Inside the Volkswagen emissions cheating

If I remember the discussions correctly, it was a means to get the AdBlue refilling cycles in line with the regular service intervals of the car as a convenience for the customers. That of course leads to the question why they couldn't just increase the size of the tank.

The story I heard around the office (and I'm not mechanically inclined at all so this could be completely wrong) was that adding the AdBlue also meant using a different air-to-fuel ratio and so by not using AdBlue the car would have better performance and more torque.

That meant VW cars would use less fuel (which is something customers can easily measure themselves, often talk about, and would influence purchases) and feel more powerful when taking them for a test drive and comparing with other cars in a similar range which (probably) do meet emissions standards. So the incentive may have been around sales.

The best guess I have heard is that the CEO of VW was a well-known tyrant, unwilling to change course or listen to nay-sayers, and way back when this started, he had publicly promised VW could create clean diesels that would not need AdBlue. One of the troubles with AdBlue is that US laws require the pollution control system work for 100,000 miles without user servicing, so the AdBlue tank has to be big enough to last for the 10,000 mile service interval (2.5L/1000km is roughly one gallon per 1000 miles, so a 10,000 mile tank is getting close to the size of the fuel tank); the less AdBlue needed, the smaller the tank. If you can make the diesel run clean without needing any AdBlue, even better.

But as the self-assessed deadline approached and the engineers couldn't make their diesels run clean without AdBlue, they resorted to a one time, temporary fix of perverting the pollution control software, fully expecting it to be taken out soon. Unfortunately, they couldn't make their diesels run clean, and the CEO was not the type to lose face by admitting so, so the perverted software remained.

That certainly sounds plausible to me. Whether it's true or not will probably never be known publicly, even though the tyrant CEO is now gone.