Volkswagen's appalling clean diesel scandal, explained
Clean diesel cars were supposed to offer great mileage and low pollution — a tricky task
Historically, Europe has dealt with this trade-off by imposing relatively looser emissions standards on diesel cars in the pursuit of better fuel economy. Roughly one-third the passenger cars in Europe now run on diesel, and it's one reason cities like Paris have a serious smog problem. In the United States, by contrast, we've imposed far stricter rules around smog and other conventional pollutants since the 1970s, which is why diesel cars haven't caught on widely here: until recently, few could pass America's stringent NOx standards.
DUE TO HIGHER NOX EMISSIONS, DIESEL CARS DIDN'T CATCH ON IN THE US FOR MANY YEARS
Volkswagen couldn't balance performance with low pollution. So it cheated.
Since 2009, we now know, Volkswagen had been inserting intricate code in its vehicle software that tracked steering and pedal movements. When those movements suggested that the car was being tested for nitrogen-oxide emissions in a lab, the car automatically turned its pollution controls on. The rest of the time, the pollution controls switched off.
Volkswagen hasn't explained exactly why it cheated, but outside analysts have a good guess. The NOx emission controls likely degraded the cars' performance when they were switched on — the engines ran hotter, wore out more quickly, and got poorer mileage. Some experts have suggested that the emission controls may have affected the cars' torque and acceleration, making them less fun to drive. (Indeed, some individual car owners have been known to disable their cars' emission controls to boost performance, though this is against the law.)
The VW scandal exposes problems with current emission tests
Part of the problem here is that regulators usually test these vehicles under laboratory conditions, placing them on giant treadmills and requiring them to do a series of maneuvers. Because this process is predictable, it's easier to game. Combined with the fact that automakers are developing ever-more-elaborate software that can control and fine-tune engines, there are ample opportunities for fraud.
EUROPEAN REGULATORS WILL SOON START REQUIRING ON-ROAD EMISSIONS TESTING
Volkswagen is now facing serious blowback
At this point, Volkswagen has been caught red-handed and has to face the consequences. The company straight-up lied about its cars and knowingly evaded pollution limits. (Getting a precise estimate of the health damages caused by the extra pollution that resulted could be difficult, since it would depend on where the cars were located, how much extra smog actually resulted, and so forth.)
This episode also raises questions about the future of clean diesel vehicles. Clean diesel appears to be a genuinely promising technology — in theory, such vehicles could get both excellent mileage and lower emissions. But this whole scandal raises serious questions about how well automakers can actually achieve both goals in practice.