Air is about 78% nitrogen and 21% oxygen. At normal temperatures nitrogen is very unreactive, so the two gases sit side by side in the air without reacting. Fuels such as petrol and diesel are hydrocarbons, so they contain carbon and hydrogen, not the nitrogen that ends up in these pollutants.
Inside a car engine, the spark or the compression of the fuel and air mixture makes the temperature very high. At this temperature nitrogen and oxygen from the air can react together to form oxides of nitrogen, such as nitrogen monoxide, NO. The word equation is: nitrogen + oxygen → nitrogen monoxide.
Oxides of nitrogen are pollutants. They can cause acid rain and breathing problems. A cooler flame makes fewer oxides of nitrogen than a hotter flame.