Predictive thermodynamic model of the performance of a stationary spark-ignition engine running on natural gas

In order to reduce the emissions of polluting gases and contribute to the efficient use of energy, natural gas has gained relevance due to its cleaner burning when compared to gasoline and diesel oil. In this work, a zero-dimensional thermodynamic model was developed to predict the performance, for the entire operating conditions, of a 11.7-L, six-cylinder, turbocharged spark-ignition engine used as genset and running on natural gas. This zero-dimensional model covers the closed period of the engine operating cycle. Firstly, this engine was tested in seven (7) different operating conditions to obtain the mass fraction burned and the pressure curves inside the cylinder. The combustion process was modeled using the simple Wiebe function, whose parameters were obtained from the seven (7) curves of the mass fraction burned through the Levenberg-Marquardt optimization method. Correlations were developed to estimate the Wiebe function parameters, as a function of pressure and temperature in the intake manifold, cylinder wall temperature, power and fuel consumption, for the entire operating range. The combustion efficiencies were estimated from the seven (7) experimental pressure curves with the aforementioned optimization technique. Results showed that the differences between the Wiebe function parameters obtained with the developed correlation, for the whole engine operating range, were less than 3% when compared with the Wiebe function parameters estimated for each of the seven (7) operating conditions tested. Results also showed that the thermodynamic model can be used to simulate the engine's performance, leading to errors less than 5% when compared with the measured maximum pressure and indicated mean pressure. In addition, it was found that the delay and duration of the combustion are smaller than those usually found in spark-ignition engines and that the energy release is deficient, resulting in high specific fuel consumption. This issue can be addressed by installing an equivalence ratio control system, especially in loads above 50%, thus ensuring greater control of emissions and increasing energy efficiency.