Numerical methods are commonly used for analyzing combustion characteristics. Besides, they provide an opportunity to predict emissions of an engine. In this paper, the conversion of a single-cylinder diesel engine to HCCI combustion model is investigated with AVL Boost v2021R2. The model was used to simulate the power, emission, bsfc value. In-cylinder pressure results also are obtained from the model. Verification of the model is conducted with experimental data from the literature. The differentiation between the numerical and experimental results remained below 8.9% for the power, below 6.62% for bsfc. In addition to this, the model gave maximum pressure values with an accuracy of ± 1%, and maximum HRR values with an accuracy of ± 2%. Maximum HRR values and in-cylinder pressure curves for 1200-2400 rpm were obtained with acceptable accuracy. Besides, the operating range of an HCCI engine fueled with n-heptane was investigated using a zero-dimensional single-zone model with reduced fuel chemistry. The compression ratio and inlet air temperature effect on HCCI combustion were analyzed. The increasing the air inlet temperature to 40oC from 20oC, increases the lowest air-fuel ratio about 6.6% which the engine can operate without entering the knock zone, at 17.5 CR at 1200rpm.