Why Did Volkswagen Cheat On Diesel Emissions In Its TDI Cars?

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2009 Volkswagen Jetta TDI

2009 Volkswagen Jetta TDI

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The bad news for Volkswagen just keeps on coming.

In the last week, the company has fired much of its upper management, eradicated all mention of its TDI diesels from its U.S. website, and admitted that 11 million diesel vehicles have software that enabled them to pass emission tests while emitting 10 to 35 times the permissible U.S. levels of nitrous oxides.

Yesterday the German transport ministry announced that Volkswagen had rigged emission tests on 2.8 million diesel vehicles in Germany alone. Switzerland temporarily banned the sale of VW diesels in the country.

DON'T MISS: VW Diesel Emissions Recall: What You Need To Know In 10 Questions

And there will likely be more news--much more--in the coming days and weeks. The scope and impact of the scandal is one that no automotive journalist could likely have imagined before the news eight days ago.

U.S. owners of Volkswagen TDI diesel cars are upset, angry, hurt, and betrayed. They're also fearful of what will happen to the resale value of their cars--and VW has said nothing about what will happen next.

Over all this, however, the question remains: Why did one of the three largest automakers in the world decide it had to cheat, so wantonly and blatantly, on emission standards that presumably all of its competitors met?

VW TDI badge

VW TDI badge

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To be clear, the 482,000 diesel Volkswagens in question passed the emission tests mandated by the U.S. Environmental Protection Agency.

The deception was in a piece of software that detected whether the car was being emission-tested or operating in real-world use, presumably based on steering position, relative wheel motion, and other sensor data.

If it detected that the car was being tested, its engine-control software behaved in a way that complied with the emission limits--specifically those for nitrous oxides, or NOx, a major contributor to photochemical smog.

ALSO SEE: VW Emission Cheating Could Kill Diesel Passenger Cars In U.S., Even Globally

But then, if it detected the car was being driven by an actual driver on open roads, it behaved quite differently--and emitted 10 to 35 times the nitrous oxides permitted under longstanding emission rules in place since 2008.

Why would a huge global automaker like Volkswagen circumvent U.S. emission rules rather than comply with them?

The answer comes down to a combination of cost, fuel efficiency, and driving performance.

'Diesel fuel only' caution on Audi Q7 TDI

'Diesel fuel only' caution on Audi Q7 TDI

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To understand these forces, it's important to understand the global auto market. Volkswagen AG is the only European maker among the three largest global automakers; the other two are General Motors (U.S.) and Toyota (Japan).

As such, VW is influenced by its home market. And in Europe, since the 1990s, diesel cars have made up roughly half of all passenger car sales. The numbers vary by country, but in the European Union, it's been at one of two for 10 years.

As Europe's best-selling automaker by far, Volkswagen has devoted an inordinate amount of its powertrain development effort to diesel engines.

MORE: VW, Audi TDI Diesel Cars Had 'Defeat Device' That Violated EPA Rules, 500K Cars Recalled:

The problem is that U.S. and European emission standards for diesel diverged sharply in 2008. The U.S. adopted much stricter rules (known as "Tier 2, Bin 5") on emissions of virtually every component of diesel exhaust.

Europe, on the other hand, stayed with the so-called Euro 5 standards. Its emission limits only moved up to roughly the U.S. level this year.

But while other makers--including Toyota, Honda, General Motors, and Ford--began to introduce gasoline hybrid-electric vehicles to boost their fuel efficiency, VW chose to stick with diesels.

2015 Volkswagen Golf TDI SE

2015 Volkswagen Golf TDI SE

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The depth of the company's commitment to diesel engines, even in a U.S. market that showed little natural appetite for them, can be illustrated by one anecdote.

"If U.S. car buyers were smarter and better informed," said a highly-placed Volkswagen powertrain executive to this reporter, "they'd buy our clean diesels instead of all this hybrid nonsense."

He went on to explain that diesels were naturally more cost-efficient than hybrids, because the latter required fitting two separate powertrains (gasoline and electric) and a very pricey battery pack into a car.

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