How Car Software Can Rig a Test
A complex mix of sensors, engine-management software track emissions
Car makers woo customers with promises of speed, acceleration and braking distance. Less sexy is the increasingly critical element that helps them deliver that performance: software.
In the case of Volkswagen AG , it was software that the U.S. Environmental Protection Agency says allowed about 500,000 U.S.-sold Volkswagen-made cars to pass emissions tests.
Volkswagen said on Tuesday as many as 11 million vehicles may have the same software, which U.S. regulators called a “defeat device.” It also said it would set aside €6.5-billion ($7.27 billion) to cover legal and others costs to deal with the disclosure.
New cars can have as many as 20 million lines of software code, more than many airplanes, said Bodo Seifert, a U.S.-based automotive software expert. The software controls everything from running the engine, gearbox and diagnostics to the radio and power windows. It can also help a car control the amount of pollutants it emits—by monitoring levels of things like carbon monoxide and nitrogen oxide—a byproduct of diesel-engine combustion. It can then figure out when to trap pollutants, or divert them through processes that can convert them into less-harmful substances to meet legal limits, according to Greg Schroeder, assistant director at the Ann Arbor, Mich., Center for Automotive Research.
At different stages of engine use—for instance, idling versus cruising on the highway—engine combustion can create widely different levels of pollutants. Diesel engines don’t emit much carbon monoxide, but they generally generate a greater amount of nitrogen oxide, or NOx—a component in acid rain and low-atmosphere ozone. In the U.S., where diesel passenger car sales are still comparatively modest, regulators have set far tougher NOx standards than in Europe, where diesel engines have long been more popular.
Diesel powered cars use a complex mix of sensors and sophisticated engine-management software to keep track and limit emission levels. The software can influence how much NOx is produced during combustion by regulating the mix of diesel fuel and oxygen used in the process. Cars also rely on a mix of NOx traps, which capture the pollutant, and catalysts to clean emissions and meet standards.
But the measures to reduce pollution come at a cost, automotive engineers say, including reducing power that can hamper acceleration. Limiting the use of the devices used to curtail emissions could improve fuel economy and cut noise in the car, said a European auto engineer. The engine software can control how much NOx is filtered from the exhaust.
It is unclear what the VW software in question did specifically to reduce emissions during testing. But by collating such data as tire rotation, steering, and use of the accelerator, a program could determine whether a car was being driven on the road or merely on a test bed at an emissions-testing station.
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If such software sensed a lack of steering wheel inputs, for instance, it might have been able to predict it was being tested for emissions, and divert more pollutants to traps or treatment.
Volkswagen hasn’t named the specific software or component on the engine affected. On Tuesday, it said its Type EA 189 engines—found in some diesel models like the Jetta—displayed “a noticeable deviation between bench test results and actual road use,” adding it was “working intensely to eliminate these deviations through technical measures.”
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Experts aren’t even sure at this stage whether Volkswagen manufactured the software or components involved. Calibrating a third-party component or piece of software to respond in a certain way could be enough to make up for the discrepancy, experts say.
German engineering company Robert Bosch GmbH said it made components for the Volkswagen cars now being probed. While the company said it “develops injection and exhaust-gas treatment systems that can reduce emissions in every driving situation,” the Stuttgart, Germany, company added that “responsibility for calibrating and integrating the components supplied by Bosch into the system as a whole lies with VW.”
—Christopher Alessi and Christina Zander contributed to this article.
Write to Robert Wall at robert.wall@wsj.com