The oxides of nitrogen in the exhaust emissions contain nitric oxides (No) and nitrogen dioxide (No2). The NOx emission was lower for methanol–biodiesel blend at all loading conditions due to the low cylinder gas temperature and cooling effect of the methanol blend . The NOx emission was lower due to high cetane number than diesel that Wang Resin results in reduced ignition delay. The aromatic and Poly aromatic hydrocarbons which are responsible for high flame temperature was lower for reduced NOx emission. With higher percentage of EGR, NOx concentration is lower due to less oxygen content and cylinder gas temperature . The NOx emission is reduced by 1% in C5 and increased by 2% in P5 as the high saturated fatty acids quickly reacts with N2 to produce NOx. The NOx emission is lesser for all the blends except B5 (5% bio-diesel and 95% Diesel) due to its rich oxygen content that involves complete combustion to increase the exhaust temperature . The NOx emission for diesel, WPO 10 and WPO are 12.15 g/kWh to 7.61 g/kWh, 12.16 g/kWh to 7.75 g/kWh and 14.68 g/kWh to 8.23 g/kWh respectively at low loads and full loads respectively. The cause of high NOx emission in WPO may be due to the presence of aromatic content with ring structure that results in higher adiabatic flame temperature for high heat release rate and the higher ignition delay that enhances premixed combustion . The NOx emission increased with increase in the load but lesser compared to that of diesel. The cause for less emission may be due to the high oxygen content and low combustion temperature with extended combustion but at high loads the increase combustion temperature causes more NOx emission . The NOx emission was higher for TCC than SCC and HCC due to better mixture formation and larger part of the combustion is completed before top dead center for POME and its blends compared to diesel due to their lower ignition delay . Hydrogen dual fuel operation increases NOx emission compared with normal CI engine operation motor output increased due to several factors such as higher combustion chamber pressures and temperatures recorded during hydrogen dual fuel operation and higher pressure-rise rate indicates faster combustion rates which results in higher NOx formation rates. CO2 emissions are reduced by a similar extent for all dual fuel modes compared with normal engine operation as a result of an increased overall hydrogen–carbon ratio . The NOx emission was higher than the neat diesel fuel due to the excess oxygen that forms with the nitrogen in presence of high exhaust gas temperature and is to be increased by 10% by 30% biodiesel. This can be rectified by proper injection timing adjustment and provision Exhaust Gas Recirculation (EGR) . The NOx emission was higher for both cotton methyl ester and diesel fuel in coated engine due to the excess oxygen content and the thermal barrier which causes a raise in the gas temperature. In coated engine the NOx emission was increased by 6.5% for diesel engine and by 6.5–7.4% for cotton methyl ester compared to uncoated engine . For the CME (Castor Methyl Ester) blends B0, B05, B10 and B20 the NOx emissions are 683, 686, 638 and 694 ppm respectively at 145.29 kN load. The reason was vegetable oil contains nitrogen and the combustion is improved due to the presence of oxygen in the blend . The NOx emission for B20, B70 and pure biodiesel increased by 6%, 9% and 12% compared to diesel fuel. Due to the excess oxygen that reacts with the surrounding N2 to produce oxides of nitrogen .