There are two main types of combustion engines widely available today: diesel and gasoline. And there are real trade-offs to each.
Diesel engines have long been popular in Europe, and one of their major advantages is fuel economy. Diesel fuel contains more energy per gallon than gasoline, and the diesel engines work more efficiently. Put it together, and the typical diesel car can travel up to 30 percent farther on a gallon of fuel than its gasoline counterpart.
But there's a catch. While diesel cars get better mileage and emit fewer carbon-dioxide emissions, they also emit more nitrogen oxides (NOx), which help form smog, and particulate matter, which can damage lungs. Both types of pollution can have serious health effects.
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.
Since 2009, however, things have changed. The Obama administration has been ratcheting up fuel-economy standards in the United States, which puts a higher premium on mileage. At the same time, diesel technology has been gradually getting cleaner through a combination of lower-sulfur fuel, advanced engines, and new emission-control technology. So automakers have shown a renewed interest in "clean diesel" cars that, in theory, don't suffer from that trade-off between performance and pollution.
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.
A second type of diesel exhaust control uses a scrubber (Wikipedia)
Diesel Exhaust Fluid (sold as AdBlue) is a 32.5% solution of urea, (NH2)2CO, in water. When it is injected from a separate tank into the hot exhaust gas stream, the water evaporates and the urea thermally decomposes to form ammonia and isocyanic acid:
The isocyanic acid hydrolyses to carbon dioxide and ammonia:
The NOx are catalytically reduced by the ammonia (NH3) into water (H2O) and nitrogen (N2), and these are then released through the exhaust: