© 2022 Elsevier LtdThis study examines combustion and emission characteristics of a converted-engine, which suggests that a diesel engine with high compression ratio is transformed to a spark-ignition engine through natural gas addition coming from the intake port. The pollutant emissions such as NOX for the converted-engine pollutant is generally higher than that of a diesel-engine along with higher in-cylinder temperature. Those challenges can be overcome through new and novel combustion methods such as colorless distributed combustion (CDC). Colorless distributed combustion is a method that provides ultra-low pollutant emissions, a more uniform thermal field and reduced combustion noise along with mitigated flame instability. CDC is basically achieved by reducing oxygen concentration in the oxidizer. That decrease can be provided with recirculation of hot combustion products into the oxidizer. In order to achieve CDC, N2 as the diluent was entrained into the fuel-oxidizer mixture from 23 % to 13 % O2 concentrations by mass. The converted-engine representing a tractor engine operated at 2300 rpm and full load was investigated numerically to obtain its performance, combustion and emission characteristics under CDC conditions through ANSYS Forte 3-Dimensional analysis program. G-equation combustion model with methane chemical kinetic mechanism consisting of 29 species and 171 equations, and RANS k-e turbulence model were selected for all modelings. According to the results predicted, it is demonstrated that for the converted engine there has been an improvement of up to 29% in terms of performance. In addition, it is concluded that a 97.66% decrease in NOX emission has been achieved without reducing the combustion performance of the converted engine under CDC.