First there was Diesel engines
Then low sulphur diesel fuel, to reduce sulphur.
Then particulate filters were added to reduce particulates. These steadily improved as standards went up from EU3 to today.
Then came EGR, exhaust gas recirculation, putting exhaust back into the combustion to reduce the combustion temperature. Lower combustion temps mean lower NOx.
Now for EU6 lab tests we have LNT, lean NOx traps, that chemically trap NOx from leaving the exhaust. It is chemically converted in trap 'regeneration' to harmless gasses and water.
The alternative to LNT is SCR. SCR uses adblue to chemically react with NOx in the exhaust converting it to harmless chemicals such as nitrogen gas.
LNT alone is better than SCR alone in the lab, and there's no ongoing cost. That's why current Hyundai EU6 diesel cars all have LNT but do not require adblue.
Other manufacturers have gone down the SCR alone route.
Commercial vehicles with their lower rpm and lower combustion temperatures (as they are not trying to replicate to feel of a petrol car with high revs) achieve pretty good NOx emissions with SCR in the real world. Cars generally don't.
In the real world an LNT combined with SCR bring emissions right down under almost all driving conditions and that's on its way very soon. Both are needed for the RDE without fudge factors being applied.
Currently being tested and developed alongside electric, plug in, fuel cell and conventional hybrid is the 48v mild hybrid which can be added at fairly low cost.
The downside of EV is that it only moves the emissions from the point of use of the car to the point of generation of the electricity, and that isn't always as clean or sustainable as it could be.
My last journey used 160 tonnes of fuel.