Do Volkswagens have secret urea compartments? The car company has been found cheating at NOx emission tests in the USA which makes their cars (diesels more specifically) look much more environmentally friendly than they really are. But what does the "defeat device" look like? In many reports it is just a piece of software that kicks in when the car senses it is part of an emission test. The Guardian initially reported it was al down to an injection of urea into the engine at the time of testing. That would mean a secret urea compartment?
Time to catch up with the specialists here, here and here. The summary: all modern Volkswagen diesels use so-called BlueTec technology for emissions control. It is basically an urea injection system to the exhaust. It works by selective catalytic reduction and the basic reaction taking place is 4NO + 2(NH2)2CO + O2 -> 4N2 + 4H2O + 2CO2. Get rid of NOx and have nitrogen, water and carbon dioxide returned.
The problem is the diesel exhaust fluid, a diesel needs a lot of it (1 gallon for 1500 miles?) and it is expensive (NYT). From a consumer point of view what money is saved in terms of fuel is spent just as easily on exhaust fluid. The Volkswagen solution: kill the urea injection unless at the emissions control testing facility. That is where the software comes in. And how does a Volkswagen know it is in such a facility in the first place? By registering a non-zero speed at zero displacement.
The LA Times in 2008 jokingly suggested urine should replace urea to make the technology economically feasible, If only VW had listened (link)