The formation of NOx emission in a diesel engine is strongly dependent on the combustion temperature, along with the concentration of oxygen presents in the combustion process [25]. In the present study, water is introduced into the intake air by using an adapted carburetor during intake stroke and it partially evaporates and mixes up with the fresh air. Thus, the presence of water vapor causes a decrease in the temperature and a rise in the specific heat of the intake charge. This usually results in lower combustion temperatures and lower NOx emission [4,5,9,21].