For decades, automakers have been caught between building an engine that squeezes a lot of energy out of the fuel it burns and one that has low emissions. It’s not an easy tension to resolve. “Negotiating both fuel consumption and emissions is a hard tradeoff,” says Anna Stefanopolou, professor of mechanical engineering at the University of Michigan.When engineers at Volkswagen allegedly inserted a few lines of code into the diesel cars’ electronic brains to circumvent emissions testing, they found a solution to this existential automotive conflict. Drivers got low emissions during the test, and high performance the rest of the time. The only problem: It’s way outside of the rules. The company might have gotten away with it, too, if it hadn’t been for those pesky engineers—and the basic chemistry of the diesel engine.According to the US EPA, those lines of code hid the fact that nearly half a million diesel VWs in the US spewed up to 40 times more nitrogen oxide from their tailpipes than testing indicated. Volkswagen has now confirmed that the problem actually affects approximately 11 million diesel cars worldwide. Diesel engines use a different mix of fuel than gasoline engines and don’t use spark plugs to induce combustion—relying instead on highly compressed, heated air and fuel injected as droplets. If a diesel engine doesn’t get enough oxygen to combust the fuel, it’ll emit all kinds of gunk—nitrogen oxides, uncombusted fuel, and particulate matter (soot, basically). Read more.