With the declining energy resources and increase of pollutant emissions, a great deal of efforts has been focused on the development of alternatives for fossil fuels. One of the promising alternative fuels to gasoline in the internal combustion engine is natural gas [1–5]. The application of natural gas in current internal combustion engines is realistic due to its many benefits. The higher thermal efficiency due to the higher octane value and lower exhaust emissions including CO2 as a result of the lower carbon to hydrogen ratio of the fuel are the two important feature of using CNG as an alternative fuel. It is well known that computer simulation codes are valuable economically as a cost effective tool for design and analysis of the engine operations. In the present work the use of an exiting spark ignition engine to run on both gasoline and CNG is evaluated by thermodynamic simulation of the engine cycle. The stepwise calculations for pressure and temperature of the cylinder at compression process, ignition delay time, combustion and expansion processes have been considered. The first law of thermodynamics is applied for all steps and Newton-Raphson method is used for the numerical solution. Temperature dependent specific heat capacity and as a result specific enthalpy, entropy, internal energy and specific Gibbs functions are calculated in each step. Two zones model for the combustion process simulation has been used and the mass burning rate is predicted by considering the propagation of the flame front spherically. The performance characteristics including power, IMEP, ISFC, thermal efficiency and emissions concentration of SI engine on both gasoline and CNG fuel are determined by the model. In order to validate the model, the results are compared with the corresponding experimental data. It is found that the simulated results show reasonable agreement with the experimental data.

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