How can changes to the combustion process reduce nitrogen oxide emissions?

Nitrogen oxides ($N{O}_x$) are primarily formed during combustion when temperatures are high enough to cause atmospheric nitrogen to react with oxygen. To reduce these emissions, combustion processes can be modified to lower peak flame temperatures, as lower temperatures inhibit the formation of these pollutants.12

One common method is exhaust gas recirculation (EGR). In this system, a portion of the exhaust gases is directed back into the combustion chamber. When mixed with the fuel-air charge, these gases act as a diluent, which lowers the overall combustion temperature and consequently reduces the production of nitrogen oxides.2

Additionally, modern engines utilize computerized control systems to optimize the combustion process. By precisely managing the air-fuel mixture, these systems improve combustion efficiency and prevent the excessively high temperatures that lead to increased $N{O}_x$ generation. While these modifications are effective for air pollution control, they are sometimes implemented at the cost of slight reductions in overall engine efficiency.2