Pollutant formation is strongly dependent on the fuel-air ratio distribution in the spray:

• NO is formed in the high temperature burned gases in the flammable region. Maximum burned gas temperatures result close to stoichiometric air-fuel ratio and these contribute maximum to NO formation.
• CO is formed in fuel rich mixtures in the flammable region.
• Soot forms in fuel-rich spray core where fuel vapour is heated by the hot burned gases .
• Unburned HC and oxygenated hydrocarbons like aldehydes originate in the region where due to excessive dilution with air the mixture is too lean at the spray boundaries. In excessive lean mixtures combustion process either fails to begin or does not reach completion. Towards the end of combustion, fuel in the nozzle sac and orifices gets vaporized, enters the combustion chamber and contributes to HC emissions.