Abstract: This study presents a detailed mathematical modeling and simulation study of CNG-diesel dual fuel compression ignition engine cycle processes using EES (Engineering Equation Solver) software. By means of a step-by-step simulation approach, the values of pressure, volume and temperature at all the state points in the ideal, air-diesel and CNG-diesel dual fuel cycles are computed. Various performance parameters such as thermal efficiency, power output, mean effective pressure and network output are evaluated for chemically correct (Y = Ycc ) air fuel mixtures assuming adiabatic combustion and full load operation for diesel and CC CNG-diesel fuel modes. CNG available in National Capital Territory of Delhi has chemical composition with methane content of 84.5% on Mole basis. In this study, the impact of other two main constituents of CNG, i.e., Ethane (C2H6 ) and Propane (C3H8 ) on the CNG-diesel dual fuel engine parameters is also analyzed. The predicted 2 6 3 8 results of various performance parameters of dual fuel cycle using 15% diesel fuel and 85% CNG fuel with 100% methane (CH4) in it and with 84.5% methane, 12.5% ethane and 3% propane are computed and presented 4 graphically. Predicted results of cycle simulation indicate that conversion of diesel to CNG-diesel is possible with no change in compression ratio, engine design and engine basic structure.