The demand for optimization of engine design and operating conditions in order to achieve fuel economy and attend strict emission legislation leads to development of engine simulation tools. The virtual testing tools provide results in a short time with low costs, and enable larger variation on the design and operational conditions. Nowadays engine simulation is performed by commercial software or open source models. In one hand, commercial software are able to simulate complex quasi or multi-dimensional models and have an intuitive interface with the user. The model validation can be extremely difficult since specific model details are not known. In the other hand, the open source models are appropriate for thermodynamic models capable of predicting in-cylinder data and pollutant emissions. The validation process in this type of tool is usually simple since all assumptions of modeling are known. However, the open source models usually do not have an interface and only the programmer can use the tool. Thus the novelty of the work is present the modeling of in-cylinder closed phase of a diesel engine with a graphical user interface (GUI). The tool predicts incylinder pressure, temperature, cycle efficiency, output power and the formation of six species of combustion products including carbon dioxide (CO2) and carbon monoxide (CO). The results are validated against experimental data.The present work aim to modeling in cylinder thermodynamic process between intake valve closing (IVC) and exhaust valve opening for diesel engine fueled with a GUI. The simulated and experimental results are in a good agreement.