What causes NOx emissions in a SI car??

Question

What are the specific reasons that cause high nox emissions in a car?? Does ari/fuel ratio affect NOx emissions??

Answer 1

I don't know where the others got there information. NOx is produced when combustion temperatures exceeds 2500degrees. The main cause of this is advanced timing, lean fuel mixture and excessive carbon deposits in the combustion chamber. EGR valves help combat the formation of NOx and the catalytic converter helps clean up the output.
So to answer your question, YES air/fuel will effect NOx.

Answer 2

Incomplete combustion does not contribute to high NOx emissions. Incomplete combustion rather leads to high Carbon Monoxide (CO) emissions.

Its actually the opposite. Complete combustion results in high temperatures. The high temperatures result in burning ambient nitrogen in the air. This is usually prevalent in high compression engines, particularly those fitted with Turbochargers where forced air induction allows more complete combustion that there is actually excess air that is not used in the combustion process.

So yes, air/fuel ratio affect NOx emissions. Some engines are fitted with EGR valves (Exhaust Gas Recirculation) to correct this. In this system, some of the exhaust gas is deliberately reintroduced into the system in an effort to reduce the efficiency of the engine at peak load. This reduces engine combustion temperatures and thereby reducing NOx. Another way of reducing NOx emissions is to advance engine timing by a few degrees. This allows some of the unburned fuel to carry the excess heat out of the engine during low speed.

Answer 3

In a "nut shell"
Incomplete combustion.