Here is an extract from an article on the Volkswagen mess by Alexander Noack, posted on the GREASEnergy web site.
The main point is that the technology to meet emissions standards and retain very good fuel mileage is known and available. But Volkswagon chose not to use it to save a couple of hundred buck per car in order to increase their profit margin.
From this short explanation you will understand easily that the first three mentioned emissions (HC, CO, particles) will simply work in the opposite way of the fourth emission (NOx). And this is more or less the bad news for the diesel engine engineers: the better your engine efficiency is, the higher the nitrogen oxide (NOx) emission – if we talk about the raw emissions.
The good news is that there is an existing technical solution for that used on the market for more than 5 years. After torturing the engines with high exhaust gas recirculation (EGR) rates over decades, there was introduced the selective catalytic reduction converter (SCR) technology, which may fix the nitrogen oxides emissions to very low levels below the limits. Diesel engines made a big step back to real low fuel consumption (decreasing by –minus 10-20%) by keeping the low nitrogen oxide (NOx) emission levels.
But why the Volkswagen engines didn’t show these good results at the International Council on Clean Transportation ICCT which conducted a real drive emission test (RDE). Technically it is definitely possible, but Volkswagen simply tried to reduce production cost of their units (roughly 150-300 USD/unit) by not using the SCR technology with an additional AdBlue (NH4 liquid) tank, which is necessary for a high level of nitrogen oxide reduction in the SCR units and is commonly found in many heavy vehicles, as well as other brands of passenger vehicles.
Instead of this they equipped some of their cars with the less efficient NOx-storage catalytic converter (NSC) and used a so called defeat device, which is software recognizing a set of criteria that indicates the vehicle is in a test procedure.