Volkswagen AG admitted on Tuesday that 11 million of its diesel vehicles worldwide use the software that helped the carmaker cheat on federal emissions tests. This revelation comes as the U.S. Department of Justice launches a criminal probe against the company and officials in European Union and South Korea open their own investigations. Despite the backlash against the German automaker, VW’s use of emission control devices isn’t illegal or even surprising. The company’s downfall was instead using this technology to try to game the U.S. Environmental Protection Agency’s secretive emissions certification process.

Most cars use software-controlled systems to adjust engine emissions when the vehicle needs more engine power to climb a steep incline or pull a heavy load. Called auxiliary emission control devices (AECDs), they can also dial up emissions controls to keep leadfoot drivers from abusing warrantee-protected engine parts. Algorithms programmed into these devices determine when a vehicle needs a boost by analyzing engine temperature and revolutions per minute, vehicle speed, transmission gear and other conditions.

For additional Scientific American coverage of the Volkswagen emissions scandal, see Volkswagen Uses Software to Fool EPA Pollution Tests

A carmaker or engine manufacturer’s legal staff typically negotiates AECD performance with the EPA or directly with a state agency such as the California Environmental Protection Agency's Air Resources Board (ARB). Carmakers must meet emissions parameters established by such agencies in order to sell their vehicles in the U.S.  

These negotiations sound mundane but they are highly secretive. More stringent emissions controls make vehicles less fuel efficient, which consumers don’t like. And the ability to ease such restrictions can confer a competitive advantage, says Daniel Carder, director of the Center for Alternative Fuels Engines and Emissions at West Virginia University. “You don't want your competitors to know your strategy by letting them know what your AECD [settings] are,” he says. “If you’re able to successfully negotiate your AECDs with the government and your competitor wasn’t, then you’ve got a leg up.” Carder and his colleagues tested VW’s vehicles at the request of the International Council on Clean Transportation, a nonprofit research group, and submitted their findings to the EPA and ARB in 2014. VW admitted to the regulators earlier this month that it had installed so-called emissions “defeat devices” on several of its models.

When car companies exceed their negotiated emissions limits, it is often an oversight, Carder says. The carmaker may have failed to prepare its vehicles to deliver appropriate emissions levels under all testing conditions or they simply were not informed of the exact conditions—similar to not telling students exactly what an exam will cover so they have only a vague idea of what to expect, he adds. Emissions testing could be improved by providing carmakers with greater transparency regarding the conditions they will face during testing.

Despite any problems with the current approach to emissions testing, VW’s problems clearly were not the result of an oversight. The company surreptitiously installed additional AECDs in five types of four-cylinder diesels—the Audi A3, Beetle, Golf, Jetta and Passat—that were not specified in their application for emissions certification. These systems included an algorithm that used steering wheel position, vehicle speed, engine operation duration and barometric pressure to determine when the vehicle was being tested so it could dial back emissions to meet EPA standards. During normal driving conditions, however, those vehicles' engine power would be ramped up by reducing the effectiveness of the emission control system. This generated up to 40 times the amount of smog-producing nitrogen oxides (NOx) permitted by the U.S. Clean Air Act.

VW came clean when the EPA and ARB said they would not certify the company’s 2016 model year vehicles until it explained its skewed emissions readings and ensured the 2016 vehicles would not have the same problems. (pdf) Now VW faces several investigations, a management shakeup and up to $18 billion in EPA fines.