All factors facilitating and accelerating the reaction between oxygen and nitrogen increase NOx formation. Thus,the main factor in NOx formation is temperature. However, engine speed, combustion chamber content, combustion chamber homogeneity,and mixture density in the combustion chamber are also factors. In fact, according to Fig. 5(a) and (b), NOx emission increases from low to medium engine speed and then decreases above medium engine speed in the UE (a) and the CE (b).