NO is the major component of NOx in diesel engines and it is the reaction product of nitrogen at high temperature and oxygen enrichment simultaneously. Two major NO formation mechanisms are thermal NO and Prompt NO. However prompt NO is prevalent only in fuel rich systems and it is not a major source of NO for diesel engines. Only thermal NO based on Zeldovich’s mechanism is predominant and it is shown as; O + N2 ↔ NO + N, N + O2 ↔ NO + O, N + OH ↔ NO + H. Higher post-flame temperatures and oxygen concentrations during the combustion process results in high thermal NO formation rates [6]. When the oxygen level is increased to 27% the NOx level increases by three times the ambient air as it is shown in Fig. 8. This is the major drawback of oxygen enriched combustion; however, this can be controlled and kept within the reasonable limit by adjusting the fuel injection timing, after treatment techniques [17,19].