Crank-rocker (CR) engine is a new kind of combustion engine that has a curved-cylinder combustion chamber. To enhance CR engine development and performance, high accuracy modeling which accurately predicts combustion characteristics is required. Due to the difference in cylinder configurations, the first empirical-model used to predict the combustion characteristics of a conventional engine may not be applicable. Currently, only a single correlation model has been proposed for CR engine. The main aims of this study are to improve the existing developed CR engine model and to investigate the impact of various heat transfer correlations in predicting combustion characteristics using single-zone combustion modeling. The model has been improved by including a sub-model of the specific heat ratio which varies with temperatures. Four cases of models incorporating various heat transfer correlations namely Woschni, Sitkei, Hohenberg, and Annand have been simulated. Then, the results were analyzed and compared with experimental data. Further simulations have been carried out to predict maximum pressures, temperatures, heat transfer coefficients, etc. under various loads. Annand's model was found to yield the best prediction for the combustion characteristics and it is considered the best choice concerning the accuracy and other criteria such as time and ease of usage. © 2021 THE AUTHORS