Volkswagen admitted to installing software designed to cheat emissions tests in about 11 million cars worldwide. While the cars passed emissions tests, they released nitrogen oxide up to 40 times the level allowed by federal law. Here's what you need to know about the scandal. (Jorge Ribas/The Washington Post)

Volkswagen has admitted it designed cars to cheat pollution tests, but the trouble with diesel emissions probably goes beyond just one automaker, according to tests of other manufacturers.

Road tests of more than a dozen popular models from several manufacturers showed that the raw nitrogen oxide emissions from the cars were on average seven times European standards, according to a little-noticed October report from the same outfit that flagged the VW problems.

Most of the models in the October report have not been publicly identified. One of the cars tested is the BMW X3, which had emissions calculated at 11 times the European limit, according to the testing company.

The failure of so many cars indicates that designing a “clean” diesel engine may be challenging and that more than one company may have engaged in deliberate manipulation of the testing.

“We have no information or data that that is going on — but it’s certainly the right question to be asking,” said John German of the International Council on Clean Transportation, which produced the October report as well as the report that led to the unraveling of Volkswagen’s deception. “It’s a question that every country should be addressing.”

Martin Winterkorn, shown in Berlin in March, on Wednesday stepped down as Volkswagen’s chief executive. (Jochen Luebke/AP)

Fears that the diesel troubles may go beyond one company have roiled financial markets in recent days, and for some companies, the news, as corporate events go, has been cataclysmic. Volkswagen chief executive Martin Winterkorn resigned this week, and with multiple investigations of the company underway, more executives are expected to depart.

For BMW, the news that the BMW X3 had fared poorly on an emissions test, first reported by German outlet Autobild, led to a 5 percent drop in the company’s share price Thursday.

“The BMW Group does not manipulate or rig any emissions tests,” BMW said in a statement Thursday. “We observe the legal requirements in each country and fulfill all local testing requirements.”

Indeed, experts said it could very well be that the high levels of real-world emissions reflected in the ICCT report show not that firms are cheating, but only that it’s all too easy to design a car that passes governmental lab tests for emissions. Once outside the lab, those cars fail in real-world conditions.

For scientists who specialize in studying auto pollution, the attention in recent days culminates years of mounting suspicions that many diesel vehicles are emitting more pollution than the governmental lab tests seem to indicate.

Their suspicions were aroused because, while emissions standards for cars have been growing more stringent in Europe, where diesel cars are far more popular, scientists were not seeing parallel improvements in urban air quality.

Nick Molden, chief executive of Emissions Analytics, a British firm, said that in recent years, Europe had put in place two new standards for tailpipe emissions, each one stricter than before.

“But there was no improvement in air quality,” Molden said. “That was the alarm bell. These results suddenly explained why every major European city has an air quality problem.”

The problem, he said, is that the governmental certification test in Europe “is so gentle. It’s 20 minutes in a lab.”

In those tests, the car tires run on rollers known as dynamometers. The speeds are low, the acceleration is slight, and there are no hills. Under such conditions, it’s much easier for an engine to meet emissions standards.

To see how well the diesel cars would fare outside the lab, the ICCT collected the real-world results for 15 cars and reported them in October. Most of the cars emitted nitrogen oxides — or NOx — at rates several times the European standard.

“This report presents strong evidence of a real-world NOx compliance issue . . . both for European Union and U.S. test vehicles,” according to the October white paper from the group.

Molden’s company, and the other testers used by the ICCT, run the cars for much longer out on the roads in what they describe as typical conditions.

“You get these big disparities between the test and the real world,” he said.

It was Molden’s firm that conducted the testing of the BMW X3. That data, which was analyzed by ICCT, showed that its emissions were 11 times over the European standard, he said.

But while the BMW X3 emitted more than some, Molden cautioned against drawing conclusions about whether the company was intentionally cheating. Other BMW models had performed very well.

“We find that each manufacturer has good ones and bad ones,” Molden said. “People have to be very careful about leaping to conclusions about a company based on one bad test.”

Other experts similarly cautioned against drawing inferences about corporate behavior, instead saying that the tests need to be more rigorous.

“Some degree of excess NOx emissions is likely to be a widespread problem for diesels, but I would be surprised if other automakers cheated the way VW has,” said John M. DeCicco, a research professor at the University of Michigan Energy Institute. “Only a major testing program will let us know for sure.”