To meet the high real-time computation requirements for the Diesel cycle process within the cylinder of a marine engine digital twin model, a single-zone real-time rapid prediction model with variable step-size explicit calculation was proposed. To improve computational efficiency, calculation points were discretized, step sizes for each phase were allocated according to precision and stability requirements, and physical equations were explicitly formulated to minimize computational errors. The model simplification involved adopting a polytropic process and improving the calculation method of specific heat ratio for the compression and expansion model, reducing combustion zones, simplifying combustion into a single-step chemical reaction, and optimizing combustion calculation points. The validation of the model was based on a 320 mm medium-speed marine engine. The results show that the calibrated peak pressure error is less than 2%, while the predicted peak pressure error is less than 3%. Both the errors for CA10（the crank angle corresponding to 10% of total heat release）and CA50（the crank angle corresponding to 50% of total heat release）are less than 1.5°. The computation time for the model is within 0.25 ms, enabling the model to achieve accurate predictions while simultaneously meeting the requirements for real-time performance.