Berkeley Lab Research News


Auto Study Suggests Unexpected Source Of Emissions Control System Failure

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By Alan Chen,

April 26, 1996

BERKELEY, CA -- When an automobile's emissions control system fails, it may be because that model is more prone to failure than most others, according to a new study by researchers at Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of Michigan. This finding goes against conventional wisdom that most auto emissions control failures are caused by owners who don't maintain their cars properly or deliberately disable their emissions systems.

Tom Wenzel of the Lab's Energy & Environment Division, and Marc Ross of the University of Michigan conducted the new study. Their results may provide clean air regulators with more cost-effective strategies to reduce air pollution from autos than the current system of individual auto inspections.

The Clean Air Act sets standards for emissions of carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons (HC) from new automobiles. The Environmental Protection Agency (EPA) tests automobile models for compliance with these standards. Since the mid-1960s, emissions of CO and HC from cars measured in these "on-cycle" lab tests have come down 96 percent. At the same time, cars in actual use emitted only 75 percent less CO and HC, while Americans drove twice as many miles per year--resulting in a roughly 50-percent net drop in actual auto emissions. Vehicle travel is growing so rapidly that the trend of decreasing total emissions could reverse without additional regulatory steps.

One problem with using the on-cycle tests to predict a car's real emissions is that the tests measure a car's performance in the lab, not on the road. Road emissions tend to be higher because drivers run their cars at higher speeds and accelerations than in the on-cycle tests. The EPA does not extensively test emissions from cars after they have been on the road for a few years, when the performance of emission controls has begun to degrade.

Wenzel and Ross analyzed thousands of observations of car tailpipe exhaust measured with a remote sensing system used in California. The system's infrared beam measures the CO, CO2 and HC emissions directly from the tailpipe as the car drives by, while a video camera records the car's license plate. Researchers from the University of Denver gathered this data for the California Air Resources Board in 1991. They set up the system at several sites in California. State Department of Motor Vehicles records provided each car's vehicle identification number, which encodes the make, model, engine type, and other technical data for each vehicle.

Wenzel and Ross studied 4,000 observations of 3,000 model year 1987-1989 cars that were two-to-five years old. Their aim was to determine the probability that a car on the road had malfunctioning emissions controls. They expected that these "young" cars, which were within the manufacturers' warranty period for emission control components, would have a low probability of malfunction.

"What we found surprised us" says Wenzel, "About 10 percent of these cars showed CO emissions that were 50 times higher than for properly functioning cars. This is a very high rate for young cars." They also found that the malfunction rate varied widely by vehicle model; some models had few or none, while others had failure rates of up to 30 percent.

The data did not show a correlation between manufacturer and probability of malfunction. In fact, their data suggests that the manufacturers whose models were among the worst identified also had very clean models. The five worst-performing models were low-priced models of Asian manufacturers: an average of 22 percent of the models in this category had malfunctioning emissions controls, compared to 6 percent for all the other models. At the same time, the mid- and high-priced models of the same manufacturers had very low failure rates. Most domestic models performed well, falling into the middle of the range of malfunction percentages.

Another finding of the Berkeley Lab study is that malfunction rates are higher in vehicles with certain technologies. With few exceptions, carbureted automobiles were more likely to malfunction than fuel-injected ones.

Wenzel and Ross confirmed their results from the remote sensing data by examining laboratory tests of in-use cars. The same five models dominated the models that failed the lab tests. The researchers are seeking additional databases of tests on in-use cars to further support their results. "If they're confirmed, these findings have important policy implications," says Wenzel. Current policy focuses on identifying individual vehicles for repair, through regularly scheduled vehicle inspections. A controversy is currently raging between the states and EPA over what is the most cost-effective system to identify individual high-emitting vehicles: a network of remote sensors or more detailed lab-type tests. The results from this study may shift the focus of the debate.

"The data indicate that emission controls problems are not necessarily the fault of drivers not properly maintaining their cars. Certain models appear to have more problems than others. Policies that focus on improving the durability of emission controls on all models may therefore be more cost-effective in reducing emissions than the proposed approaches," according to Wenzel.

This analysis is part of a larger study at several institutions to better understand the effects of real driving conditions on emissions. Ross and others at the University of Michigan, Michael Wang of Argonne National Laboratory and Wenzel recently produced a study, "Real-World Emissions from Model Year 1993, 2000, and 2010 Passenger Cars," which quantifies the actual lifetime emissions of current vehicles, and projects the effect of current policies on the emissions of future vehicles. Wenzel and Ross are also collaborating with the University of California at Riverside and Ford Motor Company to develop an improved computer model that predicts how second by second changes in vehicle operation affect tailpipe emissions. Such a model can help assess transportation policies at several levels, from evaluating the impact of local transportation system improvements to developing a more accurate emission inventory for an entire region. The efforts of Wenzel, Ross and their colleagues are providing the scientific information that regulatory bodies such as local air quality districts, the California Air Resources Board, and EPA need to reduce emissions and improve air quality using the most cost-effective measures possible.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified research and is managed by the University of California.