PhotoPlenty of consumers are outraged that Volkswagen systematically cheated on emissions tests, but few understand how and why they did it.

The BBC reports the software installed in the vehicles was able to detect when they were being tested and altered performance during those brief periods so that the vehicles didn't give off as much toxic NOx, the main pollutant in diesel exhaust.

Why, exactly, did VW think it had to go to such lengths to bypass the emissions standards? And how is a diesel engine different from one powered by gasoline?

Researchers at Empa’s Laboratory of Automotive Powertrain Technologies explain that gasoline engines employ a three-way catalytic converter that breaks down more than 98% of NOx in the exhaust. Unfortunately, it doesn't work on diesel engines.

Power plant technology

In order to meet the same pollution standards as gasoline engines, diesel engines use a method originally developed for the denitrification of power plant exhaust gases. The first system was put on trucks; it has only recently migrated to cars.

This system uses a technology bearing the tradename “AdBlue” to convert the NOx into harmless nitrogen via various chemical reactions in a special SCR (selective reduction) catalyst that has been optimized for NOx reduction.

And here's where VW may have run into trouble. According to Empa director Christian Bach, SCR systems are considerably more complex than a conventional three-way catalytic converter in gasoline engines. Here's how VW describes its SCR system on its website.

Little margin for error

AdBlue is carried in a separate tank in the car and needs to be topped off every now and again, usually while the vehicle is being serviced. The AdBlue dosage needs to be set precisely to the amount of NOx emitted by the engine.

If the dosage is too low, it doesn't reduce Nox emissions enough to meet the standard. Too high a dosage and the result is undesirable ammonia emissions. In short, diesel SCR systems can yield unpredictable results.

As an added complication, Bach says at temperatures below 200 degrees C (392 degrees F), AdBlue tends to form residues that eventually clog up the SCR catalyst. That means the systems must be adjusted specifically to the various engine types and expected load change – i.e. drivability – and optimized, which is a complex, and thus expensive, process.

Is there a better way? While some are ready to write off “clean diesel” technology, Bach and his team of researchers are in pursuit of one that might work better. For example, two doctoral students on Bach’s team are experimenting with AdBlue injection procedures using laser-based measuring techniques.

“The better we understand these technologies in detail, the more cleanly diesel vehicles will run on the streets,” Bach said.

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