expensive and inconvenient emission test which I pass. What are you doing about all those trucks?” This question is posed to many air pollution control authorities.
There is an emission measure for trucks called the Society of Automotive Engineers J1667 test, also known as the “snap idle” test. It is widely disliked and inaccurate. The truck must be pulled over and stopped with the engine idling. The tester mounts an opacity monitor on the hot exhaust pipe, then persuades the driver to floor the accelerator and allow the motor to go to its maximum allowed RPM. Too much light absorption (opacity) and the vehicle fails. The test only determines smoke opacity. The government standard for pollution, by contrast, is in units of smoke mass and the two do not correlate well. Oxides of nitrogen, an important component of diesel exhaust, are not measured at all. And the driving mode is one that never occurs under normal circumstances.
At the University of Denver we have invented a less intrusive and better test: the drive through, Streamlined, Heavy-duty Emission Test (SHED) measures all the pollutants of importance in mass emissions units. It allows the truck to simply accelerate in its normal way with its normal load. A full report on the successful testing of the SHED in Vancouver, Canada was released to the public in April.
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The SHED technique can measure realistic truck exhaust emissions in 20 seconds or less and the truck does not even have to stop. The system places a 50 ft. long roof over the roadway which captures some of the exhaust. Under the roof 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 first sample in the tube is from the entering truck.
But, as the truck drives along, more exhaust is added to the previous exhaust sample which is running parallel to the truck such that, when the truck leaves, an integrated sample of air-diluted exhaust passes through the blower and into a suite of calibrated monitoring instruments. These instruments measure the masses of all the pollutants compared to the mass of carbon dioxide and thus the mass of pollutant relative to the mass of fuel burned. This fuel-specific mass emissions rate is closely comparable to the federal emissions standards which are in units of emissions per brake horsepower hour. The close comparison arises because modern diesel engines all use almost the same mass of fuel per brake horsepower hour. If they used a lot more fuel this would show up in their gas mileage and performance and no trucking company would buy such an engine.
The world’s first demonstration of a full-scale SHED was successful and was carried out by the University of Denver in collaboration with Texas A&M University. It was sponsored by the North Central Texas Council of Governments because their constituents posed the question that starts this article.
The Vancouver study was the second. In a week, more than 1000 heavy-duty truck emissions were measured by SHED. Results showed that a majority of the Vancouver diesel fleet measured were well maintained and met the emission standards and regulations under which they were built. Particularly important is the fact that the newest heavy duty diesel vehicles have almost undetectable smoke emissions and their nitrogen oxide emissions are many times lower than the older trucks on the road. There were also some trucks with high emissions. One was emitting 60 times the average smoke emissions and its carbon monoxide emissions, normally low in diesel vehicles, would have only been normal for an early 1970s pickup truck. Repair would save the owner significantly in his diesel fuel bill and save the citizens of Vancouver the danger of inhaling potentially carcinogenic diesel exhaust.
The SHED results were presented in April to the Coordinating Research Council On-Road Mobile Source Emissions Conference.
Potential applications include improved estimates of the impact of truck emissions on air quality and fast emission screening at drive through locations such as weigh stations, transit terminals and border crossings. The North American Free Trade Agreement allows trucks from Mexico to deliver directly to destinations in the USA. But it doesn’t happen. One reason is that the non-US trucks might be more polluting than the well-regulated USA fleet. Border crossing loads now are emptied from one truck and refilled into another, significantly increasing the cost and time of transportation.
With a SHED, a cross-border truck’s emissions could be monitored efficiently. With a requirement to stop and accelerate over 100 yards in less than 25 seconds and then stop again, the truck’s power to drive and stop its load are also tested. Assessing the truck weight in motion is available technology. This application could speed up and lower the cost of cross border goods shipping and allow the USA to fulfill its unmet treaty obligations while preserving public safety.
Years ago, we developed a technique to carry out an automobile emission test, using remote sensing, in about one second as the car drove by. As a result of that work, when the 1990 Clean Air Act Amendments passed, they required on-road emission monitoring as a component of a vehicle emission testing program.
This congressional requirement has, however, been more or less ignored, except in Colorado. Almost 300,000 drivers each year receive 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 do not have to report for their biennial emission test.
In summary, it’s time to use in all 50 states the technology that Colorado has been using for many years to painlessly measure auto emissions. And it’s time to use the new and improved technology available to test truck emissions, lower transportation costs, and meet our NAFTA treaty obligations.