NOx emission from diesel engines
Many Diesel engines exceed the legal limits for nitric oxides. Often a value of over 500 mg/km NOx is reached.
Nitric oxides of the chemical composition NO are invisible, however, the composition NO2 is undiluted visible as a brown gas.
In the following experiment, an amount of 1 liter NO is converged into the appropriate amount of NO2, by addition of oxygen; noticeable at the appearance of the brown gas. [Video]
This reaction happens by combustion of nitric oxides in the engine. Due to dilution of the nitric oxides with other exhausts, the color is not visible.
The amount of 1 L nitric oxides is already reached after a 2.5 km trip by emission of 500 mg/km, with a Euro-Norm 5 car (180 mg/km) after 7 km and with a Euro-Norm 6 car (80 mg/km) after approx. 16.5 km.