For the improvement of the calibration process in CI engine, it is required to optimize both fuel injection timing and its rate by using the model based control (MBC) on ECU (on-board) during cycle-by-cycle calculation. The on-board MBC is consisted of 6 calculation models, such as a gas flow, a spray, a formation of mixture, combustion, an ignition delay, and a heat loss. The present study developed the on-board gas flow model, and our successive study discussed the on-board heat loss model that could estimate the wall temperature of cylinders, because the heat loss is significantly related to the turbulence of in-cylinder gas flow. Since gas flows are changed with each position in an in-cylinder, it was modeled by varying the combination of four kinds of flows, such as a swirl flow, a squish flow, an axial direction flow and an injection flow. For the calculation of the on-board gas flow, we improved the new model to discretize the calculation points and to reduce the calculation load, referred to as the discretized gas flow model. Furthermore, the wall temperatures of cylinders were estimated by using the discretized gas flow model, whose root mean square error was 0.9 %, compared with the results from the 3-D CFD simulation. In addition, the calculation time for the model was less than 40 μs, which corresponded to the time within 1 cycle.